Determining Sexual Selection In Extinct Species
January 30, 2013

Sexual Selection Can Be Inferred From The Fossil Record

Brett Smith for - Your Universe Online

According to a new study in the journal Trends in Ecology and Evolution, determining if certain extinct animals had sexually selective traits is possible, despite the fact that we cannot observe their behavior.

Many animals have sexually selective traits that are used or displayed in pursuit of a mate, male peacocks have their colorful plumage and mallard ducks have a distinct green coloration to their head feathers. Some of these traits are accompanied with a behavior. For example, male moose have large antlers that they use to battle for the right to mate.

Because we are able to observe moose behavior, we know that the antlers are used in the pursuit of a mate, and not for hunting or other behaviors. Unfortunately for paleontologists, it is impossible to observe extinct species and see how they used various traits and features to determine if they could be sexually selective.

However, a new review from a group of U.K. and Australian researchers suggest that determining sexual selection for the feature of some extinct animals is very much possible.

“We see much evidence from the fossil record suggesting that sexual selection played a major role in the evolution of many extinct groups,” said study co-author Darren Naish, of the University of Southampton. “Using observations of modern animal behavior we can draw analogies with extinct animals and infer how certain features improve success during courtship and breeding.”

In the study, Naish and his colleagues posit that fossils hold many clues to the existence of sexual selection in extinct species – citing features clearly used for fighting, bone damage from duels, and readily displayable ornamentation, like the fan-shaped crests on some dinosaurs.

They also note the unique differences between males and females of a species, referred to as ℠sexual dimorphism´, as the most obvious way to positively identify sexually selected traits. The study mentions American alligators, which have different pelvic structures between the sexes, as an example of sexual dimorphism.

The researchers also noted that even a lack of sexual dimorphism in the fossil record doesn´t mean that selective mating didn´t take place.

“Even when there is genuinely no morphological difference between the sexes, sexual selection might still be in operation via the phenomenon of mutual sexual selection,” they wrote. “This occurs when members of both sexes show mate choice and both exhibit sexually selected structures.”

According to the review, features that are observed in mature adults, yet that are absent from the younger members of the species, indicate that their purpose might be linked to reproduction or mating behaviors.

“Some scientists argue that many of the elaborate features on dinosaurs were not sexually selected at all,” said Naish, who is from the university´s Vertebrate Paleontology Research Group.

“But as observations show that sexual selection is the most common process shaping evolutionary traits in modern animals, there is every reason to assume that things were exactly the same in the distant geological past.”

In their conclusion, the researchers acknowledge the limited amount of information the fossil record provides, yet they insist that “strong inferences” can be made about sexually selected characteristics in certain species.