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Endangered Pacific pocket mice, native to Southern California, were once thought to be extinct until a tiny remnant population was rediscovered in the mid-1990s.

San Diego Zoo Wildlife Alliance established a conservation breeding and reintroduction program to save the species from extinction. Though there has been significant success with breeding and reintroduction, the species is still at risk of losing genetic diversity, which reduces its survival and reproduction.

In a study in Science, San Diego Zoo Wildlife Alliance researchers demonstrate how genetic rescue can be used as an effective strategy for the conservation of this species. This strategy includes introducing Pacific pocket mice from genetically distinct populations for breeding purposes to boost genetic diversity and, in turn, the health of the population.

The research findings provide a contrast to commonly held perceptions about the risks of outbreeding depression that currently limit the use of genetic rescue in conservation programs.

"When species are restricted to small, isolated populations, genetic erosion can lead to poor health. Our study examined the trade-offs between genetic erosion and outbreeding depression in Pacific pocket mice, and we find that the benefits of genetic rescue outweigh the risks of keeping these populations in isolation," said lead author, Aryn Wilder, San Diego Zoo Wildlife Alliance Conservation Genetics researcher.

The researchers suggest shifting the focus from maintaining the genetic uniqueness of populations to maximizing the genetic health of the species. They also highlight the important role that zoos and managed care facilities play in preventing species extinction.

"Erosion in diversity seen in wild populations was reversed when we mixed mice from different populations," said Wilder.

"The genetically healthier population had higher survival and reproductive success. Although different numbers of chromosomes carried by the mice from different populations increase the risk of incompatibilities in the mixed breeding program, the non-mixed mice have even lower fitness, indicating a greater risk of extinction if the populations remain isolated."

San Diego Zoo Wildlife Alliance researchers have been studying the factors, including genomic variation, that maximize the health of populations and ensure the successful production of the fittest offspring for release into the wild. San Diego Zoo Wildlife Alliance brought in 49 mice from the wild and produced more than 700 mice at its facility, including 94 births last year.

In 2024, San Diego Zoo Wildlife Alliance and partners also began releasing Pacific pocket mice to a second site, resulting in 100 pups born in the wild at that site.

With two-thirds of Earth's species undergoing population decline, the implications of this research extend beyond Pacific pocket mice and demonstrate the value of genetic rescue for species facing extinction. It also highlights how zoos can serve a vital role in conservation of at-risk species.