Category: defence (Page 3 of 4)

Ant larvae help to resist hostile take-over

When the nest of the ant Formica fusca is taken over by a parasitic queen of another species, the colony is lost. But the larvae help to limit the damage, according to Unni Pulliainen and colleagues.

An ants’ nest contains a large workforce serving the queen, which has the exclusive task to reproduce. Worker ants feed the queen and take care of her offspring, keep the nest clean and defend it. Their diligence attracts the attention of queens of other ant species that have not yet workers and for that reason could use some help. The black ant Formica fusca often suffers from such queens, which may invade a nest to exploit the workforce – thereby destroying the colony. But the host may resist, Unni Pulliainen and colleagues report.

Parasite

If a hostile queen tries to enter a nest of Formica fusca, which lives in clear-cut forest areas and along forest edges in Europe and parts of southern Asia and Africa, the workers may detect her and kill her. But that doesn’t always happen; sometimes, they accept her.

Once she’s inside, she can go on. She kills the resident queen or queens – in Formica fusca, a few queens usually live together in one colony – and she will start laying eggs. The workers have to raise her offspring as if they were the offspring of their own queen. The foreign queen, which outlives the workers, gradually acquires her own workers, while the original workers die. By temporarily parasitizing the Formica fusca colony, she founds her own.

Sabotage

But the enslaved workers can limit the damage by sabotaging. The workers can remove the foreign eggs. And the orphan ant larvae seem to help.

Ant larvae sometimes eat ant eggs, and Pulliainen wanted to know if Formica fusca larvae might be keen to consume the eggs of a foreign queen. In experiments, she offered larvae one egg each, either of their own queen or of a foreign queen, which belonged either to a parasitic species or to an innocent species that never invades other ants’ nests.

The larvae never consumed an egg of their own queen. But when they were given an egg from a parasitic queen, they consumed it in one in ten cases; eggs of an foreign innocent queen were consumed less often.

Future

The feeding behaviour of the larvae, albeit not very spectacular, may help to limit the damage. The eggs are nutritious and their consumption may increase the orphan larvae’s chance of survival. Male larvae can leave to reproduce as adults. And some of the female larvae will be future queens, which may found a new colony elsewhere. Female larvae destined to become workers can be successful too. They are not able to mate, but they can produce some sons, as sons develop from unfertilized eggs. The colony may be lost, but some larvae still have a future.

Willy van Strien

Photo: Formica fusca. Mathias Krumbholz (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Sources:
Pulliainen, U., H. Helanterä, L. Sundström & E. Schultner, 2019. The possible role of ant larvae in the defence against social parasites. Proceedings of the Royal Society B 286: 20182867. Doi: 10.1098/rspb.2018.2867
Chernenko, A., M. Vidal-Garcia, H. Helanterä & L. Sundström, 2013. Colony take-over and brood survival in temporary social parasite of the ant genus Formica. Behavioral Ecology and Sociobiology 67: 727-735. Doi: 10.1007/s00265-013-1496-7
Chernenko, A., H. Helanterä & L. Sundström, 2011. Egg Recognition and Social Parasitism in Formica Ants. Ethology 117: 1081-1092. Doi: 10.1111/j.1439-0310.2011.01972.x

Crayfish avoid light when renewing their armour

Normally, the red swamp crayfish is rather fearless. But if it has to replace its carapace with a new one, its bravery disappears, as Julien Bacqué-Cazenave and colleagues report.

Crustaceans do not have a skeleton inside their body, like we do. Instead, they have a carapace, an external skeleton. This sturdy box in which they are packed protects them from physical harm. But there is a drawback: the carapace limits body growth. That is why the animals must, from time to time, replace their carapace with a larger one. The old one is shed, a new one is formed.

That is no trifle, as Julien Bacqué-Cazenave and colleagues show.

Process takes a month

The researchers wanted to know how the red swamp crayfish, Procambarus clarkii, is doing during a moult. The species originally occurs in Mexico and the south of the United States and has been introduced in many other places; it has settled as an exotic species in Europe.

Its moult is a lengthy and complex process. The chitin, of which the carapace consists, is secreted by the epidermis and the carapace is attached to it. So, it is must be separated from the epidermis, which has to form a new one. The attachments of the muscles that are anchored to the armour have to be transferred.

As soon as the old carapace is shed, the new one is exposed. This leaves the crayfish unprotected and vulnerable, as the newly formed carapace is thin and fragile in the beginning. It has to thicken and harden before it can protect the animal. The entire process of moulting takes about a month: two weeks before the old armour is shed and two more weeks until the new armour has hardened.

Anxious

The red swamp crayfish normally is courageous, but during the month of moulting, especially during the third week, it is not at ease, as experiments conducted by Bacqué-Cazenave show. He tested the animals every two or three days in a plus-maze with two illuminated and two dark arms. Crayfish that did not experience any stress spent 40 percent of their time in the illuminated part of the plus-maze. But when they were about to shed their carapace, they began to avoid the light a few days in advance, and the first week after moulting they stayed in the dark areas almost continuously. From earlier work, the researchers knew that the animals behave like this when they are anxious.

The aversion to light was indeed associated with moulting, according to tests in which the animals were given a hormone that initiates the moulting process, a so-called ecdysteroid. But when the animals were also given a tranquilizer, they did not avoid the illuminated areas. From this, the researchers conclude that the light aversion is an anxiety reaction.

Obviously, the period of moult is hard. But when it is over, the crayfish is safe in its armour for the next two to six months.

Willy van Strien

Photo: Andrew C (Wikimedia Commons, Creative Commons CC BY 2.0)

Source:
Bacqué-Cazenave, J., M. Berthomieu, D. Cattaert, P. Fossat & J.P. Delbecque, 2019. Do arthropods feel anxious during molts? Journal of Experimental Biology 222: jeb186999. Doi: 10.1242/jeb.186999

Hovering guards of bee colony position themselves orderly

It is difficult for a robber bee to stealthy approach a colony of the stingless bee Tetragonisca angustula, because hovering guards will detect it. These guards arrange themselves in an organized manner, Kyle Shackleton and colleagues show.

Workers of the stingless bee Tetragonisca angustula defend their colony extraordinary well against their enemies. Some workers are dedicated guards; they are heavier than other workers and have longer legs. Such specialised soldier caste is not known in other bee species. And while during the day always guards are standing in or near the nest entrance, there are often also some hovering in front of it to keep an eye on the access route, especially in the afternoon. Such an airmobile brigade is unique too.

Robbery

The most important enemy is the robber bee Lestrimelitta limao. Robber bee workers do not collect nectar and pollen from flowers themselves, but get it from colonies of other species. They also steal food that is prepared for the larvae and nest constructing material. Tetragonisca angustula, with its large colonies, is vulnerable. No wonder, then, that there are guards that keep an eye on what is near the nest. It is important to deal with an approaching single robber bee at once, because it is a scout. It will recruit hundreds of others for a raid that will last for hours or days.

As more hovering guards are active, such a flying intruder is detected earlier and intercepted at a greater distance from the nest. The guards recognize the robber bee from its odour and colour; it is black and smells like lemon. The guards wrestle it to the ground by clamping to an antenna or wing. They are not able to kill it, because it is three times heavier than a they are. But they may stop it.

Maximal field of view

Often only a few hovering guards are hanging in front of the nest. Kyle Shackleton and colleagues now show that these guards do not choose their position randomly, but in a coordinated way. If two guards  are hovering, there will usually be one on the left and one of the right side of the access route to the nest. In case of three guards, it rarely occurs that all of them hover at the same side. And four guards mostly are distributed evenly; sometimes sometimes three guards hover at the one side and one at the other side, and it hardly happens that all four guards are at the same side. Because of this coordinated distribution, the hovering guards have a maximal field of view and they will discover an approaching flying enemy as fast as possible.

In case of immediate danger, more guards will be hovering in front of the nest. An even distribution between left and right is less important in that case, because together they will have a good view anyway. There is no surveillance at night, for in the evening the bees close the nest entrance with wax.

Willy van Strien

Photo:
A Tetragonisca angustula hovering guard bee next to a nest-entrance. Bibafu (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Sources:
Shackleton, K., D.A. Alves & F.L.W. Ratnieks, 2018. Organization enhances collective vigilance in the hovering guards of Tetragonisca angustula bees. Behavioral Ecology 29: 1105-1112. Doi: 10.1093/beheco/ary086
Grüter, C., C. Menezes, V.L. Imperatriz-Fonseca & F.L.W. Ratnieks, 2012. A morphologically specialized soldier caste improves colony defense in a neotropical eusocial bee. PNAS 109: 1182-1186. Doi: 10.1073/pnas.1113398109
Grüter, C., M.H. Kärcher & F.L.W. Ratnieks, 2011. The natural history of nest defence in a stinngless bee, Tetragonisca angustula (Latreille) (Hymenoptera: Apidae), with two distinct types of entrance guards. Neotropical Entomology 40: 55-61. Doi: 10.1590/S1519-566X2011000100008
Van Zweden, J.S., C. Grüter, S.M. Jones & F.L.W. Ratnieks, 2011. Hovering guards of the stingless bee Tetragonisca angustula increase colony defensive perimeter as shown by intra- and inter-specific comparisons. Behavioral Ecology and Sociobiology 65: 1277-1282. Doi: 10.1007/s00265-011-1141-2

Scale worm deceives its enemy by detaching glowing scales

If the fifteen-scaled worm is attacked by a lobster, green lights appear in the water. They are scales and tail segments that the worm released to escape, as Julia Livermore and colleagues show.

It is a cheerful sight: animals that emit light, such as fireflies. In marine habitats, this sight is much more common than on land, because most light artists live in dark water. They use their light to defend themselves against predators or to lure prey, or they perform a light show to seduce a partner.

The fifteen-scaled worm, Harmothoe imbricata, uses its light for defence, Julia Livermore and colleagues write. The worm, three to six centimetres in length, occurs everywhere on the northern hemisphere, from the inter-tidal zone to a depth of a few kilometres. On its back it carries fifteen pairs of disc-shaped scales for protection, which flash green light when the worm is irritated. It can also detach the scales, upon which they float through the water as little lights for some time. If it is in great danger, it also releases its posterior body segments, which then also emit light.

Deceived

The researchers show how this behaviour enables the worm to actually escape from its predators, crabs and lobsters. They brought worms into the lab and conducted experiments in which they exposed a worm to the American lobster, Homarus americanus, or the green shore crab, Carcinus maenas; a refuge for the worm to hide was provided.

If an enemy approaches, a worm sometimes tries to slowly swim away unseen. If that fails, its scales will flash and / or the worm detaches scales and sometimes also segments of its tail, which then emit light. Because of the movements of the animals, the parts will swirl in the water. The predator is deceived: it goes after the lights and grabs them – and it will eat the dropped scales and tail segments.

In the meantime, the worm gets a chance to safely escape, as it turns out. Especially when it drops tail segments, its chance of survival is high. The swirling lights therefore function as an effective defence, but also an expensive one: the worm sacrifices protective scales and sometimes also a piece of its tail. That will regenerate, but it takes a while: a few days for the scales, a few weeks for the tail. But the sacrifice may save its life.

Willy van Strien

Photo: Harmothoe impar, a scale worm that is closely related to Harmorhoe imbricata. Saxifraga-Eric Gibcus

The researchers filmed experiments with a crab predator

A flashlight fish uses light to lure prey

Sources:
Livermore, J., T. Perreault & T. Rivers, 2018. Luminescent defensive behaviors or polynoid polychaete worms to natural predators. Marine Biology 165: 149. Doi: 10.1007 / s00227-018-3403-2
Verdes, A. & D.F. Gruber, 2017. Glowing worms: biological, chemical, functional diversity or bioluminescent Annelids. Integrative and Comparative Biology 57: 18-32. Doi: 10.1093 / icb / icx017
Plyuscheva, M. & D. Martin, 2009. On the morphology of elytra as luminescent organs in scale worms (Polychaeta, Polynoidae). Zoosymposia 2: 379-389. Doi: 10.11646 / zoosymposia.2.1.26

When under attack, skink exposes its entire blue tongue

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

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

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

Reflexive recoil

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

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

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

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

Willy van Strien

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

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

Covered with sponges, a crab is poorly visible

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.

Velcro

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.

Defence

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

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

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