lactation at hot temperature: a test of heat dissipation limitation in mice divergently selected for BMR

The HDL (Heat Dissipation Limitation) hypothesis posits that mammalian energy budgets (SuSMR, Sustained Metabolic Rate) are limited by the ability to dissipate metabolic heat. The HDL hypothesis has often been tested in lactating mice but rarely in females, systematically differing in SuSMR. Here, we used lines of laboratory mice divergently selected for Basal Metabolic Rate (BMR) and effectively co-selected for SuSMR. We exposed lactating females to 23°C and 30°C and manipulated their heat dissipation abilities by fur shaving. Exposure to 30°C did not affect the high BMR mice' litter mass but increased litter mass in the low BMR mice. Fur shaving did not affect litter mass. However, it decreased body temperature (Tb) by 0.2°C in the shaved mice, independent of line affiliation and ambient temperature. In both lines exposed to 30°C, the Tb increased by 0.2°C, while Daily Energy Expenditures (DEEs - a proxy of SuSMR) decreased by 20% and still was higher in the high BMR mice. These results do not support the HDL hypothesis. Low SuSMR individuals may benefit from higher ambient temperatures because of reduced costs of thermoregulation. It may change the course of natural selection towards reducing SuSMR and BMR.

Methods
Dataset was collected in January 2018 for the 53rd generation of laboratory mice (Mus musculus) artificially selected towards either high and low basal metabolic rate (respectively H-BMB and L-BMR lines; details in Książek et al. 2004) at the Faculty of Biology of the University of Bialystok. We conducted one experiment, assessing the response of lactating mice to exposure to an ambient temperature of 23°C and 30°C and fur shaving as a heat dissipation manipulation procedure. Before the proper experiment, 25 randomly chosen females from both line types were implanted intraperitoneally with loggers (type DST nano-T, size 6 mm x 17 mm, weight 1.3 g, Star Oddi Logging Life Science, Iceland) to control body temperature (Tb) every 10 minutes throughout lactation (± 0.01°C; records between 9.00 and 10.30 a.m. were deleted from analyses because of disturbances caused by human activity). Prior to the implantation of the loggers, females were anesthetized with xylazine (40 mg of xylazine per kilogram of body mass) and ketamine (100 mg of ketamine per kilogram of body mass). Before the pregnancy, we controlled females' body mass and body temperature. Then, females raised at 23°C were mated with non-sibling males from the same line type, and when pregnancy was detected by the increase in body mass, males were removed from the cages. The experiment began on the parturition day when mother mice with their natural litters were randomly exposed to an ambient temperature of 23°C or 30°C. On the 6th and 10th days of lactation, mother mice from both line types, lactating at both ambient temperatures, were dorsally shaved with an electric clipper (Remington). On the 12th day of lactation we controlled litter mass (as a proxy of parental effort), litter size (number of pups per litter) and female body mass. Between the 12th and 14th day of lactation, we assessed daily energy expenditures (DEEs), metabolizable energy intake (MEI) and milk energy output (MEO) by doubly labelled water method (DLW). Analyses were carried out by Doubly-Labelled Water Laboratory at the University of Aberdeen. The experiment was terminated on the 14th day of lactation. We used 57 females of laboratory mice (originating from a separate family): 30 from the L-BMR line type (6 with fur and 6 shaved at 23°C and 8 with fur and 10 shaved at 30°C) and 27 from the H-BMR line type (7 with fur and 7 shaved at 23°C and 6 with fur and 7 shaved at 30°C).

Differences in litter mass, litter size, maternal Tb, maternal body mass, DEEs, MEI and MEO at the peak of lactation were tested by three-way ANOVA/ANCOVA, in which a line type affiliation (L-BMR or H-BMR), an ambient temperature (23°C or 30°C) and manipulation (shaved or unshaved) were the main fixed effects. Litter mass on the 12th day of lactation was applied as a covariate in the case of Tb and DEE analysis. Significance was tested at P = 0.05. Statistical analyses were carried out using the STATISTICA version 13.3 package.