Devi Stuart-Fox is an evolutionary biologist at the University of Melbourne who has spent the past decade investigating colour change in lizards. She's now leading an international team of researchers in an investigation into how they achieve colour change, why they do it, and what it costs them. I had a chat to her about what she's discovered, and what she hopes to discover, about the incredibly complex language of colour change in nature.
Your research showed that the primary use of colour change in chameleons isn't camouflage. What is it?
There are a lot of species of chameleon, and some have really advanced capacities for colour change, and in others it's really very limited. So the big question was, why has this ability evolved to such a remarkable degree in some species and not in others?
What we were able to show is the species that do change colour the most actually have the most conspicuous social displays. So there's been selection for them to use these really bright, flashy colours in their communication and social displays, therefore there was selection on their ability to change colour. We suggested that the ability to change colour evolved for that reason, rather than simply for camouflage. Because why change colours, why not just be very camouflaged against your backgrounds?
We were wondering whether the ability to change colour could be related to the number of different backgrounds they would need to match, or the particular habitat they are found in. We found that there is no relationship, so that's why we said it hasn't been driven by camouflage. They obviously use it for camouflage - they match very well - but it's a limited range of colours they have to match. Whereas in their social displays when they change colours, they've got pinks and oranges, greens and blues, and a wide range of ultraviolet colours that are visible to chameleons but not to us.
Do bearded dragons use colour change in a similar way?
Females use colour to signal whether they'll accept courtship or aggressively reject male advances, and the males use it in territorial displays and also in courtship displays to female. We don't know what specific colours mean, in some cases dark colours can be a symbol of submission, but in other cases, like the black beard in [Central bearded dragon] males, it's a symbol of dominance. So different colours mean different things in different contexts.
It's interesting that the female dragons have developed a specific mechanism based on a colour signal to ward off the males.
I'm particularly interested in precisely that. I've been studying it in the Lake Eyre dragons - they occur only in the Lake Eyre salt pan, which is the most barren habitat you could be in. Usually it's the males that are a brighter version of the females, but in this species both sexes are really well matched to their backgrounds, and in the breeding season, the females develop really bright patches of orange on their bellies. What was in the literature before was that these patches develop when the females are rejecting the males, but what we showed was that the patches developed when they're receptive, and the males see the orange on the female's throat and absolutely harass them.
But the females are only receptive for a very brief period of time, and the males will just continuously harass them. So to escape from that, the females will, as a last resort, flip onto their backs, [which means] they can't actually mate, and they show these bright orange belly patches. The belly patches don't seem to attract the attention of predators, because firstly, predators are quite rare out on these salt pans, and secondly, we showed through experiments that predators seem to avoid flipped over females with bright orange patches because they've never seen anything like it. They don't even recognise them as potential prey. And when the females are pregnant, the size of the patches increase. So they can use these orange colours to signal that they're receptive, or when they combine it with the flipping over behaviour, to signal that they're not receptive, to try and avoid harassment. So the colours can be used in very different contexts when they're combined with different behaviours.
What's the energy cost for changing colours, especially quickly? Surely it's got to be pretty significant?
It's a big unknown. No one's tried to quantify the metabolic or physiological costs of colour change. We know that it involves the movement of pigments in the cells, and that can be controlled by hormones, and in chameleons it's controlled directly by signals from the brain by neurotransmitters. But it's very hard to test because what you have to do is have lizards that are changing colour a lot and look at things like metabolic rate or loss of body condition versus lizards that aren't allowed to change colour at all. We assume that it's costly, because otherwise, why hasn't it evolved more widely?
What are some of the strangest things you've encountered in the field?
I love my job because I feel like it takes me to all sorts of interesting parts of the world. I'm just totally fascinated by all the amazing ornaments and colours that you see in nature, every time you watch a wildlife documentary, there are just so many bizarre ornaments and behaviours and that's what really grabs me and I've been lucky enough to make a career studying it.
In lizards one of the most amazing things are the gliding lizards that you get through Southeast Asia with rib cages that extend out from their sides and are covered by a gliding membrane so they can glide phenomenal distances. I got a National Geographic grant to work on those gliding lizards because they have quite colourful wings and dewlaps - which is the male throat fan that they signal madly to each other with. You get lots of species co-occurring in the gliding lizards, so it could have to do with recognising members of their own species. It could be also to do with - and this is what we're testing - some colours working better as a signal in different habitats. A green dewlap in a green forest doesn't work very well if you're trying to draw the attention of another lizard, and in fact, ultraviolet colours don't work so well because there's not much ultraviolet light in deep, dark forests. Whereas ultraviolet colours and black, work really well in open environments and things like reds, oranges and yellows work really well in closed environments, because the forest environments are quite rich in light in those wavelengths.
It sounds like colour change is an incredibly complex language.
It's hard to know what a lizard understands from it, it's a little bit of a tip of the iceberg, [but] we can get an inkling through pretty labour intensive experiments!
You recently received the 2013 L’Oréal-UNESCO International Special Fellowship for mid-career female scientists, what do you plan to do with it?
It was a big surprise, actually. It's a prize, so it's not specifically research money. I've got two small kids, and I do quite field-intensive research, so I drag my kids along on field trips, and my husband often comes too. So our plan is to use the money to help with those costs. I'm very lucky to be able to do that, I prefer not to be away from my kids for extended periods, so it is a real luxury to be able to take them on field trips. It really struck me when I was in France for the Women in Science Week, there were 15 recipients of the Early Career Fellowships, and they're mostly women from developing countries going to do a fellowship in developed countries and five of the 15 already had kids, and several of them were leaving them behind with their [grand]parents. There's so much talk about combining career with family and the reality is that it doesn't really happen for a lot of women. I just feel really fortunate to have been able to combine family and career without having to be away for my kids for long periods.
Chen, I., Stuart-Fox, D., Hugall, A., & Symonds, M. (2012). SEXUAL SELECTION AND THE EVOLUTION OF COMPLEX COLOR PATTERNS IN DRAGON LIZARDS Evolution, 66 (11), 3605-3614 DOI: 10.1111/j.1558-5646.2012.01698.x
Jessop, T., Chan, R., & Stuart-Fox, D. (2009). Sex steroid correlates of female-specific colouration, behaviour and reproductive state in Lake Eyre dragon lizards, Ctenophorus maculosus Journal of Comparative Physiology A, 195 (7), 619-630 DOI: 10.1007/s00359-009-0437-4
Stuart-Fox, D., & Moussalli, A. (2008). Selection for Social Signalling Drives the Evolution of Chameleon Colour Change PLoS Biology, 6 (1) DOI: 10.1371/journal.pbio.0060025
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