Due to comfort nursing, however, had we relied solely on our observations of nursing behavior, our estimates of the average age at which infants were weaned would have been a few years higher this suggests that fecal stable isotopes can be used to provide a more accurate and biologically meaningful assessment of weaning. The weaning process was complete when infants were between 4 and 4.5 years old, which is in line with reports of weaning established through observations at other chimpanzee sites. Second, we found that the weaning process for chimpanzees at Ngogo started when infants were around 1 year old, after which there was a gradual decrease in reliance on maternal milk.

Most of our older study subjects (between 4 and 7 years old) were weaned individuals that were observed to continue making nipple contacts regularly, presumably for comfort rather than nutrition. First, we discovered the presence of comfort nursing in chimpanzees. We reported several key findings of using fecal stable isotopes to track the diets of chimpanzee infants.

We could tell when a weanling became nutritionally independent when her stable isotope profile was nearly identical to that of her mother’s. Newborns and mothers initially showed very different isotopic profiles that decreased as infants grew older.

We compared the stable carbon and nitrogen isotope values of mothers and their offspring and attributed major isotopic differences to the maternal milk ingested by infants. We dehydrated the fecal samples on site using a solar “poop” dehydrator and stored them on silica, in a freezer, until we took them to the University of Calgary’s Isotope Science Laboratory for analyses. We collected several hundred fecal samples from mothers, juvenile siblings, and infants varying in age from birth to seven years old, as some individuals continued to make nipple contacts up to this age. We assessed observational limitations by using a novel fecal stable isotope approach to physiologically determine the timing of age-related feeding transitions and to measure the relative contributions of maternal milk to adult foods in the diets of wild chimpanzee infants at Ngogo, Kibale National Park, Uganda. A major challenge in quantifying infant diets and in identifying the timing of feeding transitions comes from our inability to know from observations alone whether infants are actually drinking maternal milk when they make nipple contacts, and to distinguish between mouthing and ingesting of vegetation. In wild primates, it is difficult to precisely measure what infants are eating. Understanding the diets and feeding patterns of nonhuman primate infants will help us establish early life history parameters in our hominin ancestors, which can shed light on the diversity of contemporary infant feeding practiced between and within human populations. To better understand the evolutionary contexts under which our diverse strategies of infant feeding evolved, we can look to nonhuman primates. For example, the optimal ages at which infants ought to be completely weaned or begin consuming solid foods are regularly questioned by researchers and in the media. Humans vary considerably in our strategies of infant feeding.

Feeding chimpanzee meme