It’s been a long time coming.  We committed to contributing our efforts to save the endangered Pacific Pocket Mouse (PPM) 10 years ago.  Last week, we reintroduced our first group of 50 mice back into the wild, at Laguna Coast Wilderness Park, creating a fourth population of the species and taking a major step forward towards recovery.

What we knew 10 years ago was that this tiny species of mouse was threatened with extinction because they’re preferred habitat was the same as ours, the Southern California coast, no more than 3-5 miles from the ocean.  There were only 4 populations remaining, 3 on Camp Pendleton and 1 at the Dana Point headlands.  To facilitate recovery, the agencies had prioritized population creation so that a single catastrophic event wouldn’t wipe them out.  Enter, San Diego Zoo Institute for Conservation Research. 

First of all, why do we care?  Why would the world famous San Diego Zoo lend their expertise to save the Pacific pocket mouse?  It’s a mouse, right?  We at San Diego Zoo Global are dedicated to saving species across the globe, including those here in our own backyard and we have many thriving local conservation programs. Personally, I would save them because I love nature and appreciate its intrinsic value and well, because it’s the right thing to do. But for those needing some convincing, I would say, the Pacific pocket mouse is not your typical house mouse that you find under kitchen cupboard. 

In fact, they are no more closely related to those mice than humans are to monkeys. They are the smallest mouse in North America, live right here in Coastal Southern California and are found nowhere else in the world. They are awe inspiring in their own right. They are adapted to our dry environment and can survive without ever having to drink any water. To do this, they get their water needs met almost exclusively from what their bodies can extract from seeds, their kidneys are specialized for water retention, and even their cheek pouches, which they use to collect and transport seeds into the burrows, are located outside of their mouth to prevent water loss. All of these adaptations will serve them well through our drought. They have excellent low frequency hearing and can even detect the silent sound of an owl approaching.  And, frankly, they are cute.  

“Cuteness” aside, they play a critical role the function of the Coastal Sage Scrub ecosystem in which they live. They are primary seed dispersers, prey for many species and their digging activities increase soil hydrology and nutrient cycling. Essentially, they support a functioning Coastal California Sage Scrub ecosystem which is one of 35 global biodiversity hotspots, areas that contain the highest species richness across the globe.

Our early field research indicated that we couldn’t create a new population through translocation, wild to wild transfer of mice, because the source populations were too small and removal of 50 mice would put them in jeopardy. And, one of the 4 remaining populations has not been detected in recent years, leading us to conclude that we are down to 3.  So, after years of serious consideration, planning, permitting, financing and building, the PPM Conservation Breeding/Reintroduction Program was born. We brought 30 mice into captivity from 2012-2014. Through dedication of our amazing staff who became nocturnal for 6 months of the year to make sure that we caught females in peak reproductive condition, careful observation, research, and care, we grew the captive population from 30 to 110 by the end of 2015. A major feat. 

At the end of the reproductive season last year, we decided that it was time to prepare for reintroduction. In the ensuing months, we planned and prepared, considering everything we could to facilitate a smooth transition back into the wild. We ran genetic models to select the ideal release group and picked a group of alternate mice in case some of the selected mice couldn’t be released. We developed a diet transition plan to ensure that the mice slated for release would be familiar with native seeds.  We conducted predator aversion training to inculcate recognition and avoidance of native predators. We stopped breeding females in the release cohort to ensure that they would be in top physical condition for release. And, we observed everyone.  If they were judged as too human friendly…willing to crawl into our hands, they didn’t get to go as they wouldn’t fare well out in nature. 

We chose the release site with our PPM partners back in the fall of 2014. Laguna Coast Wilderness Park is ideal for the Pacific Pocket Mice. It’s within the species historic range and it meets all of their habitat requirements. It’s truly amazing that there is still high quality Coastal Sage Scrub habitat within 3-5 miles from the ocean that hasn’t been developed. It’s been restored over the last decade after a century of cattle ranching and was preserved thanks to the Irvine Company and local desire for natural areas.  It’s expansive and gorgeous. 

In the weeks before the reintroduction, we prepared the release site for the mice. Besides predators, one of the biggest challenges when reintroducing a species is getting the released animals to stay where you put them. So, we installed a temporary perimeter fence to keep terrestrial predators out and pocket mice in. We also designed and installed soft-release cages to allow the mice to acclimate to their new home for a week before releasing them. These cages have been shown to be effective at dampening dispersal with closely related species. Our soft-release cages were biodegradable so that once transferred, the mice could use them as a starter burrow and nest chamber and establish new burrows as extensions of our man-made ones.  We even had special cardboard tubes made to use as temporary burrow entrances and lined the bottom of them with sand so that the mice could easily walk in and out of them without slipping J. 

We fed the mice during the week long acclimation period and observed them to make sure that they were faring well. Together with our PPM partners on this project, we released the mice on June 13.  Over the rest of the summer we’ll slowly transition the mice to a fully wild life and check in to see how they are doing through live-trapping and remote cameras on site.  We reached a major milestone in the PPM program; the first group of mice is back in the wild. 

This is a historic moment for a local species, but it’s also a personal one. I have loved small creatures my whole life and dedicated my career to conserving them. I am deeply touched and inspired that San Diego Zoo Global has embraced this mouse. I am also amazed by the steadfast commitment of our federal, state and local partners to conserve this species. 

It took a village.  We did it everyone!

Now it’s up to the mice…