Effects of thyroid hormones and cold acclimation on the energy metabolism of the striped hamster (Cricetulus barabensis).

To examine the effects of low ambient temperature and thyroid hormones on the energy metabolism of the striped hamster (Cricetulus barabensis), adult male striped hamsters were kept at 30 °C, or acclimated to 5 °C, for 4 weeks. During this time, hamsters were treated with a synthetic thyroxine, levothyroxine sodium (LTS), the antithyroid drug methimazole, or saline solution (control). Hamster's food intake, basal metabolic rate (BMR), non-shivering thermogenesis (NST), thyroid hormones, body fat content, mitochondrial state-4 respiration, cytochrome c oxidase, and uncoupling protein 1 (UCP1) gene expression in brown adipose tissue (BAT), were measured. Both acclimation to 5 °C and LTS increased serum levels of triiodothyronine, which was associated with increased food and energy intake and BMR. Interestingly, although acclimation to 5 °C also increased NST and UCP1 gene expression in BAT, and decreased body fat content, these changes were not induced by LTS treatment. Finally, exposure to 5 °C reduced the effects of LTS on energy intake and expenditure in specific metabolic markers and organs. Together, these data illustrate that ambient temperature and thyroid hormones can have both independent, and interactive, effects on the metabolic changes in striped hamsters induced by cold acclimation.

Sustained energy intake in lactating Swiss mice: a dual modulation process.

Limits to sustained energy intake (SusEI) during lactation are important because they provide an upper boundary below which females must trade off competing physiological activities. To date, SusEI is thought to be limited either by the capacity of the mammary glands to produce milk (the peripheral limitation hypothesis) or by a female's ability to dissipate body heat (the heat dissipation hypothesis). In the present study, we examined the effects of litter size and ambient temperature on a set of physiological, behavioral and morphological indicators of SusEI and reproductive performance in lactating Swiss mice. Our results indicate that energy input, energy output and mammary gland mass increased with litter size, whereas pup body mass and survival rate decreased. The body temperature increased significantly, while food intake (18 g day-1 at 21°C versus 10 g day-1 at 30°C), thermal conductance (lower by 20-27% at 30°C than 21°C), litter mass and milk energy output decreased significantly in the females raising a large litter size at 30°C compared with those at 21°C. Furthermore, an interaction between ambient temperature and litter size affected females' energy budget, imposing strong constraints on SusEI. Together, our data suggest that the limitation may be caused by both mammary glands and heat dissipation, i.e. peripheral limitation is dominant at room temperature, but heat dissipation is more significant at warm temperatures. Further, the level of the heat dissipation limits may be temperature dependent, shifting down with increasing temperature.

Interspecific variation of thermoregulation between small migratory and resident passerines in Wenzhou.

Physiological adaptation arises from several fundamental sources of phenotypic variation. Most analyses of metabolic adaptation in birds have focused on the basal metabolic rate (BMR), the lower limit of avian metabolic heat production. In this study, we investigated thermoregulation in three passerine species; the yellow-billed grosbeak Eophona migratoria, white-rumped munia Lonchura striata and black-throated bushtit Aegithalos concinnus, in Wenzhou, China. Metabolic rate was measured using the closed-circuit respirometer containing 3.5 L animal chambers. Body temperature (Tb) was measured during metabolic measurements using a lubricated thermocouple. The minimum thermal conductance of these species was calculated by measuring their Tb and metabolic rates. The yellow-billed grosbeak remained largely normothermic, and the white-rumped munia and black-throated bushtit exhibited variable Tb at ambient temperatures (Ta). Mean metabolic rates within thermal neutral zone were 2.48±0.09 O2(mL)/g/h for yellow-billed grosbeaks, 3.44±0.16 O2(mL)/g/h for white-rumped munias, and 3.55±0.20 O2(mL)/g/h for black-throated bushtits, respectively. Minimum thermal conductance of yellow-billed grosbeak, white-rumped munia and black-throated bushtit were 0.13±0.00, 0.36±0.01, and 0.37±0.01 O2(mL)/g/h/℃, respectively. The ecophysiological characteristics of these species were:(1) the yellowbilled grosbeak had relatively high Tb and BMR, a low lower critical temperature and thermal conductance, and a metabolic rate that was relatively insensitive to variation in Ta; all of which are typical of cold adapted species and explain its broader geographic distribution; (2) the white-rumped munia and blackthroated bushtit had high thermal conductance, lower critical temperature, and relatively low BMR, all which are adapted to warm environments where there is little selection pressure for metabolic thermogenesis. Taken together, these data illustrate small migratory and resident passerines that exhibit the different characteristics of thermoregulation.