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Some male mice fiercely guard females within their territories, while others seek out quick flings

In a comprehensive study conducted at the Max Planck Institute for Evolutionary Biology, researchers tracked 244 wild-derived male house mice—yes, the kind you'd find in barns or basements—over their entire lives, up to 11 months, in real-world-style enclosures.

The study is in the Journal of Animal Ecology and was led by Fragkiskos Darmis, with team members Anja Guenther and Alexandros Vezyrakis.

They discovered that while the mating strategies of the mice are flexible, they're also tied to consistent individual differences—what scientists call "personality traits." Some mice might just consistently roam and some might just defend nests.

Here's the twist: these behaviors don't need to be hardwired into DNA. Instead, they can emerge based on the male's circumstances—what evolutionary biologists call "making the best of a bad job." It's not that these males are born roamers or territorials—it's that they're adaptable.

Their behavior evolves in real-time, shaped by their body, their rivals, and their environment. When a male can't win the territory game, roaming becomes a smart backup plan.

Who wins?

Territorial males tend to reproduce more often—but at a cost. Defending the nest areas where females gather can be stressful and risky, increasing the chance of injury and likely elevating stress levels.

Roamers, on the other hand, may reproduce less frequently, but they still manage to pass on their genes—especially when competition is high. In the end, both strategies can lead to similar lifetime success.

The researchers also found that roaming males have relatively larger testes, suggesting they invest more in sperm production—perhaps to improve their chances in brief encounters. It's not just behavior that differs—physiology and energy investment do too.

The study shows that male house mice adopt different mating strategies based on their personality, body condition, and social environment. These strategies—defending a territory or roaming for mates—not only shape their chances of reproducing but also lead to different life paths. Some live longer, others take more risks.

The findings reveal how flexible and dynamic these behaviors are, and suggest that males might respond differently as environments change, offering new insight into how animals adapt and evolve.