Evolution and Biodiversity

Category: parental care (Page 1 of 3)

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

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

Wet plumage

Namaqua sandgrouse father carries water for the chicks

Male Namaqua sandgrouse fetches water for his chicks with specially adapted belly feathers

As long as the chicks are unable to fly, a Namaqua sandgrouse father will fetch water for them. Jochen Mueller and Lorna Gibson describe the specially adapted belly feathers that enable this.

As their name suggests, sandgrouse species live in dry, almost barren places. The Namaqua sandgrouse (Pterocles namaqua), for example, lives in deserts in Southwest Africa, such as the Kalahari and the Namibian desert. The birds breed up to no less than 30 kilometers from the nearest body of water. Because they mainly eat dry seeds, they have to drink. Adult birds therefore fly to waterholes in the morning and evening. This is how they survive in their arid habitat.

But their chicks can’t go with them to the waterholes for the first month. They are immediately independent after hatching; they walk and forage for food on their own. But they can’t fly yet. It was already known that sandgrouse fathers transport a supply of water for the young in specially adapted belly feathers, which trap and hold water. Now, using various microscopic techniques, Jochen Mueller and Lorna Gibson describe the structure of those feathers in detail, both in wet and dry state.


To stock up a supply of water, a Namaqua sandgrouse male steps into the water until it reaches his belly. He fluffs up his belly feathers and rocks his body, soaking the feathers. Then, he presses his belly feathers against his body and leaves. He can store an estimated 25 milliliters of water, 15 percent of his body weight. He flies back at high speed, a trip that can take half an hour. During the flight through dry desert air, some water evaporates, but a lot is still left when he arrives at his nest.

The chicks run up to him and strip the wet feathers with their beaks.

That the belly feathers have a special structure, can already be seen with the naked eye. The feathers have a broad hairy fringe along the side, except at the top. But only under the microscope does the special structure reveal itself completely.

Coiled barbules

A normal bird’s contour feather consists of a shaft on which barbs are implanted, from which barbules branch. These barbules interlock with hooklets and grooves, giving the feather a closed plane. Thanks to the hooklets and grooves, a crumpled feather can be rubbed back into shape.

Under the microscope, the barbs and barbules of the belly feathers of Namaqua sandgrouse males appear to have a different structure. The hairy fringe along the feather is formed by the outer part of the barbs being thin and flexible, and the barbules implanted on the outer part being thin and flexible too.

The inner part of the barbs, where they are attached to the shaft up to just over mid-length, is thicker and stiff. The barbules on this part branch at the upper side, make one helical curl downward and straighten out, running parallel to the barb. The coils of successive barbules intertwine and keep the feather surface closed.

That is how a belly feather looks when it is dry.

Storing water

If such feather gets wet, the picture changes. The barbules on the inner part of the barb uncurl and bend downwards perpendicularly to the feather plane, forming a dense forest of fibers. Due to the so-called capillary force, water is sucked up and held between them.

The fringe of the feather (i.e., the outer parts of the barbs and the barbules that branch from those parts) bends down and inward to the feather shaft, creating a layer to hold the water.

The Namaqua sandgrouse is one of 14 species of sandgrouse (Pterocles), all of which live on arid terrain. In all these species, the males can carry water in their belly feathers, thanks to that unique adaptation of the feather structure.

Willy van Strien

Photo: Pterocles namaqua, male. Bernard DUPONT (Wikimedia Commons, Creative Commons CC BY-SA 2.0)

On YouTube: Namaqua sandgrouse male fetches water for chicks

Mueller, J. & L.J. Gibson, 2023. Structure and mechanics of water-holding feathers of Namaqua sandgrouse (Pterocles namaqua). Journal of the Royal Society Interface 20: 20220878. Doi: 10.1098/rsif.2022.0878
Cade, T.J. & G.L. Maclean, 1967. Transport of water by adult sandgrouse to their young. The Condor 69: 323-343. Doi: 10.2307/1366197

Cuckoo duck seeks defence

Foster family protects duck eggs against birds of prey

Cuckoo duck dumps its eggs in nest of aggressive host

Young cuckoo ducks do not need any care: they are independent upon hatching. Then why does the duck burden other birds with its eggs, Bruce Lyon and colleagues wondered.

In South America a duck species occurs that, like a cuckoo, lays its eggs in nests of other bird species. The hosts then unintentionally take care of them. This is the black-headed duck, Heteronetta atricapilla, with the appropriate nickname cuckoo duck; it is a so-called brood parasite.

Bruce Lyon and colleagues wondered why the cuckoo duck dumps its eggs in other birds’ nests. They don’t require much care, apart from brooding. After hatching, the young are immediately independent. That is a big difference with all other brood parasites, such as the common cuckoo. These species have young that have to be fed and protected for weeks, so it is very profitable for parents to outsource the care. But how does the cuckoo duck profit?

Easy prey

The shedding of parental duties may have to do with the danger of predation, Lyon hypothesized. If the cuckoo duck were to make its own nest, it would be close to water. And in such nest, eggs are easy prey for avian predators, especially the chimango caracara. This was shown in experiments in which the researchers placed chicken eggs in a self-made, unguarded nest. Within a few days, all eggs were gone.

Unless they placed the nest in a colony of brown-headed gulls. In that case, hardly any egg was stolen.

This gull is one of the hosts in whose nests the cuckoo duck dumps its eggs. In Argentina, where the study was conducted, two other important hosts occur, the red-fronted coot and the red-gartered coot. Like the brown-headed gull, they are aggressive birds that are capable to defend their nests fiercely. Is that the reason why the cuckoo duck chooses them to care for its offspring?


It seems to be. The duck eggs are indeed quite safe with these fierce foster parents, the researchers noted. Admittedly, it may happen that foster parents recognize a foreign egg and throw it out of the nest. But if they accept the egg, it almost always remains undisturbed and hatches. This very high chance of survival upon acceptance far outweighs the risk of rejection.

The researchers do not know exactly how much the cuckoo duck gains. They could not determine how many eggs would survive in a self-defended nest, because it never builds a nest. But related duck species that do incubate and guard their own eggs lose quite a lot to birds of prey.

Willy van Strien

Photo: black-headed duck couple. Cláudio Dias Timm (Wikimedia Commons, Creative Commons BY-SA 2.0).

Lyon, B.E., A. Carminati, G. Goggin & J.M. Eadie, 2022. Did extreme nest predation favor the evolution of obligate brood parasitism in a duck? Ecology and Evolution 12: e9251. Doi: 10.1002/ece3.9251

First migration trip in Caspian tern

Father teaches young bird how to travel

Caspian tern father accompanies young during first autumn migration

Young Caspian terns learn from their father how to migrate to the wintering grounds. When, in following years, they make that autumn trip independently, they remember their fathers’ lesson, Patrik Byholm and colleagues show.

A young Caspian tern that is born at the end of May along the coast of Finland or Sweden, will migrate to West Africa at the end of summer to hibernate there. Its father’s job is to guide it on that first journey, Patrik Byholm and colleagues noted.

The researchers wanted to know how information about autumn migration – route and stopover sites – is passed on from one generation to the next. To find out, they equipped birds with GPS tracking devices.

The Caspian tern, Hydroprogne caspia, is found in many places in the world. In Europe, it also breeds along the Black Sea and the Caspian Sea, and in North America along ocean coasts and the great lakes. Some birds from Finland and Sweden make a stop in the Netherlands during their migration to Africa. They travel singly or in small family groups, which are single-parent families.

Reduced tempo

The collected travel data shows that couples that started a nest with two or three eggs in spring and raised their young together, leave each other after the breeding season. They travel separately, sometimes weeks apart, to the wintering area.

Young birds travel with one of the parents, and mostly, that is the father. They cannot travel safely on their own: young terns that for one reason or another lose contact with parents, are captured by birds of prey. So, they stay close to their father during the trip. He teaches them the route and knows good stopover sites, where the birds can roost and forage during the migration. The lesson is learned: the young birds will follow the same route southwards in the years that follow, using the same stopover sites.

Fathers that accompany one or a few young, will adjust their tempo a bit. They progress less quickly than adult birds traveling alone. This is mainly because young birds take more time to rest.

After arrival, the bond between father and young loosens and parental care finishes. They gradually spend less time without each other, and after a month or two they stop seeing each other at all. Sometimes, a young travels a little further south, in the company of another congener.

Willy van Strien

Photo: Colony of Caspian tern. Dmitry Mikhirev (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Byholm, P., M. Beal, N. Isaksson, U. Lötberg & S. Åkesson, 2022. Paternal transmission of migration knowledge in a long-distance bird migrant. Nature Communications 13: 1566. Doi: 10.1038/s41467-022-29300-w

Increasing efficiency in brood parasite

Cuckoo catfish improves its timing

Cuckoo catfish, Synodontis multipunctatus, improves efficiency by practizing

It is not easy for a cuckoo catfish to get its eggs adopted by intended host parents, because these are wary. But it learns the trick by experience, as Holger Zimmermann and colleagues show.

Cuckoo catfish dump their eggs at host parents to let them take care of their offspring: they are brood parasites. That seems easy and, in a way, it is, because the eggs can develop safely without the real parents having to worry. But they do have to bring them to the host parents, and that is not so easy. In that sense, cuckoo catfish spend more effort for their offspring than most fish, which simply lay eggs and leave them behind.

They have to practice the art of parasitism, Holger Zimmermann and colleagues write. The cuckoo catfish (Synodontis multipunctatus) is, as far as we know, the only fish species that, like a cuckoo, relegates the raising of its offspring to others. It lives in Lake Tanganyika in Africa.


It takes advantage of species of cichlids that have the most extensive form of parental care, the so-called mouthbrooders. In these species, mothers take the fertilized eggs in the mouth and keep them there until they hatch, after a few weeks.

During spawning, such mouthbrooding cichlids circle around each other and release eggs and sperm; in between acts, they defend the spawning site against intruders.

But a group of cuckoo catfish may intrude. They consume some cichlid eggs before the mother has been able to collect them and drop a few eggs themselves and fertilize them. The cichlid mother panics and collects her eggs as fast as she can; in her haste, she also takes up catfish eggs.

The catfish must interfere at exactly the right time, when the female cichlid is busy laying eggs; it’s a matter of seconds. By experience, they learn to improve the timing of egg laying and fertilization, Zimmermann shows with experiments in tanks, in which he exposed cichlids (4 males and 12 females) to three cuckoo catfish pairs.

Sharper timing

The researchers searched for host parents that have no resistance against the underwater cuckoo. With resilient host parents, the learning process of the parasite would not show up. They selected the mouthbrooder Astatotilapia burtoni, which lives in Lake Tanganyika and is known to the catfish. But they took a population from a neighbouring river, where the cuckoo catfish does not occur. The chosen host parents have no innate defences against cuckoo catfish, nor do they learn to avoid it, but they do behave aggressively towards any fish that disturb the spawning to predate on eggs.

Unexperienced cuckoo catfish almost never managed to get their eggs taken up by these host parents. Only 3 percent of their attempts succeeded. But after some time – in the experiments after four months, about 30 attempts – things got much better: more than 25 percent of the attempts now was successful. That success rate did not increase further. Experienced catfish also managed to consume more eggs of the host parents in the brief time available.

The improvement was possible because the parasites learn to lay and fertilize their eggs at precisely the right time, as behavioural observations showed. In addition, groups of catfish improve the coordination of their intrusive act.

Host parent is loser

Most attempts fail, though, even in experienced cuckoo catfish, because the vigilant cichlids outsmart their enemy. But that does not matter, because the profits for the parasite are large if the action does succeed. A host mother then carries on average five parasite eggs. The catfish will hatch sooner than the cichlids, and the young catfish devour some cichlid embryos.

The host mother is the loser. She is abused and produces fewer young of her own.

Willy van Strien

Photo: Cuckoo catfish. Calwhiz. (Via Flickr, CC BY-NC-ND 2.0)

Cichlids from Lake Tanganyika have learned to coexist with cuckoo catfish

Zimmermann, H., R. Blažek, M. Polačik & M. Reichard, 2022. Individual experience as a key to success for the cuckoo catfish brood parasitism. Nature Communications 13: 1723. Doi: 10.1038/s41467-022-29417-y

Content with second place

European pied flycatcher may prefer to be a concubine

Female pied flycatcher may become secondary mate of a male

A high quality male is so desirable that a female pied flycatcher may be willing to become his secondary mate – as long as it is not too hard to take care of the young without his assistance, Simone Santoro and colleagues write.

Like most passerine birds, the European pied flycatcher (Fidecula hypoleuca) is mainly socially monogamous. But some males have a secondary female. This concubine gets little help from him when raising the young, but in good years, when food is abundant, that may not be a major problem, Simone Santoro and colleagues argue.

Short breeding season

The males are the first to return from the wintering area in Africa, mid-April. They look for a suitable nest hole, which can be a tree cavity or nest box, and defend a small territory around it. Once a male occupies a good place, he tries to attract a female to breed with. Females visit a number of males before making their choice.

A couple is then busy for about five weeks. She lays five or six eggs and starts breeding when the clutch is complete. Both parents feed the young until they fledge, and dad defends the family. The breeding season covers the months of May and June; only one clutch can be raised in this period. But some males want more.

Good genetic quality

To get more, an ambitious male will have to occupy a second nest site and attract another mate. If successful, he will have to divide his paternal efforts over two nests. The research group, which works in Spain, had already shown how things go.

Males that succeed in starting a second nest are birds that have arrived and started breeding early, and that are able to defend two nests against rivals. These are strong males: of high genetic quality and in good condition. Such male stays with his first mate during the week that she is laying eggs. When she starts incubating, he tries to seduce to a second female. Usually, a second nest is located close to the first one.

When the young hatch in the first nest, he goes there to help feeding them. The primary female gets his full attention. Only when that first nest has fledged does he offer his services to the second nest.

So, the secondary female is worse off, as she has to feed the kids on her own for a while: that is hard work and she will see fewer young fledge. But, on the other hand, these young inherit a good genetic quality from their father. That is why a female may prefer to be the secondary mate of a high quality male rather than the only mate of a low quality male.

Fat and lean years

Particularly later in the season – when desirable single males are not available anymore -the choice to become a secondary female can turn out fairly well, because the time interval between father’s first and second brood will be larger and he will start helping on the second nest earlier.

Now, the researchers show that the availability of food also matters.

Because secondary females have to work harder than females in a monogamous relationship, their chance of survival is lower. (That is also true for primary females. Apparently, the situation is not ideal for them either, but it isn’t their choice.)

However, the lower survival rate of secondary females is an average over years; the researchers followed the birds for 26 seasons. The survival rate varies from one year to the next. In good years, a secondary female has less difficulty raising her young and her chance to survive is almost as high as that of a female in a monogamous relationship. To assess whether a year was good or bad, the researchers considerd the percentage of young that survived and fledged. A good year probably is a year in which food is abundant. In such year, a female can more easily accept a secondary position.

And sometimes. she does, as it turns out: in fat years it is more common for a male to have two families than in lean years. But even then, monogamous relationships remain the majority.

Willy van Strien

Photo: Caroline Legg (Wikimedia Commons, Creative Commons CC BY 2.0)

Santoro, S., P. Fernández‑Díaz, D. Canal, C. Camacho, L.Z. Garamszegi, J, Martínez‑Padilla & J. Potti, 2022. High frequency of social polygyny reveals little costs for females in a songbird. Scientific Reports 12: 277. Doi: 10.1038/s41598-021-04423-0
Canal, D., L. Schlicht, J. Manzano, C. Camacho & J. Potti, 2020. Socio-ecological factors shape the opportunity for polygyny in a migratory songbird. Behavioral Ecology 31: 598–609. Doi: 10.1093/beheco/arz220

Help is helpful in Seychelles warbler

Older mothers and their offspring benefit

Older Seychelles warbler mother and her young benefit from help

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

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

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

Long-term care

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

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

Unless she has female helpers.


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

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

And what about dad?

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

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

Willy van Strien

Photo: ©Charlie Davies

Seychelles warblers help each other also in another way

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

Successful clutch? Leave!

Plovers abandon successful family to re-nest elsewhere

Plovers, including snowy plover, leave their family when successful

A successful marriage triggers divorce, at least in plovers. That is because a parent that deserts may achieve higher reproductive success, Naerhulan Halimubieke and colleagues noted.

A bird male and female that successfully raised young had better stick together, you might think, as they have proven to be a good team. And after a disappointing breeding result, it is best for them to split up and find another mate, with which things may go better. Such is the behaviour in most species of birds in which pairs form to breed.

But in plovers, it is the other way round, Naerhulan Halimubieke and colleagues write. A pair of plover parents often will divorce after successfully producing chicks. And after nest failure, male and female stay together to try again. For these birds, this is the best strategy.

New brood

The researchers had previously found this pattern – divorce if successful, stick together if failing – in the snowy plover Charadrius nivosus, a ground-nesting bird which lives on sandy beaches. A clutch consists of three eggs in a shallow scrape, which are incubated by both parents. When the chicks hatch, they leave the nest immediately. They find their own food and only need warmth and protection from their parents. A single parent can easily provide this. It is therefore not necessary for both parents to stay with the young until they are completely independent, after about a month.

That is why one of the parents may leave its successful family to find a new mate and initiate another brood. Deserting parents gain time, taking advantage of the breeding season as much as possible; their behaviour results in more offspring on average within a breeding season.

Females desert more often than males, probably because there is a small surplus of adult males, so that on average, females meet a new mate sooner.

A completely different situation arises when a snowy plover brood fails, which in most cases is caused by a predator that detected the nest. The best strategy for the parents in that case is to stay together and start a new nest immediately.

Other plovers

Now, this appears to apply to other species of plovers as well, all of them shorebirds. Halimubieke and colleagues examined eight species. Populations with greater breeding success exhibit a higher rate of divorce within a breeding season than populations with less success, they noted. And within populations, couples with a successful clutch split up more often than couples that see their clutch fail.

Also over years, these birds are not necessarily faithful to their mates. When a new breeding season is coming, they start to nest as soon as possible without worrying too much about mate selection. Having as much offspring as possible – that’s the most important thing.

Willy van Strien

Photo: snowy plover, Charadrius nivosus. Lisa Mcgloin (Wikimedia Commons, Creative Commons CC BY 3.0)

Halimubieke, N., K. Kupán, J.O. Valdebenito, V. Kubelka, M.C. Carmona‑Isunza, D. Burgas, D. Catlin, J.J.H. St Clair, J. Cohen, J. Figuerola, M. Yasué, M. Johnson, M. Mencarelli, M. Cruz‑López, M. Stantial, M.A. Weston, P. Lloyd, P. Que, T. Montalvo, U. Bansal, G.C. McDonald, Y. Liu, A. Kosztolányi & T. Székely, 2020. Successful breeding predicts divorce in plovers. Scientific Reports 10: 15576. Doi: 10.1038/s41598-020-72521-6
Halimubieke, N., J.O. Valdebenito, P. Harding, M. Cruz‐López, M.A. Serrano‐Meneses, R. James, K. Kupán & T. Székely, 2019. Mate fidelity in a polygamous shorebird, the snowy plover (Charadrius nivosus). Ecology and Evolution. 9: 10734-10745. Doi: 10.1002/ece3.5591

True pregnancy

During gestation, pot-bellied seahorse males provision the embryos

Pregnant pot-bellied seahorse males provision the embryos

Seahorses are viviparous, and it is the males that are pregnant. In pot-bellied seahorse, Hippocampus abdominalis, males even provide the embryos with nutrients, Zoe Skalkos and colleagues discovered.

Some fish species are viviparous. In most cases, young fish are born from the mother, but in seahorses the father plays a unique role. He incubates the fertilized eggs in a fleshy, enclosed brood pouch until the offspring can live independently. In daddy’s pouch, the embryos are safe from small predators and pathogens. The pregnant father controls the water quality in the pouch; the highly vascularised pouch skin supplies oxygen and waste products are removed.

Males of pot-bellied or big-belly seahorse, Hippocampus abdominalis, that lives around Australia and New Zealand, also transport nutrients to their embryos, Zoe Skalkos and colleagues report.

Complex brood pouch

When seahorses mate, the female transfers her eggs into her partner’s brood pouch, which he has inflated by filling it with seawater. He fertilizes the eggs immediately and carries them until the young fish can be released. The developing embryos consume the large amount of high protein yolk that the eggs contain.

Pot-bellied seahorse is a large species, up to 35 centimeters long, and exhibits the most complex form of male pregnancy among seahorses. Young embryos are deeply embedded into the pouch’s lining tissue; some are completely covered. The embryos can survive on the amount of food that the yolk contains, according to experiments in which they developed outside a brood pouch. But young fish that are raised in this way exhibit stunted growth and suffer increased mortality. That is why the researchers wondered whether the pregnant father transports nutrients to his hundreds of young via the pouch wall.


To find out, they compared the dry weight of newly fertilized eggs of pot-bellied seahorse with that of newborns, which are released after a gestation period of about 24 days. They also determined the fat content of eggs and newborns. From previous research, they knew that cell constituents that transport fats are produced in large quantities in the brood pouch of males during gestation. Fat is the primary source of energy for the embryos and they need a lot of it.

If the father would not supply nutrients to the embryos, the dry weight of newborn fish would be lower than that of newly fertilized eggs. That is because embryos consume the food supply that the mother provided; they gain weight, but part of the mass is lost by metabolism. The weight loss is estimated to be 30 to 40 percent.

However, as it turned out, newborns have the same dry weight as newly fertilized eggs. Also fat contents were similar. Most likely then, the father provides nutrition to his offspring, especially fats, to replace what is lost.

Pregnant in every sense

Pipefish are closely related to seahorses. Also in pipefish, fathers carry the embryos, although not all pipefish species possess a highly developed, enclosed brood pouch. In some pipefish species, as was known, pregnant males transport a small amount of nutrients to the embryos. Now, this also appears to happen in at least one seahorse species.

These fish dads are going through a pregnancy in every sense. However, compared to that of mammals, their pregnancy is not entirely complete, because the fish mothers still provide most nutrients to the embryos. But it certainly is extraordinary.

Willy van Strien

Photo: Pot-bellied seahorse mating. Elizabeth Haslam (Wikimedia Commons, Creative Commons CC BY 2.0)

Watch a video on courtship and birth in pot-bellied seahorse

Skalkos, Z.M.G., J.U. Van Dyke & C.M. Whittington, 2020. Paternal nutrient provisioning during male pregnancy in the seahorse Hippocampus abdominalis. Journal of Comparative Physiology B 190: 547-556. Doi: 10.1007/s00360-020-01289-y
Whittington, C.M., O.W. Griffith, W. Qi, M.B. Thompson & A.B. Wilson, 2015. Seahorse brood pouch transcriptome reveals common genes associated with vertebrate pregnancy. Molecular Biology and Evolution 32: 3114-3131. Doi: 10.1093/molbev/msv177

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