This month's featured zoology books
at Amazon.com:

Turtles, Tortoises, and Terrapins: Survivors in Armor

Turtles & Tortoises

Turtles of the United States and Canada

Michael's Blog: A Day in the Life of a Zoologist

Desert Tortoise Conservation Research (posted April 21, 2006)

OK, enough about birds (for now). Although I've been doing a lot of bird projects lately, my first love is herpetology - specifically the study of turtles and tortoises. I've been studying endangered turtles and tortoises since 1991, when I began by assisting in the study of the state endangered yellow mud turtle in northeast Missouri. I've also worked with yellow mud turtles in Illinois, gopher tortoises in southern Mississippi, and most recently, with desert tortoises in the Mojave Desert in southern California. I just joined a research team to investigate desert tortoises last year, but the project has a 15-year history. Since 1991, hatchling tortoises have been raised in large predator-proof pens located in natural desert tortoise habitat. Over the years, female desert tortoises were captured from surrounding areas and released into the enclosures, allowed to lay eggs, and then released at their point of capture. The eggs developed and hatched in the enclosures, and the hatchlings were studied by teams of zoologists examining aspects of hatchling and juvenile ecology. This past year, the team I joined obtained funding to release the juveniles that had been raised in the enclosure over the years. Our team leader (a professor at UCLA), designed a research plan to examine whether releasing the juvenile tortoises to relocation sites outside of the pens could be used as a conservation strategy to replenish tortoise populations. Desert tortoise populations have declined significantly over the past 30 years, due to a variety of factors, including degradation and loss of habitat to development and disturbances (including off-road vehicle use and introduction of non-native invasive plants), introduced diseases (mostly Mycloplasma agassizii that causes Upper Respiratory Tract Disease), and increases in predator populations (mainly common ravens that prey heavily upon hatchling and juvenile tortoises). Conservation strategies need to be developed to help the tortoise populations recover. Our hope is that the research we're conducting will provide one method that can be used to help in the recovery of the populations.

OK, enough about the background of our research, now here are some specifics. There were about 60 juvenile tortoises living inside of the predator-proof pens when we started the research project in August 2005. In early September 2005, we captured 43 and separated them into treatment groups, including four size groups, two distance groups, and two release groups. The size groups will allow us to determine whether tortoises of different sizes are more or less vulnerable to predation. We know that juveniles are particularly succeptible to predation by ravens; perhaps we can determine if there's a size threshold beyond which the juveniles can escape predation by these birds. The distance groups included those released at 500 vs. 1000 meters from the predator-proof pens; the release groups included those released into open areas vs. those released into "halfway houses" - smaller pens located 500 meters away from the larger enclosures in which the juveniles were raised. These two treatments were used to examine whether we could overcome the tortoises' attempts to try to travel back to the pens in which they were raised. Most studies show that turtles have a "homing" ability - that is, when released outside of their home range, they try to get back to the area they know (which is why you should never relocate a turtle that you find crossing the road into a new area). It seems that turtles (and probably many other animals) have a mental "map" of their home range so that they can find resources and mates. Our thought was that if we used halfway house pens, the tortoises relocated within them would imprint to the area, and after a period of time, we could remove the pens and allow the tortoises to move about as they pleased. We wanted to compare their ability to "take" to the new area with those that were released into the open outside of the halfway houses. The behavior of these animals was also compared to those of the tortoises that were released at the long distance site, located 1000 meters from the pens. Perhaps relocating tortoises at greater distance would help in overcoming the "homing" behavior. We've been using radiotelemetry to track the locations of the animals on a weekly basis.

So since early September 2005, we've been tracking the movements of the juvenile tortoises. In early February, in the middle of winter, we removed the halfway houses while the tortoises were hibernating. As of this spring, we've seen a few of the larger juveniles move distances of up to a mile from their release sites, but interestingly, they've not moved towards the predator-proof pens. The smallest juveniles, which may be the most vulnerable to predation by ravens, have moved the smallest distances from their release points. We've been unable to determine whether the halfway houses resulted in different movement patterns, but preliminary data suggest there's no difference between the treatment groups. We've lost none of the juveniles to predation so far. We've got funding through the summer of 2007; we'll start work on publishing the data after the termination of the study. To be absolutely sure if relocating pen-raised juvenile tortoises would be an effective conservation tool to replenish desert tortoise populations, we'd have to track the progress of the animals until they reached maturity. But budgetary contraints probably won't allow us to carry the project on for that length of time (tortoises grow slowly, and take 15 years or so to mature). That's it for now, I'll give another update as the study progresses.

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