The evolution of vocalizations with specific "meanings" in songbirds
The assignment of arbitrary sounds with culturally defined meanings is a key building block in the development of language. However, a similar phenomenon can be found in the non-human animal world. Some animals, ranging from non-human primates to birds, use certain alarm signals in association with particular predator types, which allows others to respond appropriately without themselves observing the predator. How such communication systems arise is difficult to understand, because both the form of the signals and how others interpret them must develop in tandem. Are such context-specific signals truly arbitrarily assigned to stimuli? This project addresses these issues by evaluating the evolution of vocalizations used by a group of related species. Members of these species use a particular type of alarm call that is very similar across species when they observe cuckoos that seek to lay their eggs parasitically in their nests, but use species-specific alarm calls when confronting other types of threats. The project will develop theoretical mechanisms to explain why the evolution of the cuckoo-specific vocalizations is so constrained across species in comparison to other types of vocalizations and test the assumptions and predictions using field experiments.
This project involves collaboration between mathematicians and field biologists, exposing theoreticians to real world, testable problems and training field biologists in techniques of mathematical modeling so that their experimental research can be better planned and informed. The principal investigator will employ and train undergraduate research assistants in field techniques and basic research.
Language is a fundamental defining characteristic of human intelligence, but its cognitive basis and evolutionary origin remain mysterious, in part because humans are the sole extant linguistic apes. Comparative psychologists and neuroscientists have used animal models to understand the neural substrate and origin of two defining aspects of human language, the semantic association of sounds with socially relevant meanings and the vocal learning process by which humans learn words and associate them with meanings. My research utilizes songbird vocalizations—both alarm calls and songs—to investigate the evolution 1) of vocal signals having semantic meaning (alarm calls) and 2) of vocal learning itself (songs). First, I studied the evolution of alarm calls with specific "meanings" in 9 species of closely related songbirds breeding in India, Japan, and Sweden. Using an evolutionary analysis of the acoustic form and behavioral function of alarm calls, I showed that alarm calls associated with specific threats may evolve initially as warnings directed at the threat itself. These associations lead to stabilizing selection on the acoustic form of such alarm calls, which may make it easier for eavesdroppers to associate specific alarm calls with specific types of threats, promoting the evolution of context-specific signals (Wheatcroft and Price 2014 Behav. Ecol. in press). Second, I studied how the context in which an alarm call is produced relates to how it sounds through analyzing context-dependent variation in the alarm calls of 15 species of songbirds in India and Sweden. I demonstrated that the repetition rate of alarm calls is faster across a wide-variety of songbird species when they confront predators. Faster repetition rates increase the defensive responses of both naïve nestlings and experienced adults, suggesting that universal perceptual biases in birds facilitate recognition of the context in which signals are produced, similarly to how humans speaking different languages are able to recognize when one another is scared or happy based on the tone of their voice (Wheatcroft Anim. Behav. in revision). Lastly, I have found that nestling birds from two co-occurring and hybridizing species innately recognize and respond to the songs of their own species. This implies that the vocal learning process in birds is guided by innate perceptual biases that focus the attention of young individuals onto socially relevant signals, suggesting a connection between mate recognition and the evolution of vocal learning and between the genes controlling vocal learning and the speciation process by which two populations become reproductively isolated. My ongoing work investigates the neural and genetic basis of these biases and the social and ecological factors leading to their evolution (Wheatcroft and Qvarnström in preparation). My research on context-dependent variation in alarm calls has increased our understanding of how alarm signaling in birds depends on both learned and innately recognized features, while my ongoing work on the innate basis of vocal learning has the potential to elucidate the brain regions and genes underlying innate learning preferences and how these genes evolve and affect the buildup of reproductive isolation across species. These insights have the potential to broaden our understanding of the speciation process and, conversely, the origin of vocal learning itself. My previous and current work involves extensive experiments in the field, during which I have supervised bachelor and master student projects, trained research assistants, and worked with local officials and community members. I have disseminated my results through presentations at international conferences and publishing papers and my work increasingly involves inter-disciplinary collaborations with geneticists and neuroscientists, leading to truly integrative research.
