The Power of Infochemicals in Mediating Individualized Niches
Authors: Caroline Müller, Barbara A. Caspers, Jürgen Gadau, Sylvia Kaiser
Journal: Trends in Ecology & Evolution (2020)

Individual differences are central in behavior, ecology, and evolution, because they determine which selective factors are relevant for an individual in interaction with its environment; yet ecological niches have rarely been studied on an individual level.The subset of the species' niche realized by an individual is called its individualized niche and is implemented by three major processes: choice, conformance, and construction.Infochemicals can differ between individuals and empower the realization of the individualized niche. Combining hormones and semiochemicals under the term infochemicals highlights their common function in information transfer within or between organisms.We give several examples how chemical mediators, including hormones, pheromones, and allelochemicals, are essential in the realization of an individualized niche across the animal kingdom. Infochemicals, including hormones, pheromones, and allelochemicals, play a central role in mediating information and shaping interactions within and between individuals. Due to their high plasticity, infochemicals are predestined mediators in facilitating individualized niches of organisms. Only recently it has become clear that individual differences are essential to understand how and why individuals realize a tiny subset of the species' niche. Moreover, individual differences have a central role in both ecological adjustment and evolutionary adaptation in a rapidly changing world. Here we highlight that infochemicals act as key signals or cues and empower the realization of the individualized niche through three proposed processes: niche choice, niche conformance, and niche construction. Infochemicals, including hormones, pheromones, and allelochemicals, play a central role in mediating information and shaping interactions within and between individuals. Due to their high plasticity, infochemicals are predestined mediators in facilitating individualized niches of organisms. Only recently it has become clear that individual differences are essential to understand how and why individuals realize a tiny subset of the species' niche. Moreover, individual differences have a central role in both ecological adjustment and evolutionary adaptation in a rapidly changing world. Here we highlight that infochemicals act as key signals or cues and empower the realization of the individualized niche through three proposed processes: niche choice, niche conformance, and niche construction. Ecological niches (see Glossary) have long been considered as n-dimensional hypervolumes of populations or species [1.Hutchinson G.E. Concluding remarks.Cold Spring Harb. Symp. Quant. 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Oxford University Press, 2011Crossref Google Scholar]. Thus, for a comprehensive ecological niche concept, the concept of the individualized niche has to be included. For such integration we need to understand the ecological and physiological circumstances that modulate the resulting phenotypic and fitness differences [5.Smith B.R. Blumstein D.T. Fitness consequences of personality: a meta-analysis.Behav. Ecol. 2008; 19: 448-455Crossref Scopus (871) Google Scholar]. The interaction of an individual with its environment results in individualized niches via three major processes, namely niche choice, niche conformance, and niche construction (Figures 1 and 2). When an individual chooses the environmental conditions that best match its phenotype, it shows niche choice. Niche conformance occurs when an individual adjusts its phenotype to a given environment. Niche construction is the process whereby an individual modifies its environment according to its individual needs, which can also affect other species [6.Clark A.D. et al.Niche construction affects the variability and strength of natural selection.Am. Nat. 2020; 195: 16-30Crossref PubMed Scopus (10) Google Scholar]. All three processes have consequences for the individual and its offspring and facilitate a better match between the individual and its environment, enhancing the individual's fitness. For successful niche realization, appropriate communication both within and between individuals is crucial. The most ancient way of communication occurs via the use of chemicals and chemoreception. Such chemicals may be hormones, which convey information transfer within an individual, or semiochemicals, which mediate information between individuals. Whereas infochemicals have traditionally been seen as synonymous with semiochemicals [7.Dicke M. Sabelis M.W. Infochemical terminology: based on cost-benefit analysis rather than origin of compounds.Funct. Ecol. 1988; 2: 131-139Crossref Google Scholar], we propose to use the term infochemicals for both hormones and semiochemicals because of their unifying function in information transfer (Figure 2). Due to their evolutionary age, omnipresence, and high plasticity, infochemicals are predestined to be involved in processes leading to individualized niches, but this link has so far been neglected. We suggest that phenotypic adjustments via hormones and the information coded in semiochemicals are key factors mediating individualized niche realization. Integrating these factors into a comprehensive concept of the individualized niche will allow a deeper understanding of when, how, and why individual phenotypes are adaptively modulated by environmental factors or how and why individuals modulate their environment to meet their individual needs. Here, we focus on the role of infochemicals in the realization of individualized niches in the animal kingdom. Our conceptual approach may likewise apply to all living beings.Figure 2Infochemicals That Mediate Information within and between Individuals.Show full captionDifferent classes of infochemicals and their impacts on niche choice, conformance, and construction. In these processes, hormones mediate information within an individual, pheromones between conspecifics, and allelochemicals between individuals of different species. Each individual realizes its individualized niche.View Large Image Figure ViewerDownload Hi-res image Download (PPT) Different classes of infochemicals and their impacts on niche choice, conformance, and construction. In these processes, hormones mediate information within an individual, pheromones between conspecifics, and allelochemicals between individuals of different species. Each individual realizes its individualized niche. 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In most cases, endocrine cells respond quickly to environmental influences and thus change their secretion of hormones within seconds or minutes. However, the duration of behavioral responses to such hormonal changes can vary from seconds (e.g., fight and flight responses) to months or even years (e.g., organizational effects of hormones on sexual behavior) [11.Kelly A.M. Vitousek M.N. Dynamic modulation of sociality and aggression: an examination of plasticity within endocrine and neuroendocrine systems.Philos. Trans. R. Soc. B Biol. Sci. 2017; 372: 20160243Crossref PubMed Scopus (24) Google Scholar]. Hormones are important drivers of niche choice, conformance, and construction (Figure 2, Table 1). In insects, juvenile hormone titers entrain individual niche choices in dependence of the environment experienced during a certain developmental stage. For example, resource availability impacts hormone titers that determine horn size in dung beetles (Podischnus agenor) and horn size influences the behavioral niche choice of the individuals [12.Eberhard W.G. Beetle horn dimorphism - making the best of a bad lot.Am. Nat. 1982; 119: 420-426Crossref Google Scholar]. In locusts (e.g., Schistocerca gregaria), individual-specific information about the environment likewise affects juvenile hormone titers. These titers influence phase transition from solitary to gregarious behavior, which is accompanied by further behavioral, morphological, and physiological changes [13.Tanaka S. Endocrine mechanisms controlling body-color polymorphism in locusts.Arch. Insect Biochem. Physiol. 2001; 47: 139-149Crossref PubMed Scopus (79) Google Scholar,14.Verlinden H. et al.Endocrinology of reproduction and phase transition in locusts.Gen. Comp. Endocrinol. 2009; 162: 79-92Crossref PubMed Scopus (64) Google Scholar]. In vertebrates, individual hormone titers, especially of glucocorticoids, are central in niche choice. For example, in eider ducks (Somateria mollissima) the degree of corticosterone reactivity influences nest-site selection, with consequences for fitness [15.Seltmann M.W. et al.Boldness and stress responsiveness as drivers of nest-site selection in a ground-nesting bird.Ethology. 2014; 120: 77-89Crossref Scopus (13) Google Scholar].Table 1Examples of Infochemicals That Mediate Niche Choice, Conformance, and Construction by IndividualsChemicalsEffectsSpeciesRefsNiche choiceaNote that the authors of the respective studies may not have named these processes niche choice, conformance or construction.HormonesJuvenile hormonesDifferent horn size and behavior involved in reproductive strategiesOnthophagus taurusDung beetle[12.Eberhard W.G. Beetle horn dimorphism - making the best of a bad lot.Am. 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Ornithol. 2019; 160: 893-905Google Scholar]CorticosteroneNest-site selectionSomateria mollissimaEider duck[15.Seltmann M.W. et al.Boldness and stress responsiveness as drivers of nest-site selection in a ground-nesting bird.Ethology. 2014; 120: 77-89Crossref Scopus (13) Google Scholar]PheromonesUnknownUsage of different parts of the host plantIps duplicatus and I. typographusBark beetles[27.Schlyter F. Anderbrant O. Competition and niche separation between 2 bark beetles - existence and mechanisms.Oikos. 1993; 68: 437-447Crossref Scopus (60) Google Scholar]2,6,10-trimethyl-3-oxo-6,10-dodecadienolide and unknownOdor preference for conspecifics over heterospecificsSaccopteryx bilineata, S. lepturaSac-winged bats[28.Caspers B. et al.Odour-based species recognition in two sympatric species of sac-winged bats (Saccopteryx bilineata, S. leptura): combining chemical analysis and odour preference tests.Behav. Ecol. Sociobiol. 2009; 63: 741-749Crossref Scopus (32) Google Scholar]UnknownLarval rearing site preferenceRanitomeya variabilisPoison frog[29.Schulte L.M. et al.The smell of success: choice of larval rearing sites by means of chemical cues in a Peruvian poison frog.Anim. Behav. 2011; 81: 1147-1154Crossref Scopus (54) Google Scholar]UnknownRisk assessment of intrusion based on scent marksMus domesticusHouse mice[31.Hurst J.L. Beynon R.J. Scent wars: the chemobiology of competitive signalling in mice.Bioessays. 2004; 26: 1288-1298Crossref PubMed Scopus (327) Google Scholar]AllelochemicalsGlucosinolatesFeeding and oviposition stimulants of Brassicaceae plantsFeeding specialist herbivores[42.Hopkins R.J. et al.Role of glucosinolates in insect-plant relationships and multitrophic interactions.Annu. Rev. 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Rep. 2012; 2: 615Crossref PubMed Scopus (84) Google Scholar]AllelochemicalsUnknownDifferential performance on different parts of the same host plantMacrosiphoniella tanacetaria, Uroleucon tanacetiTansy aphids[45.Jakobs R. et al.Aphid infestation leads to plant part-specific changes in phloem sap chemistry, which may indicate niche construction.New Phytol. 2019; 221: 503-514Crossref PubMed Scopus (21) Google Scholar]Low-molecular weight, non-olefinic hydroxy-carboxylic acidMorphological changes due to the presence of predator cuesDaphnia pulexWater flea[48.Tollrian R. Vonelert E. Enrichment and purification of Chaoborus kairomone from water - further steps towards its chemical characterization.Limnol. Oceanogr. 1994; 39: 788-796Crossref Scopus (54) Google Scholar]UnknownInduced predator defense in tadpolesBufo bufoCommon toad tadpole[47.Hettyey A. et al.Predator-induced changes in the chemical defence of a vertebrate.J. Anim. Ecol. 2019; 88: 1925-1935Crossref PubMed Scopus (9) Google Scholar]Niche constructionaNote that the authors of the respective studies may not have named these processes niche choice, conformance or construction.HormonesJuvenile hormoneCaste determinationApis melliferaHoneybee[64.Huang Z.-Y. et al.Hormonal regulation of behavioural development in the honey bee is based on changes in the rate of juvenile hormone biosynthesis.J. Insect Physiol. 1991; 37: 733-741Crossref Scopus (141) Google Scholar]Juvenile hormoneDecision between foraging and egg laying in foundress associationsPogonomyrmex californicusCalifornian harvester ant[65.Dolezal A.G. et al.Worker division of labor and endocrine physiology are associated in the harvester ant, Pogonomyrmex californicus.J. Exp. Biol. 2012; 215: 454-460Crossref PubMed Scopus (47) Google Scholar]Estradiol, progesterone, testosterone, prolactinNest-building behaviorOryctolagus cuniculusRabbit[66.González-Mariscal G. 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Hormonal regulation of horn size in dung beetles influences the individual's behavioral niche choice, but could also be considered as niche conformance, because beetles adopt a mating strategy matching their horn size. The role of hormones in regulating niche conformance is well studied in guinea pigs (Cavia aperea f. porcellus). Living in different social environments during adolescence triggers different neuroendocrine responses in males that lead to adaptive shaping of their behavioral phenotypes [16.Sachser N. et al.The adaptive shaping of social behavioural phenotypes during adolescence.Biol. Lett. 2018; 14: 20180536Crossref PubMed Scopus (14) Google Scholar]. The frequency and intensity of social interactions during adolescence modulate testosterone secretion, influencing cortisol responsiveness and thus aggression (Figure 1B). Individuals with high or low aggressiveness conform to different social situations and use either resource defense or queuing strategies, realizing thereby distinct individualized social niches. An involvement of hormones in niche construction is observed, for example, in bees (Megalopta genalis, Bombus impatiens) to adjust their social organization. Dominant females can modulate changes in juvenile hormone levels in their daughters via social interactions or resource limitation of their offspring, leading to further physiological and behavioral changes in their daughters. These changes make the daughters more likely to become helpers or workers rather than reproductives [17.Kapheim K.M. Nutritional, endocrine, and social influences on reproductive physiology at the origins of social behavior.Curr. Opin. Insect Sci. 2017; 22: 62-70Crossref PubMed Scopus (18) Google Scholar]. In various vertebrate species nest-building behavior, often leading to niche construction, is regulated by different hormones [18.Murakami G. Distinct effects of estrogen on mouse maternal behavior: the contribution of estrogen synthesis in the brain.PLoS One. 2016; 11e0150728Crossref PubMed Scopus (12) Google Scholar, 19.González-Mariscal G. et al.Mothers and offspring: the rabbit as a model system in the study of mammalian maternal behavior and sibling interactions.Horm. Behav. 2016; 77: 30-41Crossref PubMed Scopus (37) Google Scholar, 20.Hall Z.J. et al.A role for nonapeptides and dopamine in nest-building behaviour.J. Neuroendocrinol. 2015; 27: 158-165Crossref PubMed Scopus (19) Google Scholar]. Given that nest-building behavior varies among individuals [20.Hall Z.J. et al.A role for nonapeptides and dopamine in nest-building behaviour.J. Neuroendocrinol. 2015; 27: 158-165Crossref PubMed Scopus (19) Google Scholar], niche construction must be considered at the level of the individual. In summary, in a rapidly changing environment and a world with dynamic global changes, the ability of an individual to respond quickly to such changes and thus the ability to realize individualized niches is crucial for survival and reproduction. Hormones are a key factor in the realization of individualized niches. Their crucial role is particularly evident in view of the current discussion that selection might act on the (endocrine) mechanisms of behavior rather than on the behavioral traits themselves [21.Trillmich F. et al.Understanding the evolution of personality requires the study of mechanisms behind development and life history of personality traits.Biol. Lett. 2018; 14: 20170740Crossref PubMed Scopus (20) Google Scholar]. 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Thus, pheromones can influence niche choice, ensuring aggregation of different species at distinct areas of their host trees. Likewise, several species decide which territories or breeding sites to use based on the presence of infochemicals. For example, female sac-winged bats (Saccopteryx bilineata) prefer to be in the vicinity of the odors produced by conspecifics over heterospecifics [28.Caspers B. et al.Odour-based species recognition in two sympatric species of sac-winged bats (Saccopteryx bilineata, S. leptura): combining chemical analysis and odour preference tests.Behav. Ecol. Sociobiol. 2009; 63: 741-749Crossref Scopus (32) Google Scholar] (Figure 1A). Poison frogs (Ranitomeya variabilis) choose water-filled cavities of plants for tadpole rearing based on the presence of chemical cues released by conspecifics [29.Schulte L.M. et al.The smell of success: choice of larval rearing sites by means of chemical cues in a Peruvian poison frog.Anim. 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Qualitative changes in the pheromones need to be accompanied by changes in their perception [33.Lee D. et al.Selection and gene flow shape niche-associated variation in pheromone response.Nat. Ecol. Evol. 2019; 3: 1455-1463Crossref PubMed Scopus (15) Google Scholar] and may provide some of the flexibility required to maintain functioning intraspecific communication. This may be particularly important when individuals adjust to new environmental conditions, that is, show niche conformance, potentially leading to niche-associated variation in pheromone signaling and perception [33.Lee D. et al.Selection and gene flow shape niche-associated variation in pheromone response.Nat. Ecol. Evol. 2019; 3: 1455-1463Crossref PubMed Scopus (15) Google Scholar]. If divergence in signals between populations increases, reproductive isolation may occur [34.Zhang Q.H. et al.Molecular and functional characterization of a candidate sex pheromone receptor OR1 in Spodoptera litura.Insect Sci. 2017; 24: 543-558Crossref PubMed Scopus (11) Google Scholar]. In social insects, differential production of and responses to infochemicals are an important mechanism for nestmate recognition and regulation of division of labor, resulting in task-specific niches. For example, CHC-based queen pheromones are crucial in regulating reproductive division of labor in colonies of ants, bees, wasps and termites [35.Holman L. Queen pheromones and reproductive division of labor: a meta-analysis.Behav. Ecol. 2018; 29: 1199-1209Google Scholar]. Group-specific odors also carry important information in social vertebrates. For example, odors differ among groups in spotted hyenas (Crocuta crocuta) [36.Theis K.R. et al.Evidence for a bacterial mechanism for group-specific social odors among hyenas.Sci. Rep. 2012; 2: 615Crossref PubMed Scopus (84) Google Scholar] or European badgers (Meles meles) [37.Noonan M.J. et al.Knowing me, knowing you: anal gland secretion of European badgers (Meles meles) codes for individuality, sex and social group me