Colour-changing chameleons evolved to stand out, not blend in

Blogging on Peer-Reviewed ResearchChameleons aren’t exactly known for being showy. Indeed, they are so synonymous with blending in that we use the term ‘social chameleon’ to refer to people who are at home in any social setting. But new research suggests that this reputation needs a rethink. The chameleon’s ability to change colour evolved not to blend in, but to stand out.

Chameleon headChameleons are a group of small lizards that are almost synonymous with camouflage. Common folklore has it that their vaunted ability to change their skin colour allows them to go undetected in a variety of environments.

Certainly, their default colours match their surroundings well. But Devi Stuart-Fox and Adnan Moussalli from South Africa have found that the changing hues they are best known for evolved for communication not disguise. They allow chameleons to make themselves incredibly but temporarily noticeable to mates and rivals, while remaining inconspicuous for the rest of the time.

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Evolutionary arms race turns ants into babysitters for Alcon blue butterflies

Blogging on Peer-Reviewed ResearchIn the meadows of Europe, colonies of industrious team-workers are being manipulated by a master slacker. The layabout in question is the Alcon blue butterfly (Maculinea alcon) a large and beautiful summer visitor and its victims are two species of red ants, Myrmica rubra and Myrmica ruginodis.

Myrmica rubra and an Alcon blue butterfly caterpillarThe Alcon blue is a ‘brood parasite’ – the insect world’s equivalent of the cuckoo. David Nash and European colleagues found that its caterpillars are coated in chemicals that smell very similar to those used by the two species it uses as hosts. To ants, these chemicals are badges of identity and so similar are the caterpillars that the ants adopt them and raise them as their own. The more exacting the caterpillar’s chemicals, the higher its chances of being adopted.

The alien larvae are bad news for the colony, for the ants fawn over them at the expense of their own young, which risk starvation. If a small nest takes in even a few caterpillars, it has more than a 50% chance of having no brood of its own. That puts pressure on the ants to fight back and Nash realised that the two species provide a marvellous case study for studying evolutionary arms races (which I’ve blogged about before here).

Theory predicts that if the parasites are common enough, they should be caught in an ongoing battle with their host, evolving to become more sophisticated mimics, while the ants evolve to become more discriminating carers. The two species make a particularly good model for this because their geographical ranges overlap in a fractured mosaic.

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Assassin bugs deceive spiders with coat of many corpses

Blogging on Peer-Reviewed ResearchThe animal world is full of charlatans. Some have bodies shaped by natural selection to fade into the background or resemble other harmful species. Yet others, like chameleons and octopuses, have the rare ability to actively change their colour or shape to actively hide themselves from view.

Assassin bugMany species disguise themselves through their behaviour rather than their bodies; like human soldiers in camouflage gear, they don special suits to remain inconspicuous.

Decorator crabs, for example, coat their shells with a collection of sea anemones, algae, corals and sponges, held on with Velcro-like bristles while other crabs actively carry these living masks with specially modified legs. These species have the cartoonish air of a man carrying a pot plant in front of him while sneaking past on tip-toes. But some charlatans are not so amusing.

Robert Jackson and Simon Pollard from the University of Canterbury have been studying a pretender with a much more gruesome disguise – the ant-snatching assassin bug Acanthaspis petax, which covers itself with the corpses of its own prey.

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Cuttlefish tailor their defences to their predators

Blogging on Peer-Reviewed ResearchThe best communicators know to cater to their audiences, and cuttlefish are no different. A new study shows that these intelligent invertebrates can target their defensive signals to the hunting styles of different predators.

CuttlefishCuttlefish and their relatives, the octopuses and squid, are expert communicators whose incredible skins can produce a massive range of colours and patterns. Cuttlefish mostly use these abilities to blend into the background but they can also startle and intimidate predators by rapidly changing the display on their dynamic skins.

Keri Langridge and colleagues from the University of Sussex, watched young cuttlefish as they were threatened by three very different predators – juvenile seabass, dogfish (a type of shark) and crabs. A glass partition protected the cuttlefish from any actual harm but gave them full view of the incoming threats.

She found that the cuttlefish only ever used startling visual displays when they were faced by seabass, which hunt by sight. As the fish approached, the young cuttlefish suddenly flattened their bodies to make themselves look bigger and flashed two dark eye-spots on their backs to startle the predator. This pattern is called a ‘deimatic display’ and it was used in 92% of encounters with seabass.

There’s a video of the deimatic display after the jump…

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Fake cleaner fish dons multiple disguises

Guess which is which? (The top one is the real deal)

Blogging on Peer-Reviewed Research

Nature is rife with charlatans. Hundreds of animals have evolved to look like other species in order to fool predators into thinking they’re more of a threat, or to sneak up on unsuspecting prey. In the Indo-Pacific lives a fish that does both and has the rare ability to switch between different disguises – the bluestriped fangblenny.

Common though it is, mimicry is usually restrictive and most fakers are stuck with one disguise. Until a few years ago, the only known animal that could switch between different acts was the amazing mimic octopus, which contorts its flexible body to look like seasnakes, lionfish, flounders and other poisonous underwater denizens.

Cleaner and faker

In 2005, Isabelle Cote and Karen Cheney from the University of Queensland discovered that a small reef fish called the bluestriped fangblenny (Plagiotremus rhinorhynchos) is also a dynamic mimic.

Its model is the bluestreak cleaner wrasse Labroides dimidiatus, an industrious species that provides a cleaning service for other reef visitors by picking off parasites and mucus from hard-to-reach places. The fangblenny’s intentions are less welcome. Its resemblance to the helpful wrasse allows it to get close enough to mount quick attacks on larger fish, biting off scales and skin (see image below for why it got it’s name).

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Foul-tasting ant parasitises the colonies of other species

An ant nest is sheltered, well defended and stocked with food, but one that takes time to build and protect. Which is why some species of ants don’t bother to do it themselves – they just squat in the nests of others.

These ants are ‘social parasites’ – they don’t feed off their hosts’ tissues, but instead steal their food, sleep in their homes and use their resources. They’re like six-legged cuckoos

Formicoxenus nitidulus escapes its hosts’ larger jaws by tasting foul.An ant colony is too dangerous a target to victimise lightly and the social parasites use several tricks to stop their hosts from ripping them apart. Some escape reprisal by chemically camouflaging themselves, either by mimicking their hosts’ odour, or by acquiring it through contact.

This specialised strategy ties the parasite’s fates into those of its host. Both are caught in an evolutionary arms race, with the hosts becoming more discriminating and the parasites’ deception becoming more accurate. But Stephen Martin from the University of Sheffield has found one ant species with a completely different and more flexible strategy – it tastes really, really bad.

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Moths mimic each others’ sounds to fool hungry bats

Impressionists are a mainstay of British comedy, with the likes of Rory Bremner and Alistair MacGowan uncannily mimicking the voices of celebrities and politicians alike. Now, biologists have found that tiger moths impersonate each other too, and they do so to avoid the jaws of bats.

A distasteful tiger moth, part of a syndicate of mimics. Some creatures like starlings and lyrebirds are accomplished impersonators, but until now, we only had anecdotal evidence that animals mimic each others’ sounds for defence.

Some harmless droneflies may sound like stinging honeybees, while burrowing owls deter predators from their burrows by mimicking the distinctive warning noises of deadly rattlesnakes.

In tiger moths, Jesse Barber and William Conner from Wake Forest University, North Carolina, have found the first hard evidence of acoustic mimicry in animals. Tiger moths are hunted by bats, which use ultrasonic clicks – echolocation – to home in for the kill.

Moths are tuned into the sounds of these clicks and respond with their own ultrasonic sounds, created by vibrating special membranes called ‘tymbals’ on their abdomens (see a Quicktime video of the tymbals in action) .

The sounds are multi-purpose – they may startle the bats, or jam their transmissions. But according to Barber and Conner, they also carry a message – they say “Don’t eat me, I won’t taste very nice.”

Mimicking moths

The duo worked with two species of bats – red bats (Lasiurus borealis), which eat butterflies and moths and big brown bats (Epstesicus fuscus), which dine mostly on beetles, but will take the occasional moth.

A bat captures a tethered moth.They raised bats in the lab and trained them to hunt live but tethered moths (right), while using high-speed video cameras to capture the split seconds of the attack (Quicktime video). They also recorded the various ultrasonic clicks used by both moths and bats.

Over five nights, Barber and Conner presented the bats with one of two species of foul-tasting tiger moths, the dogbane tiger moth (Cycnia tenera) and the polka-dot wasp moth (Syntomeida epilais).

The two species look very different, but they do absorb poisons from the plants they ate as caterpillars. All the bats learned to avoid the first species, and when they were shown the other, they avoided it too even though they had never seen it before (Quicktime video).

But they had heard it, or at least, a moth that sounded very much like it. To show that this was the case, Barber and Conner silenced the second species of moth by surgically removing their tymbals. The bats eagerly attacked the now-muted moths, although they quickly spat them out again in distaste.

Fake mimics

This was clear evidence of a type of impersonation called Mullerian mimicry, where two or more distasteful or dangerous creatures provide predators with the same warning. But the tiger moths also show Batesian mimicry, where a delicious and defenceless animal pretends to be a more noxious one.

Cycnia tenera, a distasteful tiger moth, and a member of a syndicate of mimics.Barber and Conner repeated their experiment and trained the bats on the unpalatable dogsbane moth. But then, they offered them the milkweed tiger moth (Euchates egle; right), a moth without any chemical defences and a tasty treat for a bat. It didn’t matter – the bats were put off by its pretense and avoided it like the others.

However, some bats did wise up. Red bats specialise in catching butterflies and moths, and three of them started to attack the vulnerable milkweed tiger. After a day, they had learned that these charlatans were in fact quite edible (Quicktime video).

When they were offered the unpalatable dogsbane tiger again, they avoided it (unless its tymbals were removed). They had quickly learned to tell the difference between the sounds of the impostors and the real deals.

The moths’ charades are likely to be a common strategy. There are, after all, 11,000 species of tiger moths worldwide, many of which live in overlapping geographical ranges. Other insects too, including hawkmoths and tiger beetles, respond to approaching bats with ultrasonic clicks. For all we know, the night skies around us are full of the cacophony of impostors and impressionists.

Reference: Barber & Conner. 2007. Acoustic mimicry in a predator-prey interaction. PNAS 104: 9331-9334.

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Related posts on bats:
Bats create spatial memories without making new brain cell
Bats: internal compasses and record-breaking tongues

And a post on the best mimic of all, the mimic octopus:
The mimic octopus – a master of disguise