Captive-born Released Pacific Pocket Mice are Reproducing!

Reintroducing endangered animals back to the wild is a difficult task, especially when they are a tiny prey species, and once in the wild they are viewed as snacks.  Endangered Pacific pocket mice (PPM) are no exception.  Over the last few years, we conquered breeding of this species in captivity, a feat to be sure, and were able to produce enough mice to reintroduce them for the first time ever in 2016.  But, for a successful reintroduction, our founders have to survive in the face of predators, competitors, drought, fire, extreme temperatures, and sometimes heavy rain.  And, if they do survive, they must build a home and reproduce so that wild mice are born, and the release population can grow and stabilize.

In May of this year, we released 25 captive born mice into our release site in Laguna Coast Wilderness Park. Initially we had only planned for one 2017 release, but our record breeding this year at the conservation breeding facility allowed us to release a second group of 24 mice in July.

After each release, we monitored the tiny mice through remote camera footage, and live trapping. The camera footage showed us that the mice were settling into their new home; they were digging burrows, foraging, and sandbathing (rubbing their scent on the ground to “talk” to each other). One month after the May release, we trapped to see how many of our founders survived, to assess their health, and to look for evidence that we had met the next milestone… reproduction. 

All mice that we recaptured were doing well; their fur was glossy, their weights were good, and they were feisty, and wary. We were thrilled to find that some females were pregnant, and others were lactating.  We know from our time with PPM in captivity that gestation for the Pacific pocket mouse is around 24 days, and after birth, the pups stay with their mother in underground burrows until they are weaned.  So, our finding lactating females meant that our founder mice were mating, and producing young.  But, a lot can happen in the wild during their first few weeks of life, and not all pups born survive to emergence.  We would have to wait and trap again later in the summer to see if we reached our next milestone, successful production of wild-born offspring in the wild.

When we went back out in August to find out how our mice were doing, we were ecstatic to find newly emergent wild born PPM!  In fact, our success far exceeded our expectations.  We caught 40 new wild born mice! This is the first time reproduction of captive born PPM in the wild has been confirmed. Besides being smaller, and a little awkward, you can tell young PPM from adults by their grey pup fur.  Their presence on the release site is evidence that we have officially entered the growth phase following release.

This month we cut small holes at the base of the fence at the release site to allow the PPM to expand their range, and other animals to move in to diversify the small mammal community.   We believe that taking a gradual approach to completing the reintroduction process will increase the likelihood of creating a self-sustainable viable wild population.  If all goes well overwinter, we may be able to remove the fence next spring.

PPM usually go underground in early October, and emerge ready for the next breeding season in early March; therefore, we will have to wait until next year to see how the release population is doing.  Reintroduction success takes time, but we are well on our way!  There are currently only three PPM populations left in the world, if we are successful, we will have established a fourth , moving this species a little farther from the brink of extinction.