Category: defence (Page 2 of 4)

Attractive dark eyes

29 June 2020 / wvanstrien

Thanks to black irises, female guppy escapes from predator

By drawing the attention of a predatory fish to the eyes and turning the head away as soon as it strikes, a guppy female manages to evade. Robert Heathcote and colleagues report this hitherto unknown escape strategy.

When Trinidadian guppies detect a predatory fish, they will approach and inspect it to find out if it is hungry and dangerous. The colour of their irises may change when they do; normally the irises are silver, but then they often turn black, making the eyes more salient. It doesn’t seem profitable to draw an enemies attention to the head, so Robert Heathcote and colleagues wondered why guppies blacken their eyes. Is it to deter the enemy? Or is it to divert its attack? But then, how does it work?

By conducting a series of experiments, they found the answer: the colour change is part of a successful escape strategy.

Wild Trinidadian guppies, Poecilia reticulata, live in northeastern South America. One of their enemies is the cichlid Crenicichla alta, a predatory fish that ambushes its victims.

First, the researchers exposed wild guppies to a visually-realistic model of this predatory fish in a tank, and observed whether they blackened their eyes. As it turned out, large individuals do. These are mostly females, which are larger than males on average.

Attack

The predatory fish is not deterred by those dark eyes, as became evident from the next series of experiments, this time with live predatory fish and models of guppies with either black or silver irises. The cichlid attacks guppies with black eyes as often as those with silver-coloured irises. So, the first possible explanation fails.

The researchers also investigated at what target the predator lunges when attacking its prey. When irises are silver-coloured, the predator aims at the broadest part of the body, they discovered. In dark-eyed fish, the attack is diverted to the front. So, colour change of the irises appears to be a diversion strategy. But the predatory fish grasps both types of models, with black and silver irises, just as easily, so a guppy doesn’t benefit from a dark eye colour in itself.

Matador

However, colour change does help when combined with a critically timed evasive manoeuvre, as the last tests showed. In these tests, living guppies were placed in a tank with a living cichlid, but were separated from it by a transparent acetate screen, keeping them from danger. From the movements of the fish, filmed with a high-speed camera, the researchers were able to calculate, for each attack, the probability that the predator would have caught the victim in real conditions, without screen.

The moment the predatory fish strikes, a guppy quickly pivots around an imaginary vertical axis, accelerates and swims away. The imaginary axis runs through the broadest part of the body (more precisely, through the centre of mass), roughly the point that the cichlid aims for in a victim with silvery, less conspicuous irises. This part of the body hardly moves during the rotation. If the predatory fish directs its attack at that point, its chances to succeed are high, as the analysis showed.

The head, on the other hand, immediately leaves its position during the rotational movement. If the predatory fish charges at that part of the victim – like it does in a prey with black irises – it usually misses out.

By blackening its eyes, a guppy thus increases its chance to escape with a quick manoeuvre. The researchers compare this escape strategy – drawing the enemy’s attention to a particular point and then moving it quickly away – with the behaviour of a bullfighter, the matador with his red cape. Such escape strategy was previously unknown in animals.

Only females

For the strategy to be successful, the distance between the eye and the broadest part of the body must be sufficiently large. Males are too small. Males also have a striking eye-sized black spot on their body, which makes it more difficult to draw the predator’s attention to the head. So it makes no sense for males to blacken their irises in presence of a predatory fish, just increasing the detection risk. Accordingly, they don’t.

But females can trick their enemies by making their eyes stand out. The predator aims for her attractive eyes. And they’re gone.

Willy van Strien

Photo: Guppy, Poecilia reticulata, female with silver iris. H. Krisp (Wikimedia Commons, Creative Commons, CC BY 3.0)

Source:
Heathcote, R.J.P., J. Troscianko, S.K. Darden, L.C. Naisbett-Jones, P.R. Laker, A.M. Brown, I.W. Ramnarine, J. Walker & D.P., 2020. A matador-like predator diversion strategy driven by conspicuous coloration in guppies. Current Biology, online June 11. Doi: 10.1016/j.cub.2020.05.017

Expensive defence

18 June 2020 / wvanstrien

Ladybird cannot deal with all enemies at once

When a ladybird has to defer predators regularly, it is less able to resist pathogens and parasites, Michal Knapp and colleagues write.

When threatened, ladybird beetles try to avoid being eaten by excreting a yellow, smelly and bitter-tasting liquid from their legs. This reduces the appetite of hungry insects, lizards, birds or small mammals. The liquid is haemolymph, the insect variant of blood. You can see the phenomenon by provoking a ladybird.

But you shouldn’t do that, because ‘reflex bleeding’ decreases the ability to fight pathogens and parasites, as Michal Knapp and colleagues report.

They conducted experiments with the harlequin ladybird, Harmonia axyridis. The species originally lived in East Asia, was introduced in Europe and North America and nowadays also occurs in South America and Africa.

Precious blood

Haemolymph is an expensive means to scare away enemies. It contains nutrients, as well as blood cells, proteins and other compounds that ladybirds need to eliminate pathogens and parasites. The harlequin ladybird uses, among other compounds, the substance harmonine, which has a strong antimicrobial effect. Each bleeding causes a loss of these valuable components.

To measure the effect of this loss, Knapp triggered reflex bleeding in ladybirds twice a week, during three weeks. Contrary to his expectations, the treatment did not affect the survival of the beetles, and they did not lose weight.

He also, during a month, triggered newly hatched females daily to bleed, and found that their reproductive capacity was unaffected. In their first month of life, they produced as many eggs as females that were untreated. They started laying eggs a few days later, though, especially after losing a high volume of haemolymph. That may be of little importance, however, as the beetles live for months.

Costs

But bleeding, the defence mechanism against predators, comes at the expense of the resistance to other enemies, as it turned out. The concentration of blood cells and proteins in haemolymph had decreased. The concentration of harmonine and similar compounds has not been measured, but other research indicates that it also will have decreased.

Indeed, haemolymph of ladybirds that bled was found to inhibit bacteria less strongly. Probably, these ladybirds are less resistant to parasites as well, as blood cells take part in defence, but this has not been investigated.

Ladybirds successfully deploy constituents of haemolymph against all types of enemies – but they cannot fight them all at once at full power. If they have to deal with hungry predators frequently, their resistance to pathogens and parasites is reduced.

Willy van Strien

Photo: Harlequin ladybird, Harmonia axyridis. Timku (via Flickr, Creative Commons CC BY-NC-SA 2.0)

Source:
Knapp. M., M. Řeřicha & D. Židlická, 2020. Physiological costs of chemical defence: repeated reflex bleeding weakens the immune system and postpones reproduction in a ladybird beetle. Scientific Reports 10: 9266. Doi: 10.1038/s41598-020-66157-9

Synchronous calling for safety reasons

15 May 2020 / wvanstrien

Pug-nosed tree frog male refrains from calling first

As soon as a pug-nosed tree frog male starts calling, other males in the neighbourhood follow suit. After a short time of noise, it is quiet again for a long period. Henry Legett and colleagues found an explanation for this pattern.

In pug-nosed tree frog, aka Panama cross-banded tree frog (Smilisca sila), males face a difficult dilemma. The frog lives in Central America. In order to reproduce, males have to attract a female by calling, which they do in the evening from a location along or above a water stream. But their calls reveal their presence not only to females, but also to their natural enemies, the fringe-lipped bat (Trachops cirrhosus) and midges. The enemies use sound to localise their victims.

According to Henry Legett and colleagues, the frog males reduce the risk by creating an auditory illusion in their enemies.

That illusion arises by the way in which animals, including humans, process sound. If, with a short interval (milliseconds), two or more identical sounds are produced by sources that are close to each other, we will perceive this as one sound, which originated at the source that uttered it first. So, we ignore reflections that occur in a furnished room or a forest, hearing sounds clearly as a consequence. The priority given to the first sound is called the precedence effect.

Followers

Because of this effect, pug-nosed tree frog males that call nearly synchronously with another male, can hide from their enemies’ ears. And, according to playback experiments by the researchers, this works out pretty well. They used two speakers that almost simultaneously produced the call of a male; alternately, one or the other speaker was leading. The response of bats, midges and female frogs was observed.

As results suggest, a male that closely follows another male calling runs a smaller risk of being captured by a bat and attracts less mosquitoes than the predecessor.

So following pays off – at least as far as safety is concerned. But what about reproduction? If females also have more difficulty finding followers, males won’t benefit from auditory hiding.

But as it turns out, chances are not too bad. The precedence effect is strong in other frog species, such as the túngara frog (Engystomops pustulosus), which inhabits the same region and whose males also call at night, but not synchronously. Compared to túngara frog females, the effect is weak in pug-nosed tree frog females. They chose following males less often than predecessors, but the difference is small. Also followers are approached by females.

Silence

The question remains why any tree frog male is the first to start the synchronous calling. After all, being the predecessor, its attractive power to females is only a bit stronger, while it is more likely to be eaten and bitten.

On the other hand, someone has to do it. If all males would remain silent, nothing happens. But the restraint of males to be the first explains the long periods of silence, interspersed with short, sporadic bouts of calls.

Willy van Strien

Photos:
Large: Pug-nosed tree frog Smilisca sila. Brian Gratwicke (Wikimedia Commons, Creative Commons CC BY 2.0)
Samll: fringe-lipped bat. Karin Schneeberger alias Felineora (Wikimedia Commons, Creative Commons CC BY 3.0)

Sources:
Legett, H.D., C.T. Hemingway & X.E. Bernal, 2020. Prey exploits the auditory illusions of eavesdropping predators. The American Naturalist 195: 927-933. Doi: 10.1086/707719
Tuttle, M.D. & M.J. Ryan, 1982. The role of synchronized calling, ambient light, and ambient noise, in anti-bat-predator behavior of a treefrog. Behavioral Ecology and Sociobiology 11: 125-131. Doi: 10.1007/BF00300101

Joint forces against brood parasite

7 April 2020 / wvanstrien

When yellow warbler is warning, red-winged blackbird will attack

The yellow warbler utters a specific alarm call when a brood parasite is nearby. The red-winged blackbird picks up the signal and attacks, as Shelby Lawson and colleagues write. Together, the birds protect their nests.

A bird’s nest with eggs or young is vulnerable. One of the dangers is that a heterospecific bird will lay an egg in it and charge the parents with the care of a foster young, like the cuckoo does. The red-winged blackbird, which breeds in wet areas in North and Central America, runs such risk. Here the brown-headed cowbird is the ‘cuckoo’, the brood parasite.

Although a young cowbird, unlike a cuckoo chick, does not eject its foster brothers and sisters out of the nest, its presence is to their detriment. The foreign chick demands so much attention that the legitimate young will suffer and starve or fledge in a bad condition.

So, the red-winged blackbird must keep the cowbird out of its nest. It takes advantage of the vigilance of the yellow warbler, another passerine bird that is visited by the cowbird, Shelby Lawson and colleagues show. The yellow warbler, in turn, takes advantage of the aggression of the redwing.

Defense

When yellow warblers detect a brown-headed cowbird, they utter a specific alarm signal, a ‘seet’ call. Upon hearing that call, all females respond appropriately: they immediately return to their nest (if they were not already there), repeat the seet and sit tightly on their clutch. As a consequence, a cowbird has no access.

Yellow warblers utter the seet call only in response to the brood parasite and only during the breeding period. To warn of predators, they have a different signal, and upon hearing that call, females will change perches and remain alert, but they won’t return to the nest. The combination of the specific alarm signal for brood parasites and the appropriate response of females is unique.

The researchers wondered whether red-winged blackbirds eavesdrop on that specific signal and take advantage of it. They play backed different sounds nearby redwings’ nests and observed their responses.

Both redwing males and females became aggressive upon hearing the seet of yellow warblers and attacked the speaker. They reacted as heated as in response to the chatter of brown-headed cowbirds. Also the call of a blue jay, a nest predator, aroused their aggression. Apparently, the response to the seet call is a general defence against various dangers that threaten a nest. The birds neglected the song of an innocent songbird.

Chatter of other redwings elicited the strongest defence response; the birds seem to consider conspecifics that invade their territory to be the greatest risk.

Together

The yellow warblers’ signal to warn of brood parasites is picked up by red-winged blackbirds, which respond by approaching the danger. This is to the benefit of yellow warblers: previous research had shown that their nests suffer less from parasitism by cowbirds if they breed in the neighbourhood of red-winged blackbirds. Redwings and yellow warblers often nest in loose aggregations; together they are able to resist the brood parasite.

So far, the red-winged blackbird appears to be the only bird species that understands and responds to yellow warblers’ warning of brood parasites.

Willy van Strien

Photos:
Large: Red-winged blackbird. Brian Gratwicke. (Wikimedia Commons, Creative Commons CC BY 2.0)
Small, upper: Female brown-headed cowbird. Ryan Hodnett (Wikimedia Commons, Creative Commons CC BY-SA 4.0)
Small, lower: Male yellow warbler. Mykola Swarnyk (Wikimedia Commons, Creative Commons CC BY-SA 3.0)
https://commons.wikimedia.org/wiki/File:Yellow_Warbler_Setophaga_aestiva_m_Toronto1.jpg

Researchers tell about their work on YouTube

Sources:
Lawson, S.L., J.K. Enos, N.C. Mendes, S.A. Gill & M.E. Hauber, 2020. Heterospecific eavesdropping on an anti-parasitic referential alarm call. Communications Biology 3: 143 . Doi: 10.1038/s42003-020-0875-7
Gill, S.A. & S.G. Sealy, 2004. Functional reference in an alarm signal given during nest defence: seet calls of yellow warblers denote brood-parasitic brown-headed cowbirds. Behavioral Ecology and Sociobiology 5671-80. Doi: 10.1007/s00265-003-0736-7
Clark, K.L. & R.J Robertson, 1979. Spatial and temporal multi-species nesting aggregations in birds as anti-parasite and anti-predator defenses. Behavioral Ecology and Sociobiology 5: 359-371. Doi: 10.1007/BF00292524

Upside-down jellyfish stings at a distance

10 March 2020 / wvanstrien

Mucus contains numerous stinging-cell structures

The water around upside-down jellyfish is dangerous for small animals and itching for snorkelers. Mobile cell structures, released by the jellyfish, are responsible, as Cheryl Ames and colleagues show.

The upside-down jellyfish Cassiopea xamachana doesn’t swim like jellyfish normally do, but settles upside down on muddy soils of mangrove forests, seagrass beds or shallow bays, its eight oral arms with exuberantly branched flaps facing upward. These jellyfish occur in warm parts of the western Atlantic Ocean, the Caribbean Sea and the Gulf of Mexico, often in large groups.

The habit of lying on the bottom is not the only odd trait of this animal. It is also unusual in hosting unicellular organisms inside its body, the so-called zooxanthellae. Like plants, these organisms convert carbon dioxide and water into carbohydrates and oxygen, using energy from sunlight. They donate part of the carbohydrates to the jellyfish in exchange for their comfortable and safe accommodation.

And then there is a third peculiarity: the water surrounding a group of upside-down jellyfish ‘stings’, as snorkelers know. Cheryl Ames and colleagues discovered how the upside-down jellyfish is responsible.

Mobile cell structures

The carbohydrates that upside-down jellyfish receive from the resident microorganisms are the main source of energy. But the jellyfish also need proteins. That is why they supplement the diet with animal food.

To capture prey, jellyfish use stinging cells. These cells contain stinging capsules, ‘harpoons’, and are filled with a poison blend; the harpoons are able paralyze or kill small critters. Their stings also scare off enemies.

Upside-down jelly has stinging cells on its oral arms. The animal is pulsating, causing water movements that drive prey to the arms, where it is trapped. But, unlike other jellies, the upside-down jellyfish also is able to sting at a distance. How?

If prey is around or if the jellyfish is disturbed, it releases large amounts of mucus, which contain microscopic spherical bodies with an irregular surface, as the current research shows in detail. The bodies consist of an outer cell layer, with stinging cells and ciliated epithelial cells. The content is gelatinous like the jellyfish itself; often zooxanthellae are present, but whether they are active and provide carbohydrates is unknown.

Killing

The cell structures, which the researchers have termed cassiosomes, are produced in large quantities on the jellyfish’s arms. Whenever disturbed, the jelly starts emitting them after five minutes in a mucus cloud and continues for hours. Thanks to the cilia, the spherical bodies are motile. They swim around in the mucus for fifteen minutes and then sink down. They go on rotating and displacing for days, and gradually become smoother and smaller to eventually disintegrate after ten days.

The cassiosomes are capable of killing prey animals, laboratory tests show. Brine shrimp, for example, is often instantly killed upon contact with the cell structures.

While doing their work, the researchers experienced that the water in the test tanks was indeed stinging.

Of all peculiarities that upside-down jellyfish possess, this may well be the strangest: loose jellyfish pieces that remain alive for days independently of the main body, move around and help capture prey and scare enemies. The researchers now know that a few closely related jellyfish species release similar small ‘grenades’.

The cell masses in the mucus of upside-down jellyfish had been seen before, at the beginning of the twentieth century, but were thought to be parasites. Nobody could not fancy by that time that it was jellyfish tissue.

Willy van Strien

Photo: Bjoertvedt (Wikimedia Commons, Creative Commons CC BY-SA 4.0)

Source:
Ames, C.L., A.M.L. Klompen, K. Badhiwala, K. Muffett, A.J. Reft, M. Kumar, J.D. Janssen, J.N. Schultzhaus, L.D. Field, M.E. Muroski, N. Bezio, J.T. Robinson, D.H. Leary, P. Cartwright, A.G. Collins & G.J. Vora, 2020. Cassiosomes are stinging-cell structures in the mucus of the upside-down jellyfish Cassiopea xamachana. Communications Biology 3: 67. Doi: 10.1038/s42003-020-0777-8

From reliable source?

11 February 2020 / wvanstrien

Nuthatch transmits indirect information only partially

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

Nuthatches 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

Peaceful together

24 December 2019 / wvanstrien

Dangerous bullet ant and defensive 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.

The 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

Hidden eggs

30 August 2019 / wvanstrien

Blue tit covers her clutch in case of danger

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

Vine avoids spider mites

17 May 2019 / wvanstrien

Tendrils curl away from herbivore-infested plants

When the Asian climbing plant Cayratia japonica stretches its tendrils to other plants, it is careful. The tendrils withdraw as soon as they detect the presence of spider mite, as Tomoya Nakai & Shuichi Yano observed.

The Asian vine Cayratia japonica is an excellent climber: in America, where it was introduced, it is known as bushkiller. Tendrils of the plant coil around stems of neighboring plants, enabling the vine to grow towards the light. The tendrils grab onto everything they can.

Well, not everything really. The tendrils withdraw when they touch upon a plant that is infested with two-spotted spider mite, Tomoya Nakai & Shuichi Yano show. Two-spotted spider mite or red spider mite (Tetranychus urticae) is a small arachnid hat sucks up plant sap from leaves, which often don’t survive it. The mites occur on hundreds of plant species. If their number at some place is too high, they will walk to another place. As they follow each other’s trails, a group will soon aggregate at this new site.

Spider mite web

Because of its physical contact with other plants, a vine could easily get infested by these harmful critters. But Cayratia japonica appears to have an effective way to prevent mites from invading. As soon as a tendril touches a plant that is occupied by mites, it withdraws and curls away from the infested plant. The researchers could show this in the lab, by placing a number of vines each next to a bean plant that was either clean or bearing many mites. They filmed the movement of the vine’s tendrils using time-lapse photography, making one film frame per minute.

The next question was: what cue does a tendril use to detect the presence of spider mite? Does it pick up the volatile compounds that a bean plant releases into the air when infested? Or does it feel the web with which the mites cover the plant surface to be safe underneath from predators?

Experiments showed that the volatile compounds released by infested bean plants have no effect on the stretching tendrils. But mite silk does: after contact with a spider mite web, the tendrils immediately withdraw. Nakai and Yano also tried spider silk, but the tendrils did not respond to it. The vine thus responds directly and specifically to the presence of spider mite.

This reduces the chance that mites disperse in groups from support plants to the climbing plant. A few of them will cross over during the short contact, but they are not save without the web and will disappear.

Willy van Strien

Poto: 石川 Shihchuan (via Flickr. Creative Commons CC BY-NC-SA 2.0)

Source:
Nakai, T. & S. Yano, 2019. Vines avoid coiling around neighbouring plants infested by polyphagous mites. Scientific Reports 9: 6589. Doi: 10.1038/s41598-019-43101-0

Suicidal repair team

2 May 2019 / wvanstrien

Young aphids die when closing a hole in their nest

Japanese aphids, Nipponaphis monzeni, inhabit galls on hazel. A hole in the gall wall would mean the end of the colony living there, were it not for aphid soldiers that give their lives to close it. Mayako Kutsukake and colleagues show how.

The Japanese aphid Nipponaphis monzeni is a social species, living in colonies. Juveniles, called nymphs, serve as soldiers for a period before they become adults and reproduce. It is their task to defend the nest, which is located in galls on the branches of evergreen witch hazel (Distylium racemosum), and to repair it in case of damage.

To close a hole, they show a spectacular and unique behaviour. In a self-destructive action, they discharge their body fluid to plug the gap. The liquid solidifies, forming a scab. Mayako Kutsukake and colleagues were curious about the mechanism.

Vulnerable nest

Colonies of Nipponaphis monzeni are founded by females that reproduce parthenogenetically. A colony of sisters is formed that are genetically identical and produce identical daughters.

The aphids induce the hazel on which they live to form a closed, hollow tumour, a gall. The animals inhabit this gall, sucking plant sap from the inner wall; in this phase, they are wingless. The gall remains small for a long time, but after three to five years it begins to grow rapidly during spring months and the following summer, it is fully grown – up to eight centimetres long – and home to thousands of aphids.

Winged aphids then appear in autumn. They make an opening in the wall and fly away to a second host tree, an oak, where they mate and produce a new generation of colony foundresses.

A full-grown gall has a lignified, hard wall, offering safety. But during growth, the wall consists of soft plant tissue and the nest is vulnerable. Moth caterpillars consuming hazel tree leaves easily tunnel into such gall, ingesting aphids as well. The soldiers will not tolerate this and attack the enemy: they climb onto it and sting it to death with their mouth parts.

But the hole that the caterpillar gnawed in the gall wall still remains. It has to be closed, otherwise enemies or pathogens may invade, or the nest may desiccate.

Skilful plastering

Japanese researchers had already shown how the soldier nymphs repair the hole with a self-sacrificing behaviour. Dozens or hundreds of them gather around the hole and eject large amounts of white body fluid (hemolymph, which is comparable to our blood) through two tubes on the abdomen. They mix the secretion with their legs and skilfully plaster it over the hole. Some soldiers are buried, others are locked out in the process. And all shrivel after losing their body fluid and will die.

Any way, the hole is fixed; the plug hardens and turns black. As a result, the colony is likely to survive the damage. After the sealing, the gall wall is healed, as the soldiers trigger the tree to cover the plasterwork on the inside by regenerating plant tissue.

Coagulation

Now, Kutsukake investigated the substances with which the soldiers repair a hole. The body fluid, she shows, contains many peculiar large cells of a hitherto unknown type that are packed with fat droplets and the enzyme phenoloxidase; the fluid contains long proteins and tyrosine, an amino acid.

When the soldiers discharge their body fluid, the cells rupture and the fat globules are released; the soldiers plug the gap immediately with a soft lipidic clot. At the same time, the other components come into contact with each other, and a coagulation process starts in which the proteins are linked to form a network that reinforces the lipid plug so that it becomes a scab.

The researchers assume that the process is derived from the process by which wounds heal. But in soldier nymphs’ hemolymph, the components are accumulated in extremely large quantities, far beyond what is necessary for wound healing.

With their unique repair behaviour, the soldier nymphs of Nipponaphis monzeni exhibit extreme altruism to defend the colony: they give their lives. Thanks to this sacrifice, a large part of their family survives. Otherwise the entire colony would have been lost.

Willy van Strien

Photos : ©Mayako Kutsukake
Large: Nipponaphis monzeni soldier nymphs plastering their hemolymphe over a hole
Small: gall in which Nipponaphis monzeni lives

On YouTube, the researchers show how soldiers fix a hole in the gall wall

Sources:
Kutsukake, M., M. Moriyama, S. Shigenobu, X-Y. Meng, N. Nikoh, C. Noda, S. Kobayashi & T. Fukatsu, 2019. Exaggeration and cooption of innate immunity for social defense. PNAS, 15 april online. Doi: 10.1073/pnas.1900917116
Kutsukake, M., H. Shibao, K. Uematsu & T. Fukatsu, 2009. Scab formation and wound healing of plant tissue by soldier aphid. Proceedings of the Royal Society B 276: 1555-1563. Doi: 10.1098/rspb.2008.1628
Kurosu, U., S. Aoki & T. Fukatsu, 2003. Self-sacrificing gall repair by aphid nymphs. Proceedings of the Royal Society London B (Suppl.) 270: S12-S14. Doi: 10.1098/rsbl.2003.0026

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