By Peter Moon | Agência FAPESP – Salvator merianae, the black and white tegu, also called the Argentine giant tegu, is the largest species of tegu lizard and one of the largest lizards found in the Americas, measuring as much as 2 m in length. Moreover, at the time of writing, it is the only warm-blooded lizard known to science. The discovery of its enhanced capacity for heat production and conservation is an important contribution to our understanding of how endothermy may have evolved in the ancestors of mammals and birds (and dinosaurs).
This is the conclusion of a study conducted by Brazilian and Canadian biologists at São Paulo State University’s Bioscience Institute in Rio Claro, Brazil (IBRC-UNESP), with support from FAPESP, and published in Science Advances.
The study initially involved four tegus, two males and two females. They were kept in captivity, and their body temperatures and heart rates were continuously monitored for a year.
Tegus in the southern hemisphere spend most of the fall and winter (early April through early September) hibernating in burrows without feeding. They then emerge from dormancy to begin the reproductive season.
Measurements of the tegus’ internal temperatures showed that during dormancy they sustained a very low metabolism at approximately 17°C, which is ambient temperature in the burrow. When they emerged from dormancy to enter the reproductive cycle, their internal temperatures began varying during the day. For most of the night, their average internal temperature was 24°C. After daybreak, two hours of basking in the sun raised their average internal temperature to 35-37°C. In late afternoon, the temperature started to fall again, reaching a low in the small hours of the night. During the reproductive season, however, the temperature remained higher, being maintained several degrees above the burrow temperature.
The researchers were surprised by these results. “This can’t be right,” they thought, according to Denis Andrade, one of the biologists on the team. “There must be something wrong with the equipment. Tegus are ectotherms.”
What they had expected is that the tegus’ body temperatures would match the ambient temperature a few hours after they retreated to their burrow at all times of the year, in line with the normal pattern for reptiles in general. “That was our premise. The findings were a complete surprise. It was a paradigm shift,” Andrade said.
What could the source of heat be to change the animals’ temperatures? There were only two possibilities: an external source, which had to be the sun, or an internal source, which could only be their own metabolism.
To confirm the hypothesis that tegus are warm-blooded animals after all, the team performed a new experiment in which ten animals were confined in a constant-temperature environmental chamber and monitored. Their body temperatures continued to exceed ambient temperature under these conditions, even without exposure to sunlight.
Because there was no external heat source, these results convinced the researchers that the tegus were indeed producing heat endogenously by increasing their metabolic rate and expending more energy.
Under natural conditions, this internal production of heat combined with a reduction in heat lost to the environment enabled the tegus to maintain body temperatures as much as 10°C above ambient temperature. The strategy is identical to that used by mammals and birds to regulate body temperature.
This species of lizard is now known to be endothermic, at least during the reproductive season. However, the origin of the internal heat it produces remains a mystery.
What is the source of thermogenesis in tegus? Is Salvator merianae really the first warm-blooded reptile known to science? “That’s a hard question,” Andrade said. “The tegu uses its metabolism to regulate internal temperature during the reproductive season, but behaves like an ectothermic animal during the rest of the year. You could say it’s a halfway house, part ectothermic and part endothermic.”
The tegu’s ability to regulate body heat could be an independent evolutionary adaption in Tupinambis, the genus to which this species belongs. It could be shared by several genera in the same family, Teiidae. Or it could be an even wider phenomenon common to several families in the order Squamata. To prove any of these hypotheses, the experiment would have to be repeated with various species of scaled reptiles around the world. This is indeed what many scientists will now begin to do, motivated by the results of the IBRC-UNESP project.
If temperature control were found to be common to all lizards, this could mean the tegu’s thermoregulation is an evolutionary relic, an adaptation that may have evolved over 200 million years ago in a common ancestor of all scaled reptiles.
Metabolism and reproduction
What could be the reason for the evolution of endothermy? Given that tegus display internal temperature regulation only during the reproductive season, could the evolution of endothermy be linked to the evolution of reproductive investment in mammals and birds?
This theory was first suggested in 1998 by Colleen G. Farmer, a biologist at the University of Utah in the US. “The idea that increased metabolic activity is linked to reproduction was first raised by Farmer. Our data appears to corroborate her hypothesis,” Andrade said.
The next question the researchers asked was whether thermoregulation was different in male and female tegus because endothermy was only apparent during the mating season. “You would expect the rise in temperature to be pronounced in females owing to colossal investment in egg and gamete production. However, we didn’t detect any differences between the sexes. Males had the same endothermic capacity,” Andrade said.
A possible explanation is that reproduction involves much more than gamete and egg production, including many other hormonal, morphological and behavioral changes. Male tegus, for example, are highly territorial and expend a great deal of energy fighting with other males during the mating season, according to Andrade.
In 2015, it was discovered that the opah (Lampris guttatus), a large fish that lives in the deep cold waters of the North Atlantic and North Sea (average temperature 5°C), can keep its whole body as warm as 15°C. “Many wasps, moths and bees, which are traditionally considered ectothermic, use thermogenesis to regulate body temperature,” he said.
There are also mesotherms, midway between cold- and warm-blooded animals. They burn energy from within to regulate body heat independently of ambient temperature, but like reptiles, amphibians and fish, they do not hold it constant as a mammal or bird would do. Examples of mesotherms include the great white shark, tuna and the leatherback turtle.
As for the tegu, it can definitely be considered seasonally endothermic. In other words, “it just got harder to distinguish between ectothermic and endothermic animals”, Andrade said.
The article “Seasonal reproductive endothermy in tegu lizards” (DOI: 10.1126/sciadv.1500951) by Andrade et al., published in Science Advances, can be read at http://advances.sciencemag.org/content/2/1/e1500951.full.