No evidence for a trade-off between reproductive investment and immunity in a rodent

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.

Figure 1. Changes of body mass (a) and dry matter intake (DMI) (b) over time.

RMR (c) on day 13 of lactation in lactating voles. Values are means ± s.e.m. N represents non-lactating group, and L₀ represents lactating group. Significant difference between groups is indicated by an asterisk if P<0.05.

Figure 2.The. The effects of lactation on serum total IgG (a), PHA response (b), serum anti-KLH IgM (c) and anti-KLH IgG (d) in N and L₀ group.

Values are means ± s.e.m. Significant difference between groups is indicated by an asterisk if P<0.05.

Figure 3. Changes of litter mass (a) during lactation.

L represents large litter size group, and S represents small litter size group. Changes of litter mass (b) after manipulation during lactation. E represents enlarged litter size group, C represents non-manipulated litter size group, and R represents reduced litter size group. Changes of dry matter intake (DMI) (c) over time and RMR (d) on day 13 of lactation. Values are means ± s.e.m. Significant differences in litter mass between L and S group or litter mass and food intake between E, C and R are indicated by an asterisk if P<0.05. A pound sign indicates significant differences in DMI between E and R group, and a double asterisk indicates significant differences between E and R group, and between C and R group if P<0.05, whereas significant differences in RMR between E, C and R are indicated by different letters if P<0.05.

Figure 4. The effect of litter size on serum total IgG (a), PHA response (b), serum anti-KLH IgM (c) and serum anti-KLH IgG (d) in L and S group.

Values are means ± s.e.m. Significant difference between groups is indicated by an asterisk if P<0.05.

Figure 5. The effect of litter size manipulation on serum total IgG (a), PHA response (b), serum anti-KLH IgM(c) and serum anti-KLH IgG (d) in E, C and R group.

Values are means ± s.e.m. Significant difference among groups is indicated by an asterisk if P<0.05.