Ungulates, Tooth Size and Longevity

One of the more interesting areas of paleoanthropological research concerns the timing of growth and development. For example, in macaques infancy is from birth to 1.4 years, childhood from 1.4-3.2 years, and adolescence from 3.2-5.8 years. In captivity macaques can live to be 30 years old, or more. In chimps the figures are; infancy birth – 3.3, childhood 3.3-6.5, childhood 6.5-11.4. In captivity chimps can live to be in excess of 60 years old. For humans the figures are: infancy birth – 5.9, childhood 5.9-11.3, adolescence 11.3-18. Each of these stages can be defined based on a combination of tooth eruption and epiphyseal closure (which is also helpful to forensic anthropologists). Presumably, the common ancestor of chimps and humans followed the chimp pattern of development and one of the things paleoanthropologists would like to explain is how and when the human pattern evolved. I mention this because an interesting paper has been published in The American Naturalist which seems relevant to the above issues.

The paper looks at the relationship between body size, reproductive strategies, and durability of somatic structures (in this case teeth). As Carranza and Perez-Barberıa explain it:

Although sexual selection … is not the only process driving sexual size dimorphism …, it appears that male-male contests for mating success are involved in most cases of sexual size dimorphism in ungulates … Teeth should increase in size in relation to body size if they are to maintain performance and durability (for discussions on scaling allometry of teeth size with body mass, see Fortelius 1985; Lucas 2004). However, if reproductive life span decreases by means other than teeth efficiency, as in male polygynous ungulates, tooth size and durability may not evolve at the same rate as body mass.(Note: ellipses refer to omitted references – afarensis)

To test this idea, Carranza and Perez-Barberıa examined lower premolar and molar occlusal surface area (length * width of occlusal surface) and body mass in 123 species of ungulates, both male and female. The ungulates were divided onto dimorphic and nondimorphic groups and compared the two groups. After controlling for phylogeny, they found that although occlusal surface area increased with increasing body size, the increase was smaller in dimorphic species than in nondimorphic species. In the discussion section of the paper Carranza and Perez-Barberıa state:

The main evolutionary implication of our results is that natural selection appeared to be unable to maintain the scaling of molar size when sexual selection produced an increase in body size in males. Reproductive success of female ungulates depends mainly on longevity …, whereas reproductive success of males is based mostly on mating opportunities resulting from direct male-male competition, especially in polygynous systems … The fact that male reproductive success depends on winning fighting contests reduces the probability of reproduction beyond prime age. As a result, a common consequence of sexual selection based on male-male competition in ungulates is the shortening of reproductive life span of males relative to females…

Primates can also be broken down into dimorphic and nondimorphic species and, like the ungulates, this can be related back to male-male competition related to polygynous mating systems. It would be interesting to see this analysis repeated using primates. Especially because some of the early hominins were dimorphic in size apparently up to and including Homo erectus, which raises the interesting question of when male-male competition stoped being expressed in terms of body size, etc. (perhaps culture diverted sexual selection into less biologically costly ways of competing?). One added wrinkle, though, is trends in the evolution of teeth among hominins – but that, in itself, would require several more lengthy posts…