Reproductive suppression and cooperative breeding in dwarf mongooses, Helogale parvula

Scott Richard Creel, Purdue University

Abstract

Dwarf mongooses are small, social carnivores, distributed throughout woodland and savanna regions of sub-Saharan Africa. Within a pack, only the oldest individual of each sex normally reproduces. Most adults of both sexes mate during synchronized estrous periods, although mating rates are lower among subordinates. Subordinate females are normally (88%) precluded from becoming pregnant by a series of rank-dependent interactions between ovarian hormones and mating behavior. Because estrogen levels vary continuously (and inversely) with dominance, high-ranking (older) subordinates sometimes become pregnant. Many of their offspring do not survive, probably due to infanticide by the alpha female, but some apparently do. Subordinate males have urinary androgen levels indistinguishable from those of dominants, and are reproductively suppressed primarily by aggressive interference by higher-ranking males. Temporal variation in urinary androgens does not relate clearly to levels of aggressive or reproductive behavior, among dominants or subordinates. Moreover, variation in urinary androgens does not follow patterns predicted by consideration's of Helogale's systems of mating and parental care. Suppressed subordinates cooperate in all aspects of raising the dominants' young, including guarding, grooming, carrying, feeding and even nursing them. Evolutionary cost/benefit analysis shows that helping to raise the dominants' young yields a higher net inclusive fitness benefit than would attempting breeding, for subordinate females younger than four years, and for males younger than three. These correspond to the modal ages at which subordinates of each sex switch from helping to breeding. Cost benefit analysis also suggests that pregnancies among subordinate females are not chance failures of reproductive suppression, but constitute an alternate reproductive strategy used by older, high-ranking subordinates to maximize inclusive fitness. Subordinate pregnancies are an evolutionary compromise between complete despotism by the alpha female, and complete egalitarianism. This sort of 'power-sharing' by dominants is presumably used to retain helpers that have a high expectation of breeding successfully elsewhere, and blurs the distinction often made between species with and without reproductive suppression. Other topics addressed include the interface between theory and measurement of inclusive fitness, and the measurement of inclusive fitness in populations with overlapping generations.

Degree

Ph.D.

Advisors

Waser, Purdue University.

Subject Area

Ecology|Anatomy & physiology|Animals|Forestry

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