Bottlenose dolphins' maximum speed halved by pregnancy (BBC Nature 24.XI.2011)
By Victoria GillScience reporter, BBC Nature
Pregnant dolphins are not able to sweep their tails
through as wide an arc
As for many mothers-to-be, the late
stages of pregnancy can be extremely awkward for dolphins, say scientists.
Gliding along beneath the ocean, it might seem that these streamlined marine
mammals are unaffected by the slight swell of carrying a baby.
But a study has revealed that the females' top swimming speed is almost
halved when they are close to giving birth.
The findings are published in the Journal of Experimental
They reveal just how much the animals invest in carrying their offspring.
Lead researcher Shawn Noren, from the Institute of
Marine Science at the University of California Santa Cruz, US, was originally
interested in how baby dolphins learned to swim.
But while she was diving with the animals in Hawaii, and filming their
behaviour, she became fascinated by how the females coped with the physical
demands of pregnancy.
Dr Noren captured more than 30 hours of footage of two female dolphins
swimming. She studied these animals for the period from 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. Dr Noren used her footage to measure exactly how this affected
the animals' movement.
"When a pregnant animal is swimming at 1.7 metres per second," she explained,
"it has the same drag force acting on it as a non-pregnant dolphin swimming at
3.4 metres per second.
"So the pregnant dolphin can only go half the speed as the non-pregnant
dolphin before it gets the same drag force."
Dr Noren also wanted to know if the animals changed the way they moved in
order to compensate for this additional drag.
Tracing the animals' movements, she found that pregnant females reduced the
size of the arc through which they swept their tails - the up and down sweeping
motion that propels the dolphins through the water.
"Pregnant animals had a 13% reduction in the [size] of this stroke," Dr Noren
explained. "This might be because of the way the foetus sits so far back in its
body... so the abdomen area is too stretched and taught here [and] it limits
This slowed the animals down significantly. Their top
speed was restricted to approximately 13km/h (8mph), whereas the maximum swim
speed of non-pregnant dolphins is more than 22km/h (14mph).
To put that into context, the hunting speed of a mammal-eating killer whale -
one of the dolphins' natural predators - is estimated to be in the range of
"So you can see how pregnant animals would be much more vulnerable," said Dr
Dr William Sellers, a zoologist from the UK's University of Manchester, said
he was surprised by the magnitude of 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."
He explained that the extra drag meant that the pregnant animals would need
twice as much energy to move around.
"Dolphins are this amazing streamlined shape, and it's clear that even a
small change in that shape affects that streamlining very badly."
He added that, in order to develop effective ways to protect dolphins and
safeguard their environment, scientists needed an in-depth understanding of
their ecology. Πηγή