Seasonality and interindividual variation in mandrill feeding ecology revealed by stable isotope analyses of hair and blood.

Mandrills are large-bodied terrestrial forest primates living in particularly large social groups of several hundred individuals. Following these groups in the wild to assess differences in diet over time as well as among individuals is demanding. We here use isotope analyses in blood and hair obtained during repeated captures of 43 identified free-ranging mandrills (Mandrillus sphinx) from Southern Gabon, to test how dietary variation relates to the season as well as an individual's age and sex. We measured the stable carbon (δ13 C‰) and nitrogen (δ15 N‰) isotope ratios in 46 blood and 214 hair section samples as well as from a small selection of mandrill foods (n = 24). We found some seasonal isotopic effects, with lower δ13 C values but higher δ15 N values observed during the highly competitive long dry season compared to the fruit-rich long rainy season. Variation in δ13 C was further predicted by individual age, with higher δ13 C values generally found in younger individuals suggesting that they may consume more high canopy fruit than older individuals, or that older individuals consume more low canopy foliage. The best predictor for δ15 N values was the interaction between age and sex, with mature and reproductively active males revealing the highest δ15 N values, despite the observation that males consume substantially less animal food items than females. We interpret high δ15 N values in these mature male mandrill blood and hair sections to be the result of nutritional stress associated with intense male-male competition, particularly during mating season. This is the first study showing isotopic evidence for nutritional stress in a free-ranging primate species and may spark further investigations into male mandrill diet and energy balance.

Maternal effects and the endocrine regulation of mandrill growth.

Maternal effects can influence offspring growth and development, and thus fitness. However, the physiological factors mediating these effects in nonhuman primates are not well understood. We investigated the impact of maternal effects on variation in three important components of the endocrine regulation of growth in male and female mandrills (Mandrillus sphinx), from birth to 9 years of age. Using a mixed longitudinal set (N = 252) of plasma samples, we measured concentrations of insulin-like growth factor-I (IGF-I), growth hormone binding protein (GHBP), and free testosterone (free T). We evaluated the relationship of ontogenetic patterns of changes in hormone concentration to patterns of growth in body mass and body length, and determined that these endocrine factors play a significant role in growth of both young (infant and juvenile) and adolescent male mandrills, but only in growth of young female mandrills. We also use mixed models analysis to determine the relative contribution of the effects of maternal rank, parity, and age on variation in hormone and binding protein concentrations. Our results suggest that all of these maternal effects account for significant variation in hormone and binding protein concentrations in all male age groups. Of the maternal effects measured, maternal rank was the most frequently identified significant maternal effect on variation in hormone and binding protein concentrations. We suggest that these endocrine factors provide mechanisms that contribute to the maternal effects on offspring growth previously noted in this population.