̽��ֱ�� of Cambridge - mimicry

Cuckoos mimic 'harmless' species as a disguise to infiltrate host nests

fpjl2 — Wed, 10 Jun 2015 13:26:21 +0000

Brood parasites are reproductive cheats that evolve ways of duping other birds into raising their young. Examples such as mimicry of host eggs, chicks and fledglings by brood parasitic eggs, chicks and fledglings are amongst the most iconic examples of animal deception in nature.

New research shows that adult brood parasitic female cuckoo finches have evolved plumage colours and patterns to mimic a harmless and abundant species, such as southern red bishops, to deceive possible host birds and reduce the risk of being attacked when approaching host nests to lay their eggs.

Researchers say this is the first time that "wolf in sheep's clothing" mimicry has been shown to exist in any adult bird.

While other brood parasites watch the movements of their host victims by hiding in nearby foliage, the openness of the African savannahs mean that mimicking a plentiful and nontoxic species might be the best way cuckoo finches have of sneaking up on host nests without raising the alarm.

However, the researchers found that the most common victim of the cuckoo finch, the tawny-flanked prinia, has evolved an awareness of the cuckoo finch's disguise and takes no chances - acting with equal aggression towards a female cuckoo finch and bishop alike.

Prinias attacked female cuckoo finches and female bishops equally, and increased the rate of egg rejection after seeing either a female cuckoo finch or female bishop near the nest. Egg rejection involves physically removing the parasitic egg from their nest, allowing them to salvage the majority of their reproductive effort.

At the study site in Zambia, the researchers found a consistently high rate of parasitism by cuckoos among the prinia population, with almost a fifth of all prinia eggs hatching as fledgling cuckoo finches. Cuckoo finches usually remove at least one egg on parasitism, and their hatchlings will out-compete all the host's young.

Researchers say these rates of parasitism might explain the willingness of prinias to attack anything that looks like a dangerous female cuckoo finch and reject more eggs when the risk of parasitism is high. But, the cost of this strategy can be high: during the researchers' experiments, some of the eggs rejected by prinia were their own, triggered by nothing more than a harmless bishop bird that resembles the mimetic cuckoo finch.

"Our findings suggest that female cuckoo finches are aggressive mimics of female bishops, and that prinia hosts have responded to this successful deception with generalised defences against cuckoo finches and harmless bishops alike. This suggests these prinias have decided that it's best to 'play it safe' when the risk of parasitism is high because they can't distinguish between the two species" said Dr William Feeney from Cambridge ̽��ֱ��'s Department of Zoology, who led the research.

"While other brood-parasite species monitor host behaviour from concealed perches in nearby trees, cuckoo finches must seek host nests in open grasslands and savannahs. In such exposed circumstances, resembling an abundant and harmless model may allow female cuckoo finches to remain unnoticed when monitoring hosts nests at a medium range," he said.

̽��ֱ��research is published today in the journal Proceedings of the Royal Society B.

To investigate the cuckoo finch's disguise, the research team conducted plumage and pattern analysis using cuckoo skins from the Natural History Museum in Tring. They compared plumage to the cuckoo finches closest evolutionary relatives (Vidua finches), as well as with the skins of similar-looking birds (bishops) that share the same habitat.

In both human and bird visual systems, they found that the plumage of a female cuckoo finch is far closer to the bishops and other species in the weaver family than to those of its closest relative, the Vidua finches.

̽��ֱ��researchers also investigated the reaction of prinia breeding pairs to models of female cuckoo finches and bishop birds, as well as the males of both species.

While prinias had very little reaction to the males, the female cuckoo finch and the harmless female bishop bird both received similarly high levels of alarm calls and group attacks from the prinia, known as 'mobbing'.

̽��ֱ��researchers then did a final experiment where they presented a male bishop, female bishop and female cuckoo finch and then placed a fake egg in their nest. They found that after seeing the harmful female cuckoo finch or harmless (but similar-looking) female bishop, they increased their rate off egg rejection compared to when they saw a male bishop near their nest.

Added Feeney: "This study is interesting as it's the first time anyone has quantitatively tested for 'wolf in sheep's clothing' mimicry in any adult bird, and also suggests that this type of mimicry is used by brood parasites to deceive hosts at all stages of their nesting cycle."

First time ‘wolf in sheep’s clothing’ mimicry has been seen in birds. Host birds have evolved a general counter-strategy in which they defend against all birds with the mimicked plumage - cuckoos and harmless species alike.

It's the first time anyone has quantitatively tested for 'wolf in sheep's clothing' mimicry in any adult bird

William Feeney

Claire Spottiswood

Cuckoo finch on the left and a bishop bird on the right

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Birds ‘cry hawk’ to give offspring chance to escape predators

fpjl2 — Wed, 03 Jun 2015 08:35:20 +0000

New research has found that the 6 gram brown thornbill mimics the hawk alarm calls of neighbouring species to scare a nest predator by convincing it that a much bigger and scarier predator – the brown goshawk – is on its way.

Currawongs, which raid the nests and hunt the chicks of thornbills, are also prey to goshawks. Although currawongs normally benefit from listening in on hawk alarm calls of other species, thornbills exploit this and turn it against them.

As well as issuing their own hawk alarm call, thornbills mimic those of the local species to create the impression of an impending hawk attack, which in turn distracts the pied currawong - a predator 40 times larger than the thornbill - providing thornbill nestlings with an opportunity to escape.

While animals often mimic dangerous or toxic species to deter predators, the thornbill is a surprising example of a species mimicking another harmless species to trick a predator.

̽��ֱ��research, conducted by scientists at the ̽��ֱ�� of Cambridge and the Australian National ̽��ֱ�� (ANU), is published today in the journal Proceedings of the Royal Society B.

“ ̽��ֱ��enormous size difference between a tiny thornbill and a 0.5kg goshawk might make it difficult for thornbills to mimic hawk vocalisations accurately, limiting them to mimicking the chorus of hawk alarm calls given by small local species instead,” said Jessica McLachlan, a PhD student from Cambridge’s Department of Zoology, who co-authored the study.

“As hawks are silent when hunting, the alarm calls of local species may be the only sound that warns of a hawk’s presence,” she said.

̽��ֱ��researchers studied the thornbills and currawongs living in and around the Australian National Botanic Gardens in Canberra. They devised a series of experiments in which they placed stuffed currawongs in front of thornbill nests to test when thornbills use such trickery, followed by experiments testing how currawongs respond to the calls of thornbills.

They found that thornbills used their own and mimicked hawk alarm calls when their nests are under attack. They also found that currawongs delayed attacks for twice as long when mimetic and non-mimetic alarm calls were played together as opposed to non-mimetic calls played alone.

“Distracting a currawong attacking the nest could give older thornbill nestlings a chance to escape and hide in the surrounding vegetation,” said Dr Branislav Igic from ANU, who led the study.

“It’s perhaps the thornbills best nest defence in this circumstance because physical attacks on the much larger currawong are hopeless,” Igic said.

Inset image: Pied currawong. Credit: Jessica McLachlan

Surprising finding shows that thornbills simulate a ‘chorus of alarm’ to distract predators by convincing them something scarier is on its way.

As hawks are silent when hunting, the alarm calls of local species may be the only sound that warns of a hawk’s presence

Jessica McLachlan

Brown thornbill

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Cuckoos impersonate hawks by matching their 'outfits'

fpjl2 — Thu, 17 Oct 2013 09:11:54 +0000

New research shows that cuckoos have striped or “barred” feathers that resemble local birds of prey, such as sparrowhawks, that may be used to frighten birds into briefly fleeing their nest in order to lay their parasitic eggs.

By using the latest digital image analysis techniques, and accounting for “bird vision” - by converting images to the spectral sensitivity of birds - researchers have been able to show for the first time that the barred patterns on a cuckoo’s breast may allow it to impersonate dangerous birds of prey. This might enable cuckoos to frighten other avian hosts into leaving their nests exposed.

̽��ֱ��latest findings, published today in the journal Animal Behaviour, expand the cuckoo’s arsenal of evolutionary deceptions, which include egg mimicry and chick mimicry that allow it to trick other birds into incubating its eggs.

Importantly, the study shows that a wide variety of cuckoos have adapted different plumage patterns depending on the area they inhabit so that they match a local bird of prey species.

While scientists have previously looked at links in plumage patterns between the common cuckoo and Eurasian sparrowhawk, the new research shows that this type of impersonation of a more dangerous animal – called ‘Batesian mimicry’ – may be far more widespread in cuckoos. In addition, the dangerous bird of prey that cuckoos resemble goes beyond sparrowhawks to include such raptors as bazas and harrier-hawks - depending on the species prevalent in the cuckoo’s neighbourhood.

“There is no benefit in looking like a dangerous species your target is not familiar with,” said lead researcher Thanh-Lan Gluckman from Cambridge’s Department of Zoology.

“We first established similarity in plumage pattern attributes between cuckoos and raptor species, and then showed that cuckoos look nothing like species from a different geographical area.”

̽��ֱ��cuckoos also use their crafty ‘hawk impression’ to allow them to fly ‘under the radar’, undetected as they scope out potential nests in which to deposit their parasitic eggs.

“ ̽��ֱ��barring on their plumage helps cuckoos conceal themselves while searching for potential nests, then when they approach, the host of the nest may mistake a cuckoo for a raptor coming to get them – giving them unfettered access to lay eggs,” Gluckman said.

While previous studies have focused on Batesian mimicry in the common cuckoo and Eurasian sparrowhawk, this is the first time that the plumage patterns of cuckoos have been analysed using digital image analysis techniques. ̽��ֱ��study suggests that this form of mimicry may be widespread among many cuckoo species, and that they may be mimicking a variety of different types of birds of prey.

̽��ֱ��researchers were surprised to find no pattern matching between cuckoos and raptors that live in different geographical areas, showing that the visual similarity is highly localised to species in the immediate vicinity.

“These findings underscore the importance of using digital image analysis to objectively quantify plumage patterning in mimicry – it is important not to make assumptions about even simple patterns such as these,” added Gluckman.

“We hope this encourages other researchers to examine the function of barred plumage in parasitic cuckoos and raptors the world over.”

Another interesting finding is that of the African cuckoo-hawk, a raptor so named because of its visual resemblance to cuckoos. This study objectively shows that the naming was an apt one, given that a local cuckoo matched the African cuckoo-hawk in all of the pattern attributes measured.

One of the earliest observers of the cuckoos’ invasive guile was Ancient Greek philosopher Aristotle, who noted some 2,300 years ago that it “lays its eggs in the nest of smaller birds”.

Evolutionary trick allows cuckoos to mimic the plumage of birds of prey, and may be used to scare mothers from their nests so that cuckoos can lay their eggs. Mimicry in cuckoos may be more much more widespread than previously thought.

There is no benefit in looking like a dangerous species your target is not familiar with

Thanh-Lan Gluckman

Thanh-Lan Gluckman/Gabriel A. Jamie

Left: Cuckoo compared to cuckoo-hawk plumage. Right: Cuckoo

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Yes

How the butterflies got their spots

bjb42 — Thu, 04 Feb 2010 00:00:00 +0000

Heliconius, or passion-vine butterflies, live in the Americas - from the southern United States to southern South America. Although they cannot interbreed, H. melpomene and H. erato have evolved to mimic one another perfectly.

These delicate butterflies have splashes of red and yellow on their black wings, signaling to birds that they contain toxins and are extremely unpalatable. They mimic one another's colour and pattern to reinforce these warning signals.

Scientists have studied these butterflies since the 1860s as a classic case of evolution in action, but only now is modern sequencing technology unlocking the underlying genetics.

̽��ֱ��Cambridge-led team of researchers from UK and US universities, which has been breeding the butterflies in Panama for the past decade, has been searching for the genes responsible for the butterflies' wing patterns and the answer to the question of whether the same genes in two different species are responsible for the mimicry.

According to Dr Chris Jiggins of the Department of Zoology at the ̽��ֱ�� of Cambridge, one of the authors of the study: " ̽��ֱ��mimicry is remarkable. ̽��ֱ��two species that we study - erato and melpomene - are quite distantly related, yet you can't tell them apart until you get them in your hand. ̽��ֱ��similarity is incredible - even down to the spots on the body and the minute details of the wing pattern."

That the two species have evolved to look exactly the same is due to predation by birds. " ̽��ֱ��birds will try anything that looks different in the hope that it's good, so they learn that certain wing patterns are unpalatable and avoid them, but anything that deviates slightly from what they've experienced before is more likely to be attacked," he explains.

These butterflies have been studied since Darwin's day because they are such a striking example of adaptation. For years, scientists have pondered whether when different species evolve to look the same, they share a common genetic mechanism.

According to Jiggins: "It's interesting because it tells us how flexible evolution is. If you get the same wing pattern evolving independently in different populations, do you expect the same genes to be involved?"

Because there are thousands of genes in the butterflies' genome, most scientists felt it was unlikely that the same genes should be involved. But the results of this study suggest that this is, in fact, the case.

̽��ֱ��new results - published today in two parallel papers in the journal PLoS Genetics - show that the regions of the genome associated with the wing patterns are very small - akin to genetic "hotspots".

"This tells us something about the limitations on evolution, and how predictable it is. Our results imply that despite the many thousands of genes in the genome there are only one or two that are useful for changing this colour pattern. It seems like evolution might be concentrated in quite small regions of the genome - or hotspots - while the rest of it does not change very much," says Jiggins.

This is not the only unexpected element of the study. ̽��ֱ��team was also surprised that the obvious candidate genes - such as those involved in colour or wing pattern in other species - do not seem to be involved in the passion-vine butterflies' mimicry.

According to Jiggins: "We think it's more likely to be some novel method of cellular signaling, which is quite intriguing and could be important in many other insect species."

̽��ֱ��next stage of the research is to look at other traits, such as behaviour, because the butterflies have preferences for particular colours and use wing patterns to select mates. "It seems the same regions of the genome control this behaviour as well as the wing pattern. We'd like to understand this," he says.

̽��ֱ��results are published in PLoS Genetics on 5 February 2010.

How two butterfly species have evolved exactly the same striking wing colour and pattern has intrigued biologists since Darwin's day. Now, scientists at Cambridge have found "hotspots" in the butterflies' genes that they believe will explain one of the most extraordinary examples of mimicry in the natural world.

It's interesting because it tells us how flexible evolution is.

Dr Chris Jiggins

whologwhy from Flickr

BUTTERFLY

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Yes

On the wings of a butterfly

bjb42 — Fri, 01 May 2009 15:16:36 +0000

On reading Henry Walter Bates’ 1862 account of his travels in the Amazon, Charles Darwin was captivated not only by Bates’ description of the stunning diversity of butterfly species and wing patterns found in the Amazonian jungle, but also by the impressive mimicry between unrelated species. Darwin wrote: ‘It is hardly an exaggeration to say, that whilst reading and reflecting on the various facts given in this Memoir, we feel to be as near witnesses, as we can ever hope to be, of the creation of a new species on this earth.’¹

Bates hypothesised that mimicry evolved to confuse predators. Edible butterflies, for instance, copied the wing patterns of toxic species so that predators would avoid eating them. He also described what looked like evolution in action: he observed a continuum, from variable species, in which different wing patterns were found together in the same locality, through to related species with different wing patterns. Now, 150 years later, modern science has taken this to another level, with new research that aims to study the predictability of evolution by revealing the genetic basis of wing pattern mimicry.

̽��ֱ��importance of pattern

We now recognise that not only do edible species mimic nasty ones (today called Batesian mimicry), but that several nasty species can also benefit from mimicking one another (Müllerian mimicry) – bees and wasps being a familiar example. Many of the Amazonian butterflies described by Bates are in fact Müllerian mimics, and the best-studied group are the genus Heliconius, the passion vine butterflies. Work by Dr Chris Jiggins’ group in the Department of Zoology has focused on Heliconius butterflies as a case study in evolutionary biology.

By testing the role of Heliconius wing patterns in the wild, Dr Jiggins and others have confirmed Bates’ hunch: changes in wing pattern play a big role in determining how successful the butterflies are in both mating and avoiding being eaten. Using flapping models with different patterns, the researchers have shown that the butterflies choose to mate with individuals that look the same as themselves; because of this, over time, different patterns are likely to split into new species. In addition, hybrids between populations with different patterns have intermediate patterns that are not recognised by predators as harmful and therefore suffer disproportionately from attacks, reinforcing the split into new species.

This dual role of wing patterns in signalling both to predators and to potential mates makes pattern a ‘key trait’ for speciation. As Bates suggested, shifts in wing patterns do indeed lead to the evolution of new species.

Selection signatures

One of the current hot topics in evolutionary biology is to what extent we can predict the genetic path of evolution. One particular Heliconius species (Heliconius melpomene) is an ideal system in which to address this question because it has many geographic populations with very different colour patterns.

A major new project focusing on the genetic basis of wing patterns has commenced in the Jiggins lab with funding from the Biotechnology and Biological Sciences Research Council (BBSRC), Royal Society, Leverhulme Trust and Natural Environment Research Council (NERC).

Over the past decade, the researchers have been collecting different forms of H. melpomene from around South America, carrying out genetic crosses at a field station in Panama. These crosses have shown that dramatic differences in colour pattern are controlled by just a handful of genes and that these genes are clustered together on four out of the 21 Heliconius chromosomes. ̽��ֱ��genes act as wing pattern ‘switches’, turning on and off the presence of major pattern elements, such as a large red forewing band. ̽��ֱ��challenge is to find out precisely what these genes are and how they work.

In collaboration with the Wellcome Trust Sanger Institute, regions of the butterfly genome are being sequenced to identify the specific nature of the pattern switches. ̽��ֱ��expectation was that the switches would correspond to well-known genes, perhaps controlling wing development or colour pigments. In fact, the two genomic regions studied so far each contains around 20 genes, none of which is known for its involvement in these processes. This is in itself exciting as it implies that novel mechanisms of pattern determination are operating; current research is focused on determining which, of all these genes, are having an effect in the butterfly.

Genetics of mimicry

What attracted Darwin and others to mimicry as a case study in evolution is its repeatability – the same patterns evolve in distantly related species. A key question for an evolutionary geneticist is therefore whether the patterns are generated by the same genetic mechanisms, or different ones. Again, Heliconius butterflies are a good system to study this.

Heliconius melpomene co-mimics another species, Heliconius erato, all over the neotropics – in any location you care to look you will find that the two species have evolved identical patterns. Recently, in collaboration with research groups in the USA, it has been shown that pattern switches in the two species are controlled by the same regions of DNA, such that genes at identical locations in the genome code for either a red forewing band or a yellow hindwing bar. This implies that evolution of the same mimicry patterns in the two species has been made easier by a shared genetic system. While predation against abnormal wing patterns drives the evolution of mimicry through Darwinian natural selection, a shared developmental system may bias the raw materials in favour of certain kinds of patterns.

Of course, the link between wing pattern adaptation and speciation requires changes in behaviour. ̽��ֱ��mating preferences of divergent populations need to evolve in order to match their wing patterns. Remarkably, crossing experiments currently being carried out in Panama show that the genes underlying these changes in behaviour are closely associated with colour pattern genes. It seems that there are ‘hot spots’ in the genome for evolutionary change, influencing traits as diverse as wing patterns and mating preference.

An enduring example

It is an exciting time to be studying butterfly mimicry. ̽��ֱ��combination of population genetic, developmental and behavioural approaches is starting to answer the issues Darwin and Bates themselves debated – questions that were posed at the very dawn of evolutionary biology. Throughout the intervening decades, Heliconius butterflies have persisted as an example of evolution in action. With the imminent sequencing of the Heliconius melpomene genome, they will no doubt continue to be so for some time yet. Charles Darwin would surely have approved.

¹_{[Darwin, C.R.] 1863. [Review of] Contributions to an insect fauna of the Amazon Valley. By Henry Walter Bates, Esq. Transact. Linnean Soc. Vol. XXIII. 1862, p. 495. Natural History Review 3, 219–224.}

For more information, please contact the author Dr Chris Jiggins (c.jiggins@zoo.cam.ac.uk) at the Department of Zoology.

Since Darwin’s time, Amazonian butterflies have intrigued biologists as examples of evolution in action.

One of the current hot topics in evolutionary biology is to what extent we can predict the genetic path of evolution.

Dr Mathieu Joron

Heliconius melpomene

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Yes