Sexual Dimorphism In Pinnipeds Began Around 20 To 27 Million Years Ago

April Flowers for – Your Universe Online
Pinning down the answer to a widely-asked question is one of the most exciting things a researcher can do. For Thomas Cullen, who completed the research as part of his Master’s degree at Carleton University, this experience came early in his career. Cullen, working with Canadian Museum of Nature paleontologist Dr. Natalia Rybczynski, made a ground-breaking discovery about pinnipeds — the suborder that makes up seals, sea lions, and walruses.
The findings, published in Evolution, relate to the sexual dimorphism in a variety of pinniped species. The differences in appearance between males and females of a species — including color, shape, size and structure — are considered sexual dimorphism. In many species of pinnipeds, males are often much larger — sometimes as much as twice as large — as the females. This size difference has implications for how they mate and behave.
The Steller’s Sea Lion and Northern Fur Seal are examples of dimorphic pinnipeds that mate in a harem, with one male presiding over a larger community of female mates. For non-dimorphic pinnipeds, such as the Ringed Seal, this is not a typical mating behavior. These observations underline the intimate link between sexual dimorphism and mating style.
Why sexual dimorphism exists in many pinniped species, and when this trait evolved, are questions that have puzzled researchers for a long time. When the research team examined fossils of an extinct pinniped, they discovered an incontrovertible answer to the question of when. Cullen, who is now at the University of Toronto, examined the fossil at the Canadian Museum of Nature before analyzing the data in the lab of Professor Claudia Schröder-Adams.
“We were examining a fossil of a pinniped that was previously thought to be a juvenile, but we looked at it again and found that, based on its skull structure, it was likely an adult,” says Cullen. Combined with analysis comparing this fossil to others of the same species and modern dimorphic species, this discovery proved that the fossil belonged to a sexually dimorphic species.
The pinniped fossil, Enaliarctos emlongi, was discovered in the late 1980s off the coast of Oregon. During Cullen’s study, the fossil was on loan to the Canadian Museum of Nature from the Smithsonian Institution.
Danielle Fraser, Carleton PhD candidate, collaborated with Cullen to compare the fossil to modern seal skull structure. The evidence revealed that sexual dimorphism existed in seals somewhere between 20 million and 27 million years ago, near the base of all pinniped evolution and much longer ago than was previously thought. This discovery, which has implications for the past and future of the species, made Cullen one of the first to pin down a timeline for the phenomenon.
“Early pinnipeds likely formed harems, the way a sea lion would,” says Cullen. “Up to this point, it was not widely expected that early pinnipeds would behave this way.”
Once the question of when sexual dimorphism in pinnipeds emerged, Cullen and his team tackled the question of why it happened. “Our interpretation is that these changes were happening at a time when the Earth was experiencing major climate and ocean circulation changes. Harem colonies were likely located at ocean upwelling sites that concentrate nutrients in otherwise nutrient-poor water. We think that this environmental factor, this concentration of large numbers of pinnipeds into one area, pressured them into developing the harem mating system and sexual dimorphism.”
The study results shed light on the history of pinnipeds and have major implications for the future of the species when taken in the modern context of climate change.
“Climate change today appears to be having an effect on the Arctic and Antarctic more than on the temperate and equatorial latitudes. Most Arctic and Antarctic pinnipeds aren’t really sexually dimorphic, and we think this is because the water in those areas is quite nutrient rich. The pinnipeds there didn’t have that selection pressure to form harem behavior because of the wide availability of nutrients. Going forward, if the effect of climate change is increased water temperature in the Arctic and Antarctic, it would suggest that the nutrient levels will be reduced. This could put more pressure on pinnipeds in the polar regions areas to form colonies and, as a result, harem behavior.”
The findings present some of the earliest evidence in marine mammals of Charles Darwin’s theories of sexual selection in evolution. Until now, there has been a relative lack of new data on sexual dimorphism in the fossil record.
“This paper shows that the fossil record can be really useful in answering evolutionary questions that could otherwise not be addressed,” says Cullen. “It also shows that a combination of modern and fossil analysis is crucial to thoroughly addressing evolutionary problems. We were really lucky to have access to a specimen of this nature.”