A leadership role for post-menopausal killer whales

Male killer whale (Orcinus orca) near Tysfjord, Norway
Wikimedia Commons (CC)

The male killer whale above may have an impressive dorsal fin, but when it comes to leadership, older females play a greater role. A recent study of a resident population found that females with an age of 35 or older led the pod, especially when food resources were scarce.

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. 

Walking with whales

University of California Press Oakland CA 2014
ISBN 978-0-520-27706-9

This highly readable book by Hans Thewissen recounts a scientific odyssey in search of the ancestors of whales. Almost all the relevant fossils were found in Pakistan and western India. Dr. Thewissen worked there under trying conditions and in the shadow of political conflicts (his first field expedition was foreshortened by the 1991 Gulf War). He succeeded, however, in uncovering an evolutionary series of walking whales and eventually in demonstrating their affinity to even-toed ungulates (Artiodactyla).

The key discovery (here), made independently by Gingerich (here), was that ancestral whales possessed the double pulley astragalus (ankle bone) that is a defining feature of artiodactyls. The author does a very good job of explaining this and other anatomical details. Indeed, this book was written for the general reader and its author is at pains to explain the necessary terminology. He also gives fascinating insights into several aspects of cetacean biology such as the way in which aquatic mammals, including fossil and living whales, propel themselves through the water. 

The relation of whales to artiodactyls, with hippopotami as the closest living relatives, was first proposed on the basis of molecular phylogenetics (e.g. here). Because there is no fossil record for hippopotami, Dr. Thewissen does not find this analysis very useful. He prefers to keep Cetacea as a separate order even though this makes Artiodactyla paraphyletic.

Of particular interest to me are Dr. Thewissen's studies of a series of dolphin embryos and his proposed research on embryos of the bowhead whale.

This is a deeply personal book and no attempt is made to hide the author's prejudices. He is so concerned about Japanese whaling that he missed an opportunity to collaborate with Seiji Ohsumi, the grand old man of cetacean research who has done seminal work in reproductive biology.

Ohsumi S: Comparison of maturity and accumulation rate of corpora albicantia between the left and right ovaries in cetacea. Sci Rep Whales Res Inst 1964; 18:123-148.