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Polyandry is common: female house mice, and females from many other species, often mate with more than one male, producing litters with multiple fathers.
Context matters: the advantage of multiple paternity appears under standard, less nutritious food conditions—seen as larger litters.
Open question remains: females show this behaviour even when food is high quality, where the benefit is not evident.
Many animals do something that still surprises researchers: females often mate with more than one male. This behaviour—polyandry—has long raised a blunt question. Why divide offspring among multiple fathers, and does it help mothers or young survive?
New work at the Max Planck Institute for Evolutionary Biology addresses this puzzle using house mice. The study suggests that the value of multiple mating is not fixed, but shaped by the environment—especially by the quality of the food available to the mother.
To test this, the researchers established multiple experimental semi-natural enclosures, each housing hundreds of male and female mice in settings designed to closely resemble wild habitats. Every individual was tracked throughout its lifetime using tiny subcutaneous RFID tags. Food quality differed across enclosures: some mice received a high-quality diet, while others were provided with a standard, less nutritious diet. Over four years, the team monitored 255 litters produced by more than 100 females, recording mating events and the number of fathers contributing to each litter to examine reproductive strategies under different resource conditions.
The pattern was clear. Around one-third of litters had multiple fathers in both environments. But the measurable benefit—larger litter sizes—appeared only under the poorer, standard-quality food conditions. In high-quality environments, females produced large litters regardless of how many males fathered them.
Beyond mice, the findings underline a broader point: reproductive behaviour can be an answer to circumstance. When resources are of lower quality, females may mate with multiple males in a way that increases the odds that some offspring survive—an approach known as bet-hedging. When food is plentiful, such strategies may be less necessary, yet females nevertheless tend to show this behaviour. The question now is why.
Looking ahead, the results open a line of inquiry into how animals adjust mating behaviour in response to changes in food quality, population density, and other ecological pressures—insights that may help explain variation in mating systems, parenting strategies, and life histories across species facing rapidly changing environments.
Fragkiskos Darmis
Doctoral Researcher
Max Planck Institute for Evolutionary Biology
Research Group Behavioural Ecology of Individual Differences
Darmis, F., Guenther, A. Environmental quality shapes the fitness payoffs of multiple paternity. BMC Ecol Evo 25, 134 (2025). https://doi.org/10.1186/s12862-025-02478-5
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