From reliable source?

Nuthatch transmits indirect information only partially

red-breasted nuthatch eavesdrops on black-capped chickadee

The red-breasted nuthatch understands the alarm call of black-capped chickadees perfectly. But it doesn’t propagate all the information that it contains in its own call, as Nora Carlson and colleagues show.

An owl that is perched on a tree branch during daytime does not pose an immediate threat to songbirds. Yet, they would rather not have it in their neighbourhood. By making a lot of fuss with a group, which is called mobbing, they try to bully the predator away.

This behaviour is also exhibited by the red-breasted nuthatch from North America. If the bird is aware of an owl being around, it will recruit conspecifics to participate in mobbing. In its mobbing call, it encodes how dangerous the owl is that has to be chased away, as Nora Carslon and colleagues write. At least: if the nuthatch itself observed the enemy.

Aroused

That is because not all owls pose similar threats. The great horned owl, a large bird about half a meter in length, is not agile enough to easily catch a songbird; it is therefore not very threatening. The small, agile northern pygmy owl is much more dangerous.

Accordingly, nuthatches react differently to hearing either great horned owl or pygmy owl, as appeared from playback experiments in which the researchers exposed the songbirds to the calls of both predators. Upon hearing a pygmy owl, the mobbing call of nuthatches consists of shorter, higher-pitched calls that are uttered at higher rate than after hearing a great horned owl. Their conspecifics then are more aroused and exhibit mobbing behaviour for longer and more intensively – in this case against the speakers that were used by the researchers.

Consequently, the songbirds spend their time and energy mainly in chasing away the most dangerous enemies.

Eavesdropping

black-capped chickadee encodes threat level in its alarm callNuthatches not only rely on their own ears; they also make use of the vigilance of other songbirds and eavesdrop on their alarm calls.

The researchers had shown previously how they respond appropriately to mobbing calls of black-capped chickadees, which also encode whether they face a less dangerous great horned owl or a more dangerous northern pygmy owl. When nuthatches hear chickadees calling in response to pygmy owl, they make more fuss and they will also produce more mobbing calls than when they hear chickadees’ response to great horned owl. So, they understand the message of chickadees very well.

But despite that understanding, nuthatches don’t propagate in their own mobbing call the level of danger according to chickadees, like they do after observing the enemy themselves. If the information is from chickadees, they will not indicate how dangerous the enemy is; their mobbing call is intermediate in call length, pitch and rate at high and low risk.

Less reliable

And perhaps, this is not so bad. Although nuthatches and chickadees share many predators, they are not equally vulnerable to those enemies, due to their different lifestyles. How chickadees perceive and communicate the threat of different enemies can differ from how nuthatches would estimate the level of danger, making the information obtained from chickadees a bit less reliable.

Willy van Strien

Photos
Large: red-breasted nuthatch. Cephas (Wikimedia Commons, Creative Commons CC BY-SA 3.0)
Small: black-capped chickadee. Shanthanu Bhardwaj (Wikimedia Commons, Creative Commons CC BY-SA 2.0)

Sources:
Carlson, N.V., E. Greene & C.N. Templeton, 2020. Nuthatches vary their alarm calls based upon the source of the eavesdropped signals. Nature Communications 11: 526. Doi: 10.1038/s41467-020-14414-w
Templeton, C.N. & E. Greene, 2007. Nuthatches eavesdrop on variations in heterospecific chickadee mobbing alarm calls. PNAS 104: 5479-5482. Doi: 10.1073_pnas.0605183104
Templeton, C.N., E. Greene & K. Davis, 2005. Allometry of alarm calls: black-capped chickadees encode information about predator size. Science 308: 1934-1937. Doi: 10.1126/science.1108841

Sparkling camouflage

Jewel beetles are invisible thanks to gem-like wings

jewel beetle is invisible in vegetation

The green jewel beetle Sternocera aequisignata is protected against the gaze of predators not only by its colour, but also by its iridescent shine, Karin Kjernsmo and colleagues demonstrate.

dress embellished with wing cases of jewel beetlesJewel beetles were often to be seen in the ballrooms of Victorian England. That is, their wing cases, the hardened front wings; they were applied on expensive dresses as decoration and glittered like gems. With such dress, a lady could show up.

The beetle wings are so beautiful because they are shiny and iridescent, that is, their colour changes when they are illuminated or seen at different angles. The entire body of the beetle has that iridescent shine. It is an effect of the nanostructure of its exoskeleton, with multiple layers reflecting light. The wing cases of jewel beetles are durable and colourfast, and it is no wonder that they have often been used in jewellery and clothing; jewellery with jewel beetle wings is still being made today.

Surprisingly, jewel beetles don’t have their sparkling appearance to stand out, but to hide in plain sight, Karin Kjernsmo and colleagues prove.

Nail varnish

A well-known jewel beetle is the Asian emerald green Sternocera aequisignata. The beetles are on the menu of birds and should not catch the eye when they dwell in vegetation. Green is a well protective colour. The researchers wondered whether the iridescence makes the beetles still more difficult to detect.

They first wrapped dead mealworms with either a wing case of Sternocera aequisignata or a model. Five different models were used: pieces of resin shaped like a wing and varnished with green, blue, purple or black nail polish, and a high-gloss photo of a wing case that had the different colours, but not the iridescence of real wings. In a forest environment, they pinned the mealworms on plants. It was clear that birds found mealworms wrapped with real beetle wings less often than mealworms wrapped with one of four different wing models. So, wrapped with wing cases, the prey was more safe. Only the black models offered the same safety.

Glossy background

The next question was whether birds had greater difficulty detecting the iridescent wing cases, or whether they refrain from taking them. Human test subjects answered this question. They were asked to walk past plants on which wing cases and the five different models had been placed and to search for the objects. It turned out that the real wing cases were harder to detect than the models, again with the exception of the black ones. On a glossy background, such as a wet leaf, the real beetle wings did not stand out at all.

The conclusion is that the iridescent wing cases of jewel beetles that shine so brightly on ball gowns have a camouflaging effect on plants. Perhaps that explains why iridescent colours are common among insects. Just like the colour black.

Willy van Strien

Photos
Large: Sternocera aequisignata. Ian Jacobs (via Flickr, Creative Commons CC BY-NC 2.0)
Small: 19th century dress embellished with wing cases of jewel beetle. B (via Flickr, Creative Commons, CC BY-NC-SA 2.0)

Sources:
Kjernsmo, K., H.M. Whitney, N.E. Scott-Samuel, J.R. Hall, H. Knowles, L. Talas & I.C. Cuthill, 2020. Iridescence as camouflage. Current Biology, online January 23. Doi: 10.1016/j.cub.2019.12.013
Eluwawalage, D., 2015. Exotic fauna and flora: fashion trends in the nineteenth century. International Journal of Fashion Design, Technology and Education 8: 243-250. Doi: 10.1080/17543266.2015.1078848

American coot takes care of the small

The younger, the brighter, the more food

Coot chicks' ornaments tell parents what age they are

In contrast to their parents, young coots have a striking appearance. Bruce Lyon and Daizaburo Shizuko discovered how their ornamentation helps the parents to optimally feed the offspring.

Young American coots, Fulica americana, have fancy heads: a red beak and bare patches of red skin with papillae, surrounded by a crown of orange-yellow modified feathers. That ornamentation is puzzling; because of predators, you would rather expect young coots to be unobtrusive. Bruce Lyon and Daizaburo Shizuko figured out what function their colourful appearance serves.

Starvation

If the clutch of a pair of American coots is complete, it is certain that not every egg will result in an independent young. The water birds lay nine eggs per nest on average, and although almost all of these eggs will hatch, only three or four young eventually reach independency. Four chicks is the maximum that the parents can feed. As a consequence, less than half of the chicks can survive.

The case is settled during the first ten days after the last egg has hatched. The young coots leave the nest immediately after hatching and swim to the parents to be fed, every chick trying to get attention. It is an unfair competition, because the siblings do not hatch at the same time; the first chick may be eleven days older than the last. The oldest chicks are larger, not only because they are older, but also because the first eggs laid by a female are larger. It is easy for them to keep up with their parents, while the youngest coots run a high risk of being too slow and starving.

The parents don’t interfere with this rivalry between their young.

Favourites

But after ten days, things will change, as the researchers had discovered before during their research in Canada. The size of the coot family then is reduced to a number that the parents can handle, and they shift strategies. They are now going to pay attention to the small ones and offer them most of the food that is available. Each parent chooses one of the chicks to favour; a favourite is always one of the youngest.

The eldest chicks also want to be fed, but they are already able to find their own food. They are tousled by their parents when they come begging: they are grasped by the neck and shaken. They then will give up.

In this way, the parents give full attention to the chicks that need it, making sure that all chicks that survived the period of sibling rivalry can grow up. Thanks to this preferential treatment, the youngest chicks in a coot family will gain the same weight as the oldest ones.

Age

The researchers also had observed earlier that the most brightly coloured young were preferentially fed by the parents and more likely to be chosen as a favourite. Now, they link the chicks’ colour to their age. The later an egg’s position in the laying order, they show, the brighter coloured the chick will be. This is probably because the mother adds more dye to the yolk as she has already laid more eggs. So this appears to be the function of the ornamentation: it indicates to the parents which chicks are the youngest and need food aid the most.

Sometimes a coot is extremely aggressive to a chick. In that case, this is not its own young, but another coot’s. Coot females often dump an egg in the neighbours’ nest in an attempt to increase the breeding success. But the intended foster parents recognize such foreign chick and will tackle it hard. Its chance to survive is very small.

Willy van Strien

Photo: American coot with chicks. M. Baird (Wikimedia Commons, Creative Commons CC BY 2.0)

Sources:
Lyon, B.E. & D. Shizuka, 2019. Extreme offspring ornamentation in American coots is favored by selection within families, not benefits to conspecific brood parasites. PNAS, online Dec 30. Doi: 10.1073/pnas.1913615117
Shizuka, D. & B.E. Lyon, 2013. Family dynamics through time: brood reduction followed by parental compensation with aggression and favoritism. Ecology Letters 16: 315-322. Doi: 10.1111/ele.12040
Lyon, B.E., 1993. Conspecific brood parasitism as a flexible female reproductive tactic in American coots. Animal Behaviour 46: 911-928. Doi: 10.1006/anbe.1993.1273

Peaceful together

Dangerous bullet ant and defensive bee tolerate each other

the bullet ant Paraponera clavata and a stingless bee tolerate each other

The bullet ant is not a friendly animal, the stingless bee defends its nest fanatically. Still, these two fighters live smoothly together, Adele Bordoni and colleagues report.

Just like honey bees, stingless bees are social insects. They construct their nest in a cavity, but are unable to dig out their own cavity. So, they exploit an existing one, and they often choose a bigger nest of other social insects, for instance termites. This offers a convenient home, because the host guarantees a proper nest climate.

stingless bee Partamona testacea builds its nest in an ants' nestThe small stingless bee Partamona testacea, which occurs in the Amazon in South America, builds its nest in an ants’ nest. That may be the nest of harmless fungus growing leaf cutter ants, but they also inhabit nests of the bullet ant Paraponera clavata, as Adele Bordoni and colleagues report. A weird choice at first sight, because the bullet ant is not quite friendly.

Large jaws

The bullet ant will aggressively attack as soon as it feels threatened. Its sting is known to be one  of the most painful experiences you can have in nature. In addition, it hunts for insects, which it preys upon, and it has large jaws. If you also realise that the bee is much smaller, you would expect it to avoid the nest of bullet ants. But instead, it enters it to make a home.

And things are going well, Bordoni shows. In the lab, the researchers placed a bullet ant and a bee together in a petri dish. The fierce ant behaved only a little aggressively and did not attack the bee. If the bee was from a nest within the ant’s nest, the ant was even less aggressive. Biting and stinging were highly uncommon.

Resin

Conversely, stingless bees also are tolerant. They defend their colony fanatically, as the researchers observed at an ants’ nest with inhabiting bee colony; the bullet ant builds its nest at the base of a tree. When they introduced an ant at the bees’ nest entrance, bee workers grabbed that ant, dragged it deeper inside the nest and covered it with resin, so that it was not able to move anymore.

But a bullet ant will not enter a bees’ nest voluntarily. An ant may pass the entrance, where always bee guards are present to deter invaders. And then the bees will not attack. When a bullet ant passes by, the guards were seen to retreat and to reposition when the ant was gone. When the ant passing by and the bee are from different ants’ nests, the bee guards reposition faster; in that case, they are a bit more vigilant.

Protection

Apparently, the dangerous bullet ant and the defensive stingless bee Partamona testacea recognize each other as familiar species, and they also discern individuals of an associated nest from foreigners. They probably know each other’s body odour. They live smoothly together without bothering each other, and it is to the bees’ advantage that the ants protect and defend their nest; maybe, the bees participate in nest defence with their vigilant guards.

Willy van Strien

Photos:
Large: Paraponera clavata. Graham Wise (Via Flickr. CC BY-NC-ND 2.0)
Small: nest entrance of Partamona testacea ©Giorgia Mocilnik

Source:
Bordoni, A., G. Mocilnik, G. Forni, M. Bercigli, C.D.V. Giove, A. Luchetti, S. Turillazzi, L. Dapporto, & M. Marconi, 2019. Two aggressive neighbours living peacefully: the nesting association between a stingless bee and the bullet ant. Insectes Sociaux, online November 30. Doi: 10.1007/s00040-019-00733-9

Leaf cutters prevent traffic jams

Take no heavy load when traffic flow is high

leaf cutter ants carry small leaf fragments on crowded trails

When the number of workers on foraging trails is high, leaf cutters maintain the flow by carrying only small pieces of leaf with them, Mariana Pereyra and Alejandro G. Farji-Brener show. Otherwise traffic jams would arise.

The fungus that leaf cutter ants grow in their gardens needs fresh plant material continuously to grow on. And so ant workers walk up and down trails that are cleared and maintained free of debris. They leave the nest to cut leaf fragments from plants and return with a piece in their jaws.

Sometimes ants carry extra-large leaf fragments, causing them to move slowly. That is cumbersome when the trail is crowded, because then a slow ant may hinder the flow. Accordingly, when many ants are walking on the path, they only take small loads with them, Mariana Pereyra and Alejandro Farji-Brener write.

Truck-driver effect

In earlier research, Farji-Brener and colleagues had shown that workers of the leaf cutter Atta cephalotes sometimes carry a strikingly large piece of leaf, up to twice the normal size, to deliver a large gain at the nest. But such extra large burden also has disadvantages; a heavily loaded ant runs slower and hinders the ants that come behind her carrying a normal load. Their walking speed may be reduced by up to 50 per cent. So, a traffic congestion may form behind a heavily loaded worker; the researchers call it the truck-driver effect. It slows down the entire column.

A slow ant on the trail is especially obstructive when it is busy, because in that case, ants walk close together and cannot overtake a slow colleague. At high ant flows, the biologists observed relatively few ants with a heavy load. Is that because the ants are so ‘wise’ not to enter a busy path with a heavy load?

Steady flow

Pereyra and Farji-Brener now answered that question in another species, Acromyrmex crassispinus. They offered workers pieces of ‘leaf’: filter paper soaked in orange juice. They presented pieces of normal size and of extra large size and observed what choice the ants made when different numbers of ants were walking on the trail. And indeed: only at low ant flows, workers selected extra large pieces of paper; when many ants were running, they only picked up the smaller pieces.

Various reasons are thinkable for avoiding large pieces; they make it more difficult to manoeuvre in case of obstacles, the chance of collisions is greater and a heavily loaded ant is more vulnerable to predators. But the fact that ants tend to ignore the large parts at high ant flows suggests that they also do so in order not to obstruct traffic. In this way, leaf cutters optimize colony performance. All going at the same speed: on a busy path, that is the best way to keep a steady flow.

Just like on highway.

Willy van Strien

Photo: Atta cephalotes ©Alejandro Farji-Brener

Sources:
Pereyra, M. & A.G. Farji-Brener, 2019. Traffic restrictions for heavy vehicles: Leaf-cutting ants avoid extra-large loads when the foraging flow is high. Behavioural Processes, online November 25. Doi: 10.1016/j.beproc.2019.104014
Farji-Brener, A.G., F.A. Chinchilla, S. Rifkin, A.M. Sánchez Cuervo, E. Triana, V. Quiroga & P. Giraldo, 2011. The ‘truck-driver’ effect in leaf-cutting ants: how individual load influences the walking speed of nest-mates. Physiological Entomology 36: 128-134. Doi: 10.1111/j.1365-3032.2010.00771.x

A large bill to cool down

Tufted puffin may be overheated after flying

Flying is strenuous in tufted puffin

Flying is strenuous for a tufted puffin and it causes high heat production. After landing, the bird has to cool down. The bill is used to dissipate excess heat, Hannes Schraft and colleagues report.

Puffins get their food from sea. They have relatively short wings that enable them to dive and swim under water, looking for fish and other prey. But the wings are less suitable for flying. To stay airborne and move forward, the wings have to flap in high frequency; the breast muscles work hard and much heat is produced.

Outside the breeding season, puffins usually stay at sea. But when they have a young to raise, they have to travel regularly between nest and sea. When a puffin approaches the nest with a mouth full of fish to feed its young, it is often overheated.

bill helps tufted puffin to cool downThe birds have a strikingly large bill, which helps to get rid of excess heat during and after the flight, as Hannes Schraft and colleagues show. The object of their research was the tufted puffin, which breeds along the northern Pacific Ocean, for instance on the coasts of Alaska and Kamchatka and on the Kuril Islands.

Network of blood vessels

With a special camera, the researchers made infrared images of birds that rested after landing; they did so from a distance of five to ten meters, in order not to disturb the animals. They took images of bill and back every two minutes. From these images, they were able to calculate temperature and heat exchange.

The temperature of the back remained constant, but the bill gradually cooled down; after half an hour its temperature had decreased by about 5°C. Also the heat exchange gradually decreased after landing, and the portion dissipated via the bill became smaller; a puffin also loses heat through legs and feet. Apparently, the bill is most important for cooling shortly after flight.

The bill dissipates heat thanks to an extensive network of blood vessels; warm blood cools down a bit when circulating through these vessels, just as in a cassowary’s helmet.

The tufted puffin bears the white-yellow tufts to which it owes its name only during the breeding season, when both males and females are decorated. A pair produces one young per year, and the tasks are divided: both mom and dad will breed and feed their young.

Willy van Strien

Photos:
Large: Kuhnmi (Wikimedia Commons, Creative Commons CC BY 2.0)
Small: Matthew Zalewski (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

See how the cassowary manages to stay cool

Source:
Schraft, H.A., S. Whelan & K.H. Elliott, 2019. Huffin’ and puffin: seabirds use large bills to dissipate heat from energetically demanding flight. Journal of Experimental Biology 222: jeb212563. Doi:10.1242/jeb.212563

White bellbird is the noisiest

Female runs a risk of hearing damage

White bellbird sings the loudest call

To seduce a female, a male white bellbird calls out to her so loudly at close range, that she may suffer hearing damage, Jeffrey Podos and Mario Cohn-Haft think. Still, she has to expose herself to the deafening noise.

Not all songbirds have a pleasant song. There are also squeakers, males that call as loudly as possible. Their call definitively is impressive. Up to now, the South American screaming piha, which emits an ear-splitting lashing sound that is characteristic for South-American rainforest, held the record for the loudest bird call.

But now, it turns out not to be the noisiest; it is surpassed by the white bellbird from the northeast of the Amazon. Its call can be three times as loud as that of the screaming piha, Jeffrey Podos and Mario Cohn-Haft discovered. The song consists of two tones and sounds like a horn.

Males of screaming piha and white bellbird do not invest time in raising their young; breeding and feeding are females’ tasks. Males are free and try to mate as many females as possible. To outdo each other in attractiveness, they scream, often in loose groups.

The screaming piha relies completely on its vocalization, as it has a dull appearance. But in the white bell bird, the eye also is to be satisfied. The males are white and have a long black fleshy wattle on their forehead, which dangles along their beak.

Extremely loud

The louder the screaming piha and white bellbird scream, the shorter their call will last, as investigation by Podos and Cohn-Haft showed. Apparently, it is demanding to make such a loud noise. So, females can deduce what a male’s quality is from the volume it produces. Females aim to mate a high-quality male, because that will yield healthy, strong offspring. Moreover, sons of such father will also be able to scream loudly, and so be attractive.

To assess the males’ quality on base of their sound volume, females have to come close to them. For bellbird females, which approach a male up to a meter distance, that is no fun, the biologists think. The males have two versions of their song: they usually shout roughly at the level of the screaming piha. But they are able to call even more loudly, like a pneumatic drill, no less than three times as loud as a screaming piha. A bellbird male is able to produce this sound because of its sturdy muscular body.

Risk

When a female approaches a male closely, he will choose the extremely loud version. He sings the first tone in a crouched position, head and tail bent downwards, his back towards her. Then he swivels around in a split second to blast the second, loudest tone right in her face.

She anticipates,  and flutters away when he is about to erupt, but still she is so close that she might suffer hearing damage.

Despite that risk, a female will still join different males, in order to be able to make a choice. It is in his interest to shout as loudly as possible to present himself favourably; it is in her interest to expose herself to that deafening noise, so that she is able to assess his quality.

Willy van Strien

Photo: White bellbird, singing male. ©Anselmo d’Affonseca

Watch and listen to a screaming white bellbird

Compare the sound of screaming piha and white bellbird

Source:
Podos, J. & M. Cohn-Haft, 2019. Extremely loud mating songs at close range in white bellbirds. Current Biology 29: R1055–R1069. Doi: 10.1016/j.cub.2019.09.028

Exit through head plug

Dead host helps parasitoid wasp escape from crypt

Parasitoid wasp Euderus set manipulates its host into performing a nasty task

The parasitoid wasp Euderus set lays its eggs near oak gall wasps that develop within their gall. The parasitoid larva will consume its host. But first, the larva manipulates it into performing a nasty task. Otherwise the parasitoid would be buried alive in the oak gall.

The North American parasitoid Euderus set is a natural enemy of gall wasps that develop within galls on oak trees.  It does not attack all oak gall wasps species; hundreds of oak gall wasp species live in North America. But at least seven species fall victim, as Anna Ward and colleagues report.

The researchers discovered the wasp several years ago and named this ‘crypt-keeper wasp’ after Seth, the Egyptian god of darkness and chaos. According to some sources, Seth killed his brother Osiris by trapping him in a tailor-made sarcophagus and throwing him into the Nile. The behaviour of the parasitoid  wasp is as naughty. One of the victims is the oak gall wasp Bassettia pallida, and the researchers described what happens to the galler when Euderus set appears on the scene.

Head stuck

The gall wasp female lays her eggs under the bark of young oak branches. A branch then is induced by the gall wasp to form a separate crypt for each egg, in which the wasp will develop into a larva, pupa and adult. A gall develops in the branch. The adult gall wasp has to chew its way out through woody tissue and bark.

The researchers found holes in oak branches through which an adult gall wasp had emerged. But they also discovered holes in which the head of a gall wasp was stuck. It was a mystery: why did the gall wasp sometimes get stuck?

On inspection, they found a stranger in the chamber behind stuck gall wasp heads: a larva or pupa of a parasitoid, which had consumed the gall wasp partially or completely. That parasitoid was Euderus set. In some cases, the stuck gall wasp head was pierced; the chamber behind such head was empty, except for the remains of the gall wasp.

Nasty task

Here is what happens, according to the authors: a female parasitoid lays an egg in the chamber of a developing gall wasp; after hatching, the parasitoid larva will eat its gall wasp host when it has reached adult stage. But first, it makes the host do some work. The parasitoid induces the young gall wasp to excavate an emergence hole that is narrower than normal. As a result, the gall wasp gets stuck as soon as its head reaches the surface; the head plugs the exit hole. The parasitoid then consumes its host entirely, pupates, emerges as adult parasitoid and leaves the chamber via the empty body and stuck head of the gall wasp.

Rescue

How the parasitic wasp manipulates the behaviour of its host, is still unknown. But it is to its advantage, because there is little chance that it can chew its own way out through woody plant tissue and bark, as experiments showed. Without a passage in the form of the empty gall-wasp body and head, the parasitoid wasp would be buried alive.

Now, Ward showed that not only Bassettia pallida, but at least six other oak gall wasp species can be attacked by Euderus set. They live in similar galls that are integrated with an oak branch or leaf and that have no structures to keep enemies out, such as spines. This makes makes them vulnerable to Seth.

Willy van Strien

Photo: Andrew Forbes

On YouTube, the research group explains how parasitoid Euderus set manipulates its host

Sources:
Ward, A.K.G., O.S. Khodor, S.P. Egan, K.L. Weinersmith & A.A. Forbes, 2019. A keeper of many crypts: a behaviour-manipulating parasite attacks a taxonomically diverse array of oak gall wasp species. Biology Letters 15: 20190428. Doi: 10.1098/rsbl.2019.0428
Weinersmith, K.L., S.M. Liu, A.A. Forbes & S.P. Egan, 2017. Tales from the crypt: a parasitoid manipulates the behaviour of its parasite host. Proc. R. Soc. B 284: 20162365. Doi: 10.1098/rspb.2016.2365

Smart offer

How parasitic thorny-headed worm reaches the right host

On parasitized Gammarus shrimp, an orange dot is visible

When fresh water shrimp is parasitized by thorny-headed worm, the parasite is visible from the outside as an orange dot. Thanks to this striking spot, fish will easily detect the shrimp and ingest it, whereupon the parasite completes its development in the fish. According to Timo Thünken and colleagues, only fish that are suitable as hosts preferentially swallow infected shrimp.

The thorny-headed worm Pomphorhynchus laevis is a parasite with a complex life cycle, which takes place in fresh water. During the first part of that cycle, it develops within fresh water shrimp Gammarus pulex, after the shrimp ingested parasite eggs from the water. The parasite develops to a certain stage, the cystacanth.

thorny-headed wormWhen the parasite has reached that stage, Gammarus no longer can serve as a host. The parasite has to switch to fish to be able to complete its life cycle. In the new host, the parasite hooks onto the intestinal wall, matures and reproduces. Female parasites produce eggs that are released together with fish faeces, completing the cycle.

The switch from shrimp to fish can happen in only one way: fish must ingest parasitized shrimp. Timo Thünken and colleagues show how the parasite manages this process.

Manipulation by thorny-headed worm

Normally, Gammarus pulex, no more than 2 centimetres in length, try to avoid being swallowed by fish. The shrimp hide in darkness, avoid areas with fish odour and have an inconspicuous colour.

But a parasitic thorny-headed worm that reached the cystacanth stage will intervene. It changes the behaviour of the host that it no longer needs; the shrimp leave darkness and show a preference for water with fish odour. Moreover, the mature cystacanth turns orange, being visible from the outside as an orange dot on the host.

Parasitized Gammarus seem to offer themselves as prey to fish: fish will easily encounter them and detect them. And indeed, they consume many parasitized shrimp, as was shown earlier in three-spined stickleback. For Gammarus, this is the end of the story, but for the parasite the future is opened.

At least …. if it has ended up in a suitable host. Not all fish species that prey upon Gammarus are a suitable host for the parasite. It will not survive in fish that exhibit an effective immune response. Manipulating Gammarus confers a lower net benefit if it also increases the chance of the parasite to end up in the wrong host.

Barbel suitable, brown trout not

Now, Thünken shows that the manipulation is effective: only suitable host fish ingest a relatively large amount of parasitized Gammarus.

He discovered this in experiments in which he painted an orange dot on unparasitized shrimp, so that they looked like shrimp carrying a ripe cystacanth. He then offered these shrimp, together with unpainted conspecifics, to a number of fish species. The painted shrimp were not really parasitized, and so they behaved the same as the unpainted ones. In this way, Thünken was able to check whether all fish species, just like stickleback in the earlier experiments, preferentially eat coloured prey.

In another experiment, he fed parasitized Gammarus to fish. Four months later, he checked if the fish were carrying living parasites, in order to assess which fish species are suitable hosts.

One of the fish species used, barbel, mainly consumes Gammarus with an orange dot, as it turned out, so this fish will easily get infected with the parasitic thorny-headed worm. This is beneficial for the parasite, because barbel turned out to be a very suitable host.

Brown trout, on the other hand, was as likely to swallow painted Gammarus as unpainted shrimp; the colour change had no effect on this fish. That’s also beneficial, because brown trout turned out not to be a host in which the parasite can survive. The same findings – indifferent to the colour change, poor host – applied to two other fish species, perch and ruffe.

Beneficial

Conclusion: an orange dot on Gammarus has an effect on fish that can serve as host of the horny-headed worm, barbel as well as stickleback in the earlier tests. These fish consumed colour Gammarus relatively often. But for unsuitable fish – brown trout, perch and ruffe – it makes no difference whether their prey has an orange spot or not. So, the dot increases the chance that the parasite will switch to a suitable host without increasing the risk that it will end up in the wrong fish.

How the link between the fish’s sensibility to the prey colour and its suitability to act as host might have arisen, is another question which has not yet been answered.

Stickleback

Stickleback are suitable hosts, but they do not fully meet the pattern. In the new experiments, not all stickleback seem to preferentially consume Gammarus with an orange dot; some even avoided them. With regards to this fish species, the colour alteration of Gammarus can be counterproductive.

According to the researchers, this is because this small fish suffers more from parasitic infection than the other species, which are considerably larger. Stickleback living in an environment in which thorny-headed worm is abundant are likely to avoid infection by skipping coloured Gammarus prey from their diet, warned by the orange colour. For larger fish species, on the other hand, avoiding parasitic infection is not important enough to let prey go.

Willy van Strien

Photo’s: © Nicole Bersau/Uni Bonn
Large: fresh water shrimp Gammarus pulex with thorny-headed worm Pomphorhynchus laevis visible as orange dot
Small: adult thorny-headed worm

Sources:
Thünken, T.,  S.A. Baldauf , N. Bersau , J.G. Frommen & T.C.M. Bakker, 2019. Parasite-induced colour alteration of intermediate hosts increases ingestion by suitable final host species. Behaviour, online July 19. Doi: 10.1163/1568539X-00003568
Kaldonski, N., M.J. Perrot-Minnot, R. Dodet, G. Martinaud & F. Cézilly, 2009. Carotenoid-based colour of acanthocephalan cystacanths plays no role in host manipulation. Proceedings of the Royal Society B: 276: 169-176. Doi: 10.1098/rspb.2008.0798
Baldauf, S.A., T. Thünken, J.G. Frommen, T.C.M. Bakker, O. Heupel & H. Kullmann, 2007. Infection with an acanthocephalan manipulates an amphipod’s reaction to a fish predator’s odours. International Journal for Parasitology 37: 61-65. Doi: 10.1016/j.ijpara.2006.09.003
Bakker, T.C.M., D. Mazzi & S. Zala, 1997. Parasite-induced changes in behavior and color make Gammarus pulex more prone to fish predation. Ecology 78: 1098-1104. Doi: 10.1890/0012-9658(1997)078[1098:PICIBA]2.0.CO;2

Hidden eggs

Blue tit covers her clutch in case of danger

When a predator is around, female blue tits will hide their eggs

Are there any signs indicating that a predator is nearby? In that case, it is more likely that blue tit females will conceal the eggs, Irene Saavedra and colleagues show.

During the egg-laying period, blue tit females add a new egg to their clutch every day, and it was known that they sometimes deposit nest material on the eggs. When the clutch is completed, they start incubating. From that moment on, they no longer will cover the eggs, but are sitting on them continuously. Their male partners will bring them food.

Why do some females take the trouble to cover their clutch during the egg-laying period? One of the reasons, Irene Saavedra and colleagues hypothesized, may be to hide the eggs from predators. Blue tits breed in tree cavities, and also use nest boxes. A closed nest is safer than an open nest, such as that of a blackbird: larger predators cannot enter. But perhaps blue tit females take extra protective measures if needed.

Pungent

Experiments confirmed the hypothesis. During the egg-laying period, the biologists placed a piece of absorbent paper soaked with the urine and the anal gland fluid of a ferret, a marten-like predator, in a number of nest boxes; they pushed it between the floor and the nest. Such paper emits a strong scent. They already knew that blue tits recognize that scent and realize that it indicates danger. As a control, they placed a piece of paper with lemon scent or odourless wet paper in other nest boxes.

The blue tit mothers responded to the pungent predator’s smell. If a next box contained the scent, the chance that the occupant covered her clutch was higher than if a lemon odour was present or no odour at all. So, covering the eggs appears to be a measure to protect them if a predator is nearby; the tits may however have additional reasons to cover their clutch.

Whether the concealment helps in practice has not yet been investigated. It will not always do, because if a predator searches the nest thoroughly, he may find the hidden eggs.

Willy van Strien

Photo: N.P. Holmes (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Sources:
Saavedra, I. & L. Amo, 2019. Egg concealment is an antipredatory strategy in a cavity-nesting bird. Ethology, 5 augustus online. Doi: 10.1111/eth.12932
Amo, L., I. Galván, G. Tomás & J.J. Sanz, 2008. Predator odour recognition and avoidance in a songbird. Functional Ecology 22: 289-293. Doi: 10.1111/j.1365-2435.2007.01361.x