Adaptive Prolonged Postreproductive Life Span in Killer Whales

摘要

The evolution of a prolonged postreproductive life span has attracted considerable interdisciplinary attention, primarily because of the long postmenopausal life span seen in humans (7). Two mechanisms have been proposed to underpin prolonged postreproductive life span: (i) an epiphenomenon of increased longevity, in which evolutionary benefits accrue only during the reproductive phase with no additional fitness benefits after the last reproductive event, and (ii) an evolved adaptation in which postreproductive life span increases the survival of an individual's genes, increasing its inclusive fitness. There is mounting evidence that post-reproductive human females increase the reproductive success and survival of their offspring (2, 3). Although helping in some postreproductive female cetaceans (4) suggests an adaptive benefit, there has been no evidence that a similar phenomenon occurs in nonhuman animals. With multigenerational demographic records based on photographic censuses (1974 to 2010) of the Southern and Northern resident killer whale (Orcinus orca) populations in coastal waters off Washington state, USA, and British Columbia, Canada [see (5) for details], we used a Cox proportional hazards model (6) to examine the consequences of a mother's death on ofispring survival. This unique data set consisted of 589 individually identifiable animals, of which 297 died during the study period (6).