Age-related reproductive effort in reindeer (Rangifer tarandus): evidence of senescence.

Age-specific rates of survival and reproduction in iteroparous organisms remain a key issue in population ecology. How patterns of survival vary with age in large herbivores is well known; much less is known regarding age-specific patterns of reproduction. The senescence hypothesis predicts a progressive loss of function accompanied by decreased performance with age. The terminal investment hypothesis predicts that the proportion of resources a mother invests (i.e. effort relative to costs) is expected to increase toward the end of her life-span, when the number of offspring she can expect to rear in the future is low. Assuming that the costs remain stable, the terminal investment hypothesis predicts increased reproductive effort with age. We used data on body weight of 1,956 semi-domestic reindeer (Rangifer tarandus) calves from females with known age and weight to test these two (not mutually exclusive) hypotheses. Body weight of calves clearly decreased after 7 years of age was reached (also after controlling for female body weight), which was in support of the senescence hypothesis. We can conclude that either the terminal investment hypothesis is wrong, and/or the reproductive costs increase with age in female reindeer.