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Why do male Peninsular rock lizards change colour?!
Remember our previous article on male peninsular rock lizards? (Click here if you haven't!) We had seen that males change colour to yellow on their backs and red on their sides (fighting colours) to display aggression towards other males. On the other hand, to catch the attention of females, they change colour to bright orange on their backs and jet black on their sides (courtship colours). Such bright colours could advertise the quality of males and help females to choose the best male of the lot.
Male peninsular rock agamas (lizards) in different colourations
However, don’t such bright colours also attract predators? Is there a risk to displaying these bright colours, and if there is, how have these lizards slithered their way out of it?
Dr. Madhura Amdekar and Dr. Maria Thaker of the Indian Institute of Science wanted to study the risks of predation associated with each male colouration. First, they wanted to know which of the male colourations were the most conspicuous to the predators; and second, they wanted to understand lizards in which colourations were most likely to be predated upon in the field.
Which colourations are easier to spot?
Before going forward, it’s important to note that animals don’t see colours the way we humans do. This means that colours which stand out for us may not necessarily stand out for other animals. Moreover, rock lizards are found on a variety of surfaces like grey walls, brown soil, and red rocks. Each of these background colours may contrast differently with the colours of lizards. For example, a grey lizard will camouflage perfectly against grey walls, but it might stand out on red rocks!
The background colours will contrast differently with lizard colours. Here, to us, the female has blended in perfectly on the grey rock, while the orange-black male stands out!
Hence, it was important for the researchers to first understand how predators perceive the different colourations of these lizards, and which colourations were the most likely to catch the attention of predators against different backgrounds. To understand this, they measured/quantified the colours in the lab and then tested their predictions on field.
First, they caught male and female lizards in the field and brought them to the laboratory. When the males were allowed to hang out around the females, they displayed their courtship colours and turned orange - black. Researchers recorded these colours using a spectrophotometer.
To measure the fighting (yellow-red) colours, researchers held the males in their hands for a few minutes. The stress caused by the handling makes males display their fighting colours just as aggression does. For comparison, researchers also measured the default dull brown colour of the females.
When these measurements were analysed using models of predator vision, it was confirmed that the colours of courtship, that is, orange and black colour contrasted the most with each other. The courtship colour state (orange-black) was therefore the most conspicuous for predators (dogs, raptors, snakes) and lizards, irrespective of the background colour. On the other hand, fighting colour (yellow-red) and the female brown was the least conspicuous. This means that when males display the courtship colours, it not only makes it easy for females to spot and stalk them, but they also catch the eyes of predators!
What are the risks of predation in the wild?
These results sound ideal in theory. However, what really happens in the wild? How do we determine the risk associated with a colouration in the natural environment? Any theoretical study is best confirmed by performing experiments in the field.
"How do the colours play out in the field!?" This is one of the field sites of this study
To understand this, researchers came up with the ingenious idea of making wax models of the rock lizards representing all colour states. They made male models of the 3 colour states: courtship colour (black – orange), fighting colour (yellow-red), and female brown. They also made female models, also in the regular female brown. The female model was made to determine whether predators discriminate based on size difference, given that males are almost twice the size of females!
The 4 different types of wax models: 3 male models and a female model (L-R) Picture by: Anuradha Batabyal
Location of the 6 selected field sites in and around Bangalore
The researchers made hundreds of these wax models and set them out in the field. They selected 6 sites in and around Bangalore where rock lizards are regularly found. Between March and April 2018, they visited each site for 5 consecutive days. On the morning of each field day, 15 models of each of the four types was left at the selected site on different surfaces. So each site had 60 models overall. Since the models were placed out in the field, the natural predators of agamas had access to them and hence, opportunities to attack. Additionally, because these were wax models, any injuries were easily imprinted and noticeable. The researchers returned every evening to retrieve the models and note which models had been attacked.
Wax models in courtship colour (left) and aggression colour (right) placed on field. Strings were used to anchor the models to the ground (Pictures: Pritha Kundu)
It seems odd to expect predators to attack the models – it’s like expecting people to shoot at mannequins. But just like we might get fooled by pillows under blankets, many models had actually been moved or bitten! In fact, a couple of them were even bitten by other agamas (males compete and can bite each other during combat)!
Wax models attacked by predators and other lizards. Black circles show the damaged parts of the models. Pictures: Pritha Kundu
As expected, the models with the courtship colours were attacked the most by predators. Surprisingly, the predators didn’t care about what surface the models were placed on! And they didn’t care about size either! All that mattered was the contrast within the colourations of the models. That is, the contrast between the orange and black of the courtship colours, and that between the yellow and red of the fighting colours!
Why did agamas evolve the ability to change colour?
Using conspicuous colours to attract mates is a recurring phenomenon across the animal kingdom. However, there is often a cost associated with this: it attracts predators. There are many solutions to this problem - solutions that have evolved differently, over time, in various species of animals. One such solution, as seen in the male rock lizards, is the ability to change colours!
A male changing colour in presence of a female (Original duration of video: 5 mins). To know more about this exciting colour changing behaviour and other acrobatics shown by peninsular rock lizards, check our article here
Researchers believe that this ability to change colours has evolved in order to get the best of both worlds: more mates and fewer predators. This means that male rock lizards display their conspicuous courtship colours only when there are females around, to attract their attention for mating. And when it’s time to avoid being killed by predators, they switch to colours which predators find difficult to detect! Thus, they are likely to change colour to not only reap the benefits of courtship colours, but also to minimize the costs of predation. And so, the rock agamas and their dynamic colours have proved to be yet another wonder of the blind process of evolution!
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Source: Amdekar, Madhura S., and Maria Thaker. "Risk of social colours in an agamid lizard: implications for the evolution of dynamic signals." Biology letters 15.5 (2019): 20190207.
Link to research paper: https://royalsocietypublishing.org/doi/pdf/10.1098/rsbl.2019.0207
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Article contributed by Prahlad Saldanha, edited by Nikita Gupta, Devica Ranade.
Huge thanks to the first author of the paper, Madhura Amdekar, for making sure the facts in the article are accurately represented.
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