Baby Bumps Slow Dolphins Down

For bottlenose dolphin moms-to-be, baby bumps are literally a drag. A new study shows that the animals pay a considerable price for their prenatal girth, which causes them to swim slower as they face more resistance from the water. The finding offers a first look at how pregnancy diminishes performance in cetaceans and may have implications for dolphin conservation efforts.

Swimming for two. Heavily pregnant bottlenose dolphins beat their tail fins faster (blue wavy line) and swim more slowly to compensate for increased drag.
Credit: Shawn Noren

From scorpions to humans, most animals have a hard time getting around in late pregnancy. Past studies have found that altered or diminished physical performance before giving birth is common across species. Some birds are even rendered flightless shortly before they lay their eggs. But few studies have looked at why pregnancy changes performance.

Biologist Shawn Noren of the University of California, Santa Cruz, didn't intend to answer that question when she joined a pod of bottlenose dolphins at Dolphin Quest Hawaii, a marine research and public education center. She planned to focus on how newborn dolphins learn to swim. But with two of the center's dolphins just 10 days from delivering their calves, Noren took the opportunity to examine their motion as well.

For most of 10 days, Noren sat submerged in the Dolphin Quest lagoon, becoming "one with the pod," she says. Armed with SCUBA gear and a digital camcorder, she videotaped the pregnant dolphins as they swam parallel to her camera. She filmed them again after they gave birth, returning at regular intervals from one to 24 months later. Noren then combed through her hours of footage, digitally analyzing the movies to calculate the pre- and post-birth sizes of the dolphins, as well as their swimming speeds and movement patterns.

She found that when the two dolphins had a bun in the oven, the frontal surface area—the surface of the animal that cuts through the water—increased 43% for one and 69% for the other. That extra area dramatically increased drag, meaning that a pregnant dolphin gliding at a comfortable 1.7 meters per second met with the same water resistance as a nonpregnant dolphin going twice as fast. In addition, the pregnant dolphins' increased fat stores made them more buoyant, so the moms-to-be had to work harder to dive for prey.

The pregnant females' tailbeat—their tail fins' up-and-down motion as they swim—also changed, losing about 13% of its amplitude. To compensate, the moms beat their tail fins faster, but they still averaged only 38% of their post-pregnancy speeds, Noren reports today in the Journal of Experimental Biology.

That pregnancy reduced performance isn't surprising, says Noren, who admits she wondered why no one else had done such a study before. But the magnitude of the effect did surprise her. "It was definitely bigger than I had initially thought."

Marine biologist Frank Fish of West Chester University in Pennsylvania is also impressed. "Absolutely everything is impacted by these animals getting a bit thick in the middle," says Fish, who was not involved in the study. "We always expected pregnancy would affect dolphins' motion," he says. "But no one up to this point had done the work to actually show there's a hydrodynamic penalty for being pregnant."

That penalty could spell disaster when dolphins encounter predators and certain commercial fishing vessels, Noren says. In some places, tuna fishing involves chasing down the dolphins that often swim near tuna, catching both, and then releasing the dolphins. If trapped by such methods or by a predator, slower pregnant females would be less able to escape or catch up with their pod.

"Conservation efforts aren't seeing the recovery they expected, and maybe that's because calves and pregnant females are getting left behind," Noren says. She suggests that alternative fishing methods or even reducing the speed of the chase may help improve a pregnant dolphin's chances.