The secret love lives of mice

Male mice fall into two camps when it comes to love: some fiercely guard the females within their territory, while others roam in search of quick flings

To the point

• Territorials defend nests: Some males set up camp and guard areas where females gather.
• Roamers seek out females: Other males wander around, trying to mate with any available female.
• Many switch strategies: Males may change between territorial and roaming tactics based on their age, strength, and social dynamics.
• Heavy males hold ground—until they can’t: Larger males are better at defending territories, but may start roaming if competition increases or females are hard to find.

In a comprehensive study at the Max Planck Institute for Evolutionary Biology, led by Fragkiskos Darmis, with Anja Guenther and Alexandros Vezyrakis, researchers tracked 244 wild-derived male house mice—yes, the kind you’d find in barns or basements—over their entire lives, up to eleven months, in real-world-style enclosures. They discovered that while these mating strategies 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 behaviours don’t need to be hardwired in 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 behaviour 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 behaviours are, and suggest that males might respond differently as environments change, offering new insight into how animals adapt and evolve.