Conspicuous, but also undetectable

Brightly coloured frog is invisible against background

Colourful poison dart frog is invisible from a distance

In spite of its bright colouration, predators have difficulty detecting the poison dart frog Dendrobates tinctorius from a distance, according to research by James Barnett and colleagues. The colourful animal turns out to have a cryptic colouration.

The bright colour patterns of poison dart frogs function as a warning signal to predators: don’t eat me, I’m poisonous. Natural enemies learn that they’d better leave these colourful prey alone.

Yet, the striking appearance of these frogs does not offer them complete protection, as James Barnett and colleagues point out. An inexperienced predator that doesn’t yet understand the message may attack and kill such frog. Moreover, some predators are insensitive to the poison, and others are so hungry that they ignore the warning and take the risk. So, a poison dart frog needs additional protection.

It has. Additional protection is provided by the same bright and distinctive colour pattern, which appears to function, surprisingly enough, as a cryptic colour that minimizes detectability. At least, this is the case in the poison dart frog Dendrobates tinctorius.

Camouflage

At close range, the frog clearly stands out against its natural background of leaf litter on the soil of rainforests in French Guiana, the researchers show, thanks to colours that are hardly found in that background: yellow and blue. The salient colouration is a clear signal.

But from a distance, things are different. Predators no longer are able to discern the pattern and the colours blend together to form an average hue that matches the background colour. So, the colouration turns out to function as distance-dependent camouflage; it makes the frog invisible to birds, snakes and mammals that are not at very close range.

Model frogs

That may be hard to believe, but experiments show that it really is like that. The researchers made frogs of plasticine (modelling clay) and gave their models either a natural colour pattern or painted it plain yellow or brown-and-black. They assembled different backgrounds: leaf litter, paper with a leaf litter print, bare soil and paper in a homogeneous colour. In the field, they put model frogs on different backgrounds and assessed how often wild avian predators attacked these models.

As expected, background had no effect on the number of attacks on yellow models, while brown-and-black animals were safer on leaf litter or paper with a leaf litter print than on other backgrounds.

And what about the frog models with a natural colouration?

Just like the brown-and-black animals, they had the best chance to remain undetected on a leaf litter background.

The bright colour pattern of the poison dart frog Dendrobates tinctorius thus has a dual function. At close range, it is a warning signal, while the colours blend to form a cryptic colour when viewed from a distance.

Willy van Strien

Photo: Dendrobates tinctorius. ©James B. Barnett

Source:
Barnett, J.B., C. Michalis, N.E. Scott-Samuel & I.C. Cuthill, 2018. Distance-dependent defensive coloration in the poison frog Dendrobates tinctorius, Dendrobatidae. PNAS, online June 4. Doi: 10.1073/pnas.1800826115

Posted in camouflage, colour | Leave a comment

Startling

When under attack, skink exposes its entire blue tongue

Blue-tongued skink deters attack by protruding its blue tongue

As their name indicates, blue-tongued skinks possess a blue tongue. The lizards sometimes protrude it to show the striking colour. Arnaud Badiane and colleagues explain why.

The blue-tongued skinks from Australia, Indonesia and Papua New Guinea have a cryptic colour which protects them from hunting predators. But sometimes, they suddenly expose their large tongue, which catches the eye because of a striking blue colour. This behaviour seems odd, as it reveals the animals’ presence after all.

Now, Arnaud Badiane and colleagues argue that also this tongue display offers protection from predators, just because the blue colour stands out against the background. A blue-tongued skink uses its tongue as a defensive strategy at the last moment, they say, when a predator is about to strike. The sudden appearance of the blue tongue startles or overawes the enemy – offering the skink a chance to escape.

Reflexive recoil

The researchers substantiate their arguments with experiments in which individuals of the northern blue-tongued skink, Tiliqua scindoides intermedia, were approached by models of predators: a snake, a monitor lizard, a bird, a fox or, as a control, a piece of wood.

The tested skinks behaved normally until such a predatory enemy came very close. When attack was imminent, they suddenly opened their mouth widely and showed the entire tongue by sticking it out as far as possible. To a piece of wood the threatened skinks responded less strongly than to a predator model. To a bird and a fox they protruded their tongue most often, and to a fox or a snake they exposed the largest area of their tongue. In order to increase the shock effect, they inflated their body and hissed.

The back of the tongue has the most intense colouration, and the tested skunks exposed this part when an enemy was in close proximity. The blue colour is detectable to the visual system of the natural enemies.

Predators cannot learn to ignore such suddenly exposed blue flag, the biologists assume: a recoil reflex is inevitable. They still have to investigate how predators respond in reality. If they really are startled, blue-tongued skins in distress would rightly rely on their tongue as the last defence.

Willy van Strien

Photo: northern blue-tongued skink, Tiliqua scindoides intermedia. ©Shane Black

Source:
Badiane, A., P. Carazo, S.J. Price-Rees, M. Ferrando-Bernal & M.J. Whiting, 2018. Why blue tongue? A potential UV-based deimatic display in a lizard. Behavioral Ecology and Sociobiology 72: 104. Doi: 10.1007/s00265-018-2512-8

Posted in defence | Leave a comment

Small bites, huge gulps

Thousands of copepods are ingested each day by little auk

feeding in little auk is similar to that of whale

Little auks have a way to gather food that is unusual in birds, Manfred Enstipp and colleagues show. The birds feed ‘almost like a whale’, as the title of  their publication suggests.

The little auk (or dovekie), a seabird species of arctic regions which gathers its food while diving, does not have easy prey to catch. It feeds mainly on copepods (small crustaceans) and it needs an estimated 60,000 prey items per day. It is difficult to capture these tiny animals, as they respond to threats with fast movements. It’s difficult to grasp something as small like copepods under water anyway: the object is pushed away when you try. How does a little auk get all the copepods it needs?

Just by opening the beak while swimming and filtering the prey from the water, Manfred Enstipp and colleagues assumed. But when they filmed little auks with a high-speed camera in a test pool containing copepods and analysed the footage, their idea turned out to be wrong.

Whale

Little auks, as the researchers observed, search for copepods with their eyes. Upon prey detection, they go after it, stretching their neck. They extend their gular pouch so that under pressure arises in the oral cavity, and open their bill slightly. As a consequence, a big gulp of prey-laden water is sucked in. The bird then retains the prey while it expels the excess water through the nostrils and from the back of the bill. The whole procedure takes about half a second. By capturing prey in quick succession – and with some luck it sometimes has two items in one trial – a little auk gathers its food.

Many fish species use this ‘suction-feeding’ method to catch prey, but for a bird it is unusual. It is reminiscent of the way baleen whales feed: they take in large quantities of water and expel it through the baleens, retaining the plankton.

Willy van Strien

Photo: Allan Hopkins (via Flickr, Creative Commons CC BY-NC-ND 2.0)

Source:
Enstipp, M.R., S. Descamps, J. Fort & D. David Grémillet, 2018. Almost like a whale – First evidence of suction-feeding in a seabird. Journal of Experimental Biology, online May 29 mei. Doi: 10.1242/jeb.182170 

Posted in feeding, predation | Comments Off on Small bites, huge gulps

Collateral benefit

Bird disperses eggs of stick insects it swallowed

brown-eared bulbul disperses eggs of stick insects

Some stick insects are even more like plants than you might think at first glance. Just like plant seeds, the eggs can be dispersed by a bird, Kenji Suetsugu and colleagues show.

Stick insects are perfectly camouflaged: they do not stand out among the plants. Yet insect-eating birds are able to find them and will eat them. And that is the end of the story for such tiny animal.

Well, it may not be, Kenji Suetsugu and colleagues report. If an unfortunate female stick insect is carrying mature eggs, a few of these appear undamaged in the bird’s droppings, and some may even hatch.

Younsters

The researchers, working in Japan, point out that the eggs of stick insects resemble plant seeds: they have the same size and colour and feel the same thanks to a hard shell. Hence their suggestion that the eggs might survive passage through a bird’s digestive tract like plant seeds do. Many plant species produce fruits that are eaten by birds or other animals; the seeds remain intact, are excreted and germinate. Is something similar possible for the eggs of stick insects?

eggs of stick insect after passage through a bird's digestive tractTo find out, they mixed mature eggs of three stick insect species with an artificial diet and fed this to a brown-ear bulbul, one of the main predators of the insects. Afterwards, they examined the bird’s droppings under a stereomicroscope and discovered a small number of intact eggs, and from some of these eggs a young stick insect hatched later on.

Such scenario is also possible when a bird swallowed a gravid female, the authors think. The youngsters that hatch after passage through a bird’s guts would have to find an appropriate food plant to live on, but that is always the case. Normally, a female just drops her eggs to the ground and does not provide any care.

Parthenogenetic

young stick insect, hatched from egg that passed through a bird's gutsSo, sick insects not only look like plants, but they also exhibit a surprising plant-like trait: dispersal of offspring by birds, which is unique in insects.

Dispersal by an avian predator is only possible for species that reproduce parthenogenetically, for in that case females carry eggs that can develop without fertilization. A number of stick insect species exhibit parthenogenesis, including the species that were studied here.

Flyways

Dispersal of insect eggs via a bird’s digestive tract is not entirely comparable to dispersal of plant seeds. Plants produce fruits that have to be eaten to disperse their seeds. In contrast, a female stick insect has no intention to be captured by a bird to have her eggs transported – by being camouflaged, she tries to prevent just that. But if she is unlucky enough to become a bird’s meal, it is a collateral benefit if some eggs survive and young hatch, if only a few.

The hard eggs probably have not evolved to facilitate avian dispersal, the authors suggest, but to decrease the risk of attack by parasitoid wasps, which lay their eggs in other insects’ eggs.

Stick insects are immobile. Thanks to the birds they may reach new places to live. An interesting question is whether distribution patterns in the insects, to be unravelled by DNA research, overlap with birds’ flyways; that would strengthen the idea that the eggs are sometimes dispersed like plant seeds.

Willy van Strien

Photos
Large: Brown-eared bulbul (tongue visible). Alpsdake (Wikimedia Commons, Creative Commons BY-SA 4.0)
Small: stick insect (Ramulus irregulariterdentatus) eggs that passed through a bird’s digestive tract and a young stick insect that hatched from such egg. ©Kenji Suetsugu

Source:
Suetsugu, K., S. Funaki, A. Takahashi, K. Ito & T. Yokoyama, 2018. Potential role of bird predation in the dispersal of otherwise flightless stick insects. Ecology, online May 29. Doi: 10.002/ecy.2230

Posted in growth and development, predation | Comments Off on Collateral benefit

Cuckoo catfish in search of foster parents

Cichlid mothers see through the deceit, but pay a high price

cuckoo catfish parasitizes on cichlid species

The cuckoo catfish tries to dump its eggs among those of fish of the cichlid family. Cichlids usually avoid being used as foster parents, Radim Blažek and colleagues report, but by becoming too cautious, they often reject their own eggs as well.

Just as the cuckoo lays its eggs in the nests of songbirds, manipulating them into raising their young, the cuckoo catfish transfers the care for its offspring to other fish. Several species of the cichlid family are the involuntary foster parents of this ‘underwater cuckoo’. Cichlid mothers breed their eggs in the buccal cavity to protect them from predators. They also carry the newly hatched young in their mouths, and when the fry are able to swim around freely, they continue to pick them up in case of danger during the first few weeks.

Bad ending

The cuckoo catfish (Synodontis multipunctatus) exploits the dedicated behaviour of these mouthbreeders and approaches them while they are spawning. The female of a mating couple repeatedly lays a few eggs and collects them quickly in her mouth. She then picks up sperm from the male to fertilize the eggs in her mouth.

When a group of cuckoo catfish disturbs such spawning pair, the cichlids will chase off the invaders. Still, the catfish often manage to eat some cichlid eggs and place a number of fertilized eggs of their own among the cichlid eggs. The cichlid mother will gather also the catfish eggs in her mouth.

Then, things can go wrong for her own brood. The eggs of the cuckoo catfish are smaller than the eggs of the cichlid mother and hatch earlier, and the young catfish will devour their stepsiblings. Later, when the young parasites are released, the host mother will continue to protect them as if they were her own offspring.

Defence

Now, Radim Blažek and colleagues show that the cichlids are not entirely defenceless against the brood parasite. The cuckoo catfish only occurs in Lake Tanganyika in Central Africa, and the many cichlid species that inhabit the lake have been dealing with this enemy for millions of years. The researchers investigated how one of them, Simochromis diagramma, responds to the behaviour of the cuckoo catfish. They compared its response with the behaviour of cichlids from other African lakes, which have never been in contact with the underwater cuckoo.

They maintained different cichlid species separately in aquariums with a group of catfish and first assessed how often the catfish succeeds in imposing its eggs to the cichlids.

When housed with Simochromis diagramma, the cichlid from Lake Tanganyika, the cuckoo catfish was much less successful than with cichlids from other lakes. Only 5 percent of the Tanganyika females were found to have parasite’s eggs in their mouths 12 hours after spawning. Apparently, because of the shared evolutionary history, this cichlid species has learned to resist the brood parasite.

Rejection

To find out how Tanganyika cichlids deal with the cuckoo catfish in more detail, the biologists then artificially infected breeding cichlid females with parasites by injecting six fertilized eggs of the cuckoo catfish into their mouths. They used two cichlid species: Simochromis diagramma from Lake Tanganyika and Haplochromis aeneocolor from Lake George in Uganda. Next day, they determined whether the females had retained the foreign eggs. Also, in additional experiments, they assessed how often artificially parasitized females eventually released young cuckoo catfish and / or their own young.

The cichlid from Lake George was easily misled: only 7 percent of the females rejected the introduced parasite’s eggs instead of retaining them. As a consequence, the eggs of cuckoo catfish had a high survival chance: 86 percent developed successfully.

The Tanganyika cichlid, however, was not fooled into raising foreign young: nearly all females (90 percent) rejected foreign eggs and only 13 percent of the cuckoo catfish eggs survived. In those cases where the cuckoo catfish eggs hatched, individual experience reduced the damage. Cichlid females that had already given birth to young catfish earlier now managed to save some of their own young.

High price

The cichlids from Lake Tanganyika have learned to cope with the parasite and to see through their deceit. They rarely pick up foreign eggs. Nevertheless, the presence of the cuckoo catfish lowers their reproductive success because, if these cichlids detect foreign eggs, they become so choosy that they also reject some of their own eggs, as the experiments showed. So, they pay a high price to keep the parasite out.

There are about one hundred species of brood parasites among birds. In fish, the cuckoo catfish is the only one known to display cuckoo behaviour.

Willy van Strien

Photo: Cuckoo catfish Synodontis multipunctatus. ©Institute of Vertebrate Biology, Brno (Czech Republic)

See a short video documentary of National Geographic on cuckoo catfish

Source:
Blažek, R., M, Polačik, C. Smith, M. Honza, A. Meyer & M. Reichard, 2018. Success of cuckoo catfish brood parasitism reflects coevolutionary history and individual experience of their cichlid hosts. Science Advances 4: eaar4380. Doi: 10.1126/sciadv.aar4380

Posted in parasitism | Comments Off on Cuckoo catfish in search of foster parents

Protected and aggressive mimics

Young false cleanerfish relies on disguise to bite other fishes

False cleanerfish resembles bluestreak cleaner wrasse

It is beneficial to look like cleanerfish, Misaki Fujisawa and colleagues show. By mimicking cleanerfish, false cleanerfish prevent attack by predatory fish. Also, they can approach other fishes and take a bite – but only small individuals show such behaviour.

The bluestreak cleaner wrasse, Labroides dimidiatus, lives on coral reefs where it offers cleaning services: the cleanerfish removes blood sucking parasites from the skin of other fishes. There is also a pretender, the false cleanerfish Aspidontus taeniatus. It has the same appearance as the cleaner wrasse, but it does not clean other fishes.

Misaki Fujisawa and colleagues wondered why it resembles the bluestreak cleaner wrasse. The disguise, or mimicry, helps to escape from predatory fish, which don’t consume useful cleaner fish and don’t detect the deceit. So there is a protective function. But maybe the mimicry has a second benefit. False cleaners may be able to approach fishes undisturbed in order to attack them. In that case, the disguise would also enable aggressive behaviour.

To find out, the researchers observed the behaviour of the false cleaners in the coral reefs around Sesoko Island in Japan.

Biting fins

The false cleanerfish mainly feeds on benthic animals; it bites off pieces of the tentacles of tubeworms or soft parts of the boring clam. In addition, it bites the fins of other fishes and steals eggs from the nests of damselfish (Pomacentridae).

The resemblance to the bluestreak cleaner wrasse is profitable for fin biting, as it gives the false cleaner a chance to approach unsuspecting victims, even if it behaves a little differently. Whereas a cleaner wrasse invites clients for a cleaning service by performing a zigzag dance, a false cleaner does not: it usually approaches its victims from behind, makes a sudden dart and bites off a piece of the caudal fin. So, the disguise indeed seems to have a second function, next to the protection that it offers the false cleaners against predatory fish.

Less dangerous

But only small individuals rely on their disguise to aggressively approach other fishes, as Fujisawa shows. As the false cleaners grow bigger, they continue feeding on benthic animals, but in addition to exploiting this main food source, they gradually switch from fin biting to egg predation. To raid nests, they often form small groups, and then the mimicry loses its effect, as cleanerfish always operate solitary or with their mate.

The false cleaners have good reason to change feeding tactics, as fish eggs are more nutritious than pieces of fin. But for small false cleaners it is difficult and risky to raid nests, because the eggs are guarded by damselfish parents, which will attack each enemy fiercely and without hesitation. Large false cleaners can elude attacks by swimming fast, but for small fish this is too dangerous. Biting fins is more feasible to them – thanks to their innocent appearance.

Willy van Strien

Photo: Two adult cleaners (middle and right) and an adult mimic (left) at Sesoko Island, Okinawa, Japan. ©Misaki Fujisawa

Sources:
Fujisawa, M., Y. Sakai & T. Kuwamura, 2018. Aggressive mimicry of the cleaner wrasse by Aspidontus taeniatus functions mainly for small blennies. Ethology, 19 april online. Doi: 10.1111/eth.12743
Cheney, K.L., A.S.Grutter & R. Bshary, 2014. Geographical variation in the benefits obtained by a coral reef fish mimic. Animal Behaviour 88: 85-90. Doi: 10.1016/j.anbehav.2013.11.006
Kuwamura, T., 1983. Reexamination on the aggressive mimicry of the cleaner wrasse Labroides dimidiatus by the blenny Aspidontus taeniatus (Pisces; Perciformes). Journal of Ethology 1: 22-33. Doi: 10.1007/BF02347828

Posted in mimicry | Comments Off on Protected and aggressive mimics

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

Posted in parental care | Comments Off on Why dad leaves his family

Cooling down

Blowfly blows bubbles to prevent overheating

blowfly cools down by bubbling behaviuour

 

A blowfly often extrudes a liquid bubble between its mouth parts and then takes it back. By exhibiting this bubbling behaviour, it gets rid of excess heat, Guilherme Gomes and colleagues show.

How can a buzzing blowfly avoid getting overheated? Few people will ever have wondered, but as it happens, Guilherme Gomes and colleagues did. And they discovered that the oriental latrine blowfly Chrysomya megacephala manages to lower its body temperature by blowing a bubble.

At high air temperatures, a blowfly can expel a drop of liquid out of its oral cavity and hold it with its mouthparts. As some liquid evaporates, the droplet will rapidly cool down, whereupon the fly will take it in. The cycle is often repeated, and a droplet may tidally move out and back up to six times until eventually the fly swallows it and its body temperature decreases. The liquid is a complex mix of fluids derived from ingested meals and saliva.

Daytime and night-time

The blowfly applies this trick during the day when ambient temperature exceeds 25 °C. At that temperature, the animal is warmed-up and busy, so that its muscles produce a lot of heat, which makes cooling necessary. When it gets really hot, above 30 °C, the fly becomes inactive and no longer generates heat. It then stops blowing bubbles.

At night, it bubbles more than during the day to stay cool, in order to decrease its metabolism and save energy.

If air humidity is high, the liquid will not evaporate well and a bubble will not cool down. If the fly still expels a drop, it will not re-ingest it, but spit it out instead.

Small animals only

Cooling down by extruding a droplet is only feasible in small animals, and a number of insect species seem to exhibit such behaviour. For larger animals, it is impossible to produce and handle a liquid drop that is large enough for a cooling effect. To us, it would make no sense to trying it – fortunately. We cool down by sweating, which is impossible to an insect because of its chitinous exoskeleton and wax covering.

Willy van Strien

Photo: Blowfly Chrysomya megacephala. gbohne (via Flickr, Creative Commons CC BY-SA 2.0)

Source:
Gomes, G., R. Köberle, C.J. von Zuben & D. V. Andrade, 2018. Droplet bubbling evaporatively cools a blowfly. Scientific Reports 8: 5464. Doi: 10.1038 / s41598-018-23670-2

Posted in warm, cold | Comments Off on Cooling down

Help when needed

Crab spider makes itself useful on infested flower

crab spider hunts prey on flowers

Lying in ambush on a flower, a crab spider will grab every visitor and eat it. Its victim may be a useful guest, such as a bee, as well as a harmful one. In buckler mustard, the presence of a crab spider turns out to be beneficial if flowers are infested by caterpillars, Anina Knauer and colleagues show.

Crab spiders have an effective way to acquire food. They reside in a flower, usually inconspicuously as their colour matches that of the flower, and wait for visitors to arrive. They grab them with the two pairs of large legs to which their name refers, kill them with a poisonous bite and eat them. They can handle prey that is much larger than they are. It is a disadvantage for a plant when such a spider settles on a flower, you would guess, for many flower visitors that they hunt are useful visitors, such as bees that pollinate the plant to enable it to set seed; it would be a disaster if those pollinators could not do their job.

But Anina Knauer and colleagues show that the presence of a crab spider can be a blessing. That is because a flower also gets visitors with bad intentions, and a resident crab spider can eliminate them. Therefore, they discovered, a flower will attract crab spiders in case of unwelcome visitors.

buckler mustard attracts crab spider if infested by caterpillarsThe researchers investigated how the presence of the crab spider Thomisus onustus affects the fitness of the plant it usually occurs on, buckler mustard (Biscutella laevigata), an alpine herb with yellow flowers and fruits that look like spectacles. The plant interacts with several insect species that are potential prey for the spider. The scent of the flowers attracts bees that take care for pollination in exchange for nectar. But the seed setting often fails because the flowers are consumed by caterpillars of the diamondback moth (Plutella xylostella). What happens when a crab spider is present?

The researchers conducted experiments in which they placed three caterpillars on flowers of plants with or without a crab spider every morning and counted the caterpillars in the evening. On plants with a crab spider, most of the caterpillars disappeared – apparently, they were eaten by the spider -, and after four weeks, as a consequence, those plants had suffered much less damage than plants without a spider and developed seeds normally. The crab spider rescued the flowers.

In the field, the researchers also found, plants call the voracious spider for help when the flowers are infested. This call is chemical: infested flowers emit increased amounts of one of the scent compounds, beta-ocimene. The crab spider is attracted by that compound and will settle on such flowers. Indeed, a larger proportion of plants with caterpillars is occupied by a crab spider compared to plants without a spider. So, plant and spider have a mutualistic relationship: an infested plant asks for help and receives it, while the spider that comes to the rescue gets a meal.

But what about the bees, which are the most important pollinators? Aren’t they in danger when a spider is present? They hardly are, as it turns out. They usually detect the presence of a spider on a flower, despite its camouflage, and avoid a visit, and the spider almost exclusively feeds on caterpillars. Still, despite the reduced visit rates of bees, the flowers set seed. Apparently, there is no lack of pollen. The presence of the spider therefore turns out to be beneficial for plants that are infested by caterpillars.

High in the mountains Thomisus onustus does not occur, while buckler mustard does. Upon attack by caterpillars, plants of highland populations increase the amount of beta-ocimene to a much less extent than plants of lowland populations.

Willy van Strien

Photos:
Large: Thomisus onustus (not on buckler mustard). Paco Gómez (via Flickr, Creative Commons CC BY-SA 2.0)
Small: buckler mustard. Isidre blanc (Wikimedia Commons, Creative Commons CC BY-SA 4.0)

Source:
Knauer, A.C., M. Bakhtiari & F.P. Schiestl, 2018. Crab spiders impact floral-signal evolution indirectly through removal of florivores. Nature Communications 9: 1367. Doi: 10.1038/s41467-018-03792-x

Posted in mutualism | Comments Off on Help when needed

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

Posted in parental care | Comments Off on Inequality