Bubble on the head

Water anole rebreathes exhaled air when submerged

Water anole re-uses exhaled air

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.


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.


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

Photo: submerged water anole with bubble on snout. ©Lindsey Swierk

On You Tube, the researchers show it here and here

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

Asian honey bee discourages its enemy

hornets are predators of Asian honey bee

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.

Asian honey bees kill hornet in a heat ballIt 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.


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

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)

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

Acid gulp

Ant swallows its own formic acid to stay healthy

Tnaks to formic acid, Formicinae ants are healthy

Formic acid appears to be a great help for ants to prevent infection from contaminated food, Simon Tragust and colleagues discovered. A gulp after each consumption increases their survival chance.

People like sweet desserts, but for ants of the subfamily Formicinae it is different. They take a gulp of formic acid after eating or drinking, Simon Tragust and colleagues witnessed.

This is remarkable, because formic acid is an aggressive substance. Formicinae ants produce it in a venom gland that has an opening at the tip of the abdomen. They were known to spray it at predators, such as birds, spiders, and insects, to defend themselves, and this is understandable. But swallowing?


Tragust and colleagues had shown previously that Formicinae ants use their acid not only against predators, but also against pathogens. Workers apply it in combination with resin to keep an entomopathogenic fungus (Metarhizium brunneum) out of their nest.

Also, they use formic acid to keep the brood clean. If they detect pupae covered with spores of the pathogenic fungus, they clean them and cover them with formic acid, which they had taken up from the abdominal gland opening into the mouth.

If fungal spores have already germinated on a pupa and the fungus has penetrated the cuticle, workers unpack the infected pupa from its cocoon, bite holes in the skin and inject formic acid. In this way, they prevent the fungus from growing and forming spores that will contaminate the rest of the colony. The pupa does not survive the treatment, but it would have been killed by the fungus anyway.

Crop acidity

Now, a new application of formic acid comes to light: Formicinae ants swallow their own formic acid after eating or drinking something. Tragust deduces this from tests in the lab with Florida carpenter ant, Camponotus floridanus. He offered ants honey water or plain water and saw them lick their abdominal tip afterwards. Apparently, they then took up acid into the mouth and swallowed it, as Tragust showed that the contents of their crop, just before the stomach, became very acidic.

Perhaps, the idea was, workers take formic acid to kill bacteria that may be present on food. And that was the case, as became clear from tests in which workers were given food that was contaminated with a pathogenic bacterium species (Serratia marcescens). In ants that then took a gulp of formic acid, bacteria did not survive the crop environment and the rest of the intestinal system remained clean. Ants that were prevented from taking in acid, were at greater risk of a deadly infection.

Only bacteria that thrive in acidic environments survive the acidic crop, and such bacteria populate the ants’ intestines. But these are beneficial bacteria that help digest food. The acid appears to be an excellent remedy against pathogenic microbes.

Fortunately, we don’t have to take an extremely sour dessert like Formicinae ants, because our stomach keeps itself acidic.

Willy van Strien

Photo: Carpenter ant, Camponotus cf. nicobarensis. ©Simon Tragust

Ants also use formic acid to keep fungus out of nest

Tragust, S., C. Herrmann, J. Häfner, R. Braasch, C. Tilgen, M. Hoock, M.A. Milidakis, R. Gross & H. Feldhaar, 2020. Formicine ants swallow their highly acidic poison for gut microbial selection and control. eLife 9: e60287. Doi: 10.7554/eLife.60287
Pull, C.D., L.V. Ugelvig, F. Wiesenhofer, A.V. Grasse, S. Tragust, T. Schmitt, M.J.F. Brown & S. Cremer, 2018. Destructive disinfection of infected brood prevents systemic disease spread in ant colonies. eLife 7: e32073. Doi: 10.7554/eLife.32073
Tragust, S., B. Mitteregger, V. Barone, M. Konrad, L.V. Ugelvig & S. Cremer, 2013. Ants disinfect fungus-exposed brood by oral uptake and spread of their poison. Current Biology 23: 76-82. Doi: 10.1016/j.cub.2012.11.034

Saved by decoration

Bird doesn’t attack the spider, but its web decoration instead

Cyclosa monticola and its web decoration

Thanks to a striking web decoration, the spider Cyclosa monticola escapes from hungry birds, as Nina Ma and colleagues show. The birds’ attacks fail.

The spider Cyclosa monticola, which is common in East Asia, constructs an elaborate web. Not only does it consist of sticky threads, but it also carries a striking, linear band of trash, containing moults, prey remains, and pieces of leaves and stems. According to Nina Ma and colleagues, this detritus decoration diverts attacks of predators, especially birds, away from the spider.

The spider is in the centre of its web, hardly visible in the decoration band, which extends in both directions. The animal’s colour is similar to the colour of the decoration. Birds cannot distinguish the colours of the spider from that of decorating trash.

Fine scissors

The researchers wondered whether Cyclosa monticola, a tasty snack for many birds, is safer as a result. In order to find out, they exposed spider webs to domestic chickens chicks, one web per chick. Some chicks were given a web with the resident spider present in it; from half of these webs, the researchers had removed the decoration with fine scissors without damaging the web. For comparison, other chicks were given either an empty web or a web with decoration only. Most chicks quickly pecked at whatever (spider and/or decoration) they saw. The researchers were interested in their first target.

Spiders that had been robbed of their web decoration were grabbed by the chick in almost all cases. Spiders that had kept their decoration survived much more frequently. In this situation, the chick usually did not peck at the spider, but at the trash, and the spider dropped down quickly to escape. Obviously, the decoration did confer safety.

More attractive

Although the spider is hardly detectable among the decoration trash, the protection was not only due to camouflage, as the researchers argue. Because in that case, attacks would be random and the risk for the spider to get captured would be equal to its size relative to the size of the decoration. However, the chance of being captured was lower and independent of the size of the decoration band. Apparently, the detritus decoration is more attractive to birds to peck at and diverts their attention away from the spider.

So, web decoration is an effective defence strategy.

The question now is whether insect prey will not avoid the structure. After all, the trick of a spider web is that insects don’t discern the threads and get caught in the web. But research on another spider species that adorns its web shows that insects are attracted to the decoration – and get stuck in the web. The web decoration of Cyclosa monticola may also have this effect; if so, it would have a double function.

Willy van Strien

Photo: web with Cyclosa monticola and detritus decoration. ©Shichang Zhang

Ma, N., L. Yu, D. Gong, Z. Hua, H. Zeng, L. Chen, A. Mao, Z. Chen, R. Cai, Y. Ma, Z. Zhang, D. Li, J. Luo & S. Zhang, 2020. Detritus decorations as the extended phenotype deflect avian predator attack increasing fitness in an orb‐web spider. Functional Ecology, online July 16. Doi: 10.1111/1365-2435.13636
Tan. E.J., S.W.H. Seah, L-M.Y.L. Yap, P.M. Goh, W. Gan, F. Liu & D. Li, 2010. Why do orb-weaving spiders (Cyclosa ginnaga) decorate their webs with silk spirals and plant detritus? Animal Behaviour 79: 179-186. Doi: 10.1016/j.anbehav.2009.10.025

Attractive dark eyes

Thanks to black irises, female guppy escapes from predator

Guppy female blackens eyes when in danger

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.


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.


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)

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

Ladybird cannot deal with all enemies at once

Harlequin ladybird cannot resist 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.


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)

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

Pug-nosed tree frog male refrains from calling first

pug-nosed tree frog males call almost synchronously

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.


fringe-lipped bat is susceptible for auditory illusionBecause 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.


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

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)

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

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

Red-winged blackbird eavesdrops on yellow warbler's alarm call

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.

Brown-headed cowird parasites on nests of songbirdsA 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.


Yellow warbler utters specific alarm call when brood parasite is presentWhen 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.


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

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)

Researchers tell about their work on YouTube

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

Mucus contains numerous stinging-cell structures

Upside-down jellyfish releases mucus containing stinging cell masses

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.


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)

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?

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.


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.


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

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)

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