Thermal environment affects hair coat development of Norway rats

No insulating effect of obesity

The development of obesity may be aggravated if obesity itself insulates against heat loss and thus diminishes the amount of food burnt for body temperature control. This would be particularly important under normal laboratory conditions where mice experience a chronic cold stress (at ≈20°C). We used Scholander plots (energy expenditure plotted against ambient temperature) to examine the insulation (thermal conductance) of mice, defined as the inverse of the slope of the Scholander curve at subthermoneutral temperatures. We verified the method by demonstrating that shaved mice possessed only half the insulation of nonshaved mice. We examined a series of obesity models [mice fed high-fat diets and kept at different temperatures, classical diet-induced obese mice, ob/ob mice, and obesity-prone (C57BL/6) vs. obesity-resistant (129S) mice]. We found that neither acclimation temperature nor any kind or degree of obesity affected the thermal insulation of the mice when analyzed at the whole mouse level or as energy expenditure per lean weight. Calculation per body weight erroneously implied increased insulation in obese mice. We conclude that, in contrast to what would be expected, obesity of any kind does not increase thermal insulation in mice, and therefore, it does not in itself aggravate the development of obesity. It may be discussed as to what degree of effect excess adipose tissue has on insulation in humans and especially whether significant metabolic effects are associated with insulation in humans.

Thermal environment affects morphological and behavioral development of Rattus norvegicus

Norway rats are widely distributed throughout diverse climates from the tropics to the arctic. As a result, the environmental air temperature experienced during postnatal life varies widely and should influence the development of body morphology and thermal preference. To quantify this relation, rats were housed from birth to adulthood in cool (17 degrees C), moderate (25 degrees C), and warm (33 degrees C) environments. Body morphology measures were recorded weekly and thermal preferences were assessed every 3 weeks. Compared to moderate-housed rats, cool-housed rats developed shorter ears, shorter tails, and a lower tail-length to body-length ratio. Warm-housed rats developed less mass, shorter bodies, and a higher tail-length to body-length ratio than moderate-housed rats. By postnatal day 42, cool-housed rats established and then maintained a preference for warmer air temperatures and warm-housed rats established and then maintained a preference for cooler air temperatures. These results quantify morphological and behavioral developmental plasticity by the Norway rat in response to the thermal environment.