Birth rates can be a good indicator of the viability and health of great ape populations. While apes reproduce slowly as a rule, richer and more stable food resources lead to faster production of offspring, putting members of these endangered species on a path towards population growth. In Kanyawara, chimpanzee birth rates have increased noticeably over the past 2 decades. Our alpha female, Outamba, has produced infants every 3.5 years, a startling 2 years faster than the average wild chimpanzee. But is there a downside to having all of these babies? Life history theory suggests that, because mothers have limited resources to devote to reproduction, they may face tradeoffs between producing many offspring and investing in the quality of each. While these tradeoffs are almost inevitable, evidence for them has eluded anthropologists studying humans and other primates.
In a recent issue of the Proceedings of the National Academy of Sciences, the Kibale Chimpanzee Project team tested this prediction by examining the influence of reproductive rates on offspring growth. We found that an immature chimpanzee’s size had a strong positive relationship with the interval to its next sibling’s birth. In other words, when mothers could afford to reproduce faster, they did so at the expense of their offspring’s growth. Remarkably, chimpanzee juveniles did not manage to make up for these early growth deficits later on, but remained small throughout their adolescent years. These tradeoffs could come about because mothers who reproduce quickly exhaust their supplies of energy to feed infants. Alternatively, when a mother produces two offspring in quick succession, she might withdraw nutritional support from the first in order to feed the second. Our data support the second interpretation. It was mothers like Outamba, who were in the best energetic condition during lactation, that produced the smallest offspring. Instead of using surplus energy to enhance their current offspring, these mothers appeared to bank it so that they could afford to produce a new infant sooner. Because wild chimpanzee mothers cannot afford to feed two infants at the same time, they have to wean one infant in order to feed the next. The weaned infants then have to feed themselves, even if they have not yet learned how to be efficient foragers or developed the digestive system to fully process raw plant foods. For humans, this is less of a problem. Human infants continue to receive nutritional support from their mothers and other family members long after they finish nursing, and cooked foods are both calorie rich and easy to digest.
KCP is dedicated to research that is non-invasive. One of the big challenges that we have faced is how to estimate the body size and energetic condition of our animals without weighing them. Weighing is both impractical and disruptive, as it would require baiting scales with food. Over the years, we have developed novel assays for urinary biomarkers of muscle mass and energy balance that enabled us to complete this study. With this toolkit, we can begin to answer many new questions about growth and body size. Stay tuned!