̽��ֱ�� of Cambridge - predation

What did Megalodon eat? Anything it wanted — including other predators

sc604 — Thu, 23 Jun 2022 16:32:36 +0000

New research involving the ̽��ֱ�� of Cambridge shows that prehistoric megatooth sharks — the biggest sharks that ever lived — were the ultimate top predators, operating higher up the food chain than any other marine predators through history.

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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Motion dazzle: spotting the patterns that help animals outsmart predators on the run

Anonymous — Wed, 09 Sep 2015 10:34:48 +0000

Many animals use the colours and patterns on their bodies to help them blend into the background and avoid the attention of predators. But this strategy, crypsis, is far from perfect. As soon as the animal moves, the camouflage is broken, and it is much easier for a predator to see and catch it. So how do animals protect themselves when they’re on the move?

Researchers are exploring whether high-contrast patterns during motion, such as stripes and zigzags, may be distorting the predator’s perception of where the animal is going. But, as little is known about such “motion dazzle”, we have built an online game to help shed light on it.

Lessons from war

̽��ֱ��idea is that it may be more effective for animals to focus on preventing capture, rather then preventing detection or recognition, is actually more than 100 years old. It was naturalist Abbott Thayer who suggested that high-contrast patterns may distort the perceived speed or direction of a moving object, making it harder to track and capture.

Such motion dazzle patterns were actually used in World War I and II, where some ships were painted with black and white geometric patterns in an attempt to reduce the number of successful torpedo attacks from submarines. However, due to many other factors affecting wartime naval losses, it is unclear whether motion dazzle patterns actually had the desired effect.

HMS Argus displaying a coat of dazzle camouflage in 1918. wikimedia

What about the natural world? Zebras have bold stripes, and scientists have debated the function of their patterns since Darwin’s time. A recent modelling study suggested that when zebras move, their stripes create contradictory signals about their direction of movement that is likely to confuse predators. There are potentially two visual illusions responsible for this, which could form the basis of motion dazzle effects: the wagon wheel effect and the barber pole illusion.

̽��ֱ��wagon wheel effect is named after Western movies, where the wheels on wagons often appear to be moving backwards. This is because the visual system takes “snapshots” over time and links them to create a continuous scene, in the same manner as recording film. If a wheel spoke moves forward rapidly between sampling events, it will appear to have moved backwards as it will be misidentified as the following spoke.

Wagon Wheel effect explained.

̽��ֱ��barber pole illusion (also known as the aperture effect) occurs because the moving stripes provide ambiguous information about the true direction of movement. These illusory effects produced by stripes could therefore lead to difficulties in judging the speed and movement of a moving target. However, the zebra study was entirely theoretical and didn’t test whether striped patterns actually affected the judgements of real observers.

Dazzle Bug

Surprisingly, the first experimental tests of the effectiveness of motion dazzle patterns weren’t carried out until recently. Some studies have shown that strikingly patterned targets can be more difficult to catch than targets with other patterns in studies using humans as “predators” playing touch screen computer games. However, other studies have found no clear advantage for motion dazzle patterns So although patterns can affect our perception of movement, it’s still not clear which are most effective at doing so.

Can you see the spider? Crypsis can be pretty effective - as long as you don’t move. J Kelley, Author provided

We are addressing the question of which patterns are best for avoiding predators during movement using Dazzle Bug – an online game that asks players to imagine themselves as a predator, trying to catch a moving bug as fast as possible. Each bug has a different body pattern as well as a random pattern of movement. Bugs with easy to catch patterns will disappear, whereas those that are particularly tricky to catch will survive ––just like in nature. Over time, the patterns on the bugs' body will evolve so that they become harder to catch with each successive generation.

This citizen science project will allow us to see what patterns are most effective at evading capture. We can then use these results to look at what visual effects these patterns have, and to see whether these patterns match up with those found on real animals in the wild.

Our findings will offer insight into the role of stripes, which are common in many species. While these patterns may have evolved to confuse the visual perception of a predator, they may also be a result of other selection pressures, such as attracting a mate or regulating body temperature. If striped patterns survive and evolve in the game, this would provide strong evidence that these patterns do act to confuse human predators, perhaps by producing the illusions described above. As motion perception seems to be highly conserved across a wide range of populations, these illusions may occur for many other predators too.

If we find that patterns other than stripes – such as speckles, splotches or zigzags – are most effective in preventing capture, this then leads to new and interesting questions about how these patterns may act to confuse or mislead. Whatever the outcome, Dazzle Bug will provide insight into how bodily patterns may have evolved to help animals to survive life on the go.

Laura Kelley, Research Fellow, ̽��ֱ�� of Cambridge

This article was originally published on ̽��ֱ��Conversation. Read the original article.

̽��ֱ��opinions expressed in this article are those of the individual author(s) and do not represent the views of the ̽��ֱ�� of Cambridge.

A new online game is helping researchers explore whether high-contrast patterns during motion, such as stripes and zigzags, help to protect animals from predators.

Dazzle Bug asks players to imagine themselves as a predator, trying to catch a moving bug as fast as possible

Laura Kelley

Eric Dietrich/wikimedia

Zebras on the run can razzle-dazzle their enemies

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Here’s looking at you: research shows jackdaws can recognise individual human faces

jeh98 — Tue, 11 Aug 2015 15:28:20 +0000

Researchers Alex Thornton, now at the ̽��ֱ�� of Exeter, and Gabrielle Davidson carried out the study with the wild jackdaw population in Madingley village on the outskirts of Cambridge. They found that the jackdaws were able to distinguish between two masks worn by the same researcher, and only responded defensively to the one they had previously seen accessing their nest box.

Over three consecutive days Davidson approached the nest boxes wearing one of the masks and took chicks out to weigh them. She also simply walked past the nest boxes wearing the other mask. Following this she spent four days sitting near the nest boxes wearing each of the masks to see how the jackdaws would respond.

̽��ֱ��researchers found that the jackdaws were quicker to return to their nest when they saw the mask that they had previously seen approaching and removing chicks to be weighed, than when they saw the mask that had simply walked by.

They also tended to be quicker to go inside the nest box when Davidson, wearing the mask, was looking directly at them rather than looking down at the ground.

“ ̽��ֱ��fact that they learn to recognise individual facial features or hair patterns so quickly, and to a lesser extent which direction people are looking in, provides great evidence of the flexible cognitive abilities of these birds,” says Davidson. “It also suggests that being able to recognise individual predators and the levels of threat they pose may be more important for guarding chicks than responding to the direction of the predator’s gaze.”

“Using the masks was important to make sure that the birds were not responding to my face, which they may have already seen approaching their nest boxes and weighing chicks in the past,” she adds.

Previous studies have found that crows, magpies and mockingbirds are similarly able to recognise individual people. However, most studies have involved birds in busier urban areas where they are likely to come into more frequent contact with humans.

Jackdaws are the only corvids in the UK that use nest boxes so they provide a rare opportunity for researchers to study how birds respond to humans in the wild. Researchers at Cambridge have been studying the Madingley jackdaws since 2010.

“It would be fascinating to directly compare how these birds respond to humans in urban and rural areas to see whether the amount of human contact they experience has an impact on how they respond to people,” says Davidson.

“It would also be interesting to investigate whether jackdaws are similarly able to recognise individuals of other predator species – although this would be a lot harder to test.”

̽��ֱ��study was enabled by funding from Zoology Balfour Fund, Cambridge Philosophical Society, British Ecological Survey, and BBSRC David Philips Research Fellowship.

Inset images: Mask (Elsa Loissel).

Reference:

Davidson, GL et al.,Wild jackdaws, Corvus monedula, recognize individual humans and may respond to gaze direction with defensive behaviour Animal Behaviour 108 October 2015 17-24.

When you’re prey, being able to spot and assess the threat posed by potential predators is of life-or-death importance. In a paper published today in Animal Behaviour, researchers from the ̽��ֱ�� of Cambridge’s Department of Psychology show that wild jackdaws recognise individual human faces, and may be able to tell whether or not predators are looking directly at them.

̽��ֱ��fact that they learn to recognise individual faces so quickly provides great evidence of the flexible cognitive abilities of these birds

Gabrielle Davidson

Richard Woods

Jackdaws on nest box

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