Could This Tiny Mouse’s Genes Actually Save It from Extinction?

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Organisms have been evolving to adapt to changing environmental conditions ever since life on Earth began. Those species that can adapt fast enough will survive; those that cannot will die out. Survival is that simple and that brutal. Against a backdrop of the looming demise of many animal species thanks to climate change, habitat loss, and human influence, one little creature’s genes are shifting! Could the Pacific pocket mouse be evolving so fast that it will be one of nature’s survivors?

Meet the Pacific Coastal Mouse

Pacific pocket mice ( Perognathus longimembris pacificus) are a subspecies of little pocket mice and live in the dry grasslands and coastal sage scrub of the southwestern United States. They are one of the smallest rodents in North America and can range from 4.3 to 6 inches in length from their nose to the tip of their tail. Their coat is brown of varying shades on the top and lighter underneath. These little guys are nocturnal and hide in burrows during the day. At night, they venture out to feed on seeds, which they store in external, fur-lined cheek pouches.

The Plight of the Pocket Mouse

Of all the little pocket mouse subspecies, this is the most coastal. At one time, they could be found along the Southern California coast from Los Angeles to the Mexico border in San Diego County. Now, however, only three isolated populations remain. One is at Dana Point in Orange County, managed in collaboration with the Center for Natural Lands Management. A further two populations are on Marine Corps Base Camp Pendleton, managed by the U.S. Department of Defense, referred to as South San Mateo and Santa Margarita.

Pacific pocket mice require a specific habitat with fine-grained, sandy soils found on coastal strands, dunes, river alluvium, and sage scrub within 2.5 miles of the ocean. To survive, they also need abundant forb cover (herbaceous flowering plants) and moderate to high amounts of open ground. Dense thatch, high shrub cover, and non-native grasses make survival difficult for them.

Sadly, extensive coastal urban development has taken over much of this type of habitat in the mouse’s historic range. Habitat loss significantly reduced the Pacific pocket mouse population, and by the 1970s, they had apparently disappeared altogether. A population was rediscovered in the early 1990s, leading to the species being federally listed as endangered under the US Endangered Species Act.

How Genes Can Help or Hinder Animals on the Verge of Extinction

When a species is reduced to just a few isolated populations, the choice of mate is limited. Therefore, the animals start to inbreed, and you end up with a lot of animals that are genetically very similar . This is a problem because adaptation to environmental changes relies on genetic diversity. You need a few ‘different’ individuals that can cope with the new situation to thrive and breed so that the species as a whole can become able to deal with the change. If all individuals in the population are genetically the same, there are no ‘different’ individuals to drive this change.

Experts feared that this was the case for the Pacific pocket mice. They were concerned that inbreeding within populations isolated by habitat fragmentation and urban development had resulted in what is called genetic homozygosity. This means lots of mice that are genetically the same. Without the ability to adapt to environmental changes caused by climate change, the mouse’s future was looking bleak.

Studying Pacific Pocket Mouse Genes

A recent study has assessed the genetic variation and adaptive potential of the Pacific pocket mouse. Researchers analyzed both historical and contemporary genomic sequences. In particular, they looked for climate-associated genes such as the ability to cope with minimum and maximum temperatures, vapor pressure deficits, and dew point temperature. These genes influence the mouse’s ability to cope with changes in environmental temperature and moisture.

In these mice, scientists identified 14 genes associated with temperature and moisture resilience. These likely control biological processes in the circulatory system, such as the muscle control of vasodilation and vasoconstriction (widening and narrowing blood vessels near the skin’s surface) and heart rate regulation to control heat loss.

What Is Encouraging About This Endangered Mouse’s Genes?

As anticipated, this study showed that the Pacific pocket mouse had lost a lot of genetic variation over the last one hundred years. Importantly, however, some critical climate-associated variation remains. When scientists reintroduced mice from a conservation breeding program back into the wild and monitored their offspring over multiple generations (they have one or two litters a year), they discovered a very encouraging finding.

The reintroduced mice underwent genetic changes, shifting toward the gene frequencies predicted for the climate of their new habitat. There was enough genetic variation to create an adaptive potential that could cope with environmental changes, specifically temperature and moisture. It is exciting to find such adaptive variation in an endangered species and even more encouraging to see it responding to different climatic conditions in the space of just a few years.

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