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Suicidal care

27 October 2017 / wvanstrien

Spiderlings consume their aunts as well as their mothers

Females of the social spider Stegodyphus dumicola behave altruistically: not only mothers, but also virgin females show suicidal maternal care and are consumed by the spiderlings, Anja Junghanns and colleagues report.

The South-African spider Stegodyphus dumicola lives in large groups. Females construct a communal nest of silk and plant material with capture webs attached, collaborate in nest defence and share their prey. But when it comes to reproduction, tasks are divided: less than half of the females will mate and get offspring.

It was already known that the mothers take care of their young; they construct an egg sac, tend and guard it for a couple of weeks and when the young spiders have hatched, they regurgitate food for them. Eventually, they are even consumed by their offspring. Anja Junghanns and colleagues asked whether virgin females contribute to brood care, and to what extent. They composed groups of mated females and virgins, marked them with different colours and observed their behaviour.

Risky job

The virgins do perform maternal tasks, they noticed. Like the mothers, they guard the eggs, be it less intensive; instead, they engage more in prey capture than mothers, which can be a risky job. When the spiderlings have hatched, the unmated females, just like the genetic mothers, perform extreme care, regurgitating food – and being consumed eventually.

That willingness to perform ‘suicidal care’ for the young of other females can be explained by the high genetic relatedness among group members. A group mostly starts with a single mated female. Her offspring stay around and mate and reproduce within the nest, resulting in extreme inbreeding; sometimes a small group splits off. A group can grow to a size of more than a thousand members. By helping to care for other females’ offspring, virgin females enhance growth and survival of the spiderlings. Because of the high relatedness, the virgins that provide such help gain almost as much reproductive success as when they would have produced young themselves.

Moreover, they have no better option. The males of a generation mature and die early after hatching, while females mature asynchronously and many are slow. Mature males mate with females that have grown as fast as males, and for the many females that are late, no males are left and they remain unmated. So, the virgins have nothing to lose by helping.

Willy van Strien

Photo: Female with egg sac. ©Anja Junghanns

Source:
Junghanns, A., C. Holm, M. F. Schou, A.B. Sørensen, G. Uhl & T. Bilde, 2017. Extreme allomaternal care and unequal task participation by unmated females in a cooperatively breeding spider. Animal Behaviour 132: 101-107. Doi: 10.1016/j.anbehav.2017.08.006

Cryptic leaf colour

13 October 2017 / wvanstrien

Camouflage protects alpine plants from herbivory

In the high mountains of China, Corydalis plants can be found with leaves that are coloured like stone. That is no coincidence: plants without a stone colour are easily detected by butterflies and devoured by caterpillars, show Yang Niu and colleagues.

The leaves of the alpine plant Corydalis hemidicentra don’t have a fresh green colour; instead, they have the colour of stones: they are either dark grey, reddish brown or greyish green. That is unusual, but it is for a good reason. The plants grow on bare and open stony ground in the very high mountains of southwest China. A normal green leaf colour would attract plant-eating insects, while a cryptic colouration protects the plants from herbivores.

Butterflies’ eyes

The main enemies of the mountain plants are Apollo butterflies, such as Parnassius cephalus. Butterfly females search for a Corydalis plant, which they locate visually, and lay their eggs on the rocks next to it. After emergence, the caterpillars find their meal ready to eat and they consume the plant almost completely.

The colour of the leaves of Corydalis hemidicentra almost always match against the background: where the rock is grey, the leaves are grey too; reddish brown plants grow on reddish brown scree; and greyish green plants are found among greyish green stones. Yang Niu and colleagues show that the colour of the plants is similar to the background colour not only to our eyes, but also to butterflies’ eyes. The cryptic colouration arises because the leaves not only contain green pigment (chlorophyll), as normal, but also red pigment (anthocyanin) and air-filled spaces that are white, and the leaf colour is genetically determined.

Pollinators

Previously, Niu had studied another alpine plant, Corydalis benecincta, of which a green and a grey morph exist. He had found that Apollo butterflies detect the green plants much more easily, and as a the consequence, most green plants are damaged by caterpillars, while grey plants often escape. When plants escape from the enemy, their colour is unimportant: greyish green plants perform as well as green plants. Also in Corydalis hemidicentra non-camouflaged individuals will disappear by herbivory, while camouflaged plants survive. That is why the leaf colour of the plants matches against the background.

While camouflage makes the plants invisible for butterflies, they need to be found by pollinators. Thanks to the strikingly coloured flowers – light blue in Corydalis hemidicentra, purplish pink in Corydalis benecincta – they are easy to find to them. But those flowers don’t appear until the plants are no longer at risk, that is: after the period when butterflies are laying their eggs.

So, not only many animals are camouflaged against their surroundings, but there are also plants with background matching leaves, especially in bare mountain areas. In a well-grown area, plants that are attractive to herbivores are camouflaged best by a normal green colour.

Willy van Strien

Photos: ©Yang Niu

Sources:
Niu, Y., Z. Chen, M. Stevens & H. Sun, 2017. Divergence in cryptic leaf colour provides local camouflage in an alpine plant. Proceedings of the Royal Society B 284: 20171654. Doi: 10.1098/rspb.2017.1654
Niu, Y., G. Chen, D-L. Peng, B. Song, Y. Yang, Z-M. Li & H. Sun, 2014. Grey leaves in an alpine plant: a cryptic colouration to avoid attack? New Phytologist 203: 953-963. Doi: 10.1111/nph.12834

Useful cigarette butts

26 September 2017 / wvanstrien

House finch has to accept harmful side effects

Smoked-trough cigarette filters are noxious, still some bird species add them to their nest lining, where the nicotine will repel blood-sucking parasites. They do so only when they need to, as Monserrat Suárez-Rodríguez and Constantino Macías Garcia show.

Spent cigarette filters are popular among some bird species, for instance the house finch. The birds weave cellulose fibres from discarded butts into the lining of their nests, together with more conventional soft materials such as feathers, fur or cotton. Monserrat Suárez-Rodríguez en Constantino Macías Garcia wondered whether the birds collect cellulose from butts accidently, or whether they do it to protect their young against blood-sucking parasites: lice and ticks. From earlier research, they knew that ectoparasites are repelled by nicotine, and the more smoked-through cigarette butts could be found in a nest, the smaller the amount of parasites was. Weight gain and fledging success of young increased with the proportion of cellulose from butts in the nest lining.

But they also knew that the butts are harmful to adult birds and their offspring. Next to nicotine, the butts contain more than 400 different substances such as heavy metals and insecticides, many of which are toxic. The substances may enter the birds’ bodies through the skin or the lungs.

Damage

The research team had analysed blood samples of parents and young and found nuclear abnormalities in many red blood cells (in contrast to human red blood cells, those of birds contain a nucleus with dna). The larger the proportion of butts in the nest lining, the more genotoxic damage was seen. Red blood cells live for only two to four weeks, so the damage may have no serious consequences. But other cells types likely are damaged too. The question is whether the benefits of adding cigarette butts to the nest lining – less parasites, resulting in better growth – are large enough to outweigh these costs.

The answer will depend on how much the butts are needed to fight off parasites.

Ticks

Now, experiments reveal that house finches act accordingly: they bring more smoked-through cellulose fibres from cigarette butts to their nests if parasites are present than if they’re not. The researchers removed the nest lining from a number of nests shortly after the young hatched, and added a piece of felt instead; by doing so, they removed the bulk of the tick population from the nest as well. They measured the amount of butts in the original lining. They added living ticks to some of the artificial felt nest linings, dead ticks to other linings and nothing to the remaining linings. After the young fledged, they collected the artificial linings to investigate how much butts the parents had added.

It appeared that the birds collected more butts if the researchers had added living ticks to their nest, so when it was useful to bring butts. Also birds that had brought a large amount of butts into their original nest lining, collected many butts now as well; apparently, they had experienced a high parasitic load during incubation.

The birds don’t collect cigarette butts randomly, the conclusion is, but in response to the presence of ectoparasites; so, it is a form of self-medication.

Willy van Strien

Photo: house finch male feeding young. Susan Rachlin (Wikimedia Commons, Creative Commons CC BY 2.0)

Sources:
Suárez-Rodríguez, M. & C. Macías Garcia, 2017. An experimental demonstration that house finches add cigarette butts in response to ectoparasites. Journal of Avian Biology, online September 1. Doi: 10.1111/jav.01324
Suárez-Rodríguez, M., R.D. Montero-Montoya & C. Macías Garcia, 2017. Anthropogenic nest materials may increase breeding costs for urban birds. Frontiers in Ecology and Evolution 5: 4. Doi: 10.3389/fevo.2017.00004
Suárez-Rodríguez, M. & C. Macías Garcia, 2014. There is no such a thing as a free cigarette; lining nests with discarded butts brings short-term benefits, but causes toxic damage. Journal of Evolutionary Biology 27: 2719–2726. Doi: 10.1111/jeb.12531
Suárez-Rodríguez, M., I. López-Rull & C. Macías Garcia, 2013. Incorporation of cigarette butts into nests reduces nest ectoparasite load in urban birds: new ingredients for an old recipe? Biology Letters 9: 20120931. Doi: 10.1098/rsbl.2012.0931

Double deceit

14 September 2017 / wvanstrien

Female cuckoo chuckle call is embarrassing for songbirds

By first laying her egg secretively and then giving a loud chuckle call while leaving, a female cuckoo doesn’t seem to behave in a consistent way. But her call adds to her trickery, as Jennie York and Nicholas Davis show.

A female common cuckoo that lays an egg in the nest of a songbird, for instance a reed warbler, behaves as secretly as she can, because if the intended foster parents detect her presence, they will chase her away; and if she has laid her egg already, the parents will either try to eject it or leave their clutch to start a new one somewhere else. With a cuckoo young in the nest, their own young cannot survive. So, a cuckoo visits the nest and quickly dumps her egg when the owners are away, mostly within a minute.

Vigilant

But while she tries to be unseen when laying, she gives a conspicuous chuckle call when flying away afterwards – quite different from the ‘cuck-oo’ call of the male. This seems paradoxical, as the songbirds may notice her presence after all. Why is she seeking their attention now? Jennie York en Nick Davis answer this question.

They reasoned that a calling female cuckoo may be mimicking the call of a sparrowhawk, to which it is quite similar. If the parents hear that sound, they are concerned about their safety. They become vigilant and scan the surroundings to detect the predator, and their attention is diverted away from their clutch. If they perceive a foreign egg in the clutch, they respond in the same way as when they have seen a female cuckoo on their nest: they try to eject the foreign egg or leave the clutch. But when worrying about their own safety, they will pay less attention to their clutch and may overlook a foreign egg.

Less attention

York and Davis could demonstrate that this idea is right. A few meters from reed warblers’ nests, they placed speakers and play backed the call of a male cuckoo, the call of a female cuckoo, the call of an Eurasian sparrowhawk, or the call of an Eurasian collared dove, a harmless bird; they recorded the songbirds’ responses. The results are clear: the sound of a male cuckoo or a dove elicited no response, while the call of a sparrowhawk provoked vigilance – as did the call of a female cuckoo. So, it appears that indeed a female cuckoo vocally mimics a sparrowhawk. Also great tits and blue tits, which are not exploited by cuckoos as foster parents for their young, get alarmed by the female cuckoo’s chuckle.

After such a frightening experience, reed warblers pay less attention to their clutch, as further experiments revealed. When the researchers exposed the reed warblers to the calls again, put a foreign egg in the nests and checked the nests afterwards to see whether this egg was accepted or rejected, they discovered that parents that had been exposed to the call of a sparrowhawk or a female cuckoo were less likely to notice the foreign egg than birds that had heard a male cuckoo or a collared dove.

So, a chuckling female cuckoo deceits the foster parents twice, first by secretly laying her egg and then by vocally mimicking a sparrowhawk, tricking the victims into defending themselves instead of their clutch, while in fact the clutch is in danger.

Willy van Strien

Photo: Trebol-a (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Source:
York, J.E. & N.B. Davies, 2017. Female cuckoo calls misdirect host defences towards the wrong enemy. Nature Ecology & Evolution, online September 4. Doi: 10.1038/s41559-017-0279-3

Floral dress

7 September 2017 / wvanstrien

Pollinators are deceived by flower mimicking crab spider

The spider Epicadus heterogaster is coloured strikingly like a flower, and bees are lured by the colour to become prey, as Camila Vieira and colleagues show. Its masquerade is completed by a conspicuous abdomen, mimicking a flower’s shape.

The crab spider Epicadus heterogaster, which lives in Brazil, always seems to be dressed in a carnival costume that makes it look like a flower: it has a white, yellow or purple body colour and conspicuous abdominal protuberances. By mimicking a flower, it attracts insects that use to visit flowers to collect nectar, meanwhile pollinating the flowers. All it has to do next, is extend its legs and grab the victims – and the pollinators become prey.

Now, Camila Vieira and colleagues present proof that the crab spider’s colour attracts pollinators.

Sunscreen

Like many flowers, Epicadus heterogaster has an ultraviolet component in its body colour. We cannot see that colour, but insects do and some insects prefer it. The spider’s colour stands out clearly against the green leaves on which she awaits her visitors.

In order to demonstrate that the spiders’ colour indeed lures insects, the researchers treated anesthetized females with sunscreen that blocks ultraviolet light. When they applied the sunscreen on a female’s back, passing pollinators no longer saw an ultraviolet colour and didn’t approach the spider; in contrast, they avoided it. But when the sunscreen was applied not on the dorsal side, but on the ventral side of a spider, it remained attractive to pollinators. Its flowerlike appearance undoubtedly contributes to the deceit.

Inconspicuous

Juvenile female spiders are coloured like adults, also mimicking a flower, but they exploit their disguise in another manner. They’re not sitting on a leaf to attract pollinators, as they are too small to be of any interest to them anyway, and by being conspicuous, they would attract predators. Instead, youngsters are observed mostly on flowers, where they are perfectly camouflaged.

Willy van Strien

Photo: Alex Popovkin (Wikimedia Commons, Creative Commons CC BY 2.0)

Epicadus heterogaster on YouTube

Source:
Vieira, C., E.N. Ramires, J. Vasconcellos-Neto, R.J. Poppi & G.Q. Romero, 2017. Crab spider lures prey in flowerless neighborhoods. Scientific Reports 7: 9188. Doi: 10.1038/s41598-017-09456-y

Hypocritical behaviour

4 September 2017 / wvanstrien

Cheating cleaner wrasse cares about its reputation

In some circumstances, cleaner wrasse attract clients by presenting themselves as more friendly than they really are – and subsequently provide them a bad service, as research by Sandra Binning and colleagues reveals.

Anyone who runs a business should care about a good reputation, and the bluestreak cleaner wrasse, Labroides dimidiatus, definitely does. These fish try hard to be seen as cooperative partners, especially when they are exploitative partners in reality; in the words of Sandra Binning and colleagues, they have reputation management abilities.

The cleaner fish occupy a cleaning station on a reef and offer a cleaning service to other fish: the cleaners eat ectoparasites off the surface of their clients. It is a flourishing business, with cleaners on average being involved in two thousand interactions a day with more than a hundred clients; some clients come for inspection many times a day. Both partners benefit from the interactions – clients are cleaned, cleaners earn a meal – so, they have a mutualistic relationship. But a cleaner may cheat its client by taking a bite from its mucus, a layer that protects against abrasions or ultraviolet radiation, instead of removing parasites. A cleaner prefers eating the mucus over parasites, as it is more nutritious.

Temptation

Mostly, cleaners provide honest services. They just have to, because if they bite a client, that client will either leave and visit another cleaning station or chase the cleaner away from its station, making it impossible to continue its work for a while; and some clients are predatory fish that may ingest a cheating cleaner. Also, there are bystanders, waiting clients or potential clients, that eavesdrop on ongoing interactions and that notice when a cleaner bites its current client, as that client will involuntary make a short jolt in response. For the audience, the occurrence of a jolt is a reason to avoid that cleaner.

So, most cleaners behave in a decent manner. They will not bite, especially not when other fish are witnessing. Still, some cleaner wrasse sometimes behave badly, mainly females in the spawning season, when their energy demands are high and, accordingly, the temptation to cheat is high.

Friendly

As biting cleaners run a risk to lose their clients, they care about their reputation: they behave friendly towards small clients and provide these clients tactile stimulation by hovering above them and touching them with their pelvis fins.

Clients like this stimulation; it reduces their stress. Cooperative cleaners use to perform this behaviour infrequently to persuade a client to have itself inspected and cleaned, or to reconcile after a bite and prevent the client from fleeing or becoming aggressive, or to appease predatory fish. It is costly behaviour, because cleaners cannot eat while stroking a client.

But a biting cleaner that titillates a small client, does so for another reason, as the researchers found out before. Small clients offer less parasites and less mucus than larger clients. To stimulate them in order to keep friends with them would hardly be worth the effort, and in most cases it is unnecessary. Instead, the service of a cheater towards small clients is meant to attract larger observing clients, by showing them how friendly it behaves. When a large potential client sees the high service quality that the small client receives and approaches to be cleaned, the cheater can take a big bite of that large client’s mucus.

Witnesses

Now, the researchers show that biting cleaners only perform their hypocritical behaviour when it pays, that is: when many clients and many competing cleaners are around to witness. In such circumstances, cheaters often stroke small clients and bite the large clients that are misled by this nice behaviour. But when just a few clients and a few competitors are around, they will bite their small clients as well. They may be friendly, but only when an audience is looking.

Willy van Strien

Photo: Bluestreak wrasse, Labriodes dimidiatus, and clients. Keith Wilson (via Flickr, Creative Commons CC BY-NC 2.0)

Sources:
Binning, S.A., O. Rey, S, Wismer, Z. Triki., G. Glauser, M. C. Soares & R. Bshary, 2017. Reputation management promotes strategic adjustment of service quality in cleaner wrasse. Scientific Reports 7: 8425. Doi: 10.1038/s41598-017-07128-5
Pinto, A., J. Oates, A. Grutter & R. Bshary, 2011. Cleaner wrasses Labroides dimidiatus are more cooperative in the presence of an audience. Current Biology 21: 1140-1144. Doi: 10.1016/j.cub.2011.05.021
Bshary, R. & A.S. Grutter, 2006. Image scoring and cooperation in a cleaner fish mutualism. Nature 441: 975-978. Doi: 10.1038/nature04755
Bshary, R., 2002. Biting cleaner fish use altruism to deceive image-scoring client reef fish. Proc. R. Soc. Lond. B 269: 2087-2093. Doi: 10.1098/rspb.2002.2084
Bshary, R. & M. Würth, 2001. Cleaner fish Labroides dimidiatus manipulate client reef fish by providing tactile stimulation. Proc. R. Soc. Lond. B 268: 1495-1501. Doi: 10.1098/rspb.2001.1701

Successful imitation

21 August 2017 / wvanstrien

Jumping spider cheats predators by walking like an ant

The jumping spider Myrmarachne formicaria successfully imitates the walk of ants, misleading larger spiders that hunt for critters. Predators often refrain from attacking the jumping spider, as much as they are reluctant to attack ants, as Paul Shamble and colleagues observed.

The jumping spider Myrmarachne formicaria, only a few millimetres long, resembles an ant; it is, for instance, pretty thin. This resemblance is functional. Many predators, especially larger spiders and wasps, will readily grasp a small spider, but mostly refrain from attacking an ant, as this prey may bite, spray acid or sting to defend itself and is often assisted by nest-mates. By mimicking an ant, a small spider may protect itself against predators.

Not trivial

But a spider that aims to be mistaken for an ant not only has to look like an ant, but it also has to move like an ant.

That is not trivial, Paul Shamble and colleagues point out. Spiders walk on eight legs and ants on six, and the legs are driven differently. Spiders walk along a pretty straight line, while the path of ants is looped – unless they follow an odour track laid down by their nest-mates; in that case they walk a highly regular zig-zag route along the trail. Jumping spiders, to which Myrmarachne formicaria belongs, hunt while walking; they typically stalk their prey carefully, making long pauses. Ants, on the contrary, move on continuously. When jumping spiders approach a prey, they leap towards it from a distance; ants don’t jump.

A jumping spider thus has to modify its movements significantly to mimic the behaviour of an ant. Is Myrmarachne formicaria able to do that?

Using three high-speed cameras, the researchers filmed Myrmarachne formicaria, non-mimecic jumping spiders and ants walking across a glass surface and analysed their gaits. The analysis revealed that Myrmarachne formicaria imitates the walk of an ant very well. The spider walks on eight legs like other spiders do, but it moves its legs in an ant-like manner. It imitates the zig-zag behaviour of an ant that follows an odour trail. It does make pauses, but only very short ones. When stationary, it raises its forelegs, pretending to be an ant with a pair of antennae and three pairs of legs. The researchers never observed it leaping.

Animations

Finally, based on the video recordings, Shamble produced animations of an ant, a non-mimic jumping spider and Myrmarachne formicaria, presented these animations to a large predatory spider and observed its response. The predator was attracted to all of these targets, but not to the same extent. It attacked a jumping spider target more often than an ant target, and, importantly, it was not more likely to attack an ant mimic than an ant.

So, Myrmarachne formicaria cheats large spiders by imitating the appearance as well as the behaviour of an ant, protecting itself from these predators. It would be interesting to know whether predators with better visual capacities, like shrews, birds, lizards or toads, are also misled by this mimicry.

Willy van Strien

Photo: Jeff Burcher (via Flickr. Creative Commons CC BY-NC-ND 2.0)

Source:
Shamble, P.S., R.R. Hoy, I. Cohen & T. Beatus2, 2017. Walking like an ant: a quantitative and experimental approach to understanding locomotor mimicry in the jumping spider Myrmarachne formicaria. Proc. R. Soc. B 284: 20170308. Doi: 10.1098/rspb.2017.0308

Trapped, encased, killed

11 July 2017 / wvanstrien

Snails use their shells as a weapon against parasitic worms

Parasitic roundworms that invade a snail’s shell may be trapped, encased and fixed permanently to the inner layer of that shell, as Robbie Rae shows.

Thanks to its shell, a snail is protected against damage, predators, heat and cold, drought and rain. But there is more, as Robbie Rae discovered. The snail also uses its shell as a defence system to eliminate parasitic roundworms (nematodes). These parasites attack snails since snails appeared on earth, about 400 million years ago. It is obvious that snails had to evolve a defence mechanism against these enemies, but until now, no defence mechanism was known.

Encapsulation

In his lab, Rae exposed grove snails (Cepaea nemoralis) to the nematode Phasmarhabditis hermaphrodita for several weeks. This bottom dwelling animal, less than 2 millimetres long, is able to penetrate and kill many snail and slug species, but some snails are resistant, as for instance the grove snail. Rae studied the interaction between the grove snail and the worms to find out how the snail eliminates the parasites.

It turned out that the cells on the inner layer of the shell do the job. They adhere to an invading worm, multiply, and swarm over the parasite’s body until it is entirely covered. Engulfed by the cells, it is fused to the inside of the shell and dies. By this procedure, grove snails not only encapsulate this lethal roundworm, but they use the immune reaction also to kill other, less dangerous nematodes, as experiments showed.

In nature, this is common practice. Rae collected grove snails and white-lipped snails (Cepaea hortenis) from the wild and observed that many snails had different species of roundworms attached to their inner shell surface, up to 100 worms in one shell. Also the garden snail (Cornu aspersum) – like the other two snail species an inhabitant of Western Europe – uses its shell to eliminate invading worms by encapsulation.

Old defence

Finally, he examined a large number of snails from museum collections, to conclude that many snails of many different species had nematodes attached to their shells. Trapped worms proved to be fixed permanently; they even can be found in snails that died a few hundred years before. As this defence mechanism is found to be widespread among the large and old clade of terrestrial snails and slugs, it must have evolved about 100 million years ago. Even some slug species eliminate parasitic roundworms by this mechanism. During evolutionary history, their shells have become reduced and internalised, but in many species they retained the ability to trap, encase and kill roundworms.

The vineyard snail (Cernuella virgata) is one of the species that is unable to get rid of the roundworm Phasmarhabditis hermaphrodita. Apparently, the parasite evades its immune reaction in one way or the other. As many slug species are also susceptible to this parasite, it is formulated into a biological control agent to be used against herbivirous slugs.

Willy van Strien

Photos:
Large: grove snail, Cepaea nemoralis. Kristian Peters (Wikimedia Commons, Creative Commons CC BY-SA 3.0)
Small: nematodes fixed to the inner layer of a grove snail’s shell. © Robbie Rae

Source:
Rae, R., 2017. The gastropod shell has been co-opted to kill parasitic nematodes. Scientific Reports 7: 4745. Doi: 10.1038/s41598-017-04695-5

Percussion

5 July 2017 / wvanstrien

Palm cockatoo drums with self-fashioned drumstick

With a female listening, palm cockatoo males may repeatedly strike a hollow branch or trunk with a stick. Robert Heinsohn and colleagues heard that the birds have good rhythm and that every male has his individual drumming style.

A palm cockatoo male from North Australia can produce different sounds while erecting its crest. That is impressive, but there is something that really stands out: it may start drumming.

Regular pulse

When a male is going to perform, it breaks off a twig, removes the leaves, trims it to approximately 20 centimetres, grasps it in one of both foots and starts beating repeatedly on a hollow branch or trunk. Instead of a stick, it may use a seed pod of a particular tree (Grevillea glauca, the bushman’s clothes peg) after adjusting the shape with its beak. It may continue drumming for a while, producing a sequence of up to 90 taps.

It is remarkable that the intervals between the taps don’t occur at random intervals; instead, the cockatoos produce a regular pulse, as Robert Heinsohn and colleagues assessed. They also noticed that each male has its individual, consistent style; some males have slow drumming rates, whereas others drum at a faster rate, or insert short sequences of faster drumming in the performance occasionally.

Solo

It is not known yet which function the performance might have. Palm cockatoos form monogamous pairs which occupy a large territory. The sound does not travel far enough to be heard by the neighbours, so a male cannot communicate with them by drumming; he always is playing solo. As most performances are attended by the female, the music probably is meant for her, and it may be a male’s way to inform its partner about its condition or age; the birds may live more than 50 years. We don’t know whether the females like the percussion and what rhythm they prefer.

Willy van Strien

Photo: Christoph Lorse (Via Flickr. Creative Commons CC BY-NC-SA 2.0)

The researchers explain their work on You Tube;
short fragment of a drumming cockatoo

Source:
Heinsohn, R., C.N. Zdenek, R.B. Cunningham, J.A. Endler & N.E. Langmore, 2017. Tool-assisted rhythmic drumming in palm cockatoos shares key elements of human instrumental music. Science Advances 3: e1602399. Doi: 10.1126/sciadv.1602399

Glue-coated silk

30 June 2017 / wvanstrien

Ground spider immobilises prey with sticky threads

Many ground spiders can handle large or dangerous prey. They swathe their victims’ legs with threads that are coated with a tough glue, as Jonas Wolff and colleagues show.

Sticky threads

Ground spiders (Gnaphosidae) do not capture their prey by building a web, but hunt for them on the ground. For some of them, this is a risky venture because they aim for large or dangerous prey, such as ants and other spiders. Jonas Wolff and colleagues found out how they managed to capture these prey.

The researchers recorded the behaviour of a number of ground spiders during an attack with a high speed camera and analyzed the footage. When a ground spider perceives a prey, they observed, it first tries to grab it with its front legs. If it fails because the victim is large or dangerous, it quickly switches to another tactic and applies sticky silk threads to the soil and to the opponent’s legs and mouth parts to immobilise it; sometimes, an entangled prey spider still tries to defend itself by biting – hunting is never without risk.

Other spider species don’t produce such sticky silk threads. Spider silk is made in glands, and comparing the silk glands of ground spiders that swathe the legs of their prey with those of other species, the researchers found clear differences.

Spiders possess different types of silk glands, each of which makes another type of silk. The glands produce a liquid mixture of silk proteins and have a tapering duct ending in a nozzle-like opening (the spigot) on a spinneret; spiders have one to four pairs of spinnerets on their abdomen and make threads by pulling the protein mixture from the spigots.

In most spider species, the largest pair of spinnerets bears the spigot of a single large ampullate gland. This gland produces the strong silk of which draglines are made: the structural threads of webs of web-building species and the drop lines that spiders make when they drop. In addition, there are the spigots of numerous tiny piriform glands; they produce short glue-coated fibres of which spiders spin attachment disks to link the threads of their web at the intersections, or to anchor a dragline to, for instance, a tree.

Modified glands

In ground spiders that use sticky threads to capture their prey, the silk glands are strongly modified. The ampullate glands are relatively small, while the piriform glands are enlarged and have wide spigots. Ground spiders entangle their prey with the silk threads pulled from these enlarged piriform glands. The threads are coated with a layer of ductile, tough glue, perfect for swathing and immobilising struggling prey.

So, ground spiders developed a novel use of piriform silk, and the morphology of the silk glands is adapted to this novel use. The consequence is that they are not able to make functional draglines and fully functional attachment disks. Like many other spider species, they build a silk shelter in which they hide (of yet another type of silk), but they can’t anchor that shelter to the substrate as well as other spider species do. That’s the price they have to pay for their exclusive hunting method.

Willy van Strien

Photo:
Mouse spider Scotophaeus blackwalli. Richard Pigott (via Flickr; Creative Commons CC BY-SA 2.0)

Source:
Wolff, J.O., M. Řezáč, T. Krejčí & S. Gorb, 2017. Hunting with sticky tape: functional shift in silk glands of araneophagous ground spiders (Gnaphosidae). Journal of Experimental Biology 220: 2250-2259. Doi: 10.1242/jeb.154682

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