Daytime declines in rumen–reticulum fill of male white-tailed deer (Odocoileus virginianus) from south Texas

Understanding rumen–reticulum fill dynamics gives us insight into how environmental conditions influence animal behaviors that affect when animals access forage. We examined whether there were daytime declines in rumen–reticulum fill in response to high ambient temperatures in free-ranging white-tailed deer (Odocoileus virginianus (Zimmermann, 1780)) on a diurnal scale. We hypothesized that to reduce heat-producing digestive processes in the heat of the day, rumen–reticulum fill and digestion in the rumen–reticulum would decline throughout the day. We collected the rumina and reticula of adult males and females across 30 h in two autumns in south Texas, USA. We also measured dry digesta mass, organ mass, and crude protein and acid detergent fiber concentrations of rumen–reticulum contents. In males, dry digesta mass declined throughout the day and crude protein concentrations were greatest when wet digesta and organ mass were the largest. Additionally, declines in dry digesta and wet digesta mass coincided with declines in crude protein concentrations and organ mass. Females did not display declines in rumen–reticulum fill (wet mass). Females were probably less vulnerable to heat than males because of their smaller body size. Male white-tailed deer foraged primarily during the cooler nights to reduce foraging and digestion during the heat of the day.