Category: defence (Page 1 of 4)

Hidden eyes

3 February 2025 / wvanstrien

Eye spots are effective only from a refuge

The eyespots of a spicebush swallowtail caterpillar protect the animal from hungry birds – but only when the caterpillar is concealed, Elizabeth Postema shows.

An older caterpillar of the spicebush swallowtail butterfly Papilio troilus has an appealing swollen head with two eyespots. It is obvious that these eyespots serve to deter predators. But they do so only under special conditions, Elizabeth Postema writes.

The swallowtail occurs in North America; the caterpillars live on leaves of trees and shrubs such as sassafras and American tulip tree. A common predator of caterpillars is the black-capped chickadee (Poecile atricapillus), a tit species.

Frightening

A spicebush swallowtail caterpillar sitting openly on a leaf is visible; it is green, but in a slightly different shade than the leaf. The eye spots make it even more conspicuous. That is why a caterpillar hides during the day to escape detection by hungry tits and other predators. Lying on the midrib of a leaf with its head turned towards the tip, it exudes silk. The silk dries and shrinks, forcing the leaf to fold around the caterpillar.

But what is the use of eyespots to a hidden caterpillar? Postema assumed that these eyespots are important when a bird peers into a leaf roll or picks it open. It will then suddenly see a snout with two eyes – an imitation of a snake – and be startled by it. Two eyespots that suddenly appear, the idea is, have a completely different effect than two eyespots that are continuously visible from far away.

Caterpillars of modelling clay

She tested this hypothesis with artificial caterpillars. She made hundreds of green caterpillars of modelling clay with and without eyespots and attached them to tree leaves, which she then folded around the caterpillar or not. So, there were four experimental groups: visible caterpillar without eyespots, visible caterpillar with eyespots, hidden caterpillar without eyespots and hidden caterpillar with eyespots. After five days, she looked for her artificial caterpillars and checked whether they showed bite marks from birds.

As expected, she discovered that a leaf roll offers protection. Caterpillars that were sitting on a leaf in the open were attacked more often than caterpillars that were hidden.

Eye spots did not help the visible caterpillars: tits did not care. But caterpillars in a leaf roll – which were already safer – were even better off with eyespots. Eye spots made the chance of an attack smaller. Postema’s assumption appears to be correct.

The conclusion is that spicebush swallowtail caterpillars protect themselves by combining eyespots with a refuge, so that the ‘eyes’ suddenly pop out in the event of acute danger.

Young caterpillars have no eyespots, but use a different defense strategy: predators overlook them because they are brown and resemble bird droppings.

Willy van Strien

Photo:
Large: caterpillar of spicebush swallowtail Papilio troilus. NCBioTeacher (Wikimedia Commons, Creative Commons, Public Domain)
Small: adult spicebush swallowtail. Robert Webster (Wikimedia Commons, Creative Commons CC BY-SA 4.0)

Source:
Postema, E.G., 2024. Eyespot peek-a-boo: Leaf rolls enhance the antipredator effect of insect eyespots. Journal of Animal Ecology, online December 25. Doi: 10.1111/1365-2656.14232

Beetle mimics noxious moths

31 May 2024 / wvanstrien

Bats leave tasty tiger beetle alone

Tiger beetles imitate ultrasonic sounds of unpalatable tiger moths

By mimicking the ultrasonic sound of noxious tiger moths, nocturnally active tiger beetles protect themselves against foraging bats, as Harlan Gough and colleagues write.

Some tiger beetles fly at night. It means that they must be afraid of hunting bats, predators that search for prey by emitting ultrasonic (very high-pitched) clicks and deducing from the reflected sound where a tree or a building is – or where a tasty insect snack is flying. This so-called echolocation allows bats to ‘see’ in the dark. Many insects perceive the ultrasonic clicks of an attacking bat and respond by fleeing or diving to avoid the enemy.

There are a few tiger beetles that react differently: they produce an ultrasonic sound in response to an approaching bat. Harlan Gough and colleagues wanted to know why.

Ultrasonic response

The only other insects known to respond with ultrasonic sound to a hunting bat are moths; an estimated 20 percent of moths responds, at a pitch that bats hear well. The sounds have several effects. Some moths disrupt the reflected bat sound by their calls, so that the bat no longer can interpret the noise. Other moths warn with their sound that they are distasteful or poisonous; once a bat has tasted such a species, it will leave it alone from then on. And other, non-poisonous moths benefit from this: they imitate the sound of a noxious species so that a bat let them also go.

And what about the tiger beetles that produce ultrasonic sound in response to a bat? What do they achieve by doing so?

The researchers tested nineteen tiger beetle species, beetles from the Cicindelidae family, from southern Arizona (USA). They exposed the beetles in the lab to the ultrasonic clicks of a bat that is about to attack. Seven of these nineteen species responded with their own ultrasonic sound, all being species that are active at night. The other twelve species stay put at night and therefore do not need to defend themselves against bats.

Tiger beetles and tiger moths

Do tiger beetles flying at night disrupt the echolocation of bats by jamming? No, the authors write, because that would require a more intensive sound (in technical terms: a higher duty cycle) than the beetles can produce.

Is their ultrasonic sound a warning that they are unpalatable? That is also not the case. The beetles do contain a repellent substance, benzaldehyde, which has an almond scent. But bats still like to eat them, as is evident from experiments with the big brown bat, Eptesicus fuscus. Apparently, the concentration of benzaldehyde is too low to deter this predator. The substance may help against small enemies such as ants and robber flies.

Unpalatable tiger moth warns bat predator with ultrasonic sound

Maybe they imitate the ultrasonic sound of noxious moths? To evaluate that hypothesis, the researchers compared the sounds of tiger beetles with existing sound records of sympatric tiger moths, moths of the subfamily Arctiinae, some of which are poisonous. And yes: the sound produced by tiger beetles is similar to that of poisonous tiger moths. The beetles seem to practice acoustic mimicry.

Wings

Moths that produce ultrasonic sounds do so in diverse ways. They have special structures, such as tiny combs. Tiger beetles produce sounds by brushing their beating hindwings along the back edges of the rigid forewings, the elytra. Normally, they hold the elytra up during flight, but to make sound, they lower them slightly.

Definitive evidence for acoustic mimicry by tiger beetles is still lacking. This would require behavioural tests with bats to find out whether they indeed ignore the beetles after having tasted an unpalatable tiger moth.

Willy van Strien

Photos:
Large: Ellipsoptera marutha, Aridland Tiger Beetle, is one of the species that mimics tiger moths. Laura Gaudette (Wikimedia Commons, Creative Commons CC BY 4.0)
Small: unpalatable tiger moth Cisthene martini, Martin’s Lichen Moth. Ken-ichi Ueda (Wikimedia Commons, Creative Commons CC BY 4.0)

Sources:
Gough, H.M., J.J. Rubin, A.Y. Kawahara & J.R. Barber, 2024. Tiger beetles produce anti-bat ultrasound and are probable Batesian moth mimics. Biology Letters 20: 20230610. Doi: 10.1098/rsbl.2023.0610
Barber, J.R., D. Plotkin , J.J. Rubin, N.T. Homziak, B.C. Leavell, P.R. Houlihan, K.A. Miner, J.W. Breinholt, B. Quirk-Royal, P. Sebastián Padrón, M. Nunez & A.Y. Kawahara, 2022. Anti-bat ultrasound production in moths is globally and phylogenetically widespread. PNAS 119: e2117485119. Doi: 10.1073/pnas.2117485119
Corcoran, A.J., W.E. Conner & J.R. Barber, 2010. Anti-bat tiger moth sounds: form and function. Current Zoology 56: 358-369. Doi: 10.1093/czoolo/56.3.358

Egg signature

21 August 2023 / wvanstrien

African cuckoo stands little chance with fork-tailed drongo

African cuckoo is not successful with fork-tailed drongo

African cuckoo females lay their eggs in nests of fork-tailed drongos. They mimic drongo eggs very accurately – and yet drongos recognize more than 90 percent of cuckoo eggs, Jess Lund and colleagues show.

South of the Sahara lives the African cuckoo, Cuculus gularis, which, like the common European cuckoo, lays eggs in the nests of other bird species (one egg per nest) with the intention that foster parents will raise their chicks. The brood parasite targets only a few bird species, of which the fork-tailed drongo, Dicrurus adsimilis, is one of the most important.

But the cuckoo has hardly any success with this important host species, Jess Lund and colleagues show. The intended foster mother usually notices the deception because she has put a ‘signature’ on her own eggs for verification.

It is the outcome of the long evolutionary history that is shared by African cuckoo and fork-tailed drongo. There is a major conflict between both bird species, because the brood parasite fully depends on the services of the foster parent, and the burden on the foster parent is enormous.

Arms race

It starts with the fact that an African cuckoo female destroys a drongo egg after arriving to lay an egg in the nest of a fork-tailed drongo couple. The cuckoo chick finishes the job. It hatches first and pushes the drongo eggs out of the nest; if a chick happens to have hatched already, it is also thrown out. The foster parents lose their entire clutch. And they are busy for weeks with the demanding care of the foster chick.

This conflict with major interests created an arms race. The drongo learned to recognize cuckoo’s eggs and to reject them. In response, the cuckoo developed eggs that increasingly resembled drongo eggs. Currently, the mimicry is almost perfect: in the eyes of drongos, cuckoo eggs look exactly like drongo eggs.

Individual signature

Drongo eggs are hugely variable. The background colour ranges from white to reddish brown, and the eggs can be immaculate, speckled, or blotched. Between eggs of the African cuckoo, the same variation exists. The mimicry is excellent on population level, and the African cuckoo seems to be ahead in the arms race.

But in reality, the fork-tailed drongo is the winner.

That is because a drongo female consistently produces eggs with the same look. Each female has her own characteristic colour and pattern. She puts, as it were, a distinctive signature on each egg for verification: I laid this one. A cuckoo female lays eggs that fall within the drongo variation, but she lays them randomly. Chances are small that she lays an egg in the nest of a drongo female that produces exactly the same egg type. The cuckoo egg usually is aberrant.

Protected

Conducting experiments and using models, the researchers predict how likely it is that a fork-tailed drongo will recognize and reject an egg of the African cuckoo in her nest. And that is more than 90 percent! Without individual egg signatures, that chance would be much smaller. So, the strategy of drongos – great variation between clutches, great uniformity within clutches – is an excellent response to the almost perfect mimicry of cuckoos, protecting the drongo effectively against the brood parasite.

And so, the African cuckoo has little success with this host. A cuckoo’s egg seldomly is accepted. If you consider that about one in five drongo nests is lost during breeding, the brood parasite has an extremely low reproductive success. But apparently, that low success is enough for the species to survive.

Willy van Strien

Photo: African cuckoo. Alastair Rae (Wikimedia Commons, Creative Commons CC BY-SA 2.0)

Sources:
Lund, J., T. Dixit, M.C. Attwood, S. Hamama, C. Moya, M. Stevens, G.A. Jamie & C.N. Spottiswoode, 2023. When perfection isn’t enough: host egg signatures are an effective defence against high-fidelity African cuckoo mimicry. Proceedings of the Royal Society B, online 26 July. Doi: 10.1098/rspb.2023.1125
Stoddard, M.C., R.M. Kilner & C. Town, 2014. Pattern recognition algorithm reveals how birds evolve individual egg pattern signatures. Nature Communications 5: 4117. Doi: 10.1038/ncomms5117

Venom with a history

9 August 2023 / wvanstrien

Asp caterpillar defends itself with bacterial protein

Asp caterpillar Megalopyge opercularisdelivers a painful sting

Flannel moth caterpillars have a venom that is unique among moths and that causes excruciating pain, deterring predators. Andrew Walker and colleagues unlocked the surprising origin of this venom.

Caterpillars of flannel moths have a cuddly appearance: they have a ‘fur’ of long, often curly hairs. But it is not a good idea to touch them, because spines are hidden under the hairs that inject a venom when touched. The result is excruciating pain that can last for hours or days. Flannel moths form the family Megalopygidae, which has about 250 species that live in North, Central and South America. Their caterpillars are known as asp caterpillars or puss caterpillars.

Certain proteins in the toxic blend of the caterpillars are responsible for the pain. These proteins have a special evolutionary history, Andrew Walker and colleagues discovered.

Holes

The researchers were curious about the composition and mode of action of asp caterpillar venom. They took a closer look at two species: the southern flannel moth Megalopyge opercularis and the black-waved flannel moth Megalopyge crispata. First, they were surprised to find that the toxic proteins, which they call megalysins, closely resemble toxic proteins from disease-causing bacteria, such as Clostridium. The bacterial proteins are harmful because they puncture victims’ cells. And in experiments, the toxic proteins of asp caterpillars turned out to do exactly the same: they punch holes in animal nerve cells. The nerve cells then fire signals that cause the sensation of pain.

There are more species of butterflies and moths with venomous caterpillars, but they have very different types of venom. The venom of the Megalopygid family is unique among the Lepidoptera. Isn’t it strange that caterpillars of this family make the same type of toxic proteins as bacteria? Is that a coincidence?

Defence

No, it’s not. An ancestor of butterflies and moths once obtained genes that code for pore-forming proteins from bacteria, and the butterflies and moths conserved these genes (horizontal gene transfer between species occurs seldomly in evolution). Apparently, the proteins are useful for them, but what function they have is not yet known. In any case, they are not used as venom.

That is, except for members of the Megalopygid family. They restored the function of these proteins as venom, with which caterpillars defend themselves against their predators.

Asp caterpillar is mimicked by bird

And that works great. Once an animal has tried to handle an asp caterpillar and got stinged, it will leave similar critters alone henceforth. Young of the cinereous mourner (Laniocera hypopyrra, a South American passerine bird) take advantage of this. They convincingly mimic the appearance and behaviour of an asp caterpillar, and without being venomous themselves, they still deter predators.

Flannel moths aren’t the only animals that use this type of pore-forming bacteria-derived proteins as venom. Some centipedes, cnidarians and fish do as well.

Willy van Strien

Photo: asp caterpillar of southern flannel moth Megalopyge opercularis. Judy Gallagher (Wikimedia Commons, Creative Commons CC BY 2.0)

Researchers explain their work on YouTube

Sources:
Walker, A.A., S.D. Robinson, D.J. Merritt, F.C. Cardoso, M.H. Goudarzi, R.S. Mercedes, D.A. Eagles, P. Cooper, C.N. Zdenek, B.G. Fry, D.W. Hall, I. Vetter & G.F. King, 2023. Horizontal gene transfer underlies the painful stings of asp caterpillars (Lepidoptera: Megalopygidae). PNAS 120: e230587110. Doi: 10.1073/pnas.2305871120
Londoño, G.A., D.A. García & M.A. Sánchez Martínez, 2015. Morphological and behavioral evidence of Batesian mimicry in nestlings of a lowland Amazonian bird. The American Naturalist 185: 135-141. Doi: 10.1086/679106

Cleaning ants are successful

14 February 2023 / wvanstrien

Metarhizium fungus makes fewer victims

Ants defend themselves against disease-causing Metarhizium fungus by grooming off fungal spores from each other. Prolonged exposure to that cleaning behaviour makes the fungus less deadly, Miriam Stock and colleagues show.

Metarhizium fungus can quickly spread throughout an ant nest because the ants easily infect each other with fungal spores. But the animals take action to inhibit the pathogen. That does not leave the fungus unaffected, Miriam Stock and colleagues show with experiments.

To counteract the fungus, ants can disinfect nest and brood (eggs, larvae and pupae) with a mixture of formic acid, which they produce in a poison gland, and tree resin. In addition, a sick ant stays away from the brood and spends more and more time outside the nest so as not to endanger its nest-mates. And the animals keep each other clean. If spores of the fungus land on an ant, her nest-mates either groom off the spores, risking infection themselves, or spray them with formic acid.

New spores

These caring nest-mates should act quickly. The spores attach on the affected ant and germinate, after which nothing can be done anymore. The fungus penetrates the body to develop, eventually killing the ant. Then the fungus appears on the cadaver forming spores that make new victims in the next infection cycle.

Conducting experiments with the Argentine ant, Linepithema humile, Stock shows that timely care does indeed help; the presence of other ants reduces the chance that an ant dies after contact with fungal spores.

But, as it turns out, cleaning also causes changes in the fungus.

Metarhizium-fungus adapts

The trials consisted of series in which the Metarhizium fungus passed repeatedly via spores from a dead ant to a new victim. In half of these series, the infected ant was held isolated, in the other half she was accompanied by two nest-mates that could remove the fungal spores. Conducting a final test after ten infection cycles, the researchers allowed the fungus to infect either an isolated ant or an ant with company.

In the final test, fungal lines that had grown on isolated ants caused a lot of mortality among newly infected ants when they did not receive care from others. But fungal lines that had infected ants that were in company of other ants – that could groom them -, had changed. They formed twice as many spores, but nevertheless made fewer victims among ants they came into contact with, even if there were no nest-mates around to help. These fungal lines had become less deadly.

Essential component

And there was something else: the spores of those ‘social fungal lines’ were less well detected and removed by the ants. The researchers discovered that these spores produced less ergosterol; this is a compound that occurs in all fungi and that, apparently, arouses the ants. So, the ‘social fungus lines’ evade defence by the ants.

But this comes at a cost. Ergosterol is an essential component of the spore membrane. The fact that the ‘social lines’ have lower levels of this important component probably explains why they are less deadly.

So, grooming each other to remove Metarhizium fungus spores as ants do is useful in two ways. It works immediately if ants quickly remove spores from a nest-mate, saving her from death. And in the longer term, it makes the fungus less dangerous.

Willy van Strien

Photo: Argentine ants exchanging food. Davefoc (Wikimedia Commons, Creative Commons CC BY-SA 4.0)

See also: ants disinfect their nest with a mixture of resin and formic acid

Sources:
Stock, M., B. Milutinović, M. Hoenigsberger, A.V. Grasse, F. Wiesenhofer, N. Kampleitner, M. Narasimhan, T. Schmitt & S. Cremer, 2023. Pathogen evasion of social immunity. Nature Ecology & Evolution, online February 2. Doi: 10.1038/s41559-023-01981-6
Brütsch, T., G. Jaffuel, A. Vallat, T.C.J. Turlings & M. Chapuisat, 2017. Wood ants produce a potent antimicrobial agent by applying formic acid on tree-collected resin. Ecology and Evolution 7: 2249-2254. Doi: 10.1002/ece3.2834
Bos, N., T. Lefèvre, A.B. Jensen & P. D’Ettore, 2012. Sick ants become unsociable. Journal of Evolutionary Biology 25: 342-351. Doi: 10.1111/j.1420-9101.2011.02425.x
Chapuisat, M., A. Oppliger, P. Magliano & P. Christe, 2007. Wood ants use resin to protect themselves against pathogens. Proceedings of the Royal Society B 274: 2013-2017. Doi: 10.1098/rspb.2007.0531

Colour meanings

3 August 2022 / wvanstrien

Aegean wall lizard with white throat is more brave

An Aegean wall lizard with striking throat colour will run off fast when a predator looms, Kinsey Brock and Indiana Madden write.

In Aegean or Erhard’s wall lizard, Podarcis erhardii, different colour morphs exist: the animals have either a white, yellow or orange throat. The lizards can be found on walls in South-eastern Europe, in a dry landscape with tough shrubs. They have several predators: snakes, birds, and mammals.

When a predator appears, a lizard will flee. But that implies that it must stop what it was doing: sunbathing or foraging for food. For that reason, it will not leave until necessary. Kinsey Brock and Indiana Madden wanted to know whether the three colour morphs have a similar flight initiation distance. They checked the distance they could approach a lizard before it ran away.

Careful

The throat colour of the Aegean wall lizard is genetically determined. Most animals, males and females alike, have a white throat; yellow and orange are less common. There are also individuals with mosaic throat colours, but they are rare. Brock and Madden investigated lizards with plain throat colour on the Greek island of Naxos.

You can get most closely to the white-throated wall lizards, they found; lizards with an orange throat run off earliest; yellow-throated animals are in between.

So, animals with an orange throat are the most careful. They also stay closest to a refuge: a crevice in a wall or dense vegetation. And once they fled, they are slower to reappear than animals with yellow or white throats.

It is in line with lab research showing that white-throated males are the most aggressive, bold, and brave.

Striking colour

An orange-throated Aegean wall lizard probably is more wary because it is more detectable. The grey-brown blotchy body has a camouflage colour, but a yellow, and especially an orange throat stands out against the background. This makes it easier for a predator to discover a lizard with an orange throat, so, in turn, it must flee earlier to escape from the enemy.

Willy van Strien

Photo: Male Podarcis erhardii with white throat. Gailhampshire (Wikimedia Commons, Creative Commons CC BY 2.0)

Source:
Brock, K.M. & I.E. Madden, 2022. Morph‑specific differences in escape behavior in a color polymorphic lizard. Behavioral Ecology and Sociobiology 76: 104. Doi: 10.1007/s00265-022-03211-8

Detering owls by buzzing

30 May 2022 / wvanstrien

Greater mouse-eared bat mimics the sound of bees and wasps

Owls avoid the buzzes of angry bees and wasps. The greater mouse-eared bat takes advantage of that fear by mimicking the sound, Leonardo Ancillotto and colleagues show.

A greater mouse-eared bat in stress behaves weird: it buzzes like a startled group of bees or wasps. Leonardo Ancillotto and colleagues noticed this when they handled the animals during their research. They wondered whether the bats mimic the sound of alarmed bees and wasps when they feel threatened by a potential predator to deter it. It was worth a study.

The greater mouse-eared bat, Myotis myotis, occurs in most European countries. Its enemies are owls, which are nocturnal like the bats.

Larynx

To find out, the researchers first analysed sound recordings of buzzing bats and compared that to the buzzing sounds that several species of bees and wasps produce when they are harassed and defend their nests. Among those species were honeybee (Apis mellifera) and hornet (Vespa crabro). And yes: the buzzing sounds were similar, especially to the ears of an owl.

The similarity is remarkable because the sound is created in different ways. Bees and wasps buzz by beating their wings, while bats produce the sound with the larynx.

Next, the researchers conducted playback experiments in which they broadcasted the buzzing sounds of honeybee, hornet or greater mouse-eared bat to a number of barn owls and tawny owls. The buzzing of the bat was most similar to that of honeybee and hornet. In addition, these insects live in tree cavities, in which owls are interested. As control, they broadcasted the communication calls of another bat species, the European free-tailed bat (Tadarida teniotis).

Experience

The owls moved away from loudspeakers that emitted buzzes, whether these were produced by honeybee, hornet, or greater mouse-eared bat. Bat communication calls, in contrast, attracted them. Wild owls, which may have encountered angry bees or wasps and suffered painful stings, were even more averse to buzzing sounds than owls that had been raised in captivity.

Does it make sense that owls, which are nocturnal animals, are afraid of bees and wasps, which are active during the day? Yes, that fear is conceivable. Honeybees fly until late evening in summer and hornets may fly at night, under moonlight or artificial light. Barn owls appear already at dusk, and when they have hungry young to feed, tawny owls sometimes even hunt during the day.

Apparently, the owls are afraid of bees and wasps and the bats delude them. Buzzing like bees or wasps, acoustic mimicry, may be all they can do to escape from their predator.

Willy van Strien

Photo: Greater mouse-eared bat. Kovács Richárd (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Source:
Ancillotto, L., D. Pafundi, F. Cappa, G. Chaverri, M. Gamba, R. Cervo & D. Russo, 2022. Bats mimic hymenopteran insect sounds to deter predators. Current Biology 32: R408-R409. Doi: 10.1016/j.cub.2022.03.052

Valves closed

10 February 2022 / wvanstrien

Blue mussels learn to avoid parasites

Blue mussels adapt their behaviour when parasitic larvae are nearby, according to research by Christian Selbach and colleagues.

During food intake, blue mussels, Mytilus edulis, run a risk. The bivalve molluscs feed by filtering water. It enters through an inlet and flows over gills, which not only take oxygen from the water, but also food particles, mainly plankton. These particles get stuck on a mucous layer and are transported to the stomach. The water exits through an outflow opening.

With the inflow of water, mussels may ingest larvae of a harmful parasite.

Mussels that encountered the parasite before, have learned to be more careful. If they notice the presence of parasites in the water, they close their valves and stop filtering to avoid further infection, Christian Selbach and colleagues show.

Intermediate host

The parasite, the fluke (or trematode) Himasthla elongata, has a complicated life cycle in which mussels are indispensable. The cycle starts in a bird that lives near or at sea, such as an oystercatcher, common eider, or scoter; in these animals, adult parasites thrive. They mate and produce eggs that end up in the water with the bird’s faeces. The eggs hatch and the larvae, so-called miracidia, are eaten by common periwinkles; the small snails are the first intermediate host.

In the snails, the parasites develop into the next larval stage, the cercariae, which also end up in the seawater. These are the larvae that infect filtering mussels, which are the second intermediate host. Mussels live in the tidal zone, near the coast, where they can form large shell reefs.

After ingestion by mussels, the parasitic larvae form cysts, a resting stage. Infected mussels grow poorly and are vulnerable to predation by oystercatcher, eider or scoter. And that completes the circle: those birds are the primary host. Once a bird has eaten infected mussels, the parasites mature, and the story starts all over again.

Shut off

If infective larvae are present in the water, mussels cannot help ingesting them when filtering. The only thing they can do to avoid infection is to stop taking in water. But that has a price, because it also means that they cannot take in oxygen and food.

Yet they stop, according to Selbach’s experiments in which he exposed mussels to infective larvae. But they have to learn it.

Mussels that have no previous experience with the parasites go on filtering when they are exposed to larvae. But mussels that met the parasite before and got infected, now shut themselves off. They reduce filtration activity and close the valves with the adductor muscles, which costs energy. But apparently, it would be worse to ingest another dose of parasitic larvae.

Now, it would be interesting to find out how the mussels notice that there are infective larvae around; that is still unclear.

Willy van Strien

Photo: blue mussel. Inductiveload (Wikimedia Commons, public domain)

Source:
Selbach, C., L. Marchant & K.N. Mouritsen, 2022. Mussel memory: can bivalves learn to fear parasites? Royal Society Open Science 9: 211774. Doi: 10.1098/rsos.211774

Bubble on the head

3 June 2021 / wvanstrien

Water anole rebreathes exhaled air when submerged

Some Anolis lizard species can stay underwater for a while without drowning. Thanks to a layer of air around their water-repellent skin, they continue to breathe, Chris Boccia and colleagues write.

The water anole, Anolis aquaticus, is not a fast lizard. But it often manages to escape from a predator, such as a larger lizard, snake, or bird. In case of danger, it splashes into the water to be out of sight. Although it may reappear only after more than fifteen minutes, it does not suffer from breathlessness. That is because it makes good use of the air it has with it, Chris Boccia and colleagues show.

The water anole from Costa Rica is one of more than 400 Anolis lizard species which occur in tropical America. Some species, including this one, live close to water and often submerge. The researchers studied how these semi-aquatic species survive submersion and how they differ from species that always remain on dry land.

All Anolis species appear to have a water-repellent skin. If they get into the water, a thin layer of air forms between water and skin across the body surface. In other words, they do not get wet like other lizards. As a consequence, no air bubbles up to the water surface to escape when an anole exhales underwater, as in other animals. Instead, the exhaled air is incorporated into the air layer around the body. This is visible as an air bubble near the nostrils. In the water anole, that bubble appears on top of its snout.

Bubble

Semi-aquatic species like the water anole use that trapped air. They re-inhale it. And exhale, and inhale, five times at least.

How does that help?

Breathing is necessary to take up oxygen from the air into the blood and to get rid of carbon dioxide. That gas exchange occurs in the lungs. The carbon dioxide exhaled by a diving anole dissolves easily from the air bubble in the water. So, it gets rid of that waste gas.

Also, with each breath, it takes up oxygen from the air bubble, the researchers show: the oxygen content of the bubble slowly decreases. The oxygen supply may be partly replenished if the air that comes from the lungs where it lost oxygen mixes with air that did not pass through the lungs: the air layer around the skin and air from mouth, nose, and windpipe.

Patience

And the bubble might act like a gill; perhaps it absorbs oxygen from the water. That will not be enough for a long stay underwater. But it might extend the maximum dive time a bit. A possible indication for this supplemental oxygen is that the oxygen content of the air bubble decreases more and more slowly over time. But that may also be explained by a lowered metabolism underwater, and thus less oxygen consumption.

Terrestrial Anolis species occasionally reuse expired air when submerged, but they do not do so routinely and not for as long as the water anole and other semi-aquatic species – that have to sustain rebreathing until the predator’s patience is gone.

Willy van Strien

On You Tube, the researchers show it here and here

Source:
Boccia, C.K., L. Swierk, F.P. Ayala-Varela, J. Boccia, I.L. Borges, C.A. Estupiñán, A.M. Martin, R.E. Martínez-Grimaldo, S. Ovalle, S. Senthivasan, K.S. Toyama, M. del Rosario Castañeda, A. García, R.E. Glor & D.L. Mahler, 2021. Repeated evolution of underwater rebreathing in diving Anolis lizards. Current Biology, online May 12. Doi: 10.1016/j.cub.2021.04.040

Hornets deterred

31 December 2020 / wvanstrien

Asian honey bee discourages its enemy

Hornets are dangerous predators of the Asian honey bee. The bees try to avert danger by making approaching hornets know they have been seen, as Shihao Dong and colleagues describe. Or by covering the nest entrance with animal faeces, as Heather Mattila and colleagues show.

The Asian honey bee, Apis cerana, is threatened by dangerous hornets, more than the European honey bee. Such large wasp with strong jaws and venomous sting can hover in front of a colony of honeybees, plucking foraging bee workers from the air to consume them.

And worse: hornets can operate in groups, enter a bees’ nest, kill any adult bees that do not flee and take possession of the larvae and pupae, which they bring to their own nest to feed their offspring. Like honeybees, hornets live in social groups with a queen laying eggs and workers taking care of her offspring.

So, a visit from hornets is something that should definitely not occur.

Asian honey bees have developed different defence mechanisms. The bees inform an approaching hornet that they are ready to defend themselves, as Shihao Dong and colleagues report. So, a surprise attack is not possible. Or they smear animal faeces around the entrance of their nest to frighten off the enemy, Heather Mattila and colleagues show.

I see you

Hornets are especially dangerous in autumn, when the brood in their nests needs a lot of animal food.

A hornet that detects a colony of Asian honey bees cannot enter it immediately. The nest entrance is too small and it is monitored by bee guards that alert their nest mates if necessary. But the hornet can apply a chemical scent mark to the nest to recruit dozens of colleagues, and collectively they can enlarge the nest opening by chewing and invade. The bees have to prevent that from happening. They have to deter the first hornet, the scout, and avert a group attack.

That is possible by showing an approaching hornet that it has been seen. Asian honey bees in China display a so-called I-see-you signal: when an Asian hornet, Vespa velutina, approaches the nest, bee guards will shake their abdomen. Guards copy this movement from each other, even without seeing the hornet with their own eyes, and the behaviour attracts more guards. The closer the hornet approaches or the faster it flies, the faster the swinging motion becomes, up to more than 30 sweeps per minute.

It repels the hornet. Because if the bees spot it in time, they are able to attack and kill it, as was already known. They enclose it in a dense ball of tens or hundreds of bees. The bees vibrate their flight muscles, so that the temperature in the ball rises to about 47°C, a temperature that the bees just endure, and the carbon dioxide content rises. The hornet succumbs.

But it is better if it doesn’t get that far, because killing a hornet in such heat ball takes a lot of time and energy. Not all bees survive the heat balling. Hence, the bees first try to discourage the enemy.

Sullied

The Asian hornet is a small species, and not the most dangerous one for the Asian honey bee. It does not perform mass-attacks and does not enter a bees’ nest. More threatening are the Asian giant hornet, Vespa mandarinia, and the related Vespa soror.

To discourage the larger hornets, Asian honey bees take more pains than for the smaller species, as it seems. In Vietnam, they manage to keep the large hornet Vespa soror away from their nest by applying mounded spots of animal poo around the entrance. When workers notice a hornet or its chemical scent mark, they look for a pile of animal dung, pick up a clump of it with their mouth parts, carry it to the nest and stick it close to the entrance. Upon detecting the smaller Asian hornet, Vespa velutina, near their nest, they don’t do this.

A sullied entrance acts as a deterrent: hornets leave faster and are less likely to land on the nest and enlarge the entrance opening. The researchers are not yet sure why animal poo has this repellent effect.

Odour mark masked

In northern Japan, honeybees smear chewed plant material around the entrance of their nest after spotting an Asian giant hornet, Ayumi Fujiwara’s research showed. It could well be that the smell of the stuff masks the chemical odour mark of the hornet. And maybe foetid poop does as well.

Willy van Strien

Photos:
Large: Japanese yellow hornet, Vespa simillima xanthoptera, at the nest of Asian honey bees, Apis cerana. Takahashi (Wikimedia Commons, Creative Commons CC BY-SA 2.1 JP)
Small: Asian honey bees forming a heat ball around two hornets. Takahashi (Wikimedia Commons, Creative Commons CC BY-SA 2.1 JP)

Sources:
Dong, S., K. Tan & J.C. Nieh, 2020. Visual contagion in prey defence signals can enhance honest defence. Journal of Animal Ecology, online November 20. Doi: 10.1111/1365-2656.13390
Mattila, H.R., G.W. Otis, L.T.P. Nguyen, H.D. Pham, O.M. Knight & N.T. Phan, 2020. Honey bees (Apis cerana) use animal feces as a tool to defend colonies against group attack by giant hornets (Vespa soror). PLoS ONE 15(12): e0242668. Doi: 10.1371/journal.pone.0242668
Fujiwara, A., M. Sasaki & I. Washitani, 2016. A scientific note on hive entrance smearing in Japanese Apis cerana induced by pre-mass attack scouting by the Asian giant hornet Vespa mandarinia. Apidologie 47: 789-791. Doi: 10.1007/s13592-016-0432-z
Tan, K., Z. Wang, H. Li, S. Yang, Z. Hu, G. Kastberger & B.P. Oldroyd, 2012. An ‘I see you’ prey-predator signal between the Asian honeybee, Apis cerana, and the hornet, Vespa velutina. Animal Behaviour 83: 879-882. Doi: 10.1016/j.anbehav.2011.12.031

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