Timing of Estrus in Fallow Deer Is Adjusted to the Age of Available Mates.

The ability to manipulate the timing of ovulation should be a tool available to females during their choice of mates. Females should advance estrus when preferred males are available and delay estrus when only nonpreferred males are available. However, ovulation in some female mammals can be induced by the presence of males. Female estrus could then be either a result of a better stimulus provided by the preferred male or simply manipulation of estrus by the female to reflect her preference. Evidence is therefore required that female estrus coincides with the availability of preferred mating partners, that females actively choose among males, that avoidance of nonpreferred males results in a cost, and that nonpreferred males are in fact capable of reproduction. We observed two experimental groups of fallow deer, one with only socially mature males, and one with subadult but physiologically mature males. Females with mature males conceived earlier (13 of 15 gave birth) than females with subadult males. The former also conceived earlier than they did in the previous year, whereas the latter conceived later (13 of 14 gave birth) than in the previous year. Females avoided subadults more than mature males, and they lost more weight than females with mature males. Because subadult males are capable of reproduction, females not reproducing with them incur two costs: energetic loss and delayed reproduction. Given these costs, we suggest that females receive a benefit from not responding to the advances of young males that would outweigh these costs. This benefit may be to adjust ovulation to the availability of preferred mates.