title: The evolutionary ecology of complex signalling systems in ungulates - diversity and flexibility in a changing world
Creators: name: D'Ammando, Giacomo
Publisher: name: University of Liverpool Repository
Year: 2021

description: Understanding the ultimate and proximate causes of signal diversity, complexity, and flexibility is a key issue in the study of animal communication. In this thesis, I investigated the factors affecting inter- and intraspecific variation in the visual and acoustic displays of wild ungulates, by adopting a two-pronged approach based on (1) comparative analyses with phylogenetic controls, and (2) playback experiments in the field. Comparative analyses provided the opportunity to test the validity of functional hypotheses proposed to explain the extreme diversity of sexual signals observed across ungulates (Chapter 2 and Chapter 3). Playback experiments, on the other hand, allowed for a fine-scale examination of (i) the adaptive significance of complex vocalizations (Chapter 4); and of (ii) the impact of anthropogenic disturbance on the flexibility of antipredator signalling systems (Chapter 5). In Chapter 2, I used the comparative method to identify the evolutionary drivers of courtship display complexity in male bovids and cervids. Specifically, I tested the role of multiple selection pressures in shaping the evolution of elaborate gestural signals. I found that the size of gestural repertoires (intended as a proxy for complexity) was positively correlated with the potential for male polygyny, and with the number of sympatric, closely-related heterospecifics. These patterns point to sexual selection and species recognition as the main promoters of complexity in courtship displays. Moreover, my results showed that larger male body mass was associated with smaller gestural repertoires, possibly due to energetic constraints imposed to movements in very large species. In Chapter 3, I asked why sexual dimorphism in two morphological signalling traits, namely colouration and pelage ornaments, is absent in a large number of highly polygynous bovids, contrary to expectations from sexual selection theory. I therefore tested whether lack of dimorphism could be explained by unsustainable fitness costs of ornamentation from sex-biased predation in species forming social groups containing multiple males and females (“mixed-sex herds”). Supporting this interpretation, I found that morphological dimorphism was promoted by intense male mating competition, but reduced by the propensity of sexual aggregation. Hence my results indicate that social integration in mixed-sex herds likely represents a powerful evolutionary force limiting the acquisition of dimorphic ornaments, in direct contrast with the positive drive exerted by sexual selection. In Chapter 4, I examined the adaptive value of the complex roaring display of male impala antelopes (Aepcyeros melampus). The roaring display is a signal of territorial advertisement, and consists of combinations of snorts and grunts. However, similar snorts are also used in isolation as alarm calls. Using playback experiments, I therefore focused on (i) establishing whether alarm and “advertisement” snorts where indeed the same calls; and on (ii) examining the function of snorts in the roaring display. I found that male impala reacted with similar risk-aversive responses to both alarm and “advertisement” snorts, indicating equivalence in the message conveyed by the two calls (as also supported by acoustic analyses). By contrast, grunts elicited aggressive behaviour. Roaring displays (snorts + grunts) also triggered aggressive reactions; the presence of the snorts, however, decreased the latency of male impala to respond appropriately to the following grunts. This suggests that snorts, which are originally alarm calls, have acquired a secondary role in roaring displays as “attention-grabbing elements” to the grunting component. My results therefore indicate that vocal complexity can also evolve via the co-option of pre-existing calls for secondary, derived functions. In Chapter 5, I aimed at determining whether human shields against natural predators along the borders of the Maasai Mara National Reserve (Kenya) could have reduced the alarm call responsiveness of two common antelopes, the topi (Damaliscus lunatus) and the Thomson’s gazelle (Eudorcas thomsonii). Contrary to my predictions, I found that both topi and gazelles showed stronger rather than weaker alert reactions to playbacks of conspecific alarm calls in areas exposed to human disturbance. I propose that this could be explained by the eventual association of alarm calls with low-intensity but unpredictable threats linked with human activities, which require careful evaluation by receivers. In summary, these four studies highlight how various social, ecological, and anthropogenic factors have contributed to the diversification of ungulate signalling phenotypes, and might provide useful insights to evaluate the effects of global changes on the behaviour of wild ungulate populations.