Most of us remember our first sortie into a swimming pool supported by buoyancy aids. But while children's flotation devices are designed to give them support, buoyancy can be a major hurdle for some diving species; they have to swim hard to overcome its effects. And some animal's buoyancy isn't a constant impediment, it can vary significantly between individuals and from season to season as an animal gains and loses fat. How diving animals adapt as their buoyancy changes wasn't clear until Yuuki Watanabe and his colleagues from Japan and Russia attached data loggers to freely swimming Baikal seals,ranging in weight from 54 kg to 83 kg, to see whether each seal had a unique dive signature that reflected their different densities(p. 3269).
Securing detachable data loggers to three animals in Lake Baikal, the team let the animals swim free. After a nail-biting wait, the data loggers bobbed back to the surface a day later and the team retrieved them with the help of a VHF signal before downloading the animals' stroke pattern data. Sure enough,the seals had very different dive signatures. Two of the seals descended with little effort, yet swam hard to return to the surface. The third seal returned to the surface with ease, while swimming with fast strokes while diving. The animals were all using different swimming stroke rates during descent and ascent, probably in response to their different body densities.
But how would the animals adapt to a seasonal change in buoyancy? The team secured a 1.5 kg lead weight, designed to fall off after 24 hours, to a seal and monitored its diving behaviour for 72 hours. But the experiment seemed doomed when the VHF signal failed and it couldn't be retrieved. Fortunately, a passing tourist retrieved the valuable data logger and returned it to a relieved Watanabe two weeks later.
Analysing the precious data, the team saw that the animal's stroke pattern changed significantly after the weight detached and the animal's `fatness'suddenly increased. While carrying the weight, the seal's density was high, as if it carried little fat, so the animal glided during its descent while swimming continually to return to the surface. However, after the weight fell off the seal had to adapt to its apparent fat increase by raising its stroke rate while diving, to overcome its increased buoyancy, and alternating swimming and gliding stroke patterns to return to the surface. The seal had drastically changed its swimming style in response to its altered buoyancy.
Curious to know how the 1.5 kg lead weight compared with a real change in the animal's fat deposits, the team calculated that carrying the lead weight was equivalent to the seal losing 14% of its fat, a realistic loss for animals marooned on shore when moulting and raising young. So, the diver's drastic swimming style change is probably a realistic reflection of the adjustments the seals make on a seasonal basis.