Maternal investment in relation to sex ratio and offspring number in a small mammal – a case for Trivers and Willard theory?

Abstract

Optimal parental sex allocation depends on the balance between the costs of investing into sons vs. daughters and the benefits calculated as fitness returns. The outcome of this equation varies with the life history of the species, as well as the state of the individual and the quality of the environment.We studied maternal allocation and subsequent fecundity costs of bank voles, Myodes glareolus, by manipulating both the postnatal sex ratio (all-male/all-female litters) and the quality of rearing environment (through manipulation of litter size by −2/+2 pups) of their offspring in a laboratory setting.We found that mothers clearly biased their allocation to female rather than male offspring regardless of their own body condition. Male pups had a significantly lower growth rate than female pups, so that at weaning, males from enlarged litters were the smallest. Mothers produced more milk for female litters and also defended them more intensively than male offspring.The results agree with the predictions based on the bank vole life history: there will be selection for greater investment in daughters rather than sons, as a larger size seems to be more influencial for female reproductive success in this species. Our finding could be a general rule in highly polygynous, but weakly dimorphic small mammals where females are territorial.The results disagree with the narrow sense Trivers & Willard hypothesis, which states that in polygynous mammals that show higher variation in male than in female reproductive success, high-quality mothers are expected to invest more in sons than in daughters.

A trade-off between current and future sex allocation revealed by maternal energy budget in a small mammal

“…We have previously demonstrated that maternal allocation in terms of offspring defence and milk production in the bank vole is increased for enlarged compared with reduced litters, as well as for daughters compared with sons [22]. Our current results show that maternal energy budget, i.e.…”

“…Significant fecundity costs of raising enlarged litters in (semi)natural conditions have been previously observed in the bank vole [23,31,53]. However, in ad libitum food conditions, delays in pregnancy have been the only clear fecundity costs found for mothers raising concurrent litters [22,54]. Mice that are pregnant while lactating are usually not found to compromise suckling pups in favour of the gestating litter (e.g.…”

“…Details of the manipulation of litter size and sex ratio were described in our previous study [22]. In short, after parturition, newborn pups were immediately sexed, individually marked with toe codes and manipulations carried out.…”

“…To explore the potential effect of the maternal energy budget on future offspring, we looked at the number of pups, their sex [22] and body mass (electronic scale, +0.01 g) at birth in second litters. In six out of 44 litters, we lacked complete information on sex and mass of the pups and thus sample size in some analyses is smaller.…”

“…In six out of 44 litters, we lacked complete information on sex and mass of the pups and thus sample size in some analyses is smaller. To assess potential conflict between maternal allocation into the two generations of the offspring, allocation into the simultaneously gestating litter was studied in relation to the mean mass of pups weaned in the manipulated litters (data from [22]). …”