In this episode, we dive into a fascinating question in animal behavior:Why do males and females in some species move away from home at different rates?
A study by ecologist Tadao Hirota looks at this question using the cabbage white butterfly (Pieris rapae crucivora)—a very common butterfly in Japan. In this species, females tend to leave their birthplace and spread out to new areas, while males mostly stay put. The study uses a window bird feeder and computer simulations to understand why this sex difference evolved.
The research highlights two key factors:
1. When females mate before leaving, everything changes.
According to the article, female butterflies often mate in their home habitat before dispersing (page 1). This means they can lay eggs both at home and in new locations. This “spread-out” strategy helps protect their offspring if the environment suddenly changes or their home habitat becomes unsuitable.
2. Unpredictable environments make female dispersal even more important.
The study shows that in fluctuating environments, places where habitat quality changes unpredictably, females benefit even more from spreading their eggs across multiple areas (page 2). This acts as a kind of risk-spreading or “bet-hedging” strategy, increasing the chances that at least some offspring survive.
Meanwhile, males don’t gain the same benefit from dispersing. Since females carry and deposit the eggs, males can spread their genes effectively without leaving home. In the models, males often evolved to stay in their natal habitat, especially when dispersal was risky or costly (page 4).
What does this all mean?
Hirota’s results show that female-biased dispersal can evolve simply because females mate before dispersing and live in unstable environments. This combination pushes females to explore and males to stay home, no complicated mating system required.
This work not only explains butterfly behavior but also sheds light on why sex-biased dispersal is so common across animals, from insects to birds to mammals.
