Hair progesterone reflects past reproductive activity in the invasive Pallas's squirrel Callosciurus erythraeus

Integrated approaches to physiology and ecology offer powerful tools to assess individual- and population-level health. Although hair glucocorticoid analysis is widely used as a physiological indicator, hair progesterone has recently attracted attention as a potential measure of reproductive health that may integrate information on past reproductive activity. Here, we examined the assay validation and biological utility of hair progesterone analysis as an indicator of long-term reproductive activity in Pallas's squirrel Callosciurus erythraeus. A hair progesterone enzyme immunoassay was validated and used to compare hair progesterone concentrations (HPC) between sexes and age classes. The effects of current and past reproductive activity (pregnancy and number of placental scars), body condition, age, and season (hair non-growth vs. growth season) on adult female HPC were investigated. The analytical assay validation was successful. Adult females had significantly higher HPC than juvenile males, adult males, and juvenile females. Although there was no effect of current reproductive activity, past reproductive activity (as indexed by the number of placental scars) significantly influenced adult female HPC, such that female Pallas's squirrels with more placental scars had higher HPC. There was also a significant positive impact of season on HPC, and females with better body condition tended to have higher HPC. Hair progesterone is therefore an integrative physiological indicator of past reproductive activity in Pallas's squirrels, with potential applications in the prediction of population dynamics to inform the management of this invasive squirrel.