Kin selection theory (Hamilton 1964) is a widely invoked explanation for the evolution of cooperation in animal societies. A major assumption of this theory is that individuals must discriminate among individuals based on genetic relationships to bias helpful behaviors toward kin. In the context of alarm calls, a common danger signal, kin selection is expected to favor inclusive fitness through increased attention to closely related (indirect benefits) and reliable (direct benefits) callers. Here we quantified individual differences in acoustic properties of alarm calls (i.e., chatters) for members of two long-term populations of California ground squirrels. We used Raven Pro to measure the acoustic properties of chatters produced by marked individual squirrels over time. As expected, the acoustic properties of calls were explained by the age, sex, and location of callers. Beyond this, individuals’ calls were repeatable across the study, providing evidence for individuality in call structure. Our findings suggest these animals have the potential to discriminate among individual callers based on acoustic information produced in alarm calls. These findings are important because individuals could support a suite of other behavioral processes important for social animals that include parent-offspring interactions, mate choice, and recognition of social partners.
