Bird doesn’t attack the spider, but its web decoration instead

Thanks to a striking web decoration, the spider Cyclosa monticola escapes from hungry birds, as Nina Ma and colleagues show. The birds’ attacks fail.

The spider Cyclosa monticola, which is common in East Asia, constructs an elaborate web. Not only does it consist of sticky threads, but it also carries a striking, linear band of trash, containing moults, prey remains, and pieces of leaves and stems. According to Nina Ma and colleagues, this detritus decoration diverts attacks of predators, especially birds, away from the spider.

The spider is in the centre of its web, hardly visible in the decoration band, which extends in both directions. The animal’s colour is similar to the colour of the decoration. Birds cannot distinguish the colours of the spider from that of decorating trash.

Fine scissors

The researchers wondered whether Cyclosa monticola, a tasty snack for many birds, is safer as a result. In order to find out, they exposed spider webs to domestic chickens chicks, one web per chick. Some chicks were given a web with the resident spider present in it; from half of these webs, the researchers had removed the decoration with fine scissors without damaging the web. For comparison, other chicks were given either an empty web or a web with decoration only. Most chicks quickly pecked at whatever (spider and/or decoration) they saw. The researchers were interested in their first target.

Spiders that had been robbed of their web decoration were grabbed by the chick in almost all cases. Spiders that had kept their decoration survived much more frequently. In this situation, the chick usually did not peck at the spider, but at the trash, and the spider dropped down quickly to escape. Obviously, the decoration did confer safety.

More attractive

Although the spider is hardly detectable among the decoration trash, the protection was not only due to camouflage, as the researchers argue. Because in that case, attacks would be random and the risk for the spider to get captured would be equal to its size relative to the size of the decoration. However, the chance of being captured was lower and independent of the size of the decoration band. Apparently, the detritus decoration is more attractive to birds to peck at and diverts their attention away from the spider.

So, web decoration is an effective defence strategy.

The question now is whether insect prey will not avoid the structure. After all, the trick of a spider web is that insects don’t discern the threads and get caught in the web. But research on another spider species that adorns its web shows that insects are attracted to the decoration – and get stuck in the web. The web decoration of Cyclosa monticola may also have this effect; if so, it would have a double function.

Willy van Strien

Photo: web with Cyclosa monticola and detritus decoration. ©Shichang Zhang

Sources:
Ma, N., L. Yu, D. Gong, Z. Hua, H. Zeng, L. Chen, A. Mao, Z. Chen, R. Cai, Y. Ma, Z. Zhang, D. Li, J. Luo & S. Zhang, 2020. Detritus decorations as the extended phenotype deflect avian predator attack increasing fitness in an orb‐web spider. Functional Ecology, online July 16. Doi: 10.1111/1365-2435.13636
Tan. E.J., S.W.H. Seah, L-M.Y.L. Yap, P.M. Goh, W. Gan, F. Liu & D. Li, 2010. Why do orb-weaving spiders (Cyclosa ginnaga) decorate their webs with silk spirals and plant detritus? Animal Behaviour 79: 179-186. Doi: 10.1016/j.anbehav.2009.10.025

Thanks to black irises, female guppy escapes from predator

By drawing the attention of a predatory fish to the eyes and turning the head away as soon as it strikes, a guppy female manages to evade. Robert Heathcote and colleagues report this hitherto unknown escape strategy.

When Trinidadian guppies detect a predatory fish, they will approach and inspect it to find out if it is hungry and dangerous. The colour of their irises may change when they do; normally the irises are silver, but then they often turn black, making the eyes more salient. It doesn’t seem profitable to draw an enemies attention to the head, so Robert Heathcote and colleagues wondered why guppies blacken their eyes. Is it to deter the enemy? Or is it to divert its attack? But then, how does it work?

By conducting a series of experiments, they found the answer: the colour change is part of a successful escape strategy.

Wild Trinidadian guppies, Poecilia reticulata, live in northeastern South America. One of their enemies is the cichlid Crenicichla alta, a predatory fish that ambushes its victims.

First, the researchers exposed wild guppies to a visually-realistic model of this predatory fish in a tank, and observed whether they blackened their eyes. As it turned out, large individuals do. These are mostly females, which are larger than males on average.

Attack

The predatory fish is not deterred by those dark eyes, as became evident from the next series of experiments, this time with live predatory fish and models of guppies with either black or silver irises. The cichlid attacks guppies with black eyes as often as those with silver-coloured irises. So, the first possible explanation fails.

The researchers also investigated at what target the predator lunges when attacking its prey. When irises are silver-coloured, the predator aims at the broadest part of the body, they discovered. In dark-eyed fish, the attack is diverted to the front. So, colour change of the irises appears to be a diversion strategy. But the predatory fish grasps both types of models, with black and silver irises, just as easily, so a guppy doesn’t benefit from a dark eye colour in itself.

Matador

However, colour change does help when combined with a critically timed evasive manoeuvre, as the last tests showed. In these tests, living guppies were placed in a tank with a living cichlid, but were separated from it by a transparent acetate screen, keeping them from danger. From the movements of the fish, filmed with a high-speed camera, the researchers were able to calculate, for each attack, the probability that the predator would have caught the victim in real conditions, without screen.

The moment the predatory fish strikes, a guppy quickly pivots around an imaginary vertical axis, accelerates and swims away. The imaginary axis runs through the broadest part of the body (more precisely, through the centre of mass), roughly the point that the cichlid aims for in a victim with silvery, less conspicuous irises. This part of the body hardly moves during the rotation. If the predatory fish directs its attack at that point, its chances to succeed are high, as the analysis showed.

The head, on the other hand, immediately leaves its position during the rotational movement. If the predatory fish charges at that part of the victim – like it does in a prey with black irises – it usually misses out.

By blackening its eyes, a guppy thus increases its chance to escape with a quick manoeuvre. The researchers compare this escape strategy – drawing the enemy’s attention to a particular point and then moving it quickly away – with the behaviour of a bullfighter, the matador with his red cape. Such escape strategy was previously unknown in animals.

Only females

For the strategy to be successful, the distance between the eye and the broadest part of the body must be sufficiently large. Males are too small. Males also have a striking eye-sized black spot on their body, which makes it more difficult to draw the predator’s attention to the head. So it makes no sense for males to blacken their irises in presence of a predatory fish, just increasing the detection risk. Accordingly, they don’t.

But females can trick their enemies by making their eyes stand out. The predator aims for her attractive eyes. And they’re gone.

Willy van Strien

Photo: Guppy, Poecilia reticulata, female with silver iris. H. Krisp (Wikimedia Commons, Creative Commons, CC BY 3.0)

Source:
Heathcote, R.J.P., J. Troscianko, S.K. Darden, L.C. Naisbett-Jones, P.R. Laker, A.M. Brown, I.W. Ramnarine, J. Walker & D.P., 2020. A matador-like predator diversion strategy driven by conspicuous coloration in guppies. Current Biology, online June 11. Doi: 10.1016/j.cub.2020.05.017

Ladybird cannot deal with all enemies at once

When a ladybird has to defer predators regularly, it is less able to resist pathogens and parasites, Michal Knapp and colleagues write.

When threatened, ladybird beetles try to avoid being eaten by excreting a yellow, smelly and bitter-tasting liquid from their legs. This reduces the appetite of hungry insects, lizards, birds or small mammals. The liquid is haemolymph, the insect variant of blood. You can see the phenomenon by provoking a ladybird.

But you shouldn’t do that, because ‘reflex bleeding’ decreases the ability to fight pathogens and parasites, as Michal Knapp and colleagues report.

They conducted experiments with the harlequin ladybird, Harmonia axyridis. The species originally lived in East Asia, was introduced in Europe and North America and nowadays also occurs in South America and Africa.

Precious blood

Haemolymph is an expensive means to scare away enemies. It contains nutrients, as well as blood cells, proteins and other compounds that ladybirds need to eliminate pathogens and parasites. The harlequin ladybird uses, among other compounds, the substance harmonine, which has a strong antimicrobial effect. Each bleeding causes a loss of these valuable components.

To measure the effect of this loss, Knapp triggered reflex bleeding in ladybirds twice a week, during three weeks. Contrary to his expectations, the treatment did not affect the survival of the beetles, and they did not lose weight.

He also, during a month, triggered newly hatched females daily to bleed, and found that their reproductive capacity was unaffected. In their first month of life, they produced as many eggs as females that were untreated. They started laying eggs a few days later, though, especially after losing a high volume of haemolymph. That may be of little importance, however, as the beetles live for months.

Costs

But bleeding, the defence mechanism against predators, comes at the expense of the resistance to other enemies, as it turned out. The concentration of blood cells and proteins in haemolymph had decreased. The concentration of harmonine and similar compounds has not been measured, but other research indicates that it also will have decreased.

Indeed, haemolymph of ladybirds that bled was found to inhibit bacteria less strongly. Probably, these ladybirds are less resistant to parasites as well, as blood cells take part in defence, but this has not been investigated.

Ladybirds successfully deploy constituents of haemolymph against all types of enemies – but they cannot fight them all at once at full power. If they have to deal with hungry predators frequently, their resistance to pathogens and parasites is reduced.

Willy van Strien

Photo: Harlequin ladybird, Harmonia axyridis. Timku (via Flickr, Creative Commons CC BY-NC-SA 2.0)

Source:
Knapp. M., M. Řeřicha & D. Židlická, 2020. Physiological costs of chemical defence: repeated reflex bleeding weakens the immune system and postpones reproduction in a ladybird beetle. Scientific Reports 10: 9266. Doi: 10.1038/s41598-020-66157-9