Glow in the dark

Flashlight fish turns headlights on to catch prey

flashlight fish turns headlights on to find prey

It is difficult to find food in the dark. But the splitfin flashlight fish Anomalops katoptron has no problem: it turns its headlights on when it hunts on zooplankton, as Jens Hellinger and colleagues report.

Only in complete darkness, the splitfin flashlight fish Anomalops katoptron will leave its hiding place. During daytime the fish, which lives in shallow coral reefs in the Pacific, resides in cavities and cracks in the reef where it is invisible to its predators, thanks to its dark colour. But in dark moonless nights it ventures to the open water to forage in a school of conspecifics. The diet consists of swimming zooplankton, prey that is difficult to find in the dark.

But Anomalops katoptron has a light organ under each eye that emits blue light, Jens Hellinger and colleagues point out. The light is produced by symbiotic bacteria that live densely packed within these organs. The bacteria have got a safe place to live in, in exchange for producing light.

Blinking

The bacteria glow continuously, but the fish can turn his lights off by rotating them, exposing their dark backsides instead of the transparent sides. During the day, the lights are almost always off, otherwise the fish would be visible in spite of its dark colour. Occasionally, he blinks.

When the splitfin flashlight fish is active, at night, he blinks more often, Hellinger observed when he studied a number of fish in a tank in the laboratory, and the lights shine about half of the time. And if the fish detects prey, it has its lights on almost continuously.

Many animal species exist that emit light, particularly in the sea, and their luminescence has several functions. Most luminescent species emit light to chase off or embarrass predators. Anglerfish lure prey: their dorsal fin is modified to a ‘fishing rod’ with a luminous bulb that attracts little creatures. And still others lure or recognize partners by flashing patterns; male ostracods, for instance, perform a spectacular light show to attract females, much like fireflies do on land.

Until now, it was not clear where the splitfin flashlight fish Anomalops katoptron uses its light for. It now turns out that it is mainly to detect prey in the dark.

Photo: California Academy of Sciences (via Flickr. Creative Commons CC BY-NC-ND 2.0)

Source:
Hellinger, J., P. Jägers, M. Donner, F. Sutt, M.D. Mark, B. Senen, R. Tollrian & S. Herlitze, 2017. The flashlight fish Anomalops katoptron uses bioluminescent light to detect prey in the dark. PLoS ONE 12: e0170489. Doi:10.1371/journal.pone.0170489

The trick of a snake

Puff adder sticks out its tongue to lure a frog

puff adder extends its tongue to lure a frog

A South African frog that perceives and approaches a tasty worm may be deceived. The worm may turn out to be the tongue of a snake, as Xavier Glaudas and Graham Alexander write, and if it is, the frog is in trouble.

The venomous puff adder (Bitis arietans), which lives in South Africa, hunts its prey by lying in ambush. Mostly nocturnal, camouflaged and hidden in the vegetation, it waits unobtrusively until a victim comes close, and then it strikes. But the striking range is only ten centimetres, so a prey often will stay out of reach.

Luring prey

But the snake uses a trick, Xavier Glaudas and Graham Alexander noticed when they reviewed  a large amount of video recordings they had made of puff adders in ambush in the field. A puff adder often lures a prey by extending and moving the black tongue, the two points spread. The tongue then looks like a squirming worm and apparently, a frog is easily deceived. It hops closer to inspect the snack, and as soon as it comes within striking range, the snake will try to seize it. The frog that thought to find a meal is eaten himself.

Is the puff adder really mimicking a worm by extending the tongue to lure prey? According to the researchers, it does. The snake only extends its tongue if there is a frog or a toad close by, they argue; it doesn’t upon perceiving the presence of other prey, such as a mouse that doesn’t eat worms. Also, snakes use their tongues to sample odours, but chemosensory tongue flicks only take half a second while ‘lingual luring’ bouts take much more time.

The puff adders also wave their tails, and according to Glauda and Alexander that behaviour is also performed to lure prey. But they don’t have any recordings to show this, because their camera had been focussed on the heads of the animals.

Willy van Strien

Photo: Joachim S. Müller (via Flickr, Creative Commons CC BY-NC-SA 2.0

Xavier Glaudas explains his research

Source:
Glaudas, X. & G. J. Alexander, 2017. A lure at both ends: aggressive visual mimicry signals and prey-specific luring behaviour in an ambush-foraging snake. Behavioral Ecology and Sociobiology 71:2. Doi: 10.1007/s00265-016-2244-6

Selfknowledge in red-backed fairy-wrens

Only old and bright males seek extra-pair mates

in red-backed fairy-wrens old and bright males seek extra-pair matings

Red-backed fairy-wren males know how to behave to maximize their fitness, Denélle Dowling and Michael Dowling show. A male that has a bright breeding plumage invests in courting extra-pair females, whereas a man with a dull appearance invests in mate guarding.

Like most songbirds, red-backed fairy-wrens, which live in Australia, form socially monogamous pairs, and a couple may stay together for years. This doesn’t mean that the birds are faithful, however: roughly half of the young is not sired by the social father. Adultery is the rule.

In the breeding season, a male may adopt one of two alternative strategies. He can either invest in seeking extra-pair copulations to gain extra-pair offspring in addition to within-pair offspring, or he can stay on his own territory to defend it the against other couples together with his mate, to help provision the young – and to keep other males away from his mate to minimize the risk of being cuckolded.

Preference

What strategy is the best strategy? Jenélle Dowling and Michael Webster argued that the answer differs among males. It just depends on how attractive a male is to other females.

And the males differ greatly in attractiveness. Some have a bright, black and red plumage, while others have a dull, brownish plumage, much like a female. Almost all males aged more than two years are brightly coloured, among young males about half is bright. It was already known that females prefer bright males. Also, they prefer old males, as their age indicates that they are of good quality.

So, old black-red males are the most attractive ones. The best strategy for them will be to foray to neighbouring territories and court other females, Dowling and Webster assumed, as they stand a real chance to succeed. For dull males, on the contrary, it will be better to stay with their partner, as other females will be reluctant to copulate with them. Moreover, a dull male runs a high risk of being cuckolded by his social partner whenever an attractive male approaches her when she is alone. Young black-red males can try to gain extra-pair copulations, but they will be less likely to succeed than old males.

The researchers set up an investigation to find out whether red-backed fairy-wren males act in their best interests. And it turns out that they do. Old black-red males frequently leave to search for extra-pair females, while brown males mostly remain on their territory. Young black-red males adopt an intermediate strategy.

Cuckolded

DNA analyses of parents and young birds revealed that black-red males (young and old) sire more extra-pair young, as expected, but less within-pair young than dull males; apparently, bright males are cuckolded more often.

The latter result is not undisputed. In other studies, including that of Jordan Karubian, brown males were found to have less young than black-red males and to be cuckolded more frequently, even though they guarded their mate closely. But still, the best strategy for them is to stay on their territory. Otherwise, they will probably be cuckolded even more.

Willy van Strien

Photo: Red-backed fairy-wren, bright male. Jim Bendon (Wikimedia Commons, Creative Commons CC BY-SA 2.0)

Sources:
Dowling, J. & M.S. Webster, 2017. Working with what you’ve got: unattractive males show greater mateguarding effort in a duetting songbird. Biology Letters 13: 20160682. Doi: 10.1098/rsbl.2016.0682
Karubian, J., 2002. Costs and benefits of variable breeding plumage in the red-backed fairy-wren. Evolution, 56: 1673-1682. Doi: 10.1111/j.0014-3820.2002.tb01479.x

Good friend

Sea anemone grows better with a shy clownfish around

shy clownfish is better partner

The strength of the mutualistic interactions between sea anemones and clownfish depends on the personality of the fish, Philip Schmiege and colleagues report. A shy, cautious partner benefits a sea anemone more than a bold, venturous fish.

Clownfish (also called anemonefish) are safe between the tentacles of sea anemones. The anemones, animals related to jellyfish, hold off the clownfish’s predators with their stinging, toxic nematocysts. The clownfish are insensitive to these cells. Conversely, clownfish chase and bite guests who intend to nibble at the tentacles with which the sea anemones gather their food. So, sea anemones and clownfish are partners that protect each other.

Fertilize and refresh

But the fish deliver extra services. Many sea anemones harbour unicellular microorganisms that, like plants, are able to capture sunlight and to use it to convert carbon dioxide into carbohydrates. They feed these carbohydrates to the anemones in exchange for residence. The microorganisms derive their nutrients from waste products of the clownfish. Moreover, by moving, the fish refresh the water around the sea anemones continuously, guaranteeing the availability of oxygen. By fertilizing the unicellular inhabitants and refreshing the water, clownfish promote the growth of sea anemones.

Now, clownfish, like many other animal species, have personalities. There are bold, venturous individuals as well as shy, passive ones. It matters to the sea anemones what personality the fishes have that associate with them, Philip Schmiege and colleagues assumed. And they proved to be right.

The researchers brought some wild-caught orange clownfish (or clown anemonefish, Amphiprion percula) into the lab, a species living along coasts of Australia, Asia and Japan. Also, they had grown bubble-tip anemones (Entacmaea quadricolor). Though this is not a natural partner of the fish, they associate readily in the lab.

In each of sixty tanks, the researchers placed one anemone and one or two fish; in the field, zero to six fish associate with one anemone. They measured the size of the anemones and kept track of their growth. Also, they examined how bold or shy each fish was by videotaping its activities every day during twenty minutes and assessing from the footage if he ventured away from the sea anemone. The more time a fish spent away from his partner, the bolder its personality.

Shy fish

After eighteen months, the biologists noted a difference in the growth of the sea anemones. Anemones associated with a shy fish had grown better than anemones with a bold partner. Apparently, shy fish supply better services. Because they remain in close proximity to their host anemone, they fertilize its unicellular inhabitants better and refresh the water more efficiently. And perhaps the anemones dare to expand their tentacles for longer periods of time to catch food as long as there is a clownfish nearby.

In conclusion: the strength of mutualistic relationships, of which the association between sea anemones and clownfish is an iconic example, depends on the personality of the partners.

Willy van Strien

Photo: Orange clownfish © Philip Schmiege

Source:
Schmiege, P.F.P., C.C. D’Aloia, P.M. Buston, 2017. Anemonefish personalities influence the strength of mutualistic interactions with host sea anemones. Marine Biology 164: 24. Doi: 10.1007/s00227-016-3053-1

Prepared to have a meal

Even a sedentary antlion has a capacity for learning

Antlion larva builds a pitfall

All that an antlion larva has to do once he has made his pitfall, is sit there and wait for a prey to come. Over the weeks, he learns to anticipate the arrival of a prey, Karen Hollis discovered.

Antlion larvae need a lot of food, consisting of little critters. They don’t go after their prey, but they take what comes along. While some species wait in ambush for their food, others build traps: a larva of such a species digs a funnel-shaped pit with steep walls in loose sandy soil and buries itself at the vertex, only head and jaws remaining visible. Tiny animals that wander along the edge lose their footing and tumble into the pit, from which it is difficult to escape. And when a nearby prey fails to fall in, the antlion larva tosses sand to him, so that the victim is disoriented, stumbles and comes down in a sand avalanche.

Vibrational signal

Once an antlion has dug his pit – which is a big job -, he only needs to wait until a prey is trapped. That’s all, and yet such a buried larva is learning something, as Karen Hollis reports.

adult antlion is a winged insectWorldwide, there are a few thousand species of antlions, many of them with larvae that dig a pit on a sheltered place, for instance under overhanging branches. Ants are a common prey. Adult antlions are graceful, winged insects.

Hollis and colleagues show that larvae learn to perceive when a prey is approaching. They brought a number of larvae into the laboratory and housed each of them in its own sand-filled plastic bowl. Half of the larvae received a prey item each day at a randomly determined time, always a few seconds after the researchers had dropped some sand grains beside their pits. This was an imitation of the natural situation: an animal that approaches a pit, causes a similar vibrational signal. The other half were presented with a daily prey item at the same time as the first group, but received a vibrational cue at a different, randomly selected time. The larvae were treated as described until they pupated.

Prepared

If a victim falls into the pit, an antlion larva will pick it up, drag it under the sand, bite and deliver an immobilizing poison and digestive enzymes. He then sucks the liquefied prey contents and throws the empty exoskeleton out. If it is necessary, he will repair his trap.

Antlions that used to get their prey each day after a vibrational cue, began to prepare themselves after receiving this cue, the experiments revealed. They responded faster than the untrained larvae when a prey arrived and extracted the contents of the prey at a higher rate and more efficiently, probably because they started to produce digestive enzymes earlier. Apparently, they had learned to associate the vibrational cue with the gain of a prey, in contrast to the other group.

Other researchers, Karolina Kuszewska and colleagues, reported that antlion larvae can learn to distinguish between large and small prey, as large prey causes stronger vibrations. Antlions abandoned a small prey when they noticed that a large one was approaching.

Faster

Because antlions learn to anticipate the capture of a prey, they are able to handle it efficiently, the authors conclude. This is advantageous: in the laboratory, the larvae that learned to associate the vibrational cue with prey grew faster, were bigger and pupated sooner than the larvae that had not been given the opportunity to learn. Trained larvae thus shorten the larval stage, which is the most vulnerable stage of their life cycle because larvae are exposed to wind and rain and accessible to predators. Moreover, the more food a female larva consumes, the bigger and stronger the eggs she will produce later as adult.

So, even an animal that captures prey with a sit-and-wait strategy proves to be able to improve this strategy by learning.

Willy van Strien

Photos:
Large: an antlion larva, probably Myrmeleon formicarius. Aiwok (Wikimedia Commons, Creative Commons CC BY-SA 3.0)
Small: adult Myrmeleon formicarius. Gilles San Martin (Wikimedia Commons, Creative Commons CC BY-SA 2.0)

Watch a video of an antlion and its pitfal

Sources:
Hollis, K.L., 2016. Ants and antlions: The impact of ecology, coevolution and learning on an insect predator-prey relationship. Behavioural processes, online December 6. Doi: 10.1016/j.beproc.2016.12.002
Kuszewska, K., K. Miler, M. Filipiak & M. Woyciechowski, 2016. Sedentary antlion larvae (Neuroptera: Myrmeleontidae) use vibrational cues to modify their foraging strategies. Animal Cogntion 19: 1037-1041. Doi: 10.1007/s10071-016-1000-7
Hollis, K.L., F.A. Harrsch & E. Nowbahari, 2015. Ants vs. antlions: An insect model for studying the role of learned and hard-wired behavior in coevolution. Learning and Motivation .50: 68-82. Doi: 10.1016/j.lmot.2014.11.003

Sprouted food

Common ragworm caches seeds and consumes the seedlings

The common ragworm sprouts seeds

Common ragworms bury seeds of cordgrass for future use: when the seeds have germinated, the worms eat the sprouts. Sprouting seeds is a newly discovered gardening strategy in animals, as Zhenchang Zhu and colleagues point out.

The seeds of cordgrass, Spartina species, are protected by husks that make them inedible for the common ragworm, Hediste diversicolor. Yet these worms take the trouble to drag the large seeds to their burrows and pull them inside. This is deliberate behaviour, according to Zhenchang Zhu and colleagues. The buried seeds will germinate and the worms can eat the sprouts. In fact, the seeds are a nutritious dietary supplement for the ragworms, which feed mainly on low-quality sedimentary organic matter, because they are high in protein and vitamins.

Waiting

Common ragworms live in the seabed of intertidal flats. Each animal inhabits a self-made burrow in the sand or the mud.The species is native to the north-east Atlantic.

In experiments, the researchers noticed that the ragworms wouldn’t eat intact cordgrass seeds. But if sprouted seeds were offered, they did eat them. Experiments also showed that the worms grew much better when given a diet including cordgrass sprouts than on a diet without sprouts.

It is striking that they cache the seeds, because that behaviour pays off only in the long term. The cordgrass produces seeds from October to March, and these seeds will germinate from April until July. So, the worms have to wait a few weeks or months before their stored food supply will be usable.

Seed dispersal

Sprouting seeds to consume the seedlings is a form of agriculture. More examples of agriculture in animals exist, such as the well-known fungus gardens of ants and termites. The sprouting strategy of common ragworms, however, differs from fungus gardening. While the fungus is the main food source for the ants and termites, the sprouts are a superior supplementary food for the ragworms: superfood instead of staple food.

Also, a mutual relationship exists between termites or ants and the fungus they grow: the animals are dependent on their crop, but the fungus is also dependent on its growers. Common ragworms and cordgrass, in contrast, have a predatory relationship, as the cached seeds are eaten after germination. The ragworms may help in seed dispersal, though. Buried seeds will not be displaced by water currents, retain their viability and can produce new plants when the ragworm that cached them dies or is eaten itself. This often happens, as common ragworms have many predators: birds like avocets and curlews, and fish like plaice and sole.

The authors suggest that common ragworms and their relatives may bury seeds of plants like seagrass and glasswort to sprout them as well.

Willy van Strien

Photo: Common ragworm. © Jim van Belzen

Source:
Zhu, Z., J. van Belzen, T. Hong, T. Kunihiro, T. Ysebaert, P.M.J. Herman & T.J. Bouma, 2016. Sprouting as a gardening strategy to obtain superior supplementary food: evidence from a seed-caching marine worm. Ecology 97: 3278-3284. Doi: 10.1002/ecy.1613

Mini garden

Some arboreal ants grow useful plants

A Squamellaria major plant on macaranga, grown by ants

Gardening is an art – and there are ants that master this art. On the branches of trees they cultivate plants to live in or to strengthen their nests, as research teams of Guillaume Chomicki and Jonas Morales-Linares report.

Many ants and plants are partners in a mutualism: the plants provide the ants with a place to live or with nectar, and the ants deposit their droppings as fertilizer or protect the plants from herbivorous insects. Some tropical arboreal ants go a step further and cultivate the plants they live with. As these plants grow upon tree branches (they are epiphytes), it is more difficult for them to obtain nutrients than it is for plants that root in the soil, so the ant-plant mutualism is a good strategy. Many of the ant-grown plants are completely domesticated and would perish without the ants.

Seed collection

Philidris nagasau, native to Fiji, inhabits the hollow stems of Squamellaria species, bulb-shaped plants that grow on trees. The ants live nowhere else, and six Squamellaria species are always inhabited by these residents. The ant fertilizes the plants, as Guillaume Chomicki and colleagues had previously shown.

workers of Philidris nagasau inspect seedlings of SquamellariaNow, they discovered that the ant makes sure that plants are available by farming them. The researchers observed ant workers collecting exclusively the seeds of these six Squamellaria-species, and not those of any other species. They take them out of the unripe fruits, insert them in fissures and cracks in the bark of a tree and patrol the planting sites. Soon after, the seeds germinate and seedlings appear on the tree, and as soon as they form a cavity, a few ants will enter it, likely to leave their droppings. By doing so, they grow the plants they need to live in.

So, this ant-plant mutualism is more intimate than previously thought. The plants need the ant partner not only for nutrition, but also for seed dispersal.

Beautiful flowers

A different kind of plant nurseries can be found in Central and South America: conspicuous little gardens that hang from some trees. They are the overgrown carton nests of certain ant species. The ants collect seeds of epiphytes and insert them in the walls of their nest, where of the seeds germinate and grow up. The plant roots strengthen the nest and take up water when it rains, so that the nests don’t disintegrate. In return, the ants fertilize the plants and protect them against herbivorous insects. Some plants are exclusively dispersed by the ants and only germinate in an ant nest.

Hanging garden of Azreca gnavaAzteca gnava from southern Mexico and Panama is such a gardening ant. His gardens are frequently found in plantations, as Jonas Morales-Linares and colleagues report, mostly on cocoa, mango, sapote and orange trees. The gardens contain twelve plants on average, typically of two or three different species. Two plant species that cannot live outside these gardens are the bromeliad Aechmea tillandsioides and the orchid Coryanthes picturata. Apparently, the gardening ants have a good taste, for these plants have beautiful flowers.

Three million years

The ant Camponotus femoratus of the Amazonian lowland forest plants similar gardens. Mutualism is obligate for the plant Peperomia macrostachya, that only lives in the nests of this ant. Elsa Youngsteadt and colleagues showed that Camponotus femoratus is the only ant species to collect the seeds of this plant. The ant takes them from the plant, from the soil or from the feces of birds and mammals that have eaten the fruits. Probably, the seeds emit volatiles that only only Camponotus femoratus appreciates. The ant inserts many Peperomia seeds in the walls of its nest. Each seed has only a small chance to germinate there, but the seeds that are not brought into the ant’s nests have no chance to sprout at all.

According to Chomicki, Philidris nagasau in Fiji descends from ancestors that, just like their American colleagues, made carton nests in trees and planted seeds in the wall. But at some time, Philidris nagasau stopped making nests and planted the seeds in the bark instead; at roughly the same time Squamellaria species developed the hollow, bulbous stems that can house the ants. So, ant and plants co-evolved; their co-evolution started about three million years ago.

Willy van Strien

Photo’s:
Large: a Squamellaria major plant, grown by ants on macaranga. © Guillaume Chomicki
Small 1: workers of Philidris nagasau inspecting seedlings. © Guillaume Chomicki
Small 2: hanging garden of Azteca gnava. © Jonas Morales-Linares

Sources:
Chomicki, G. & S.S. Renner, 2016. Obligate plant farming by a specialized ant. Nature Plants 2: 16181. Doi: 10.1038/nplants.2016.181
Chomicki, G., Y.M. Staedler, J. Schönenberger & S.S. Renner, 2016. Partner choice through concealed floral sugar rewards evolved with the specialization of ant-plant mutualisms. New Phytologist, online May 9. Doi: 10.1111/nph.13990
Morales-Linares, J., J.G. García-Franco, A. Flores-Palacios, J.E. Valenzuela-González, M. Mata-Rosas & C. Díaz-Castelazo, 2016. Vascular epiphytes and host trees of ant-gardens in an anthropic landscape in southeastern Mexico. The Science of Nature 103: 96. Doi: 10.1007/s00114-016-1421-9
Youngsteadt, E., J. Alvarez Baca, J. Osborne & C. Schal, 2009. Species-specific seed dispersal in an obligate ant-plant mutualism. PLoS ONE 4: e4335. Doi: 10.1371/journal.pone.0004335

Too busy to care

A successful daddy longlegs is a bad dad

Serracutisoma proximum, two males fighting

In Serracutisoma proximum, a species belonging to the harvestmen, the mother guards her eggs until the young have hatched. A male can do the job as well, but he does so only when he has nothing better to do, as Louise Alissa and colleagues report.

Many species of harvestmen (or daddy longlegs) exhibit a form of parental care: they protect their eggs. This is also true for Serracutisoma proximum, a species that inhabits the Atlantic forests of Brazil.

At the start of the breeding season, males try to establish a territory. They perform ritualised fights for the possession of an attractive area: two males face each other, extend their strongly elongated second pair of legs and try to hit each other. If one of them gives up and leaves, the other is the owner of the territory.

Harem

This male then has to wait for a female to arrive. When this happens, he copulates with her and she lays her eggs after fertilising them internally with his sperm. For a month, she will then stay and guard the clutch. That improves the survival, as conspecifics and other predators will eat many of the eggs when unattended.

Serracutisoma proximum, male tending eggsOn rare occasions, a female deserts or dies. The owner of the territory can take her place and tend the eggs, but there may be more important things for him to do, Louise Alissa and colleagues realised. A second female may visit the territory, and maybe still another. Around a successful male, a harem forms. The territorial male will copulate which each newcomer female as to increase his number of offspring, and he cannot tend a clutch and court newcomer females at the same time.

And that’s not all. There is also a need to guard a female after copulation until she has laid her eggs because of sneaker males, a second type of males (minors) that exist in addition to the territorial males (majors). Sneakers don’t fight for territories (and their second pair of legs is not strongly elongated), but they invade the territories of other males, especially when there are several females present, and try to furtively copulate with one of them to fertilise some of her eggs. The females lay almost all their eggs on the first day after mating with the territorial male, so continuous vigilance during this period is most important. Thereafter, sneaker males still have a chance to sire some young because a few late eggs appear during the next two weeks.

Deserted eggs

Because a successful male must pay attention to newcomer females, he will have less time to guard deserted eggs than a less successful male, the researchers hypothesized. To prove, they removed a egg-tending female from a number of territories that they had been observing for ten days. They then inspected these territories regularly to see whether the owners cared for the orphaned eggs.

As expected, males with only one or two females in their territory guarded the deserted eggs pretty well, while more successful males typically spent less time with the clutch. Males can do only one thing at a time.

Willy van Strien

Photos: ©Bruno A. Buzatto.
Large: two territorial males fighting
Small: caring male

Sources:
Alissa, L.M., D.G. Muniz & G. Machado, 2016. Devoted fathers or selfish lovers? Conflict between mating effort and parental care in a harem-defending arachnid. Journal of Evolutionary Biology, online November 7. Doi: 10.1111/jeb.12998
Munguía-Steyer, R., B.A. Buzatto & G. Machado, 2012. Male dimorphism of a neotropical arachnid: harem size, sneaker opportunities, and gonadal investment. Behavioral Ecology 23: 827-835. Doi:10.1093/beheco/ars037
Buzatto, B.A., G.S. Requena, R.S. Lourenço, R. Munguía-Steyer & G. Machado, 2011. Conditional male dimorphism and alternative reproductive tactics in a Neotropical arachnid (Opiliones). Evolutionary Ecology 25: 331-349. Doi: 10.1007/s10682-010-9431-0
Buzatto, B.A. & & G. Machado, 2008. Resource defense polygyny shifts to female defense polygyny over the course of the reproductive season of a Neotropical harvestman. Behavioral Ecology and Sociobiology 63: 85-94. Doi: 10.1007/s00265-008-0638-9

Gravid frog

Mother transfers nutrients to the young in her dorsal pouch

female Gastrotheca excubitor has a brood pouch on her back

Young of the live-bearing frog Gastrotheca excubitor are well taken care of. The mother carries them in a brood pouch on her back and provides them with nutrients, as Robin Warne and Alessandro Catenazzi show.

In many frog species, females lay their eggs in the water and then leave them alone. After hatching, the tadpoles take care of themselves. Parents of other species look after their young to some extent: the father or mother sees to it that the eggs, which are deposited above a water body, remain moist, or the parents carry the tadpoles to a suitable pool to grow up and bring them some food.

Brood pouch

A few species go even further: parents retain the young in or on their body until they have developed into froglets. One of them is Gastrotheca excubitor, a terrestrial marsupial frog of Central and South America. A mother carries the young in a brood pouch on her back and she transfers nutrients to them, Robin Warne and Alessandro Catenazzi report.

A male clasps a female that is about to lay eggs (amplexus) and fertilizes them when they come out. He then leads them into her dorsal brood pouch, a skin fold with an entrance at the rear. It accommodates more than ten eggs.

During pregnancy the brood pouch is sealed, so the eggs and, later, the hatchlings cannot take up oxygen from their environment. Respiratory gas is exchanged with the mother, as was already known, through the numerous blood vessels in the brood pouch membrane which folds over each egg. The eggs absorb oxygen through the egg membrane and hatchlings breathe through their large external and well vascularised gills that have fused to a ‘bell’. All nutrients the embryo needs are contained in the yolk, the idea was, as they are in other frog species.

Nutrients

Larva of Gastrotheca excubitorBut Warne and Catenazzi hypothesized that a mother provides her young also with nutrients, using the extensive network of blood vessels. Because she retains them for a long period – until they have fully developed into froglets – the amount of yolk may be insufficient, they reasoned. And experiments showed that this is true. The researchers fed pregnant frogs with insects that were chemically labeled (with rare isotopes of carbon and nitrogen, 15N and 13C), and found that the amounts of isotopes increased in the embryos. In addition, the developing froglets gained weight. So, nutrients must be transferred from mother to young.

More species of live-bearing frogs exist. In some species, the males swallow fertilised eggs and brood them in their vocal sac. There are also species in which the females ingest the eggs, which develop in their stomachs, and species in which froglets develop in the oviducts. Transfer of nutrients, however, is rare.

Willy van Strien

Photos © Alessandro Catenazzi
Large: mother with one of her emerged young
Small: larva

Sources:
Warne, R.W. & A. Catenazzi, 2016. Pouch brooding marsupial frogs transfer nutrients to developing embryos. Biology Letters 12: 20160673. Doi: 10.1098/rsbl.2016.0673
Wake, M.H., 2015. Fetal adaptations for viviparity in amphibians. Journal of Morphology 276: 941-960. Doi: 10.1002/jmor.20271

Who is the prey?

Assassin bug sneaks in the web to eat the deadly spider

giraffe-assassin-bug-f-soley

Thanks to a stealthy hunting tactic, the giraffe assassin bug is able to prey on web building spiders. Fernando Soley unravelled how the bug manages to approach a spider unnoticed.

With their sticky webs and their venom, web building spiders are formidable predators of insects and other small animals. Webs and venom also make a powerful defence and it is nearly impossible for small animals to prey on spiders. Still, some critters feed on these predators, risking their lives.

One of the most remarkable spider eating species is the giraffe assassin bug, Stenolemus giraffa, an inhabitant of the northern, tropical part of Australia. It has a weird appearance, as a long ‘giraffe’s neck’ (pronotum) extends between the head with part of the thorax and the rest of the thorax with the thin abdomen. Antennae and front legs are placed at one end of the neck, middle legs and hind legs at the other end. The bug lives on rock escarpments and is often found associated with spider webs because it is araneophagic: it preys on web building spiders. After grasping one, it inserts the tip of its rostrum (piercing mouthparts) into the spider’s abdomen to feed on the contents. Feeding takes an hour or longer.

Cautious

How is this delicate looking bug able to approach and attack a dangerous spider in her web? To find out, Fernando Soley observed the bugs under natural conditions and conducted experiments in the lab.

The giraffe bug spends nearly half his time on stalking resting spiders, as Soley noticed. If he perceives one, he first tries to access it without touching the web, for the spider will perceive any vibration of the threads. Hanging from the rock on middle and hind legs the enemy cautiously progresses. If he is close enough, he bends to his prey and stretches his forelegs to grab it. The long legs and giraffe neck facilitate this way of hunting.

But often, he cannot reach the spider without invading the web. Soley made artificial webs of spider’s silk and placed them in front of a laser vibrometer in order to measure the vibrations produced when a bug steps on a thread. It turned out that those vibrations are very soft and hardly detectable. Because of the long body and the long legs, the bug’s weight is distributed over a large surface area. Besides, it moves slowly, pausing after each step.

Wind

Great difficulties arise when the spider rests at the opposite side of the web. Then the assassin bug has to break some threads next to the spider with its forelegs to be able to capture it. He does so very carefully. To prevent the ends from snapping back after the break and alerting the spider, the bug holds on to the loose ends for a while, causes them to sag and then releases them carefully. After releasing the first loose end, he waits several seconds or even minutes before he releases the second one to space out the vibrations in time. In this way, Soley reports, the bug strongly attenuates the vibrations produced, making them often indistinguishable from background noise.

If a wind blows, things are easier, as vibrations are less conspicuous and the bug can move faster. So, it prefers to breaks threads in the presence of wind.

Long rest

In spite of this stealthy behaviour, there is a risk that the spider notices the steps of the giraffe assassin bug or the breaking of threads and either escapes or attacks. Occasionally, the hunting bug becomes prey himself. The success rate of his attacks is only 20 per cent, but if he succeeds, rewards for his patience and careful behaviour are high. After having overpowered and eaten a spider, the bug can rest for days, digesting this meal.

Willy van Strien
This is an update of an earlier version in Dutch

Photo: Giraffe assassin bug Stenolemus giraffa. © Fernando Soley

Sources:
Soley, F.G., 2016. Fine-scale analysis of an assassin bug’s behaviour: predatory strategies to bypass the sensory systems of prey. Royal Society Open Science 3: 160573. Doi: 10.1098/rsos.160573
Soley, F.G. & P.W. Taylor, 2012. Araneophagic assassin bugs choose routes that minimize risk of detection by web-building spiders. Animal Behaviour 84: 315-321. Doi:10.1016/j.anbehav.2012.04.016
Soley, F.G., R.R. Jackson & P.W. Taylor, 2011. Biology of Stenolemus giraffa (Hemiptera: Reduviidae), a web invading, araneophagic assassin bug from Australia. New Zealand Journal of Zoology 38: 297-316. Doi: 10.1080/03014223.2011.604092