Researchers at UC Davis have recently published with the Cornell Center for Conservation Bioacoustics demonstrating that certain components of a monkey’s identity can predict how they communicate. Inter-individual differences have become an exciting field of research in recent years, as biologists set out to quantify how variation in a trait among individuals of a species may be acted upon by natural selection. Animals may develop behavioral ‘signatures’ or ‘repertoires’ that represent individuality when it is favored by evolution. In species where animals tend to perform the same behaviors repeatedly, differences in how individuals perform can be indicative of animal identity or status. Dr. Dena Clink, Allison Lau, and Dr. Karen Bales set out to investigate what parts of a coppery titi monkey’s (Plecturocebus cupreus) identity predicts differences in their communication signals. These monkeys are an ideal candidate for investigating individual differences since they use complex communication (they sing!) to communicate with their partner and defend their territory from other groups. Since these monkeys are naturally pair bonded and choose one mate to live and breed with exclusively, identifying your partner’s vocalizations may be important and provide potential fitness advantages.

For this study, authors recorded the morning duet vocalizations of 74 titi monkeys at the California National Primate Research Center. The authors then looked at a visual representation of those calls and measured variability in multiple calls from each individual using spectrogram analysis (see the image above for an example). Once all sound files were processed, the authors used linear modelling to assess which individual traits of the titi monkeys predicted variability in their calls. As predicted, their results showed that both age and the length of partnership influenced titi monkey call variability. As monkeys get older, the rate at which they repeat notes slows down. The duration of calls was also predicted by age as well as how long they had been with their partner. The final and most exciting result from this study showed that an individual’s vocalizations were more similar to their partner than any other monkey in the study. These results combined show that age effects how a monkey sings, but their partner and characteristics of their relationship with their partner also have an effect! This study provides further evidence that primate vocal communication is plastic and changes throughout the lifetime. Future studies will characterize what drives these changes in communication between partners and at what time scales these changes are occurring. Further, the evidence for vocal similarity between partners found in this study sets the stage for future work aimed at understanding how partner identity and quality are related to these differences in vocal communication.

For more information:

Clink, D. J., Lau*, A. R., & Bales*, K. L. (2019). Age-related changes and vocal convergence in titi monkey duet pulses. Behaviour, 156(15), 1471-1494.

*Denotes UC Davis ABGG-affiliated authors

Cover Photo Source: Zweer de Bruin