Abstract
Heterospecific alarm calls may provide crucial survival benefits shaping animal behaviour. Multispecies studies can disentangle the relative importance of the various processes determining these benefits, but previous studies have included too few species for alternative hypotheses to be tested quantitatively in a comprehensive analysis. In a community-wide study of African savannah herbivores, we here, for the first time to our knowledge, partition alarm responses according to distinct aspects of the signaller–receiver relationship and thereby uncover the impact of several concurrent adaptive and non-adaptive processes. Stronger responses were found to callers who were vulnerable to similar predators and who were more consistent in denoting the presence of predators of the receiver. Moreover, alarm calls resembling those of conspecifics elicited stronger responses, pointing to sensory constraints, and increased responsiveness to more abundant callers indicated a role of learning. Finally, responses were stronger in risky environments. Our findings suggest that mammals can respond adaptively to variation in the information provided by heterospecific callers but within the constraints imposed by a sensory bias towards conspecific calls and reduced learning of less familiar calls. The study thereby provides new insights central to understanding the ecological consequences of interspecific communication networks in natural communities.
Highlights
Most studies investigating the role of communication in animal behaviour have focused on single-species groups [1,2]
Responsiveness increased with grass height (response probability (M2.1), duration (M2.3), head-ups (M2.5) and scratches (M2.6); figure 2f ), wind speed (response probability (M2.1), latency (M2.2) and duration (M2.3)) and proximity to the caller (latency (M2.2)), whereas no significant effects were found of proximity to cover, or group size. These results support that the environmental context can affect alarm responses (H6). These findings show that the responses of African savannah herbivores to heterospecific alarm calls are shaped by a range of factors which are partly adaptive, as indicated by the effects of body size similarity, caller consistency and grass height which affects predation risk, and partly non-adaptive, notably depending on the acoustic similarity between the conspecific and heterospecific calls
We established the information content of alarm calls from the community of African savannah herbivores and quantified species-specific alarm responses in order to test the relative importance of different adaptive and non-adaptive processes
Summary
Most studies investigating the role of communication in animal behaviour have focused on single-species groups [1,2]. To determine species-specific differences in alarm responses, we modelled response probability as the binary response variable in a logistic regression model with receiver species ID, call type (conspecific/heterospecific/control), their interaction, grass height, proximity to cover, distance to speaker, wind speed and group size as explanatory variables (n 1⁄4 2433 experiments). To assess the adaptive value of alarm calls (H2 – H6), we analysed the probability to respond to heterospecific alarm calls using a binomial mixed effect model with logit-link function with the following explanatory variables: receiver’s body size, body size ratio (including linear and quadratic terms as we expected the highest responsiveness to callers of the same size), the interaction between the receiver’s body size and the body size ratio (linear and quadratic term), caller consistency, call reliability, acoustic similarity and abundance of the caller. Response strength was analysed using separate loglinear mixed models for latency (M2.2, n 1⁄4 1529 experiments), duration (M2.3, n 1⁄4 1429 experiments) and speed of head-lifting (M2.4, n 1⁄4 1466 experiments), and generalized linear mixed effect models with negative binomial distribution for the number of head-ups and scratches (M2.5 and M2.6, n 1⁄4 1380 experiments); the explanatory variables and the random factor were the same as in the previous model
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