Reproductive state alters vocal characteristics of female North American red squirrels (Tamiasciurus hudsonicus)

Female advertisement of reproductive state and receptivity has the potential to play a large role in the mating systems of many taxa, but investigations of this phenomenon are underrepresented in the literature. North American red squirrels (Tamiasciurus hudsonicus) are highly territorial and engage in scramble competition mating, with males converging from spatially disparate territories to engage in mating chases. Given the narrow estrus window exhibited in this species, the ubiquitous use of vocalizations to advertise territory ownership, and the high synchronicity of males arriving from distant territories, we hypothesized that female vocalizations contain cues relating to their estrous state. To test this hypothesis, we examined the spectral and temporal properties of female territorial rattle vocalizations collected from females of known reproductive condition over 3 years. While we found no distinct changes associated with estrus specifically, we did identify significant changes in the spectral characteristics of rattles relating to both female body mass and reproductive state relative to parturition. To the best of our knowledge, this is the first evidence of changes in vocal characteristics associated with late pregnancy in a nonhuman mammal.