̽��ֱ�� of Cambridge - camouflage

Lizards camouflage themselves by choosing rocks that best match the colour of their backs

jeh98 — Mon, 25 Jan 2016 10:01:00 +0000

Resting out in the open on rocks can be a risky business for Aegean wall lizards. Out in these habitats they have nowhere to hide and their backs, which show varying shades of green and brown between individuals, are dangerously exposed to birds hunting in the skies above.

New research by Kate Marshall from the ̽��ֱ�� of Cambridge’s Department of Zoology and Dr Martin Stevens from the ̽��ֱ�� of Exeter’s Centre for Ecology and Conservation, published today in Scientific Reports, shows that individual lizards are able to choose their resting spot wisely and select a rock in their natural environment that will make their backs less conspicuous to avian predators.

“This suggests that wild individual lizards can choose to rest on the rock they will most resemble, which enhances their own degree of camouflage against visually-oriented predatory birds,” says Marshall. “This is the first result of its kind in wild animals, and in lizards specifically.”

“One intriguing puzzle remains: how do the lizards ‘know’ how camouflaged their own backs are to a bird against a particular rock?” She adds.

Other types of lizard, such as chameleons and geckos, are able to rapidly change colour in a matter of seconds or minutes to better match their background environment and avoid being spotted by approaching predators. Aegean wall lizards, which are widespread across the South Balkans and many Greek islands, are unable to do this. Instead, this new research shows that they enhance their level of camouflage to hunting birds by choosing to rest on rocks that are more similar in colour to that of their own backs.

Birds see the world differently from you or I: for example, they are able to see ultraviolet light whereas we cannot, which means they perceive colour (and camouflage) in a very different way. Marshall and her colleagues used visual modelling to test how conspicuous individual lizards would be to a bird’s eye against the backgrounds they had chosen to sit on.

Marshall and her field assistant and co-author, Kate Philpot, found that on each island individuals showed better colour matching against their own chosen rock backgrounds than against other lizards’ rock backgrounds, as perceived by avian predators such as the crows and raptors abundant in their study sites.

“This strongly suggests that lizards rest on backgrounds that heighten their own camouflage to reduce the risk of being attacked by birds, and that individual behaviours have an important role in enhancing camouflage across different microhabitats,” says Marshall. “Our findings appear to be the first demonstration of this occurring in wild populations as viewed by likely predators.”

̽��ֱ��researchers also found that lizards’ resting site choices that heightened individual camouflage were more evident on islands with higher numbers of predatory bird species, suggesting that this behavioural defence is more likely to evolve in riskier environments. It was also more apparent in female lizards, probably because males have a conflicting need to stand out against the rocks to attract mates.

“As for the puzzle over how the lizards ‘know’ how camouflaged their own backs are against a particular rock - one theory is that it is under genetic control, while another possibility is that it develops in early life through learning from other lizards and from experience,” says Marshall.

“Although we don’t know what the exact mechanism is yet, we hope to uncover some clues in future research. It would also be interesting to look at whether lizards can adjust their choice of rock not just for camouflage but also to aid thermoregulation (basking site choices) and sexual signalling,” she adds.

“Our study shows that there is much more to camouflage than just an animal’s appearance - how individuals behave and what backgrounds they choose to sit on can have a major bearing on how effective their camouflage will be. This is something that needs much more research in future,” says Stevens.

This research shows that individual animals’ behaviours can increase their chances of survival by allowing flexible, real-time adjustments to the many different microhabitats encountered in the wild. Marshall suggests that it also emphasises the importance of considering broader environmental contexts, such as predation risk, as well as the perceptual abilities of natural observers like predators in studies of animal behaviour.

Reference:

Marshall, K et al. ‘Microhabitat choice in island lizards enhances camouflage against avian predators’ Scientific Reports 25 January 2016.

Inset images: Aegean wall lizards resting on rocks (Kate Marshall).

New research shows wild Aegean wall lizards found on Greek islands choose to sit on rocks that better match their individual colouring. This improves camouflage and so reduces the risk of being attacked by birds when they sit out in the open, raising the intriguing question of how the lizards know what colour they are.

One intriguing puzzle remains: how do the lizards ‘know’ how camouflaged their own backs are to a bird against a particular rock?

Kate Marshall

An Aegean wall lizard resting on a rock

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Love’s Labours: study shows male lizards risk becoming lunch for a bird in order to attract a mate

jeh98 — Tue, 22 Sep 2015 14:47:17 +0000

In the animal kingdom, the flashiest males often have more luck attracting a mate. But when your predators hunt by sight, this can pose an interesting problem.

Like many species, lizards use bright colours for sexual signalling to attract females and intimidate rival males. A new study published in Ecology and Evolution by Kate Marshall from the ̽��ֱ�� of Cambridge’s Department of Zoology and Martin Stevens from the ̽��ֱ�� of Exeter’s Centre for Ecology and Conservation has provided evidence that this signalling comes at a cost.

Using models that replicated the colouration of male and female wall lizards found on the Greek islands of Skopelos and Syros, they found that the male lizard models were less well camouflaged against their habitat and more likely to fall prey to bird attacks.

Marshall, lead author of the study, explains: “we wanted to get to the origins of colour evolution; to find out what is causing colour variation between these lizards. We wanted to know whether natural selection favours camouflage, and whether the conflicting need to have bright sexual signals might impair its effectiveness.

“It has previously been assumed that conspicuous male colours are costly to survival, but this hasn’t been tested before among these specific lizards living on different islands, and in general rarely in a way that takes into account the particular sensitivities of avian vision.”

Birds see the world differently from you or I: they are able to see ultraviolet (UV) light whereas we cannot, which means they perceive colour (and camouflage) in a very different way. To test whether the males really are more visible to feathered predators, the researchers had to develop clay models that accurately replicated the lizards’ colour to a bird’s eye.

Using visual modelling, Marshall and her colleagues painstakingly tested around 300 colour variations to find ones that matched the male and female colours in order to make the 600 clay lizards used in the study.

Marshall comments: “it was important to get a clay colour that would be indistinguishable from a real lizard to a bird’s eyes: we even tried using a paint colour chart, but they all reflected too much UV. To us the models may not look like very good likenesses, but to a bird the models should have looked the same colour as the real lizards.”

Marshall and her field assistant, Kate Philpot, placed the male and female lizard models in ten sites on each of the two islands and checked them every 24 hours over five days to see which had been attacked by birds.

“ ̽��ֱ��models that had been attacked showed signs of beak marks, particularly around the head, and some had been decapitated,” explains Marshall. “We even found a few heads in different fields to the bodies.”

“ ̽��ֱ��fact that the birds focused their attacks on the heads of the models also shows us that they perceived them as real lizards because that is how they would attack real prey,” she adds.

At the end of the study, the researchers found that the models with male colouration had been attacked more than the models with female colouration.

Marshall and the team also tested how conspicuous the models were against their real backgrounds using further modelling of avian vision, and found that the male models were less camouflaged than the females.

“In females, selection seems to have favoured better camouflage to avoid attack from avian predators. But in males, being bright and conspicuous also appears to be important even though this heightens the risk of being spotted by birds,” says Marshall.

However, it is not entirely a tale of woe for the male Aegean wall lizard. Despite being attacked more than the females by predatory birds, 83% of the male lizard models survived over the course of the five-day experiment. Marshall explains that this may indicate that males have colour adaptations that balance the contradictory needs to attract a mate and to avoid becoming lunch.

“In past work we’ve found these lizards have evolved bright colours on their sides, which are more visible to other lizards on the ground than to birds hunting from above,” explains Marshall. “ ̽��ֱ��visual system of lizards is different again from birds, such as through increased sensitivity to UV, so the colour on their backs is more obvious to other lizards than to birds. Such selective “tuning” of colours to the eyes of different observers might provide at least some camouflage against dangerous predators that sneakily eavesdrop on the bright signals of their prey.”

“With these models we were only able to replicate the overall colour of the lizards rather than their patterns, so it would be interesting to investigate further whether these patterns affect the survival rates of lizard models,” she adds. “It would also be great to apply this type of experiment to other questions, such as how different environments affect the amount of predation that prey animals experience.”

Reference: Marshall, K et al. “Conspicuous male coloration impairs survival against avian predators in Aegean wall lizards, Podarcis erhardii” Ecology and Evolution (September 2015). DOI: http://onlinelibrary.wiley.com/doi/10.1002/ece3.1650/full

̽��ֱ��research was enabled by funding from the Biotechnology and Biological Sciences Research Council, the British Herpetological Society, the Cambridge Philosophical Society, and Magdalene College, Cambridge.

Inset images: Tetrahedral plot of avian vision (Kate Marshall et al); Models showing signs of bird attack (Kate Marshall et al); Males, females and their corresponding models (Kate Marshall et al).

New research shows male lizards are more likely than females to be attacked by predators because the bright colours they need to attract a mate also make them more conspicuous to birds.

̽��ֱ��models that had been attacked showed signs of beak marks, particularly around the head, and some had been decapitated

Kate Marshall

Female (left) and male (right)

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Predators might not be dazzled by stripes

fpjl2 — Wed, 12 Aug 2015 09:22:43 +0000

Stripes might not offer protection for animals living in groups, such as zebra, as previously thought, according to research published today in the journal Frontiers in Zoology.

Humans playing a computer game captured striped targets more easily than uniform grey targets when multiple targets were present. ̽��ֱ��finding runs counter to assumptions that stripes evolved to make it difficult to capture animals moving in a group.

“We found that when targets are presented individually, horizontally striped targets are more easily captured than targets with vertical or diagonal stripes. Surprisingly, we also found no benefit of stripes when multiple targets were presented at once, despite the prediction that stripes should be particularly effective in a group scenario,” said Anna Hughes, a researcher in the Sensory Evolution and Ecology group and the Department of Physiology, Development and Neuroscience.

“This could be due to how different stripe orientations interact with motion perception, where an incorrect reading of a target’s speed helps the predator to catch its prey.”

Stripes, zigzags and high contrast markings make animals highly conspicuous, which you might think would make them more visible to a predator. Researchers have wondered if movement is important in explaining why these patterns have evolved. Striking patterns may confuse predators and reduce the chance of attack or capture. In a concept termed ‘motion dazzle’, where high contrast patterns cause predators to misperceive the speed and direction of the moving animal. It was suggested that motion dazzle might be strongest in groups, such as a herd of zebra.

‘Motion dazzle’ is a reference to a type of camouflage used on ships in World Wars One and Two, where ships were patterned in geometric shapes in contrasting colours. Rather than concealing ships, this dazzle camouflage was believed to make it difficult to estimate a target's range, speed and heading.

HMS Argus (1917) wearing dazzle camouflage.

A total of 60 human participants played a game to test whether stripes influenced their perception of moving targets. They performed a touch screen task in which they attempted to ‘catch’ moving targets - both when only one target was present on screen and when there were several targets present at once.

When single targets were present, horizontal striped targets were easier to capture than any other target, including uniform colour, or vertical or diagonal stripes. However, when multiple targets were present, all striped targets, irrespective of the orientation, were captured more easily than uniform grey targets.

“Motion may just be one aspect in a larger picture. Different orientations of stripe patterning may have evolved for different purposes. ̽��ֱ��evolution of pattern types is complex, for which there isn’t one over-ruling factor, but a multitude of possibilities,” said Hughes.

“More work is needed to establish the value and ecological relevance of ‘motion dazzle’. Now we need to consider whether colour, stripe width and spatial patterning, and a predator’s visual system could be important factors for animals to avoid capture.”

Anna Hughes has written a blog post on this research for the journal publisher BioMed Central. Above story adapted from a BioMed Central press release.

New research using computer games suggests that stripes might not offer the ‘motion dazzle’ protection thought to have evolved in animals such as Zebra and consequently inspired ship camouflage during both World Wars.

Motion may just be one aspect in a larger picture. Different orientations of stripe patterning may have evolved for different purposes

Anna Hughes

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Zebras, Serengeti National Park, Tanzania

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