Marsupial frogs

Female Brazilian tree frog (Flectonotus pygmaeus) with
brood pouch enclosing the developing embryos
(Mauricio Rivera Correa ShareAlike 2.5)

What happens when frogs abandon their amphibious lifestyle for a more terrestial one? Frogs in the Family Hemiphractidae from South and Central America have evolved some ingenious solutions.

In hemiphractids, the embryo develops on the back of the mother either in a mucous-filled depression or in a closed pouch - as shown above for a Brazilian tree frog (the embryos are under the bumps).

Froglet of a marsupial frog (Gastrotheca ovipera) showing
the external gills. From Nathan 1932 (here)

The embryos may develop into tadpoles and be released to water-filled cavities in plants or skip the tadpole stage and develop directly into froglets. In species of the genus Gastrotheca, embryos have 1-2 pairs of external gills that serve for respiratory gas exchange with maternal tissues in the brood pouch. This would satisfy most people's definition of a placenta. The gills are shed around the time of birth.

Marsupial Frogs by William E. Duellman 2014
Johns Hopkins University Press ISBN 978-1-4214-1676-5

The biology of marsupial frogs is described in painstaking detail in this new book - the destillation of a lifetime's work by William E. Duellman (details here). It is superbly illustrated but at USD 120 a bit pricey. More than half the content comprises species accounts and no doubt it will find a place on the bookshelf of specialists. But it is well worth checking out for its insights into the reproductive biology of frogs. Who knew, for example, that the oocytes of Flectonotus pygmaeus have up to 2000 nuclei reduced during oogenesis to a single one?

For other fun facts on frog reproduction see this video.

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.