A year with lizards: Egg incubation temperature has long term impacts on bearded dragons

Jun 19, 2019
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At the time of writing, it is June 2019, and I plan to submit my PhD thesis in two months…yes, in 2 months!

The last 3 years have flown by and it seems like an age since I was a MSc student in the cold-blooded cognition lab, at the University of Lincoln, investigating how subtle differences in incubation environment can influence bearded dragons.  

As a fresh faced undergraduate, I first spoke to Anna Wilkinson (my MSc research supervisor) about potential projects that I would like to do. I had envisioned monkeys. I didn’t know in what capacity, but I knew that I wanted to do something with monkeys.

I ended up working with lizards…in hindsight this was a blessing. 

To start with bearded dragons are awesome. Originally from Central Australia, bearded dragons are a social species of lizard that get their name from their large beard, which turns dark black when they are angry and/or horny. Bearded dragons, like all animals, have their own individual personalities and they are surprisingly interactive with you, the experimenter.

An angry bearded dragon
Angry bearded dragon

One bearded dragon for example really did not like men (for some reason I wasn’t included in this category, probably the long hair). She would puff up her beard and head bob which in bearded dragon language is the equivalent of saying ‘you, me, outside!’ (there was one particular security guard who used to patrol the building that she really didn’t like).

Like many lizards, bearded dragons are oviparous, meaning that they lay their eggs outside of their body. This means that their eggs are completely reliant on external factors to incubate them. Egg incubation temperature can influence reptiles in a variety of different ways and the most famous way of course is sex. For example, green turtles eggs incubated below 27°C will all be male and those incubated above 31°C will be female.

For my project I was interested in how the incubation environment influences bearded dragon behaviour and cognition. I had 14 lizards, half of which were incubated at a hotter temperature (30°C) and half a colder temperature (27°C). Egg incubation temperature does not influence bearded dragon sex determination at these incubation temperatures, and we had an even split of males and females between the ‘hot’ and ‘cold’ groups.

Green turtles have temperature dependant sex determination
Green turtles have temperature dependant sex determination

In my first experiment (which was published recently in the journal ‘Behavioural Process’ click here for link) we found that lizards incubated at a cooler incubation temperature grew significantly quicker than those incubated at hotter temperatures. Despite this, we found that lizards incubated at hotter temperatures were better at foraging, even though lizards incubated at cooler temperatures were larger.

So why is this?

Well honestly…… we don’t know. We speculate in the paper, but what I found so impressive was that these subtle differences in incubation temperature were still affecting the lizard’s behaviour 18 weeks after they had emerged!

However, our later research trumped this.

When the lizards were 10 months old we tested them in a ‘novel-object experiment’. We placed the lizards in a familiar environment with a novel object (in this case a toy fire engine #science) and recorded how much time they spend in proximity to it. This was to assess lizard ‘personality’ and in this case measured whether lizards were bold or shy. What we found was that the lizards incubated at the hotter temperatures were bolder than those incubated at cooler temperature (10 months after hatching!).

So hot lizards have personality?? (A genuine suggested paper title at one point,click here for the paper).

When the lizards were fully grown, we found that egg incubation temperature can influence lizard cognition. Lizards can socially learn from one another. In a previous experiment it was shown that lizards can open a sliding door, but only after first observing another lizard completing the task. Lizards that did not observe a conspecific successfully opening the door could not do this.

We tested our bearded dragons in the exact same experimental set up and found that the bearded dragons incubated at cooler temperatures were more successful, and quicker at opening the door than lizards incubated at hotter temperatures (click here for paper). Again, we found that a lizard’s ability to learn was different as a direct result of egg incubation temperature…a year and a half after they emerged as hatchlings (see video summary from Reuters above).

The impacts of egg incubation temperature on bearded dragon growth, behaviour and cognition are clearly long lasting!

This research is (to me a least) interesting, but it’s fairly niche and its application might not seem obvious. However, as human induced environmental change continues, subtle differences in incubation environment could have profound impacts on the phenotypes of reptiles and indeed other animals.  

Our own research here is rather limited but does show the potential for long-term impact of incubation environment on reptiles.  

Flexibility of phenotype, as a results of egg incubation environment, could be advantageous to reptiles as it can allow them to modify their behaviour or size in response to climate change. It’s also possible that climate change is occurring too quickly, meaning reptiles and other animals will not be able to adapt to changing environments.

Ultimately, we don’t know what the impact of climate change on reptiles will be. Could climate change reptile phenotypes without us even noticing?

Harry Siviter

PhD, Royal Holloway University

1 Comments

Harshad Mayekar 28 days ago

May sound like a silly question, but did you test some lizards which were not given the necessary training but observed for acquiring the meal worms? In other words are there any exploratory studies for novel objects in lizards? Your work is really interesting, great!