March of the Penguins, the Oscar-winning documentary, showed how the emperor penguins endure their incubation and fast for four dark and bitterly cold months each year. The tight huddling among these South Pole penguins is a key energy-saving mechanism that allows them to endure their extremely harsh conditions.
A team of scientists that had already shown that emperor penguins who are free ranging in their colony spend about 50 percent of their time in dense huddles and drop their average metabolic rate by 25 percent has questioned whether this drop is due to a process similar to hibernation. Entering into the colony with bulb thermometers, earlier investigators had indeed found that huddling penguins maintain a lower rectal temperature than birds which were isolated from the colony (35.7°C vs. 37.9°C, respectively). However, a sustained drop in deep body (core) temperature would be in direct conflict with the requirements for successful egg incubation. Therefore, energy savings accrued from huddling might rely on mechanisms other than a lower body temperature.
To better understand this mechanism, the researchers conducted the first recordings of deep body temperatures in free ranging birds throughout their breeding cycle by using long-term implanted data loggers. The researchers sought to assess whether male emperor penguins lower their deep body temperature during breeding and incubation. Additionally, using external devices during pairing and visual observations of implanted males during incubation, they sought to study deep body temperature variations when the birds were huddling.
The authors of the study entitled "Body Temperature Changes Induced by Huddling in Breeding Male Emperor Penguins," are conducted by Caroline Gilbert, Yvon Le Maho and André Ancel, all from the Institut Pluridisciplinaire Hubert Curien, Département Ecologie, Physiologie et Ethologie and Département Interactions Physique, Chimie et Vivant, Centre National de la Recherche Scientifique and Université Louis Pasteur, Strasbourg; and Martine Perret at the Département d'Ecologie et Gestion de la Biodiversité, Centre National de la Recherche Scientifique and Muséum National d'Histoire Naturelle, Brunoy, France. Their findings appear in the January, 2007 edition of the American Journal of Physiology - Regulatory, Integrative and Comparative Physiology.
The research was conducted between April and August 2001 at the emperor penguin colony of Pointe Géologie, near the French station of Dumont d'Urville, Adélie Land, Antarctica. Light durations for the colony site varied from two to 11.5 hours. At the mid-point of the pairing period, between April 22 and May 5, 2001, five pairs of emperor penguins were captured and had devices ("loggers") surgically implanted to record the animals internal temperatures. Pairs were selected in which the males had sufficient body fuel reserves that would ensure success in their incubating task. After implantation, both mates were individually marked with colored strips and released together.
Male and female penguins implanted with the loggers were observed daily in the colony using binoculars, at a distance of at least 10 to 20 meters. Breeding success of the entire colony during 2001 was low at 42 percent. Out of the five pairs studied, three were feeding their chicks until October 2001. The experimental pairs in the study resumed with the expected breeding cycle, and their breeding success was similar to penguin pairs without device implantation.
This study is the first to provide long-term deep body temperature records of unrestrained breeding emperor penguins at their colony. Deep body temperatures of the five free-ranging penguins throughout their breeding cycle were on average 36.7 ± 0.3°C. During the pairing period, deep body temperature decreased progressively from 37.5°C to 36.5°C, associated to a significant temperature drop of 0.5°C during huddling. In case of egg loss, body temperature continued to decrease to 35.5°C, with a further 0.9°C decrease during huddling. By contrast, a constant core temperature of 36.9°C was maintained during successful incubation, even during huddling, suggesting a trade-off between the demands for successful egg incubation and energy savings. The average core temperature measured (36.7°C) was on average 1°C lower than those previously reported for captive emperor penguins during rest within their thermo neutral zone, of about 37.5°C to 38°C. This temperature range corresponds to the core temperature recorded at mid-pairing (from 37.1 to 38.1°C). A core temperature reduction during the pairing period to 36.5°C and a mean core temperature of 36.7°C (when averaged over the entire breeding cycle), may therefore amount to significant energy savings during the breeding cycle of emperor penguins. However, it cannot explain the already observed 25 percent drop in average metabolic rate, which suggests that emperor penguins depress their metabolic rate by 50 percent during half of the time when they are in dense huddles.
Metabolic Rate in Dense Huddles May Become Depressed
The researchers have also found that the ambient temperature in a dense huddle increases up to 35°C. Yet there is no rise in the body temperature of these birds although the body temperature of individuals exposed to only 20°C in the laboratory goes up to 40-41°C. The team believes that a possible explanation for the constancy (or slight decrease) of core temperature inside the dense huddles, in contrast to the expected temperature rise, is the depression of metabolic rate. Such depression could be achieved by entering sleep. In fact, during tight huddles, birds were observed with their eyes closed. It is known that the proportion of sleep increases during the fast of emperor penguins, and that sleep is associated in penguins with an eight percent drop in metabolic rate.
The findings suggest that tight huddling allows a down-regulation of body temperature by 1°C in roaming birds when compared with animals isolated from the colony. The maintenance of a constant core temperature of about 37°C by those birds incubating indicates that they have a slightly higher metabolic rate than birds that failed their incubation. However, in both incubating and non-incubating birds, most of the energy conservation can be explained by a depression in the metabolism which is associated with the reduction of cold-exposed body surfaces. This depression explains why the birds neither suffer from hyperthermia despite the very high ambient temperatures within the huddle, nor become hypothermic due to the harsh cold.
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