ABSTRACT High temperatures and heatwaves are rapidly emerging as an important threat to many aspects of physiology and behavior in females during lactation. The body's capacity to dissipate heat is reduced by high ambient temperatures, increasing the risk of hyperthermia. Exposure to wind, a pervasive environmental factor for most terrestrial animals, is known to increase heat loss, but its effects on the reproductive performance of small mammals remains unclear. In the present study, the effects of wind on the energy budgets, resting metabolic rate and milk energy output (MEO) were measured in lactating Swiss mice at 21 and 32.5°C. Females kept at 32.5°C had a significantly lower resting metabolic rate, food intake and MEO, and lighter offspring, than those kept at 21°C. However, exposure to wind increased the asymptotic food intake of females kept at 32.5°C by 22.5% ( P <0.01), their MEO by 20.7% ( P <0.05) and their litter mass by 17.6% ( P <0.05). The body temperature of females kept at 32.5°C was significantly higher during lactation than that of females kept at 21°C, but this difference was reduced by exposure to wind. These findings suggest that exposure to wind considerably improves reproductive performance, increasing the fitness of small mammals while undergoing hot temperatures during heatwaves.

ABSTRACT The capacity to get rid of excess heat produced during hard work is a possible constraint on parental effort during reproduction [heat dissipation limit (HDL) theory]. We released hard-working blue tits ( Cyanistes caeruleus ) from this constraint by experimentally removing ventral plumage. We then assessed whether this changed their reproductive effort (feeding rate and nestling size) and levels of self-maintenance (change in body mass and innate immune function). Feather-clipped females reduced the number of feeding visits and increased levels of constitutive innate immunity compared with unclipped females but did not fledge smaller nestlings. Thus, they increased self-maintenance without compromising current reproductive output. In contrast, feather clipping did not affect the number of feeding visits or innate immune function in males, despite increased heat loss rate. Our results show that analyses of physiological parameters, such as constitutive innate immune function, can be important when trying to understand sources of variation in investment in self-maintenance versus reproductive effort and that risk of overheating can influence innate immune function during reproduction.

ABSTRACT Heat dissipation has been suggested as a limit to sustained metabolic effort, e.g. during lactation, when overheating is a possible risk. We tested this hypothesis using mice artificially selected for either high or low BMR that also differed with respect to parental effort. We used fixed size cross-fostered families and recorded litter mass daily until the 14th day of lactation. Midway through the experiment (day 8) half of the mothers from each group had fur from the dorsal body surface removed to increase their thermal conductance and facilitate heat dissipation. Our results showed that neither high nor low BMR mouse lines benefited from increasing their thermal conductance at peak lactation. On the contrary, growth of the litters reared by the low BMR females was compromised. Thus, our results do not support the heat dissipation limitation hypothesis.

ABSTRACT Little is known about the phylogenetic variation of avian evaporative cooling efficiency and heat tolerance in hot environments. We quantified thermoregulatory responses to high air temperature ( T a ) in ∼100-g representatives of three orders, namely, the African cuckoo ( Cuculus gularis , Cuculiformes), lilac-breasted roller ( Coracias caudatus , Coraciiformes) and Burchell's starling ( Lamprotornis australis , Passeriformes). All three species initiated respiratory mechanisms to increase evaporative heat dissipation when body temperature ( T b ) approached 41.5°C in response to increasing T a , with gular flutter observed in cuckoos and panting in rollers and starlings. Resting metabolic rate and evaporative water loss increased by quantitatively similar magnitudes in all three species, although maximum rates of evaporative water loss were proportionately lower in starlings. Evaporative cooling efficiency [defined as the ratio of evaporative heat loss (EHL) to metabolic heat production (MHP)] generally remained below 2.0 in cuckoos and starlings, but reached a maximum of ∼3.5 in rollers. The high value for rollers reveals a very efficient evaporative cooling mechanism, and is similar to EHL/MHP maxima for similarly sized columbids which very effectively dissipate heat via cutaneous evaporation. This unexpected phylogenetic variation among the orders tested in the physiological mechanisms of heat dissipation is an important step toward determining the evolution of heat tolerance traits in desert birds.

SUMMARY The heat dissipation limit theory suggests that heat generated during metabolism limits energy intake and, thus, reproductive output. Experiments in laboratory strains of mice and rats, and also domestic livestock generally support this theory. Selection for many generations in the laboratory and in livestock has increased litter size or productivity in these animals. To test the wider validity of the heat dissipation limit theory, we studied common voles ( Microtus arvalis ), which have small litter sizes by comparison with mice and rats, and regular addition of wild-caught individuals of this species to our laboratory colony ensures a natural genetic background. A crossover design of ambient temperatures (21 and 30°C) during pregnancy and lactation was used. High ambient temperature during lactation decreased milk production, slowing pup growth. The effect on pup growth was amplified when ambient temperature was also high during pregnancy. Shaving fur off dams at 30°C resulted in faster growth of pups; however, no significant increase in food intake and or milk production was detected. With increasing litter size (natural and enlarged), asymptotic food intake during lactation levelled off in the largest litters at both 21 and 30°C. Interestingly, the effects of lactation temperature on pup growth where also observed at smaller litter sizes. This suggests that vole dams trade-off costs associated with hyperthermia during lactation with the yield from investment in pup growth. Moreover, pup survival was higher at 30°C, despite lower growth, probably owing to thermoregulatory benefits. It remains to be seen how the balance is established between the negative effect of high ambient temperature on maternal milk production and pup growth (and/or future reproduction of the dam) and the positive effect of high temperatures on pup survival. This balance ultimately determines the effect of different ambient temperatures on reproductive success.

SUMMARY Seals have adapted to the high heat transfer coefficient in the aquatic environment by effective thermal insulation of the body core. While swimming and diving, excess metabolic heat is supposed to be dissipated mainly over the sparsely insulated body appendages, whereas the location of main heat sinks in hauled-out seals remains unclear. Here, we demonstrate thermal windows on the trunk of harbour seals, harp seals and a grey seal examined under various ambient temperatures using infrared thermography. Thermograms were analysed for location, size and development of thermal windows. Thermal windows were observed in all experimental sessions, shared some common characteristics in all seals and tended to reappear in similar body sites of individual seals. Nevertheless, the observed variations in order and location of appearance,number, size and shape of thermal windows would imply no special anatomical site for this avenue of heat loss. Based on our findings, we suggest that, in hauled-out seals, heat may be transported by blood flow to a small area of the wet body surface where the elevation of temperature facilitates evaporation of water trapped within the seals' pelages due to increased saturation vapour pressure. The comparatively large latent heat necessary for evaporation creates a temporary hot spot for heat dissipation.