For Dolphins, Pregnancy Comes With a Price

A bigger body means increased drag, slower speeds and greater vulnerability to predators

Bottlenose dolphins are good swimmers
Bottlenose dolphins are good swimmers Courtesy of flickr user Willy Volk

Human females often find late pregnancy to be a bit of a drag, as they waddle about trying to accommodate a big baby belly, but they can take comfort in knowing that dolphins probably have it worse. Pregnancy for them really is a drag, physics-wise, and they may find it harder to catch food or avoid becoming a predator’s meal, according to a new study in the Journal of Experimental Biology.

Researchers from the University of California at Santa Cruz and the Southwest Fisheries Science Center studied two female bottlenose dolphins at Dolphin Quest Hawaii, measuring and weighing them and diving with them, recording them as they swam, starting from a week or so before each gave birth and continuing their observations for two years. Pregnancy, they found, had serious consequences for movement through water.

Pregnant females may appear as streamlined as their non-pregnant counterparts, perhaps a bit fatter, but their bellies increase their frontal surface area by 51 percent, which greatly increases drag. They also can’t sweep their tails as far as when they’re not pregnant, so they have to change their gait, sweeping faster to compensate. During pregnancy, dolphins also increase their stores of fat to prepare for lactation after they give birth, but the fat makes them more buoyant and they require more energy to dive. As a result of all of these changes, pregnant females swim slower. “Two to three meters per second is a comfortable speed for most bottlenose dolphins,” says the study’s lead author, Shawn Noren of U.C. Santa Cruz, “but these pregnant animals did not feel comfortable going beyond that.”

The dolphins in the study were captive animals, so their lack of speed was no more than an inconvenience. But for dolphins in the wild, the inability to swim fast could be deadly, the scientists say. Dolphins’ main predators–sharks and orcas–can easily swim at speeds greater than the maximum reached by the pregnant animals. And a dolphin’s pod may be little help if all their friends have swum away. “Ultimately,” the scientists write,” the results of this study support the notion that reproduction is a costly endeavour that may increase energetic expenditure, increase risk of predation and decrease longevity.”

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