The emperor penguins breed at -40�C, in Antarctica's winter, in a scene that looks like a frozen planet. It's hard to imagine living things can endure these temperatures, but breeding and fasting for four months ...

Scientists had already shown that the tight huddling among these big penguins, which is practiced during 50 % of their time spent in the breeding colony, is a important energy-saving behavior that permits them to endure these extremely harsh conditions.
They also decrease their metabolism by 25 %, so researchers were puzzled if it really was a process of hibernation, as previous research indicated that huddling penguins keep a lower rectal temperature than birds outside the mating season (35.7�C vs. 37.9�C). But a prolonged decrease in body (core) temperature would impede successful egg incubation.

A new study, at a colony in Ad�lie Land (Antarctica) assessed body core temperatures in emperor penguins throughout their breeding cycle by using long-term implanted data loggers.

5 pairs of emperor penguins were captured and had devices ("loggers") surgically implanted to record the animals's internal temperatures in April-May 2001, during the mating season.

The entire colony had a breeding success of 42 % and 3 of the 5 pairs fed their chicks until October 2001. Body core temperatures of the five penguins throughout their breeding cycle were on average 36.7 � 0.3�C. During the pairing time, body core temperature dropped progressively from 37.5�C to 36.5�C, with a more important drop of 0.5�C during huddling.

If the egg was lost, body temperature continued to drop to 35.5�C, with a further 0.9�C decrease during huddling. A successful incubation kept a constant core temperature of 36.9�C, even during huddling, thus there is a trade-off between the demands for successful egg incubation and energy savings.

The core temperature drops during the incubation period to 36.5�C and a mean core temperature of 36.7�C (when averaged over the entire breeding cycle) means a lot of energy saved during the breeding cycle of emperor penguins.

But this does not explain the already observed 25 % decrease in metabolism, which suggests that emperor penguins depress their metabolic rate by 50 % during half of the time when they are in dense huddles.

The team has also discovered that the ambient temperature in a dense huddle rises up to 35�C, a fact not associated with body temperature rise even if these penguins exposed to only 20�C in the laboratory rose up their body temperature to 40-41�C.

A possible cause of core temperature decrease inside the dense huddles, in contrast to the expected temperature rise, is the depression of metabolism, possibly through sleep. It has already been shown that the sleep proportion rises during the fast of emperor penguins, and this is linked to an 8 % drop in metabolic rate.