Endangered Mice Nestle into New Homes

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Female #14 fills her cheek pouches with seed.

It’s early morning, and I have just finished feeding the pocket mice various seeds, spinach, and a twig of buckwheat. I watch as the mice collect seeds in their small cheek pouches (fur-lined cavities next to both sides of their mouth) before burying them somewhere in their enclosure. Often the mice cache the seed in their burrow where they sleep. They then begin moving sand and nesting material to get their sleeping chambers ready. One female has made an elaborate nest that is probably 10 times bigger than her body. As she peeks out and looks at me one last time before going to sleep, I think about how much we will learn from this species in the coming months, all of which will be crucial to future reintroductions to the wild.

The Pacific pocket mouse (PPM) is critically endangered, and only three populations remain in San Diego County. This tiny creature is important to the coastal sage scrub community because it disperses native plant seeds and is a prey item for various species including coyotes, owls, and snakes. PPMs have been historically found within 2.5 miles of the southern California coast, so they have, unfortunately, lost much of their habitat to human development.

We worked through the night to collect Pacific pocket mice for the conservation breeding program.

This summer, we spent several weeks collecting 22 PPMs for the new Pacific Pocket Mouse Conservation Breeding Facility at the San Diego Zoo Safari Park. It was quite an undertaking! Since PPMs are nocturnal, we opened traps at sunset, checked traps twice per night, and finished at sunrise. We strategically collected animals from different locations to maximize the genetic diversity of our breeding population. We also wanted an equal number of males and females to contribute to the gene pool. Often while we were checking traps, we would not only be crossing our fingers in hopes that we caught a PPM, but more specifically, we wanted a male!

Each morning, after closing traps, we drove our precious cargo to the Safari Park where veterinarian Jack Allen would be waiting to examine the mice. Like all other animals in the collection at the Safari Park, the PPMs were quarantined at the hospital to ensure individuals were not exhibiting signs of disease. (See post, Pocket Mice Arrive)

Individuals vary in how they build their nests. Some mice fill the PVC burrow we provide with bedding, while others (like female #19) custom build a nest outside of the PVC tubes.

Today, all of the mice are out of quarantine and are setting up their enclosures to their liking and nesting into their new homes. With the help of San Diego Zoo Global veterinarians, geneticists, reproductive physiologists, nutritionists, population biologists, and mammal keepers, we hope to create a successful breeding program for this subspecies so we can ultimately reintroduce animals to the wild and move toward recovery of the species.

In the fall, pocket mice begin hibernating for the winter and are no longer reproductive. We will begin breeding the pocket mice next spring, in step with their natural rhythm. Interestingly, even if we do not induce hibernation by lowering the temperature or removing food (which we will not do), the mice will go through approximately 24 hours of torpidity (short-term hibernation). In fact, we have already seen individuals drop into torpor.

There is a lot of work ahead and major goals to accomplish, including experiments to examine mate choice, anti-predator behavior, and foraging behavior. But for now, I will relish in the quiet moments with this rare, fascinating mammal.

Maryke Swartz is a senior research technician with the San Diego Zoo Institute for Conservation Research. Read her previous post, Protecting Kangaroo Rats.