Pregnant Dolphins Must Overcome More Than Impending Motherhood
November 25, 2011

Pregnant Dolphins Must Overcome More Than Impending Motherhood

A recent study, published in the Journal of Experimental Biology, finds that pregnant bottlenose dolphins - especially in the later stages - find their swimming speed almost halved and tail movement restricted, Victoria Gill Science reports for BBC Nature.

Originally interested in how baby dolphins learned to swim, lead researcher Shawn Noren, from the Institute of Marine Science at the University of California Santa Cruz, was diving with the animals in Hawaii, and filming their behavior, and was intrigued by how the dolphins coped with the physical demands of pregnancy.

She studied these animals for a period 10 days before they gave birth until two years after they had given birth to their calves. During their 12 months of pregnancy, dolphins develop a characteristic bump in their abdomen. Noren used more than 30 hours of film footage to measure exactly how this affected the animals´ movement.

“When a pregnant animal is swimming at 1.7 meters per second,” she explained, “it has the same drag force acting on it as a non-pregnant dolphin swimming at 3.4 meters per second. So the pregnant dolphin can only go half the speed as the non-pregnant dolphin before it gets the same drag force.”

Noren also measured the animals´ girth and calculated their frontal surface area, and realized that the pregnancy had a colossal impact, increasing their frontal surface area by an enormous 51 percent. And when Noren measured the drag experienced by the animals as they glided through the water, she discovered that it doubled when the mothers were close to delivery.

The dolphins also were found to have increased fat stores in preparation for lactation that increased their buoyancy. “The buoyancy issue is going to be problematic when you are going down on a dive to capture prey and they are going to need extra energy to overcome that buoyant force”, says Noren.

“So, pregnancy had a dramatic effect on the dolphin´s hydrodynamics, but had it changed their swimming style? Did the pregnant dolphins move with a different gait?”

Zoologist William Sellers, from the UK´s University of Manchester, was surprised by the cost to dolphins of carrying a baby. “It´s not surprising that being pregnant has costs,” he said, “all mammals invest a lot in their offspring, but putting an actual number on it... gives us an idea of what it´s like to be a dolphin,” Sellers told BBC.

“Dolphins are this amazing streamlined shape, and it´s clear that even a small change in that shape affects that streamlining very badly.”

Sellers added that this research could increase the effectiveness of conservation efforts and that scientists needed a more in-depth understanding of their ecology.


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