Understanding the evolution of traits involves analyzing how they affect fitness. Challenging examples are alternative reproductive tactics, as observed in many invertebrates and vertebrates. A team of scientists from the University of Zurich and the University of Florida analyzed what influenced female house mice breeding tactics to either raise a litter solitarily or communally with one or several other females. They used a detailed 8-year data set on a population of several hundred individually known house mice, ranging free in a barn in Switzerland and followed from cradle to grave. Unexpectedly, communal breeding proved to decrease average fitness in the free-living study population. Nevertheless, the tactic increased over the years and with increasing population density. Interestingly, females in inferior condition (low body weight) were more likely to rear litters communally. Communal breeding therefore persists in the population as a ‘best-of-a-bad-job’ tactic – females of inferior condition and when exposed to high reproductive competition choose to rear litters communally to improve their probability to breed.
The team gained those insights by applying a holistic demographic approach that incorporates not only individual reproductive success, but also the females’ state and age dependent survival and breeding probabilities, as well as the transitions among alternative breeding states – with surprising results and novel insights. Manuela Ferrari, postdoc and leading author of the study, explains the approach: “We calculated season-specific litter sizes at weaning for solitary and communally breeding females. We further used multi-state capture-mark recapture models (MSCMR) to estimate survival probabilities and how likely females transitioned between different breeding states. We then constructed matrix population models with these parameters to assess the population-dynamic consequences of the choice of breeding solitarily or communally”. Since the study population grew substantially over the 8 years, the scientists were able to address whether selection on breeding tactics differed depending on population density. Communal breeding increased with density, but rather surprisingly the analyses revealed selection against communal breeding at higher densities. Furthermore, when simulating populations in which all females were either breeding solitarily or communally the team observed that a purely solitarily breeding tactic outcompeted a communally breeding one both at low and at high population densities. The females’ body weight as a measure of their condition and reproductive competitiveness helped to explain this unexpected result. When in inferior condition, and at increasing population densities, females were more likely to rear litters communally. The persistence of communal breeding in the population can thus be explained as a ‘best-of-a-bad job’ tactic, even though it reduces fitness and should be selected against.
The study elucidates an exciting case study of female breeding decisions that proved to be density- and condition-dependent. It reveals that alternative breeding tactics can mediate the costs of reproductive competition and result in growing populations even at high densities. Barbara König, senior author and head of the long-term study on house mice in Switzerland even speculates: “This novel insight on communal breeding in house mice might explain the long-standing ecological puzzle of population outbreaks as is frequently observed in Australian farms”.
Ferrari, M., Lindholm, A.K., Ozgul, A. et al. Cooperation by necessity: condition- and density-dependent reproductive tactics of female house mice. Commun Biol 5, 348 (2022). https://doi.org/10.1038/s42003-022-03267-2
Department of Evolutionary Biology
and Environmental Studies