From so simple a beginning

Evolution and Biodiversity

Page 6 of 19

Bubble on the head

3 June 2021 /

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.

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

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

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

Royal matchmaker

27 May 2021 /

Ant worker transports young queen to suitable mates

Cardiocondyla elegans worker carries queen to new nest to mate

Workers of the ant Cardiocondyla elegans make sure that their queen sisters will meet unrelated males, Mathilde Vidal and colleagues show.

Young queens of the ant Cardiocondyla elegans do not leave the natal nest on their own to be inseminated outside. Although they have wings, they make no nuptial flights like queens in many other species. They stay inside. Males have no wings, and they too remain in the natal nest. And so it happens that young queens, or gynes, mate with males that were born in the same nest. But workers intervene to promote outbreeding, Mathilde Vidal and colleagues discovered.

Cardiocondyla elegans, which is mainly found along the Mediterranean, contructs underground nests on river banks. Hundreds of workers, dozens of young queens and a few males share a nest, headed by one fertile queen. This mother queen has mated with several males and stored their sperm to fertilise eggs.

As a consequence, the gynes and males in her nest are full and half siblings. If they mate with each other, this is inbreeding, and prolonged inbreeding has negative effects on the lifespan of mother queens, the survival of brood and the ratio of females (workers and young queens) to males.

Vidal had observed workers walking through the field between nests, carrying a winged young queen on their back. She wondered whether this queen transport could be a way to promote outbreeding. Behavioral observations and genetic research confirmed this idea.

New contacts

The researchers discovered that gynes do indeed mate in the natal nest. But it doesn’t always stop there. A worker regularly drags a young queen out of the nest, takes her head between its jaws and carries her on its back to another nest, where it drops her into the nest entrance.

Just below that entrance, as it turned out, is a chamber with hundreds of young queens and males. It is obvious what is happening there. The nest chambers with mother queen and brood are much deeper, 1 to 2 meters below the surface. By dropping a young queen into the entrance of an alien nest, a worker brings her into contact with males that are not (half) brothers. The nest where a young queen is delivered also profits because males can mate not only with (half) sisters, but also with an unrelated queen.

After delivery, a queen is sometimes picked up again and taken to still another nest.

Hibernation

Young Cardiocondyla elegans queens hibernate in the natal nest or in the nest to which they were brought. They retain their wings. In spring they leave, usually on foot, to found a nest on their own, with a stored stock of sperm from several males: (half) brothers and, if they have been transported to another nest, alien males. Once they have started their own nest, they lose their wings.

Apparently, young queens are able to disperse on their own. Then why don’t they leave the natal nest to mate elsewhere?

Maybe it’s quicker and safer to be carried, Vidal thinks. Workers know the surroundings. Passing by other nests, they walk in a straight line to a nest that they have selected. Often, they will deliver another gyne there later on.

Inhabitants of a preferred nest are no family, so the matchmakers select unrelated partners for their queen sisters. This behaviour is a new and surprising way to promote outbreeding, the researchers state.

Willy van Strien

Photo: Cardiocondyla elegans: worker carrying young queen. ©Mathilde Vidal

Sources:
Vidal, M., F. Königseder, J. Giehr, A. Schrempf, C. Lucas & J. Heinze, 2021. Worker ants promote outbreeding by transporting young queens to alien nests. Communications Biology 4: 515. Doi: 10.1038/s42003-021-02016-1
J-C. Lenoir, A. Schrempf, A. Lenoir, J. Heinze & J-L. Mercier, 2007. Genetic structure and reproductive strategy of the ant Cardiocondyla elegans: strictly monogynous nests invaded by unrelated sexuals. Molecular Ecology 16: 345-354. Doi: 10.1111/j.1365-294X.2006.03156.x

Flying high

19 May 2021 /

Great reed warbler avoids being roasted by the sun

Great reed warbler flies extremely high during migration

During migration, the great reed warbler may climb to extreme altitudes, Sissel Sjöberg and colleagues show. Presumably, that is to control their body temperature.

Imagine: the great reed warbler, a medium-sized songbird, climbs up to 6 kilometers above ground level during migration. The discovery of Sissel Sjöberg and colleagues also surprised the researchers themselves.

The birds breed in Europe and overwinter in tropical Africa; in both locations, they stay in reed beds. So, they make a big trip twice a year. They typically fly at night and use the day to rest and eat.

Nonstop

But when great reed warblers cross the Mediterranean, they cannot land. And when they fly over the Sahara Desert, it makes no sense to go down, because there is nothing edible there. Across such barriers, they continue to fly during daytime, traveling more than 30 hours non-stop if necessary.

To find out more about flight behaviour during migration, Sjöberg equipped several birds with data loggers that record various data during migration: light, ambient temperature, and altitude. In addition, the data loggers register the movements of the birds: whether they are flying, resting, or moving on the ground in search of food.

The birds fly at night at an altitude of 2400 meters on average, it turned out, which is already quite high. But if they prolong their fly into daytime, they go more than twice as high. At dawn they quickly climb to about 5400 meters, up to more than 6000. At dusk, they descend in an equally short time.

Why do they do that?

From an altitude of 1500 meters on, temperature and wind speed are the same between day and night; the temperature at 2400 meters is about 14 °C. So, that is no reason for going higher during daytime.

Heat radiation

A difference between day and night is the presence of the sun. The great reed warblers, the bodies of which are already warm due to their flapping flight, cannot stand the solar heat radiation, Sjöberg thinks. It puts them in danger of getting overheated. At 5400 meters it is 9 °C below zero; there, they will not be overheated by solar radiation. So, when the sun is shining, the birds better fly much higher.

An additional advantage may be that during the day, the birds have a better view of the landscape when flying higher. Also, they are out of reach of birds of prey, especially Eleonora’s falcon, which hunts up to 3500 meters.

The biennial migration is a great achievement. The long flying periods in which the great reed warbler goes extremely high during daytime make it extra impressive. Hats off.

Willy van Strien

Photo: Great reed warbler in breeding habitat. Zeynel Cebeci (Wikimedia Commons, Creative Commons CC BY-SA 4.0)

Source:
Sjöberg, S., G. Malmiga, A. Nord, A. Andersson, J. Bäckman, M. Tarka, M. Willemoes, K. Thorup, B. Hansson, T. Alerstam & D. Hasselquist, 2021. Extreme altitudes during diurnal flights in a nocturnal songbird migrant. Science 372: 646-648. Doi: 10.1126/science.abe7291

Hanging baskets

19 April 2021 /

Ants grow plants in tree nests

Well-maintained ant garden of Camponotus femoratus and Crematogaster levior

Some ants have ant gardens. In their nests, one or more plant species are flourishing. The ants are good gardeners, as a Brazilian research team shows: they select the plants carefully and protect them.

In the forests of the Amazon region, you can see hanging baskets, balls from which plants grow. They are the nests of ants that live in trees. These ant gardens  benefit both parties. The plants belong to species that do not root in the soil but grow on tree branches (or in an arboreal ant’s nest), so-called epiphytes. They benefit because the ants disperse the seeds and fertilize the plants that germinate. The ants benefit because the plant roots strengthen the nest and make it rainproof.

Most of the hanging gardens in the Amazon region belong to two ant species that live together: Camponotus femoratus and Crematogaster levior. They share nests and foraging trails but keep their broods separated. A Brazilian research group describes how well this ant duo takes care of their gardens.

Division of tasks

The two species have divided the tasks. Crematogaster levior goes out to get food. The researchers think that it also is the one that, within a colony, takes the initiative to create a new nest; a colony contains on average 17 nests. Crematogaster levior workers are in the majority, especially in young nests; in initial nests, they are even the only ones.

But Camponotus femoratus is the stronger and more aggressive of the two. He constructs and defends the nests.

This species is also the one that collects seeds of the desired plants and puts them in the cardboard nest wall. He is picky: of the many epiphyte species that grow in South America, the ants only use a handful. Only one or two species are grown per nest.

The most common garden plant is Peperomia macrostachya. Probably, the ants are fond of it because, in addition to nectar glands, flowers and fruits, it also has oil glands. Oil is a hard-to-find part of the diet, so these glands are valuable. Among other plants used are Philodendron species.

Maintenance of ant gardens

The ants take good care of the plants. If a leaf is damaged, experiments showed, workers of Camponotus femoratus will gather there; they are triggered by volatile substances that are released upon damage. So, they arrive at places where herbivore insects are gnawing and they can chase them away. Especially damage to the precious Peperomia macrostachya provokes a rapid influx of many workers.

In addition, the ants prune ‘weeds’. The walls of an ants’ nest are an attractive growing place for many epiphytes because they are rich in nutrients. But the ants prevent the growth of unwanted plants that would compete with the garden plants. If the wrong seeds stick to the wall, the ants will remove them, and if the wrong plants germinate, they will cut the stem or leaves.

No wonder that the garden plants flourish and the hanging baskets look good.

Willy van Strien

Photo: Garden with Philodendron of the ant duo Camponotus femoratus and Crematogaster levior. ©Ricardo Eduardo Vicente

More about gardening ants: mini garden

Sources:
Pereira, A.A., I.V. da Silva & R.E. Vicente, 2021. Interaction between epiphytic chemical allelopathy and ant‑pruning determining the composition of Amazonian ant‑garden epiphytes. Arthropod-Plant Interactions, online April 9. Doi: 10.1007/s11829-021-09825-5
Dacquin, P., F. Degueldre & R.E. Vicente, 2021. Relative colony size of parabiotic species demonstrates inversion with growth. Insectes Sociaux, online January 2. Doi: 10.1007/s00040-020-00798-x
Vicente, R.E., W. Dáttilo & T.J. Izzo, 2014. Differential recruitment of Camponotus femoratus (Fabricius) ants in response to ant garden herbivory. Neotropical Entomology 43: 519-525. Doi: 10.1007/s13744-014-0245-6

New body

17 March 2021 /

Loose head regenerates a complete Elysia sea slug

Elysia sea slug can grow new body from head

Sea slugs Elysia marginata en Elysia atroviridis can decapitate themselves and regrow a new body from the loose head, Sayaka Mitoh en Yoichi Yusa show. A bizarre phenomenon. Why do they do it, and how do they survive?

Sayaka Mitoh and Yoichi Yusa must have been dumbfounded when seeing sea slugs that they kept in their lab, species Elysia marginata, sever their heads from their bodies. The loose heads moved around, as they report. After a day, the wounds were closed. In some cases, especially in young sea slugs, things got even crazier: the head began to feed; after a week, a new body started to grow and in three weeks it was complete.

The loose bodies also moved for a while, sometimes even months, but eventually they decomposed. No new head appeared on any loose body.

Parasite

There are more animals that can regrow a missing body part, such as a lizard that shed its tail or a fiddle crab that lost a claw. But this – regenerating almost an entire body – is very extreme. These sea slugs even have a groove behind the head as a predetermined breakage plane for self-decapitation. Why do they do it?

In any case, it is not to escape from a predator, like a lizard sheds its tail when a predator grasps it. The sea slugs take hours to separate body from head; that is not effective to avoid predation. And when the researchers simulated an attack by teasing them, nothing happened. The animals have a different defence mechanism against predators: they are poisonous.

The reason for self-decapitation became clear by observations on wild-caught specimens of a related species, Elysia atroviridis. Once in the lab, some of them shed the whole body, and these specimens turned out to contain a parasite, a copepod of the genus Arthurius. It is a large parasite that occupies almost the entire body of its host. In fact, a parasitized sea slug has already lost its body. If it sheds it, it will get rid of the parasite while losing almost nothing more.

Chloroplasts

But how does it survive without organs such as heart and kidneys? This has to do with a special property of sacoglossan sea slugs, to which Elysia belongs, the researchers suppose. They extract chloroplasts from algal food and incorporate them in special cells that line their highly branched digestive gland. The head also contains chloroplasts. Thanks to the chloroplasts, which they need to survive, these sea slugs can endure a period without food, it was known.

It is a mystery how exactly they utilise the chloroplasts. The chloroplasts continue to do what they do in plants: they convert carbon dioxide into carbohydrates with the help of sunlight, a process called photosynthesis. Whether the sea slugs can survive on sunlight as a result, just like plants, is a matter of debate.

Regardless, it may well be thanks to the chloroplasts that a loose head of Elysia marginata and Elysia atroviridis survives.

No eternal life

Parasitized Elysia sea slugs shed their worthless bodies. But they only manage to grow a new one from the head when they are young. The loose head of an older specimen does not feed and does not grow, but will die within ten days. Shedding and regrowing a body is not a recipe for eternal life.

Willy van Strien

Photo: Elysia marginata. Budak (via Flickr, CC BY-NC-ND 2.0)

The research explained on YouTube

Sources:
Mitoh, S. & Y. Yusa, 2021. Extreme autotomy and whole-body regeneration in photosynthetic sea slugs. Current Biology 31: R233-R234. Doi: 10.1016/j.cub.2021.01.014
Wägele, H., 2015. Photosynthesis and the role of plastids (kleptoplastids) in Sacoglossa (Heterobranchia, Gastropoda): a short review. Aquatic Science & Management 3: 1-7. Doi: 10.35800/jasm.3.1.2015.12431

False alarm

4 March 2021 /

Superb lyrebird male tries to prolong a female’s visit

a male superb lyrebirds show is also used to manipulate females

A male superb lyrebird can deceive a female into believing that danger is imminent, Anastasia Dalziell and colleagues think. This increases the chance that she will stay for a while and copulation ensues.

Superb lyrebirds are masters of imitating all possible sounds, and males make clever use of that talent. When a female pays a visit, a male imitates the sound of a flock of alarmed songbirds, creating the illusion that a predator is nearby, Anastasia Dalziell and colleagues write. Then she might stay a little longer than she really wanted.

The superb lyrebird (Menura novaehollandiae), one of the largest passerine species, lives in the forests of South East Australia. Males and females do not form breeding pairs; each bird lives in its own territory. Females have one young per year, which they raise on their own.

Just watching

During the breeding season, males make themselves as attractive as possible. They construct mounds in their territory, where they sing their songs. Sometimes they sing a special song accompanied by a dance according to fixed rules, as Dalziell described earlier, throwing their decorative tail feathers over their body and head.

The purpose of a male’s show, of course, is to attract females and get them to copulate, because every mating can result in a young that he sires. Females visit several males before making their choice. As a result, a female may come to watch a displaying male, but refrain from mating with him in the end.

That is not what he intended.

Illusion of danger

When she is about to leave without copulating, he adds a new element to his song, which is remarkably similar to the sound of a flock of aroused songbirds.

Small songbirds get aroused when they detect a predator, such as a snake, large lizard, roosting owl, or perched hawk. They utter alarm calls to recruit others and harass the enemy together. The researchers show how accurately a male lyrebird mimics such mobbing flock of multiple simultaneously calling birds. Even the noise of beating wings is incorporated into his song. It is so accurate, that small songbirds are misled and approach to join.

The predators that arouse the small birds are dangerous for lyrebirds too. Therefore, the researchers hypothesize, the song creates the illusion that danger is imminent; the female is inclined to stay, and there is a chance that a copulation will happen.

Deception

A male superb lyrebird also utters this cacophony during mating. This job is not done in a few seconds, as it is other birds. Only after havng sat on her for more than half a minute, does he transfer his sperm. From the moment he mounts her until the end, he mimics the sound of a mobbing flock to prevent her from leaving too soon. During mating, he beats his wings in front of him, obscuring her view. She is unable then to assess whether it is safe to go, the authors suggest.

In conclusion, the superb lyrebird male uses his song not only to advertise his good health and condition, as is usual, but also to get a female to stay by giving a false signal of danger. That is deception.

Whether a female will stay longer because of the false alarm, the researchers don’t know. To find out, they would have to do experiments, which would be difficult for this species.

Willy van Strien

Photo: Courting male, covered with his tail. Kim Edol (via Flickr, CC BY-NC-ND 2.0)

Anastasia Dalziell about her research on YouTube

Sources:
Dalziell, A.H., A.C. Maisey, R.D. Magrath & J.A. Welbergen, 2021. Male lyrebirds create a complex acoustic illusion of a mobbing flock during courtship and copulation. Current Biology, online February 25. Doi: 10.1016/j.cub.2021.02.003
Dalziell, A.H., R. A. Peters, A. Cockburn, A.D. Dorland, A.C. Maisey & R.D. Magrath, 2013. Dance choreography is coordinated with song repertoire in a complex avian display. Current Biology 23, 17 juni online. Doi: 10.1016/j.cub.2013.05.018

Shrimp keepers

24 February 2021 /

Longfin damselfish have their algal farms fertilized

Longfin damselfish grows algae with help of shrimp

Every intruder is chased away from the algal farm of the longfin damselfish, but a swarm of opossum shrimp is allowed stay and even receives protection. For good reason, Rohan Brooker and colleagues report.

The longfin damselfish (Stegastes diencaeus) is an aggressive, territorial fish that lives on coral reefs. It grows its own food by creating a farm of a few square meters where palatable algae grow. It tends its algae and defends its territory fiercely; all animals are chased off.

Or rather: almost all animals. Rohan Brooker and colleagues show that during the day, a swarm of opossum shrimp (Mysidium integrum) can be found in many algal farms. A farmer not only tolerates the shrimp’s presence, but it also protects them from their predators, although it takes some extra effort. Apparently, the tiny animals are worth it.

Flourishing farm

Why would an algae-farming longfin damselfish care about the shrimp, Brooker wondered. Although the fish supplements its algae diet with some small animals, it does not eat these shrimp. Perhaps, Brooker supposed, the shrimp are fertilizing the algae with their feces.

And that turned out to be the case. A farm with a swarm of shrimp does better than a farm without such swarm, thanks to nutrients excreted bythe shrimp. It hosts more large brown algae that form a structure on which turf-algae, which the damselfish prefers to eat, grow well. This translates into a better condition of the fish: damselfish with a shrimp swarm on their farm have a larger energy reserve than colleagues with a non-fertilized farm.

Brookers conducted his research on coral reefs off the coast of Belize, Central America.

Domesticated

Thus, the farming fish benefits for its flourishing crop, the shrimp for its guarded refuge. There are no shrimp swarms to be found outside farms during daytime. The shrimp leave the farm at night, when it is safe, to filter food from the water at the surface. Then they return to their permanent residence.

The fidelity to this place is so strong that young shrimp remain in their parents’ farm. Therefore, the authors consider the relationship between longfin damselfish and opossum shrimp as an early stage of domestication. The fish ‘keep’ the shrimp as livestock.

Willy van Strien

Photo: Longfin damselfish Stegastes diencaeus. Mark Rosenstein (Wikimedia Commons, Creative Commons CC BY-SA 4.0)

The research explained on YouTube

Source:
Brooker, R.M., J.M. Casey, Z-L. Cowan, T.L. Sih, D.L. Dixson, A. Manica & W.E. Feeney, 2020. Domestication via the commensal pathway in a fish-invertebrate mutualism. Nature Communications 11: 6253. Doi: 10.1038/s41467-020-19958-5

Help is helpful in Seychelles warbler

28 January 2021 /

Older mothers and their offspring benefit

Older Seychelles warbler mother and her young benefit from help

It requires great efforts of a female Seychelles warbler to raise a young. When she gets older, she is unable to sustain her work pace. The presence of helpers then compensates for the decline in care, as Martijn Hammers and colleagues show.

Adult Seychelles warblers often stay in the territory of their parents. Some of them, mostly females, make themselves useful by helping when the parents start brooding again. The helpers share in the reproductive success of their parents because new offspring is kin; they gain experience and may lay an egg in the nest themselves. But do the parents need their help?

When the female is older, they certainly do, according to long-term research by Martijn Hammers and colleagues on Cousin, an island that belongs to the Republic of Seychelles.

Long-term care

Seychelles warblers form breeding pairs. The female lays an egg once a year and both parents take care of the young. But the female does more. She incubates, and when the young bird has hatched, she brings it food at a higher rate than the male; the birds eat insects. The care continues for a long time, a young bird remains dependent on its parents for three to four months.

Young mothers are in good condition: although they work hard, they stay healthy. But as a female gets older – that is, from the age of 6 years on – her ability to provide care declines. She provisions her kid at a lower rate than when she was younger. Consequently, it has a smaller chance to survive the first, critical year. The elderly mother herself is ageing, and the chance that she will die increases sharply every year.

Unless she has female helpers.

Compensation

Female helpers make life a lot easier for an elderly brooding female. She will provision her young less frequently. Thanks to the help, it will still get enough and the chance that it will survive is high. The presence of helpers thus compensates for the lower work paceof an elderly mother. She also benefits herself: senescence sets in later and proceeds more slowly, increasing the chance that she survives and can breed once again.

For a young mother, it does not make much difference whether female helpers assist: her chance to survive is very high anyway, and that of her young is quite good. And the nice thing is that older females, which really benefit from help, are more likely to have helpers in their territory. For they have breeded more often, and therefore have a greater chance that at least one young stayed with her and will assist.

And what about dad?

We are doing a Seychelles warbler male wrong by saying that he does not incubate and provisions less intensively. He has a different task: to protect the nest against egg predators. Anyway, his contribution does not decline as he gets older, as in females. Perhaps it is easy to sustain his early pace, which is lower than that of a young female.

The chances for a young to survive, therefore, do not depend on its father’s age. Also, males age later than females. Consequently: whether a breeding pair really benefits from help of helpers does not depend on the father’s age, but only on the age of the mother.

Willy van Strien

Photo: ©Charlie Davies

Seychelles warblers help each other also in another way

Sources:
Hammers, H., S.A. Kingma, L.A. van Boheemen, A.M. Sparks, T. Burke, H.L. Dugdale, D.S. Richardson & J. Komdeur, 2021. Helpers compensate for age-related declines in parental care and offspring survival in a cooperatively breeding bird. Evolution Letters, online January 20. Doi: 10.1002/evl3.213
Hammers. M., S.A. Kingma, L.G. Spurgin, K. Bebbington, H.L. Dugdale, T. Burke, J. Komdeur & D.S. Richardson, 2019. Breeders that receive help age more slowly in a cooperatively breeding bird. Nature Communications 10: 1301. Doi: 10.1038/s41467-019-09229-3

Sound amplifier

14 January 2021 /

Small tree cricket calls from a window

small tree cricket male calls from a window to amplify its sound

Those that are small must be smart. At least that applies to tree crickets. Males with a softer call than others have an effective strategy to amplify their sound, as Rittik Deb and colleagues show.

Small crickets face a problem. To attract females, cricket males raise their forewings and rub them together. The forewings are leathery and provided with a comb. The rubbing causes them to vibrate, and as vibrating membranes, they produce sound waves: the familiar chirping. But at the wing edges, the sound waves are attenuated, softening the sound. Especially for small species, with small wings, that effect is significant. For instance for tree crickets (Oecanthus species), with wings only about one centimeter long.

Small and soft calling males of tree cricket Oecanthus henryi, which lives in India, have a unique method to amplify their sound, Rittik Deb and colleagues report: they turn a leaf into a sound amplifier.

Pear-shaped hole

The chirping noise is extinguished at the wing edges because waves at the front and back side are in opposite phase. That is because when air is compressed by the vibrating wings at the front side (creating a wave peak), it expands at the back (wave trough) and vice versa. At the edges, waves cancel each other out because of these opposite phase, resulting in a softer sound.

Tree cricket males can prevent this by separating the waves from the front and back side with a baffle. They do this by cutting a small window in a leaf and taking place in it to sing, with the head directed to one side, the abdomen to the other, and the raised forewings perpendicular to the body, in the plane of the leaf.

Previously, the researchers had shown that tree crickets can create a nearly perfect window in one go. They make such hole only in larger leaves of their host plant, Hyptis suavevolens. That makes sense, because large leaves produce a clear effect. The best place for the hole is in the center, but there the midrib runs. The leaf would wither when the tree crickets pierced it. So, they cut the hole close to the center, next to the midrib. And they make the hole pear-shaped, fitting the male with raised forewings, with the edges of wings and window close to each other.

The baffle is a good sound amplifier. The call is louder than it would be without it. For females, this is more attractive. In addition, the sound travels further, reaching more females.

More sperm

Yet not all tree cricket males take the effort to make such sound amplifier. Now, the researchers show that mainly the smaller ones with a soft sound do it, and explain why they do it.

By singing with a self-made baffle, small and soft calling males increase females’ attendance, as expected. With this sound amplifier, they may attract about six times as many females per night as without baffling, the researchers calculated. That is a considerable gain. Large and loud calling males can attract more females with a baffle too. But it doesn’t help them much, because even without sound amplification, they get as many mates as they can handle. They do not need to attract more females.

Large and loud males therefore call in the standard way: from the edge of a leaf. But other males make a window to amplify the sound of their call. This makes them appear larger than they really are, and females are misled. They mate longer with louder calling males – either large males or small, baffling ones – so that these males can transfer more sperm. By making a baffle, small and soft calling males increase their reproductive success, which would otherwise be quite low.

Willy van Strien

Photo: Oecanthus henryi. ©Rittik Deb

Sources:
Deb, R., S. Modak & R. Balakrishnan, 2020. Baffling: a condition-dependent alternative mate attraction strategy using self-made tools in tree crickets. Proceedings of the Royal Society B 287: 20202229. Doi: 10.1098/rspb.2020.2229
Mhatre, N., R. Malkin, R. Deb, R. Balakrishnan & D. Robert, 2017. Tree crickets optimize the acoustics of baffles to exaggerate their mate-attraction signal. eLife 6: e32763. Doi: 10.7554/eLife.32763

Hornets deterred

31 December 2020 /

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.

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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