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Study: Social Networking Could Make Relocated Desert Tortoises Sick

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A desert tortoise | Photo: Joshua Tree National Park/Flickr/Creative Commons License

A segment for KCET's award-winning TV show "SoCal Connected" has been produced in tandem with this story. Watch it here now.

Development projects in the California desert routinely use relocation as a way of dealing with any federally Threatened tortoises they find on site, but the process has long been controversial due to potential harm to the tortoises. Now a new study suggests that moving tortoises into new locations may actually cause outbreaks of disease in both the tortoises that are moved and those who welcome the newcomers into their new neighborhoods.

Though wildlife managers often worry that relocating wildlife can cause disease outbreaks by spreading pathogens, the study suggests that moving large numbers of tortoises into a new territory might cause disease outbreaks even if both the host population and the new population already carry the pathogens.

That can happen because handling involved in relocation causes stress, which can reduce resistance to disease. But the study's authors suggest that the new tortoises' exploration of their new stomping grounds might also play a role, by changing what we might as well just succumb to the obvious clickbait temptation and call the reptiles' social networking habits.

In the study, the results of which were published in November in the journal Animal Conservation, U.S. Geological Survey biologist Christina Aiello and seven colleagues examined tortoises that were relocated in 2008 from the U.S. Army's Fort Irwin near Barstow to lands nearby in the Mojave Desert.

Aiello et al studied data from the tortoises' radio transponders, then constructed diagrams of the interactions the translocated and resident tortoises had with one another during the weeks following translocation.

Desert tortoises ordinarily keep to set territories throughout their lives, with physical interactions a relatively rare occurrence. But in the first 20 days following the introductions of the translocated tortoises into their new habitat, interactions between tortoises jumped dramatically in number as the newcomers explored their new surroundings and residents investigated the new, strange tortoises in town.

This diagram from the paper in Animal Conservation represents interactions among the tortoises in the study. The top row shows interactions among tortoises at nearby control plots with no new tortoises, while the bottom row shows the plots to which new tortoises were moved. From left to right, the four pairs of diagrams show interactions in the 15 days before translocation (T0), then for three ten day periods (T1 through T3) immediately following the new tortoises arrival.

On those control plots, interactions between tortoises were few and far between. Same went for the recipient plots before the new tortoises arrived. But as soon as the new tortoises were moved to the recipient plots, the tortoises started social networking fast and furious -- or at least as fast and furious as they could.

That's important because the main disease threatening desert tortoises in the wild, called Upper Respiratory Tract Disease or URTD, is thought to be caused by organisms called Mycoplasmas, which are spread by social contact between live tortoises. It's generally assumed that even if tortoises recover from Mycoplasma infections, they still harbor the pathogen and remain susceptible to future outbreaks.

(That's the main reason it's so important that captive tortoises never be released back into the wild, as captive tortoises have a much higher rate of URTD.)

Since stress causes torts to be more susceptible to URTD, then the likely nature of the interactions between resident and incoming tortoises wouldn't help: tortoises are solitary critters, and unless they're mating encounters between adults are usually antagonistic.

Twenty days after the beginning or introductions of new tortoises, as shown in T3 in the diagram, that social networking started to die down. But Aiello et al suggest that even twenty or thirty days of increased interaction might be enough to change the relationship between the tortoises and the Mycoplasma pathogens they carry around in their bodies, perhaps promoting new outbreaks or URTD.

The authors suggest that this social netowrking aspect of URTD susceptibility might be a good reason for wildlife agencies to rethink their policy on translocation:

Current [USFWS desert tortoise] translocation guidelines recommend the movement of animals in good physical condition that do not exhibit moderate to severe clinical signs of disease. This policy makes a number of assumptions: released animals with latent or mild infections will not progress to more virulent and transmissible infections, will integrate uniformly into the resident population, and will exhibit natural levels of contact and transmission. How infections in the translocated and resident population manifest and transmit through the population after release will largely depend on whether translocation alters disease parameters.

In other words, the feds' current guidelines for moving tortoises out of the way of our projects might not adequately protect desert tortoises from disease. If that's the case, then developing the desert might be even more harmful to tortoises than we thought.

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