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)

Sources:
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.

Supplement

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

Sources:
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

Antibacterial treatment

When the eggs are brown-coloured, hoopoe male is more helpful

Hoopoe male will feed its partner when she paints the eggs brown

A brooding hoopoe female repeatedly smears a dark substance from her preen gland onto the eggs. The browner the eggs, the more helpful her partner will be. That is because the colour has meaning to him, Silvia Díaz Lora and colleagues think.

During incubation, a dark-coloured, foul-smelling substance is produced in the preen gland (uropygial gland) of hoopoe females, which is greatly enlarged during this period. It was already known that a female smears the material onto the eggs with her beak. The colour that the eggshells get as a result determines how frequently the male will feed her, Silvia Díaz Lora and colleagues now report.

This has to do with the hatching success of the clutch that is to be expected. If prospects are good, he will invest a lot. If not, he will save energy for the next breeding event. The egg colour is an indication of the expected success.

In the breeding season, hoopoes establish a territory, form pairs and breed in tree cavities. Both parents take care of the nestlings until they fledge. But before nestlings appear, the clutch has to be incubated, which is the females’ task. They don’t leave the nest during this period, and they also stay with the young during the first week. Their partners provide them with food.

Symbiotic bacteria

The preen gland secretion that the females spread onto their eggs protects the embryos, thanks to bacteria that live in the gland during the breeding period and produce compounds that inhibit pathogenic bacteria. When an egg is covered with gland secretion, pathogens are prevented from penetrating the egg shell. This increases the chance the eggs will hatch, and the higher the density of beneficial bacteria, the better the result.

The mother smears the antimicrobial substance onto each egg soon after laying and repeats the procedure until the young hatch. Microscopic cavities on the eggshell enhance adhesion of the substance. Newly laid eggs are light bluish-gray, but they turn dark and greenish-brown by the treatment.

Embryos of other bird species are protected by an extra layer in the egg. But the hoopoe has its own, unique way.

Later on, also the nestlings’ preen gland will produce the brown secretion. The substance has another function: the stench deters predatory enemies. Outside the breeding season, the preen gland of females and fledged young, like that of males, is small. The bacteria are disappeared and the gland produces a white, colourless fat, which the birds use to preen their plumage.

Colour difference

Among hoopoe females, there is a difference in the colour of the substance they paint the eggs with. And what is important for this story: that difference is related to the amount of bacteria that are present in the preen gland. Without bacteria, the substance is red, with bacteria it is brown. The browner the colour, the higher the bacterial density – and the stronger the antimicrobial activity.

That means: the browner the eggs, the better the embryos are protected against infections.

When bringing food to their mate, males see the colour of the eggs. The researchers wanted to know whether they adjust their feeding effort to this colour. On the basis of video recordings at a number of nests, they investigated at what frequency the male came to the nest and what prey he carried. With a spectrometer, they measured the colour of the clutch and they sampled the female’s preen gland contents to assess the density of bacteria.

Willingness

Results are appealing. If the eggs were brown-coloured, the male frequently fed his partner while she was incubating. If the eggs were more reddish, he only worked this hard if her body condition was good. Apparently, hoopoe males are willing to invest a lot in a clutch if it is promising because the mother colours the eggs with a potent antibacterial material or because she is healthy.

Once the eggs had hatched, father’s behaviour no longer depended on the egg colour. There are probably other factors that determine his diligence at that stage, such as the begging behaviour of the young.

Wealth

But there is no proof yet that this story – if eggs are brown, a hoopoe male will work harder – is really true. An alternative possibility is that a father who brings in more food for its partner simply has a richer territory than a father who brings less. Thanks to lots of available food, the mother is in better condition and able to maintain a larger population of bacteria in her preen gland, which colours the secretion brown and gives the eggs good protection. So: the quality of the territory may determine both how much food the male brings in and the colour of the eggs.

To find out what is true, the researchers would have to do experiments in which they paint hoopoe eggs darker and see whether males respond by working harder. For now, it looks like they adjust their effort to the chance that the eggs will hatch successfully. But only if such experiments corroborate this, we can be sure.

Willy van Strien

Photo: Eurasian hoopoe, Upupa epops. Imran Shah (Wikimedia Commons, Creative Commons CC BY-SA 2.0)

Sources:
Díaz Lora, S., T. Pérez-Contreras, M. Azcárate-García, M. Martínez-Bueno, J.J. Soler & M. Martín-Vivaldi, 2020.  Hoopoe Upupa epops male feeding effort is related to female cosmetic egg colouration. Journal of Avian Biology, online June 20. Doi: 10.1111/jav.02433
Martín-Vivaldi, M., J.J. Soler, J.M. Peralta-Sánchez, L. Arco, A.M. Martín-Platero, M. Marínez-Bueno, M. Ruiz-Rodríguez & E. Valdivia, 2014. Special structures of hoopoe eggshells enhance the adhesion of symbiont-carrying uropygial secretion that increase hatching success. Journal of Animal Ecology 83: 1289-130. Doi: 10.1111/1365-2656.12243
Soler, J.J., M. Martín-Vivaldi, J. M. Peralta-Sánchez, L. Arco & N. Juárez-García-Pelayo, 2014. Hoopoes color their eggs with antimicrobial uropygial secretions. Naturwissenschaften 101: 697-705. Doi: 10.1007/s00114-014-1201-3

Scattering sand

Sea turtle creates decoy nests on the beach

sea turtle creates decoy nests

Sea turtle eggs, buried in a sandy beach, are an attractive meal for some animals. Turtle mothers confuse these enemies with a series of decoy nests, Thomas Burns and colleagues think.

For a sea turtle female, it is physically demanding to lay her eggs. She crawls from the sea onto a sandy beach, selects a suitable place, digs a hole, lays dozens of eggs in it and refills the nest cavity. You would expect her to return to the sea as soon as possible, where she can move more easily and is safer.

But she doesn’t, as Thomas Burns and colleagues report. She first scatters sand around the refilled egg chamber. And then she starts to travel a convoluted path over a large area, further and further away from the nest, stopping periodically to scatter sand again. Only after having done this at several scattering stations, she will leave the beach. What is this extra effort good for?

Tasty

Sea turtles lay their eggs on tropical and subtropical beaches worldwide; afterwards, they don’t look after their clutches any more. The eggs develop in the warm sand, and the young turtles dig through the sand and crawl to the sea. All a mother can do for her offspring is to make sure not to betray the location of the nest, where she has dug in the sand, to predators. The tasty eggs are liked by various animals, including gulls, foxes, raccoons and wild pigs.

Biologists presumed that sea turtles scatter sand around their nests to disguise or camouflage them, so they won’t be noticed. But that can’t be the reason, Burns and colleagues argue after thoroughly studying the behaviour of leatherback sea turtle (Dermochelys coriacea) and hawksbill sea turtle (Eretmochelys imbricata). Because why, in that case, would sea turtles scatter sand also in places that are at considerable distance from the nest?

Big effort

The researchers, who worked on the islands of Trinidad and Tobago, point out that female sea turtles behave according to a fixed pattern until they have finished the nest. Thereafter, their movements become unpredictable. They take a random route on the beach, changing direction at each station.

Research also shows that scattering sand is a time-consuming and exhausting activity. The hawksbill sea turtle puts in as much energy as it does in excavating a nest hole and refilling it, and for the leatherback it is even the most strenuous activity. The hawksbill often scatters sand at more than ten stations, the leatherback may stop more than twenty times. Despite the great effort, the turtles persist: at the last stop they are as active as at the first.

The researchers’ conclusion: the sea turtles create a series of decoy nests. An natural enemy looking for eggs will mostly dig in vain and lose a lot of time. As a consequence, real nests are less easily found and therefore safer.

Willy van Strien

Photo: Eretmochelys imbricata. Gerwin Sturm (Wikimedia Commons, Creative Commons CC BY-SA 2.0)

Sources:
Burns, T.J., R.R. Thomson, R.A. McLaren, J. Rawlinson, E. McMillan, H. Davidson & M.W. Kennedy, 2020. Buried treasure—marine turtles do not ‘disguise’ or ‘camouflage’ their nests but avoid them and create a decoy trail. Royal Society Open Science 7: 200327. Doi: 10.1098/rsos.200327
Burns, T.J., H. Davidson & M.W. Kennedy, 2016. Large-scale investment in the excavation and ‘camouflaging’ phases by nesting leatherback turtles (Dermochelys coriacea). Canadian Journal of Zoology. Doi: 10.1139/cjz-2015-0240

American coot takes care of the small

The younger, the brighter, the more food

Coot chicks' ornaments tell parents what age they are

In contrast to their parents, young coots have a striking appearance. Bruce Lyon and Daizaburo Shizuko discovered how their ornamentation helps the parents to optimally feed the offspring.

Young American coots, Fulica americana, have fancy heads: a red beak and bare patches of red skin with papillae, surrounded by a crown of orange-yellow modified feathers. That ornamentation is puzzling; because of predators, you would rather expect young coots to be unobtrusive. Bruce Lyon and Daizaburo Shizuko figured out what function their colourful appearance serves.

Starvation

If the clutch of a pair of American coots is complete, it is certain that not every egg will result in an independent young. The water birds lay nine eggs per nest on average, and although almost all of these eggs will hatch, only three or four young eventually reach independency. Four chicks is the maximum that the parents can feed. As a consequence, less than half of the chicks can survive.

The case is settled during the first ten days after the last egg has hatched. The young coots leave the nest immediately after hatching and swim to the parents to be fed, every chick trying to get attention. It is an unfair competition, because the siblings do not hatch at the same time; the first chick may be eleven days older than the last. The oldest chicks are larger, not only because they are older, but also because the first eggs laid by a female are larger. It is easy for them to keep up with their parents, while the youngest coots run a high risk of being too slow and starving.

The parents don’t interfere with this rivalry between their young.

Favourites

But after ten days, things will change, as the researchers had discovered before during their research in Canada. The size of the coot family then is reduced to a number that the parents can handle, and they shift strategies. They are now going to pay attention to the small ones and offer them most of the food that is available. Each parent chooses one of the chicks to favour; a favourite is always one of the youngest.

The eldest chicks also want to be fed, but they are already able to find their own food. They are tousled by their parents when they come begging: they are grasped by the neck and shaken. They then will give up.

In this way, the parents give full attention to the chicks that need it, making sure that all chicks that survived the period of sibling rivalry can grow up. Thanks to this preferential treatment, the youngest chicks in a coot family will gain the same weight as the oldest ones.

Age

The researchers also had observed earlier that the most brightly coloured young were preferentially fed by the parents and more likely to be chosen as a favourite. Now, they link the chicks’ colour to their age. The later an egg’s position in the laying order, they show, the brighter coloured the chick will be. This is probably because the mother adds more dye to the yolk as she has already laid more eggs. So this appears to be the function of the ornamentation: it indicates to the parents which chicks are the youngest and need food aid the most.

Sometimes a coot is extremely aggressive to a chick. In that case, this is not its own young, but another coot’s. Coot females often dump an egg in the neighbours’ nest in an attempt to increase the breeding success. But the intended foster parents recognize such foreign chick and will tackle it hard. Its chance to survive is very small.

Willy van Strien

Photo: American coot with chicks. M. Baird (Wikimedia Commons, Creative Commons CC BY 2.0)

Sources:
Lyon, B.E. & D. Shizuka, 2019. Extreme offspring ornamentation in American coots is favored by selection within families, not benefits to conspecific brood parasites. PNAS, online Dec 30. Doi: 10.1073/pnas.1913615117
Shizuka, D. & B.E. Lyon, 2013. Family dynamics through time: brood reduction followed by parental compensation with aggression and favoritism. Ecology Letters 16: 315-322. Doi: 10.1111/ele.12040
Lyon, B.E., 1993. Conspecific brood parasitism as a flexible female reproductive tactic in American coots. Animal Behaviour 46: 911-928. Doi: 10.1006/anbe.1993.1273

Costs before benefits

By guarding stepkids, bee male may get the mother

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

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

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

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

Guard

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

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

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

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

Male switches

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

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

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

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

Willy van Strien

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

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

Care for everyone

Earwig mother tends foreign eggs and adopts orphans

earwig female cares for another female's offspring 

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

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

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

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

Weight loss

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

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

Orphans

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

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

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

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

Willy van Strien

Photo: European earwig female with eggs. ©Joël Meunier

Watch an earwig mother tending her eggs on You Tube

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

Curbed pregnancy

Pipefish father responds to sight of attractive female

male pipefish curb their pregnancy upon seeing an attractive female

A pipefish father with a filled brood pouch may see a female who produces better offspring than the mother of the embryos he currently carries. He will then curb his pregnancy, as Mário Cunha and colleagues show.

In pipefish, long thin fish species with a tubular snout, parental care is provided by the males. The fathers carry the fertilized eggs until the young hatch and swim away. In some species, including the black-striped pipefish (Syngnathus abaster), they even have a brood pouch, within which the water is of good quality and the eggs are protected and provided with oxygen and nutrients. Black-striped pipefish males carry eggs from three females per ‘pregnancy’ on average. They prefer to receive eggs of large partners, because then the pregnancy will result in more and larger offspring.

Now, a dilemma arises if a pregnant male encounters a female that is larger than the females whose eggs he is brooding, Mário Cunha and colleagues realized. A pregnancy that would yield more than the current one is within reach, but his pouch is already occupied. What would he do: just continue the pregnancy? Or would he break it off, end it earlier or invest less in it to save time and energy for a next brood?

Extremely attractive

To find out, the researchers brought pipefish into the lab and conducted experiments. They paired a number of males with one female of large size and waited until the embryos were developing. They then removed the mother and introduced a very large, extremely attractive female to some of the males. She was behind a transparant divider; the males could see and smell her, but physical contact was not possible.

The researchers checked how long the pregnancy lasted and measured the length of the young fish after birth. For comparison, other fathers were either allowed to remain in contact with the mother or exposed to another female that was equally sized.

embryo of pipefish Syngnathus abasterFathers who perceived a particularly large female were found to respond to this encounter. They curbed their pregnancy: its duration was shorter than that of other fathers, and the young fish that emerged were smaller. Another trial showed that it was more likely that some embryos died. So, when a male sees a very attractive partner, he will invest less in his current pregnancy.

Insensitive behaviour

In our view, that is insensitive behaviour. But it may occur if it increases the breeding success of pipefish fathers. In that case, a fast new pregnancy with high yield should compensate for the smaller number of offspring from the current pregnancy.

Willy van Strien

Photos:
Large: Syngnathus abaster. Giacomo Radi (Wikimedia Commons, GNU Free Documantation License 1.2)
Small: embryo of Syngnathus abaster near the end of the pregnancy. © Sara Mendes

Source:
Cunha, M., A. Berglund, S. Mendes & N. Monteiro, 2018. The ‘Woman in Red’ effect: pipefish males curb pregnancies at the sight of an attractive female. Proceedings of the Royal Society B 285: 20181335. Doi: 10.1098 / rspb.2018.1335

Why dad leaves his family

Conflict between moult and care in Hooded Warbler

When a Hooded Warbler male initiates moulting while still having dependent young, he often deserts

When parental care and feather replacement overlap in time, a Hooded Warbler male may abandon his family, forcing his mate to provide all remaining care for the young. The mother can handle it, as Ronald Mumme points out.

Migratory birds are under time pressure during the breeding season. Not only do they have to raise young, they also have to replace their feathers and store fat reserves to prepare for fall migration to their wintering area. Those tasks – caring, moulting and storing fat – may be in conflict.

The Hooded Warbler, which spends the summer in North America, has a very hard time, Ronald Mumme notices. Couples produce two clutches per season. The female incubates the eggs and when the young have hatched, both parents will provision them. The parents have to perform this job for about four weeks, for only then the young are independent. Hooded Warblers feed on winged insects, especially flies and mosquitoes, which they pick from the air. Before the young of the second clutch reach independency, it may already be time for the parents to initiate moult.

Indispensable tail

The problem now is that Hooded Warblers use their tail as foraging tool. The outer tail feathers have white spots that become visible when a bird spreads the feathers and that stand out against the olive-green background. By flicking its tail, a bird flushes flying insects hidden in the vegetation and captures them in the air.

But during moult, those feathers are shed simultaneously and a bird has to spend a week without a tail. It then has difficulty capturing insects, while it needs extra food because the moult is energetically demanding. And so it may happen that one of the parents leaves the family because it cannot obtain sufficient food for itself and the chicks, especially if they are so young that they cannot catch any food for themselves. Mostly, the father deserts, because males initiate moult on average two weeks earlier than females. Young, inexperienced fathers are more likely to leave than older dads.

Why males initiate moult earlier than females is not known yet.

Abandoned females

When the father deserts at the end of the season, the mother is left responsible for all remaining parental care. But apparently, she has no big problems: the chance that such a female survives the winter does not seem to decrease; the birds spend wintertime along the Caribbean coast of Central America. And in the next breeding season, she probably will choose the same mate, even though he had left her.

Willy van Strien

Photo: A male Hooded Warbler delivers food to his nestlings © Ron Mumme

Sources:
Mumme, R.L., 2018. The trade-off between molt and parental care in Hooded Warblers: simultaneous rectrix molt and uniparental desertion of late-season young. The Auk 135: 427-438. Doi: 10.1642/AUK-17-240.1
Mumme, R.L., 2014. White tail spots and tail-flicking behavior enhance foraging performance in the Hooded Warbler. The Auk 131: 141-149. Doi: 10.1642/AUK-13-199.1

Inequality

Red-capped plover invests more in young of opposite sex

male red-capped plover will provide more parental care to daughters

Fathers care more when they have daughters, mothers care more when they have sons. Such is the case in the red-capped plover, in which both parents divide the tasks of raising their two chicks, as Daniel Lees and colleagues witnessed.

In the red-capped plover, a bird that inhabits coastal areas of Australia, parents divide the care for their young. The nest is a shallow scrape on the ground wherein the female usually lays two eggs, open and exposed. The eggs are well camouflaged thanks to their yellowish-brown colour and black spots. They have to be, otherwise they would be easily detected during daytime by visually-foraging predators, such as the little raven. The female also has a protective coloration. But the male has a bright red head to impress females and help him acquire a high-quality mate. When he would incubate the eggs, his ornamentation would disclose their presence to predators.

In order to prevent this from happening, the birds have divided the breeding duties properly, Kasun Ekanayake and colleagues showed. During the day, the inconspicuous female breeds and only at night the male will take over. The one enemy that forages in the dark, the red fox, uses olfactory cues, and for this peril, it does not matter whether the father or the mother is sitting on the nest. Many clutches fall prey to the red fox, which isn’t a native species of Australia, but has been introduced and now poses a major threat to many bird and mammal species.

young red-capped plover is vulnerable to predatorsAs soon as the young red-capped plovers have hatched, they are mobile and they have to feed themselves. One of the parents is with them to keep them warm, to warn of danger and to lead them to places with food. The chicks fledge at approximately 35 days.

During the first few weeks after hatching, the chicks, which are camouflaged, are very vulnerable to predators. In that period, it is mainly the mother who accompanies them. Later, when the chicks become able to escape from danger, the father gradually takes over the care until they are independent. So, the care for the young birds seems to be equally divided between parents.

But there still is some inequality, as Daniel Lees and colleagues point out. For the division of care between parents, it matters whether they have daughters or sons.

The mother, who takes care of the chicks during the first weeks, will decrease her contribution over time at a lower rate when both young are males; in that case she continues to invest more than she would do if she had had two daughters or a son and a daughter. And the father, who gradually takes over her job, will provide more care if both young are female.

So, both parents care more for young of the opposite sex. No difference is to be seen between male and female chicks, and the researchers needed a blood sample to be able to determine the sex of the young. Apparently, however, the birds can distinguish between sons and daughters and treat them differently.

How to explain this? The researchers suggest that the parents may provide more care to young of the opposite sex because these young will not be rivals later on, on the wedding market. Fathers will then have to compete for attractive partners with their sons and mothers with their daughters. It is an possibility that still has to be investigated.

Willy van Strien

Photos:
Large: Red-capped plover, male. ©Daniel Lees
Small: Red-capped plover, chick. Benjamint444 (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Sources:
Lees, D., C.D.H. Sherman, K. Kostoglou, L.X.L. Tan, G.S. Maguire, P. Dann & M.A. Weston, 2018. Plover parents care more for young of the opposite sex. Behavioral Ecology, online April 5. Doi: 10.1093/beheco/ary052
Ekanayake, K.B., M.A. Weston, D.G. Nimmo, G.S. Maguire, J.A. Endler & C. Küpper, 2015. The bright incubate at night: sexual dichromatism and adaptive incubation division in an open-nesting shorebird. Proceedings of the Royal Society B 282: 20143026. Doi: 10.1098/rspb.2014.3026