Vine avoids spider mites

Tendrils curl away from herbivore-infested plants

Vine Cayratia japonica prevents spider mites from invading

When the Asian climbing plant Cayratia japonica stretches its tendrils to other plants, it is careful. The tendrils withdraw as soon as they detect the presence of spider mite, as Tomoya Nakai & Shuichi Yano observed.

The Asian vine Cayratia japonica is an excellent climber: in America, where it was introduced, it is known as bushkiller. Tendrils of the plant coil around stems of neighboring plants, enabling the vine to grow towards the light. The tendrils grab onto everything they can.

Well, not everything really. The tendrils withdraw when they touch upon a plant that is infested with two-spotted spider mite, Tomoya Nakai & Shuichi Yano show. Two-spotted spider mite or red spider mite (Tetranychus urticae) is a small arachnid hat sucks up plant sap from leaves, which often don’t survive it. The mites occur on hundreds of plant species. If their number at some place is too high, they will walk to another place. As they follow each other’s trails, a group will soon aggregate at this new site.

Spider mite web

Because of its physical contact with other plants, a vine could easily get infested by these harmful critters. But Cayratia japonica appears to have an effective way to prevent mites from invading. As soon as a tendril touches a plant that is occupied by mites, it withdraws and curls away from the infested plant. The researchers could show this in the lab, by placing a number of vines each next to a bean plant that was either clean or bearing many mites. They filmed the movement of the vine’s tendrils using time-lapse photography, making one film frame per minute.

The next question was: what cue does a tendril use to detect the presence of spider mite? Does it pick up the volatile compounds that a bean plant releases into the air when infested? Or does it feel the web with which the mites cover the plant surface to be safe underneath from predators?

Experiments showed that the volatile compounds released by infested bean plants have no effect on the stretching tendrils. But mite silk does: after contact with a spider mite web, the tendrils immediately withdraw. Nakai and Yano also tried spider silk, but the tendrils did not respond to it. The vine thus responds directly and specifically to the presence of spider mite.

This reduces the chance that mites disperse in groups from support plants to the climbing plant. A few of them will cross over during the short contact, but they are not save without the web and will disappear.

Willy van Strien

Poto: 石川 Shihchuan (via Flickr. Creative Commons CC BY-NC-SA 2.0)

Source:
Nakai, T. & S. Yano, 2019. Vines avoid coiling around neighbouring plants infested by polyphagous mites. Scientific Reports 9: 6589. Doi: 10.1038/s41598-019-43101-0

Suicidal repair team

Young aphids die when closing a hole in their nest

Soldier nymphs in Nipponaphis monzeni repair their nest with their body fluid

Japanese aphids, Nipponaphis monzeni, inhabit galls on hazel. A hole in the gall wall would mean the end of the colony living there, were it not for aphid soldiers that give their lives to close it. Mayako Kutsukake and colleagues show how.

The Japanese aphid Nipponaphis monzeni is a social species, living in colonies. Juveniles, called nymphs, serve as soldiers for a period before they become adults and reproduce. It is their task to defend the nest, which is located in galls on the branches of evergreen witch hazel (Distylium racemosum), and to repair it in case of damage.

To close a hole, they show a spectacular and unique behaviour. In a self-destructive action, they discharge their body fluid to plug the gap. The liquid solidifies, forming a scab. Mayako Kutsukake and colleagues were curious about the mechanism.

Vulnerable nest

Colonies of Nipponaphis monzeni are founded by females that reproduce parthenogenetically. A colony of sisters is formed that are genetically identical and produce identical daughters.

gall on hazel in which Nipponaphis monzeni livesThe aphids induce the hazel on which they live to form a closed, hollow tumour, a gall. The animals inhabit this gall, sucking plant sap from the inner wall; in this phase, they are wingless. The gall remains small for a long time, but after three to five years it begins to grow rapidly during spring months and the following summer, it is fully grown – up to eight centimetres long – and home to thousands of aphids.

Winged aphids then appear in autumn. They make an opening in the wall and fly away to a second host tree, an oak, where they mate and produce a new generation of colony foundresses.

A full-grown gall has a lignified, hard wall, offering safety. But during growth, the wall consists of soft plant tissue and the nest is vulnerable. Moth caterpillars consuming hazel tree leaves easily tunnel into such gall, ingesting aphids as well. The soldiers will not tolerate this and attack the enemy: they climb onto it and sting it to death with their mouth parts.

But the hole that the caterpillar gnawed in the gall wall still remains. It has to be closed, otherwise enemies or pathogens may invade, or the nest may desiccate.

Skilful plastering

Japanese researchers had already shown how the soldier nymphs repair the hole with a self-sacrificing behaviour. Dozens or hundreds of them gather around the hole and eject large amounts of white body fluid (hemolymph, which is comparable to our blood) through two tubes on the abdomen. They mix the secretion with their legs and skilfully plaster it over the hole. Some soldiers are buried, others are locked out in the process. And all shrivel after losing their body fluid and will die.

Any way, the hole is fixed; the plug hardens and turns black. As a result, the colony is likely to survive the damage. After the sealing, the gall wall is healed, as the soldiers trigger the tree to cover the plasterwork on the inside by regenerating plant tissue.

Coagulation

Now, Kutsukake investigated the substances with which the soldiers repair a hole. The body fluid, she shows, contains many peculiar large cells of a hitherto unknown type that are packed with fat droplets and the enzyme phenoloxidase; the fluid contains long proteins and tyrosine, an amino acid.

When the soldiers discharge their body fluid, the cells rupture and the fat globules are released; the soldiers plug the gap immediately with a soft lipidic clot. At the same time, the other components come into contact with each other, and a coagulation process starts in which the proteins are linked to form a network that reinforces the lipid plug so that it becomes a scab.

The researchers assume that the process is derived from the process by which wounds heal. But in soldier nymphs’ hemolymph, the components are accumulated in extremely large quantities, far beyond what is necessary for wound healing.

With their unique repair behaviour, the soldier nymphs of Nipponaphis monzeni exhibit extreme altruism to defend the colony: they give their lives. Thanks to this sacrifice, a large part of their family survives. Otherwise the entire colony would have been lost.

Willy van Strien

Photos : ©Mayako Kutsukake
Large: Nipponaphis monzeni soldier nymphs plastering their hemolymphe over a hole
Small: gall in which Nipponaphis monzeni lives

On YouTube, the researchers show how soldiers fix a hole in the gall wall

Sources:
Kutsukake, M., M. Moriyama, S. Shigenobu, X-Y. Meng, N. Nikoh, C. Noda, S. Kobayashi & T. Fukatsu, 2019. Exaggeration and cooption of innate immunity for social defense. PNAS, 15 april online. Doi: 10.1073/pnas.1900917116
Kutsukake, M., H. Shibao, K. Uematsu & T. Fukatsu, 2009. Scab formation and wound healing of plant tissue by soldier aphid. Proceedings of the Royal Society B 276: 1555-1563. Doi: 10.1098/rspb.2008.1628
Kurosu, U., S. Aoki & T. Fukatsu, 2003. Self-sacrificing gall repair by aphid nymphs. Proceedings of the Royal Society London B (Suppl.) 270: S12-S14. Doi: 10.1098/rsbl.2003.0026

Discrete invitation

Arabian babbler leads partner to hidden place

Arabian babbler invites partner in an unobtrusive way

Unlike other animals, the Arabian babbler keeps its sex life private. It has a subtle way to invite another bird for a concealed copulation, as Yitzchak Ben Mocha and colleagues observed.

Animals do not seek to conceal their sexual behaviour. But the Arabian babbler, Argya squamiceps, is an exception. The birds, which live in stable kin groups of two to twenty individuals, do not want to be detected when copulating. A couple that is going to mate will take care to be out of sight of their group mates: at a certain distance or behind thick vegetation.

Yitzchak Ben Mocha and colleagues describe how the birds take a partner to such hidden place without revealing their intention to the other birds.

Arabian babblers live in open, dry landscapes across the Arabian Peninsula and Israel, where each group defends a territory. Within a group, only one pair, the dominant pair, will breed. They are the parents of nearly all young in the group. The other adult group members are subordinates and help raise the young. After hatching, the young stay in the nest for two weeks. And after fledging, it takes another eight weeks until they reach independency. During this period, they need care: protection and food.

Unobtrusive

Observing a population, the researchers witnessed that the birds have a subtle way to invite another bird to copulate. They place themselves in a location that is visible to that specific bird only while holding an object in the beak; often they slightly shake their head. The object can be anything, such as a twig, leaf, fruit, small animal or eggshell. The signalling behaviour is unobtrusive, but the partner grasps the message. When he or she accepts the invitation and approaches, the initiator moves away or hides behind the vegetation and the partner will follow. If they lose contact, the initiator comes back, places itself within the other’s visual field and repeats the invitation.

Usually, a copulation follows. But when another group member appears, the  signaller drops the object and stops the mating behaviour.

The object presented is nothing special, just something that happens to be abundant. So, it is not intended to impress. Neither is it a gift; although it may be edible, that does not affect the partner’s response. The presentation is just a subtle way to invite a mate for a concealed copulation.

Crucial help

Even dominant birds, which don’t have to fear that subordinates will dare to disturb a copulation, take great care to hide their mating behaviour. Why is that? The authors offer an explanation. The care of subordinate group members is crucial for raising the offspring. Without that care, the young have a smaller chance to reach adulthood. Moreover, they gain less weight and will be less capable to acquire food once they are independent.

The dominant pair does not want to lose that precious help. With overt mating behaviour, the researchers suggest, they would cause social tension in the group and increase the chance that subordinates leave or fight, which would be undesirable. So, the parents prefer to keep peace by keeping their love life private.

Willy van Strien

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

See invitation for concealed copulation on YouTube

Sources:
Ben Mocha, Y. & S. Pika, 2019. Intentional presentation of objects in cooperatively breeding Arabian babblers (Turdoides squamiceps). Frontiers in Ecology and Evolution 7: 87. Doi: 10.3389/fevo.2019.00087
Ben Mocha, Y., R. Mundry & S. Pika, 2018. Why hide? Concealed sex in dominant Arabian babblers (Turdoides squamiceps) in the wild. Evolution and Human Behavior 39: 575-582. Doi: 10.1016/j.evolhumbehav.2018.05.009
Ridley, A.R., 2007. Factors affecting offspring survival and development in a cooperative bird: social, maternal and environmental effects. Journal of AnimalEcology 76: 750-760. Doi: 10.1111/j.1365-2656.2007.01248.x

Who dares?

Black-headed female takes the lead in Gouldian finch

Black-headed Gouldian finch is most courageous

When Gouldian finches have to drink, not every bird is willing to leave the safe trees first. There is a pattern in the order in which they venture to a waterhole, Andrias O’Reilly and colleagues show.

Drinking water is a perilous adventure for the Gouldian finch, a songbird from north-western Australia. It is a colourful species. If the birds are on the ground near a creek or pond, birds of prey will spot them easily, so they are less safe there than in the trees. But they have no choice: they have to drink. Once a day, early in the morning, they take the risk, coming in flocks. Some of them are more courageous than others, as Andrias O’Reilly and colleagues saw.

Personality

That was already known from experiments in the lab. Gouldian finches have different head colours; most birds (70 percent) have a black head, others (30 percent) a red one and a few (less than 1 percent) a yellow one. Research had shown that the colour is linked to personality: animals with a red head are more aggressive, whereas black-headed birds are more explorative and more courageous. The researchers wondered if this is also the case in the wild. If it is, a difference in courage should be visible in a risky activity, such as drinking water.

So, they observed the birds at their daily drinking party and noted what type of bird descended first to the waterhole, taking the greatest risk. They conducted their research at the end of the breeding season, when the young had fledged.

Apparently, Gouldian finches are no heroes. They will not land on the ground to drink untill other birds are present at the waterhole. If no bird is there, they will remain in the trees and wait for other birds to appear.

Risk

Only in that case, they dare as well. And then, the black-headed birds are most likely to take the lead, as it turned out. That is in agreement with what was found in the lab. The black-headed birds run less risk because they are less conspicuous than the red-headed ones, the explanation is.

Young birds have not yet full colours and are even less conspicuous. Still, juveniles let the adult birds precede; probably the young birds are more cautious because they have little experience.

At the start of the research period, mainly females are the first birds to descend to the water. Fathers primarily care for the young for a while after fledging; during this period, males will stay with the cautious young, so females have to take the lead. And the black-headed females will take it.

Willy van Strien

Photo: Gouldian finch, black-headed male. Linda de Volder (via Flickr; Creative Commons CC BY-NC-ND 2.0)

Source:
O’Reilly, A.O., G. Hofmann & C. Mettke-Hofmann, 2019. Gouldian finches are followers with blackheaded females taking the lead. PLoS ONE 14: e0214531. Doi: 10.1371/journal.pone.0214531

Costs before benefits

By guarding stepkids, bee male may get the mother

In bee Ceratina nigrolabiata, the male takes care of other males' offspring

Ceratina nigrolabiata bee males guard the nest of their female partner. This seems surprising, as the brood consists mainly of other males’ offspring, as Michael Mikás and colleagues show. Still, the males have good reason.

Bee males don’t do much. Okay, they mate with females and of course that is important, but that’s it. The females construct a nest and take care of the offspring. In solitary species, such as the species that visit a bee hotel, each female makes her own nest; social species, such as the honeybee, live in groups in which queens produce eggs and workers do the work.

There is one exception, Michael Mikát and colleagues report: in the solitary bee species Ceratina nigrolabiata, males do participate in care – but, surprisingly, mainly by protecting other males’ offspring.

Guard

A Ceratina nigrolabiata female makes her nest in the hollow stem of a plant. She goes inside, lays an egg, brings food for the larva that will hatch, closes the space by building a wall and lays another egg in the next part of the stem. Ultimately, a nest consists of six to seven cells in a row, with young in a descending stage of development when viewed from the inside out. The mother leaves when the nest is completely provisioned.

In the majority of nests in which a female is active, a male is present, as the researchers observed during their studies in the Czech Republic. When the female performs foraging trips, the male stays inside the nest to protect it from predators such as ants, driving them away when they come near. He is sitting near the entrance with the head facing inwards. When she returns, she will scratch his abdomen and he will let her pass.

The benefit for her is clear: thanks to this guard, she can leave to forage without having her nest unattended.

For him, it is different. DNA analyzes show that in most cases the brood that he protects does not contain any offspring that he fathers. So he takes care of other males’ offspring, and in general, that is not a good strategy from an evolutionary point of view.

Male switches

In fact, the bee males have no interest in the brood at all; it is the mother that captivates them. A male only has a chance to mate if he finds a female and stays with her until she is willing; in Ceratina nigrolabiata, a female will mate several times in her life. So he has to stay at her nest. While he certainly participates in care by actively protecting the brood, this stepfather care is a by-product of monopolizing a female, according to the researchers.

And indeed, if they removed a female from her nest, the male abandoned the brood.

So, every female is assured of a helpful lover. If a male disappears, his place is usually taken by another within a day.

These stepfathers are not ideal helpers, because they stay on average for only seven days, while a female needs about forty days to complete her nest. As a consequence, the male inhabitant of most nests changes one or more times, and in fatherless periods the female spends less time collecting food, staying on the nest instead. The more changes, the fewer offspring she therefore can produce. But at least she gets help, which is unique among solitary bees.

Willy van Strien

Photo: Ceratina nigrolabiata, female returns at her nest in a hollow plant stem and scratches the guarding male. ©Lukáš Janošík

Source:
Mikát, M., L. Janošík, K. Cerná, E. Matoušková, J. Hadrava, V. Bureš & J. Straka, 2019. Polyandrous bee provides extended offspring care biparentally as an alternative to monandry based eusociality. PNAS: 116: 6238-6243. Doi: 10.1073/pnas.1810092116

Care for everyone

Earwig mother tends foreign eggs and adopts orphans

earwig female cares for another female's offspring 

An earwig mother will treat another female’s eggs as caring as her own eggs, Sophie Van Meyel and colleagues write. Previously, Janine Wong and Mathias Kölliker had discovered that she is willing to accept young orphans in her family.

Earwigs are not very popular animals, but actually they are lovely creatures. The extensive and complex care that females provide to their offspring is impressive.

In late autumn, a female European earwig, Forficula auricularia, lays twenty to forty eggs in a burrow. She then remains in that nest and tends her clutch during winter. And that pays off: without her presence, almost all eggs would be lost, Sophie Van Meyel and colleagues show. A mother cleans the eggs with her mandibles to prevent growth of fungi and pathogens. She protects her clutch against predators. She ensures that the eggs will not desiccate. And she relocates them if necessary.

This nice maternal behaviour is not directed exclusively to a mother’s own clutch.

Weight loss

When the clutch of an earwig female is replaced by that of another female, she provides the same care with the same dedication, as Van Meyel witnessed when she conducted cross-fostering tests in the lab with five-day-old clutches. The eggs have their mother’s odour, so a female should be able to recognize foreign eggs. But she does not reject them. When tending eggs, a female faces a tough task, because she will not leave to forage until the young have hatched, which takes a few months. So, she will lose weight.

But strangely enough, the weight loss during winter is even greater for a female that has no eggs to tend. Apparently, food is scarce outside. A tending mother probably cannibalizes  some of her eggs to survive. This may explain that she is willing to care for a foreign clutch as well as for her own clutch, as the possession of eggs is a guarantee that she does not have to starve. She will be forgiven for consuming a small part of the clutch, since without her care almost no egg would make it through the winter months.

Orphans

The young hatch in early spring. Earwigs do not go through a complete metamorphosis with larval and pupal stages, but the juveniles resemble adult animals. They are nymphs.

After hatching, the nymphs usually stay in their burrow for a week. The mother protects them, regurgitates food for them and accompanies them when foraging at night. The nymphs can do without that care; they are mobile shortly after hatching and can search for food independently. But they do better if their mother attends them during the first week.

However, not every mother survives until spring, so some nymphs are orphans from the start of their life. Many such nymphs leave their natal nest during the first night. If they survive, they often join another family. Then again something remarkable happens: the mother of that family typically accepts them, and most orphaned nymphs end up well, as Janine Wong and Mathias Kölliker have shown.

Most motherless nymphs appear to choose an adoptive family with smaller juveniles. They are safe there, because when food is scarce, nymphs may cannibalize each other, preferring nonsibling smaller nymphs. By accepting foreign nymphs, an earwig family therefore runs a certain risk. Apparently,  group augmentation confers an advantage to the adoptive family that outweighs that risk, but it is not clear yet what that advantage might be.

Willy van Strien

Photo: European earwig female with eggs. ©Joël Meunier

Watch an earwig mother tending her eggs on You Tube

Sources:
Van Meyel, S., S. Devers & J. Meunier, 2019. Love them all: mothers provide care to foreign eggs in the European earwig Forficula auricularia. Behavioral Ecology, online 9 February. Doi: 10.1093/beheco/arz012
Wong, J.W.Y. & M. Kölliker M., 2013. The more the merrier? Condition-dependent brood mixing in earwigs. Animal Behaviour 86: 845-850. Doi: 10.1016/j.anbehav.2013.07.027

Young rebels

Ant larvae help to resist hostile take-over

The ant Formica fusca can resist parasites

When the nest of the ant Formica fusca is taken over by a parasitic queen of another species, the colony is lost. But the larvae help to limit the damage, according to Unni Pulliainen and colleagues.

An ants’ nest contains a large workforce serving the queen, which has the exclusive task to reproduce. Worker ants feed the queen and take care of her offspring, keep the nest clean and defend it. Their diligence attracts the attention of queens of other ant species that have not yet workers and for that reason could use some help. The black ant Formica fusca often suffers from such queens, which may invade a nest to exploit the workforce – thereby destroying the colony. But the host may resist, Unni Pulliainen and colleagues report.

Parasite

If a hostile queen tries to enter a nest of Formica fusca, which lives in clear-cut forest areas and along forest edges in Europe and parts of southern Asia and Africa, the workers may detect her and kill her. But that doesn’t always happen; sometimes, they accept her.

Once she’s inside, she can go on. She kills the resident queen or queens – in Formica fusca, a few queens usually live together in one colony – and she will start laying eggs. The workers have to raise her offspring as if they were the offspring of their own queen. The foreign queen, which outlives the workers, gradually acquires her own workers, while the original workers die. By temporarily parasitizing the Formica fusca colony, she founds her own.

Sabotage

But the enslaved workers can limit the damage by sabotaging. The workers can remove the foreign eggs. And the orphan ant larvae seem to help.

Ant larvae sometimes eat ant eggs, and Pulliainen wanted to know if Formica fusca larvae might be keen to consume the eggs of a foreign queen. In experiments, she offered larvae one egg each, either of their own queen or of a foreign queen, which belonged either to a parasitic species or to an innocent species that never invades other ants’ nests.

The larvae never consumed an egg of their own queen. But when they were given an egg from a parasitic queen, they consumed it in one in ten cases; eggs of an foreign innocent queen were consumed less often.

Future

The feeding behaviour of the larvae, albeit not very spectacular, may help to limit the damage. The eggs are nutritious and their consumption may increase the orphan larvae’s chance of survival. Male larvae can leave to reproduce as adults. And some of the female larvae will be future queens, which may found a new colony elsewhere. Female larvae destined to become workers can be successful too. They are not able to mate, but they can produce some sons, as sons develop from unfertilized eggs. The colony may be lost, but some larvae still have a future.

Willy van Strien

Photo: Formica fusca. Mathias Krumbholz (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Sources:
Pulliainen, U., H. Helanterä, L. Sundström & E. Schultner, 2019. The possible role of ant larvae in the defence against social parasites. Proceedings of the Royal Society B 286: 20182867. Doi: 10.1098/rspb.2018.2867
Chernenko, A., M. Vidal-Garcia, H. Helanterä & L. Sundström, 2013. Colony take-over and brood survival in temporary social parasite of the ant genus Formica. Behavioral Ecology and Sociobiology 67: 727-735. Doi: 10.1007/s00265-013-1496-7
Chernenko, A., H. Helanterä & L. Sundström, 2011. Egg Recognition and Social Parasitism in Formica Ants. Ethology 117: 1081-1092. Doi: 10.1111/j.1439-0310.2011.01972.x

Humboldt squid doesn’t discriminate

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

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
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Frightening days

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