Life's a drag for pregnant dolphins

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It seems that it's not only females of land-dwelling mammals that are left to waddle around in their final weeks of pregnancy.

Research published online in The Journal of Experimental Biology demonstrates that pregnancy negatively affects the swimming performance of female Bottlenose dolphins (Tursiops truncatus), so much so that they adopt a new swimming style (gait), are left open to predation as well as being in danger of being left behind by the rest of the pod.

Shawn Noren from the institute of Marine Science, University of California Santa Cruz, observed two pregnant female dolphins at Dolphin Quest, Hawai’i prior to them giving birth.

"The pregnant females had huge protrusions where the foetus was sitting towards the back end of the body", says Noren, who used SCUBA gear to film the pregnant dolphins underwater as they swam parallel to her camera between their trainers.

Filming continued until the calves reached two years, and the footage compared. Noren realised that pregnant females were slower with a restricted top speed of 3.54 metres per second, whereas they were able to swim at much higher speeds after giving birth.

"Two to three metres per second is a comfortable speed for most bottlenose dolphins,' says Noren, 'but these pregnant animals did not feel comfortable going beyond that.'

Slower swimming was not the only change. The animals’ girth was measured with the dolphins showing a 51% increase to their frontal surface area, drag was doubled when the mothers were close to delivery and, as the dolphins laid down fat in preparation for lactation their buoyancy was increased. 

"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.

The discovery that gait was affected came as a result of digitising, and comparing, the up and down beats of the dolphins’ tail fins (flukes) with the conclusion that the dolphins had to beat their flukes faster to make up for a 13% reduction in stroke length.

Noren points out that tuna are still fished using massive nets in the eastern tropical Pacific, a potential problem for pregnant dolphins. "Here is a fast speed event, so it is possible the near term pregnant females are being left behind in the chase. They are reliant on a large pod for protection and cooperative feeding and once the animal is separated it would be hard for it to find the pod again."

For further information see the paper: Noren, S. R., Redfern, J. V. and Edwards, E. F. (2011). Pregnancy is a drag: hydrodynamics, kinematics and performance in pre- and post-parturition bottlenose dolphins (Tursiops truncatus). J. Exp. Biol. 214, 4149-4157.

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