Reference: Abayarathna T., Webb J.K. (2020). Effects of incubation temperatures on learning abilities of hatchling velvet geckos. Animal Cognition 23, 613 – 620.

For most of us, the summer heat represents an annoyance and an excuse to take a break and go swimming. A heat wave is unlikely to have long-lasting effects on our lives. But what about animals that are sensitive to changes in temperature? Could high temperatures during development –a critical period in an animal’s life–cause changes in adult behavior? Because an animal’s brain is still growing while it is in an egg or a uterus, changes in its environment will likely influence brain growth and later behavior. Research by Theja Abayarathna and Jonathan Webb suggests that for velvet geckos, high heat while in the egg results in hatchlings that struggle to learn.

Why did Abayarathna and Webb decide to study velvet geckos? Lizards such as geckos are used to study how the developmental environment impacts adult behavior due to characteristics of their life history, or the species-typical experiences an organism undergoes. Because the females of most lizard species abandon their eggs after laying them, scientists can attribute any effects on hatchling behavior to the physical environment rather than parental care. In addition, lizard eggs are often laid in shallow nests that are sensitive to changes in temperature in the environment. As a result, lizards are often subjected to variations in temperature, including heat waves. This makes Abayarathna and Webb’s experiment examining how temperature affects adult behavior ecologically valid: it is something that could actually occur in the wild.

Many studies have examined how developmental temperature affects the physical characteristics, behavior, and even sex of reptiles; few have looked at how cognition is impacted. Abayarathna and Webb were interested in how temperature might affect the ability of hatchling velvet geckos to learn shelter locations that could be used to hide from predators. Impairments in this ability would have drastic impacts on the population as a whole: individuals who had difficulty learning how to find shelter would likely be eaten in the wild.

Raising temps and learning shelters

Abayarathna and Webb used the offspring of velvet geckos caught from two areas in Australia: Nowra and Dharawal National Park. Because these two areas vary in their summer temperatures and number of  heat waves, hatchlings from one area may be more resilient to the heat than hatchlings from the other.

Eggs were raised in one of two incubation conditions: warm and hot. The warm condition was meant to mirror temperatures that would be naturally found in the wild and included some intermittent heat waves. The hot condition was 2 – 3℃ higher than the warm condition, to simulate the temperatures expected by 2050 as a result of global warming.

Hatchlings were tested in the learning paradigm when they reached three to four weeks old. The learning paradigm was conducted in a Y-maze, a standard, Y-shaped behavioral test often used in animal cognition studies. Hatchlings were placed in the bottom stem of the Y-maze and given the choice to go to either arm. One arm of the Y contained an open, accessible shelter that the hatchling could enter, whereas the other had a closed, inaccessible shelter.

To simulate a predator attack, the researchers tapped the tail of the gecko with a paintbrush causing the gecko to run down the stem of the Y into (hopefully) the open shelter. This procedure was repeated for ten trials, giving the hatchlings time to learn the location of the open shelter. If a gecko learned the shelter location, they would quickly choose and run down the correct arm. In comparison, geckos with learning impairments would be slower and make more mistakes: a result that, in the wild, would likely lead to the gecko remaining exposed to the hungry eyes of predators. 

Too hot to learn

Geckos in the hot condition were worse at learning the shelter location than those in the warm condition. The researchers found no difference between hatchlings from different geographical locations, suggesting there was no benefit from having ancestors that lived in a warmer environment. These results suggest that global warming and increasing temperatures appear to be moving too fast for evolution to keep up. 

Why would an increase in heat while in the egg affect learning ability three weeks after hatching? A large amount of brain development occurs when vertebrates (mammals, reptiles, amphibians, birds) are either in utero or in eggs. For reptiles in particular, incubation temperature seems to affect pathways that control brain development. Many reptiles have temperature-dependent sex determination – the temperature at incubation determines the sex of an individual and its corresponding brain structures. Temperature also appears to affect the structure and activity of brain regions important in learning and social behaviors. Incubation at these higher temperatures likely changed the actual brain structure of the velvet geckos, ultimately impacting their ability to learn.

Life in a warming world

A key component of cognition is the ability to adapt to new situations. As global warming causes temperatures to increase across the world, animals will find themselves in novel and volatile environments. Individuals with flexible and quick cognitive abilities will be better able to adapt to the shifts in resource availability and number of predators. 

Unfortunately, this research suggests that velvet geckos will be at a disadvantage in an ever-warming world. As temperatures increase, they will be less able to learn and adapt to their new environment, potentially even leading to their extinction. How will other reptiles that are affected cognitively by temperature fare in this new world? As we enjoy our time in the sun this summer, we should stop and think about how this latest heat wave may be changing the mind, behavior, and future of our reptilian friends.

Images:

1. Wikipedia Commons. Accessed 08.10.2020. Link.
2. Created by C. Finton on 08.10.2020.