The hypothesis of the grandmother: why is there menopause?

Juan Ignacio Pérez Iglesias Updated: Save Send news by mail electrónicoTu name *

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The menopause , from an evolutionary perspective and in appearance, a anomaly . In fact, it is a phenomenon that is very rare in the animal world. In nearly all species the females remain fertile throughout their life. Ours is part of a small group of mammals in that the reproductive capacity deteriorates at an accelerated rate relative to the decline in organic general. The group is completed by the calderón tropical, the beluga , the narwhal and orca , all of them cetaceans odontocetes. The females of these species can live decades after cease to ovulate and, therefore, of being able to procreate.

The biologist american George C. Williams proposed in 1957 that menopause could be, in reality, a adaptive . He thought that from the point of view of evolution it may be more convenient for women to dedicate their efforts to support their descendants who have them a progeny greater.

As we age increases the probability of death, so that if a woman has offspring at an advanced age, it would not be unlikely that their last children would not survive to die she . In such a case, the effort that this woman had been dedicated to those past children would have been vacant, as their genes would not have been able to be transmitted to the following generations.

on the other hand, that in a population has individuals that do not play makes no sense from an evolutionary point of view. These individuals consume resources which could be used by others for the benefit of his own progeny.

For these reasons, Williams suggested that older women contribute more effectively to transmit their genes to subsequent generations by devoting their efforts to the descendants that are part of the group . That is to say, their grandchildren , instead of the sons and daughters that they might have at an advanced age.

The “ hypothesis of the grandmother ”, as it is known at present to the proposal of Williams, with supporting empirical in our species. Both groups of hunters-gatherers, as in industrial societies it has been found that grandmothers who do not play will increase the likelihood of survival of their grandchildren.

In other words, their presence in the group facilitates their genes –that have come down to the grandchildren– will endure after his own death. Told in language of darwinian, to improve its own evolutionary success (fitness) through the care provided to their grandchildren.

There are species, such as the asian elephant and possibly also the african, as we all know that the grandmothers enhance the survival of the grandchildren , although in this case it is still playing. But apart from the human species, no other animal had evidence in favor of the “hypothesis of the grandmother.” Until recently.

At the end of last year a study was published with the killer whales that documents the positive effect that they have grandmothers that do not play in the survival of their grandchildren. The results not only confirmed this effect, also showed that grandmothers who continue procreating does not provide a support similar to that provided by the menopausal women.

The importance of that period postmenopausal is so extensive has on humans and orcas would have driven, according to the specialists, an increase in longevity of the two species. The increased survival of grandchildren during the years of life added would have more than compensated for, and in evolutionary terms, the cessation of procreation.

Juan Ignacio Pérez Iglesias is a Professor of Physiology, University of the Basque Country / Euskal Herriko Unibertsitatea (spain)

This article was originally published on The Conversation