Disarmed, but not impotent

Disabled cactus bug produces more sperm

male Narnia femorata that dropped a leg grows larger testes

With their enlarged hind legs, male cactus bugs fight with each other to defend a territory or to achieve access to a female. What will become of a male that lost one of those weapons, Paul Joseph and colleagues wondered.

The leaf-footed cactus bug Narnia femorata can drop (autotomize) a leg when this leg is grasped by a predator, entrapped or damaged. Thanks to such self-amputation the bug survives the incident, but from now on it has only five legs left to stand on and to walk on; a leg that is lost is not regenerated. For a male, it is extra annoying if it has to sacrifice one of its two hind legs, because it uses them to fight with other males for the possession of a territory or the access to a female. However, if it loses a hind leg before it is fully grown, it can compensate for it, write Paul Joseph and colleagues.

cactus bug narnia femorata preferably feeds on cactus fruitsIn the southwest of the United States, Mexico and parts of Central America, the bugs live on cacti, for instance on the prickly pear cactus Opuntia mesacantha. They feed on the plants, preferably on the ripe fruits, and females lay their eggs on them, selecting parts with ripe fruits.

Males try to defend a territory on a cactus. If an intruder shows up, both males position themselves rear to rear to display, kick and wrestle with their hind legs until one of them gives up. In the presence of a female – when there is a lot at stake – the fight is fiercer, and the male with the largest hind legs will be the winner. The hind legs of males are real weapons, they are enlarged and serrated.

A male that loses one of its hind legs is in problems. It cannot defeat an intact rival and the chance that it will mate a female has decreased considerably. But it may compensate for its disability, Joseph hypothesized, by growing larger testes. This would be possible if the leg is lost before the male is full-grown; bugs don’t go through a complete metamorphosis with a pupal stage, but they grow gradually.

In order to find out whether juvenile males grow larger testes after losing a hind leg, Joseph experimentally induced juvenile bugs to drop a leg by grasping the leg with a pair of forceps and tickling with a small paintbrush, mimicking what can happen in the wild. As expected, after such treatment the testes grew extra large, while everything else developed as it normally does.

And is it useful to have enlarged testes? The researchers paired disabled and untreated males each with a female for 24 hours. Afterwards, they counted how many eggs the females laid and how many of them hatched, meaning that they had been fertilized. They noticed that most females produced about twenty eggs, independent of whether or not they had mated. Clutches of females that had been paired with an untreated male were more likely to contain eggs that hatched than clutches of females with a disabled partner. Apparently, males that dropped a hind leg less often succeeded in mating.

But if disarmed males managed to mate, they fertilized a larger proportion of the eggs. Their enlarged testes produced more sperm, and so they sired more offspring than intact males.

In conclusion, males can compensate for the loss of a weapon by investing more in testes growth – but only if they lose it when still young. Otherwise, it is just bad luck.

Willy van Strien

Photos
Large: leaf-footed cactus bug Narnia femorata; male that dropped a hind leg. ©Christine Miller
Small: leaf-footed cactus bug male on cactus fruit. Cotinis (via Flickr; Creative Commons CC BY-NC-SA 2.0)

Sources:
Joseph, P.N., Z. Emberts, D.A. Sasson & C.W. Miller, 2017. Males that drop a sexually selected weapon grow larger testes. Evolution, 20 november online. Doi: 10.1111/evo.13387
Procter, D.S., A.J. Moore & C.W. Miller, 2012. The form of sexual selection arising from male-male competition depends on the presence of females in the social environment. Journal of Evolutionary Biology 25: 803–812. Doi: 10.1111/j.1420-9101.2012.02485.x

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In its own bubble

Alkali fly manages to stay dry in very wet water

Alkali fly can enter the alkaline Mono Lake

Each insect would drown in Mono Lake, a saline soda lake in California. Each insect, except for the alkali fly, which has unique adaptations to survive the extreme environment, as Floris van Breugel and Micael Dickinson show.

Only bacteria, algae and brine shrimp tolerate the saline water of Mono Lake in California- and the alkali fly Ephydra hians, which flourishes here. It is an amazing critter. The larvae develop in the water, feeding on the algae. Adult flies, which occur in great numbers along the shores, regularly crawl into the water to feed on algae or to lay their eggs. They are the only adult insects that are able to dive into the briny water and emerge alive, Floris van Breugel and Michael Dickinson report.

Several insect species exist that survive submersion in the water of lakes or streams, thanks to a water-repellent layer of hydrocarbons (waxy substances) on their cuticle and tiny hairs that trap a layer of air. But they would be wetted and drown in Mono Lake. That is because the water of this lake contains a large amount of sodium carbonate, a salt known as baking soda, the presence of which renders the insects incapable of keeping the layer of air intact; such water is ‘wetter’ than pure water and penetrates into the layer of air. Sodium carbonate is one of the substances that make the water strongly alkaline.

But the alkali fly easily dives into the alkaline water and when it emerges, it is completely dry. The researchers show that this is possible because the diving fly possesses a very dense mat of hairs and a layer of superhydrophobic hydrocarbons which, combined, prevent wetting. Upon entering the water, a stable air bubble forms around the fly, which enables it to spend 15 minutes underwater; the bubble protects it from the hostile water and offers a supply of oxygen.

In the past 60,000 years, Mono Lake became more salt and more alkaline because it has no outlet, and as water is evaporating, the concentrations of mineral salts gradually rise. Limestone columns, named tufa, developed, along which the flies descend below water surface.

The alkali fly’s ancestors had to enter the lake to forage in a time when the lake was still fresh and algae were the only food available, the authors hypothesize. While the lake gradually became more alkaline, the fly adapted to the new conditions. Nowadays, it can safely graze underwater, as no fish predators occur. The flies are preyed upon by many birds that forage near the lake, like gulls.

When oil, from decaying organic matter or sunscreen and other cosmetics, is floating on the water, even the alkali flies are wetted and drown, in spite of their unique adaptations.

Willy van Strien

Photo: Alkali fly, pictured under water inside its protective air bubble. Floris van Breugel/Caltech

Source:
Van Breugel, F. & M.H. Dickinson, 2017. Superhydrophobic diving flies (Ephydra hians) and the hypersaline waters of Mono Lake. PNAS, online Nov. 20. Doi: 10.1073/pnas.1714874114

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Second hand meal

Sea slug consumes the food of its prey

the pilgrim hervia steals the prey of its prey

The diet of the pilgrim hervia Cratena peregrina, a sea slug, not only consists of the hydroids on which it lives; the animal also feeds on prey that was captured by the polyps, Trevor Willis and colleagues report.

The pilgrim hervia Cratena peregrina is a fairylike beautiful creature. Its white back bears tens of red protuberances with a luminescent blue tip, much like little candles. The sea slug occurs in the Mediterranean Sea and the eastern Atlantic Ocean on the branched colonies of hydroids such as Eudendrium ramosum and Eudendrium racemosum. The colonies provide shelter, the polyps are edible and possess defensive weapons that are useful. And according to Trevor Willis and colleagues, there is even more: the polyps capture food which the sea slug then may steal and consume.

Hydroids are cinidarians, just like jellyfish, and their mouth is surrounded by tentacles which grasp their prey, mainly zooplankton. They also possess stinging cells that are able to eject a harpoon (nematocyst) with a toxic content to paralyze prey or to deter predators from attacking.

But the pilgrim hervia is not deterred. It devours polyps without being bothered by stinging cells, as has already been known for a long time. In one way or another it is protected against injury by nematocysts that are fired while it is feeding, and many nematocysts that are ingested remain undischarged. Undischarged harpoons are not digested, but remain structurally intact while passing through the digestive system; some of them are discarded in the faeces, but others are sequestered and stored in cellular vesicles (cnidosacs) at the tips of the dorsal appendages.

So, the sea slug incorporates its prey’s weapons, and it can use them to frighten off hungry fish predators. Because of the bright aposematic coloration, predators quickly learn not to touch this beautiful but unpalatable morsel.

Now, Willis shows that Cratena peregrina not only obtains second hand weapons from the hydroids, but also takes the prey that the polyps have captured. Laboratory experiments revealed that the sea slugs preferentially feed on polyps that are handling prey, for instance brine shrimp. By doing so, the sea slug ingests two food types in one bite: its prey and its prey’s prey. The second hand food, zooplankton, appears to be a substantial part of its diet – and the sea slug doesn’t have to capture this mobile prey by himself.

Willy van Strien

Photo: Cratena peregrina on Eudendrium ramosum. Français (Wikimedia Commons, public domain)

Watch the sea slug on YouTube

Sources:
Willis, T.J., K.T.L. Berglöf, R.A.R. McGill, L. Musco, S. Piraino, C.M. Rumsey, T.V. Fernández & F. Badalamenti, 2017. Kleptopredation: a mechanism to facilitate planktivory in a benthic mollusc. Biology Letters 13: 20170447. Doi: 10.1098/rsbl.2017.0447
Greenwood, P.G., 2009. Acquisition and use of nematocysts by cnidarian predators. Toxicon 54: 1065-1070. Doi:10.1016/j.toxicon.2009.02.029
Aguado, F. & A. Marin, 2007. Warning coloration associated with nematocyst-based defences in aeolidiodean nudibranchs. Journal of Molluscan Studies 73: 23-28. Doi:10.1093/mollus/eyl026
Martin, R., 2003. Management of nematocysts in the alimentary tract and in cnidosacs of the aeolid nudibranch gastropod Cratena peregrina. Marine Biology 143: 533-541. Doi: 10.1007/s00227-003-1078-8

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Camouflage suit

Covered with sponges, a crab is poorly visible

Spider decorator crab Camposcia retusa covered with sponges

The spider decorator crab Camposcia retusa adorns its legs and carapace exuberantly with sponges, probably to mislead predators, Rohan Brooker and colleagues write. The crab accumulates more decorations when it has no access to shelter.

Equipped with a lot of sponges, complemented by some algae and dead organic matter, the spider decorator crab Camposcia retusa moves around: a weird appearance. The crab is associated with tropical coral reefs in the Indian Ocean and the western Pacific Ocean. Why would this little animal, with a carapace that is a few centimetres wide, carry so much stuff that probably hampers its mobility?

According to Rohan Brooker and colleagues, a highly decorated crab is less visible to its predators. In addition, many sponges are noxious or toxic, and they may deter predators that perceive such a crab in spite of its camouflage.

The researchers wanted to learn more about the crab’s decorating behaviour. From reefs, they caught a number of crabs to study their decoration patterns. They then conducted a manipulative behavioural experiment on crabs in tanks to which they added red polyester pompoms of different sizes to see how the crabs would use them.

They found that the animals covered their carapace and the third and fourth sets of walking legs most (they have four pairs of walking legs). In the experiments, they placed the largest and heaviest pompoms only on the hind legs, which are the strongest. The chelipeds – which the crabs use for feeding and communication – and the first set of legs were hardly decorated. The parts of the body on which items are distributed are equipped with hooked seta like those of Velcro, to which pieces of sponge and other material are easily attached.

In another experiment, the crabs either got a shelter in the form of a PVC elbow in their tank or no shelter. The crabs that had no access to shelter decorated more than the crabs that had shelter, hence the conclusion that the decoration is primarily an antipredator defence. Because the animals accumulate and retain a wide range of materials, camouflage most likely is the main effect of decoration. And because they prefer to attach sponges, it may also serve as a deterrent. It would be great if the researchers now would go on to show that predators have more difficulty perceiving a prey in camouflage suit, or that they are deterred by the sponges.

Decoration occurs in many animal species, most frequently in aquatic species. The spider decorator crab Camposcia retusa is a beautiful example of this behaviour.

Willy van Strien

Photo: Patrick Randall (via Flickr, Creative Commons CC BY-NC-SA 2.0)

Three examples of decorated crabs on YouTube: 1, 2, 3

Source:
Brooker, R.M., E.C. Muñoz Ruiz, T.L. Sih & D.L. Dixson, 2017. Shelter availability mediates decorating in the majoid crab, Camposcia retusa. Behavioral Ecology, online Oct. 17. Doi: 10.1093/beheco/arx119

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Saving energy

Venus flytrap controls the trapping process in several ways

Venus flytrap has several mechanisms to save energy

Carnivorous plants should control their energy budget, otherwise the benefits of capturing insects will not compensate for the costs. The Venus flycatcher has several mechanisms to limit waste of energy, Andrej Pavlovič and colleagues discovered.

To us, it is almost impossible to catch a fly. But the Venus flytrap has no difficulty. The plant (Dionaea muscipula) occurs in North and South Carolina (United States), where it grows in sunny, wet areas on poor soil; it can grow there by ‘eating’ insects. The catch of a fly yields lots of nutrients, but the process also demands lots of energy, and the balance between yield and costs must be positive, otherwise the plant will not grow. So, it has evolved a number of control mechanisms to minimize waste of energy, as Andrej Pavlovič and colleagues point out.

The leaves of the Venus flytrap end in a two-lobed trapThe leaves are arranged in a rosette, and each leaf has a double-lobed trap at the top with a row of ten to twenty teeth at the edge of each lobe. Glands along the edges secrete a sugary substance that attracts insects. Each lobe has a few trigger hairs that respond when touched by an insect, causing the trap to snap shut rapidly. The central zone of the trap contains glands secreting enzymes that digest a trapped prey and proteins that enable the glands to absorb the nutrients that are released upon digestion.

The Venus flytrap has to invest a lot of energy to keep the traps operational and to produce lures, digestive enzymes and absorption proteins. How does the plant control these costs?

1 First of all, a trap will not snap shut until trigger hairs are touched at least twice within twenty seconds, when there is a fair chance that an insect has landed. So, a trap will not close when a for instance a wind-blown dust grain touches a trigger hair.

2 But not every animal that landed turns out to be a nice fat fly. Upon closure, small gaps between the marginal teeth allow little insects that are not worth the effort to digest them to escape. If the trap is empty, it will reopen again after a few hours. But if a large insect is encased, it will struggle in panic, and his movements induce the trap to seal hermetically. After the trigger hairs have been touched at least five times, the secretion of digestive enzymes and absorption proteins starts, and the more movements the prey makes, the more enzymes and proteins will be secreted.

3 Still, a trap may snap shut, close tightly and secrete digestive enzymes and absorption proteins in vein. This happens when it is damaged. The cause of the error is to be found in the evolution of carnivorous plants, for the habit to capture insects probably evolved from defence mechanisms against herbivorous insects. In ordinary plants, herbivory generates an electrical signal, which in turn stimulates the accumulation of plant hormones, jasmonates. These will induce the plants to synthesize toxins that harm the insects, not only locally on the place of damage, but also elsewhere in the plant, as a precaution. In carnivorous plants, such as the Venus flytrap, things have a bit changed. In these plants, the presence of an insect triggers an electrical signal that induces the accumulation of jasmonates; these hormones stimulate the secretion of digestive enzymes and absorption proteins. The electrical signal also induces the trap to close.

Now, Pavlovič conducted an experiment in which he repeatedly wounded a trap of Venus flytraps by piercing it with a needle to mimic herbivory, and noticed that the trap showed the same response as when the trigger hairs wouldhave been touched by an insect: the trap closed and jasmonates accumulated; if he continued damaging every few minutes, the traps secreted digestive enzymes and absorption proteins – to no end. But the misplaced reaction was limited to the local trap that was damaged and did not occur elsewhere in the plant, in contrast to defence reactions agains herbivores.

4 The secretion of digestive enzymes and absorption proteins does not run at full speed from the start. Only when certain substances from an enclosed prey are released, the rate of secretion increases to the highest speed. If there is no prey, the process will stop. So, the plant doesn’t waste much energy when it is misled.

After about ten days the fly is digested and the fall will reopen again.

Willy van Strien

Photos
Large: ©Andrej Pavlovič
Small: Olivier License (via Flickr, Creative Commons CC BY-NC-ND 2.0)

Watch the trapping process

Sources:
Pavlovič, A., J. Jakšová & O. Novák, 2017. Triggering a false alarm: wounding mimics prey capture in the carnivorous Venus flytrap (Dionaea muscipula). New Phytologist 216: 927-938. Doi: 10.1111/nph.14747
Böhm, J., S. Scherzer, E. Krol, I. Kreuzer, K. von Meyer, C. Lorey, T.D. Mueller, L. Shabala, I. Monte, R. Solano, K.A.S. Al-Rasheid, H. Rennenberg, S. Shabala, E. Neher & R. Hedrich, 2016. The Venus flytrap Dionaea muscipula counts prey-induced action potentials to induce sodium uptake. Current Biology 26: 286-295. Doi: 10.1016/j.cub.2015.11.057
Libiaková, M., K. Floková, O. Novák, L. Slováková & A. Pavlovič, 2014. Abundance of cysteine endopeptidase dionain in digestive fluid of Venus flytrap (Dionaea muscipula Ellis) is regulated by different stimuli from prey through jasmonates. PLoS ONE 9: e104424. Doi:10.1371/journal.pone.0104424
Pavlovič, A., V. Demko & J. Hudák, 2010. Trap closure and prey retention in Venus flytrap (Dionaea muscipula) temporarily reduces photosynthesis and stimulates respiration. Annals of Botany 105: 37-44. Doi:10.1093/aob/mcp269

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Suicidal care

Spiderlings consume their aunts as well as their mothers

In Stegodyphus dumicola, unmated females provide extreme care to the offspring of other females

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.

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

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Cryptic leaf colour

Camouflage protects alpine plants from herbivory

Corydalis hemidicentra has stone coloured leaves

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.

Apollo butterfly oviposits near Corydalis plantsThe 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.

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.

leaves of Corydalis hemidicentra match against their backgroundThe 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.

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

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Useful cigarette butts

House finch has to accept harmful side effects

House finches add cigarette butts to their nests to repel parasites

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.

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.

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

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Double deceit

Female cuckoo chuckle call is embarrassing for songbirds

female cuckoo vocally mimics a hawk

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.

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.

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

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Posted in mimicry, parasitism | Comments Off on Double deceit

Floral dress

Pollinators are deceived by flower mimicking crab spider

Epidacus heterogaster sucessfully mimics a flower

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. The 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.

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.

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

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