Individual variation in estrogenic plant consumption predicts fecal estradiol levels in the Ugandan red colobus monkey (Piliocolobus tephrosceles)

Phytoestrogens are plant secondary compounds that can bind to estrogen receptors and mimic or modulate endogenous estrogen activity, with potential downstream physiological and behavioral effects. To test whether daily variation in wild estrogenic plant consumption was associated with subsequent variation in fecal steroid hormone concentrations, we sequentially followed six individually recognized Ugandan red colobus monkeys (Piliocolobus tephrosceles) in Kibale National Park, Uganda, over an eight-month study period, collecting 1318 h of behavioral observations and 516 fecal samples. On average, 17.7% of the daily diet consisted of foods with measurable estrogenic activity. Linear mixed-effects models indicated that greater estrogenic plant consumption was associated with lower fecal estradiol concentrations the following day and higher concentrations two and three days later. Cortisol showed no significant relationship with estrogenic plant consumption at any timescale. Variation in fecal estradiol was also associated with time spent moving and feeding but not with sex or time of defecation. These results demonstrate a lagged association between daily phytoestrogen consumption and fecal estradiol excretion patterns at the individual level but do not establish a causal endocrine effect. Longer-term studies incorporating more individuals and reproductive data are needed to determine the biological significance of these associations.