Climate Change May Make These Males Sexier

September 27, 2016 | Erica Tennenhouse

Photo credit: Bernardo J. O'Connor, University of Adelaide

Population boom explained by increases in crustacean male claw and body size in warmer, more acidic waters.

Female crustaceans like a male with big claws. Lucky for them, male claw size, along with body size, are set to increase under warmer waters and more acidic oceans, according to a study published in Scientific Reports.

Scientists placed members of a species called Cymadusa pemptos — a shrimp-like marine creature in which male claws are used as a display to attract amorous females or as a weapon to fend off competing males — in tanks, and then simulated the elevated temperature and CO2 levels predicted for 100 years into the future to better understand how their reproduction will fare in the face of climate change.

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"Climate change most usually comes with predictions of severe negative impacts on population sizes, if not extinctions," says corresponding author Pablo Munguia, from the University of Adelaide, in a press release.

Yet under conditions mimicking future climate change, Cymadusa pemptos populations boomed, increasing as much as twenty-fold.

"It got even more interesting, however, when we dug deeper and found that males were much larger in size than in previous generations under cooler waters and lower CO2, and their bigger claws were disproportionately larger still,” Munguia explains. Females, on the other hand, stayed the same size.

Not only did male claws get larger under climate change, but the claws of most males in the population also became uniformly large. Munguia proposes that "sexual selection for this attractiveness trait could mean that every male was equally attractive to the females, resulting in very large numbers of females – almost 80% – becoming pregnant, causing a massive population explosion."

Resource availability is often a limiting factor when a population booms so quickly. But in this case, the future climate conditions supported growth of Cymadusa pemptos’ main food source — marine algae — giving the crustaceans unlimited energy for population growth, and for the growth of their massive claws.

"We know that climate change will be cataclysmic for many species but in some cases it will not," says Munguia. "This is the first quantitative example of how it will be beneficial for some individual species, albeit with massive consequences to the environment overall.

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