Hibernation occurs with exposure to low temperatures and, under normal conditions, occurs principally during winter seasons when there are lengthy periods of low environmental temperatures. A related form of dormancy is known as
estivation. Many animals estivate when they are exposed to prolonged periods of drought or during hot, dry summers. For all practical purposes, hibernation and estivation in animals are indistinguishable, except for the nature of the stimulus, which is either cold or an arid environment.
Many mammals and some birds spend at least part of the winter in hiding, but remain no more drowsy than in normal sleep. On the other hand, some mammals undergo a profound decrease in metabolic rate and physiological function during the winter, with a body temperature near 32°F (0°C). This condition, sometimes known as deep hibernation, is the only state in which the warm-blooded vertebrate, with its complex mechanisms for temperature control, abandons its warm-blooded state and chills to the temperature of the environment. Between the drowsy condition and deep hibernation are gradations about which little is known. The bear, skunk, raccoon, and badger are animals which become drowsy in winter. Although usually considered the typical hibernator, the bear's body temperature does not drop more than a few degrees.
The deep hibernators are confined to five orders of mammals: the marsupials, the Chiroptera or bats, the insectivores, the rodents,
and, probably, the primates. Most, if not all, of the insect-eating bats of temperate climates not only hibernate in the winter, but also drop their body temperature when they roost and sleep. The advantage of this for a small mammal with a disproportionately large heat-losing surface is obvious when conservation of energy is considered. Many rodents are deep hibernators, including ground squirrels, woodchucks, dormice, and hamster. The fat-tailed and mouse lemurs are primates that hibernate or estivate. Among birds, the poorwill (Phalaenoptilus) and some hummingbirds and swifts undergo a lowering of body temperature and metabolic rate in cold periods.
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Notice that the first discovery was only four years ago... this can be looked at in two ways:
- We don't know everything yet, and
- What other primates out there might hibernate or estivate
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Lemur Is First Known Hibernating Primate, Study SaysJames Owen in England
for National Geographic News
June 23, 2004Madagascar, a large, tropical island off the east coast of Africa, isn't the kind of place you'd expect to find a mammal that hibernates. Yet researchers say a small, nocturnal lemur resorts to the same tactic used by bears, squirrels, and dormice to survive cold northern winters—spending at least seven months of the year hibernating through harsh times brought on by drought.
The German-based team say their study is the only report of prolonged hibernation in a tropical mammal. Furthermore, writing in this week's scientific journal Nature, they claim the study provides the first proof of hibernation in any species of primate.
The study focused on the fat-tailed dwarf lemur (Cheirogaleus medius), one of a group of snout-faced primates for which Madagascar is famous. The island boasts some 32 lemur species, the highest percentage of indigenous primates anywhere on Earth.
The fat-tail dwarf lemur gorges on fruit, flowers, and insects during a short rainy season between December and February. As its body weight increases by around 40 percent, the lemur's tail swells massively with stored fat reserves. The animal, which usually weighs five ounces (150 grams), then goes into a torpor, marked by reduced activity and appetite.
Scientists previously noted that this cycle of behavior is most pronounced in western Madagascar, where the climate veers between extremes. Here the "fat-tail" was found to take to holes in tree trunks during prolonged periods of drought, when trees shed their leaves and food becomes scarce.
Led by scientists from Phillips University's animal physiology department in Marburg, Germany, the team revealed that the fat-tail's body temperature varied to an extent previously unknown in mammals. Daily fluctuations ranged almost 20 degrees Celsius (nearly 40 degrees Fahrenheit), with temperatures recorded from as low as 9.3 degrees Celsius (48.7 degrees Fahrenheit) to well over 30 degrees Celsius (86 degrees Fahrenheit).
Tree HolesBody temperatures were found to correlate closely with how well each tree hole was insulated. The researchers say this suggests the lemurs body temperatures are regulated by external conditions, as with lizards and other reptiles, instead of being maintained internally.
"They follow changes in environmental temperature as if they were a piece of stone," said Gerhard Heldmaier, from Phillips University. This, he says, is potentially of great benefit, as it helps fat-tails conserve their fat reserves.
For reasons not fully understood, most hibernating mammals usually go through periods of arousal, when they bring their body temperatures up to normal levels. "This costs them a lot of energy," said Heldmaier. He added that the lemurs studied experienced body temperatures as high as 35.9 degrees C (97 degrees F) and didnt need to go through arousal during hibernation.
The researchers say the condition of hibernation should no longer be seen as synonymous with low body temperatures, as is presently the case.
They also state: "To our knowledge, our findings are the first physiological confirmation of prolonged hibernation by a tropical mammal, as well as the first proof of hibernation in a primate."
Heldmaier added, "Hibernation has nothing to do with the cold. What animals are coping with is a seasonal scarcity of food."
He is confident that future studies will show that other primates also go into hibernation. He nominates the potto—a loris from western and central Africa—as an example.
Kirindy ForestAreas where fat-tailed dwarf lemurs resort to hibernation include the 25,000-acre (10,000-hectare) Kirindy Forest on the west-central coast of Madagascar. The focus of studies by the German Primate Center, based in Göttingen, Germany, the forest exhibits pronounced seasonal changes, with most of the trees dropping their leaves to conserve water during the long, dry season.
Other lemurs in the forest respond in similar ways to the fat-tail, according to Peter Kappeler, head of ethology and ecology at the center.
Writing in the American Museum of Natural History magazine, Natural History, Kappeler says female gray mouse lemurs may also hibernate for months, adding, "Like their fat-tailed cousins, [they] go through their complete reproductive cycle between late December and early February."
Meanwhile, Verreaux's sifakas— large, long-legged lemurs that leap high in the canopy—cope with the dry season through calorie-saving measures such as cutting distances traveled and sunbathing in the morning to bring body temperatures up.
Kappeler says the leaf-eating red-tailed sportive lemur also saves energy by reducing its activity level and metabolic rate.
He adds, "These primates offer scientists an opportunity to study how a group of related mammals respond to a range of environmental conditions—in this case, along the entire length of the world's fourth largest island."
However, scientists say they may not have long to make the most of this opportunity, because hunting and destruction of forests in this desperately poor country are putting the lemur's long-term future in jeopardy.
So we followed the tracks over to the edge and it became very clear what was going on. He was playing. He would run across the flat then leap onto the face and slide on down the mountain. He'd done this over and over. Must of been having a really good time. 
