In early 2020, south Florida experienced its coldest temperature in 10 years at just under 40 degrees Fahrenheit. Scientists at Washington University in St. Louis seized the opportunity to investigate how tropical lizards are affected by extreme climates.
When temperatures exceed a certain threshold, sleeping lizards can actually lose their grip and fall out of trees. The experts previously established that various types of lizards in Miami can tolerate low temperatures ranging from about 46 to 52 degrees Fahrenheit before they are shocked by cold.
The researchers examined lizards that had survived the record cold night in Miami, and were surprised to find that all of the species could tolerate cold temperatures down to about 42 degrees – regardless of their previous threshold.
“Prior to this, and for a different study, we had measured the lowest temperatures that six lizard species in south Florida could tolerate,” said study co-author James Stroud. “We realized after the 2020 cold event that these data were now extremely valuable – we had the opportunity to re-measure the same lizard populations to observe if their physiological limits had changed; in other words, could these species now tolerate lower temperatures?”
To investigate further, the team collected as many different types of lizards as possible in the local region, including lizards that are nocturnal. Next, the experts set out to analyze how various specimens responded to cold.
“A major unexpected result of this study was that all species converged on the same new, lower level of thermal tolerance,” said Stroud. “While there was great variation in temperature tolerance before the cold event – some, like the large-bodied brown basilisk, were very intolerant of low temperatures, while others like the Puerto Rican crested anole were more robust – we observed that all species could now tolerate, on average, the same lowest temperature.”
Stroud said that given the differences in the body size, ecology, and physiology of the lizards, the results were unexpected. The researchers noted that only one of the species examined for the study was native to the area, while the rest have been introduced to Florida over the past century.
“The shifts to tolerate significantly lower temperatures that we observed were so large that we found it unclear whether natural selection was responsible,” said Stroud. “And so in our paper we discuss other alternative processes which may also have led to this pattern.”
The findings provide brand new evidence that tropical, cold-blooded creatures, which are typically not thought to be able to adapt to rapidly changing climates, can sometimes endure conditions that exceed their established limits.
“The results of this study are surprising and unexpected. Who would have thought that tropical lizards from places like Puerto Rico and Central America could withstand temperatures near freezing?” said study co-author Professor Jonathan Losos.
“What we now need to find out is how this was accomplished. Is this evidence of natural selection, with those lizards that just happened to have a lower cold tolerance surviving and others freezing to death, or was it an example of physiological adjustment – termed ‘acclimation’- in which exposure to lower temperatures changes a lizard’s physiology so that it is capable of withstanding lower temperatures?”
Regardless, the study provides critically important insight for understanding the impacts of climate change.
“It is widely thought that tropical and subtropical species are going to be especially vulnerable to changes in temperature — particularly extreme spikes of heat or cold — as tropical areas do not typically have strong seasons,” said Stroud. “Unlike temperate species, which are adapted to summer highs and winter lows, tropical species have typically evolved in very thermally stable environments.
“While there is no doubt that climate change represents a major threat to species and ecosystems around the world, and deserves as much research attention as possible, this study provides fascinating insight and a glimpse of hope. Perhaps tropical and subtropical species can withstand more extreme climatic conditions.”
The study is published in the journal Biology Letters.