# On the biological basis of beauty

Tamra C Mendelson, Julien P Renoult, Gil G Rosenthal, David M Shuker

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Tamra C Mendelson, Julien P Renoult, Gil G Rosenthal, David M Shuker. On the biological basis of beauty. Biological Reviews, 2025, 100 (4), pp.1578-1593. 10.1111/brv.70014. hal-05343313

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On the biological basis of beauty

Tamra C. Mendelson<fnref n="1" />, Julien P. Renoult<fnref n="2" />, Gil G. Rosenthal<fnref n="3" /> and David M. Shuker<fnref n="4" />

ABSTRACT

The world around us is full of beauty. Explaining a sense of the beautiful has beguiled
philosophers and artists for millennia, but scientists have also pondered beauty, most notably
Darwin, who used beauty to describe sexual ornaments that he argued were the subject of
female mate choice. In doing so, he ascribed a “sense of the beautiful” to non-human animals.
Darwin’s ideas about mate choice and beauty were not widely accepted. Humans may
experience beauty, but assuming the same about other animals risks anthropomorphism: we
might find the tail of the peacock to be beautiful, but there is no reason to believe that
peahens do. Moreover, mate choice, resurrected as an object of serious study in the 1970s,
simply requires attraction, not necessarily beauty. However, recent advances in psychology
and cognitive neuroscience are providing a new, mechanistic framework for beauty. Here we
take these findings and apply them to evolutionary biology. First, we review progress in
human empirical aesthetics to provide a biological definition of beauty. Central to this
definition is the discovery that merely processing information can provide hedonic reward.
As such, we propose that *beauty is the pleasure of fluent information processing, independent
of the function or consummatory reward provided by the stimulus*. We develop this definition
in the context of three key attributes of beauty (pleasure, interaction, and disinterestedness)
and the psychological distinction between ‘wanting’ and ‘liking’. Second, we show how

<footnote n="1">¹Department of Biological Sciences, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland, USA 20912</footnote>

<footnote n="2">²Centre d'Ecologie Fonctionnelle et Evolutive, Université de Montpellier, Centre National de la Recherche Scientifique, Ecole Pratique des Hautes Etudes, Institut de Recherche pour le Développement, 1919 route de Mende, 34090 Montpellier, France</footnote>

<footnote n="3">³Università degli Studi di Padova, Department of Biology; Centro de Investigaciones Científicas de las Huastecas Aguazarca”</footnote>

<footnote n="4">⁴School of Biology, University of St Andrews, Harold Mitchell Building, Greenside Place, St Andrews, KY16 9TH, UK</footnote>

<footnote>*Author for correspondence (E-mail: tamram@umbc.edu; Tel.: +1 410-455-2267).</footnote>
beauty provides a new, proximate approach for studying the evolution of sexual signalling that can help us resolve some key problems, such as how mating biases evolve. We also situate beauty within a more general framework for the evolution of animal signals, suggesting that beauty may apply not only to sexual ornaments, but also to traits as diverse as aposematic signals and camouflage. Third, we outline a variety of experimental approaches to test whether animal signals are beautiful to their intended receivers, including tests of fluency and hedonic impact using behavioural and neurological approaches.

*Key words:* animal signalling, beauty, mate choice, neuroaesthetics, sexual selection.

# CONTENTS

## I. Introduction

### II. Towards a biological definition of beauty

(1) Beauty is pleasure

(2) Beauty is an interaction

(3) Beauty is ‘disinterested’

(4) Beauty and aesthetics

### III. Why does beauty matter for evolution?

(1) A proximate explanation for exaggerated ornaments

(2) Linking beauty to mating

(3) A complement to ‘quality indicators’ and Fisherian runaway

(4) Beauty as an engine for diversification

(5) Linking beauty to other visual pattern functions

(6) Beyond visual signalling

## IV. Empirically testing the beauty of sexual ornaments

(1) Measuring hedonic impact

(2) Measuring fluency

(3) Combining measures of hedonic impact and efficiency

(4) Testing for disinterestedness

## V. Conclusions

## VI. Acknowledgements

## VII. References

# I. INTRODUCTION

Beauty (see Table 1 for glossary) has puzzled scientists and philosophers for centuries, and one of the most enduring questions is its evolutionary origin. In evolutionary biology, beauty typically refers to the elaborate traits that animals use in courtship, so-called ‘sexual ornaments’ (Darwin, 1874; Prum, 2017; Ryan, 2018). But while many sexual ornaments are clearly beautiful to us, an open and fundamental question is whether and to what extent
other animals experience beauty. If beauty is only a human construct, and its attribution to other animals is merely an anthropocentric projection of our own experience, then it is unlikely to be useful in explaining the evolution and diversity of sexual ornaments. However, if we share the underlying neural mechanisms that generate our sense of beauty with other animals, and there is increasing evidence that we do (Chatterjee & Vartanian, 2016), then the study of beauty in humans will inform our understanding of the role of beauty in other animals, and *vice versa*. The objective of this review is mutually to advance these disparate fields of study by applying the burgeoning science of beauty in psychology and neuroscience (Barlow, 1961; Berridge & Robinson, 1998; Simoncelli & Olshausen, 2001; O'Connell & Hofmann, 2011; Pool et al., 2016; Berridge & Dayan, 2021; Nguyen, Naffziger & Berridge, 2021) to fundamental questions in evolutionary biology.

Arguably the most influential proponent of beauty in evolutionary biology was Darwin (1874), who used the terms “beauty” or “beautiful” over 300 times in his book *The Descent of Man, and Selection in Relation to Sex*, in which he laid out his theory of sexual selection. He described beauty as “the pleasure given by certain colours, forms, and sounds” (p. 92 (2nd)) and ascribed a “taste for the beautiful” (p. 92) to many animals other than humans. Beyond the pleasure of perception, however, beauty remained a puzzle for Darwin: “Why certain bright colours should excite pleasure cannot, I presume, be explained, any more than why certain flavours and scents are agreeable” (p. 92). Yet, he was confident that taste, while variable among individuals and over time, is to some extent predictable [“taste is fluctuating, but it is not quite arbitrary” (p. 495)].

His hypotheses met significant resistance in the sexist, outwardly puritanical culture of 19th-century Western society (Richards, 2019), and the study of sexual selection *via* mate choice did not gain traction until the 1970s (Cronin, 1991). But as modern evolutionary biologists began studying mate choice and sexual selection in natural systems, the question of beauty was left behind. If the concept was used at all in this early work, it was rarely defined, and for the most part, beauty was equated with the ornaments themselves rather than the experience of a receiver. Indeed, for many years, the study of sexual selection addressed mostly ultimate-level questions of how animal ornaments evolve, for example whether through direct or indirect selection for mate choice (Cronin, 1991; Andersson, 1994) rather than proximate-level questions about what beauty *is*. Starting in the 1980s, a focus on sensory mechanisms brought attention to the proximate basis of mate choice (West-Eberhard, 1984; Ryan, 1990; Guilford & Dawkins, 1991; Endler, 1992), mainly considering physiological mechanisms such as auditory and visual tuning, while still largely neglecting the steps between detection and mating (Fig. 1).

Today, beauty in evolutionary biology is experiencing something of a renaissance (Prum 2017; Ryan 2018), notably in two recent popular science books. In *The Evolution of Beauty*, Richard Prum (2017, p. 6) describes beauty as “encompass[ing] all of the observable features that are desirable in a mate” and advocates for a particular model of sexual selection, based on indirect selection for mate choice, as the default explanation for elaborate male ornaments. In *A Taste for the Beautiful*, Michael Ryan (2018) takes a receiver-centered view, describing
multiple potential explanations for the evolution of female mate choice as key to
understanding the evolution of sexual ornaments. In both books, beauty is defined implicitly
or explicitly as “sexually attractive” or “desirable”: a trait is beautiful if it attracts a mate. But
if the sexual ornaments of other animals can be beautiful to us, for whom they hold no broad
sexual appeal, then equating beauty with sexual attraction does not adequately capture its
meaning. Prum (2017) restricts the definition even further, defining beauty as the by-product
of co-evolution between sexual signals and preferences, which he extends to the cultural
evolution of art. But clearly not all beauty is co-evolved – a sunrise does not care how you
feel.

Here we take a proximate approach to the question of beauty, characterizing its component
phenotypes in order to understand better its origin and potential impact on biological
diversity. Drawing on literature in psychology, neuroscience, and philosophy, we propose the
following working definition: *beauty is the pleasure of fluent information processing,
independent of the function or consummatory reward provided by the stimulus*. Below, we
outline the origins of this definition, using three core attributes of beauty to elaborate on the
biology of pleasure (Section II.1), the fluency of information processing (Section II.2), and the
difference between ‘liking’ and ‘wanting’ (Section II.3). We then describe how this biologically
grounded concept of beauty can expand our current understanding of sexual signal evolution
(Section III) and generate testable predictions (Section IV) that can be applied across animals
to ask whether sexual ornaments are beautiful to their intended receivers, and if so, whether
the experience of beauty affects mate choice.

## II. TOWARDS A BIOLOGICAL DEFINITION OF BEAUTY

We cannot do justice to the rich history of the study of beauty across disciplines. Yet at least three points of consensus have emerged that can help shape a definition grounded in biology. First, beauty is pleasure. Hume (1896, p. 299), for example, wrote that “pleasure and pain [...] are not only necessary attendants of beauty and deformity, but constitute their very essence”. Second, beauty is interactive. It arises from the interaction between a stimulus and the brain of a receiver, thus being neither wholly objective nor subjective. Third, beauty is independent of the function of, or desire for, the beautiful object. Kant (1902) proposed the term “disinterestedness” to describe this attribute of beauty. To claim beauty as disinterested is not to say that enjoying beauty is not adaptive, nor that the object has no function, but rather that perceiving it is a self-sufficient source of pleasure. These three attributes of beauty are manifest in our working definition and are bolstered by empirical evidence from the fields of psychology and neuroscience, as developed below.

### (1) Beauty is pleasure

The key role of pleasure in beauty is strongly supported empirically (e.g. Brielmann & Pelli, 2019). Thus, a sense of beauty requires a nervous system with the ability to respond to a stimulus with a reward. A robust body of literature shows that animals as diverse as mammals, birds, reptiles, amphibians, fish, flies, and bees have a neural reward system. In
vertebrates, this is the phylogenetically ancient mesolimbic reward system, which contains homologous brain regions that coordinate both motivation and reward (O'Connell & Hofmann, 2011). Pleasure (also hedonic impact) in humans and other vertebrates can be traced to these regions, called 'hedonic hotspots' (e.g. Berridge & Dayan, 2021). In invertebrates, fruit flies and honeybees appear to share conserved elements of an ancient arthropod reward system; however, it is unclear whether those systems generate a positive affect associated with pleasure or hedonic impact (Søvik, Perry & Barron, 2015; Dvořáček & Kodrík, 2021). The phylogenetic breadth of such reward systems, and their ability to generate affective states that qualify as hedonic, will delineate how far a sense of beauty could extend beyond humans.

Importantly, studies in the field of neuroaesthetics confirm that the pleasure humans derive from looking at objects or listening to music that they describe as beautiful coincides with activation in the vertebrate mesolimbic reward system (Kühn & Gallinat, 2012; Chanda & Levitin, 2013; Stark, Berridge & Kringelbach, 2022; Ishizu, Srirangarajan & Daikoku, 2023). That is, brain regions known to regulate the pleasure of food and sex – consummatory rewards associated with survival and reproduction – also regulate the experience of beauty. Comparative studies confirm that these regions are activated by, for example, sex and food in rats (Peciña & Berridge, 2005; Georgiadis, Kringelbach & Pfaus, 2012) and monkeys (Bichot, Heard & Desimone, 2011), and social bonding in prairie voles *Microtus ochrogaster* (Young & Wang, 2004), among other examples. The fact that beauty and consummatory rewards activate the same brain regions does not mean they have the same function; it is now widely recognized that each region of the brain can support multiple functions (Anderson, 2007). For example, the mesolimbic reward system also plays a key role in reinforcement learning (Coddington, Lindo & Dudman, 2023) and exploration and curiosity (Gruber, Gelman & Ranganath, 2014). Like Chatterjee & Vartanian (2016), we propose that the experience of beauty in humans may have been evolutionarily co-opted from an ancestral consummatory reward circuit. If so, then a sense of the beautiful could be phylogenetically widespread.

## (2) Beauty is an interaction

The second attribute of beauty is that it arises from an interaction between a stimulus and the brain of a receiver. "It is not the object that makes the relation aesthetic but the relation that makes the object aesthetic" (Genette, 1997, p. 18). Beauty as an interaction corresponds with studies in psychology and cognitive neuroscience showing that a fundamental predictor of an aesthetic experience is the fluency with which information is processed. The psychological model of 'fluency' arguably represents the most influential modern explanation of beauty (Christensen, Ball & Reber, 2020; Vogel et al., 2020; Vissers & Wagemans, 2023; reviewed in Schwarz, 2018). It is grounded in evidence that stimuli that are experienced as 'easy' to process by the brain trigger pleasure and are thus judged as beautiful (Winkielman et al., 2003; Reber, Schwarz & Winkielman, 2004). Many features of communication signals are known to ease information processing. Some increase detectability [e.g. high visual contrast (Reber, Winkielman & Schwarz, 1998; Stevens & Ruxton, 2012)], others enhance
recognizability and memorability (e.g. prototypicality; Winkielman et al., 2006), and yet
others facilitate processing because they are redundant and predictable (Yoo, Jasko &
Winkielman, 2024). Redundant features are repeated over time or space and include
symmetry, roundness, or any form of temporal (e.g. rhythmicity) or spatial (e.g. fractals)
autocorrelation. All these features, processed fluently by the brain, have been shown to
increase stimulus attractiveness to humans and, for several of these features, to other
animals (references in Renoult & Mendelson, 2019). The mechanisms by which fluency elicits
pleasure are not entirely clear but may be a response to some combination of reduced
metabolic activity, increased processing speed, or accuracy (Winkielman et al., 2003).

For visual images, another predictably attractive pattern is one with the image statistics of
natural scenes. Image statistics include the underlying spatial patterns of a visual stimulus,
for example, the distribution and redundancy of contrast and orientation. Natural scenes
have remarkably predictable visual statistics, and numerous studies in the field of empirical
aesthetics show that people prefer images with the visual statistics of natural scenes (Spehar
et al., 2015; Graham et al., 2016; Nguyen & Spehar, 2021). Indeed, many artists unconsciously
mimic these statistics in their work (Redies, Hasenstein & Denzler, 2007; Graham & Field,
2007; Graham & Redies, 2010), and deviation from natural statistics is known to cause visual
discomfort (Penacchio et al., 2023). This preference for natural visual statistics likely exists
because brains have evolved to process natural scenes efficiently (Redies, 2007; Spehar &
Taylor, 2013). The well-supported efficient coding hypothesis of Attneave (1954) and Barlow
(1961) posits that neurons are 'tuned' (i.e. selectively respond) to redundant features of the
environment. This neural tuning allows information from the environment to be processed
accurately with fewer active neurons, thus reducing the metabolic cost of processing. Given
robust evidence for the hedonic impact (pleasure) of fluency and a preference for efficiently
processed information (Spehar et al., 2015; Renoult, Bovet & Raymond, 2016) it is likely that
efficient processing is one of the primary causes of the subjective feeling of fluency (e.g.
Redies, 2007); beauty is therefore a result of 'fluent' or 'efficient' information processing.
That is not to say that all efficiently processed stimuli are beautiful, but rather that efficient
processing is a necessary, and potentially evolutionarily ancient, component of beauty.

A critical point is that beauty is not a property of the object itself but rather resides in the
brain of the beholder. Whether a stimulus 'is beautiful' depends ultimately on the match
between its features and the neuronal tuning of the perceiver. This neuronal tuning
perspective has two important consequences. First, beautiful stimuli are not necessarily
simple. Even complex designs and shapes, such as those in elaborate sexual signals, can be
processed efficiently, if neurons are tuned to those stimulus features (Joye et al., 2016; Chang
& Tsao, 2017).

Second, this perspective accounts for both the subjective and universal characteristics of
beauty that have confounded philosophers for centuries. A stimulus that leverages neuronal
tuning developed over an individual's unique life experience will be judged beautiful by some
individuals but not others (i.e. seem subjective when assessed across individuals). This type
of plastic tuning is more typical of neurons active in later stages of cognition, like those
involved in assigning meaning. Therefore, tuning at this level would also explain the role of
conceptual fluency in beauty (Reber et al., 2004), for example why humans often find beauty
‘meaningful’ in some way. Alternatively, a stimulus tuned to neurons whose selectivity is
mostly independent of experience, and largely determined by a shared set of genes, will tend
to be ‘universally’ beautiful, at least to genetically similar individuals. Differences in neural
tuning therefore can arise *via* differences in both experience and genotype (as well as
epigenetic effects). Thus, individuals, populations, and species with different developmental
or evolutionary histories can differ in which stimuli are processed fluently or efficiently
(Renoult & Mendelson, 2019) and therefore which are judged as beautiful. These differences
have obvious implications for the evolution of phenotypic diversity (see Section III).

**(3) Beauty is ‘disinterested’**

The third attribute of beauty is that a receiver's pleasure is 'disinterested', or independent of the object's function (e.g. Eco & McEwen, 2005). Our working definition is consistent with that view, as we emphasize that the pleasure of processing a sexual ornament, or any other beautiful stimulus, is distinct from that of potential downstream events. An ornament may be beautiful, being fluently processed and thus pleasant for a receiver, but any further consequences in terms of changes in motivation for anything beyond information processing, for example whether or not the ornament leads to mating, is not relevant to its beauty (Schaeffer, 2015). This decoupling of beauty from downstream events would arise because the pleasure of processing stimuli is likely an adaptation for information gathering generally, rather than shaped to respond to any particular stimulus (Winkielman et al., 2003). Some cognitive scientists suggest that fluency is a metacognitive phenomenon that indicates progress in acquiring information (Schwarz et al., 2021; Sarasso et al., 2020b); centuries earlier, Kant (1902, p. 204) suggested something similar of beauty, which "denotes nothing in the object", but rather "a feeling that the subject has of itself and of the manner in which it is affected by the representation". Importantly, there is evidence that people generally are not aware of the source of the pleasure they feel from fluent information processing and incorrectly attribute it to the perceived object itself, rather than to the processing of their own brain (Schwarz, 2012), likely explaining the common intuition that beauty is an objective property of stimuli.

This interpretation of the philosophical concept of disinterestedness is supported by findings
in cognitive sciences that distinguish aesthetic-oriented perception from goal-oriented
perception. Notably, aesthetic perception is characterized by increased attentional
processing: paying attention to more features or certain features (lines, colours, or sounds)
that would go unnoticed during goal-oriented perception (Marković, 2012; Cupchik et al.,
2009; Sarasso et al., 2020b). Sarasso et al. (2020a) further argue that disinterestedness is
embodied in the inhibition of certain motor responses, which is a hallmark of aesthetic
evaluation. This motor inhibition allows for the refocusing of time and energy towards
perceptual processing, which thus can be more intense. Crucially, disinterestedness does not
preclude physiological or behavioural responses involved in decision-making and action; such
responses may indeed pertain to beauty as long as they aim to enhance information gathering.

A disinterested view of beauty also corresponds to the framework in modern psychology that distinguishes ‘liking’ from ‘wanting’ (Chatterjee, 2014). Berridge & Robinson (1998) first showed that rats can ‘like’ a stimulus (i.e. experience pleasure from it) independently of ‘wanting’ it. Rats deprived of dopamine lost the motivation to pursue a sugar reward – they no longer ‘wanted’ it – yet still they ‘liked’ it, as confirmed by orofacial and behavioural responses associated with hedonic reactions (tongue protrusions and paw licks). Though both liking and wanting are part of a shared reward system, they can be distinguished among other ways by associated neurotransmitters (see Section III(2)). ‘Liking’ is generally associated with the release of opioids and endocannabinoids, whereas ‘wanting’ – also called incentive salience to refer to unconditioned, typically consummatory incentives like sex or food – is driven primarily by dopamine (Berridge & Robinson, 1998; Pool et al., 2016; Berridge & Dayan, 2021). The liking–wanting framework maps well onto our working definition [see also Berridge & Kringelbach (2013) and Chatterjee & Vartanian (2016)]. First, beauty is universally agreed to trigger pleasure or hedonic impact, central to the concept of ‘liking’. Second, if beauty is a result of information processing, rather than a function of the stimulus itself, then beautiful stimuli need not be ‘wanted’ in a consummatory sense.

Importantly, disinterested is not the same as evolutionarily neutral or non-adaptive. Indeed, adaptation plays a crucial role both in tuning the brain to process environmental stimuli efficiently and to experience pleasure in gathering information; both of these adaptive processes underlie the experience of beauty. Beauty is only disinterested in so far as the receiver has no desire for the object beyond perceiving it, and possibly understanding it; receivers could make neutral or non-adaptive mating decisions in response to a beautiful ornament (Prum, 2017). Of course, beautiful stimuli may also be desired for reasons other than information processing. For example, consumers may purchase a work of art because they find it beautiful – they want to own it so they can continue processing it. They also may want it for its resale value, or to impress a potential mate or business partner (see references in Skov & Nadal, 2024), but these judgments of its value do not contradict disinterestedness in the judgement of its beauty.

Finally, not all attractive stimuli are necessarily beautiful, nor are all stimuli that elicit ‘liking’ likewise beautiful. Attraction encompasses more than just a sense of beauty, and it can be both ‘interested’ and ‘disinterested’. We suggest that beauty is a special case of attraction and ‘liking’, driven at its core by the disinterested pleasure of fluent information processing. Restricting beauty to disinterested attraction has at least two useful outcomes. First, it identifies information processing and its reward as mechanistically distinct from downstream or parallel processes, like sexual desire, generating testable hypotheses about how information ultimately generates a mate choice. Second, it emphasizes the proximate basis of attraction as distinct from its ultimate (evolutionary) function. That distinction allows for disinterestedness at the proximate level while acknowledging a central role for natural and sexual selection at the ultimate level.
### (4) Beauty and aesthetics

Fluency, whether perceptual or conceptual, is widely recognized as central to aesthetics. However, fluency alone is generally acknowledged as insufficient to explain aesthetic experiences (Belke, Leder & Carbon, 2015; Graf & Landwehr, 2017). Novelty, surprise, or a subjective perception of complexity can be, depending on the circumstances, necessary to trigger pleasure (Hekkert, Snelders & Van Wieringen, 2003; Hung & Chen, 2012; Ball et al., 2018). These experiences stimulate curiosity, evoke interest, and enhance attention – all of which enriches the aesthetic experience. Recent models of aesthetics, such as the Pleasure–Interest model of Aesthetic liking (PIA; Graf & Landwehr, 2015), have thus supplemented the original fluency-based model to incorporate these dimensions of aesthetic experience. Our definition of beauty does not include dimensions beyond fluency, because we hypothesize that many of these properties characterize aesthetics more broadly and do not apply to all experiences of beauty.

Despite a close association between beauty and aesthetics, many have emphasized the importance of distinguishing the two (e.g. Augustin, Wagemans & Carbon, 2012). Beauty necessarily entails pleasure, for example, whereas aesthetics, a more encompassing concept, can also include unpleasant experiences. The sublime, for example, involves mixed or negative emotions, such as vertigo (Gordon et al., 2017). The aesthetic experience can also include complexity, or surprise, which could make the experience of fluency more intense. Suspense is an essential component of the aesthetics of storytelling, where information is withheld in order to enhance the sense of fluency when it is eventually and suddenly received. Others propose that aesthetic reward is the *reduction* of surprise and uncertainty (e.g. Sarasso et al., 2020a; Van de Cruys & Wagemans, 2011). We contend that beauty is but one component of, or example of, an aesthetic experience.

Some stimuli can be beautiful simply due to fluent features in the automatic, stimulus-driven processing of information, while others can be beautiful if they become fluent during controlled, perceiver-driven processing after a mobilization of cognitive resources (Graf & Landwehr, 2017). The automatic experience fits philosophical accounts of beauty such as those of Thomas Aquinas or George Santayana, for whom beauty is immediate pleasure received without intermediate reasoning (Reber, 2012). It also aligns with numerous results showing that beauty can be perceived very quickly, within a timeframe compatible only with bottom-up information processing (Olson & Marshuetz, 2005; Verhavert, Wagemans & Augustin, 2018; Marzi & Viggiano, 2010). However, beauty can also be influenced by cognitive processes, and some authors have proposed that the difference between beauty and mere ‘liking’ is this mobilization of cognitive resources (Che et al., 2021). However, ‘liking’ arises for a variety of reasons other than fluent information processing. Like Redies (2015), we suggest that the automatic positive affect of fluency is a kind of ‘liking’ that can generate an immediate experience of beauty, and that additional cognitive processing also can generate an experience of beauty, either by creating or enhancing fluency. Whether other animals have aesthetic experiences more broadly construed, or whether beauty in other animals is
influenced by additional cognitive processing, are open and interesting questions. We restrict
our discussion to the immediate pleasure of fluent information processing.

### III. WHY DOES BEAUTY MATTER FOR EVOLUTION?

We thus propose a definition of beauty as the pleasure that an animal derives from fluent information processing, independent of the adaptive or maladaptive value this pleasure might have in the context of mating, or indeed any other function such as feeding or finding shelter. Taking a broader view, does it matter whether sexual ornaments are beautiful according to the proposed definition? Does this proximate concept of beauty clarify how communication evolves? We suggest that it does.

#### (1) A proximate explanation for exaggerated ornaments

Beauty was central to Darwin's theory of sexual selection *via* mate choice. Yet, for most of the 20th century, research in sexual selection neglected this proximate explanation for sexual ornaments (i.e. a 'taste for the beautiful') and focused almost exclusively on the fitness benefits of female mate choice. This focus expanded with the introduction of 'sensory exploitation' and 'sensory drive' hypotheses in the 1990s [also, e.g. 'receiver bias', 'sensory bias', 'pre-existing bias' (Ryan, 1990; Guilford & Dawkins, 1991; Endler, 1992). These related hypotheses posited in one way or another that the adaptation of sensory systems to their environments will generate biases that can drive the evolution of sexual ornamentation; beauty is an extension of these hypotheses.

Our understanding of how the brain transforms sensory information into a mating decision is still in its infancy, however, and current models explain some things well but not others. For example, conspicuousness is predicted to be a key signal design feature because it facilitates detection, and mate choice clearly requires detection. However, many ornaments are displayed by animals for whom detectability is not an issue, like highly social or lekking species (Fig. 2), and many are composed of intricate visual or rhythmic patterns that extend far beyond the need to be detected. A beauty hypothesis on the other hand posits that these patterns have evolved because they are efficiently processed, perhaps through symmetry, prototypicality, redundant spatial or temporal patterns, by mimicking patterns of the local habitat, or some combination of these. Beauty also accounts for supernormal stimuli, i.e. exaggerated versions of expected stimuli. For example, flowers of the deceptive orchid *Chiloglottis trapeziformis* are significantly larger than females of the species they imitate. Supernormal stimuli are easier to recognize and memorize, and they are preferred by receivers independently of the signalled quality, even in insects (Schiestl, 2004; Fig. 2). Much like other perceptual biases (Ryan & Cummings, 2013), beauty therefore increases our explanatory power for signal design beyond the more restricted set of signal features that facilitate detection.

#### (2) Linking beauty to mating
Whether and how the pleasure of information processing facilitates an animal making a mate choice itself is a fundamental question. Sexual ornaments in many taxa are known to increase mating success (Ryan, 1990, 1998), and they are also demonstrably attractive (Ryan & Keddy-Hector, 1992). But whether they are attractive because they are beautiful, and if so, whether beauty is a means by which ornaments increase mating success, remains to be tested. Here, we apply the liking–wanting framework to propose multiple alternative and complementary testable hypotheses to explain, at the proximate level, how a sexual ornament influences a mating decision, and how beauty would (or would not) play a role (Fig. 1).

One way a beautiful ornament could increase mating success is by providing a reward for mating in species where mating is hedonically neutral or even aversive. For instance, the hedonic reward of fluent information processing could stimulate the approach and/or inhibit the avoidance of a potential mating partner in the absence of ‘wanting’ to mate. This hypothesis would predict that males with coercive mating strategies, for example like many ducks or damselflies, would benefit from beautiful ornaments. However, for many animals, mating is a rewarding, goal-directed behaviour that both males and females ‘want’ (Paredes, 2014; Quintana, Mac Cionnaith & Pfaus, 2022). How might beauty contribute to or augment that process? One way is simply to increase the probability of approach, leading to selection of the most beautiful signaller by a chooser already hormonally primed to mate. Another is if the hedonic reward of efficient information processing amplifies the hedonic reward of mating (‘liking’) and/or the desire to mate (‘wanting’). Opioid transmission (at mu-opioid receptors) is known to regulate the pleasure of primary rewards like sex, but it can also inhibit the motivation to mate, potentially regulating sexual refractory periods (Paredes, 2014; Quintana et al., 2022). If the pleasure of efficient information processing is mediated instead by other neurotransmitters, such as endocannabinoids, beauty could have an amplifying effect on both ‘liking’ and ‘wanting’ to mate. Consistent with that hypothesis, the pleasant ‘chills’ induced by music do not appear to be regulated by opioids (Laeng et al., 2021), suggesting that beauty (of music) will not inhibit the desire to mate. Moreover, endocannabinoids increase motivation to mate in female rats (Memos et al., 2014) and humans (Lynn et al., 2020; Buchel, Miedl & Sprenger, 2018). Thus, beauty, through the action of endocannabinoids or other non-inhibitory neurotransmitters, could enhance both the pleasure of mating as well as the motivation to mate with a beautiful signaller (Fig. 1).

An alternative hypothesis is that ornaments are *not* beautiful by the proposed definition and simply trigger ‘wanting’, or incentive salience. For example, the ornament could mimic a primary reward such as food or sex: female guppy fish *Poecilia reticulata* are attracted to ornamental orange colour spots on males, presumably as a pleiotropic effect of attraction to orange-coloured food (Rodd et al., 2002). Ornaments may also directly generate incentive salience through learning, with ornaments functioning as ‘reward cues’. A reward cue is a neutral stimulus that becomes associated with a reward (the unconditioned stimulus, here mating) through learning, i.e. it is a conditioned stimulus (Pool et al., 2016; Berridge, 2007, 2023). Through conditioning, reward cues generate incentive salience just as does the unconditioned stimulus with which they are associated (Robinson et al., 2014). For example,
rats (Pfaus *et al.*, 2012) and quail *Coturnix japonica* (Cetinkaya & Domjan, 2006) learn to sexualize ('want') inanimate objects if they have been associated with a sexual reward. The ornament as a conditioned stimulus would thus trigger 'wanting', amplifying the desire to mate and increasing mating success of the signaller. Alternatively, incentive salience for an elaborate sexual ornament could be innate, arising through natural selection. For example, if ornaments are indicators of fitness benefits (e.g. direct benefits, or indirect benefits such as 'good genes') for the chooser, then a release of dopamine ('wanting') in response to the sexual ornament of a high-quality mate should be advantageous and could therefore evolve as an adaptive mate choice.

Still, each of these incentive ('wanting')-based explanations – mimic, reward cue, indicator – fall short in explaining the extravagant elaboration of many sexual ornaments, when a simple feature would suffice. Sexual ornaments that both directly generate incentive salience *and* are beautiful should be more effective at attracting a mate. For example, a mimic could generate both incentive salience and a hedonic (processing) reward, if it is a more efficiently processed representation (e.g. prototype) of the anticipated food or mate, much as painted portraits are often processed more efficiently than photographs of the faces themselves (Redies *et al.*, 2007). An ornament that is efficiently processed also would be learned more readily and thus more easily become a reward cue associated with mating. Indeed, an ornament that facilitates a combination of these processes – liking, wanting, and learning – may be the most effective at attracting a mate.

Thus, our working definition of disinterested beauty and the liking–wanting framework generates new and testable hypotheses, described above, about the relationship between elaborate sexual ornaments, mating success, and mate choice. One reason that distinguishing these hypotheses is helpful is that they stipulate different agents of selection and thus potentially different signal designs. If the function of ornaments is to be beautiful, or ‘liked’, according to the working definition, then signals should evolve to be fluently processed, with optima specified by pleasure pathways in hedonic hotspots of the brain. If ornaments are ‘wanted’, signal evolution might be directed instead or in addition towards designs that are associated with the consummatory rewards of eating or mating, with optima specified by dopaminergic pathways in areas of the brain associated with gustatory or sexual motivation.

**(3) A complement to ‘quality indicators’ and Fisherian runaway**

Like sensory bias, the beauty hypothesis of sexual ornamentation challenges ‘quality’ as the default explanation for elaborate secondary sexual traits. A pervasive, almost reflexive, assumption in the modern study of sexual selection is that a sexual ornament indicates either the health of the courter (i.e. the ornament indicates ‘good genes’ or other indirect benefits) or the direct benefit a receiver might gain from mating, like a nuptial gift. Choosers are assumed to receive these fitness benefits from mating with a courter that bears the ornament, and elaborate ornaments evolve as a result of this adaptive mate choice. By contrast, choosing a mate based solely on the beauty of its ornament(s) does not assume or predict particular fitness consequences of mate choice; indeed, a choice based on beauty
could be maladaptive (Prum, 2017). Importantly though, 'beauty' and 'quality' hypotheses for
sexual ornamentation are not mutually exclusive, the former being a proximate and the latter
an ultimate explanation for attraction. Many signal features that facilitate information
processing can also be indicators of fitness. Symmetry, prototypes (e.g. the masculinity or
femininity of faces; Thornhill & Gangestad, 2006) and fractals (Pérez-Rodríguez, Jovani &
Mougeot, 2013; Cantarero et al., 2018) have all been suggested to indicate sexual partners
with 'good genes', and there is no reason a quality indicator cannot also be beautiful (e.g.
Patricelli, Hebets & Mendelson, 2019).

A beauty hypothesis also suggests both a starting point and a direction for Fisherian runaway selection (Fisher, 1930; Lande, 1981; Kirkpatrick, 1982), i.e. towards greater, more exaggerated contrast, symmetry, rhythmicity, etc. Because efficiency can be achieved in a variety of ways, selection can ‘run away’ in any number of predictable directions, for example towards a single, saturated colour or towards an elaborate design consisting of multiple efficiently processed elements. In this way, beautiful traits may be arbitrary with respect to the direct or indirect benefits provided by the signaller, as proposed by some (e.g. Prum, 2017) (although notably not by Fisher), but they are predictable.

**(4) Beauty as an engine for diversification**

Behavioural ecologists have long posited that divergent environments can lead to divergent sexual ornaments, either because habitats differ in the properties of signal transmission (e.g. closed- *versus* open-canopy forest), or because different habitats favour different sensory biases, which in turn drive signal evolution. A beauty hypothesis identifies multiple additional dimensions along which ornaments can diversify. Environments can vary in many dimensions of spatial and temporal patterning (redundancies), and the brain can be tuned to one or more of those patterns. Sexual ornamentation therefore can evolve to mimic local patterns in potentially any sensory modality. For vision, for example, scale invariance describes the fractal nature of an image, i.e. how its pattern remains unchanged at different spatial scales (such as when zooming in or out). In darter fish (genus *Etheostoma*), the scale invariance of male nuptial colour patterns is correlated with the scale invariance of species-typical habitats, suggesting that the sexual ornament is efficiently processed and has diverged to match the pattern of the habitat (Hulse, Renoult & Mendelson, 2020; Fig. 2G). Environments can also vary in community composition, such that categories of stimuli that must be distinguished (e.g. species of congeners) will vary from place to place and over time. Because prototypicality depends on the characteristics of the categories being distinguished, environments can differ in what defines a prototypical stimulus. Thus, combining multiple dimensions of environmental variation with multiple additional ways of generating fluency (e.g. roundness, symmetry, contrast) suggests innumerable ways of producing a locally adapted, efficiently processed signal. Beauty therefore provides a powerful predictive framework for explaining both the complexity and diversity of sexual ornaments in nature.

**(5) Linking beauty to other visual pattern functions**
Beauty might also serve as a unifying explanation for visual patterning across multiple functions, including camouflage and aposematism. For example, a match between the visual statistics of an animal pattern and its habitat is a primary form of camouflage (i.e. background matching). The proposed definition of beauty predicts that background-matching camouflage patterns will be attractive because of that match to visual statistics of natural scenes, and recent evidence supports that prediction (Héjja-Brichard et al., 2023). If true, then background-matching patterns could serve as evolutionary precursors for sexual ornamentation. A mutation that increases the detectability or attractiveness of a background-matching pattern, for example by the addition of a colour or a bold outline, could create a novel design that is both detectable and pleasing to process. This pathway to sexual ornamentation challenges the conventional wisdom that natural and sexual selection are antagonistic in the evolution of sexual signals, respectively favouring camouflage *versus* conspicuousness. If background-matching patterns are attractive, then natural and sexual selection can be aligned, especially if the detectable component can be facultatively concealed, as in a collapsible fin or dewlap (Fig. 1), only detectable from a close distance (Marshall, 2000), or if it can be highlighted with a dynamic display. The darter fish described above may be one example, with a camouflaged base pattern ornamented with conspicuous coloration in the breeding season (Fig. 2).

The proposed definition also suggests that sexual ornaments could originate as aposematic patterns. Aposematic patterns are generally easy to process, so that predators can easily learn and remember them (Fig. 2). These fluently processed signals are often beautiful to us (Landová et al., 2012), and they are also attractive to conspecifics of the opposite sex (Rojas et al., 2018), for example in the poison frog *Oophaga pumilio* (Maan & Cummings, 2008) and the butterfly *Heliconius erato* (Finkbeiner, Briscoe & Reed, 2014). Thus, as for background-matching patterns, natural and sexual selection may be aligned in the origin of sexual ornamentation *via* efficiently processed aposematism. Note that aposematic patterning is also another demonstration of how liking can be distinguished from wanting, in that processing the stimulus may be pleasurable, but the resulting response is aversive, to distance oneself from the dangerous stimulus, whether snake, wasp, or lionfish.

## (6) Beyond visual signalling

Although most of the literature linking beauty with fluency in human cognitive psychology focuses on visual signals and preferences, the same principles appear to govern other sensory modalities. For example, perceptual fluency is thought to play a primary role in the pleasure of music (Reber et al., 1998). Moreover, music, an arguably 'disinterested' acoustic pleasure, activates the same regions of the brain as other rewarding stimuli like food and sex (Chanda & Levitin, 2013). Far less is known about beauty in the olfactory domain. Humans are "chemosensory idiots" (Wilson, 2014, p.), and we generally do not describe olfactory stimuli in the same transcendent terms as visual art and music. Nonetheless, variation among individuals in odorant receptor composition at the periphery, and experience-dependent changes in sensitivity to specific odorants (Jin, Pokala & Bargmann, 2016), suggest that the
efficiency of processing olfactory information may vary markedly among individuals. The proposed definition of beauty may therefore apply to olfactory stimuli as readily as to visual and acoustic stimuli.

Beauty by the proposed definition therefore can apply to any stimulus perceived by any animal with the appropriately tuned neural architecture. In that way, beauty applies to both cues and signals (Maynard Smith & Harper, 2003), whereas a co-evolutionary model, for example the Fisherian runaway process, suggests that beauty applies only to signals. A signal, by definition, evolves because of the effect that it has on the receiver of the signal; signals include sexual ornamentation, gestures, alarm calls, etc. A cue, on the other hand, is any aspect of an organism's biotic or abiotic environment that an organism perceives and may respond to but will not evolve in response, e.g. sunshine, a carcass, snow. In humans, a taste for the beautiful contributes to the creation and appreciation of art (a signal), but also to other goods whose primary function is not communication (e.g. cars, buildings, dishes, etc...). Our sense of beauty also clearly extends to cues, which in turn influences our attitude toward the environment, e.g. we are more willing to protect beautiful landscapes and species (Barua et al., 2012). Darwin proposed that beauty in other animals was limited to the pleasure of perceiving conspecifics of the opposite sex, and that only humans (and only Western, educated humans) could appreciate the beauty of a landscape or the night sky (Darwin, 1874). But given what we now know about the biology of beauty, it is reasonable to speculate that beauty influences animal behaviour not just in the choice of sexual partner, but perhaps in the choice of territory or food as well (Schaeffer, 2015). The challenge will be to evaluate how and to what extent.

# IV. EMPIRICALLY TESTING THE BEAUTY OF SEXUAL ORNAMENTS

To demonstrate empirically that a sexual ornament is beautiful according to the proposed definition requires showing that it generates pleasure because of the efficiency with which it is processed, and that this pleasure is 'disinterested', i.e. independent of downstream events like social bonding or mating. Achieving this 'gold standard' in any one system is a high bar that will likely require a series of experiments, each of which addresses one or more criteria. Here, we suggest a non-exhaustive variety of ways to (i) measure the hedonic impact of a sexual ornament, (ii) measure the fluency or efficiency with which it is processed, (iii) experimentally combine those measures, and finally (iv) evaluate 'disinterestedness' by distinguishing liking from wanting, arguably the most challenging empirical criterion.

## (1) Measuring hedonic impact

Approach behaviour is commonly used in studies of mate choice to measure attraction, or preference, inferred from metrics like low latency to approach, high frequency of approach, or a long time spent associating with a stimulus. These metrics could indicate that a stimulus has hedonic impact; logically, the animal is engaged in hedonic reward-seeking behaviour. Moreover, approach may represent an action tendency, that is, a behavioural component of an affective response (e.g. Lowe & Ziemke, 2011). However, because approach is so easily
confounded by wanting (Pool *et al.*, 2016; Rosenthal, 2017), behaviour alone is a reasonable
but insufficient demonstration of hedonic impact.

More directly, in humans, participants can report their pleasure, although care must be taken
to distinguish the pleasure of liking from any anticipatory pleasure of wanting (Pool *et al.*,
2016; Dai, Brendl & Ariely, 2010). Orofacial expressions can also be useful markers of hedonia
in humans, other primates, and rats (Berridge & Robinson, 1998; Schyns, Petro & Smith, 2007;
Peciña, 2008; but see Barrett *et al.*, 2019) and can be measured, for example using facial
electromyography to measure contraction of the cheek muscles (e.g. Korb *et al.*, 2020). In
many animals, orofacial expressions may be uninterpretable, but in others, they may be a
straightforward way to test whether a sexual ornament generates hedonic impact.

A more targeted behavioural test of hedonic impact is the conditioned place preference (CPP) paradigm (Tzschentke, 1998; Prus, James & Resecrans, 2011). CPP is used to assess the rewarding properties of stimuli, most commonly recreational drugs, but also social interactions (Lahvis et al., 2015), sexual interactions (Bedos, 2023), and social communication (Riters & Stevenson, 2012). Affective reward or hedonic impact is inferred if subjects spend more time in a location where they received the test stimulus compared to a location where they received a sham or control (i.e. their preference for a place is conditioned by the rewarding stimulus). The inference of hedonic impact is especially convincing when CPP is eliminated by blocking the transmission of opioids or cannabinoids, the neurotransmitters associated with pleasure, using a pharmacological antagonist (Prus et al., 2011; Jarosz, Sekhon & Coscina, 2006; Stevenson et al., 2020). If sexual ornaments are beautiful, they should induce CPP, and blocking opioids or cannabinoids should reduce CPP caused by a sexual ornament.

To assess hedonic impact physiologically, knowledge of the animal's reward circuitry and the technological capacity to interrogate it are key. In humans, pleasure can be traced to 'hedonic hotspots', including the ventral pallidum, nucleus accumbens, insula and orbitofrontal cortex. As noted above, it is associated primarily with the release of opioid and endocannabinoid neurotransmitters (Berridge & Dayan, 2021; Peciña, 2008; Berridge & Kringelbach, 2015; Sander & Nummenmaa, 2021). Because hedonic hotspots and neurotransmitters appear to be shared across vertebrates (O'Connell & Hofmann, 2011), researchers should be able to leverage the knowledge and methods of model systems, e.g. using state-of-the-art techniques to functionally characterize neuroanatomical regions (see Sander & Nummenmaa, 2021), to test whether the perception of sexual ornaments increases activity in hedonic hotspots and/or releases neurotransmitters associated with pleasure, at least in vertebrates. Evidence suggests that sexual ornaments do engage the reward circuitry of non-human animals, and specifically in ways that indicate hedonic impact. For example, the song of male white-throated sparrows *Zonotrichia albicollis*, and the calls of male túngara frogs *Engystomops pustulosus*, trigger activity in the nucleus accumbens and ventral striatum, respectively, of conspecific females (Hoke *et al.*, 2010; Earp & Maney, 2012).

(2) Measuring fluency
As a subjective sensation, fluency is notoriously difficult to measure (Oppenheimer 2008). In humans, fluency is often quantified through self-reports of how easy or difficult it is to process a stimulus, like recognizing a shape or understanding a text (Graf, Mayer & Landwehr, 2018). Self-reports, however, can be highly unreliable (Nisbett & DeCamp Wilson, 1977). A more repeatable proxy of fluency frequently used in psychology (Schooler & Hertwig, 2005) is the time required to complete a task, but this approach also suffers limitations, notably being influenced by motor skills.

New ways of quantifying fluency will become available by recasting fluency in terms of processing efficiency, rather than its subjective expression (Redies, 2007). Two main approaches are used to characterize processing efficiency in the brain. The first analyses brain functional connectivity through the lens of graph theory. Efficient processing then corresponds to the shortest physical path connecting brain regions activated by a stimulus (Achard & Bullmore, 2007). The second approach is to estimate the sparsity of the neural response to a stimulus. A sparse and thus efficient neural code is achieved by maximally activating the fewest possible neurons (Olshausen & Field, 2004). Both approaches rely on brain imaging techniques, in particular functional magnetic resonance imaging (fMRI) and *in vivo* calcium imaging. fMRI has been used extensively in neuroaesthetics to map regions that are differentially activated when processing beautiful stimuli (Kühn & Gallinat, 2012; Stark *et al.*, 2022; Ishizu & Zeki, 2011); however, the functional connectivity of these regions remains to be investigated. Calcium imaging is increasingly popular for both connectivity (Benisty *et al.*, 2024) and sparseness analyses (Yoshida & Ohki, 2020) because technological advances now allow the whole brain to be monitored in freely moving individuals of various species, including birds, mammals and fish (Grienberger *et al.*, 2022). Calcium imaging thus appears to be the most promising technique currently available to test directly whether sexual ornaments are efficiently processed.

As an alternative to studying brain activity directly, processing efficiency can be estimated indirectly using mathematical and computational models of perception. Deep convolutional neural networks have proved very useful for modelling vertebrate vision (Kriegeskorte, 2015; Lindsay, 2021). Using this approach, the stimulus (e.g. an image of an ornament) is processed by the network, and efficiency is estimated by analysing the activity of artificial neurons, for example their sparsity (Guest & Love, 2017). Another metric of efficiency that has been used with artificial neural networks is statistical typicality. A stimulus (e.g. the ornament of an individual) is efficiently processed if it is statistically typical, that is if its pattern of neuronal activation in the brain of the receiver is similar to that of other stimuli in the same category (e.g. the same ornament in other individuals of the population). In practice, the other stimuli are used to estimate a reference distribution of activations, and the statistical typicality of a focal stimulus is calculated as the likelihood of its activations given this reference distribution (Brielmann & Dayan, 2021; Ryali *et al.*, 2020). Statistical typicality relies on the assumption that the brain has adapted to efficiently process the most common stimuli of a category.

An even more indirect approach to measuring efficiency, yet convenient for studies in ecology and evolution, is to quantify statistical features of a stimulus (e.g., a sexual signal) that are
known to be fluently processed (Mayer & Landwehr, 2018), such as high colour contrasts, averageness, redundancy, or a match between the statistical features of an ornament and the species' preferred habitat. As noted, scale invariance is commonly used as a measure of fluency, and the scale invariance of male darter fish nuptial coloration correlates with that of species-typical habitat. Females do not show this correlation, suggesting that only the sexual ornament (male nuptial colour) is efficiently processed (Hulse et al., 2020). Thus, comparing the sparsity, statistical typicality, or other image statistics (e.g. scale invariance) of sexual signals with that of phenotypes that are *not* expected to function as sexual ornaments could test whether sexual ornaments exhibit higher sparsity or typicality, or more closely match the image statistics of natural scenes.

**(3) Combining measures of hedonic impact and efficiency**

To test whether processing efficiency is the cause of pleasure, researchers can manipulate the efficiency of a stimulus to generate a range of values (e.g. high to low sparsity, typicality, scale invariance) and evaluate their effect on hedonic impact, predicting a positive correlation between the degree of efficiency and metrics of pleasure. Experiments like these that use the most direct measures of hedonic impact (e.g. CPP, brain imaging) and efficiency (e.g. functional connectivity, sparsity) remain to be conducted in both humans and other animals, but some recent studies based on more indirect measures are compelling.

One study in humans, for example, showed that the sparsity of images of human female faces, as measured by a simple model of the primary visual cortex (V1), was positively correlated with attractiveness as judged by human males (Renoult et al., 2016). Using the same approach, Holzleitner et al. (2019) found that sparseness was the best predictor of face attractiveness when compared with body mass index, sexual dimorphism, averageness, and asymmetry. More recently, Dibot et al. (2023) used a deep convolutional neural network to model information processing in the entire ventral stream of the visual system, rather than in V1 alone, and showed that sparseness explained 24–28% of the variance in facial attractiveness. Using a metric of statistical typicality, Ryali et al. (2020) showed that typicality is a good predictor of the attractiveness of those faces.

Other studies have investigated the response of viewers to features that are known to be fluently processed. In a particularly compelling study, Spehar et al. (2015) showed that the degree of scale invariance to which people have the highest sensitivity (measured as absolute detection, discrimination, and contrast sensitivity) is the same as the dominant scale invariance of natural environments. This is one of many studies showing that the spatial statistics of natural scenes are processed fluently. Importantly, however, this study further showed that this value of scale invariance is also the most attractive to people. Applying a similar approach to other animals, Hardenbicker & Tedore (2023) found that peacock jumping spiders (*Maratus picatus*), known for their elaborate nuptial colour patterns, preferentially approached images with the average scale invariance of natural scenes, compared to two other options. These studies are thus early examples of experimental designs that combine
manipulations of fluency with proxies of hedonic impact to test for beauty in human and non-human animals.

### (4) Testing for disinterestedness

Finally, to demonstrate the ‘disinterested’ attribute of beauty, a study would need to assess whether the hedonic impact of efficient information processing is independent of motivation (‘wanting’) for associated or downstream events like social bonding or sex. Given that beauty itself may give rise to wanting – animals may be motivated (‘want’) to continue processing the beautiful ornament (Fig. 1) – this attribute may be challenging to demonstrate, as it requires the ability to distinguish different motivational targets. Moreover, sexual ornaments might be both a reward and a reward cue, or they may combine fluency and incentive salience through mimicry, and thus elicit both liking and consummatory wanting. Nonetheless, carefully designed experiments and advancing technology should allow us to examine the criterion of ‘disinterestedness’ empirically.

Approach behaviour could be used to demonstrate disinterested liking if the experimental design can rule out a desire for social bonding or mating. For example, an experiment that manipulates the efficiency of abstract patterns and demonstrates a correlation between sparsity and approach behaviour can help exclude motivation for bonding or mating, which is unlikely for an abstract stimulus. Alternatively, an experiment could manipulate the processing efficiency of an ornament and measure approach behaviour of animals not in breeding condition, thus ruling out a hormonally primed motivation for sex in explaining a preference for a more efficiently processed ornament. Finally, the same experiment could be conducted in a phylogenetic context, using one or more species whose lineage branched off before the evolutionary gain of a sexual ornament (e.g. see Basolo, 1995). If species lacking the ornament preferentially approach an efficiently processed version of the ornament, that preference is unlikely to indicate a desire to mate, as it pre-dates the evolution of the ornament.

Beyond approach behaviour, psychologists and neuroscientists have developed increasingly sophisticated techniques to tease apart ‘liking’ and ‘wanting’. For example, Dai et al. (2010) used different psychophysical methods in a study of humans to evaluate the “likability” (liking) versus “incentive value” (wanting) of faces of the same and opposite sex. Although methods used in that study would need to be modified for other animals (e.g. the likability metric required knowledge of one’s name), the same principles could apply. Orofacial expressions can also be used to distinguish ‘liking’ from ‘wanting’ (e.g. Berridge & Robinson, 1998; Korb et al., 2020). In the seminal study that established the liking–wanting distinction, dopamine signalling in rats was experimentally blocked to eliminate ‘wanting’, yet rats still showed positive orofacial responses to sugar (‘liking’) (Berridge & Robinson, 1998). Thus, assessing orofacial response to a sexual ornament after blocking dopaminergic signalling in the reward pathway will indicate whether the ornament is ‘liked’ independently of ‘wanted’, as predicted for beauty. The challenge in that case is determining which orofacial expressions, if any, are associated with fluency in a given animal (for humans, see Topolinski et al., 2009).
Finally, technological advances now allow us to distinguish ‘liking’ from ‘wanting’ via neural imaging. Haber & Knutson (2010) describe different regions of the brain associated with reward anticipation *versus* reward outcomes, discoverable *via* fMRI. Pool *et al*. (2022) used high-resolution fMRI to identify distinct regions of the ventral striatum associated with the ‘wanting’ *versus* ‘liking’ of an olfactory reward (chocolate). If the motivation for social bonding or mating can be eliminated or reduced *via* dopaminergic blockade, the use of abstract stimuli, or by testing in the non-breeding season, then the activation of brain regions associated with hedonic reward would be compelling evidence of disinterested liking.

## V. CONCLUSIONS

(1) We offer a rigorous definition of beauty grounded in biology, psychology, neuroscience, and philosophy. Beauty by this definition is an evolutionary by-product of adaptive information processing, a type of bias that can be exploited by any communication signal, from animal ornamentation to human art.

(2) This definition can be used to test whether sexual ornaments are indeed ‘beautiful’, or whether a different concept or mechanism would be better suited to explain their evolution. Beauty may in fact be limited to the human brain, and it could also encompass more than the hedonic impact of fluent information processing. Nonetheless, given the ability of the proposed definition to explain a wide range of phenomena attributed to beauty in humans, and the similarities in information processing and reward circuitry across animals, we suggest this view as a baseline for further exploration.

(3) Applying the concept of beauty to non-human animals brings us back to Darwin, who remains perhaps the most influential thinker breaking down the experiential and cognitive boundaries between humans and non-human animals. Unifying beauty across art and animals also aligns the ideas of many great scholars of beauty, including Goethe, who proposed that “Beauty is a manifestation of secret natural laws, which otherwise would have been hidden from us forever”.

## VI. ACKNOWLEDGMENTS

We thank Pietro Allegretti, Patrice David, Yseult Héjja-Brichard, David Lucarelli, Eva Pool, Lauren Ritters, Michael Ryan, Martin Skov, and an anonymous reviewer for helpful comments on the manuscript. Work was supported by the University of Padua - University of St Andrews Joint Seed Funding Scheme to G.G.R. and D.M.S., by National Science Foundation IOS-2026334 to T.C.M. and J.P.R., and by Agence Nationale de la Recherche (ANR-20-CE02-0005-01) to J.P.R.

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<table><caption>Table 1. Glossary.</caption><tr><td>Beautiful (biological)</td><td>Describing a stimulus that evokes pleasure (hedonic impact) in a receiver due to fluent information processing, independent of its function or consummatory reward.</td></tr><tr><td>Beauty (biological)</td><td>The pleasure (hedonic impact) of fluent information processing, independent of the function or consummatory reward provided by the stimulus.</td></tr><tr><td>Consummatory reward</td><td>The pleasure (hedonic impact) of consuming or engaging in a tactile manner with a stimulus, for example eating or mating. Consummatory rewards are often associated with direct fitness benefits related to survival or reproduction.</td></tr><tr><td>Efficiency (efficient coding)</td><td>Measure of how well a system performs with a minimal use of resources (time, energy). The efficient coding hypothesis proposes that the brain extracts as much information as possible at the lowest cost possible by removing redundancies in a signal.</td></tr><tr><td>Fluency (perceptual, conceptual)</td><td>The subjective experience of ease with which information is processed by the brain. Perceptual fluency refers to sensory and low-level cognitive operations involved in the encoding of stimulus features. Conceptual fluency refers to mental operations concerned with stimulus meaning and its relation to semantic knowledge structures.</td></tr><tr><td>Hedonic impact ('liking')</td><td>Positive affective valence resulting from activity in the neural reward system (see Berridge & Kringelbach, 2008). 'Liking' in quotation marks distinguishes unconscious from conscious liking.</td></tr><tr><td>Image statistics</td><td>Quantitative description of regularities derived from analysing the pixel values within an image.</td></tr><tr><td>Incentive salience ('wanting')</td><td>A motivational process regulated by dopaminergic pathways in the neural reward system (see Berridge & Kringelbach, 2008). 'Wanting' in quotation marks distinguishes unconscious from conscious wanting.</td></tr><tr><td>Information</td><td>Property of a stimulus that reduces uncertainty about the environment. Information processing describes how information is received, transmitted, coded, stored and retrieved by an artificial or biological system.</td></tr><tr><td>Mate choice</td><td>Any aspect of an animal's phenotype that leads to its being more likely to engage in sexual activity with certain individuals than with others (Rosenthal, 2017).</td></tr><tr><td>Ornament (sexual)</td><td>A trait that evolves in response to sexual selection via mate choice.</td></tr><tr><td>Pleasure</td><td>Here, synonymous with hedonic impact and 'liking' (see Berridge & Kringelbach, 2008).</td></tr><tr><td>Sexual selection</td><td>Any selection that arises from fitness differences associated with non-random success in the competition for access to gametes for fertilization (Shuker & Kvarnemo, 2021).</td></tr></table>
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Beauty and mate choice. The hedonic impact of a beautiful ornament may be one of many mutually enhancing neural processes driving mate choice. First, a receiver (here, a female lizard) encounters a signaller (a male). If the receiver is in breeding condition, its baseline state is a hormonally primed motivation to mate, magnified by (conspecific) mate recognition signals or cues. If the ornament (here, a dewlap) is beautiful, then it is 'liked' because of the fluency with which it is processed, which triggers further 'wanting' to process the signal. Social animals are also motivated ('want') to bond socially, and signallers may also offer food or shelter, triggering further 'wanting'. This combination of 'wanting' and 'liking' different motivational targets (mating, information, bonding, food, etc.) amplifies the probability of the receiver approaching the signaller. Then, multiple sources of 'liking', e.g.</figcaption></figure>
information processing, bonding, can amplify the motivation to mate as well as its consummatory reward.
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Elaborate and exaggerated visual sexual ornaments. The proposed definition of beauty provides a powerful predictive framework for the elaboration and diversity of animal signalling. Males of the social Gelada baboons *Theropithecus gelada* (A) and the lekking cock-of-the-rock *Rupicola rupicola* (B) are easily detected by conspecifics due to social behaviour but nonetheless display conspicuous sexual ornaments. Contrasting colours and patterns trigger pleasure in addition to increasing detectability. Many sexual signals are highly elaborated, which often originates from an extreme regularity in pattern, for example, vertical bars in female pipefish *Microphis deocata* (C), star-shaped tails of the sage grouse (D), dots, concentric circles, symmetrical or fractal forms, including in elaborate extended phenotypes like the nest of the pufferfish *Torquigener albomaculosus* (E), and regularities in the rhythms of sexual vocal signals. Pattern regularity may indicate developmental quality and provide an ultimate explanation for attractiveness; beauty provides a complementary proximate explanation, that regularity eases information processing and thus is pleasing to the senses. Flowers of the orchid *Chiloglottis trapeziformis* (F) are supernormal stimuli, significantly larger than females of the pollinator species they imitate, easing recognition and memorability. Males in species of the genus *Etheostoma* (G) display nuptial patterns that correspond to the spatial statistics of their habitat. The aposematic patterns of *Oophaga pumilio* (H) and *Heliconius erato* (I) are also attractive to conspecifics, argued here because they are efficiently processed by both predators and conspecifics. Copyright permission for images not yet obtained.</figcaption></figure>