WikiFindings

Postreproductive female killer whales increase survival of grandoffspring

Background

In most mammals, females reproduce until the end of their lives. However, in humans and some toothed whale species this is not the case. In these species, females have a long period of their lives after reproduction has stopped. This is known as the post-reproductive lifespan. Female killer whales stop reproducing around the age of 40 but can live for decades afterwards. Postreproductive lifespans may have evolved in order to benefit family members. The grandmother effect suggests postreproductive females increase fitness of and survival of grandoffspring. This study tested the grandmother effect in killer whales.

Findings

Grandoffspring who lost a maternal grandmother in the last two years had a 4.5 times higher risk of death than those with a living grandmother. The death of a grandmother reduced survival rates in grandoffspring of both sexes. Although this effect was seen both in those who had lost grandmothers still reproducing and those who had lost postreproductive grandmothers it was stronger in the latter. This trend was shown in grandoffspring with ages ranging from 5 to 20 years old. Benefits of post-reproductive grandmothers on grandoffspring survival were greatest when levels of prey (Chinook salmon) were low. Although it was predicted that the grandmother effect may also reduce the interval between births of their daughters there was no supporting evidence for this.

Implications

Post-reproductive grandmothers provide greater survival benefits to grandoffspring than grandmothers which are still reproducing. These benefits are present for a long period of the grandoffsprings life. This provides evidence of a post-reproductive grandmother effect. Offspring do not disperse away from their mothers in this species meaning they are regularly grouped with their grandmother. Stopping reproduction prevents reproductive conflict between grandmothers and their daughters meaning grandmothers can aid in providing food for grandoffspring, particularly when prey levels are low. This is likely due to the fact they have key leadership roles in the group. Grandmothers do not reduce the birth interval of their daughters as by increasing offspring survival, length of care is also increased. The evidence of the grandmother effect provides an explanation for the evolution of long lifespans following the end of reproduction in a non-human species.