From so simple a beginning

Evolution and Biodiversity

Amputation saves ant lives

Workers of Florida carpenter ant amputate legs when useful

With a wound on a leg, a worker of the Florida carpenter ant, Camponotus floridanus, would have only a small chance to survive if it were not for her nest mates that come to the rescue. To prevent infection, they lick the wound clean and often amputate the leg, significantly increasing the survival chance, Erik Frank and colleagues show. This care is frequently needed because colonies of this ant species fight each other intensively.

The researchers tested the ants’ medical skills by making a small cut in the leg of workers. They then injected a saline solution containing a deadly bacterium, Pseudomonas aeruginosa, into the wound. Infected ants were placed either in isolation or in a nest where two hundred workers were available.

Most of the ants that sat alone after infection succumbed to the injuries. But when placed in a nest, most ants did survive thanks to the care of nest mates. What care they provided turned out to depend on where the wound was.

If the infected wound was on the upper leg, usually one of the nest mates intervened drastically and bit off the leg at the top. If the wound was on the lower leg, this did not happen; instead, workers licked the wound thoroughly clean. In both cases, helpers chose the treatment that was most effective, as became clear in experiments with infected wounds in which the researchers amputated the affected leg.

If they amputated a leg of an ant with a wound on the upper part, then her chance to survive was as high as it was in the case of amputation by nestmates. But if they removed a leg with an injury on the lower part, it did not help: most patients died. The treatment that the ants apply in that case, extensive cleaning, is much more effective.

Why is amputation only helpful for an infection in the upper leg?

Whether an ant will survive an infection depends on how quickly the bacteria are able to spread through the body: the higher the bacterial load, the higher the mortality. The bacteria spread via the hemolymph, the insect version of blood, which flows through the legs in channels.

In the upper leg, these channels are narrower than in the lower leg, so that bacteria are less likely to enter the hemolymph. Also, the upper leg has much more muscle mass than the lower leg, and blood is pumped around by muscle movements. If the upper leg is affected, circulation is slowed down much more than if the lower leg is affected, impeding the spread of bacteria.

Consequently, when the upper leg is affected, ants have enough time to perform an amputation, which takes forty minutes at least, before the bacteria have spread. But timely amputation is unfeasible with an infection in the lower leg. Then cleaning is the best way to help a victim.

The ants also amputated the leg if the researchers injured the upper leg but injected a sterile saline solution instead of a solution with bacteria. That makes sense, because under natural conditions, in the ants’ nest, such a wound is most likely to become infected. The workers err on the side of caution.

The Florida carpenter ant is the only animal species known to apply amputation to treat conspecifics in case of injury.

The researchers previously discovered that also workers of the Matabele ant from Africa, Megaponera analis, treat infected wounds of nestmates. They do so by administering antibiotics from glands on their backs that produce a mix of antimicrobial substances. The Florida carpenter ant does not have such a built-in pharmacy. For this ant, cleaning and amputation are good alternatives.

Willy van Strien

Illustration: ©Hanna Haring

See also: the Matabele ant fights infections with self-made antibiotics

Frank, E.T., D. Buffat, J. Liberti, L. Aibekova, E.P. Economo & L. Keller, 2024. Wound-dependent leg amputations to combat infections in an ant society. Current Biology, 2 July online. Doi: 10.1016/j.cub.2024.06.021

Successful in the deep sea

Deep-sea anglerfishes flourished thanks to sexual parasitism

The pitch-dark, oxygen-lacking, cold, almost empty deep sea is a difficult environment to live in. But the common ancestor of deep-sea anglerfishes moved into this environment and the fish became highly successful from an evolutionary perspective: there are about 170 species. Chase Brownstein and colleagues describe how this animal group arised and flourished.

Deep-sea anglerfishes (Ceratioidea) may be the strangest animals around. Females are clumsy animals that can barely swim. They lure their prey with a ‘fishing rod’ growing from their heads with a luminous end. Males are much smaller than females and do not eat anything at all. They swim around looking for a mate. Upon finding a female, a male attaches onto her abdomen with his teeth and when she lays eggs, he fertilizes them. In some species, this biting results in a fusion, in which the male turns into a sperm-supplying appendage to his partner, deriving nutrition from her through a shared circulatory system: sexual parasitism.

It was this bizarre and unique method of reproduction that enabled colonization of the deep sea.

The deep-sea anglerfishes are part of the order of the anglerfishes (Lophiiformes). Their closest relatives live on seafloors, where they lie still or ‘walk’ on their pelvic fins. About 50 million years ago, the ancestor of the deep-sea anglerfishes split from such bottom dwellers and moved to the open deep sea. This happened at a time when the Earth was warmer normal, and many species in oceans went extinct. Perhaps the seafloor became less suitable as a place to live. In any case, the deep sea was a new environment where deep-sea anglerfishes underwent a period of rapid specialization and speciation.

A major problem in the deep sea is reproduction. Because there is little life, fish live in low densities. The chance of encountering a mate is small, and the chance that two conspecifics will meet each other when both are ready to reproduce is extremely small. Here, the unique method of reproduction in deep-sea anglerfishes was helpful. The researchers think that they practiced sexual parasitism from the beginning. As a result, a male only once had to find a female and it did not matter when he met her. Because he attached and did not let go, the two were assured of sex: he was ready to deliver his sperm as soon as she could lay eggs.

This is still the case in many species, but other species arose in which the male attaches to a female only temporarily.

Sexual parasitism, with dwarf males attached as sperm sacs, does not otherwise occur in vertebrates. How did it arise in deep-sea angler fishes? The researchers point to two developments that were taking place. First, there was a trend for male anglerfish to be smaller than females. Second, anglerfishes reduced their immune system, especially the acquired part, which builds up protection against specific pathogens or parasites that it has been exposed to. How these fishes do defend themselves against diseases is still unknown.

The deep-sea anglerfishes took both trends to the extreme: males are no larger than necessary to swim to a mate and produce sperm. And the acquired immune system has largely been dismantled, so that males can parasitize on females without any problems.

So, it was a fortunate combination of circumstances and characteristics that drove the deep-sea anglerfishes to the challenging deep sea and made them successful.

Willy van Strien

Photo: Female Humpback anglerfish (Melanocetus johnsonii), which belongs to deep-sea anglerfish. Fernando Losada Rodríguez (Wikimedia Commons, Creative Commons CC BY-SA 4.0)

More about tiny deep-sea anglerfish males that parasitize on females

Brownstein, C.D., K.L. Zapfe, S. Lott, R. Harrington, A. Ghezelayagh, A. Dornburg & T.J. Near, 2024. Synergistic innovations enabled the radiation of anglerfishes in the deep open ocean. Current Biology 34: 2541-2550. Doi: 10.1016/j.cub.2024.04.066

Beetle mimics noxious moths

Tiger beetles imitate ultrasonic sounds of unpalatable tiger moths

Some tiger beetles fly at night. It means that they must be afraid of hunting bats, predators that search for prey by emitting ultrasonic (very high-pitched) clicks and deducing from the reflected sound where a tree or a building is – or where a tasty insect snack is flying. This so-called echolocation allows bats to ‘see’ in the dark. Many insects perceive the ultrasonic clicks of an attacking bat and respond by fleeing or diving to avoid the enemy.

There are a few tiger beetles that react differently: they produce an ultrasonic sound in response to an approaching bat. Harlan Gough and colleagues wanted to know why.

The only other insects known to respond with ultrasonic sound to a hunting bat are moths; an estimated 20 percent of moths responds, at a pitch that bats hear well. The sounds have several effects. Some moths disrupt the reflected bat sound by their calls, so that the bat no longer can interpret the noise. Other moths warn with their sound that they are distasteful or poisonous; once a bat has tasted such a species, it will leave it alone from then on. And other, non-poisonous moths benefit from this: they imitate the sound of a noxious species so that a bat let them also go.

And what about the tiger beetles that produce ultrasonic sound in response to a bat? What do they achieve by doing so?

The researchers tested nineteen tiger beetle species, beetles from the Cicindelidae family, from southern Arizona (USA). They exposed the beetles in the lab to the ultrasonic clicks of a bat that is about to attack. Seven of these nineteen species responded with their own ultrasonic sound, all being species that are active at night. The other twelve species stay put at night and therefore do not need to defend themselves against bats.

Do tiger beetles flying at night disrupt the echolocation of bats by jamming? No, the authors write, because that would require a more intensive sound (in technical terms: a higher duty cycle) than the beetles can produce.

Is their ultrasonic sound a warning that they are unpalatable? That is also not the case. The beetles do contain a repellent substance, benzaldehyde, which has an almond scent. But bats still like to eat them, as is evident from experiments with the big brown bat, Eptesicus fuscus. Apparently, the concentration of benzaldehyde is too low to deter this predator. The substance may help against small enemies such as ants and robber flies.

Unpalatable tiger moth warns bat predator with ultrasonic sound

Maybe they imitate the ultrasonic sound of noxious moths? To evaluate that hypothesis, the researchers compared the sounds of tiger beetles with existing sound records of sympatric tiger moths, moths of the subfamily Arctiinae, some of which are poisonous. And yes: the sound produced by tiger beetles is similar to that of poisonous tiger moths. The beetles seem to practice acoustic mimicry.

Moths that produce ultrasonic sounds do so in diverse ways. They have special structures, such as tiny combs. Tiger beetles produce sounds by brushing their beating hindwings along the back edges of the rigid forewings, the elytra. Normally, they hold the elytra up during flight, but to make sound, they lower them slightly.

Definitive evidence for acoustic mimicry by tiger beetles is still lacking. This would require behavioural tests with bats to find out whether they indeed ignore the beetles after having tasted an unpalatable tiger moth.

Willy van Strien

Large: Ellipsoptera marutha, Aridland Tiger Beetle, is one of the species that mimics tiger moths. Laura Gaudette (Wikimedia Commons, Creative Commons CC BY 4.0)
Small: unpalatable tiger moth Cisthene martini, Martin’s Lichen Moth. Ken-ichi Ueda (Wikimedia Commons, Creative Commons CC BY 4.0)

Gough, H.M., J.J. Rubin, A.Y. Kawahara & J.R. Barber, 2024. Tiger beetles produce anti-bat ultrasound and are probable Batesian moth mimics. Biology Letters 20: 20230610. Doi: 10.1098/rsbl.2023.0610
Barber, J.R., D. Plotkin , J.J. Rubin, N.T. Homziak, B.C. Leavell, P.R. Houlihan, K.A. Miner, J.W. Breinholt, B. Quirk-Royal, P. Sebastián Padrón, M. Nunez & A.Y. Kawahara, 2022. Anti-bat ultrasound production in moths is globally and phylogenetically widespread. PNAS 119: e2117485119. Doi: 10.1073/pnas.2117485119
Corcoran, A.J., W.E. Conner & J.R. Barber, 2010. Anti-bat tiger moth sounds: form and function. Current Zoology 56: 358-369. Doi: 10.1093/czoolo/56.3.358

Different flower colour, different visitor

Fritillary Fritillaria delavayi has brown leaves in many places

At high altitudes in the Hengduan Mountains in southwest China, plants grow on bare, stony soil. Green leaves are very noticeable here, and to escape from herbivores such as caterpillars, many plant species have developed an unusual brown or grey leaf colour that matches the background. An example is Corydalis hemidicentra.

The fritillary Fritillaria delavayi goes one step further than other plant species: in some places not only the leaves, but also the flowers are stone-coloured. Tao Huang and colleagues wondered whether pollinators could find such camouflaged flowers.

Fritillaria delavayi is a perennial bulb plant that may have regular green leaves and a bright yellow flower. It grows at an altitude of 3700 to 5600 meters. It is easy to see why this fritillary took on camouflage colours in many places. The small bulbs are used in traditional Chinese medicine, because they contain substances that are beneficial for lung diseases. There is a great demand for them. The plant is difficult to grow because it requires cold and dry air. And so, bulbs are dug out in accessible places.

In some places, Fritillaria delavayi even has brown flowers

In such places the plant developed a stone-coloured appearance. Brown or grey plants are not noticeable, especially if also the flower is brown or grey. But the flowers must be detectable by pollinators, that transfer pollen from one flower to the pistil of another. The flowers cannot fertilize themselves.

Field observations show that two species of bumblebees come to yellow flowers to collect nectar, pollinating the flowers in the process. But they cannot perceive brown or grey flowers, so they do not visit them. How can these flowers be pollinated?

By other insects, it turns out. The camouflaged flowers of Fritillaria delavayi attract three species of the Anthomyiidae fly family. The flies are looking for nectar and pollen and sometimes mate within the flowers. They do not perceive brown or grey flowers any better than bumblebees do, but they are attracted to the smell. Huang shows that the camouflaged flowers are smaller than the yellow ones, an adaptation to the small bodies of the flies. The flies are less efficient pollinators than bumblebees, but this is compensated for by more frequent flower visits.

As a result, stone-coloured flowers set seed just as well and produce as many seeds as yellow ones. This means that camouflage is not at the expense of reproduction. And camouflaged flowers do indeed protect the plant from human collectors, according to previous research with slides: people clearly have more difficulty finding brown or grey flowers than yellow ones.

Willy van Strien

Large: Fritillaria delavayi with brown leaves and yellow flower
Small: Fritillaria delavayi with brown leaves and brown flower
©Yang Niu

See also: Corydalis hemidicentra with cryptic leaf colour

Huang, T., B. Song, Z. Chen, H. Sun & Y. Niu, 2024. Pollinator shift ensures reproductive success in a camouflaged alpine plant. Annals of Botany, 9 May online. Doi: 10.1093/aob/mcae075
Niu, Y., M. Stevens & H. Sun, 2021. Commercial harvesting has driven the evolution of camouflage in an alpine plant. Current Biology 31: 446-449. Doi: 10.1016/j.cub.2020.10.078

Nest architectural traditions

Nest of Scaptotrigona depilis with combs in corkscrew form

The brood cell complex in nests of the stingless bee Scaptotrigona depilis, which lives in South America, can have two distinct forms. In most colonies, workers build combs (plates with brood cells) horizontally one above the other, each comb starting from a central pillar: the parallel form. But in some colonies, they construct a continuous spiral comb without a central pillar: the corkscrew form. So, the workers that build the combs follow one of two possible architectural styles.

This is not a matter of hereditary makeup, Viviana di Pietro and colleagues write, nor is it an adaptation to the location of the nest or to the environmental temperature. The workers simply continue to build in the style that has already been applied, continuing a tradition.

Like honeybees, stingless bees are highly social species with queens that reproduce and workers that do the other tasks. These tasks include construction and maintenance of the nest, which they make in cavities. Workers of Scaptotrigona depilis construct combs from a mixture of wax and plant resin. They build them from the bottom up, and as said in one of two ways. They put food in each cell and close it after the queen has added an egg. The egg develops into a larva and pupa, and finally a young bee emerges.

Combs are much more often built in the parallel form than in the corkscrew form: about 95 per cent of the colonies have the parallel form.

Scaptotrigona depilis: combs in parallel form

Sometimes a colony switches from one type to another. On average, the parallel form lasts for almost two years. The corkscrew shape is maintained for a month and a half; that is much shorter, but still longer than a cohort of workers is building, namely two to three weeks. Both architectural styles are passed on from generation to generation for some period.

The researchers wanted to determine whether this is because workers are guided by the structure that already exists. They therefore conducted experiments in which they took experienced workers from one colony and placed them in another colony, the brood cell complex of which had either the familiar or the alternative form. The result was clear: workers that were placed with the type they were not familiar with, immediately continued that construction plan, instead of adhering to the building plan they were used to. Apparently, they didn’t have to learn that different architectural style from their new nest mates.

In a second experiment, the researchers changed the parallel form of the combs to the corkscrew form in a number of colonies by making a cut in the top comb from the edge to the centre and placing one end on top of the other. In most cases, the workers continued to build following the corkscrew form.

The conclusion is that not much is needed to maintain a tradition. It is sufficient if the animals are guided by what exists, in this case: they apply the building plan of the existing structure. It requires no understanding, planning or communication. The technical term for this form of self-organization is stigmergy.

Probably, the parallel form of the brood cell complex is default. The researchers think that sometimes a corkscrew shape arises by error. Instead of breaking things down, the bees than continue to build according to that model.

Willy van Strien

Large: Rare corkscrew shape comb; open cells at the margin still have to be filled
Small: Parallel combs with central pillar, the dominant form
©Viviana di Pietro

Di Pietro, V., C. Menezes, M.G. de Britto Frediani, D.J. Pereira, M. Fajgenblat, H. Mendes Ferreira, T. Wenseleers & R. Caliari Oliveira, 2024. The inheritance of alternative nest architectural traditions in stingless bees. Current Biology, online 19 March. Doi: 10.1016/j.cub.2024.02.073

Perfume makers

Male orchid bees concoct a precious perfume

A beautiful colour with a metallic sheen appears to be insufficient for male orchid bees to impress females. They must also have an attractive scent. Orchid bees (Euglossini, about two hundred species) are found in tropical America. They do not live in colonies, like the honeybee. Males occupy a territory and try to attract females.

A wonderful smell is provided by a self-made perfume, which consists of a mixture of scents that males collected from flowers, rotting wood, sap from trees and – weird enough – faeces. Males differ in the intensity and complexity of their perfume, and only a few males have an outstanding blend. That’s no wonder, because it takes a lot of time and energy to find and collect the dozens of aromatic components, most of which are scarce. Young bees in particular work fanatically to concoct their perfume, as Jonas Henske and Thomas Eltz discovered during their research on Euglossa imperialis in Costa Rica. Males of this species can live for six weeks to three months.

Hundreds of species of orchids benefit from the male orchid bees’ urge to collect scents. They are pollinated exclusively by these bees, in exchange for a precious fragrance.

Males capture volatile odours by spitting lipids onto a sweet-smelling surface. The fragrances are absorbed by the lipids. The bees then use their legs to stuff the scented fat into special pouches on their hind legs. They regularly take out the content to enrich it with new odours, creating an extensive and complex mixture of fragrances.

A male that wants to mate makes hovering flights in his territory, moving his legs in a characteristic way. Underneath its body, the right and left middle legs alternately sweep the pouches of the opposite hind leg. Tufts of hair on the middle legs absorb some fat. The fluttering wings then spread the perfume that is released.

Experiments in the lab showed that females are interested. The perfume allows them to determine whether they are dealing with a male of their own species; each species of orchid bee has its characteristic scent bouquet. The species Euglossa dilemma and Euglossa viridissima, for example, look similar, but can be distinguished by smell. This indicates that the males carefully compose their scent mixture: all components in exactly the right proportions.

The researchers had assumed that the oldest male orchid bees would have the largest supply and the most complex perfume, because they had had the most time to create it. Such a precious perfume would be evidence that its owners had lived a long time, a signal of good hereditary quality and good condition. But that idea turns out to be wrong.

To test their assumption, Henske and Eltz attracted Euglossa imperialis males, captured them, estimated their age, and sealed the perfume pouch on the right hind leg with glue. They then released the animals and waited for them to reappear at the original capture site. Half of the males returned within five days, some stayed away for more than twelve days. The researchers could capture about a third for the second time, and then they compared the contents of both perfume pouches.

Between the first and second measurement, young males had improved the perfume in the pouch on the left hind leg, which had been left open: the supply had become larger, and the perfume had a more varied composition than that in the sealed right pouch. The perfume in the left pouch of old males had deteriorated in quantity and quality.

Perhaps young orchid bee males have a sharper sense of smell, or are more efficient at finding and storing odours, the researchers think. But it may also be a trade-off. Males collect scents and court females throughout their lives, expanding their perfume supply continuously on the one hand and using bits of it up on the other. The balance can change. For males that do not have much time left to live, it may not be worth acquiring more scent; they better use the perfume supply they have.

Unfortunately, it is not known whether females prefer a more complex scent.

Willy van Strien

Photo: Euglossa species male. Thomas Shahan (Wikimedia Commons, Creative Commons BB BY 2.0)

Henske, J. & T. Eltz, 2024. Age-dependent perfume development in male orchid bees, Euglossa imperialis. Journal of Experimental Biology 227: jeb246995. Doi: 10.1242/jeb.246995
Eltz, T., C. Bause, K. Hund, J.J.G. Quezada-Euan & T. Pokorny, 2015. Correlates of perfume load in male orchid bees. Chemoecology 25:193-199. Doi: 10.1007/s00049-015-0190-9
Eltz. T., Y. Zimmermann, J. Haftmann, R. Twele, W. Francke, J.J.G. Quezada-Euan & K. Lunau, 2007. Enfleurage, lipid recycling and the origin of perfume collection in orchid bees. Proc. R. Soc. B 274: 2843-2848. Doi: 10.1098/rspb.2007.0727
Eltz, T., D.W. Roubik & M.W. Whitten, 2003. Fragrances, male display and mating behaviour of Euglossa hemichlora: a flight cage experiment. Physiological Entomology 28: 251-260. Doi: 10.1111/j.1365-3032.2003.00340.x

Suckling amphibian

Ringed caecilian feeds milk to her young

Mammals suckle their young. This behaviour distinguishes them from other vertebrates: fish, amphibians, reptiles, and birds. Yet that distinction is not watertight, as a few bird species exist that produce a kind of milk to feed their young: some pigeons and doves, some flamingos, and the emperor penguin. And now, Pedro Mailho-Fontana and colleagues report that females of the ringed caecilian, an amphibian, feed their young with something that resembles mammalian milk. Thanks to this ‘milk’, the youngsters grow rapidly.

It is not surprising that this peculiar trait, which is easily observable, is only now coming to light. The biology of caecilians is poorly known because the animals live underground. Caecilians (Gymnophiona) form a third group within the amphibians, next to frogs & toads and salamanders. They have no legs, have reduced eyes, and are blind; they have two tentacles with which they find their way underground.

The caecilian in question here is the ringed caecilian (Siphonops annulatus). It is widespread in South America. The animal grows to over forty centimetres in length. A female lays eggs, on average ten at a time, in an underground round nest chamber. She is a devoted mother: she lays coiled up with the eggs on her body, and when the young have hatched, she stays with them for another two months; then the young are independent. Until that moment, she will not even leave for a brief time to find food for herself.

A female changes colour when she is attending hatchlings. Normally, ringed caecilians are bluish grey, but a mother turns whitish grey. It was already known that this colour is caused by fat droplets accumulating in her epidermis. Every few days, the young are allowed to scrape off that skin; for this purpose, they have spoon-shaped teeth in the lower jaw. They all do at the same time, ferociously; within ten minutes it is over, and peace returns until the mother’s skin is ready for consumption again. This ‘skin feeding’ occurs in more caecilian species.

But ringed caecilians hatchlings also get other food, Mailho-Fontana discovered: milk. He kept a number of animals in the lab and filmed their behaviour.

Hatchlings often assemble near their mother’s rear end, he observed. Further research revealed that glands in the mother’s oviduct walls produce a white, viscous fluid that emerges through the genital opening, the cloaca, several times a day. The stuff is rich in fats and carbohydrates.

The young imbibe it voraciously. This ‘milk’ is a more important source of nutrition than the mother’s skin, and it is thanks to the milk that the young grow very fast, the researchers think. Their weight doubles within a week after hatching. The mother, eating nothing, loses a third of her weight.

The mother releases milk when hatchlings touch her tail, often producing high-pitched sounds, which probably is begging behaviour. She then raises her body end vertically, and the hatchlings compete for a good place. On average, three of them drink at the same time until they are fully satiated.

Viviparous caecilian species in which the young drink milk in the oviducts before birth were already known. The discovery of oviductal milk in the egg-laying ringed caecilian was unexpected.

Willy van Strien

Photo: Ringed caecilian, female with young. ©Carlos Jared

Mailho-Fontana, P.L., M.M. Antoniazzi, G.R. Coelho, D.C. Pimenta, L.P. Fernandes, A. Kupfer, E.D. Brodie Jr. & C. Jared, 2024. Milk provisioning in oviparous caecilian amphibians. Science 383: 1092-1095. Doi: 10.1126/science.adi5379
Jared, C., P.L. Mailho-Fontana, S.G.S. Jared, A. Kupfer, J.H.C. Delabie, M. Wilkinson & M.M. Antoniazzi, 2019. Life history and reproduction of the neotropical caecilian Siphonops annulatus (Amphibia, Gymnophiona, Siphonopidae), with special emphasis on parental care. Acta Zoologica. 100: 292-302. Doi: 10.1111/azo.12254
Wilkinson, M., A. Kupfer, R. Marques-Porto, H. Jeffkins, M.M. Antoniazzi & C. Jared, 2008. One hundred million years of skin feeding? Extended parental care in a Neotropical caecilian (Amphibia: Gymnophiona). Biology Letters 4: 358-361. Doi: 10.1098/rsbl.2008.0217


Tree fern Cyathea rojasiana transforms dead leaves into roots

The tree fern Cyathea rojasiana, which grows in Panama, has a crown of leaves on a trunk. Sometimes a new frond sprouts, sometimes an old leaf dies. But dying is not the end for a leaf, James Dalling and colleagues discovered: the decayed leaf gets a second life. At least: the rachis.

The tree fern grows to a height of two meters, the leaves are more than two meters long. A senescent leaf will bend down, the leaf tip touching the ground. The leaf rots away, but the rachis remains. A tree can have a ‘skirt’ of twenty to thirty of those remains. They look lifeless, but when Dalling tried to remove them, they were found to be firmly stuck in the ground.

Vascular bundles run through the rachis of a green leaf, transporting water and nutrients absorbed by the roots to the leaf tissue. These vascular bundles appear to be intact in the ground-stuck leaf remains of Cyathea rojasiana. They are surrounded by a black layer that apparently protects them from rotting. And, surprisingly, at the end, i.e., in the soil, a bunch of finely branched roots has sprouted from each vascular bundle.

The conclusion is that the former leaf rachises have been transformed into roots, in which the direction of the water flow is reversed: in green leaves, it flowed from stem to leaf tip, now it is from leaf tip to stem.

Cyathea rojasiana grows in wet, extremely nutrient-poor soil. Extra roots are not so much useful to absorb water, the researchers think, but to extract nutrients from a larger area of soil. They showed that the new roots indeed absorb nitrogen and transport it upwards.

For many plant species holds: you can stick a piece of stem or leaf in the ground, and roots will grow. But converting a leaf rachis into a root is something that, as far as we know, only Cyathea rojasiana does.

Willy van Strien

Photo: Tree fern Cyathea rojasiana with decayed leaves, now functioning as roots. ©James Dalling

James Dalling tells about his discovery on YouTube

Dalling, J.W., E. Garcia, C. Espinosa, C. Pizano. A. Ferrer & J.L. Viana, 2024. Zombie leaves: novel repurposing of senescent fronds in the tree fern Cyathea rojasiana in a tropical montane forest. Ecology e4248, 18 January online. Doi: 10.1002/ecy.4248

Sacrificing sleep

Male dusky antechinus reduces sleep in mating season

As for all species, producing as many offspring as possible is what life is all about for the dusky antechinus, Antechinus swainsonii. For males, which do not care for young, this means that they have to mate with as many females as possible, because every successful mating may increase the number of young they sire. To achieve this, only three weeks are available, because this is the period in which all females are fertile. Males experience fierce competition; as a consequence of the pressure to face this, they are twice as heavy as females.

This short and intensive mating season has a very bizarre ending for males: they all die. Females, that carry the young in a flap of skin (they have no complete pouch), stay alive and many of them experience a second reproduction season the next year. But for males, it is over after one time.

To score as many partners as possible in that single mating season, males cut back on rest, Erika Zaid and colleagues discovered.

The dusky antechinus, a species of broad-footed marsupial mice, is an insectivorous predator that lives in Australia. Before the mating season, male and female sleep an average of more than 15 hours per day. During the mating season, measurements of physical activity and EEGs show that males reduce this to 12 hours on average: 20 percent less. The increased activity, which they exhibit especially at night, is accompanied by a higher level of the male sex hormone testosterone in the blood, giving them extra time and strength to find females and get access.

Unfortunately, the researchers do not know whether males that sacrifice much sleep actually father more offspring. Also, they did not investigate whether males compensate for the lack of sleep by sleeping more deeply.

Sleeping less jeopardizes health. The concentration of corticosteroids, which suppress the immune system, increases, with ultimately fatal consequences. But because males will die soon anyway, staying healthy is no longer important. Mating more often is now a better strategy than getting enough sleep.

You might think that dusky antechinus males die after the mating season because they have been acting so unhealthy. But that is not how it works, according to the researchers. Their death is a certainty. The increase in corticosteroids hardly contributes anything to this fate, but it does ensure that they can sustain their increased activity.

Willy van Strien

Photo: Antechinus swainsonii. Catching the eye (Wikimedia Commons, Creative Commons CC BY 2.0)

Zaid, E., F.W. Rainsford, R.D. Johnsson, M. Valcu, A.L. Vyssotski, P. Meerlo & J.A. Lesku, 2024. Semelparous marsupials reduce sleep for seks. Current Biology, January 25 online. Doi: 10.1016/j.cub.2023.12.064

Coordinated rolling

Dung ball roller Sisyphus schaefferi: male pulling and female pushing dung ball

Dung ball rollers (Sisyphus species) have a striking habit. The dung beetles form pairs, take a piece of mammal droppings, construct a ball larger than themselves and roll it away in a straight line to make sure not to collide with other pairs that have taken a part of the same dung pile. When, often after demanding work, they arrive at a suitable place, they bury their ball in the soil together with an egg. The larva that will hatch from the egg is surrounded by excellent food.

Claudia Tocco and colleagues wanted to know more about the harmonious cooperation that male and female exhibit. They investigated how partners divide tasks in two species: Sisyphus fasciculatus from South Africa and Sisyphus schaefferi, that lives in North Africa, Southern Europe, and Asia Minor. In an outdoor test setup, they offered cow dung to groups of dung ball rollers. The beetles formed pairs, constructed a dung ball and started rolling.

The male, which is slightly larger than the female, is always the one driving the dung ball transport, as the researchers saw. He determines the course. He walks backwards and pulls the dung ball with his front legs. His partner walks on the other side, also backwards, with her head down and her hind legs on the ball. On flat terrain, she contributes nothing. If the male stops rolling, she also stops. So, you never see a female dragging a dung ball by herself. Conversely: if she stops, he goes on alone, rolling the dung ball as quickly and staying on course as well as a couple.

Mostly, she does move along, maintaining contact with the ball. Consequently, she can help immediately when things get difficult. And they do, because Sisyphus fasciculatus and Sisyphus schaefferi live in woodlands and forests, with all kinds of objects lying on the ground. A couple of dung ball rollers often encounters obstacles. The researchers conducted experiments to simulate these situations to see how the pair cleared them.

First, they placed two 2.6 centimeter high obstacles one behind the other on the path. That is quite a challenge, because a dung ball roller is less than a centimeter long. At these obstacles, the female no longer followed passively, as it turned out, but assisted by pushing or steering, and as a result, a pair cleared the double obstacle faster than a male working alone. Moreover, a male alone often gave up.

In a next series of experiments, the beetles were challenged with a wall of 3.9, 6.5 or 9.1 centimeters high. The higher the wall, the less likely a pair was to climb over it with the ball and the less likely it was to succeed if it tried. A male alone declined more often than a pair, but if he tried to clamber over the wall, he usually succeeded.

A couple could get over a high obstacle faster than a male alone, mainly because the female helped at the start. When the male pulled himself up along the wall and lifted the dung ball from the ground, she stabilized the ball with her hind legs and pushed it, in a headstand position. Then he worked his way up, while she hung on the ball. Despite that extra burden for the male, a pair climbed as quickly as a male alone. If he was in danger of falling, she would provide support. Once she got to the top, she became active and pushed the ball over the edge with her head. The beetles then fell down and continued their path.

Dung ball rollers manage to get their ball over difficult obstacles. Unlike the Greek mythical king Sisyphus, after whom the beetles are named. Because of his brutality towards the gods, he was punished by having to push a boulder up a slope in the underworld for eternity, while the boulder kept falling back. He could not do it alone and he did not have a partner to lend a hand.

Willy van Strien

Photo: Sisyphus schaefferi couple with dung ball, male at left side, female at right side. Daniel Ballmer (Wikimedia Commons, Creative Commons, CC BY-SA 4.0)

Watch the dung beetles’ behaviour on YouTube

Tocco, C., M. Byrne, Y. Gagnon, E. Dirlik & M. Dacke, 2024. Spider dung beetles: coordinated cooperative transport without a predefined destination. Proceedings of the Royal Society B 291: 20232621. Doi: 10.1098/rspb.2023.2621
Dacke, M., E. Baird, B. el Jundi, E.J. Warrant & M. Byrne, 2021. How dung beetles steer straight. Annual Review of Entomology 66: 243-256. Doi: 10.1146/annurev-ento-042020-102149

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