But not rats
Rats, however, do not seem to display the same degree of reduction in body temperature in response to dietary restriction, although there is evidence that some live longer when in a warm environment, a situation which puts a lesser demand on their need to generate heat. For example, Weindruch and Walford tell us of rats living their entire lives in a controlled environment where the temperature was 34°C (93.2°F). Some of these had body temperatures which were higher on average than others. Those with the lower body temperature, in this hot environment, lived an average of 100 days longer than those with high body temperatures, with no dietary modification being involved. The same phenomenon was not seen when the body temperature of rats who lived in a cooler environment (28°C/82.4°F) was measured and compared with their life spans. At this lower temperature the core temperature of animals did not seem to correlate with their length of life.

This points to a life extension advantage (for rats) in having a lower body temperature (slower metabolic rate, more thrifty energy production system), but only when local environmental temperatures are on the high side.

Another interpretation
Weindruch and Walford point out that when mice are kept in an environmental temperature of 33°C (91.4°F) they automatically reduce food intake by about a third. Is this self-chosen dietary restriction the reason for their life extension, and not the temperature reduction? And if this effect is seen in mice but not rats, what implications does it have for humans? In the book The Biology of external Starvation Volume 2(University of Minneapolis Press, 1950) Dr E. Keys describes the drop in human body temperature when enforced calorie restriction is experienced as in the Irish potato famine and (in some cases) during the Second World War. Obviously, in such circumstances undernutrition is accompanied by malnutrition, a quite different situation from that during controlled feeding of fewer calories as part of a nutritious diet. However, we do know that humans respond to calorie restriction and fasting by lowering their core temperatures.

What about external temperature influences?
John Mann (Secrets of Lik Extension. Harbor, San Francisco, 1980) discusses environmental influences on body temperature. He tells us that as far back as 1917 researchers at the Rockefeller Institute lengthened the lives of fruit flies by maintaining their environmental temperature at 6°C (42.8°F) below normal. This technique worked in fruit flies because they are cold-blooded and their environmental temperature could directly reduce their core temperatures. Many subsequent experiments involving coldblooded creatures have replicated these results.

Mammals, however, would react to a cold environment by increasing their core temperature rather than dropping it (and of course this would equal elevated metabolic rate which equals greater free radical activity - not what we are aiming for at all.)

Where cold-blooded organisms (such as tiny sea creatures called rotifers) were both calorie restricted as well as having their temperatures reduced, even greater life extension was achieved: from a normal 18 days to over 50 days. It is worth emphasizing that such a response is almost certainly linked to a reduction in free radical activity brought about by lowered metabolic activity. This is confirmed by evidence from a study performed in Canada at Concordia University, Montreal (Experimental Biology (1980) 15:335-8) which showed that when rotifers (Rotifer philodina) had vitamin E (a powerful antioxidant) added to their culture medium there was a significant increase in both their life span and their breeding potential.