Tooth wear as a practical indicator of sexual differences in senescence and mastication investment in ecology studies

Abstract

Abstract Teeth of many mammalian species do not grow further after they have emerged, nor are they replaced or repaired if they break or wear. These dental properties have been used to test evolutionary hypotheses on life expectancy relative to actual age (senescence, aging), as the animal is expected to die when all its dental crown tissue has been depleted due to the wearing effect of mastication. Females of polygynous mammals with small litter size are expected to live longer than males of the same species, as female reproductive fitness is maximised by having a long reproductive life. In contrast, maleś reproductive success is limited to the short period when they are in prime condition, as it is in this period that they are most successful in intra-sexual competition for mating opportunities. Consequently, it has been predicted that femaleś teeth should be more durable and so wear more slowly than maleś teeth. However, these predictions are affected by potential sex differences in the hardness of dental tissues, which have not been assessed in depth in the literature. I tested sexual differences in the hardness of enamel and dentine on a balanced, by sex and age, sample of 156 lower first molars of Scottish red deer of known age using Vickers micro-hardness indentation measures as a proxy of the resilience of the tooth to wear. My results clearly indicate that there are no sexual differences in enamel and dentine hardness, and that tooth hardness has a negligible effect on predicting tooth wear, which supports the use of tooth wear relative to body size as a convenient proxy for assessing sexual differences in senescence in polygynous ungulates. I warn on making conclusions from comparisons between populations or species in tooth wear when tooth hardness was controlled for.