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Female (top) and male (below) of the ostracod Cypideis salebrosa. Note the male shell is more elongated than that of the female. This is thought to reflect the need to accommodate the large male genitalia (highlighted in blue). Photo: M. João Fernandes Martins, Smithsonian

Bigger is not necessarily better – at least when it comes to the survival of species.

Too much competition for mates can drive some species to the brink of extinction, and even over the edge, according to the new study in Nature.

The analysis of millions of years of crustaceans in the fossil record determined that males can invest too heavily in beating out their competitors within their group – at the expense of the entire species.

For instance, male peacocks that are too invested in beautiful plumage to attract females, or elephant seals that grow too big to dominate their group, could put their entire species at risk.

“If devoting so much energy to reproduction made it harder for species in the past to adapt to changing circumstances, perhaps that same should apply to species we’re concerned about conserving in the present day,” said Gene Hunt, of the Smithsonian National Museum of Natural History.

The look at “sexual investment” starts with a tiny creature that populates the sea.

Ostracods are a class of tiny crustaceans that have lived in shells the size of poppy seeds for half a billion years. But there are many separate species within the class that have evolved and died out over that span.

The males of the shrimp-like fauna have longer shells, apparently to accompany bigger sex organs than their female counterparts.

The Smithsonian researchers, along with colleagues at the College of William and Mary and the University of Southern Mississippi, looked at 93 different such species over roughly 20 million years. Their look sought to establish whether species with the biggest disparity between males and females died out more frequently over the study period, from 84 million to 66 million years ago.

Extinction and preservation probabilities of each kind of ostracod was determined by a capture-mark-recapture method through the program MARK.

Their finding indicates that males much bigger than females was only an advantage to an individual – and not the group.

“We show that species with more pronounced sexual dimorphism, indicating the highest level of male investment in reproduction, had estimated extinction rates that were 10 times (greater) than those of the species with the lowest investment,” they write. “These results indicate that sexual selection can be a substantial risk factor for extinction.”

The theory is that resources devoted to a larger shell and more sperm mean are diverted away from survival in the face of pressures outside the species – and that increases sperm competition could force females to also evolve increasingly taxing counter-adaptations.

“We showed that when males are larger and more elongated than the females, those species tend to not last as long in the fossil record,” said Hunt.

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