A hatching brown anole.

Temperature is probably the most studied environmental factor that influences living things; however, you might be surprised to learn that we still don’t have a solid understanding of why things die when they get hot. If you recall your intro biology, you’ll remember that proteins and cell membranes fall apart when they get hot, and that is often the explanation for death at high temperatures. But, there are several reasons to question this explanation. For example, complex organisms (e.g. plants and animals) universally have lower heat tolerance than simple organisms (e.g. bacteria), despite using the same basic biochemical building blocks (i.e. proteins and membranes). Moreover, complex organisms often die at temperatures lower than those that cause proteins and membranes to fall apart.

One explanation has gained a lot of traction in recent years: the oxygen-and capacity-limited thermal tolerance concept (what a mouth full!). This concept posits that as your body heats up, you need more oxygen; however, you eventually get so hot that you can’t get enough oxygen to survive.  There is growing evidence that oxygen limitation explains thermal tolerance for reptile eggs. Several studies show that when eggs are incubated in low oxygen conditions, their heat tolerance is lower (e.g. Smith et al., 2015); however, we still don’t know much about embryo metabolism at near-lethal temperatures, which would vastly improve our understanding of embryo heat tolerance.

In a recent study (Hall and Warner, 2020), we (I and Dr. Dan Warner, who was recently awarded the distinction of “Outstanding Mentor” by Auburn University – well deserved) sought to better understand the factors that determine heat tolerance of reptile embryos. We used eggs from our good friend, the brown anole (Anolis sagrei). Using 1-hour heat shocks, we measured the lethal temperature of embryos (~45.3 °C). We then monitored heart rate and metabolism of eggs across temperature, including near-lethal temperatures.

Figure 1. Heart rate of brown anole eggs across temperature.

As embryos approach the lethal temperature, heart rate and CO2 production increase (Figure 1), but oxygen consumption plateaus (Figure 2). Therefore, eggs need more and more energy as they heat up, but they are eventually unable to support their energy needs via aerobic respiration. Without enough oxygen, energy production is less efficient. These data indicate that oxygen is limited at near-lethal temperatures and provides additional support for the oxygen-and capacity-limited thermal tolerance concept for reptile eggs.

Figure 2. Oxygen consumption across temperature for brown anole eggs.

Many aspects of human-induced global change cause increases in temperature (e.g. deforestation, urbanization, climate change), potentially heating lizard nests and exposing embryos to thermal stress. The results of our study make progress toward understanding how embryos respond to extreme temperatures, which is important to understand how reptile populations will respond to global change.

Hall, J.M. and Warner, D.A., 2020. Thermal sensitivity of lizard embryos indicates a mismatch between oxygen supply and demand at near-lethal temperatures. Journal of Experimental Zoology, in press. https://doi.org/10.1002/jez.2359

Smith, C., Telemeco, R.S., Angilletta Jr, M.J. and VandenBrooks, J.M., 2015. Oxygen supply limits the heat tolerance of lizard embryos. Biology letters, 11(4), p.20150113.

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