When sea lions go in search of a tasty morsel they have to rely on more than just the air in their lungs as they swim into the depths. The lungs of diving mammals, like sea lions, seals and whales, tend to collapse as they dive deeper and hold very little air, so these animals depend on oxygen stored in their blood and muscles instead. Knowing how much oxygen a diving mammal can store gives researchers a clue as to how deep they can dive, and for how long. Interested to know more about the diving ability of the California sea lion (Zalophus californianus), Michael Weise at the University of California, Santa Cruz, and his colleague Daniel Costa looked at blood and muscle oxygen stores in pups through to adults of both sexes(p. 278).

To find out how much oxygen is carried in the blood, the team took blood samples from five groups of sea lions ranging from 5 month old pups to fully grown adults. They measured the percentage of the blood that was made up of red blood cells, the amount and concentration of the oxygen-carrying pigment haemoglobin in the red blood cells, and estimated each animal's blood volume. Using these measurements, they then calculated how much oxygen a sea lion could store. Taking into account each animal's size, they found that nine month old recently weaned pups could only store half as much oxygen in their blood as adults, and didn't catch up until they were between 1.5 and 2.5 years old. This suggests that young sea lions won't be able to dive as deep or for as long as the adults, but the seas around California are very productive, so they probably don't have too much trouble finding something to eat when conditions are good.

Having shown that sea lions' ability to store oxygen in their blood improved with age, the team turned to muscle stores. They took samples from the muscles that control the fore flippers, which sea lions use to propel themselves through the water, and measured the concentration of the oxygen-holding pigment myoglobin in the fibres. `We came up with some interesting results' says Weise. The team were expecting that ability to store oxygen in myoglobin would develop at the same rate as blood oxygen storage,but instead found that myoglobin stores did not reach adult levels until 4-6 years of age. `This could be because muscle takes longer to develop than blood' says Weise, `there could be a cost to developing myoglobin'.

The next surprise came when they compared myoglobin concentration in adult males and females - they found that females had greater myoglobin concentrations than males, meaning that per gram of muscle, females can store more oxygen. The team think that there are two reasons for this difference. After mating and giving birth, females stay in Southern California with their pups, while the males travel further north. In southern California `the females have to dive deeper and longer to access prey,' explains Weise, `in the North, males can dive shallower to find food'. Males are also larger than females and have a greater blood volume, so can rely on their bulk to store the oxygen they need for their more leisurely dives. The team suspect that the females compensate for their tougher dives, smaller body size and the added strain of rearing pups by increasing their myobglobin concentration.

Weise, M. J. and Costa, D. P. (
). Total body oxygen stores and physiological diving capacity of California sea lions as a function of sex and age.
J. Exp. Biol.