Anyone who has awakened in a cold bed, with all the blankets wrapped around the sleeper next to them, will marvel at hibernating animals able slumber away the winter in icy conditions.
How do they do it? Researchers studying hibernating rodents have found two types that have evolved cold-sensing nerve cells with a diminished ability to detect temperatures below 20 degrees Celsius, which could allow their body temperature to drop for long periods of time without causing them to be stressed by the harsh conditions, thus triggering their seasonal slumber.
Scientists at Yale University in the United States found that the Thirteen-Lined ground squirrel (Spermophilus tridecemlineatus) and the Syrian hamster (Mesocricetus auratus), two common rodents that hibernate in winter, do not feel cold in the same way as non-hibernators, such as rats or mice.
Thirteen-lined ground squirrels, named for their brown and white stripes, are common in much of North America. Syrian hamsters, also known as Golden hamsters, are kept as pets in many parts of the world – but in Australia are classified as exotic rodents and not allowed into the country as a potential threat to the local ecosystem.
All rodents sense cold, but the researchers found that the somatosensory receptors – the nerve cells responsible for awareness of touch, pressure, pain, temperature, position, movement, and vibration – in these two hibernating species take a lot more to be activated compared to those in animals that do not hibernate.
In a study published this week in the journal Cell Reports, the Yale researchers, led by physiologists and neuroscientists Elena Gracheva and Sviatoslav Bagriantsev, found that the squirrels and hamsters evolved similar cold-suppressing adaptations independently.
“If these animals were to feel cold, they would not be able to hibernate because their sensory system would tell the rest of the body that they need to warm up first,” says Gracheva, the report’s senior author. “They would not be able to survive as a species.”
During hibernation, animals experience low body temperature, slow breathing and heart rate, and decreased metabolic activity.
Animals such as rodents are “deep” hibernators, but the term is often used to include animals such as bears, which have periods of torpor and metabolic suppression rather than absolute body temperature decline.
To compare the biology of animals that hibernate and those that do not, the researchers performed tests on ground squirrels, hamsters, and mice. They put the rodents on two temperature-controlled plates: one warm (30 degrees Celsius) and another that changed temperature from 20 to 0 degrees. The rodents could move between the two plates.
The researchers observed that mice always strongly preferred the warm plate. Ground squirrels and hamsters, on the other hand, did not show significant preference for the warm one, unless the cold plate approached five degrees.
Gracheva and Bagriantsev connected this behavior to a protein called TRPM8. The activation of TRPM8 leads to the sensation of feeling cold. In ground squirrels and hamsters, it is less sensitive to cold than in mice. In mice, there was an increase in activity when the temperature decreased from 30 to 10, but in the ground squirrels and hamsters, there was no change even at temperatures below 20.
The researchers analysed the differences in the amino acid sequence of the TRPM8 molecules and reverse-engineered squirrel TRPM8 so that it became sensitive to cold.
This resistance to feeling cold occurs when ground squirrels and hamsters are hibernating, and also while they are active. For example, Thirteen-lined ground squirrels can survive exposure to temperatures of two degrees for up to nine months.
“This adaptation is the perfect example of how environment can shape properties of the sensory system,” Gracheva says.
The Yale scientists are now studying how these animals behave in even lower temperatures. The next step is to study rodents at temperatures below 10 degrees Celsius. They also want to expand their study of the molecular basis for cold sensitivity.
“This process is very complex, and TRPM8 is just one part of the mechanism,” Gracheva says.