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There are many species, who are continuous breeders those species that breed year-round and whose mating periods are distributed throughout the year (e.g., humans). In continuous breeders, females are sexually receptive during estrus, at which time [[ovarian follicles] are maturing and ovulation can occur. Evidence of ovulation, the phase during which conception is most probable, is advertised to males among many non-human primates via swelling and redness of the genitalia.
Support for the challenge hypothesis has been found in continuous breeders. For example, research on chimpanzees demonstrated that males became more aggressive during periods when females displayed signs of ovulation. Moreover, male chimpanzees engaged in chases and attacks almost 2.5 times more frequently when in groups containing sexually receptive females.
- ↑ Muller, M. N. & Wrangham, R. W. (2003). Dominance, aggression, and testosterone in wild chimpanzees: A test of the 'challenge hypothesis'. Animal Behaviour, 67, 113-123.