I haven’t done one of these short, newsy posts in a while. However, I felt this one warranted the attention.
Announced today, a new paper from Glen Tattersall and colleagues (open access):
Tattersall, G.J., Leite, C.A., Sanders, C.E., Cadena, V., Andrade, D.V., Abe, A.S., Milsom, W.K. 2016. Seasonal Reproductive Endothermy in Tegu Lizards. Sci. Adv. 2:e1500951.
In another example of slow-cooked science, this paper was the culmination of over three years worth of work collecting data on tegus. For the study, the authors looked at adult black and white tegus (Salvatore merianae). Tegus are an interesting group of lizards. They are the largest members of the family Teiidae and are often referred to as the monitor lizards of the new world, due to their convergent lifestyles (highly predaceous, active foragers). Besides their varanid-like demeanor, tegus are also known for their enormous jowls, especially in the males. The jowls hold the pterygoideus muscles, the big jaw snappers, which have been shown to increase in size for males during the breeding season (Naretto et al. 2014). As reptiles, tegus have been assumed to follow the standard ectothermic lifestyle of requiring external sources of heat to warm their bodies and maintain stable body temperatures. Looking at the natural history of the animals, tegus appear to fit the mold pretty well. They have distinctive winter and summer activity levels. In the summer, the animals regularly maintained body temperatures of 32–35°C, and in the winter they let their body temperatures drop to the temperature of their burrows (15–20°C). This is all fine and good for a bradymetabolic, ectothermic lizard, but when the researchers tracked body temperatures over time they discovered something completely unexpected.
During the months leading up to and during reproduction (October–March) a distinctive circadian body temperature cycling was observed. Tegus were now routinely holding body temperatures that were up to 10°C above burrow temperatures. The strangest thing, though, was that these body temperature increases were occurring at night, while the animals slept. This last part is important, because unlike the myogenic endothermy of leatherback sea turtles, monitor lizards (Bartholomew and Tucker 1964), and female pythons (Vinegar et al. 1970, Harlow and Grigg 1984), tegus were exhibiting endogenous heat production without the aid of their muscles. At a maximum size of 2 kg, it seems unlikely that thermal inertia was playing much of a part in this increased heat production. Nonetheless, the authors tested this alternative by placing some tegus in a constant temperature enclosure for 8 days. To ensure that the increased metabolism associated with digestion (known as Specific Dynamic Action, or the Heat Increment of Feeding) were not playing a part, the animals were fasted during the entire 8 days. The results continuously showed that during the early hours of the morning, while the lizards slept, their body temperatures rose by 5–6°C. Heart rate data further corroborated these results by increasing during periods of higher metabolic rates. Incidentally, this is not the first time this has been recorded in a lizard. Tosini and Menaker (1995) recorded daily temperature cycles in the body temperatures of green iguanas (Iguana iguana) using a similar setup to Tattersall et al. However, their data only recorded a 0.8–1.8°C change in temperature, not the 5–10°C seen in the tegus.
So if it’s not body size and muscle output, then what is responsible for the heat production? This is where things get interesting. No one, including the authors, know for sure. The authors suspect that the usual suspects for mammalian and avian endothermy (the liver, intestines and heart) may be active in this role. For now it remains to be seen.
The origin of automatic endothermy
The origins of automatic endothermy (sensu Bakker 1980) have been difficult to pin down. The most popular and accepted model proposed for the evolution of automatic endothermy is that of increased aerobic capacity (Bennett and Ruben 1979). However, as I have discussed at length previously, the popularity of this model runs counter to its empirical backing. Other models such as primarily heat production (“Hot is good model”, McNab 1978), or the parental care hypothesis (Farmer 2000) have more empirical support, yet have not received as much traction (especially among paleontologists). The data presented by Tattersall et al. suggest that automatic endothermy may have started off as a seasonal event related to reproduction, which agrees well with Farmer’s initial hypothesis. There is still a lot to be done (e.g., finding out the hormone[s] responsible for the increased heat production and the location of the heat producers) and it remains possible that the heat production observed in tegus is completely unrelated to the heat production seen in mammals and birds. For now, though, only time will tell.