|Male killer whale (Orcinus orca) near Tysfjord, Norway|
Wikimedia Commons (CC)
Ovarian function in whales has been assessed by counting the number of corpora lutea and corpora albicantia in each ovary. Baleen whales ovulate from both ovaries with about the same frequency. In toothed whales, however, the left ovary tends to be more active. In some dolphins, all ovulations occur on the left side early in life; later the right ovary kicks in, perhaps because the left ovary is becoming exhausted. According to the renowned scientist Seiji Ohsumi, this pattern occurs in the bottlenose dolphin (Tursiops truncatus) and striped dolphin (Stenella coeruleoalba).
In some cetaceans, it would seem the ovaries have a limited capacity. Once it is exhausted the females enter a post-reproductive phase that has been likened to human menopause. This was documented for the short-finned pilot whale (Globicephala macrorhynchus) by Marsh and Kasuya and later for the killer whale (Orcinus orca).
A resident population of killer whales off the coast of British Columbia and Washington State has been under observations for years. Individuals can be identified in the field and their ages are known. Their main source of food is Chinook salmon. When resources were scarce the hunt was led by females of post-reproductive age.
Does this provide an explanation for "the evolution of menopause" as the authors of the new study suggest? To my thinking that is a bit of a stretch.
Ohsumi S. Scientific Reports of the Whale Research Institute 1964; 18: 123-49.
Marsh H, Kasuya t. Rep. Int. Whaling Commission (Special Issues) 1986; 8: 57-74.