When a herbivore wears away its teeth to nothing by years of chewing on abrasive foliage, its outlook is not bright. Many specialist herbivore species have independently evolved long teeth, often as part of an arms race with plants armouring themselves with silica. It is then tempting to suppose that longer teeth should lead to longer life spans; however, recent work by Vebjørn Veiberg and colleagues provides strong evidence that the reverse can be the case, with longer life-span requirements driving the evolution of longer teeth.
The research team studied two populations of roe deer living wild in forests in France. Both live in habitats dominated by oak and beech, but the forests differ in climate and productivity. Deer from the population living in the less productive environment tend to have a longer life expectancy,probably due to caloric restriction. On the other hand, those deer from the more productive forests tend to follow the `live fast, die young' strategy. They are probably more capable of producing more offspring at a younger age but also suffer a higher mortality rate.
The team wanted to test two hypotheses: the `tooth-wear' hypothesis, which relates variation in tooth height to diet quality, and the `life-history'hypothesis, which relates initial molar height to life expectancy. They measured molar heights - top of tooth to bottom of enamel - in 93 animals found dead within the study areas and correlated this with age. Veiberg's team show no difference in the rate of tooth wear between the two populations, but deer from the poor quality environment, with long life expectancies, started adulthood with longer molars despite being generally smaller animals. Thus, differences in tooth height are related to adaptations for different life expectancies rather than diet abrasiveness or rate of chewing. Deer from poor habitats have to survive long enough to reproduce effectively despite their low-calorie diet; increased initial tooth height evolves in response to this longer life-span strategy, supporting the life-history hypothesis.
While it makes some sense that individuals from a population with greater life expectancies have to be better built, this study is remarkable in that it isolates life expectancy from other factors exceptionally well, which is challenging to show in wild populations of relatively large mammals. Previous work by Juan Carranza and colleagues, published in Nature in 2004(vol. 432,p. 215), related tooth wear to life expectancy and suggested a relationship between life expectancy and initial tooth height in red deer. The males truly do `live fast, die young': not only do they have smaller teeth than females despite their considerably larger size but they also expend huge amounts of energy competing with other males to mate with females. Hence, they eat much more and wear their teeth down faster, making it difficult to attribute tooth height to life expectancy. Therefore, Carranza and colleagues couldn't reject the tooth-wear hypothesis. Veiberg's work on roe deer, by contrast, compares populations with very similar tooth wear rates, so that the tooth-wear hypothesis can be cautiously rejected.
So, what we have here is in nice agreement with ageing evolutionary theory,but it is also vaguely disturbing. That such a simple metric as initial adult tooth height might give away an organism's `design life-span', or effective`warranty period', has interesting implications. What if insurance companies could determine a similar metric for humans? Instead of looking at indicators of risk for some specific failure (such as heart disease), they could look at an apparently innocent trait and calculate our allotted life spans.
