HDL: Heat Dissipation Limitation or Hot Hamsters during lactation?

In response to high metabolic requirements such as during periods of cold exposure, high long-term physical activity or throughout lactation animals raise their metabolic rate whilst maintaining their body weight via increased food intake and energy assimilation. However, in all of these situations, there are limits to energy turnover, as heat production, physical activity and milk production cannot be raised indefinitely, even in the presence of unlimited food. Thus, animals may not only be limited by food availability in the environment, but also by metabolic ceilings, so-called maximum sustained metabolic rates (SusMRs,), also referred to as the "physiological limit". Preponderant evidence in the last decade has revealed that females may be limited by the capacity to dissipate excess heat, the heat dissipation limitation (HDL) hypothesis. According to the HDL hypothesis, the extent of the heat that is produced as a by-product of both metabolism and milk production is constraining the amount of nutrients a female can assimilate. Our proposed model, the Syrian golden hamster (Mesocricetus auratus) has a large litter size, altricial young and produces large quantities of milk during the 3- week lactation period up until the pups are weaned. Thus we propose to specifically test the hypothesis that HDL limits lactating female Syrian golden hamsters. We will conduct 3 separate experiments over a time course of 3 years. Firstly, we will alter ambient temperature across 5, 22 and 30C to investigate the effect it has on hamsters during lactation with regard to food intake and milk production. This first experiment will shed light on sustained metabolic rate in this newly established model and enable us to efficiently test the HDL hypothesis. Secondly, we will dorsally shave a group of 15 hamsters during day 4 of lactation to ascertain whether this relieves females from the limits of HDL. Lastly, we will test whether the possible building up of a body fat "capital" will affect the physiological limit while allowing them to raise three consecutive litters. The repeatability of sustained metabolic rate will thus also be tested along with the "seasonal investment hypothesis" via HDL. This newly suggested study aims to introduce a new, interesting and suitable model system into the largely "mouse-biased" science field while applying the same accuracy and experimental possibilities as in the MF1 mouse model whilst finally elucidating the extent of the HDL hypothesis and its application in laboratory and livestock animals with regard to thermal living conditions.

In response to high metabolic requirements such as during periods of cold exposure, high long-term physical activity or throughout lactation animals raise their metabolic rate whilst maintaining their body weight via increased food intake and energy assimilation. However, in all of these situations, there are limits to energy turnover, as heat production, physical activity and milk production cannot be raised indefinitely, even in the presence of unlimited food. Thus, metabolic ceilings may limit animals, also referred to as physiological limits. According to the heat dissipation limitation hypothesis, this limit is imposed by heat that is produced as a by-product of both metabolism and milk production. Our single funded project focused on studying lactating golden hamsters (Mesocricetus auratus) to elucidate their physiological limitation. Golden hamsters produce large litters of 3- 16 naked and blind young and transfer large quantities of milk to them during the 19 days lactation period. By breeding golden hamsters at three different ambient temperatures (22, 8, 30C), we observed that subcutaneous body temperatures in lactating females were significantly higher than in non-reproductive controls (F1, 866 = 128.74; p<0.0001, Ohrnberger et al., 2018b). Impressively, we observed that at hot ambient temperatures lactation comes to a standstill with very low survival of young (Ohrnberger et al., 2016b). Comparing milk energy output, food intake and energy assimilation at three ambient temperatures, we found they were highest at cold, lowest at hot and intermediate at normal ambient conditions (Ohrnberger et al. 2018b.). Due to this finding, we dorsally shaved the fur off in the mothers to test if lactation performance could be further improved by this measure (Ohrnberger et al. in prep.) We observed that female stress was significantly decreased in the shaved females due to the facilitated heat loss (F1,22 = 8.69, p = 0.0075, Ohrnberger et al. 2018a). In summary, our results suggest that increased metabolic heat production during lactation lower energy intake and milk production in golden hamsters, similarly as observed in other heat- limited rodent models such as laboratory mice, voles and Mongolian gerbils.