Press Release: January 2013

First Molar Eruption, Weaning, and Life History in Living Wild Chimpanzees

The Study in Brief

Scholars have been debating whether early humans (hominins) followed ape-like or human-like patterns of growth and development for quite some time. Others have recently questioned if developmental standards derived from captive animals are accelerated relative to wild animals, casting suspicion on most comparative data from great apes. Studies of human life history evolution often rely on broad primate-wide correlations of molar eruption age and developmental variables to assess fossil hominins, although these relationships have never been studied within the same population of primates. Using a novel photographic approach, a team of evolutionary anthropologists and wildlife photographers led by Professors Tanya Smith and Richard Wrangham, and Postdoctoral Fellow Zarin Machanda of Harvard's Department of Human Evolutionary Biology, have determined first molar (M1) eruption ages in several living chimpanzees. Our study demonstrates that healthy wild chimpanzees do not show systematic delays in M1 eruption age relative to captive chimpanzees, and both show strong similarities to early human ancestors. What is most surprising is that these ages do not correlate with weaning events or maternal reproductive states, but instead relate to the transition to an adult diet.

The Approach

To improve our understanding of dental development in humans’ closest living relative, we conducted a long-term study of dental eruption in the Kanyawara Community of chimpanzees in Kibale National Park, Uganda. This community has been continually studied since 1987 and currently includes more than 20 subadults whose ages of birth are known to within a month, as well as whose mothers’ reproductive states are known. High-resolution photographs were taken between March 2011 and December 2012 using a digital single-lens reflex (SLR) camera with high sensitivity, high frames per second, and a wide aperture lens (Figure 1, Video 1).

These images allowed us to precisely assess tooth emergence from exposed cusp tips at or above the gumline. We then compared information on tooth eruption with infant behavioral data, including feeding rates and time spent on the nipple. Urine was routinely collected from adult females and tested for hormonal indications of pregnancy.

The Kanyawara community is an ideal population to demonstrate this new non-invasive photographic method. Community members are very tolerant of photographers (Figure 2, Figure 3) as well as field assistants who record detailed behavioral observations. Individual ages and relationships are known as they have been studied for many years. The community structure is stable and does not show negative effects of human interaction or disturbance. This is important as long-term observations of wild primates facilitate very precise comparisons of anatomical, physiological, and behavioral development, providing critical comparative data for the reconstruction of human evolutionary development.

The Results

We found that five healthy infant chimpanzees erupted their mandibular M1s by or before they were 3.3 years old (Figure 4). This suggests that wild chimpanzees do not show systematic delays in M1 eruption relative to captive chimpanzees.

Moreover, these ages are nearly identical to estimated ages for early fossil hominins. All infants continued to nurse for several months after M1 appearance (Figure 5, Video 2), including two individuals whose mothers resumed reproductive cycling during their third year of life. This is important as it contradicts influential theories that M1 eruption age may be used as a reliable proxy for weaning age or birth spacing.

We observed that infants began supplementing mother’s milk with fruit by about 6 months of age, consumed the same percentage of fibrous foods as adults shortly after 1 year of age, spent the same time feeding as adults by about 3.5 years of age (Video 3), but did not cease nursing until after 4 years of age. We suggest that M1 emergence in eastern chimpanzees may relate to a key dietary transition around age 3, prior to which infants are fundamentally dependent on their mothers for support as they develop the necessary anatomy and ecological knowledge to survive on their own if necessary.

Significance of this Study

Our study is the first longitudinal investigation of tooth development and nursing behavior in a habituated population of wild primates. This is important as we can directly test hypotheses about how tooth development relates to infant feeding and maternal reproductive behavior. These hypotheses have been highly influential in reconstructions of how early hominins, such as the Australopithecus africanus Taung child, grew and developed. Chimpanzees are humans' closest living relatives, and many theories about human evolution posit that early humans were similar to chimpanzees. Prior to our research, most information about chimpanzee dental development came from captive animals or a small sample of deceased wild individuals, which has been questioned as these individuals may not be representative of living wild animals.

This study will inspire re-evaluations of previous ontogenetic analyses, and will set a standard for future studies of the relationship between skeletal development, ecology, and life history in primates. Importantly, our data do not support the 1:1 correlation between M1 eruption and weaning age previously found across 14 primate species, nor the observation that the length of nursing corresponds “reasonably well” with molar eruption age in hominoids. First molar eruption in this chimpanzee population is not coincident with weaning events such as the introduction of solid foods, resumption of maternal estrous cycling, or the termination of suckling. It appears that caution is warranted when inferring particular life history traits from M1 eruption in juvenile hominins. Our results may be informative of the development of adult feeding behavior in juvenile primates, as the age at which M1s erupt and move into place (functional occlusion) coincided with the transition to adult feeding patterns. Future studies are needed to determine if this pattern of molar eruption and feeding behavior is consistent across great apes or other primates, which may better inform our understanding of the evolution of human development.


Tanya M. Smith 
Department of Human Evolutionary Biology 
Harvard University 
Cambridge, MA 02138 
USA Phone: +1 617 496 8259
Zarin Machanda
Department of Human Evolutionary Biology 
Harvard University 
Cambridge, MA 02138 
USA Phone: +1 617 496 4262