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‘Judas’ snakes lead scientists on a high-tech Easter egg hunt for pythons

  • Scientists are exploring various technologies to address the spread of highly invasive Burmese pythons, which have devastated native mammal and bird populations across much of southern Florida.
  • Researchers who recently captured a large pregnant Burmese python did so using the “Judas” technique: the radio-tagging of adult pythons that will approach others of the opposite sex during the breeding season, “betraying” them to the research teams.
  • More recently, separate research teams have trialed the use of environmental DNA (eDNA) to determine the spatial distribution, range limits, and expansion rates of Burmese pythons in the region. They found python eDNA within a wildlife refuge, indicating that the invaded area extends further north than previously thought and that pythons are likely resident there.

Remember that 3-meter (17-foot) pregnant Burmese python recently captured by wildlife authorities in southern Florida? These snakes, invasive to North America and destructive to native wildlife, are cryptic in both their coloration and behavior, making them difficult to find. But scientists found this female with the help of some tracking tags and a cooperative male python.

Moreover, they’ve used DNA the snakes have left behind to identify areas already invaded by the pythons, which can help researchers determine the range limits of these snakes and assess efforts to control their spread.

Researchers and National Park Service staff display the full 5.1 meters (17 feet) of the pregnant Burmese python.  Wildlife officials say this snake is the largest Burmese python ever to be removed from Big Cypress National Preserve in the Florida Everglades. Image courtesy of Florida Everglades National Park Service.

Burmese pythons (Python bivittatus), native to Southeast Asia, have no predators in Florida, so the few snakes brought to the United States through the pet trade that escaped or were released have quickly reproduced and morphed into an out-of-control, continuously growing population there, likely established since before the 1990s. Pythons’ long life span and large clutches of eggs have further enabled them to spread across southern Florida.

Along the way, they’ve decimated the populations of numerous native species, eliminating large portions of the region’s small and medium-sized mammals and now suspected of turning to the iconic wading bird colonies of the Everglades wetland ecosystem.

The ‘Judas’ technique

Traditional methods of visually searching for and trapping the animals have been generally ineffective in locating pythons in the region’s inaccessible, dense, swampy vegetation.

Scientists have thus turned to a tracking strategy called the “Judas” technique, which uses a radio-tagged individual of an invasive species to approach and “betray” others of the same species, or conspecifics, to the researchers following them.

The story of Judas, the biblical disciple who betrayed Jesus to religious officials, thereby leading to Jesus’s crucifixion, typically gets an airing during Easter. Today, the name’s been adopted for a technique that scientists use to find social animals, such as invasive goats and pigs, often transported by people to islands, in an effort to eradicate the invasive exotic species and enable native plants and animals to recover.

Goats bought by people to various Galapagos islands destroy vegetation, leaving little for giant tortoises and other native herbivores. In the absence of predators, the goats have proliferated, increasing the threat to native plants and animals. They prefer to move in groups, making the “Judas” goat a feasible strategy for locating even the most wary individuals. Researchers can follow the tagged goat to the group, where they kill all but the Judas goat, who is free to find another group. Image from BBC Youtube.

Once pursued by research or management teams, these animals become extremely wary of humans, but they like being in groups and will approach a conspecific through normal social behavior.  In the case of the Burmese pythons, individual snakes in southern Florida are normally solitary and highly cryptic, but they come together between December and April to mate.

In this method, researchers implant tiny radio transmitters into each “Judas” python, locate the released tagged snakes, and monitor their movements using receivers attached to a small plane. Once relocated, researchers approach a tagged snake on foot, record the GPS coordinates of its location, and search for other pythons in the area. Over time, the researchers glean the animals’ movement patterns and use of the various vegetation types, including upland and lowland forest, marsh prairie, freshwater grasslands, coastal, and open water. They also record the number of “betrayal” events and number of individual pythons betrayed per field visit by a given tagged python.

The use of radio tags and a small plane or helicopter means the method is relatively expensive, but it offers researchers access to large female snakes, such as the recently caught individual who was carrying 73 eggs.

“This is one more tool we can add to our tool box to help us combat this invasive species,” Brian Smith, a contractor with the U.S. Geological Survey (USGS) who led a study documenting the radio-tagging of Burmese pythons in southern Florida, said in a 2016 statement. “It’s effective at a time of year when we do not catch pythons on the road, and … it provides more opportunities to catch the really big, breeding females.”

That initial study, conducted in Everglades National Park, found that python betrayal events were particularly likely to occur in lowland forest and tree islands within the massive wetland system, suggesting that researchers should target those habitat types when searching for breeding individuals.

The Judas technique is more expensive per python caught than opportunistic detection during road monitoring, and it’s applicable only during the few months when the snakes aggregate. However, it provides researchers with access to females with high reproductive potential and is effective at times of the year when road surveys aren’t possible due to flooding. The two methods can thus complement each other.

“Despite the cost [of the Judas technique], it has been the best method to help us find more, large, reproductive individuals during the breeding season,” said Kristen Hart, a USGS research ecologist, 2016 study co-author, and a graduate adviser to Smith. “Removal of these large breeders is essential if we are ever to make a dent in the python population.”


A more recent technology using species’ DNA has helped python research teams further target their searches to make better use of both the Judas technique and road surveys.

In a recent pair of studies, researchers assessed the range limits of pythons in a south Florida wildlife reserve and the tendency of pythons to target wading bird breeding sites.

The teams tested the use of environmental DNA, or eDNA, in water samples from within the Loxahatchee National Wildlife Refuge, where pythons hadn’t been detected previously and from areas south of the reserve, where pythons were known to occur.

A Burmese python up close. In the wild, these snakes average 3.7 m (12 feet) long, but can reach 7 m (23 feet). In areas where they’ve become established, sightings of many native mammals, including raccoons, rabbits, possums, and deer, have decreased by over 90 percent. Image by Brian Smith-U.S. Geological Survey.

eDNA is the genetic material from the skin, hair, scales, feces, pollen, or seeds shed by living creatures into the environment and extracted from soil or water, rather than directly from a plant or animal. Analyzing eDNA has enabled scientists to determine the presence of a particular species or describe the community of plants or animals in an area.

Scientists collect eDNA through simple water or soil samples, which makes data collection faster and more cost effective than having to extract a blood or tissue sample from an animal or plant. Analysis of the eDNA requires comparing the DNA in the samples to existing sequence data for the species of interest, usually from a reference database, as well as standard genetic analysis tools. Once reference data are available, costs associated with genetic analyses decrease rapidly over time.

Being able to quickly sample large areas can help researchers determine the presence of an invasive species in previously unstudied areas. According to the team assessing the Loxahatchee Refuge, “the ability to detect invasive species at low densities or prior to establishment is critical for successful control and eradication efforts.” The same basic genetic information can also help managers assess the success of their control or removal efforts; repeated tests that do not detect a species could signal a successful removal campaign.

The Loxahatchee researchers aimed to determine the range limits of the area invaded by pythons. They found Burmese python eDNA within the refuge, showing that the invaded area extends further north than previously thought and that pythons are likely resident there.

They found python eDNA at most sampling sites throughout their three-year collection period, which, they write in their paper, “is consistent with the pattern expected for a resident python population, as opposed to sporadic, transient individuals or alternative vectors.”

Egrets and wood storks share a tree for their nests in the Everglades wetland ecosystem. Scientists have found greater presence of established python populations near wading bird colonies than around islands with no nesting birds, suggesting that resource managers focus python monitoring and control efforts in areas of nesting wading birds. Image via pxhere, CC 0.

Similarly, scientists who collected and examined python eDNA around the region’s wading bird colonies, found that the amount of python eDNA was higher around colonies than around control islands (with no wading birds), “suggesting that pythons are attracted to wading bird colonies during the wading bird breeding season” and that the pythons’ wide distribution across the central Everglades was related to areas near active bird colonies.

They found eDNA analysis to be “the most powerful and quantitatively accurate technique currently available,” especially in habitats where accessibility is difficult and the dense vegetation makes it hard to spot pythons.


Hunter, M. E., Meigs-Friend, G., Ferrante, J. A., Smith, B. J., & Hart, K. M. (2019). Efficacy of eDNA as an early detection indicator for Burmese pythons in the ARM Loxahatchee National Wildlife Refuge in the greater Everglades ecosystem. Ecological Indicators, 102, 617-622. doi: 10.1016/j.ecolind.2019.02.058

Smith, B. J., Cherkiss, M. S., Hart, K. M., Rochford, M. R., Selby, T. H., Snow, R. W., & Mazzotti, F. J. (2016). Betrayal: Radio-tagged Burmese pythons reveal locations of conspecifics in Everglades National Park. Biological Invasions, 18(11), 3239-3250. doi: 10.1007/s10530-016-1211-5


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