Sex–biased maternal investment in voles: importance of environmental conditions

Abstract

Adaptive bias in sex allocation is traditionally proposed to be related to the condition of mothers as well as to the unequal fitness values of produced sexes. A positive relationship between mother condition and investment into male offspring is often predicted. This relationship was also recently found to depend on environmental conditions. We studied these causalities experimentally using a design where winter food supply was manipulated in eight outdoor-enclosed populations of field voles Microtus agrestis. At the beginning of the breeding season in spring, food-supplemented mothers seemed to be in a similar condition, measured as body mass, head width, body condition index and parasite load (blood parasite Trypanosoma), to non-supplemented mothers. Food supplements affected neither the litter size, the reproductive effort of mothers, nor the litter sex ratios at birth. However, food supplementation significantly increased the birth size of male offspring and improved their condition, as indicated by reduced parasite loads (intestinal Eimeria). Interestingly, mothers in good body condition produced larger male offspring only when environmental conditions were improved by food supplements. Although the adaptiveness of variation in mammalian sex ratios is still questionable, our study indicates that mothers in good condition bias their investment towards male offspring, but only when environmental conditions are favourable.

Litter size, sex ratio, and age structure of gray wolves, Canis lupus, in relation to population fluctuations in northern Belarus

“…The very early age of pups reported in our study (<1 month) suggests that sex-biased maternal investment takes place at the stage of conception, mortality in utero, and (or) during the first few weeks following birth. Sex-biased maternal investment has not been studied so far in canids, although it was investigated in ungulates (review in Hewison and Gaillard 1999), primates (e.g., Brown 2001;Ostner et al 2005), and rodents (e.g., Koskela et al 2004).…”

Maternal effects and evolution at ecological time‐scales

“…For example, the beneficial effects of large egg size on offspring body size could be more pronounced in female offspring when it influences their future fecundity (Falconer 1965) or in male offspring when it influences their attractiveness or competitive ability (Trivers & Willard 1973). In this vein, female field voles Microtus agrestis produce larger males when environmental conditions are good, whereas no such pattern is observed for female offspring (Koskela et al . 2004).…”

Local environmental factors affect reproductive investment in female edible dormice

“…This concurs with the results of Kager and Fietz (2009), who found that litter size in edible dormice increased with the degree of seed mast and in response to continuous supplemental feeding. For some small mammal species food supplementation results in no increase in litter size (Hubbs and Boonstra 1997;Koskela et al 2004;O'Donoghue and Krebs 1992) or only a slightly larger litter size (Karels et al 2000). Provided that reproduction is limited by food availability, this indicates that in these species the reproductive value of the offspring does not vary much between years, probably due to a relatively constant food supply and thus constant survival probability for the young.…”

“…Food availability is one of the most important factors affecting reproduction (Hubbs and Boonstra 1997;Karels et al 2000;Koskela et al 1998Koskela et al , 2004. Food resources are typically not distributed uniformly, but vary on a spatial and temporal scale.…”