Variation in age at M1 emergence and life history in wild chimpanzees.

Age at first molar (M1) emergence is commonly used to infer the general pace of life history in extinct ape and human species. However, there is essentially no information on variation in age at M1 emergence within species to complement the growing body of knowledge on intraspecific variation in life history. Recently reported ages at M1 emergence in five living subjects from the Kanyawara community of Pan troglodytes schweinfurthii in Uganda range from <2.5 to 3.3 years, less than the age from a single deceased Pan troglodytes verus individual from the Taï Forest in Côte d'Ivoire (~3.7-3.8 years), the only other reliable M1 emergence age for wild chimpanzees. Using standard histological methods, we determined ages at death for two wild-shot juveniles of P. t. verus from central Liberia, both with erupting M1s, and estimated their M1 emergence ages at ~4.2-4.4 and ~4.5-4.6 years, substantially later than those of the P. t. schweinfurthii individuals and outside the range of captive chimpanzees (2.1-4.0 years). The combined range of M1 emergence ages from just the small Kanyawara and Liberian chimpanzee samples thus spans nearly the entire range of values known for captive and wild great apes as a whole, a surprising result. While little is known about Liberian chimpanzee life history, data for P. t. verus from the Taï Forest and P. t. schweinfurthii from Kanyawara reveal a somewhat longer average interbirth interval in Kanyawara females. This observation is the reverse of what would be expected given the M1 emergence ages in the two populations and based on the correlation between age at M1 emergence and various life-history traits in primates as a whole. We examine these results with respect to factors impacting the different populations. Supported by the Institute of Human Origins (JK, GTS) and the USA National Science Foundation (TMS). [ABSTRACT FROM AUTHOR]