From so simple a beginning

Evolution and Biodiversity

Page 13 of 21

Humboldt squid doesn’t discriminate

13 March 2019 /

Sperm to both male and female partners

Humboldt squid male mates male and female partners

Males of the humboldt squid are generous with their sperm cells; male-to-male mating is as common as male-to-female mating, Henk-Jan Hoving and colleagues discovered.

The mating of the humboldt squid or jumbo squid, Dosidicus gigas, is peculiar. Males produce spermatophores, long narrow capsules in which sperm cells are packed, and deposit them around a partner’s beak, which is between the eight arms and two tentacles. Each spermatophore then turns itself inside out to form a so-called spermatangium, which attaches itself to the skin.

If the partner is a female, the sperm cells will be needed. When she is spawning, she will use the sperm cells to fertilize the eggs. But the males transfer their sperm packets not only to females, but also to other males, according to Henk-Jan Hoving and colleagues. And males can’t use them.

It is not possible for researchers to directly observe the mating behaviour of the squid, which occurs in the eastern Pacific Ocean, because the animals live at a depth of several hundred meters. Instead, in order to learn something about that behaviour, the team examined the buccal area of captive specimens, both males and females, and counted the implanted spermatangia. They found sperm packets attached to both females’ and males’ buccal tissues, the same number in both sexes. The motto of mating males seems to be: ‘deposit your spermatophores anywhere you can’.

The question is why they don’t distinguish between male and female partners, as sperm cells transferred to a male are wasted.

Sharp teeth

The authors offer an explanation. The animals live in large mixed schools, in which they encounter many females and males. External morphological differences between the sexes are small, and a male that is about to mate has little time to check whether the individual in front of him is female. If he doesn’t manage to deliver his spermatophores quickly between the other squid’s arms and tentacles, he is in danger to be attacked. The humboldt squid is a predator; the suckers on its tentacles are lined with sharp teeth and its mouth has sharp edges. Cannibalism occurs.

That is why a male prefers a partner that is not larger, but of similar size. Because males are on average smaller than females, he will often deposit his spermatophores on a female that is not yet sexually mature. That is okay; she will store it until she needs it. But there is a chance that he accidentally transfers his sperm to a male.

Because of this strategy – be fast and stay safe – a humboldt squid male admittedly will waste sperm. But that is not a serious drawback. A male has hundreds of spermatophores available, and no more than 80 are transferred per mating. Even if he often mistakenly chooses a same-sex partner, he can still mate many females.

Storage

A female has dozens of sperm-storage organs in the buccal membrane, the seminal receptacles. Sperm cells leave the spermatangium after mating and migrate over the female skin to those storage organs, which apparently secrete an attractant.

When spawning, a female releases millions of eggs, held together in a gelatinous spherical mass. When that mass of eggs passes her mouth, the sperm cells will leave the storage organs, swim to the egg mass and fertilize the eggs.

Willy van Strien

Photo: Foto: Humboldt squid. Rick Starr. Credit: NOAA/CBNMS (Wikimedia Commons, Creative Commons CC BY 2.0)

Sources:
Hoving, H-J.T. Fernández‑Álvarez, F.Á., E.J. Portner & W.F. Gilly, 2019. Same‑sex sexual behaviour in an oceanic ommastrephid squid, Dosidicus gigas (Humboldt squid). Marine Biology 166: 33. Doi: 10.1007/s00227-019-3476-6
Fernández-Álvarez, F.Á., R. Villanueva, H-J.T. Hoving & W.F. Gilly, 2018. The journey of squid sperm. Reviews in Fish Biology and Fisheries 28: 191-199. Doi: 10.1007/s11160-017-9498-6

Staying cool

22 February 2019 /

Southern cassowary dissipates excess heat via its helmet

thanks to its helmet, the southern cassowary can offload excess heat

How can the tropical southern cassowary stay cool at high temperatures? Danielle Eastick and colleagues show that it uses its helmet to prevent overheating.

Just because of its size and strong legs – with a dangerous dagger-like claw that can be 10 centimetres long – the southern cassowary is an impressive bird. It also wears a prominent helmet or casque. What could be its function?

Until now, that was a mystery. Perhaps the helmet amplifies the deep sounds that the bird can produce, some people thought. Or perhaps it is a decoration to seek the attention of possible partners, next to the blue head and the blue and red wattled neck, according to another assumption. Otherwise, it may protect the head when the bird is moving through dense vegetation at high speed, or involved in a fight.

But now, Danielle Eastick and colleagues come up with a different answer.

Easily overheated

The southern cassowary lives in tropical forests of New Guinea and Australia. Being a large and dark animal, it can easily become overheated at high temperatures, so it must have the possibility to offload heat. Eastick hypothesized that the helmet might offer that possibility and set out to test this idea. It turned out that she was right.

The helmet consists of fragile, spongy bone and is partly hollow; it is covered with a horn layer and has an extensive superficial network of blood vessels. Infrared images taken by a special camera revealed that the blood vessels dilate at high temperatures, and the helmet gets warm. Heat can be offloaded to the air. But when it is cold, the blood vessel walls constrict and only a small amount of blood flows into the helmet. It cools down while the heat of the animal is preserved.

The legs and the tip of the beak contribute to temperature regulation in the same way, but the helmet plays the most important role. When it is very hot, a cassowary sometimes plunges its head into the water to lose more heat.

Design

The possibility of thermoregulation had already been suggested earlier, but it was never studied extensively. There are some other tropical birds with a helmet that may help to offload heat; for instance, some hornbills have a helmet on the beak. And perhaps the helmet of some dinosaur species facilitated heat loss as well.

The fact that the helmet of the cassowary has a function for thermoregulation does not exclude that it also plays a role in partner choice. Although, to be honest, its design is not very impressive.

Willy van Strien

Photo: Paul IJsendoorn (Wikimedia Commons, Creative Commons CC BY 2.0)

Sources:
Eastick, D.L., G.J. Tattersall, S.J. Watson, J.A. Lesku & K.A. Robert, 2019. Cassowary casques act as thermal windows. Scientific Reports 9: 1966. Doi: 10.1038/s41598-019-38780-8
Naish, D. & R. Perron, 2014. Structure and function of the cassowary’s casque and its implications for cassowary history, biology and evolution. Historical Biology 28: 507-518. Doi: 10.1080/08912963.2014.985669
Phillips, P.K. & A.F. Sanborn, 1994. An infrared, thermographic study of surface temperature in three ratites: ostrich, emu and double-wattled cassowary.  Journal of Thermal Biology 19: 423-430. Doi: 10.1016/0306-4565(94)90042-6

Frightening days

15 February 2019 /

Crayfish avoid light when renewing their armour

red swamp crayfish is anxious when moulting

Normally, the red swamp crayfish is rather fearless. But if it has to replace its carapace with a new one, its bravery disappears, as Julien Bacqué-Cazenave and colleagues report.

Crustaceans do not have a skeleton inside their body, like we do. Instead, they have a carapace, an external skeleton. This sturdy box in which they are packed protects them from physical harm. But there is a drawback: the carapace limits body growth. That is why the animals must, from time to time, replace their carapace with a larger one. The old one is shed, a new one is formed.

That is no trifle, as Julien Bacqué-Cazenave and colleagues show.

Process takes a month

The researchers wanted to know how the red swamp crayfish, Procambarus clarkii, is doing during a moult. The species originally occurs in Mexico and the south of the United States and has been introduced in many other places; it has settled as an exotic species in Europe.

Its moult is a lengthy and complex process. The chitin, of which the carapace consists, is secreted by the epidermis and the carapace is attached to it. So, it is must be separated from the epidermis, which has to form a new one. The attachments of the muscles that are anchored to the armour have to be transferred.

As soon as the old carapace is shed, the new one is exposed. This leaves the crayfish unprotected and vulnerable, as the newly formed carapace is thin and fragile in the beginning. It has to thicken and harden before it can protect the animal. The entire process of moulting takes about a month: two weeks before the old armour is shed and two more weeks until the new armour has hardened.

Anxious

The red swamp crayfish normally is courageous, but during the month of moulting, especially during the third week, it is not at ease, as experiments conducted by Bacqué-Cazenave show. He tested the animals every two or three days in a plus-maze with two illuminated and two dark arms. Crayfish that did not experience any stress spent 40 percent of their time in the illuminated part of the plus-maze. But when they were about to shed their carapace, they began to avoid the light a few days in advance, and the first week after moulting they stayed in the dark areas almost continuously. From earlier work, the researchers knew that the animals behave like this when they are anxious.

The aversion to light was indeed associated with moulting, according to tests in which the animals were given a hormone that initiates the moulting process, a so-called ecdysteroid. But when the animals were also given a tranquilizer, they did not avoid the illuminated areas. From this, the researchers conclude that the light aversion is an anxiety reaction.

Obviously, the period of moult is hard. But when it is over, the crayfish is safe in its armour for the next two to six months.

Willy van Strien

Photo: Andrew C (Wikimedia Commons, Creative Commons CC BY 2.0)

Source:
Bacqué-Cazenave, J., M. Berthomieu, D. Cattaert, P. Fossat & J.P. Delbecque, 2019. Do arthropods feel anxious during molts? Journal of Experimental Biology 222: jeb186999. Doi: 10.1242/jeb.186999

Romantic sea

8 February 2019 /

Fairytale light shows of Cypridinid ostracods

ostracod produces light to escape from predator

With an amazing show of light pulses, male cypridinid ostracods try to attract a mate. Each species has its own specific show program, with either very short lasting flashes or bulbs that glow for several seconds. Nicholai Hensley and colleagues examined the chemistry behind.

It looks like a fairytale scene: dozens of blue lights dancing in the dark waters of the Caribbean Sea. The spectacle is visible to those who dive or snorkel at the beginning of the night. The light artists are ostracods of the Cypridinidae family, tiny crustaceans (less than two millimeters long) with a carapax consisting of two valves, like a clam shell.

They are also known as sea fireflies. Nicholai Hensley and colleagues study their behaviour and the chemistry behind their light.

Slimeballs

Ostracods produce light by expelling mucus containing a reactant, vargulin, and the enzyme c-luciferase, which react with oxygen in seawater emitting blue light. The ostracods use their light mainly to avoid predation. If a fish picks up an ostracod, the prey will produce a cloud of blue mucus that is pumped into the water via the gills of the fish. It makes the fish visible to its own predators. Startled, it will spit out the bite.

In ostracods of the family Cypridinidae that live in the Caribbean Sea, males use the same light reaction in a much more subtle way with a completely different purpose: they place luminescent slimeballs in the water in order to seduce a female into a mating. This courtship behaviour produces the fairytale scenes.

Train of lights

The light artist best known is Photeros annecohenae, one of the most abundant species off the coast of Belize. In the first dark hour of the night, when the sun is down and the moon is not shining, groups of males display above seagrass beds. They have to perform well, because competition is high. While there are as many females as males, most are unavailable. This is because they incubate fertilized eggs in a brood pouch, and during this period, they will not mate.

American biologists examined male courtship behaviour in the lab, using infrared light. A displaying male will first swim in a looping pattern just above the tips of the seagrass blades and place about three bright flashes of light, probably to draw attention. Then, while spirally swimming upward, it places weaker light pulses at regular intervals. It swims at high speed, slowing down when it releases a luminescent slime ball.

By doing so, it creates a train of about twelve consecutively flashing lights that can be 60 centimetres long. When finished, it descends to start a new series. Often other males join and start displaying in synchrony.

Interception

To choose a mate, females assess the light pulses that the males produce. If a female is attracted to a particular male, she will swim to him without producing any light herself. Thanks to his regular flashing pattern, she manages to meet him just above his last light pulse. Mission accomplished.

Sometimes males try to obtain a mate without producing light themselves. Instead, they intercept a female that is on her way to a performing male.

Starting a show, following another male’s show or sneaking to get a female are different tactics to acquire a mate and a male can easily switch among them.

Species-specific shows

In the Caribbean Sea, many other species of Cypridinidae also occur, and about ten species commonly live at the same place. Because they all have their own characteristic light show, a female has no difficulty finding a conspecific partner. The shows vary in the trajectory a courting male swims, the number of light pulses, the brightness of the light, the interpulse distance and time interval and the time that a pulse remains visible.

Romantic

Hensley investigated the cause of the variation in light pulse length. For although all species perform the same chemical reaction to make light pulses, the duration of the pulses varies greatly: some species, such as Photeros annecohenae, show flashes that last only a fraction of a second, others make light bulbs that continue to glow for 15 seconds.

The structure of the enzyme c-luciferase appears to vary between species, resulting in the light reaction to proceed faster in one species than in another. This determines how soon the light extinguishes. In addition, the reaction rate depends on the amount of vargulin compared to the amount of enzyme: the more vargulin, the longer it takes before it is all converted and the light disappears.

Courting males produce far less light than an animal that avoids predation. Romantic lights don’t have to be that big and bright.

Willy van Strien

Photo: Luminous cloud around a fish that intended to consume an ostracod. It will spit it out. © Trevor Rivers & Nicholai Hensley

Fifteen-scaled worm emits light to defend itself in another way

Sources:
Hensley, N.M., E.A. Ellis, G.A. Gerrish, E. Torres, J.P. Frawley, T.H. Oakley & T.J. Rivers, 2019. Phenotypic evolution shaped by current enzyme function in the bioluminescent courtship signals of sea fireflies. Proceedings of the Royal Society B 286: 20182621. Doi: 10.1098/rspb.2018.2621
Rivers, T.J. & J.G. Morin, 2013. Female ostracods respond to and intercept artificial conspecific male luminescent courtship displays. Behavioral Ecology 24: 877–887. Doi: 10.1093/beheco/art022
Rivers, T.J. & J.G. Morin, 2012. The relative cost of using luminescence for sex and defense: light budgets in cypridinid ostracods. The Journal of Experimental Biology 215, 2860-2868. Doi: 10.1242/jeb.072017
Morin, J.G. & A.C. Cohen, 2010. It’s all about sex: bioluminescent courtship displays, morphological variation and sexual selection in two new genera of Caribbean ostracodes. Journal of Crustacean Biology 30: 56-67. Doi: 10.1651/09-3170.1
Rivers, T.J. & J.G. Morin, 2009. Plasticity of male mating behaviour in a marine bioluminescent ostracod in both time and space. Animal Behaviour 78: 723-734. Doi: 10.1016/j.anbehav.2009.06.020
Rivers, T.J. & J.G. Morin, 2008. Complex sexual courtship displays by luminescent male marine ostracods. The Journal of Experimental Biology 211: 2252-2262. Doi: 10.1242/jeb.011130

Fiery character

30 January 2019 /

A brave great tit probably is either big or hungry

The corage of a great tit depends on its body size and condition

How much risk is a great tit prepared to take? The answer differs greatly among individuals. Maria Moiron and colleagues show that how much the courage the birds exhibit, depends on their size and condition.

Just like humans, animals have a personality, a stable set of coherent behavioural traits. For example, animals differ from each other in how brave they are, the extremes being an aggressive, brutal, curious and enterprising character on the one hand and a shy, cautious and withdrawn nature on the other. As biologists pointed out, these personality types also occur in the great tit (Parus major). Maria Moiron and colleagues wondered if the personality of a great tit is linked to its physical characteristics.

To find out, they weighed a number of males and measured the length of leg, beak and wing. Also, they tested the animals for their willingness to take risks by assessing how aggressive the animals behaved to another male, which intruded into their territory. They also tested how quick they were to explore an unfamiliar test cage.

Fear or courage?

Great tits differ greatly from each other in all triats measured, as it turned out. After a statistical analysis of the data, the researchers conclude that large individuals on average are less anxious than smaller conspecifics. That may be, they speculate, because a large animal has a greater chance of winning when it comes to fighting and will be hurt less severely. An alternative explanation is that it will take more risk in acquiring food because it needs more energy.

Another finding is that the condition of the birds, in the sense of their energetic reserves, also determines their behaviour. An animal in need of some food generally takes more risks than an animal that is well-fed. A hungry bird cannot afford to be careful, it has to take action, the authors explain. It is also possible that a well-fed bird is more cautious because it has more difficulty taking off in case of danger.

Conclusion: the personality of a great tit is indeed related to its physical characteristics. That is not unexpected – but it had not been demonstrated before.

Willy van Strien

Photo: Tbird ulm (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Source:
Moiron. M., Y.G. Araya-Ajoy, K.J. Mathot, A. Mouchet & N.J. Dingemanse, 2019. Functional relations between body mass and risk-taking behavior in wild great tits. Behavioral Ecology, online January 18. Doi: 10.1093/beheco/ary199

Giving everything he’s got

11 January 2019 /

Hummingbird male shines for a split second

broad-tailed hummingbird male performs spectacular dive

In order to seduce as many females as possible, a broad-tailed hummingbird male performs tight diving courtship flights. He combines movement, colour and sound into a spectacular whole, Ben Hogan and Cassie Stoddard show.

With a striking display, a broad-tailed hummingbird male (Selasphorus platycercus) tries to gain a female’s interest. He performs a number of U-shaped dives, getting down from great height (up to 30 meters!) while his wings are trilling. The lowest point of the dive is close to the targeted female, which is perched. At that point, he will give everything he’s got: he rushes past her with a top speed of more than 20 meters per second while his tail feathers produce buzzing sounds. The female perceives his iridescent gorget rapidly shifting from bright red to dark green. Then he climbs up to enable a new dive.

The show is so fast that we can’t see what exactly happens. But Ben Hogan and Cassie Stoddard made video and audio recordings of a large number of shows and analyzed them.

Blink of an eye

An entire dive takes about 6.5 seconds. At the lowest point, the small bird appears to tightly synchronize the components of the show, as the analysis revealed. As a result, top speed, buzzing sounds and colour change almost coincide, all occurring within 300 milliseconds, a human blink of the eye. When he rapidly rises again from the lowest point, the pitch of wing- and tail-generated sounds drops sharply, as when a car with a siren is passing by (the Doppler effect).

The whole is meant to make an overwhelming impression on her. But she is used to see shows like his, because all males perform them. The hummingbird males do not contribute to nest construction or care for the young, leaving all of the work to the females. They try to sire young with as many females as possible. With their tightly synchronized dive, they advertise their genetic quality, promising healthy and attractive offspring.

But is he able to seduce a female? The researchers have not yet figured out what exactly makes a show appealing and how it is performed perfectly in her eyes.

Willy van Strien

Photo: Greg Schechter (Flickr/Wikimedia Commons, Creative Commons CC BY 2.0)

Source:
Hogan, B.G. & M.C. Stoddard, 2018. Synchronization of speed, sound and iridescent color in a hummingbird aerial courtship dive. Nature Communications 9: 5260. Doi: 10.1038/s41467-018-07562-7

Escape scene

18 December 2018 /

Young aphids try to hitch a ride on adults

young pea aphid hitches a ride on an adult

Young pea aphids demand a piggyback ride on the back of an adult when they have to walk on the soil, Moshe Gish and Moshe Inbar report. But they don’t always get what they want, because the adults try to remove them.

When their host plant starts shaking and a warm, moist air is blowing over it, sap feeding pea aphids (Acyrthosphon pisum) are in danger: a mammalian herbivore is approaching. Just before the plant is eaten by the grazer, they massively drop off the plant to the ground to escape. But they are not safe there either; they can be trampled, dessicate, starve or fall prey to predators that hunt on the ground, like spiders. And so they start walking in search of a new plant.

That is not easy for young aphids (the nymphs). The ground is bumpy with clumps of soil, cracks, stones, fallen twigs and leaves. The young critters are moving much slower than adults. But there is a solution, Moshe Gish and Moshe Inbar discovered: piggybacking on a large aphid.

Tiny drama

In a series of lab experiments, the researchers simulated escape scenes. They placed about ten female aphids on a fava bean plant. Next morning, young aphids had been born; pea aphids can reproduce parthenogenetically, females giving birth to daughters without having been mated. By tapping the plant and exhaling over it, the researchers induced the aphids to drop off. At some distance from the fava bean plant, a circle of lentil plants was placed to offer a destination, the bottom was covered with soil.

A tiny drama takes place in such a case, as the researchers observed. Immediately after landing, young aphids try to climb on an adult. They also climb on green plastic beads and on dead aphids that they happen to stumble upon, but from these, they quickly disembark. If a living aphid stays immobile after they climbed on, they get off after a while too. But when their carrier starts walking, they ride to a new plant, where they arrive faster than when they would have walked the distance on their own.

But adult aphids are not really willing to help young ones.

Nuisance

On the contrary: passengers seem to be a nuisance to them. When a nymph tries to climb on an adult aphid, this aphid will often raise its body to make it difficult, or it runs away. If there are already nymphs on its body, it often stays motionless, waiting until the passengers leave. Or it repeatedly lowers its head or posterior end to the surface to get rid of them. The back is the best place for a nymph to sit on. Eventually, an adult aphid will start walking with one passenger at most.

Once it has left, after a delay, it will reach a new plant as soon as an aphid without a hitchhiker, so the walking speed is not affected by the burden. But it is energetically costly to bear it.

Willy van Strien

Photo: © Stav Talal

Also these tadpoles try to catch a ride

Sources:
Gish, M. & M. Inbar, 2018. Standing on the shoulders of giants: young aphids piggyback on adults when searching for a host plant. Frontiers in Zoology 15: 49. Doi: 10.1186/s12983-018-0292-7
Gish, M., A. Dafni & M. Inbar, 2010. Mammalian herbivore breath alerts aphids to flee host plant. Current Biology 20: R628-R629. Doi: 10.1016/j.cub.2010.06.065

Unopened flower

13 December 2018 /

Moth larva enforces self-pollination in Canada Frostweed

Canada Frostweed may be enforced to self-pollination by a moth larva

The larva of the moth Mompha capella inhabits a flower bud of Canada Frostweed and prevents it from opening, as Neil Kirk Hillier and fellow researchers show. Pollinators cannot visit the flower, which has to pollinate itself instead.

Canada Frostweed (Crocanthemum canadense), a perennial plant of Northern America, is attractive to the moth Mompha capella, which lays its eggs on it. Then something unusual happens: the plant loses control over its reproduction.

The plant produces yellow flowers that normally open just after sunrise, revealing the female pistil and male stamens. Bees and flies visit the flowers, transferring the pollen from one to the next, so that the flowers are cross-pollinated. Multiple stamens lay against the five yellow petals, retracted from around the pistil to prevent self-pollination. Within a few hours, a flower’s own pollen has disappeared and the pistil is covered with pollen from other flowers. The petals fall off, the green sepals close over the pistil and protect the developing fruit with seeds within.

But when a moth has left its eggs on the plant, the larvae that hatch from these eggs crawl into a flower bud, one larva per bud. And then things are very different, Neil Kirk Hillier and colleagues discovered.

Cap

The larvae start to eat. And they don’t do it randomly, but first feed on the bases of the still folded petals. The severed petals no longer grow and don’t unfold when the flower should open, but remain folded like a cap over stamens, pistil and developing fruit, keeping the flower closed. Pollinators cannot enter. Because the stamens are compacted around the top of the pistil, the pollen is in contact with the pistil and seeds develop through self-pollination. Almost all of them will be consumed by the larva in the end.

Frostweed duped

As a consequence, the Canada Frostweed plant produces less offspring. A yellow flower produces on average about forty seeds, and a larva saves only one or two of them. Reproduction, however, is not in immediate danger. This is because the plant not only produces a small number of yellow flowers that open, unless caterpillar disturbs the process, but also a large number of flowers without yellow petals and only four or five stamens, which appear later in the year. These flowers never open and produce seeds through self-pollination. While they make less seeds than yellow, open flowers (only six or seven seeds per flower), there are much more of them. So seeds are produced anyway.

But seeds of open flowers that develop after cross-pollination are necessary for the exchange of hereditary material. In plant populations with a high infestation rate, such exchange is limited, and genetic variation is low.

Larva safer?

The researchers don’t mention what benefit a larva gains by intervening in the flowering process. If the flower opened and had been pollinated normally, seeds to be consumed would have appeared as well. Perhaps in a closed flower, the larva is safer from predators and parasites.

Willy van Strien

Photo: Homer D. House, 1918 (Wikimedia Commons)

Source:
Hillier, N.K., E. Evans & R.C. Evans, 2018. Novel insect florivory strategy initiates autogamy in unopened allogamous flowers. Scientific Reports 8: 17077. Doi:10.1038/s41598-018-35191-z

Stealthy moth

28 November 2018 /

Bats receive attenuated echo of their calls

The moth Bunaea alcinoe applies acoustic camouflage

Scales and furry coat of the moth Bunaea alcinoe thwart the searching method of hunting bats, as Zhiyuan Shen and colleagues show. That is how it manages to make itself undetectable.

Moths, which fly around at night, have to deal with agile enemies: bats. These predators find their prey by emitting high (ultrasonic) calls and hearing the echo of their sound when it is reflected by a wall, a tree – or a moth. This ‘echolocation’ enables them to localize a moth and find out in which direction it goes, and then they can catch it.

Moths have developed different ways to escape from these enemies. Some hear the bats’ sounds and quickly change direction; others produce a sound by themselves with which they startle or confuse a hunting bat; still others have wings that distort the echo.

Another defence strategy is to absorb part of the bats’ sounds, preventing their reflection. That idea is applied by Bunaea alcinoe, the cabbage tree emperor moth which lives in Africa, as Zhiyuan Shen and colleagues show.

Vibrations

The numerous scales that cover the wings of this moth are responsible for the absorption. The scales, which look like leaves on a pedicel under a microscope, have a regular, very open nanostructure. Thanks to this structure, they can vibrate exactly at the frequencies of the bats’ sound, the researchers show. The sound waves are transferred to the scales, where they are extinguished by friction: acoustic camouflage against bat echolocation.

Butterflies, which are active during daytime and are not hunted by enemies that use echolocation, have scales on their wings with a different nanostructure, that do not vibrate at the frequency of bat calls.

Another feature is the striking hair growth on the backside of the moths. That furry coat also absorbs sound, in the way curtains and carpets do.

As a result, bats have difficulty finding this prey, Bunaea alcinoe, as only a part of the sound of their calls is reflected when it hits a moth.

Willy van Strien

Photo: Paul Wursten (via Flickr, Creative Commons CC BY-NC-SA 2.0)

Explanation by researchers on YouTube

Sources:
Shen, Z., T.R. Neil, D. Robert, B.W. Drinkwater & M.W. Holderied, 2018. Biomechanics of a moth scale at ultrasonic frequencies. PNAS, online Nov. 12. Doi: 10.1073/pnas.1810025115
Neil, T.R., Z. Shen, B.W. Drinkwater, D. Robert & M.W. Holderied, 2018. Stealthy moths avoid bats with acoustic camouflage. Journal of the Acoustical Society of America 144: 1742. Doi:  10.1121/1.5067725
Zeng, J., N. Xiang, L. Jiang, G. Jones, Y. Zheng, B. Liu & S. Zhang, 2011. Moth wing scales slightly increase the absorbance of bat echolocation calls. PLoS ONE 6(11): e27190. Doi:10.1371/journal.pone.0027190

Complex cooperation

15 November 2018 /

Weaver ant will rescue nestmate from spider web

a weaver ant on its nest

Floria Uy and colleagues show that workers of the weaver ant care about their nestmates: when an ant gets entangled in a spider’s web, others are willing to help it. The ants were already known for their ‘living sewing machine’.

The weaver ant or green tree ant, Oecophylla smaragdina, is common in the tropical parts of Asia and Australia. Its presence is clearly visible because of its nests, which stand out as balls of leaves in bushes and trees. The arboreal nests are the result of a special piece of group work, involving not only workers, but also larvae.

weaver ants building their nestTo construct a nest, the ants must glue the leaf edges together. Workers will line up, grasp the edges of two leaves (or two pieces of a long leaf) and draw them together. If the distance between the leaf edges is large, they grab each other and form living chains to bridge the gap. By then shortening the chain, they pull the leaf edges towards each other.

 

Living sewing machine

They then attach the edges firmly to each other with a ‘living moveable sewing machine’, as described by Ross Crozier. While many workers hold the leaf edges, others join with a mature larva between their jaws.weaver ants use larval silk to construct the nest The workers stimulate the larvae to spin a silk thread and move them zigzag between the leaf edges, stitching the leaves together.

In many ant species, larvae spin silk to make a cocoon in which they pupate. But in the weaver ant, their silk is used for nest construction.

A colony of weaver ants consists of multiple nests in several trees; many ants are running on trails between these nests. In peripheral nests of the colony, soldiers live that guard the boundaries of the territory and defend it against conspecific ants from foreign colonies. On average, a colony will live for eight years and can have as many as half a million inhabitants.

That is a lot, and you would be inclined to think that an ant’s life does not matter that much.

Yet, the ants are concerned when a nestmate that is in danger, Floria Uy and colleagues now show. They discovered a second example of complex cooperation within this species.

Ant in distress

If a weaver ant is trapped in a spider’s web, conspecifics may bite the threads of the web and free the victim, as Uy showed. But an alternative scenario is also possible: ants may attack a worker that is entangled in a spider’s web and kill it. When will the ants rescue a conspecific, and when will they kill it?

Uy, who conducted experiments on the Solomon Islands, put a number of ants next to an ant trail after having wrapped them in spider’s silk; in some cases, the victim was a nestmate of the ants on the trail, in other cases it was from a foreign colony.

She found that ants will always help a nestmate in distress. But for ants from a different colony, the outcome is uncertain: some are rescued, others are killed. That some of them are killed is to be expected: to the residents on the trail, they are intruders and accordingly, they are treated aggressively. The fact that some non-nestmates in distress are helped instead is surprising.

It may be a mistake, the researchers hypothesize. In the study area, ants from neighbouring colonies have a greater chance of being rescued than ants from distant colonies. Ants recognize nestmates and colony mates by smell, and neighbouring colonies may have a rather similar odour.

Willy van Strien

Photos:
Large: weaver ant on nest. Rushen (via Flickr, CC BY-SA 2.0)
Small, middle: workers building the nest. Sean.hoyland (Wikimedia Commons, Public Domain)
Small, below: nest with weaver ants. Bernard DUPONT (via Flickr, CC BY-SA 2.0)

Watch weaver ants building a nest

Sources:
Uy, F.M.K., J.D. Adcock, S.F. Jeffries & E. Pepere, 2018. Intercolony distance predicts the decision to rescue or attack conspecifics in weaver ants. Insectes Sociaux, online Nov. 3. Doi: 10.1007/s00040-018-0674-z
Crozier, R.H., P.S. Newey, E.A. Schlüns & S.K.A. Robson, 2010. A masterpiece of evolution – Oecophylla weaver ants (Hymenoptera: Formicidae). Myrmecological News 13: 57-71

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