Milk output and composition in mice divergently selected for BMR

Abstract

From an evolutionary perspective, the high basal metabolic rate (BMR) of homeotherms is hypothesised to be a by-product of natural selection for effective parental care. We estimated daily milk output during two consecutive lactation bouts in mice divergently selected for high/low BMR and applied a cross-fostered design to control for potential differences in the between-line suckling abilities of nursed juveniles. Additionally, to remedy the potential limitation imposed by the ability of mother mice to dissipate excess heat, we exposed them to an ambient temperature of 17°C during the most energetically demanding second week of lactation. We found that the mice selected for high BMR produced significantly more milk in a 24 h period in both reproductive bouts. The milk samples obtained from the high BMR females had lower protein concentration and did not differ with respect to fat. However, the concentration of the primary milk carbohydrate -lactose -was higher. Although all the above between-line differences were statistically significant, their magnitude was too small to unambiguously ascribe them as stemming from a positive genetic correlation between the physiological traits underlying BMR and lactation performance. Nevertheless, our study lends such support at least at the level of phenotypic variation.

Not that hot after all: no limits to heat dissipation in lactating mice selected for high or low BMR

“…In previous studies we found that a significant difference in parental effort between the two line types is most prominent when selected mice are lactating at an ambient temperature of 17°C, i.e., significantly lower than the housing temperature of 23°C. This difference was manifested both in higher pup growth rate (Sadowska et al, 2013) and higher milk production in the H-BMR mice (Sadowska et al, 2015a). Since higher rates of BMR are associated with high heat production, it is tempting to attribute higher reproductive output of the H-BMR type mice at low ambient temperatures to a release from the heat constraint.…”

“…with an intrinsically high basal metabolic rate (H-BMR). In our previous work on a unique model, mice from two line types, one selected for H-BMR and the other for low basal metabolic rate (L-BMR), we were able to demonstrate a significant positive correlation between parental effort (quantified as pup growth rate and milk output; Sadowska et al, 2013) with BMR (Sadowska et al, 2015a). The differences between the two line types were, however, most prominent when animals at peak lactation were exposed to a temperature lower than that of normal housing conditions (17°C versus 23°C).…”

“…For this study, 79 females (36 H-BMR and 43 L-BMR) that successfully reared one litter were paired outside their families for the second time and placed with males in plastic cages for a 2 week period. Second lactation has been shown to be more effective in terms of milk production; thus, we used animals in their second lactation for this study (Sadowska et al, 2015a). When pregnancy was detectable by an increase in body mass, the males were removed.…”

“…The remaining animals consisted of the control group. We chose the second week of lactation for fur removal as this is the more demanding period, approaching the peak of lactation at days 14-15, when pups are not only bigger but also end the period when they rely solely on milk for nutrition (Sadowska et al, 2013(Sadowska et al, , 2015a.…”

“…This would allow the mothers from both line types to: (1) achieve higher thermal conductance even at normal housing temperature (23°C), (2) avoid overheating while increasing food intake and milk production, as assumed by the HDL hypothesis, and (3) raise heavier pups compared with a control group of non-shaved mice (Król et al, 2007;Zhao, 2012;Sadowska et al, 2016). Conversely, if there is no HDL, the increased thermal conductance would impose an additional cost to the mothers, which should be especially noticeable in the L-BMR mice because of their lower capabilities to sustain a high parental effort (Sadowska et al, 2013(Sadowska et al, , 2015a.…”