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
non-human mammal.