The Elements of Evolution (48-3) Woodrats


Desert and Bryant’s woodrats are closely related and live in neighboring biomes. They can interbreed, but seldom do.

In the Mojave, desert woodrats live in the valley amid desert scrub. Up the hill is the sierra vegetation where Bryant’s woodrats reside (shown).

Making their homes and foraging in distinct habitats offers limited mating opportunities between the 2 woodrats. Another factor limits interbreeding: personality conflicts.

Desert woodrat females are intimidated by the large, aggressive Bryant’s woodrat males. But Bryant’s females, which are larger than their desert cousins, find either woodrat species appealing. The more docile desert woodrat males are pleasant enough.

For logistical and preference reasons, few hybrid mating encounters occur. Those that do create disadvantaged offspring. Only 10% of hybrid pups make it to their 1st year of adulthood, compared to 22% of desert rats and 33% of Bryant’s rats.

Being less aggressive, hybrid males do not compete well for denning sites, and falter in territorial foraging battles. This pushes them out to live in less promising places.