29 May 2015

Grannies of the Sea

­Orcinus orca, photo by S. Blanc on www.arkive.org 

What do humans and whales have in common?
        Killer whales (Orcinus orca) and the short-finned pilot whales (Globicephala macrorhynchus) have comparable post-reproductive lifespans. Just like human females, killer whales undergo menopause after they have reached a certain age. They have the longest post-reproductive lifespan of all non-human animals. Female orcas usually stop reproducing during their 30’s to their 40’s but can live until their 90’s, as compared to males that rarely live beyond their 50’s [1].

        Aside from the mortality of the females being higher, their post-reproductive stage renders a higher survival rate to their pod, especially to their male progenies. These so called “grandmothers” may serve as teachers, guides, and keepers of the clans or pods’ traditions [3]. According to a recent study that was conducted for about 35 years, females that had undergone menopausal stage exhibit a strong leadership to its pod. During periods wherein food is scarce, they are most likely to lead the group’s activities, especially when food source such as salmons are low in supply or extremely difficult to locate [2]. The mothers are able to share their long experience in the waters which gives them an advantage in foraging.

        Compared to females, males are more likely to follow their mothers as they conceive the ecological knowledge of their mothers useful. It is known that once their post-reproductively-aged mother dies, the sons have a higher probability of dying compared to the females [2]. From this, it can be said seen that the males highly depend on their mothers for their survival.

        Aside from the killer and pilot whales, there has been no evidence or data in other non-human animals that can suggest similar post-reproductive behavior [1].

            To know more about whales, visit SeaLifeBase.
[1] Foster, E.A., Franks, D.W., Mazzi, S., Darden, S.K., Balcomb, K.C., Ford, J.K.B., and Croft, D.P. (2012) Adaptive Prolonged Postreproductive Life Span in Killer Whales. Science. 337:1313

[2] Brent, L.J.N., Franks, D.W., Foster, E.A., Balcomb, K.C., Cant, M.A., and Croft, D.P. (2015) Ecological Knowledge, Leadership, and the Evolution of Menopause in Killer Whales. Current Biology. 25: 746-750

[3] Allen, S.G., Mortenson, J., and Webb, S. (2011) Field Guide to Marine Mammals of the Pacific Coast. University of California Press: Berkley, California. 261p.

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