Anolis sagrei eggs in the field. Photo by Jenna Pruett.
I would like to start by apologizing for the title of this post. I couldn’t help myself. Let’s move on.
How can organisms respond to climate change? There are basically three mechanisms: move, evolve, or acclimate via phenotypic plasticity. Plasticity is potentially very powerful because it drives changes in traits without the generational time lag and population cost of natural selection. Individuals simply adjust on the fly to prevailing conditions. To find out if plasticity can help in a changing world, the following questions have to be addressed: 1) Are relevant traits plastic? and 2) if so, how plastic are they (i.e., how much can they change)?
The thermal tolerance of most organisms is plastic to some degree, and this includes anoles. For example, if you move adult A. carolinensis housed at low temperatures to warmer conditions, their heat tolerance will increase (Corn 1971). Most of the work on thermal tolerance plasticity comes from studies of “reversible” plasticity, in which the plastic trait shift can be erased. In the A. carolinensis example, moving the individuals from the warm conditions back to the original cooler conditions would be associated with a decrease in heat tolerance. Reversible plasticity in thermal tolerance is fairly weak in lizards: on average across taxa, a 1°C increase is body temperature is associated with only about a 0.1°C increase in heat tolerance (Gunderson and Stillman 2015).
Plastic shifts can also be irreversible if they are induced at the right time in the organism’s life cycle, termed “developmental plasticity.” For example, Drosophila usually have greater heat tolerance as adults when they develop under warm versus cool temperatures (MacLean et al. 2019). Overall however, relatively little is known about the presence and strength of developmental plasticity in thermal tolerance. This is especially the case in lizards. Few studies exist and, importantly for this audience, none have focused on anoles (reviewed in Refsnider et al. 2019).
Heat tolerance of adult A. sagrei that developed as embryos under different temperature regimes.
In a new paper with Dan Warner and Amelie Fargevieille, we tested for developmental plasticity in the heat tolerance of the Cuban brown anole, Anolis sagrei (Gunderson, Fargevieille & Warner, 2020). Eggs laid by females maintained in the lab were incubated under one of three different fluctuating thermal regimes (cool, warm, and hot) that mimicked temperature dynamics measured in nests in the field. Minimum temperatures of each treatment were similar, but they differed in the maximum temperatures experienced during the day. After hatching, all lizards were raised under common garden conditions until sexual maturity, at which point we measured heat tolerance. With this design, we isolated the effect of embryonic conditions on the thermal physiology of reproductive adults. As far as we know, this is the first study to use this design in a reptile system.
We found no evidence for developmental plasticity: embryonic temperature did not influence adult heat tolerance. One conclusion that might be drawn from our work is that developmental plasticity will be of little to help to anoles as the climate warms, meaning behavioral and evolutionary processes could be particularly important in dealing with changing temperatures. Additionally, developmental plasticity may play a minor role in driving observed differences in the thermal physiology of anoles from different thermal environments, making evolutionary divergence a more likely explanation.
But these inferences must be taken with a huge grain of salt. Plasticity itself evolves, and therefore what we find in one or even a few species may not be broadly representative. We will have to wait for more data to emerge to get a clearer picture of the ecological and evolutionary implications of developmental plasticity in reptile thermal traits.