Marine mammals have been pottering around unhindered in the oceans for millions of years. But, more recently, another mammal – humans – has begun exploiting their aquatic environment. ‘The ocean is a much noisier and busier place than marine animals have previously experienced in their evolutionary histories’, says Jennifer Maresh from The University of California, Santa Cruz, USA. Maresh explains that our activities are probably disturbing the distribution of many populations in the ocean, and that will impact on the amount of effort that these predators exert to grab a meal. ‘We want to know how resilient animals are when it gets harder to find food’, says Maresh. However, vacuuming up prey in a particular location to find out how predators respond when food is scarce is not an option, so Maresh and her colleagues Dan Costa, Dan Crocker and Terrie Williams decided to make elephant seals work harder for their dinner in a different way. They attached neutrally buoyant blocks to the backs of commuting animals to find out how they adapt as they increase their exertions (p. 1485).

Maresh, Samantha Simmons and Birgitte McDonald travelled to the Año Nuevo State Park just north of Monterey Bay in California, where elephant seals go to breed. Selecting 12 animals over a period of months, the scientists injected two different types of modified water into the animals (to measure their energy expenditure) and strapped a wooden block to their backs to increase their exertion by increasing the amount of water that they must push aside while swimming. In addition, they also attached GPS trackers, time-depth recorders, accelerometers and magnetometers to the animals' backs to monitor their behaviour, before driving the animals 50 km down the coast to the Hopkins Marine Station where they were released. ‘Elephant seals have a tendency to return to their rookery’, says Maresh, who was confident that she would be able to retrieve the priceless data when the animals returned home. The team also repeated the exercise with the same animals, but this time they removed the drag block, to find out how the animals behave and how much energy they consume under normal circumstances.

After successfully retrieving the data loggers, the team was impressed to see that the unhampered animal's movements are incredibly efficient. ‘Elephant seals had low energy requirements (106.5 kJ kg−1 day−1), approaching or even falling below predictions of basal requirements’, says Maresh. However, when the animals had to drag the blocks through the water, their metabolic rates rocketed by 65% to 175.2 kJ kg−1 day−1. Yet, when the team scrutinised the seals' behaviour, they were surprised that the animals had barely altered their swimming style: they did not beat their flippers wider or faster to compensate and they continued diving to the same depths and for as long as the unhindered seals. However, the animals spent 46% longer recovering at the surface after returning from a dive with the drag block and they took longer to ascend and descend. Instead of altering their swimming style to maintain efficiency, the seals stuck to their usual technique, which became increasingly inefficient.

Explaining that increasing time at the surface puts the seals at increased risk of predation by sharks and orcas, Maresh adds, ‘Elephant seals are considered to be a relatively hardy species … that they were so easily affected by a disruption to their routine swimming behaviours was thus somewhat surprising’, and she warns that other more sensitive species are likely to be more seriously affected by human disturbance of the sea.

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Free-swimming northern elephant seals have low field metabolic rates that are sensitive to an increased cost of transport
J. Exp. Biol.