Maternal Behavior and Physiological Stress Levels in Wild Chimpanzees (Pan troglodytes schweinfurthii)

Abstract

Individual differences in maternal behavior toward, and investment in, offspring can have lasting consequences, particularly among primate taxa characterized by prolonged periods of development over which mothers can exert substantial influence. Given the role of the neuroendocrine system in the expression of behavior, researchers are increasingly interested in understanding the hormonal correlates of maternal behavior. Here, we examined the relationship between maternal behavior and physiological stress levels, as quantified by fecal glucocorticoid metabolite (FGM) concentrations, in lactating chimpanzees, Pan troglodytes schweinfurthii, at Gombe National Park, Tanzania. After accounting for temporal variation in FGM concentrations, we found that mothers interacted socially (groomed and played) with and nursed their infants more on days when FGM concentrations were elevated compared to days when FGM concentrations were within the range expected given the time of year. However, the proportion of time mothers and infants spent in contact did not differ based on FGM concentrations. These results generally agree with the suggestion that elevated GC concentrations are related to maternal motivation and responsivity to infant cues and are the first evidence of a hormonal correlate of maternal behavior in a wild great ape.

Keywords: Glucocorticoids; Maternal investment; Mother–infant; Stress.

Fig. 1

Plot of fecal glucocorticoid metabolite (FGM) concentration categorization based on the expected value for a lactating female chimpanzee in the Kasekela community on a given day of the year (January 2009–August 2013). The solid line represents the predicted relationship between log10 FGM concentrations and day of the year based on a linear regression using FGM concentrations from unpaired samples (N_samples = 629; N_females = 12; see Methods for details). The dashed line represents the 50% prediction interval from that model. Each point represents a day 2 log10 FGM concentration that was paired with day 1 behavioral data (N_{paired samples}= 122; N_females = 12). FGM concentrations that fell above the 50% prediction interval were categorized as elevated (triangles), while those that fell at or below the 50% prediction interval (shaded region) were categorized as within the expected range (circles).

Fig. 2

Mean ± SE proportion of follow time mothers in the Kasekela chimpanzee community from January 2009 to August 2013 spent (a) socially interacting (grooming or playing), (b) nursing, or (c) in contact with their infants by maternal FGM concentration category. N_expected = 89; N_elevated=33. *P < 0.05.