Backlit messages

Humboldt squid is like an e-reader

Humboldt squid creates colour patterns with backlight

It is difficult to communicate visually in the dark of the ocean, but Humboldt squid can do it, as Benjamin Burford and Bruce Robison show. It makes pigmentation patterns on its body visible by switching on backlight.

Humboldt squid or jumbo squid, Dosidicus gigas, is a social animal: individuals form groups to hunt prey, for instance lantern fish. Joint hunt requires good coordination, so that the whole group will swim in the same direction and decelerate synchronously to catch prey. And the squid manages to do so, without the animals bumping against each other or attacking each other, Ben Burford and Bruce Robison observed. Apparently, the animals communicate effectively.

Dark

This is remarkable, as the squid mainly lives in the dark. It spends the day hundreds of meters below surface and ascends to the surface only at night. So how do the animals communicate, the researchers wondered.

Squid species that live in light conditions are known to exchange messages with colour patterns on their body. Their skin contains chromatophores, small elastic bags filled with pigment that can be expanded. It was already known that Humboldt squid has chromatophores in one colour, reddish-brown. This enables it to display white-red patterns. But how can the animals show these patterns to each other in the dark?

By turning on backlight, as it turns out.

Glowing body

Burford and Robison studied the behaviour of the animals by filming during daytime at great depth with a camera mounted on a remotely operated vehicle and analyzing the footage.

In addition to chromatophores, Humboldt squid has so-called light organs, with cells that can produce light; this is called bioluminescence. Many deep-sea inhabitants have light organs in the skin, usually located at certain places, and convey messages by changing light intensity. For example, they show a pattern of spots indicating what species they are, they give a light show when courting, they flash to scare off an enemy or they lure prey with a lantern.

Humboldt squid uses bioluminescence in a different way. Its light organs are not embedded in, but located underneath the skin. And they are not located in certain places, but spread all over the body. By making its entire body glow yellow-green, Burford and Robison assume, Humboldt squid creates a backlight that reveals the white-and-red pattern of the chromatophores in the skin. It functions like an e-reader.

Deciphering the Humboldt squid

The squid has a whole repertoire of pigmentation patterns, as was already known. It can flash and flicker. It can make its caudal fins contrast with mantle, head and arms, or make the edge of the fins stand out; it can show stripes along the side of the mantle or on the arms, or create a stain between the eyes. Certain patterns are displayed only when the squid is hunting in a group, and some patterns appear in a fixed order. So, the system seems to enable complex, advanced communication.

The next challenge is to decipher that language. The camera used was not light-sensitive enough to read the patterns in detail, and it is still unknown how the animals respond to each other’s messages.

Willy van Strien

Photo: A Humboldt squid shows its colours in the lights of a remotely operated vehicle 300 meters below the surface of Monterey Bay. ©2010 MBARI

Researchers telling about their work on YouTube

Learn also about Humboldt squid mating behaviour

Sources:
Burford, B.P. & B.H. Robison, 2020. Bioluminescent backlighting illuminates the complex visual signals of a social squid in the deep sea. Proceedings of the National Academy of Sciences 117: 8524-8531. Doi: 10.1073/pnas.1920875117
Trueblood, L.A., S. Zylinski, B.H. Robison & B.A. Seibel, 2015. An ethogram of the Humboldt squid Dosidicus gigas Orbigny (1835) as observed from remotely operated vehicles. Behaviour 152: 1911-1932. Doi: 10.1163/1568539X-00003324
Rosen, H., W. Gilly, L. Bell, K. Abernathy & G. Marshall, 2015. Chromogenic behaviors of the Humboldt squid (Dosidicus gigas) studied in situ with an animal-borne video package. The Journal of Experimental Biology 218: 265-275. Doi:10.1242/jeb.114157
Benoit-Bird, K.J. & W.F. Gilly, 2012. Coordinated nocturnal behavior of foraging jumbo squid Dosidicus gigas. Marine Ecology Progress Series 455: 211-228. Doi: 10.3354/meps09664

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.

Defense

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.

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

Pollen on wings

Brush-like fireball lily is specialized for butterfly visits

Fireball lily is pollinated by large butterflies

On butterfly wings, the pollen of fireball lily, Scadoxus multiflorus, is transferred from one plant to another, as Hannah Butler and Steve Johnson show.

Like bees, butterflies are pollinators. They visit flowers to drink nectar, pick up pollen and deposit pollen grains on the pistil when visiting the next flower. Flowers pollinated by butterflies are often red or orange, because that colours are attractive to butterflies. In addition, the structure of the flowers is specialized for butterfly pollination, write Hannah Butler and Steve Johnson.

The ‘salver form’, in which petals form a platform on which a butterfly can settle while inserting its proboscis into a flower, was already known. In this model, pollen grains stick to the proboscis and the head of the butterfly. Now, Butler and Johnson describe another flower form with a different pollen transfer mechanism: the ‘brush model’.

Fluttering

A brush model plant is fireball lily, Scadoxus multiflorus, of Africa, also known as indoor plant. The stamens and pistils extend beyond the petals, and because the flowers are placed close together in an umbel, stamens and pistils of different flowers overlap. The plant cannot fertilize itself; pollen has to be brought from another plant for seeds to develop.

Large butterflies that visit the plant do the job. A frequent visitor is the mocker swallowtail Papilio dardanus, principally males. How does it transfer pollen from one plant to another?

The butterfly flutters along the inflorescence to inspect it, touching many stamens and pistils with the wings. When drinking nectar, it continues fluttering. The flat pollen grains from the stamens it touches stick between the scales at the ventral surface of the wings, as macro photos show. And part of the grains that a butterfly carries with it will fall on the pistils; the butterfly can pollinate several flowers during a single visit, even when it doesn’t drink nectar.

So, the brush-shaped umbel is a specialisation for butterfly-wing pollination. The fireball lily belongs to the amaryllis family. As it turns out, other red flowers of that plant family also have a brush model and deposit their pollen on butterfly wings.

Willy van Strien

Photo: Mocker swallowtail Papilio dardanus (male) on fireball lily Scadoxus multiflorus. ©Steven D. Johnson

Source:
Butler, H.C. & S.D. Johnson, 2020. Butterfly-wing pollination in Scadoxus and other South African Amaryllidaceae. Botanical Journal of the Linnean Society, online March 12. Doi: 10.1093/botlinnean/boaa016

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.

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

Saving room for delicacy

Cuttlefish won’t eat crab when there will be shrimp at night

cuttlefish refrains from eating during daytime when the night will bring better food

Common cuttlefish exhibits a clear food preference: shrimp. If shrimp will be available at night, it refrains from eating crab during daytime, as Pauline Billard and colleagues show.

The European common cuttlefish Sepia officinalis, which occurs in Mediterranean Sea, North Sea and Baltic Sea, consumes different types of prey, but not with the same eagerness. Shrimp are its favourite food. It likes them so much, that it will skip a crab meal if it expects shrimp to be available later on, as research of Pauline Billard and colleagues revealed.

In the lab, the researchers conducted two experiments. First, they offered cuttlefish, kept in separate tanks, a crab during daytime. Some cuttlefish received an additional shrimp every night, the others were given shrimp only on some nights, in an unpredictable way.
The cuttlefish that received shrimp every night started to lower consumption of crab during daytime, saving room for their favourite food. The animals that were not sure about getting the delicacy at night maintained the consumption of crab during the day, eating enough in any case.
When the routine was reversed between groups – so, cuttlefish that had received shrimp every night changed to an unpredictable regimen and the other way round – the animals modified their behaviour accordingly.

Hunger

Then a second, more complex experiment was done. The animals again were offered crab during daytime, but now all of them got shrimp every other night. It took some time for them to get used to the regimen, but then they adjusted their behaviour. If they had received shrimp the night before, so when there would be no shrimp the following night, they consumed crab during daytime. Conversely, if there hadn’t been any shrimp the night before, they didn’t eat much crab, expecting to get shrimp the following night.

The animals’ behaviour can’t be explained by their feeding state, because in that case they would have eaten crab when they didn’t eat shrimp the night before, being more hungry.

Willy van Strien

Photo: Common cuttlefish. Amada44 (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Source:
Billard, P., A.K. Schnell, N.S. Clayton & C. Jozet-Alves, 2020. Cuttlefish show flexible and future-dependent foraging cognition. Biology Letters 16: 20190743. Doi: 10.1098/rsbl.2019.0743

From reliable source?

Nuthatch transmits indirect information only partially

red-breasted nuthatch eavesdrops on black-capped chickadee

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

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

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

Aroused

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

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

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

Eavesdropping

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

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

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

Less reliable

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

Willy van Strien

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

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

Sparkling camouflage

Jewel beetles are invisible thanks to gem-like wings

jewel beetle is invisible in vegetation

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

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

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

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

Nail varnish

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

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

Glossy background

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

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

Willy van Strien

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

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

American coot takes care of the small

The younger, the brighter, the more food

Coot chicks' ornaments tell parents what age they are

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

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

Starvation

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

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

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

Favourites

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

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

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

Age

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

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

Willy van Strien

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

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

Peaceful together

Dangerous bullet ant and defensive bee tolerate each other

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

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

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

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

Large jaws

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

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

Resin

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

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

Protection

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

Willy van Strien

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

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

Leaf cutters prevent traffic jams

Take no heavy load when traffic flow is high

leaf cutter ants carry small leaf fragments on crowded trails

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

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

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

Truck-driver effect

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

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

Steady flow

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

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

Just like on highway.

Willy van Strien

Photo: Atta cephalotes ©Alejandro Farji-Brener

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