From so simple a beginning

Evolution and Biodiversity

Page 2 of 19

Promotion for buff-tailed bumblebee worker

If the queen is lost, a worker can take over

When the queen is lost, a buff-tailed bumblebee worker can take over

Normally, buff-tailed bumblebee workers do not mate. But if the queen disappeared, they may mate, Mingsheng Zhuang and colleagues show, enabling the colony to survive.

A bee queen mates and lays eggs; fertilized eggs develop into females, unfertilized eggs into males. Her workers, also females, refrain from reproduction; they defend the nest, care for the brood and forage for food. Thanks to this strict division of labour, a colony runs well. If workers also would produce eggs, too little work would be done. Because the offspring of the queen are related to each other, workers have indirect reproductive success. They do not have a spermatheca, the vesicle in which females store sperm after mating, and are unable to mate. Once a worker, always a worker.

At least, this is how it is in honeybees.

But it does not apply to all bee species that live in colonies with a division of labour between queen and workers, so-called ‘eusocial’ species. In bumblebees (which belong to the bees), workers do have a spermatheca.

It was a mystery why. Now, Mingsheng Zhuang and colleagues argue that bumblebee workers sometimes are promoted to queen.

Artificial insemination

Zhuang shows that workers of several bumblebee species have a spermatheca that is functional. When he artificially inseminated workers, they responded in the same way as queens. They laid fertilized eggs from which daughters emerged and founded a colony. He thinks that workers of all bumblebee species still have a functional spermatheca, even though bumblebees have existed as a eusocial group for tens of millions of years.

The logical next question is whether bumblebee workers can actually mate and function as queens. And under what circumstances they will do.

The researchers conducted much of their research on the buff-tailed bumblebee, Bombus terrestris. This species, which occurs in Europe, North Africa, and parts of Asia, has colonies that exist for one year. In the spring, each queen that has mated and hibernated starts a colony on her own. She makes a nest in the ground, lays eggs and takes care of the larvae that hatch. These larvae develop into workers. Once they are present, the queen is dedicated to laying eggs. The colony grows to a size of hundreds of workers.

At the end of the season, the queen lays eggs from which males develop, and young queens appear. Workers also will lay eggs then, which are unfertilized and produce males. Young queens leave, mate and search a place to hibernate. Males and workers die.

Replacement

Buff-tailed bumblebee workers normally do not mate. But they can, as experiments of Zhuang show, if they have been separated from the queen and egg-laying workers for a while. In this regard, they differ from young queens, which do not need such a period of isolation. And if a worker has been in the company of nest mates for more than 24 hours before isolation, a switch is not possible anymore. So, opportunities for promotion are limited. Moreover, the chance of workers surviving a mating appears to be small.

But it may be enough to be able to provide a replacement and rescue the colony if a queen dies prematurely, Zhuang and colleagues think; that chance is probably quite high. In that case, workers will lay eggs that develop into early males and if one of the workers takes over the role of queen, mating and producing daughters, the colony can finish the season. According to them, this explains why workers have retained a functional spermatheca. It is difficult to determine whether such replacement often occurs in the wild, they write. It would require locating and digging out colonies and conducting DNA research.

Smaller

Why doesn’t a worker leave the natal colony and start her own? She would have to leave soon after eclosion, meet a male and survive the mating. But workers are much smaller than queens and produce fewer eggs. Being part of a large colony as a worker will yield greater reproductive success than heading a small colony as a queen.

Willy van Strien

Photo: Buff-tailed bumble bee queen on small-leaved lime. Ivar Leidus (Wikimedia Commons, Creative commons CC BY-SA 4.0)

Source:
Zhuang. M., T.J. Colgan, Y. Guo, Z. Zhang, F. Liu, Z. Xia, X. Dai, Z. Zhan, Y. Li, L. Wang, J. Xu, Y. Guo, Y. Qu, J. Yao, H. Yang, F. Yang, X. Li, J. Guo, M.J.F. Brown & J. Li, 2023. Unexpected worker mating and colony founding in a superorganism. Nature Communications 14: 5499. Doi: 10.1038/s41467-023-41198-6

Only when the weather is cool

Lancet liver fluke turns ant into zombie, but not during the day

lancet liver fluke manipulates ant into clamping onto a blade of grass

Larvae of the lancet liver fluke, a parasite, have to transfer from ant to deer. They manipulate the behaviour of infected ants to maximize the chance of transmission, Simone Nordstrand Gasque and Brian Fredensborg report.

An ant carrying lancet liver fluke larvae is no longer itself. At the parasite’s command, it climbs up in the grass and stays there motionless. This makes it more probable for the parasite to reach the host in which it matures, a grazer. The manipulation is complex, as Simone Nordstrand Gasque and Brian Fredensborg show: an infected ant only remains high up in the vegetation when it is chilly; when it is warm, it comes back and behaves normally.

The lancet liver fluke (Dicrocoelium dendriticum, a flatworm) has a complicated life cycle with three larval stages in three different hosts; it cannot live outside a host. It develops successively in a land snail, an ant, and a grazing mammal, such as a deer, sheep, or a cow. So, it has to transfer several times.

Bile ducts

adult lancet liver fluke lives in bile ducts of grazers

Adult liver flukes live in bile ducts in the livers of grazers. They mate and produce eggs that are excreted with the feces. The eggs are picked up by a land snail that nibbles on the droppings. The eggs hatch in the snail’s body into so-called miracidium larvae. They multiply asexually and thousands of larvae of the next stage, the cercaria larvae, appear. They migrate to the snail’s lung where they are packed in slime balls.

The snail coughs up the slime balls, and then it is the turn of the next host, which also comes by itself: the slime balls are tasty snacks for ants, which take them with them to their nest. Adult ants and larvae consume the balls and become infected. In ants, the cercaria larvae develop into the next stage, the metacercaria larvae.

Sacrifice

Now comes the most difficult transmission, which is necessary to complete the cycle: from ant back to grazer. That doesn’t happen easily. Ants reside in their nest or walk around on the ground. A grazer does not take a bite of that. The cycle could stop here, but now the parasite intervenes.

The larvae – there may be hundreds of them –safely encapsulate in the ant’s abdomen. But one of them moves to a ganglion in the ant’s head. It is unclear exactly how it manages, but this larva gains control over the ant’s behaviour. Like a zombie, the ant climbs up a blade of grass for no reason and locks its jaws to the vegetation. And so, a grazer may ingest the ant with the larvae on board along with the grass.

The larva that enabled the transfer dies in the grazer’s stomach. It sacrificed itself for the others, which emerge from their capsule in a safe place, develop into adult worms and settle in the bile ducts of the grazer: the circle is complete.

It is extraordinary that a parasite changes the behaviour of its host so drastically. But the lancet liver fluke does even more: it makes sure that the change is only expressed when it makes sense.

Sophisticated

Gasque and Fredensborg conducted research into the behaviour of the European red wood ant (Formica polyctena) after infection with lancet liver fluke in woods in Denmark, where roe deer live. They show that an infected ant only stays high up in the vegetation when it is cool, i.e., early in the morning and in the evening. During the day, it unlocks its jaws, goes down and behaves like the other ants.

It turns out that the temperature determines whether an infected ant is itself or becomes a zombie. Time of day, humidity and amount of sunlight do not matter. The warmer it is, the fewer infected ants persist in their biting behaviour. Only on chilly days at the end of the season, do many infected ants stay attached to vegetation all day.

This is beneficial from the parasite’s point of view. Because on hot days an exposed ant could overheat and die, and then the parasitic larvae would not survive either. Since deer mainly graze at dusk, there is no point in taking that risk. It is better to release the ant and let it ant behave normally, and only send it back up again in the evening.

Willy van Strien

Photos:
Large: infected European red wood ant, Formica polyctena. ©Simone Nordstrand Gasque
Smal: lancet liver fluke (Dicrocoelium dendriticum), adult. D. Drew (Wikimedia Commons, Public Domain)

Source:
Gasque, S.N. & B.L. Fredensborg, 2023. Expression of trematode-induced zombie-ant behavior is strongly associated with temperature. Behavioral Ecology, online 24 August. Doi: 10.1093/beheco/arad064

Mutualism, no deception

Smelly Gastrodia orchid provides food for fly larvae

Gastrodia foetida rewards its fly visitors for pollination

For its pollination, Gastrodia foetida, attracts female flies that normally visit mushrooms to lay their eggs on. The orchid seems deceptive, but it is not, Kenji Suetsugu discovered.

Many orchids are cheaters. Whereas most plants cooperate with insects and offer them nectar as a reward for pollination, such orchids have their flowers pollinated without offering a reward in return. They lure their pollinators with false pretences. For example, some orchids mimic female insects to abuse males who want to mate and, in their futile attempts, pick up pollen from one flower and deposit it on another.

Another type of deception is perpetrated by orchids of the genus Gastrodia. They attract fly females who want to lay eggs by mimicking the smell of material in which fly larvae grow up, such as fermenting fruits or decaying mushrooms. But the promise is false, as turns out when females visit these flowers. If they lay eggs on them, which they do occasionally, the larvae that hatch die of starvation.

Gastrodia foetida is an exception, Kenji Suetsugu discovered.

Entrapped

Gastrodia foetida is a rare plant from the forests of Japan and Taiwan. As do other Gastrodia species, the plants have no normal leaves, and the succulent flower does not look like much to us: it is inconspicuous and brown. But to females of some fly species the flower is attractive because of its musty smell; foetida means stinky. A common visitor is Drosophila bizonata, a species with larvae that develop in decaying mushrooms.

Drosophila bizonata carrying pollen

When a female fly enters the flower, the hollow lip in the flower bends up to the column that carries pistil and stamens. The female is stuck in the resulting channel between column and lip. To escape from that trap chamber, she has to crawl through a narrow opening along the stamens, and then the pollen, which is packed in two clumps, gets attached to her back (unless there had been another fly before, because then the clumps are already gone). If she then visits another flower in which she is locked up again, the pollen clumps end up on the pistil and this flower will produce many seeds. In other Gastrodia species, things go in the same way.

Decomposing flowers

But unlike those other species, the stinky orchid really is a suitable place to lay eggs on. Suetsugu frequently found eggs on flowers of Gastrodia foetida that had been visited by a female fly. And surprisingly, the eggs hatch and the larvae do not die, but grow well. Three or four days after pollination, the flowers fall off, leaving only the ovary behind. As the flowers decompose on the soil, the larvae feed on the floral tissue until they mature and pupate. Two weeks after pollination they emerge as adult flies.

Although the larvae of Drosophila bizonata are mushroom eaters, these flowers apparently meet their needs.

Mutual service

It is not clear why mushroom eating fly larvae can also grow well on these flowers. It may have to do with the fact that the orchid cannot make its own sugars through photosynthesis, like normal plants, because it does not have the green leaves necessary for this process. Instead, it steals sugars from fungi. Suetsugu suggests that, as a result, the plant tissue may have chemically similarities to that of mushrooms.

In any case, Gastrodia foetida appears to have gone from deception back to mutualism with pollinators, but with a reward other than nectar. Flies pollinate the flowers, and the succulent decomposing flowers then serve as food for their larvae. It is the first time that this form of ‘nursery pollination’ has been demonstrated.

The mutualism is indispensable for the plant, but not for the fly; it still can lay its eggs on mushrooms also.

Willy van Strien

Photos: ©Kenji Suetsugu
First: Gastrodia foetida
Second: Drosophila bizonata carrying pollen on its back in de flower; the trap chamber (the column above, the lip below) is open

See also:
Gastrodia pubilabiata smells like a brood site for fly larvae, but it is not

Source:
Suetsugu, K., 2023. A novel nursery pollination system between a mycoheterotrophic orchid and mushroom-feeding flies. Ecology, online 23 August. Doi: 10.1002/ecy.4152

Shadowing behaviour

Hunting trumpetfish swims closely alongside other fish to remain undetected

By swimming aligned with other fish, the trumpetfish can approach its prey more closely.

By swimming next to another fish, the trumpetfish can approach its prey closer without them noticing. It works, Samuel Matchette and colleagues observed.

In order to ‘surprise’ its prey when attacking, a trumpetfish keeps itself invisible. The elongate, extremely slender fish, typically just over half a meter long, often waits in vertical position between corals and sponges for prey to come close enough to strike; its prey are smaller fish and shrimp. As the predator fish takes on the background colour while waiting, it does not stand out.

But that strategy is useless in water with little cover. In such scenery, the predator applies another camouflage trick: it shadows a harmless fish by swimming very closely alongside it. And that works well, as Samuel Matchette and colleagues show: prey do not see their enemy approaching.

Clear response

The trumpetfish, Aulostomus maculatus, is related to seahorses and pipefish; it lives in the western part of the Atlantic Ocean. It was already known that it often swims aligned with other fish. It arches for instance over the back of a large fish, taking on that fish’s colour. The idea already existed that in doing so, it conceals its characteristic outline to approach its prey undetected. Fish that it hides behind are usually harmless herbivores, such as parrotfish, from which prey animals do not flee. 

But the question still was: does it really work? Does it enable the predator to approach its prey more closely undetected?

On coral riffs near Curaçao, the researchers conducted tests that answered these questions in the affirmative.

They studied the defensive behaviour of bicolour damselfish (Stegastes partitus). These fish live in colonies, are on the menu of trumpetfish and react clearly when seeing a predatory fish: a few individuals inspect the predator and then all quickly seek shelter.

Models

Matchette and colleagues exposed damselfish colonies to a three-dimensional printed and painted model of a trumpetfish alone, a parrotfish alone, or a parrotfish with a trumpetfish attached. They moved those models over a colony, one at a time, and videotaped the colony’s reaction.

As might be expected, damselfish were not much startled by a parrotfish passing on its own. In contrast, they responded strongly to a trumpetfish; they inspected it intensively and then quickly retreated.

And the combination of parrotfish and trumpetfish? This did not elicit a stronger reaction than a parrotfish alone. Conclusion: bicoloured damselfish do not notice a trumpetfish that shadows a parrotfish. The camouflage trick – hiding behind moving fish – works well.

That is: as long as the parrotfish tolerates the company. It often chases the trumpetfish away.

Willy van Strien

Photo: Trumpetfish. Becky A. Dayhuff (Public Domain)

Shadowing behaviour on YouTube

Sources:
Matchette, S.R., C. Drerup, I.K. Davidson, S.D. Simpson, A.N. Radford & J.E. Herbert-Read, 2023. Predatory trumpetfish conceal themselves from their prey by swimming alongside other fish. Current Biology 33: R781-R802. Doi: 10.1016/j.cub.2023.05.075
Aronson, R., 1983. Foraging behavior of the west Atlantic trumpetfish, Aulostomus maculatus: use of large, herbivorous reef fishes as camouflage. Bulletin of Marine Science 33: 166-171.

Egg signature

African cuckoo stands little chance with fork-tailed drongo

African cuckoo is not successful with fork-tailed drongo

African cuckoo females lay their eggs in nests of fork-tailed drongos. They mimic drongo eggs very accurately – and yet drongos recognize more than 90 percent of cuckoo eggs, Jess Lund and colleagues show.

South of the Sahara lives the African cuckoo, Cuculus gularis, which, like the common European cuckoo, lays eggs in the nests of other bird species (one egg per nest) with the intention that foster parents will raise their chicks. The brood parasite targets only a few bird species, of which the fork-tailed drongo, Dicrurus adsimilis, is one of the most important.

But the cuckoo has hardly any success with this important host species, Jess Lund and colleagues show. The intended foster mother usually notices the deception because she has put a ‘signature’ on her own eggs for verification.

It is the outcome of the long evolutionary history that is shared by African cuckoo and fork-tailed drongo. There is a major conflict between both bird species, because the brood parasite fully depends on the services of the foster parent, and the burden on the foster parent is enormous.

Arms race

It starts with the fact that an African cuckoo female destroys a drongo egg after arriving to lay an egg in the nest of a fork-tailed drongo couple. The cuckoo chick finishes the job. It hatches first and pushes the drongo eggs out of the nest; if a chick happens to have hatched already, it is also thrown out. The foster parents lose their entire clutch. And they are busy for weeks with the demanding care of the foster chick.

This conflict with major interests created an arms race. The drongo learned to recognize cuckoo’s eggs and to reject them. In response, the cuckoo developed eggs that increasingly resembled drongo eggs. Currently, the mimicry is almost perfect: in the eyes of drongos, cuckoo eggs look exactly like drongo eggs.

Individual signature

Drongo eggs are hugely variable. The background colour ranges from white to reddish brown, and the eggs can be immaculate, speckled, or blotched. Between eggs of the African cuckoo, the same variation exists. The mimicry is excellent on population level, and the African cuckoo seems to be ahead in the arms race.

But in reality, the fork-tailed drongo is the winner.

That is because a drongo female consistently produces eggs with the same look. Each female has her own characteristic colour and pattern. She puts, as it were, a distinctive signature on each egg for verification: I laid this one. A cuckoo female lays eggs that fall within the drongo variation, but she lays them randomly. Chances are small that she lays an egg in the nest of a drongo female that produces exactly the same egg type. The cuckoo egg usually is aberrant.

Protected

Conducting experiments and using models, the researchers predict how likely it is that a fork-tailed drongo will recognize and reject an egg of the African cuckoo in her nest. And that is more than 90 percent! Without individual egg signatures, that chance would be much smaller. So, the strategy of drongos – great variation between clutches, great uniformity within clutches – is an excellent response to the almost perfect mimicry of cuckoos, protecting the drongo effectively against the brood parasite.

And so, the African cuckoo has little success with this host. A cuckoo’s egg seldomly is accepted. If you consider that about one in five drongo nests is lost during breeding, the brood parasite has an extremely low reproductive success. But apparently, that low success is enough for the species to survive.

Willy van Strien

Photo: African cuckoo. Alastair Rae (Wikimedia Commons, Creative Commons CC BY-SA 2.0)

Sources:
Lund, J., T. Dixit, M.C. Attwood, S. Hamama, C. Moya, M. Stevens, G.A. Jamie & C.N. Spottiswoode, 2023. When perfection isn’t enough: host egg signatures are an effective defence against high-fidelity African cuckoo mimicry. Proceedings of the Royal Society B, online 26 July. Doi: 10.1098/rspb.2023.1125
Stoddard, M.C., R.M. Kilner & C. Town, 2014. Pattern recognition algorithm reveals how birds evolve individual egg pattern signatures. Nature Communications 5: 4117. Doi: 10.1038/ncomms5117

Venom with a history

Asp caterpillar defends itself with bacterial protein

Asp caterpillar Megalopyge opercularisdelivers a painful sting

Flannel moth caterpillars have a venom that is unique among moths and that causes excruciating pain, deterring predators. Andrew Walker and colleagues unlocked the surprising origin of this venom.

Caterpillars of flannel moths have a cuddly appearance: they have a ‘fur’ of long, often curly hairs. But it is not a good idea to touch them, because spines are hidden under the hairs that inject a venom when touched. The result is excruciating pain that can last for hours or days. Flannel moths form the family Megalopygidae, which has about 250 species that live in North, Central and South America. Their caterpillars are known as asp caterpillars or puss caterpillars.

Certain proteins in the toxic blend of the caterpillars are responsible for the pain. These proteins have a special evolutionary history, Andrew Walker and colleagues discovered.

Holes

The researchers were curious about the composition and mode of action of asp caterpillar venom. They took a closer look at two species: the southern flannel moth Megalopyge opercularis and the black-waved flannel moth Megalopyge crispata. First, they were surprised to find that the toxic proteins, which they call megalysins, closely resemble toxic proteins from disease-causing bacteria, such as Clostridium. The bacterial proteins are harmful because they puncture victims’ cells. And in experiments, the toxic proteins of asp caterpillars turned out to do exactly the same: they punch holes in animal nerve cells. The nerve cells then fire signals that cause the sensation of pain.

There are more species of butterflies and moths with venomous caterpillars, but they have very different types of venom. The venom of the Megalopygid family is unique among the Lepidoptera. Isn’t it strange that caterpillars of this family make the same type of toxic proteins as bacteria? Is that a coincidence?

Defence

No, it’s not. An ancestor of butterflies and moths once obtained genes that code for pore-forming proteins from bacteria, and the butterflies and moths conserved these genes (horizontal gene transfer between species occurs seldomly in evolution). Apparently, the proteins are useful for them, but what function they have is not yet known. In any case, they are not used as venom.

That is, except for members of the Megalopygid family. They restored the function of these proteins as venom, with which caterpillars defend themselves against their predators.

Asp caterpillar is mimicked by bird

And that works great. Once an animal has tried to handle an asp caterpillar and got stinged, it will leave similar critters alone henceforth. Young of the cinereous mourner (Laniocera hypopyrra, a South American passerine bird) take advantage of this. They convincingly mimic the appearance and behaviour of an asp caterpillar, and without being venomous themselves, they still deter predators.

Flannel moths aren’t the only animals that use this type of pore-forming bacteria-derived proteins as venom. Some centipedes, cnidarians and fish do as well.

Willy van Strien

Photo: asp caterpillar of southern flannel moth Megalopyge opercularis. Judy Gallagher (Wikimedia Commons, Creative Commons CC BY 2.0)

Researchers explain their work on YouTube

Sources:
Walker, A.A., S.D. Robinson, D.J. Merritt, F.C. Cardoso, M.H. Goudarzi, R.S. Mercedes, D.A. Eagles, P. Cooper, C.N. Zdenek, B.G. Fry, D.W. Hall, I. Vetter & G.F. King, 2023. Horizontal gene transfer underlies the painful stings of asp caterpillars (Lepidoptera: Megalopygidae). PNAS 120: e230587110. Doi: 10.1073/pnas.2305871120
Londoño, G.A., D.A. García & M.A. Sánchez Martínez, 2015. Morphological and behavioral evidence of Batesian mimicry in nestlings of a lowland Amazonian bird. The American Naturalist 185: 135-141. Doi: 10.1086/679106

Two-spotted spider mite male in a hurry

He strips off her old skin to be the first to mate

A female two-spotted spider mite often is undressed by a male

When a two-spotted spider mite female is about to moult into an adult, a male is often already waiting to undress her and mate, Peter Schausbergen and colleagues write.

Males of the two-spotted or red spider mite, Tetranychus urticae, have to exert every effort to produce offspring, because only the one who is the first to copulate with a female can fertilize her eggs. So, it is important to be present as soon as a female matures. Often, a male is already around before that time, according to observations by Peter Schausbergen and colleagues.

Mites are arachnids. They start life as an egg, become a larva and then go through two nymphal stages. They moult between the stages and emerge from the old skin a bit bigger; after the last moult they are sexually mature. Females develop from fertilized eggs, males from unfertilized eggs.

Silvery appearance

A female two-spotted spider mite is often joined in the last nymphal stage by a male that claims her by sitting on top of her. He spends time and energy on guarding her, and these would be wasted if a rival appears after the last moult and succeeds in mating first. That danger is real, because a newly emerged adult female secretes pheromones that attract males. The guarding male must prevent this.

To shorten the precious waiting time and secure the first mating, a guarding male acts decisively when her final moult is coming. A day before moulting, the nymph enters a resting phase, and in the last few hours she takes on a silvery colour due to air getting between the old skin, which she will shed, and the new skin.

She initiates the moult by bulging, causing the old skin to crack along a crossline. If she is alone, she first pulls off the anterior part of the old skin and then the posterior part, exposing her genital opening. But if a male is guarding, things go different. He drums her back with his forelegs, and in response she bulges earlier. When the old skin has cracked, he quickly strips off the posterior part with his pedipalps (the ‘boxing gloves’ that also spiders also possess). And then, with a bit of luck, he will indeed be the first to mate.

Fighters and sneakers

In our view, this undressing behaviour of the male two-spotted spider mite is very indecent. But he has no choice. Prudent behaviour is punished by natural selection: if he waits patiently for her to undress herself, it is more likely that another male takes over and sires the offspring.

There are two types of guards. Some are fighters, that are often disturbed by other males when they sit on a female and dismount to fight. Others are sneakers, that are not attacked by rivals and are never disturbed. Maybe other males mistake them for females because they do not respond, or maybe they smell like females. It would be interesting to find out whether fighters and sneakers display the same pushing behaviour when the nymph they guard is about to moult.

Pest

The two-spotted spider mite is less than half a millimetre long. It feeds by piercing plant cells and sucking their contents. It is a worldwide pest on many agricultural crops. A single mite does little harm, but the bugs multiply quickly and in a brief time, there are many of them.

Willy van Strien

Photo: Two-spotted spider mite female, Tetranychus urticae. Gilles San Martin (Wikimedia Commons, Creative Commons, CC BY-SA 2.0)

Sources:
Schausberger, P., T.H.H. Nguyen & M. Altintas, 2023. Spider mite males undress females to secure the first mating. iScience, 107112, 7 July. Doi: 10.1016/j.isci.2023.107112
Sato, Y., M.W. Sabelis, M. Egas & F. Faraji, 2013. Alternative phenotypes of male mating behaviour in the two-spotted spider mite. Experimental and Applied Acarology 61: 31-41. Doi: 10.1007/s10493-013-9673-y

Skilful camouflage artist

Cuttlefish has to search for the best pattern

Common cuttlefish is a master of camouflage

The cuttlefish has an excellent camouflage ability and rapidly modifies its appearance when the background changes. But its change is indirect, Theodosia Woo and colleagues show: the cuttlefish adjusts a new skin pattern a few times before it is good enough.

To defend itself against predators, the common cuttlefish, Sepia officinalis, like many other squids, can use camouflage to blend in with its surroundings. And if a predator still detects it, it sprays ink to block the view.

The common cuttlefish lives in the North Sea, the Baltic Sea, and the Mediterranean Sea. Depending on the substrate, such as sand, rocks, or sea grass, it can take on a uniform colour, have a mottled pattern, or have large dark and light skin areas that disrupt its contours. There are countless variations, and the cuttlefish produces an appropriate camouflage against almost any background, Theodosia Woo and colleagues write.

Pigment sacs

This is possible, among other things, thanks to two or three million pigment cells in the skin, the so-called chromatophores. They come in three colours: yellow, red, and brown. The cells are closed sacs with an elastic wall, surrounded by radial muscles. When the muscles contract on command of the brain, they pull the sac open, and the colour becomes visible.

Woo showed how cuttlefish change their appearance by doing experiments in which she provided animals with a changing background; she filmed the skin at high resolution and measured the skin patterns with robust computer software. The result is remarkable. The lightning-fast transition makes it seem as if a cuttlefish realises a new matching skin pattern in one go. But it is not like that.

Confronted with a new background, a cuttlefish immediately starts to adapt its skin pattern. But after a first change, it waits shortly and then adjusts the created pattern to improve it. Then it waits again and adjusts the pattern further, until a satisfying pattern is found. So, it goes through a search process in the blink of an eye and apparently receives feedback continuously. Search trajectories are not fixed, because when the researchers offered the same background change several times, the animals followed different search trajectories and the result was also different. The difference in final skin patterns was so subtle that we cannot observe it.

Reflection

In addition to the pigment cells that were studied here, the squid skin has two more types of neurally controlled cells that enable changes in appearance. There are cells that, thanks to their nanostructure, reflect light of one specific colour, for example blue: the iridophores. And there are cells that reflect all incident light and are white in daylight: the leucophores. In addition, the skin can be smooth or rough. The sophistication of a squid skin is beyond our imagination.

All these possibilities are not only used for camouflage, but also for communication. Common cuttlefish spend spring and summer inshore to spawn, and the colours the animals display then is an attraction for divers.

Colour blind

The greatest puzzle about squids is how they are capable to mimic their environment so perfectly while being colourblind themselves. Almost nothing is known about this, but there is evidence that small light sensitive organs occur in the skin.

Willy van Strien

Photo: Young common cuttlefish. Magnef1 (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Sources:
Woo, T., X. Liang, D.A. Evans, O. Fernandez, F. Kretschmer, S. Reiter & G. Laurent, 2023. The dynamics of pattern matching in camouflaging cuttlefish. Nature, online 28 June. Doi: 10.1038/s41586-023-06259-2
Gilmore, R., R. Crook & J.L. Krans, 2016. Cephalopod camouflage: cells and organs of the skin. Nature Education 9(2): 1
Chiao, C-C., C. Chubb & R.T. Hanlon, 2015. A review of visual perception mechanisms that regulate rapid adaptive camouflage in cuttlefish. Journal of Comparative Physiology A 201: 933-945. Doi: 10.1007/s00359-015-0988-5

Desert ant builds landmark

Nest hill helps ants to return home in barren salt flats

desert ant Cataglyphis fortis has outstanding navigation skills

Often nothing is visible around a nest of the desert ant Cataglyphis fortis that could help foraging workers to return to the nest. In that case, the ants make a landmark themselves, Marilia Freire and colleagues show.

A foraging trip is a survival trip for the desert ant Cataglyphis fortis, which lives in salt pans in Tunisia; salt pans are vast bare plains where there was once water, but now only a salt crust remains. Ant workers venture out individually to search that barren plain for insects and other small critters that have succumbed to the relentless desert heat. After founding something, they must return to the nest with the loot between their jaws as quickly as possible, otherwise they will succumb themselves.

But the entrance to the underground nest is barely visible. That is why the ants build a landmark, when necessary, Marilia Freire and colleagues discovered.

Navigation

Food is scarce, so foraging desert ants often must move far from the nest to find something. They venture up to 350 meters away. Because they have excellent navigation skills, they usually return safely.

When going out foraging, a worker constantly uses an internal sun compass to keep track of the direction in which she is walking and with a kind of pedometer she measures the distance she covers in that direction. When she finds food, she has usually followed a tortuous path, but thanks to this so-called path integration she can walk back to the nest in a straight line, i.e., via the shortest possible route. At least: she closely approaches the nest.

When within a few meters, she needs visible clues to find the exact place of the nest entrance, because path integration doesn’t work perfectly. The farther an ant has gotten from the nest, the more uncertainty creeps into the route back and thus the greater the chance is that she has to search for too long and succumbs. For the very last bit of the trip homewards, she relies on nest smell.

But in the middle of a salt pan, there is nothing to be seen at all. What to do in this case?

Landmark

desert ant builds nest hill when no other visual landmarks are aroundFreire and the other researchers had noticed that the desert ants often build a hill at their nest, and that nest mounds in the middle of a salt pan are higher than on the edge, where some shrubs grow. A nest mound in the middle of a salt pan is 12 centimeters high on average (the highest they found was 30 centimeters), a nest mound at the edge only 5. So, they wondered if the mounds might serve as visual landmarks for workers returning from a foraging trip.

To find the answer, they captured ants at the nest and placed them at a distance of a few meters. Since the ants had not walked themselves, they could not use path integration. But they were placed at distances where they normally must be guided by landmarks to find the nest entrance anyway. The researchers had removed the mounds at some of the nests to see if that made a difference.

That turned out to be the case, especially for nests in the middle of a salt pan. Without a mound, ants were not able to walk directly to such nest and more often failed to find it at all. The hills therefore serve as landmarks. Next question: do ants build them specifically for that purpose? It is possible that the mounds have another main function, such as regulating the nest temperature.

Only when needed

But the researchers show that the desert ant does build its mounds as landmarks by conducting experiments in which they removed the mounds at sixteen nests in the middle of a salt pan. At eight of those nests, they placed artificial landmarks, namely two black cylinders. Three days later, the ants were found to have built a new mound at some of those sixteen nests, especially at nests without artificial landmarks, and at those nests, the mounds were taller.

Conclusion: desert ants build mounds near their nest as landmarks for foraging workers. But they only make the effort if there are no other landmarks visible, such as bushes or, in the trials, black cylinders.

Willy van Strien

Photos: ©Markus Knaden
Large: Cataglyphis fortis
Small: nest mound in the centre of a salt pan

Sources:
Freire, M., A. Bollig & M. Knaden, 2023. Absence of visual cues motivates desert ants to build their own landmarks. Current Biology 33: 1-4 (31 May online). Doi: 10.1016/j.cub.2023.05.019
Steck, K., B.S. Hansson & M. Knaden, 2009. Smells like home: desert ants, Cataglyphis fortis, use olfactory landmarks to pinpoint the nest. Frontiers in Zoology 6: 5. Doi: 10.1186/1742-9994-6-5
Wittlinger, M., R. Wehner & H. Wolf, 2007. The desert ant odometer: a stride integrator that accounts for stride length and walking speed. The Journal of Experimental Biology 210: 198-207. Doi: 10.1242/jeb.02657
Wehner, R., 2003. Desert ant navigation: how miniature brains solve complex tasks. Journal of Comparative Physiology A 189: 579-588. Doi: 10.1007/s00359-003-0431-1

Evolution of butterflies

Europe has the fewest butterfly species

Mating butterflies, pea blue

The very first butterflies on earth flew in what now is North or Central America. The caterpillars fed on leaves of bean plants, according to research by Akito Kawahara and countless others.

Butterflies can be found everywhere on earth, except Antarctica. Until now, it was poorly known where and when they originated and how they evolved.

Together with a huge team, Akito Kawahara figured this out. The researchers analyzed the DNA of almost 2300 butterfly species to draw up an evolutionary tree. They also gathered a lot of knowledge by studying museum collections and digging through field guides in all languages. This allowed them to unravel how butterflies spread over the earth and how they lived.

The diversity of butterflies is great; nowadays, there are 19,000 species worldwide. They have descended from moth ancestors. On the evolutionary tree, their branch originates about one hundred million years ago, when dinosaurs were still around. Flowering plants were already present; adult butterflies could find nectar on the flowers which, in return, they pollinated.

Caterpillars need a lot of food to grow, and the caterpillars of the first butterflies probably gnawed on leaves of bean plants.

Late in Europe

The great supercontinent of Pangea had broken into two pieces when the first butterflies appeared. Both pieces, Gondwana (Africa, Australia, Antarctica, and South America) and Laurasia (North America, Europe, and Asia), were falling apart and the parts drifted away. Originally, India was part of Gondwana, but came loose and drifted to Laurasia.

According to the research, butterflies have originated in what is now western North America or Central America. They crossed the sea to South America fairly quickly.

About seventy-five million years ago, butterflies also moved from North America to Asia, via the Bering Strait, and then spread to India and Australia, and later Africa. Overland the road to Europe was open, but butterflies took a long time to chose this direction. The reason is unclear, and the research gives no answer. Butterflies arrived in Europe ‘only’ thirty million years ago, and as a consequence, Europe has few species compared to other continents.

Caterpillar diet

The caterpillars of most species feed on plants and are quite choosy. The so-called host plants of these species usually belong to one plant family.

Some species have developed an alternative caterpillar diet. The caterpillars consume organic detritus or lichens, and some blues (Lycaenidae) even are carnivorous and eat other insects.

Willy van Strien

Photo: Pea blues (Lampides boeticus) mating. Atanu Bose Photography (Wikimedia Commons, Creative Commons CC BY-SA 4.0)

Source:
Kawahara, A.Y. et al., 2023. A global phylogeny of butterflies reveals their evolutionary history, ancestral hosts and biogeographic origins. Nature Ecology and Evolution, 15 May. Doi: 10.1038/s41559-023-02041-9

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