There are two major strategies available to mammals to survive winter: hibernation, where large animals drop their metabolic rate, and non-hibernation, where small mammals utilise fat stores to maintain their body temperature. According to Stuart Egginton from the University of Birmingham, textbooks state that the liver is the main organ of heat generation, ‘but this is based purely on the relative mass,’ he explains. Egginton realised that the liver could only be a thermogenic organ if it generated more heat then expected for its size. Egginton, David Hauton and Andrew Coney decided to test the liver's lipid oxidation profile to find out whether the liver really does keep non-hibernators warm during winter.
Focusing on rats' body temperatures and fat metabolism as the animals were cooled, the team treated some of the rats with fenofibrate (which increases fat oxidation by the liver and could maintain the rat's body temperature during cooling if liver is the source of winter warmth) and others with dichloroacetate (which inhibits the liver's ability to oxidise fats and would force the animals to rely on other forms of heat generation to maintain their temperature) before cooling the rodents. Then they compared how the fenofibrate and dichloroacetate treated animals performed with a third group of rats that had been prepared for the cold conditions by cold acclimation.
If fat metabolism by the liver was key to keeping warm, the fenofibrate treated rats should survive cooling as well as the animals that were previously acclimated to cold conditions. However, the cooled rats that had been treated with dichloroacetate would not survive cooling in good condition, as their livers cannot metabolise fats and they must rely on other, limited, energy supplies (glucose) to maintain their temperature. As the fenofibrate treated rats did not maintain their temperature as well as the well-prepared cold acclimated rats, the researchers suggested that the liver is not the critical step to heat generation.
Some other fat related tissue must be responsible for the rodents' ability to defend their body temperature. Brown adipose tissue (BAT) is a fat storage tissue especially abundant in small mammals and newborn humans. BAT is highly vascularised, full of mitochondria and burns fat to produce heat in a special way. Maybe it could provide the warmth the rodents require to survive winter in addition to its supposed role in arousal?
The team found that the BAT of cold acclimated rats took up fatty acids that were oxidised to generate heat. Amazingly, these rats were up to 12 times better at the conversion than the other rats. Additionally, while the other rats slowed their ventilation, the cold acclimated rats increased their breathing rate to better supply BAT with oxygenated blood and hence maintain their temperature while being cooled.
The authors decided that BAT is the true ‘thermogenic machinery’ for non-hibernators, and that the liver may contribute very little to thermogenesis. Scientists think BAT fat metabolism that non-hibernators use to stay warm and remain alert during cold conditions may have been one key to the evolutionary success of early mammals.
