By Matt Walker
Editor, Earth News
Large, and eating Florida's native wildlife
A cold snap last winter may have helped arrest the spread of Burmese pythons through the US state of Florida.
In recent years, the huge snakes have secured a foothold in the southeastern US, after having escaped or been released into the wild by pet owners.
But hundreds of the snakes did not survive unusually low temperatures last January, a monitoring study reveals.
Though the pythons remain a threat to biodiversity in Florida, it now appears less likely they will spread further.
Burmese pythons are one of a number of invasive species that pose a significant threat to the native wildlife of Florida and its Everglades National Park.
Invasive jewelled cichlids, Asian swamp eels and Cuban treefrogs have all colonised the unique Everglades ecosystem, often outcompeting native species.
Aerial view of the Everglades National Park
But the arrival of Burmese pythons has caused a particular stir, in part because these snakes can often reach over 3m long, and also because they have spread far beyond artificial habitats, such a back gardens and canals, that many invasive reptiles remain confined to.
The snakes also appear to prey on locally endangered species such as Key Largo woodrats and marsh rabbits.
Professor Frank Mazzotti of the University of Florida is one of many conservationists leading the effort to thwart the impact species such as the Burmese python are having in Florida.
Together with colleagues from the University of Florida, US National Parks Service and US Geological Survey, Prof Mazzotti analysed how Burmese pythons responded to a prolonged period of unusually cold weather that gripped Florida between 2 and 11 January this year.
Prior to the cold snap, the researchers implanted radio transmitters and temperature loggers into ten pythons they had captured and then released again.
The radio telemetry allowed the researchers to track the pythons' movements and fate over the winter, while the temperature loggers allowed them to record how the snakes' body temperatures fluctuated.
The results are published in the journal Biological Invasions.
Body temperatures of eight telemetered pythons fluctuated wildly between 10 to 30 degrees Celsius in the period leading up to 9 January.
They then significantly declined in the two following even colder days, often not reaching 5 degrees Celsius, indicating the snakes were unable to thermoregulate in the cold temperatures.
Later, the bodies of these dead pythons were found along with that of a ninth telemetred animal.
Overall, the researchers also found 99 other pythons, of which 59 were alive and 40 dead.
That suggests many more pythons perished unnoticed.
"A lot of pythons died," Professor Mazzotti told the BBC.
A separate group of researchers also reported in May that during the same period of cold weather, seven of nine captive Burmese pythons held in outdoor pens at a National Wildlife Research Center facility in north-central Florida died, or would have died without intervention.
However, it is still unclear exactly why.
All ten telemetred pythons behaved oddly before being found, as each was recovered on the surface.
Normally snakes are found in warmer refuges such as in burrows or other subterranean retreats.
One reason may be that Burmese pythons are naturally a tropical species, and as such are not adapted to cope with cold temperatures.
Key Largo woodrats are vulnerable to invasive pythons
Other large tropical reptiles, such American crocodiles, tend not to avoid the cold, whereas temperate species, such as American alligators, do, and will retreat to warmer refuges when temperatures fall.
A snake's ability to thermoregulate is also set early in its life, and snakes later exposed to novel temperatures often behave differently.
The Florida pythons have probably not experienced temperatures as cold as those in January this year before, say the scientists.
As such, they can be considered to be "thermally naive", and were caught out by the cold snap.
That raises the possibility that Burmese pythons might better cope with cold conditions in the future, particularly snakes that are better adapted genetically to the cold.
Because water covers much of the Everglades, there are relatively few refuges to hide in.
That suggests that, counter-intuitively, more pythons might survive further north, where temperatures are cooler but there are more artificial habitats containing warm places to hide.
However, overall, the evidence suggests that the Burmese pythons are essentially tropical reptiles that find it difficult to tolerate more temperate climates.
So Prof Mazzotti's team hypothesises that Burmese pythons are unlikely to spread as widely across the US as American alligators, which are a warm, temperate species that also ranges into tropical areas.
The cold snap also appears, for now, to have limited the snake's colonisation of Florida.
Female pythons particularly suffered, likely reducing the population's ability to grow this year.
If many juveniles also died, then the python's recovery will be even more limited.
The researchers warn though that it will be impossible to completely remove Burmese pythons from Florida.
Scientists currently use traps to find the snakes, and sometimes they use pythons to find other pythons during the mating season, following these so-called "Judas snakes".
But a combination of techniques offers the best chance of limiting their impact.
The cold spell in January therefore offers the possibility that nature as well as science might combine to control the snake's population.
Understanding how Burmese pythons respond to their new environment can also help inform the best ways to control them, and prevent other invasive species taking hold before it is too late.