Drivers of fecal triiodothyronine metabolite patterns in the wild thick-tailed greater galago (Otolemur crassicaudatus)

Abstract Understanding how animals adjust their physiological processes to environmental variation is essential for predicting their resilience under future climate change. Thyroid hormones, particularly triiodothyronine (T3), play key roles in metabolism, thermoregulation, and seasonal energy balance, yet remain understudied in nonhuman primates, especially nocturnal strepsirrhines. We aimed to 1) evaluate an enzyme immunoassay (EIA) for measuring fecal triiodothyronine metabolites (fTM) in the thick-tailed greater galago ( Otolemur crassicaudatus ), and 2) examine environmental and reproductive drivers of seasonal fTM variation in a free-ranging population. We collected fecal samples ( N = 126) for 18 months across four seasons (two dry and two wet). We used a thyroid-stimulating hormone (TSH) challenge to assess the EIA’s ability to detect changes in fTM concentrations. We used linear mixed models to test effects of climate, resource availability and reproductive stage on fTM. The TSH challenge provided preliminary support for the assay: fTM levels in the female peaked at 6.5 h, whereas the male's postinjection peak occurred at approximately 10 h. fTM concentrations increased with ambient temperature and rainfall but were not related to insect, seed, or gum availability and were highest during early lactation in both sexes; because reproduction is seasonal, we could not separate climatic and reproductive effects. Our findings suggest that thyroid activity in galagos varies with seasonal climatic conditions in both sexes, rather than increasing in the cold as expected for a thermoregulatory role, highlighting the value of noninvasive thyroid monitoring for understanding how primates cope physiologically with seasonal environments.