Menopausal Whales, Mama’s Boys, and the Conundrum of Reproductive Senescence
From a purely evolutionary perspective, our main function in life is to reproduce, with the course of our ontogeny basically being programmed to push as many copies of our genes into the next generation as possible (a view that was most famously popularized by Richard Dawkins' The Selfish Gene in 1976). In the context of social systems, ecological communities, and other external influences, however, this prerogative can become much more complicated than it seems at first.
One fascinating conundrum related to these issues is the existence of reproductive senescence: why do we even live past the point at which we can no longer reproduce? Women in particular can hang around for decades and decades of age-induced infertility. Menopause is a bit of a puzzle. What is the point of persisting, consuming resources your offspring could otherwise use, if all of your gametes have already been distributed?
There has been broad speculation as to the selective pressures that have produced that wedge of the life cycle, with the most popular being the “Grandmother Hypothesis." This theory suggests that having a grandmother around to help her daughter raise the next generation will increase the reproductive success of said daughter, helping the grandmother to promote her genes indirectly (a form of inclusive fitness). This hypothesis has been thrown about quite a bit, and is still being turned over and tested in a variety of settings.
Enter the orca whale (Orcinus orca). Female orcas stop producing offspring in their 30s to 40s, but often attain lifespans of 90 years or more. That is a very, very long time to be around consuming resources without producing calves. As such, this seems to be a fascinating species in which to examine the benefits that adult animals might obtain from the presence of a post-reproductive parent.
A previous study showed only mild support for the Grandmother Hypothesis in orcas (Ward et al. 2009). Those results indicated slight benefits to the young calves females with living mothers, yet ultimately the researchers concluded that their sample size was too small to determine whether grandmothers incurred benefits to their grandcalves’ survival. The issue obviously needed more investigation. It stands to reason that it is hard to reproduce if you are not alive, so a group of researchers led by Darren Croft from the University of Exeter set out to determine how the death of a mother orca affected the survival rates, rather than the reproductive rates, of her adult offspring. They reported their results of in the 14 September issue of Science.
The team followed 589 individual orcas as part of a long-term study, and over the course of the sampling period 297 of the animals died. The researchers then used proportional hazard and survival models to determine how much the mortality risk of an individual increased in the year after its mother’s death. The study accounted for a key natural history point: the “family life” of adult orcas differs greatly between males and females. In a pattern that is a bit unusual for mammals, neither male nor female orcas disperse, staying with their maternal family groups for their entire lives. This means that female orcas will raise their offspring within the matrilineal group, but that when male orcas mate, it is done with females of a different pod, who retain the resulting calves within their respective family group, out of the way of the father and his family. So, a mother whale can expect the offspring of her daughters to join the group and potentially compete for resources, while the offspring of her sons will be out there existing and growing on someone else’s dime.
In light of this difference in the natural history of male and female orcas, the researchers compared increases in mortality risk after the death of a mother between males and females. Their findings were significant and fascinating: the death of a post-reproductive mother (>45 years old) incurred a whopping 13.9-fold increase in mortality risk for her sons, and a 5.4-fold risk for her daughters (irrespective of the reproductive status of the daughters).
What about maternal mortality for individuals that may not have yet been post-reproductive? The death of a mother orca had no effect on her daughters that were under the age of 30, and a relatively moderate 2.7-fold increase for older daughters. The "mama's boy" trend continued, though: the death of a younger mother orca resulted in a 3.1-fold increase in mortality risk for sons under 30 years old, and a 8.3-fold spike in mortality risk for her sons above the age of 30.
Thus, it appears that the presence of a reproductive mother greatly reduces the mortality risk of her adult offspring, even long past the age that they are able to forage and fend for themselves. This fascinating insight should lead to future research on the exact mechanisms by which post-reproductive mother orcas influence the survival and reproductive success of their adult offspring. Why is it that male survival suffers so much more than female survival after the loss of a senescent mother, and how is this related to the fact that a male's offspring are not part of the family group?
It appears that the Grandmother Hypothesis may turn out to have more facets that we previously thought. Also, given that orcas are still occasionally poached and often suffer from toxic pollution and declining prey abundance, it is important to understand the effects that the loss of specific family members may have on the dynamics and success of their social group, a phenomenon that is also an issue for species such as elephants, meerkats, and some primate species.
Foster, E. A., Franks, D. W., Mazzi, S., Darden, S. K., Balcomb, K. C., Ford, J. K. B. & Croft, D. P. (2012). Adaptive prolonged postreproductive life span in killer whales Science, 337, 1313-1313 DOI: 10.1126/science.1224198
Johnstone, R. A. & Cant, M. A. (2010). The evolution of menopause in cetaceans and humans,: the role of demography. Proceedings of the Royal Society B, 277, 3765-3771. doi:10.1098/rspb.2010.0988
Ward, E. J., Parsons, K., Holmes, E. E., Balcomb, K. C., & Ford, J. K. (2009). The role of menopause and reproductive senescence in a long-lived social mammal. Frontiers in Zoology, 6, 4. doi:10.1186/1742-9994-6-4
Photo credits (click through images for links):
Orca head shot: NOAA, via Wikimedia Commons
Grandmother: Getty Images
Orca group: Anonymous contributor, Wikimedia Commons
Orca and juvenile: "Christopher" [CC-BY-2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons