Some mammals, including humans and elephants, have a long post-reproductive life. This has raised a question: given the notion of survival-of-the-fittest is a herd (or pride, or tribe) within a species made more fit, is it collectively given a reproductive benefit if, in particular, its females live for years, even decades, after the birthing of newborns has become impossible?
The Grandmother Hypothesis:
One oft discussed hypothesis, addressing this issue, is this: grandmothers are good for you. And they are good for their gene line amongst other mammals too. Individuals in a species with a complicated social life are more lively to live into their own reproductive prime if they have post-menopausal females in their genetic line than otherwise, because grandmothers share parental responsibilities for their daughters’ children.
Strange New Worlds:
Scientists associated with the University of Bristol have been studying giraffes with the grandmother hypothesis in mind. In a recent publication in MAMMAL REVIEW they have demonstrated that female giraffes live 30% of their lives in a post-reproductive state. They have given reasons for believing that the social structures within a herd of giraffes fit the grandmother hypothesis.
The two authors of the article, Zoe Muller and Stephen Harris, believe that giraffes have evolved “highly successful and complex societies, which have facilitated their survival in tough, predator-filled ecosystems.”