It has been known since Scholander's first experiments on diving animals in the 1940s that the core body temperature of diving marine mammals drops by several degrees during a dive in cold water, confirmed by more recent reports of blood and brain decreases of 2–3°C in diving seals. Researchers have suggested opposing hypotheses for this phenomenon: that the temperature drop is due to a decrease in metabolic heat production or that it is a deliberate cooling of the body to decrease metabolic rate and thus oxygen consumption. Decreasing metabolic rate and conserving oxygen allows diving animals to spend more time foraging underwater.
Either explanation for the drop in body temperature, however, leads to the question posed by Petter Kvadsheim and his colleagues at the University of Tromso, Norway: do diving mammals shiver? Normally, a decrease in brain temperature of 3°C, as seen in diving seals, will cause shivering in non-diving mammals, as these muscle contractions increase the generation of body heat. But an increase in heat production is counterproductive if an animal is trying to cool down to conserve energy, nor would it be a productive way for an animal to decrease its metabolic rate and oxygen consumption, since increasing muscle contractions will, of course, increase energy demand.
Kvadsheim et al. tested the hypothesis that shivering is inhibited in hypothermic diving seals by measuring brain and rectal temperatures, heart rate and muscle activity in three different muscles in captive-raised juvenile hooded seals (Cystophora cristata). Shivering was measured in animals kept in a cold chamber (–35°C) for three hours (or until rectal temperature dropped below 33°C), in shivering animals in cold water(3–4°C) and during a 10–15-min dive (submerged on a restraining board). The animals were also videotaped so that the irregular contractions of shivering could be distinguished from movement of the head or flippers in the muscle activity recordings.
They found that in cold air, the seals began to shiver when rectal temperatures fell below a mean of 35.3°C, a drop of 1–2°C from normal body temperatures, while brain temperatures in shivering seals before a dive averaged 37.3°C. During dives, however, shivering was clearly suppressed. Seals began to shiver again upon reemergence, with four to six times higher muscle activity between and after repeat dives compared with baseline pre-dive data. While hypoxia itself has been shown to inhibit shivering in mammals, the inhibition of the shivering response was nearly instantaneous in submerged seals, and therefore clearly a response to the dive itself rather than to any decrease in oxygen supply. This also implies that part of the `apparent oxygen debt' (increase in oxygen consumption upon resurfacing) after a dive is due to the increased energy demands of shivering and rewarming the body core. Non-diving animals in cold water also shivered,so the inhibition was a direct response to submergence, not just immersion in cold water.
It appears then that, as their body temperature drops in cold air, hooded seals shiver just like any other, non-diving mammal. And a missing piece of the diving response puzzle has been filled in. Whether inhibiting these muscle contractions helps reduce body temperature or is simply part of the reflexes that conserve energy and oxygen, diving seals, at least, don't get the shivers.