Limits to sustained energy intake. XIX. A test of the heat dissipation limitation hypothesis in Mongolian gerbils ( Meriones unguiculatus )

We evaluated factors limiting lactating Mongolian gerbils ( Meriones unguiculatus ) at three temperatures (10, 21 and 30°C). Energy intake and daily energy expenditure (DEE) increased with decreased ambient temperature. At peak lactation (day 14 of lactation), energy intake increased from 148.7±5.7 kJ day−1 at 30°C to 213.1±8.2 kJ day−1 at 21°C and 248.7±12.3 kJ day−1 at 10°C. DEE increased from 105.1±4.0 kJ day−1 at 30°C to 134.7±5.6 kJ day−1 at 21°C and 179.5±8.4 kJ day−1 at 10°C on days 14–16 of lactation. With nearly identical mean litter sizes, lactating gerbils at 30°C exported 32.0 kJ day−1 less energy as milk at peak lactation than those allocated to 10 or 21°C, with no difference between the latter groups. On day 14 of lactation, the litter masses at 10 and 30°C were 12.2 and 9.3 g lower than those at 21°C, respectively. Lactating gerbils had higher thermal conductance of the fur and lower UCP-1 levels in brown adipose tissue than non-reproductive gerbils, independent of ambient temperature, suggesting that they were attempting to avoid heat stress. Thermal conductance of the fur was positively related to circulating prolactin levels. We implanted non-reproductive gerbils with mini-osmotic pumps that delivered either prolactin or saline. Prolactin did not influence thermal conductance of the fur, but did reduce physical activity and UCP-1 levels in brown adipose tissue. Transferring lactating gerbils from warm to hot conditions resulted in reduced milk production, consistent with the heat dissipation limit theory, but transferring them from warm to cold conditions did not elevate milk production, consistent with the peripheral limitation hypothesis, and placed constraints on pup growth. * DEE : daily energy expenditure DEI : digestible energy intake DLW : doubly labelled water GEI : gross energy intake HDL : heat dissipation limitation iBAT : interscapular brown adipose tissue MEI : metabolizable energy intake MEO : milk energy output RMR : resting metabolic rate SusEI : maximum rate of sustained energy intake UCP1 : uncoupling protein 1