Endangered Rats and Mice: Unexpected Results

This juvenile San Bernardino kangaroo rat juvenile has been weighed, ear tagged, and sexed.

In October, we went out to check on how our relocated San Bernardino kangaroo rats and Los Angeles pocket mice were doing in the San Jacinto River Basin. Since our population survey in August, we provided them with extra food three times a week for three months to help them settle into their new residence and hopefully stay in the same place. We spent a week trapping at this site and expanded our grid beyond the artificial burrows we original put them in. The habitat is continuous, and we found new burrows and digging in all directions, so it is hard to know where they have moved. Ultimately, we did find fewer individuals of both species in October than in August, but there are several possible reasons for this.

I have often wondered why I agreed to study these nocturnal species. At night it is very cold, and it’s hard to find your way in the dark. You work from 5 p.m. to 5 a.m. with no sleep, which is very inconsiderate of our little critters! We had to have a park ranger escort at this site as well. However, audiobooks like the Hunger Games or Sookie Stackhouse series makes the down time pass quickly, even if I can never get used to napping in a truck. What I have realized is the cuteness of these animals never wanes, and the fulfillment of helping an endangered species is sizable. I am always happy to see them, even if they aren’t happy to see me. Yes, pocket mice are small, but they can bite! We often refer to them as angry jellybeans.

This red diamond rattlesnake thought the Sherman trap made a good hiding place!

October trapping turned out to be quite interesting with many unexpected results. I ended up with a three-foot-long red diamond rattlesnake in one of the traps. I couldn’t believe he could even squeeze himself full into it. I was lucky that I noticed how heavy the trap was and didn’t stick my hand in it! This year has had very heavy predation, and we have seen rattlesnakes, coyote scat, barn owls, and even seen some wild dogs at the site. An experiment with pocket mice and kangaroo rats showed that when barn owls are present, they often shift their habitat to bush-covered areas rather than the open habitats due to predation risk. This may mean that our kangaroo rats have moved off into the bushes, which is not where we would expect to find them or normally survey.

Most of the kangaroo rats and pocket mice we trapped were also caught during our August survey. The lower numbers in October may be due to natural dispersal, as this reserve is large and has the right habitat for both species. Originally we released many juveniles, which typically move away from their home territory when they become adults. One experiment with kangaroo rats found that juvenile males disperse earlier when they receive extra food, because they gain more weight and grow faster at a younger age. A kangaroo rat study found that males and females typically disperse around 98 feet from their birthplace. However, they have been known to disperse over longer distances of up to 1,312 feet to find a mate or better habitat.
In addition, it has been shown that pregnant kangaroo rats or females with young tend to stay in their territory. This may explain why most of the females we caught in August and October were pregnant or nursing. We recaptured an adult female that had had pups while in captivity, was nursing in August, and was again pregnant in October. It’s amazing that she has already produced possibly three litters of pups this year! So it is not surprising that a female with that many litters would rather stay in an area she knows rather than take the costly risk of searching for a new territory. When we check on the population next year, we will have to search for burrows and survey a much larger area.

We also caught a handful of new Los Angeles pocket mice, which may be locals already at the site or the grown offspring of the pregnant females we trapped in August. They grow very fast and are almost grown at 22 days old. In addition, we trapped some new San Bernardino kangaroo rat juveniles. The fact that we are finding youngsters and reproductive adults is hopeful and suggests that this habitat is suitable for their long-term success.

A Los Angeles pocket mouse is released from a trap.

In October, we caught a dramatic increase of San Diego pocket mice and common deer mice. In August, we caught 5 San Diego pocket mice, and in October, 40 had moved into this area. In addition, the number of deer mice captures increased from 2 to 30. Therefore, competition between these species may have resulted in San Bernardino kangaroo rats and Los Angeles pocket mice moving away from the site. They may have been attracted by the supplemental food, or it may suggest that these areas are more ideal for San Diego pocket mice and deer mice, as they often live in slightly different microhabitats.

We will continue to follow the survival success and population growth of the San Bernardino kangaroo rats and Los Angeles pocket mice for a year at the San Jacinto River Basin site to grow our knowledge of kangaroo rat communication and the relationship between the two species in the wild. Furthermore, we have learned many practical things about pocket mice translocation, which can be used in the future.

Christine Slocomb is a senior research technician at the San Diego Zoo Institute for Conservation Research. Read her previous post, No Night-lights for Kangaroo Rats.

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