Read the following passage and mark the letter A, B, C, or D to indicate the answer to each of the question.Hibernation is typically linked to seasonal changes that limit food supplies. It is identified by metabolic suppression, a drop in body temperature and torpor- a sleep- like state- interspersed with brief bouts of wakefulness. Though certain species of fish, amphibians, birds and reptiles are known to lie dormant during cold winter months, hibernation is generally associated with mammals, ...

Read the following passage and mark the letter A, B, C, or D to indicate the answer to each of the question.

Hibernation is typically linked to seasonal changes that limit food supplies. It is identified by metabolic suppression, a drop in body temperature and torpor- a sleep- like state- interspersed with brief bouts of wakefulness. Though certain species of fish, amphibians, birds and reptiles are known to lie dormant during cold winter months, hibernation is generally associated with mammals, according to Don Wilson, a curator emeritus of vertebrate zoology at the Smithsonian National Museum of Natural History.

Endothermic mammals- “warm- blooded” animals that generate body heat internally- need a constant energy source to keep their engines running, Wilson told Live Science. And when that energy source becomes difficult to find, hibernation can help them weather harsh conditions.

“During times of the year when that energy source is missing- especially in northern climates- one coping mechanism is to just shut down,” he said. “They’ll feed heavily during the few months when food is plentiful and build up fat, then go to sleep and live off their fat reserves”.

A special type of fat called “brown fat” accumulates in hibernating mammals, Wilson said. Bats that hibernate develop brown fat on their backs between their shoulder blades, but mammals can also store brown fat in their bellies and elsewhere in their bodies, Wilson said.

Brown fat goes a long way because the hibernating animal draws on it very slowly, reducing their metabolism to as little as 2 percent of their normal rate, according to a 2007 study published in the Journal of Neurochemistry.

Their core body temperature is also greatly reduced. It generally hovers close to the air temperature in the animal’s den but can sometimes fall as low as 27 degrees Fahrenheit (minus 3 degrees Celcius) in Arctic ground squirrels, according to Kelly Drew, a neurochemist and professor with the Institute of Artic Biology at the University of Alaska Fairbanks.

Arctic ground squirrels’ bouts or torpor last about two or three weeks, Drew told Live Science, and the animals rouse “ pretty consistently” for about 12 to 24 hours, before resuming their winter sleep. They repeat this process for up to eight months.

But even though Arctic squirrels maintain a lower body temperature than any other hibernating mammal, the changes in their bodies overall aren’t that different from those that occur in other hibernating mammals, Drew said.

“The quality of mammalian hibernation is similar from bears to hamsters to ground squirrels,” Drew said. “The distinguishing feature is how cold they get”.

What does the word “their” in paragraph 4 refer to _______?