Mating strategies in whale populations: searching strategy vs. harem strategy

This work develops a probability model for comparing two different male mating strategies in whale populations where females group together in pods and are only receptive for a fraction of the breeding season. A searching strategy and a harem strategy are modelled and the probabilities that a given female becomes pregnant derived for the two strategies and compared. The advantage one strategy has over the other in terms of pregnancy rates depends on four key parameters: the number of oestrus cycles in a breeding season (m), fraction of time in the season when a female is in oestrus (q), ratio of number of males to number of female pods (r), and the expected number of pods found by a male in a breeding season (α). Situations in which a searching strategy is best are intuitively reasonable, i.e. a high value of q and/or a high value of α. The theory is then applied to two species of social cetaceans: short-finned pilot whales and sperm whales. Non-reproductive matings are practised by short-finned pilot whales and it is hypothesized that this behaviour entices males to stay with the pod, i.e. to adopt a harem strategy. The benefits females derive from this behaviour in terms of increased pregnancy rates are then evaluated. Very limited information exists concerning the value of the relevant parameters for the two species, but what little there is, suggests that sperm whales are in the region of parameter space where the searching strategy is better, but that short-finned pilot whales are in the harem region.