Northern elephant seals are truly impressive divers. During months-long migrations, they spend most of their time underwater, diving hundreds of meters down into the sea. What's even more impressive is that these animals are breath-hold divers. This means that they must empty their lungs of air, and dive, dive, dive…until they reach their feeding grounds, where they forage for up to half an hour before making their way back up to the surface for more air. Because this type of diving is energetically expensive, seals must balance the costs of diving with the benefits of remaining at the bottom long enough to eat sufficient food to make it worth the trip. And the amount of fat a seal has determines how easily it moves up and down through the water – lean (negatively buoyant) seals fall fast, but must work harder to swim back up to the surface. While mathematical models have suggested that being neutrally buoyant, as the seals are when they are fat from months of feeding, may reduce total overall locomotor costs and allow the seals to spend more time at the bottom foraging, testing this prediction in nature on deep-diving, long-term migrating marine mammals has proved challenging. But that didn't stop Taiki Adachi, at The Graduate University for Advanced Studies in Tachikawa, Tokyo, Japan, and his team of international collaborators from seeking the answer to the question, are fatter seals more efficient foragers?
Adachi and his team developed an instrument (a stroke logger) that allowed them to record acceleration and pitch angles at high sampling rates and attached these, along with data loggers and transmitters, to female northern elephant seals (Mirounga angustirostris) before the seals journeyed on two foraging migrations – a 2 month post-breeding season migration and a 7 month post-molting season migration. They also recorded the seals’ body masses and external physical measurements. Using stroke loggers, the authors estimated the body composition of the seals based on the rate of vertical passive descent or ascent (drift rate) of the seals, as drift rate is strongly correlated to body density.
The researchers found that short post-breeding migration did not result in a shift from negative to neutral buoyancy – that is, the seals did not amass enough fat to change how quickly they fell through the water column, or how hard they had to work to get back up through it. However, by the end of recording during the long post-molting migration, the seals were fatter, more buoyant and had fewer swimming strokes per trip than when they were leaner. The big reward for the increase in fat was that it allowed the seals to spend more time at the bottom of the sea foraging. The authors suggest that a reduction in oxygen consumption during the dive may allow the seals more energy to use during their time foraging.
Many marine mammals have similar diving and foraging strategies to these seals, and the authors propose that there could be potential locomotor costs for other marine mammals when straying from the optimal body composition. Although future studies including measurements of metabolic rate would provide a more complete picture of the energetic costs of diving and foraging in marine mammals, one thing seems clear – for these seals, it pays to be fat.