Why don't different species have sex more often?

Finally, once hybrids are born, they tend not to make it to adulthood as often as rats from the two parent species, keeping their numbers down. The problem seems to be that, as in-betweeners, there’s really no place for them to go in either species’ territory. The researchers think that they may not be aggressive enough to compete with purebred Bryant’s woodrat males for dens and territory in the hills, and aren’t able to acclimate to desert life in the valley. This could lead them to wander off in search of greener pastures, leaving them out of range of females to mate with and exposing them to predators.

Because the rats live at such a sharp ecological divide and there’s little transitional habitat for the hybrids to call home, it looks like the story of these woodrats will continue to be a tale of two species. It might not stay like that for long, though. As the climate changes, physical and ecological barriers are altered or disappear, and species change or expand their ranges. This can bring once-isolated species into contact, with the potential for mating and hybridization.

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