The Bruce effect revisited: is pregnancy termination in female rodents an adaptation to ensure breeding success after male turnover in low densities?

Abstract

than year-born and multiparous females. Taken together, our results suggest that the Bruce effect may be an adaptive breeding strategy for rodent females in cyclic populations specifically at low densities in the increase phase, when isolated, overwintered animals associate in MF pairs. During population lows infanticide risk and inbreeding risk may then be higher than during population highs, while also the fitness value of a litter in an increasing population is higher. Therefore, the Bruce effect may be adaptive for females during annual population lows in the increase phases, even at the costs of delaying reproduction.

Male‐mediated prenatal loss: Functions and mechanisms

“…24 Finally, the Bruce effect in bank voles (Myodes glareolus) occurs more frequently in circumstances that may be associated with greater infanticide risk. 78 Together, these results support the hypothesis that females may express the Bruce effect in a facultative, context-dependent manner. The details of this context-dependence need to be more fully explored within a comparative framework.…”

“…In meadow voles, pregnancy termination following exposure to nonsire males is less likely in highly aggressive than in less aggressive females, and is more likely after exposure to highly aggressive than to less aggressive males . Finally, the Bruce effect in bank voles ( Myodes glareolus ) occurs more frequently in circumstances that may be associated with greater infanticide risk . Together, these results support the hypothesis that females may express the Bruce effect in a facultative, context‐dependent manner.…”

The scent of infanticide risk? Behavioural allocation to current and future reproduction in response to mating opportunity and familiarity with intruder

“…Gustafsson et al (1980, 1983) assumed lactational anoestrus in a laboratory colony of caged bank vole pairs, since only 2% of females ( n  = 640) conceived during lactation. Meanwhile, in semi-wild experimental bank vole populations in large outdoor enclosures, we observed that 40% of non-gravid, lactating females (Eccard and Ylönen 2003, n  = 39 females) and 9 out of 10 of non-gravid, lactating females (re-analysing data reported in Eccard et al 2017) were conceiving within days after being release to enclosures, while still accompanied by their several days-old litter.…”

“…3, Table 2) revealed a general response to the male scents independent of familiarity, which may indicate a general interest in mating and future reproduction among the females. After missing the post-partum oestrus, female bank voles seem to be able to conceive during lactation (re-analysed from Eccard and Ylönen (2003) and Eccard et al (2017), but see Gustafsson et al 1980, 1983) and interaction with male scents and males may trigger a behavioural oestrus, during which females were reported to actively visit males for mating (Klemme et al 2011). In the control treatment, we found a negative correlation between litter size and the total time spent at the scent area, as well as a negative correlation with the longest visit at the scent area (Fig.…”

“…Several counterstrategies among females against male infanticide have been identified (summarised in Lukas and Huchard 2014), including promiscuity to confuse paternity, directly attacking potential perpetrators (Ylönen and Horne 2002), avoiding infanticidal individuals, aggression, and territoriality (Elwood and Kennedy 1990; Palanza et al 1994; Agrell et al 1998), as well as early termination of pregnancy to reduce the potential damage (Bruce effect, Bruce 1959; Eccard et al 2017). Counterstrategies against infanticide come with a cost for the female, however, as these increase the risk of injury and energetic investments (Agrell et al 1998; Ylönen and Horne 2002).…”