It's Gettin Hot in Hurrr...So Take Off All Your Scales

Who turned on the heat!?

It comes to no surprise for me that when the issue of climate change is brought up, often times, someone will turn to me and say, “Boy! Those lizards you’re studying must be the only animals who will benefit climate change!” I mean, after all, yes, lizards are ectothermic organisms, that is, they depend on their environmental habitat and temperature to thermoregulate in order to maintain proper functioning processes (digestion, escaping predation, or even on a molecular level such as protein synthesis). And while our little scaly friends can function in a wider range of temperate environments than we can (8 – 38 ºC vs. 36 – 39 ºC), lizards are very sensitive to rising temperatures above their preferential temperature (T_b). “Whoa! But Kat, what is a lizard’s typical T_b?” Well, I’m glad you asked! While T_b can range based on their environment, it is safe to say that in mild climates such as that of the sunny central coast of California the T_b of lizards lies somewhere around 35 – 39 ºC. Once a lizard begins to go beyond the zone of T_b, we start to hit critical temperatures that could lead to deleterious effects on function, proteins falling apart (denaturation), enzyme malfunction, and even death.

In fact, Cowles and Bogert (1944) defined a key variable for measuring the upper thermal limit of a species as the critical thermal maximum (CTMax), or the body temperature at which animals lose motor function just below lethal temperatures. If habitats become hot enough that lizards will exceed their CTmax for significant portions of the day, local extinctions are likely to occur. Yikes! “But seriously, Kat, is this something that we need to worry about? Won’t lizards just find a way to acclimate or something?” Negatory, ghostrider. In fact, a recent study showed that in 2050, nearly 18% of all species will go extinct (Thomas et. al., 2004), and within that, 20% of all lizard populations will go extinct as a result of climate change (Sinervo et al., 2010). Lizards are very sensitive to micro and macro changes in temperature, and at the rate that climate change is occurring, lizards can not readily adapt to these increasing temperatures.    

*Que dramatic end of the world simulation (push play)*

Okay, okay, yes, while the future of climate change and how this impact will exacerbate the decline in populations globally is exceptionally important, we need to understand exactly what is happening. How can we truly understand how rising global temperatures are going to speed up the extinction of our slithery and scaly friends if all we know is that if they heat up they will die? This is where the star of today’s show is introduced: The Western Fence Lizard (Sceloporus occidentalis) (fig. 1).    

Figure 1. Western Fence Lizard (Sceloporus occidentalis)

If you are a regular to the great outdoors here in the central coast of California, you might recognize these lizards scurrying across trails, between dorms at Cal Poly, or just hanging out in your garden. Because these little guys are so accessible and plentiful, researchers have been studying them and their thermal tolerance for many years! 

In fact, get this! A study done at California Polytechnic State University, San Luis Obispo showed that there was a connected between CTmax and oxygen (Shea et al., 2016). To break it down, red blood cells have a protein called hemoglobin on it. Hemoglobin binds with oxygen and is carried throughout the body to all the necessary organs and tissues that utilize it for normal functioning. When temperature increases, an organism’s metabolism is going to increase as well. In response to this, the body’s heart rate and breathing rate will increase. What’s happening is that the body’s demand for oxygen is increasing with temperature and therefore blood being pumped throughout the body is increased in order to meet that demand. In this study, Shea et al. (2016) placed S. occidentalis in a specialized machine called the Controlled Reptile Oxygen and Climate System (CROCS) (fig. 2) to measure CTmax. I know it sounds intense, but really, it’s a machine that has seven chambers (6 of them will hold lizards with temperature probes in their cloaca and 1 will have a temperature sensor for the other chambers). The chambers heat up 1 ºC/minute and while the chambers heat up, observers stand by and rotate each chamber to flip the lizard onto it’s back. Once the lizard reaches it’s CTmax, the lizard will no longer be able to flip back over. This is called a loss of righting response (LORR). Essentially, what’s happening is that the lizard has heated up far too much and the body’s supply of oxygen has can not meet the demand and muscle function begins to shut down. 

Figure 2. Schematics for Controlled Reptile Oxygen and Climate System (CROCS). A scuba tank of (1) compressed gas containing a known concentration of oxygen is routed through a first stage regulator (2) into a needle valve (3) to control the flow of air. The normally closed solenoid-actuated valve (4) controls air entering into the heating system.When the valve is open, air flow into the heater box is measured by a pressure gauge (5). Air flows into an insulated heater box (6) containing a heater (7) wrapped in aluminum tubing (8) and a digital temperature sensor (9) that monitors the box temperature. Heated air then flows through a diffuser (10) that distributes the air evenly into seven chambers (11) by silicone tubing. Each chamber contains an additional plastic diffuser (12) at the entrance to help disperse the air evenly through the chamber. The central chamber contains a temperature sensor (13) to monitor chamber temperature. Each of the six test chambers contains lizards, 14) and temperature probes (15) inserted into the lizards' cloacae to measure body temperatures. Fiber glass insulation (16) prevents heat loss from the outermost two chambers. An Arduino microcontroller (17) collects data from 13 and 15 and controls a solenoid relay (18) and a heater relay (19) to maintain the desired heating rate of 1 °C/min. Data output (20) was saved locally on a laptop. CROCS is powered via 120 V AC power (21) (Shea et al., 2016)

But wait! Theeeeere’s more! Shea et al. (2016) tested CTmax under different oxygen concentrations to test the limited oxygen hypothesis. This hypothesis, not well studied as a interaction with thermal tolerance is what was discussed earlier where researchers hypothesize that when the body is raised to high temperatures, the supply of oxygen to the body does not meet the demand and the body starts to fail. Okay, so what were the results when they tested randomly selected lizards under limited, normal, and above average oxygen concentrations? Limited oxygen concentrations (hypoxia) showed a significant decrease in LORR than normal and above average oxygen concentrations (hyperoxia) (Fig. 3)! This is significant evidence that finally shows a correlation between thermal tolerance and the oxygen limitation hypothesis. With climate change occurring in our every day lives, lizards in higher altitudes (limited oxygen) will not be able to within rising temperatures than lizards that are at sea level (normal oxygen). 

Figure 3. Effect of differing oxygen concentrations on the thermal tolerance of S. occidentalis.

Before the onset of LORR, there are two other displays that the lizards will exhibit: 1) gaping (mouth opening and closing without any change in breathing patterns) and 2) panting (mouth open and heavy lateral movement of the thoracic cavity) (Cowles and Bogert, 1944). While there appeared to be an association for a sex difference between male and female panting threshold, there were not significant sex differences for the LORR (fig. 4).

Figure 4. Females had a higher panting threshold than males for all oxygen treatments

The study of CTmax is just the beginning to understanding how climate change will impact our lizard species. Schall et al. (2010) showed that lizards naturally infected with malaria (Plasmodium mexicanum) not only affected reproduction and overall fitness, but also greatly impacted their thermal tolerance. Infection with the malaria parasite causes an overall decrease in red blood cells in the host and therefore, with a lower blood count, there is a lowered oxygen carrying capacity. It is unclear whether or not infection with P. mexicanum at higher elevations will be detrimental to more species than species at lower elevations given that oxygen is limited. Further, it is also unclear how stress affects the different stages of parasitemia and if the stress of rising global temperatures will lower CTmax. Understanding this aspect of disease and upper thermal limits will help us all understand just how critical climate change is on the world around us.    

Author: Kat Ivey

References

Cowles, R.B. and C.M. Bogert. 1944. A Preliminary Study of the Thermal Requirements of Desert Reptiles. Bulletin of the American Museum of Natural History. 83(5): 261–296.

Schall, J.J., Bennett, A.F., Putnam, R.W., Series, N., N. Sep. 2010. Lizards Infected with Malaria : Physiological and Behavioral Consequences. Science. 217(10): 1057–1059.

Shea, T.K., DuBois, P.M., Claunch, N.M., Murphey, N.E., Rucker, K.A., Brewster, R.A., and E.N. Taylor. 2016. Oxygen concentration affects upper thermal tolerance in a terrestrial vertebrate. Comparative Biochemistry and Physiology. 199: 87–94.

Sinervo, B. et al. 2010. Erosion of Lizard Diversity by Climate Change and Altered Thermal Niches. Science. 19: 894–899.

Thomas, C.D. et al. 2004. Extinction risk from climate change. Nature. 427: 145–148.