Abstract
Life history theory assumes there are trade-offs between competing functions such as reproduction and immunity. Although well studied in birds, studies of the trade-offs between reproduction and immunity in small mammals are scarce. Here we examined whether reduced immunity is a consequence of reproductive effort in lactating Brandt's voles (Lasiopodomys brandtii). Specifically, we tested the effects of lactation on immune function (Experiment I). The results showed that food intake and resting metabolic rate (RMR) were higher in lactating voles (6≤ litter size ≤8) than that in non-reproductive voles. Contrary to our expectation, lactating voles also had higher levels of serum total Immunoglobulin G (IgG) and anti-keyhole limpet hemocyanin (KLH) IgG and no change in phytohemagglutinin (PHA) response and anti-KLH Immunoglobulin M (IgM) compared with non-reproductive voles, suggesting improved rather than reduced immune function. To further test the effect of differences in reproductive investment on immunity, we compared the responses between natural large (n≥8) and small litter size (n≤6) (Experiment II) and manipulated large (11–13) and small litter size (2–3) (Experiment III). During peak lactation, acquired immunity (PHA response, anti-KLH IgG and anti-KLH IgM) was not significantly different between voles raising large or small litters in both experiments, despite the measured difference in reproductive investment (greater litter size, litter mass, RMR and food intake in the voles raising larger litters). Total IgG was higher in voles with natural large litter size than those with natural small litter size, but decreased in the enlarged litter size group compared with control and reduced group. Our results showed that immune function is not suppressed to compensate the high energy demands during lactation in Brandt's voles and contrasting the situation in birds, is unlikely to be an important aspect mediating the trade-off between reproduction and survival.
Highlights
Reproduction and self-maintenance are important for fitness and both require substantial energy investment [1,2,3,4,5,6]
resting metabolic rate (RMR) in lactating voles was significantly increased by about 100% compared with non-lactating voles (F1, 10 = 13.147, P = 0.05; Fig. 1c) on day 13
In contrast to our a priori predictions almost all the immune components that we measured were not suppressed by increased reproductive investment
Summary
Reproduction and self-maintenance are important for fitness and both require substantial energy investment [1,2,3,4,5,6]. Prolactin is required for the ongoing maintenance of milk secretion [15] and the regulation of hyperphagia and metabolic process during lactation [16]. These two hormones may play an important signal driving counterbalance between reproduction and immune function [4]. The high cost of lactation requires that energy intake must increase, or that the allocation of energy to other functions reduces [19]. Sustained energy intake during late lactation might be limited intrinsically by aspects of an animal’s physiology [9,11,12,20]; other physiological functions would be down-regulated
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