The objectives of the project were to augment existing reptile translocation projects occurring on U.S. Department of Defense (DoD) installations using traditional translocation techniques with either soft release or environmental enrichment and to demonstrate the value of these technologies relative to traditional translocation techniques. For example, translocation programs that currently capture, move, and release animals from areas of high-risk to low-risk, provide an opportunity to incorporate soft release technologies. For translocation programs that maintain animals in captivity for prolonged periods, the researchers can implement environmental enrichment technologies. Their goals are to clearly define success criteria following the suggestions of Hall and Fleischman (2010) and to compare the success of soft release and environmental enrichment approaches with standard protocols.
The specific objectives were to demonstrate how soft release could improve the survival and decrease the post-release movements of Eastern Massasauga Rattlesnakes (Sistrurus catenatus) on Camp Grayling and Texas Horned Lizards (Phrynosoma cornutum) on Tinker Air Force Base. At each of these installations, animals are captured and moved from active training ranges or construction areas and hard released (i.e., direct, unrestrained release without spending time in an acclimation pen) into suitable habitat. Here, they augmented these efforts by adding a soft release component, which allowed them to compare the survival of soft- and hard-released individuals. Similarly, they calculated movement indices (home range size and daily movement rate) of soft- and hard-released individuals and compared them with the movements of control resident individuals using generalized linear models or Kruskal-Wallis tests. They predicted that soft-released animals will move less frequently and occupy smaller home ranges than hard-released animals and that these space use and movement parameters will be similar to those of resident animals.
They also demonstrated how environmental enrichment influenced the survival and growth of Eastern Box Turtles (Terrapene carolina) on Fort Custer. By rearing box turtles in complex and challenging enriched captive conditions compared to unenriched, simplistic captive conditions allowed them to assess how different rearing conditions affected the survival and behavior of translocated animals post-release. They compared the post-release survival of turtles using known-fates modeling. Additionally, they used general linear models to compare growth rates, temperatures, and dispersal of enriched and unenriched turtles post-release to assess the predictions that enriched captivity better prepares captive individuals to naturally forage and reduced the propensity to leave the release area, respectively.
Holding the animals in outdoor, naturalistic enclosures at the release site for some period of time before release allowed the individuals to become acclimated to the area and may have encouraged them to develop site fidelity to the area, which may have reduced the tendency to try to disperse and find their way back to their capture locations. Their goal in constructing soft release enclosures was to provide animals with complex, safe enclosures situated within suitable habitat that were also escape proof. Soft release enclosures were constructed at the release sites in suitable habitat known to support resident animals. Because of the different behaviors of the target species, the size and construction of the soft release pens varied between demonstrations. For instance, because Eastern Massasauga Rattlesnakes are poor climbers, they constructed open-topped soft release pens. Additionally, the pen site contained several active hibernacula used by resident animals. The topography inside the pen allowed for animals to seek higher ground on soil and vegetation hummocks or to go into shallow depressions that often held water. The pen had two removable doors that could be opened and closed as needed. When no animals were in the enclosure, they kept the doors open to allow small mammals and lizards (prey) to move in and out of the enclosure. When animals were contained within, the doors were closed. After the two-week retention period ended, the doors were opened and the animals were allowed to disperse at will, as opposed to forcefully removing them from the enclosure.
They constructed a similar enclosure for Texas Horned Lizards, which are also not climbers. In the enclosure, they provided drinking water and created a large sand mound for thermoregulation, burrowing, and oviposition by gravid individuals. Additionally, they used sugar water to lay trails to attract ants (prey for lizards) to the inside of the enclosure to ensure that soft-released lizards had sufficient access to food. However, they built this pen to provide some protection from avian predators as enclosed lizards could be vulnerable to predation by crows or hawks. Thus, they used fine-mesh wildlife netting to create a ceiling and prevent predators from accessing the pen.
Environmental enrichment can be designed to target development of many types of beneficial species-specific brain characteristics. Here, they demonstrated how environmental enrichment enclosures can be simply and easily designed to target ecologically-relevant, species-specific behaviors to improve individual survival post-release. Previous efforts have shown that environmental enrichment can provide reptiles with social interaction, structural complexity, thermal heterogeneity, and spatially dispersed, live prey items.
Enriched box turtles were communally housed in deep Rubbermaid® stock tanks with naturalistic features designed to mimic vegetation and substrate commonly used by wild box turtles. Unenriched turtles were housed individually in comparably simplistic enclosures consisting of a tall transparent plastic tub with reptile cage carpet and a piece of plastic shelf liner resting on the carpet. They provided these turtles a small plastic hide box and kept tubs on a slight angle to hold fresh-standing water in the lower end for drinking and soaking.
The type and amount of food provided to individuals at each feeding was similar between rearing treatments. However, they predominantly fed enriched turtles by scattering food throughout their enclosures to promote active foraging, whereas unenriched turtles were provided food on petri dishes, placed in the same spot in enclosures at each feeding. They initially fed live blackworms (Lumbriculus variegatus) and mealworms (Tenebrio molitor). They then transitioned turtles to live superworms (Zophobas morio) and then solely to live redworms (Eisenia foetida) after several months. They also offered fresh mixed greens (excluding spinach) and Zoo Med Gourmet Box Turtle Food—a commercial diet consisting of pellets and dehydrated mealworms, strawberries, and mushrooms. Turtles were offered fresh food five days per week, and they dusted food with calcium powder three days per week. They also provided enriched turtles with cuttlebones to chew on. Fresh water was provided ad libitum.
For the Texas Horned Lizards, survival analyses indicated that soft release was a viable technique for juveniles but was ineffective for adults. Soft-released juveniles had remarkably high annual survival (55%) compared to residents (29%). However, only 5% of soft-released adult lizards survived the year compared to an estimated annual survival rate of 57% for resident adults. These results suggest that juveniles may be a better age class to target for soft release because they have yet to develop an affinity to an areas whereas adults may display homing behavior after being translocated. When they evaluated the effects of all of the factors on the survival of all 55 Eastern Massasaugas snakes tracked, they did not find compelling evidence that any of the models adequately explained patterns in survival at this site. For the Eastern Box Turtles, their observed survival rates were higher than anticipated and higher than has been reported for similarly aged box turtles.
Their demonstration found relatively modest benefits from soft release translocation. Both hard-released and soft-released individuals were at a survival disadvantage relative to resident animals. However, there was strong evidence from Texas Horned Lizards that soft release may be most beneficial to juvenile individuals relative to adults. This may be because juveniles have yet to establish site fidelity and on release at a new site, do not try to home to their capture location. Future endeavors may experience the most effective results focusing on young age classes for translocation studies.
Their demonstration relied on a two-week soft release time period (i.e., individuals were kept in soft release enclosures for two weeks before being released). This may have been an insufficient amount of time for animals to acclimate to the new study area. Efforts aimed at using soft release for longer periods of time might benefit from larger pens, more complex pens, and a larger number of pens that can accommodate more individuals than the pens they used in their demonstration. Holding animals in pens for longer duration may entail other challenges such as the need to feed or provide other resources to enclosed animals, more intensive husbandry, or additional permits from regulatory agencies.
They also found modest benefits to environmental enrichment. While enriched animals were more inexpensive to care for, they experienced relatively modest increases in growth post-release and the researchers did not document an increase in survival. Because this method was more inexpensive than traditional methods and there were no costs to the enriched animals, the researchers do not see any reason not to adopt this methodology. However, they specifically chose a species that was relatively easy to care for in captivity. Animals with more complex life histories, larger body sizes, and greater space needs may be far more challenging to enrich in captivity.