Florida’s Everglades are home to an estimated 300,000 Burmese pythons, an invasive species that has significantly reduced populations of native mammals such as raccoons, opossums, and marsh rabbits. However, in Big Cypress National Preserve, some young pythons are being killed by native predators, including alligators, cottonmouths, and other mammals.
One notable case involved a juvenile python that died after attacking prey larger than itself. The animal implicated in that death was a hispid cotton rat, a small rodent that has not experienced the same population decline as other mammals in the region.
The first Burmese python in Everglades National Park was recorded in 1979 when a 12-foot snake was found dead on the Tamiami Trail. Though Hurricane Andrew is sometimes cited as the starting point of the invasion, the U.S. Geological Survey reports that pythons were reproducing in the Everglades as early as the 1980s. The snakes likely came from the exotic pet trade, through escapes or releases by owners. The first python nest in the park was not found until May 2006.
A 2018 USGS study of roughly 400 Florida pythons found that most are related as first or second cousins, suggesting a small founding population. At least 13 of the snakes also carried DNA from the Indian python, a species that favors drier habitat. Researchers believe the two species may have interbred before reaching Florida, potentially making the snakes more adaptable to a range of conditions.
Pythons are now distributed across more than 1,000 square miles of southern Florida. Scientists have detected their DNA in water samples from 25 of 28 sites in a USGS survey of the greater Everglades. Another survey found pythons present at nearly 90 percent of tree islands where wading birds nest.
A USGS examination of 1,716 pythons collected between 1995 and 2020 identified 76 different prey species, mostly mammals and birds. Road surveys in Everglades National Park have documented declines of 85 to 100 percent in sightings of raccoons, opossums, bobcats, and rabbits compared with pre-python years.
A controlled study involving radio-collared marsh rabbits found that pythons caused 77 percent of rabbit deaths inside the park within 11 months. The killing increased once weekly average temperatures exceeded about 80 degrees Fahrenheit. At control sites where pythons were rare, bobcats and coyotes did most of the hunting. The rabbits in the park bred, but by December no droppings could be found. The pythons effectively eliminated a small, fast-breeding animal, something ecological theory would not usually predict.
The impact has extended beyond prey animals. A University of Florida study found that a species of swamp mosquito that once fed mostly on deer, raccoons, and other medium and large mammals had shifted its diet. By 2016, those mammals accounted for less than 2 percent of its blood meals. With the larger mammals reduced, the mosquito turned to whatever remained, primarily the hispid cotton rat.
Efforts to control python numbers have had limited results. Florida’s annual Python Challenge drew 907 participants in 2026, who removed 280 snakes. State contractors have removed more than 19,000 pythons since 2017. The USGS has concluded that eradicating pythons from South Florida is not possible with current tools.
The Conservancy of Southwest Florida has used radio-tagged male “scout snakes” to locate breeding females. In the 2025 to 2026 season, 40 scout snakes helped remove 177 pythons weighing over 8,000 pounds from a 200-square-mile area of Collier County. Captured females carried an average of 70 eggs each, and about a quarter contained white-tailed deer remains.
A record cold spell in January 2010 killed nine of 10 radio-tagged pythons in Everglades National Park. But among 99 untagged pythons checked afterward, 59 were still alive. The survivors continued breeding.
The hispid cotton rat stands out for its resilience. It is a stocky rodent with coarse fur, described by scientists in 1825 from a specimen collected along the St. Johns River. It is not a true rat; its closest relatives are voles and muskrats. Females can carry litters of five to seven pups and can conceive again within hours of giving birth. In South Florida’s warm climate, breeding can continue year-round. Individual cotton rats rarely live long, but the population refills rapidly.
University of Florida researchers fitted 115 cotton rats with radio collars in a 2023 study published in the Journal of Mammalogy, tracking them in areas with many pythons and areas with few. Survival did not differ significantly between the two areas. Where pythons were common, birds of prey caused about 49 percent of cotton rat deaths, native reptiles about 24 percent, and pythons about 16 percent. Across the whole study, pythons accounted for 11.8 percent of all deaths. This is described as compensatory mortality: cotton rats produce more offspring than an area can support, and most die young regardless of cause. When pythons take some, they mostly replace deaths that would have occurred anyway.
The cotton rat also appears to recognize pythons as a threat. A 2025 experiment published in the Journal of Animal Ecology used feeding trays and life-size replicas of a python, a rattlesnake, a coyote, and an unfamiliar object. Raccoons showed caution only around the coyote model. Cotton rats reacted to all three predators, including the python. The researchers attribute this to the cotton rat’s long history with native snakes, which have always been part of the danger. Raccoons and marsh rabbits lack this recognition, a gap ecologists call prey naivete. Snakes as large as pythons had not lived in the eastern United States for millions of years, so those species had no defense prepared.
The cotton rat’s role in the Everglades virus cycle has drawn concern. Everglades virus, a native pathogen in the Venezuelan equine encephalitis complex, can cause encephalitis in people. The mosquito species that carries it feeds primarily on cotton rats. In 1979, cotton rats supplied 14.7 percent of that mosquito’s blood meals. By 2016, they supplied 76.8 percent. Where mosquitoes bit cotton rats more often, more of them carried the virus, and infection rates rose roughly threefold. At one site in Big Cypress with only three mammal species present, every blood meal identified came from rodents, and that site had the highest infection rate in the study.
Scientists describe the earlier situation as a dilution effect. Deer and marsh rabbits were bitten frequently but appear to be poor hosts for the virus, so many bites produced no transmission. With those animals largely gone from many areas, the mosquitoes turned to cotton rats. The same paper draws a parallel to Lyme disease, where ticks in the southern United States feed heavily on lizards that are poor hosts.
In 1993, a man in South Dade County was hospitalized with hantavirus pulmonary syndrome with kidney involvement. Investigators trapped rodents in the area, and 13 percent of cotton rats tested positive for hantavirus antibodies. A new virus, named Black Creek Canal virus, was isolated from them. No second human case was found in the hospital surveillance that followed.
The researchers note that more infected mosquitoes does not automatically mean more human cases, since the risk depends on how often people are bitten. The practical guidance for visitors to the Everglades has not changed: avoid mosquito bites and avoid stirring up rodent droppings in closed sheds or cabins. The data, however, shows a risk moving in the wrong direction because of the python invasion.
With eradication of pythons not currently possible, the more realistic path is continued removal. Every python taken out gives deer, marsh rabbits, and raccoons a chance to recover. Those species were the ones that helped absorb mosquito bites and limit virus transmission. Populations of raccoons and marsh rabbits near suburbs and towns have held up better against pythons, and researchers suggest those edge populations could help keep human risk down. Native predators are also adjusting, with a Florida cottonmouth found to have consumed a young radio-tagged python. The cotton rat’s survival has made it a persistent host in the virus cycle, and the broader consequences of the python invasion continue to unfold.
