Forced adoption

Cuckoos, catfish and even mushrooms take diverse ways to stay with their hosts. Cheating, punishment and survival mechanisms

Yonat Ashhar and Noam Levithan Galileo

The neighbors are not good in my eyes.

How will I sit here, mother of twenty rabbits,

With a promiscuous cuckoo the boys?

All her sons grew up in foreign nests,

Everyone is deserted, everyone is promiscuous.

What lesson will my children learn from this?

(The rabbit snorts at the cuckoo, from "A Apartment for Rent" by Leah Goldberg, Poalim Library, 1959)

Most of the time, when we think of parasites, we think of small, nasty animals that live on and inside the bodies of larger animals and feed on them. But there is also another form of parasitism, in which the parasites do not exploit the bodies of their hosts, but rather their social customs - thus guaranteeing themselves or their offspring food and protection. Social parasites are expert tricksters and the best known example is, of course, the cuckoo. As the rabbit points out, many of the cuckoo species (although not all) do not bother to build a nest and raise their young themselves, but lay their eggs in the nests of other birds, and trust them to raise the young. In Israel, the cuckoos usually sneak their eggs into the nests of gray crows. It should be noted that the cuckoos, despite being the most famous social parasites, are certainly not the only ones. Different species of birds, including the indicator (Ovil, Indicator), the cowbird (Molothrus) and even a species of duck also deserve the rabbit's scolding. And as we will see later, this custom is not limited to birds only.

The Cuckoo 's Nest

Social parasitism may sound sympathetic in relation to intestinal worms, for example, but it also costs surrogates dearly. Not only do the adoptive parents - against their will - contribute their time and money to the raising of a foreign chick, in many cases the parasitic chick hatches before its "siblings", the legal residents of the nest, and makes sure to throw them out even before they hatch. This is how the chick ensures that all parental care will reach him and only him. But in doing so, he does something else, unintentionally - since the damage caused to the parents is so great, he ensures that eventually the bird species used as surrogates will evolutionarily develop defense mechanisms, learn to recognize the foreign eggs and throw them out of the nest.

Cuckoos trust that most birds follow a few simple rules of thumb: If it's in your nest and it's egg-shaped, incubate it. If it is in your nest, making noises and barking - give it food. The shape, color and even the size of the egg or the chick are usually not taken into account, and thus you can sometimes see small birds devotedly feeding a cuckoo chick many times larger than themselves! But the parasitic behavior means that an error in identification may be critical, and natural selection will favor birds that are able to avoid it. Indeed, experiments have shown that birds living near cuckoos or other parasitic birds tend to recognize foreign eggs that have been put into their nests and throw them out, unlike birds living in areas where such parasitism is not common.

The parasitic birds of course fight back, and produce eggs that imitate the eggs of the surrogates. But some of them also have another, darker method of making sure that their chick is taken care of properly.

About 15 years ago, researchers from Florida conducted a study on the bird Protonotaria citrea, one of the New World's entanglements. This woodpecker is a host of the wall-headed cowbird (Molothrus ater). The cowbird chicks do not throw the nestlings out of the nest, and grow together with them. The nestlings, for their part, almost never recognize the parasitic eggs and do not remove them from the nest. The researchers made nests for the tusks on poles coated with oil, to prevent their usual predators from reaching them. In more than 95% of the nests, the chicks reached maturity - the tangled chicks and also, if the nest was a target for the cattle birds, the parasitic chicks. A few years later, as part of another experiment, the researchers removed the cowbird eggs from the tangle nests in which they were laid. The researchers discovered that when they did this only about 60% of the nestlings reached maturity and the rest of the nests were destroyed. Who destroyed the nests, given that the usual predators couldn't get to them? The researchers suspected the parasitic birds, and went to check it out. It turned out that the cowbirds do raid the nests, but they don't do it randomly. When the researchers left the parasitic eggs in the nest, only 6% of the nests were destroyed, compared to 56% of the nests from which the eggs were removed. As for the nests where no cowbird eggs were laid, 20% of them were destroyed. There is an opinion that the raids on these nests were also done in order to increase the reproductive capacity of the parasitic bird - when the nest is destroyed, the parents usually build a new nest and lay more eggs, thus creating another opportunity for the cowbird to lay its own egg in the nest. It should be noted that the cattle bird does not feed on eggs or chicks.

The researchers concluded that the cattle birds do not abandon the eggs completely, but check whether the adoptive parents take care of them, and "take revenge" on the parents who throw the eggs out. This form of behavior has received the appropriate name "the mafia method", and it may be very effective, for two reasons: first of all, a pair of terns whose nest was destroyed after throwing away the parasitic egg may learn the lesson, and not do it again in the event that cowbirds again lay an egg in the nest (and indeed Studies were done that showed that the surrogates "learned a lesson"). Second, from an evolutionary point of view, nestlings that recognize the foreign eggs and throw them out will be less successful in rearing their offspring as their nests will be destroyed. This may lead to the fact that natural selection will rather prefer birds that will not bother to identify the parasitic eggs: although raising a foreign chick will cost them time and resources, this is better than demolishing the entire nest, considering that, as mentioned, the cowbird chicks do not throw their "brothers" out of the nest.

The mafia behavior is not, apparently, characteristic only of the cattle bird - even the cuckoos themselves are able to take revenge on those who throw their eggs from the nest. A study conducted in Spain on the crested cuckoo (Clamtor glandarius), which is also common in Israel, found that they used to raid the nests of their local host by wagging their tails. Even in this case, the absolute majority of attacks were made on nests where the foreign eggs were thrown from the nest.

Full to the brim

Sneaking a foreign egg into the nest is one thing, but there are animals that take social parasitism one step further - and sneak their eggs into the mouths of their hosts. It is a species of catfish, Synodontis multipunctatus, known as "cuckoo catfish", and there is a good reason for that. This fish lives in Lake Tanganyika in Tanzania, which is also home to many species of tilapia. In some of these fish, the mothers incubate the eggs inside their mouths, until the fry hatch and are able to swim on their own. About 25 years ago, researchers from Kyoto University in Japan noticed a strange thing: they caught several dozen tilapia females with eggs in their mouths, but these were not tilapia eggs, but catfish eggs.

This is how the breeding method of the cuckoo catfish was discovered: these fish follow the tilapia fish, and when a pair of tilapias begins the work of reproduction - the catfishes join the celebration. It is important to note here that the fertilization of both species of fish is in vitro, that is, the female lays the unfertilized eggs in the water and the male scatters the sperm over them, thus fertilizing them. Immediately after the tilapia eggs are fertilized, and before the female has time to put them in her mouth, the female catfish lays her eggs among the tilapia eggs, and the male catfish fertilizes them. Thus, the eggs that reach the female tilapia's mouth are a mixture of her eggs and the foreign eggs.

Similar to the custom of cuckoos, the eggs of the cuckoo catfish also hatch before the eggs of their hosts. In their case, the catfish do not throw the "legal" eggs out of their mother's mouth, but do something that may seem even more horrifying to us: they eat them. In this way, the cuckoo catfish ensures that its offspring will enjoy both a protected place to develop in, and a hearty first meal when they hatch. What does the tilapia think when it opens its mouth and a school of catfish comes out? One can only guess.

A mushroom in the kindergarten

In another case of social parasitism, the "adopted" offspring do not just belong to another biological species, but to another kingdom. The hosts here are not birds or fish, but social insects - termites.

Different species of termites of the genus Reticulitermes lay elongated, white eggs, but round and brown balls are often found together with them. These "termite balls", as they are called, are nothing but fungi of the genus Fibularhizoctonia. They are packed in a structure called a sclerotium, which allows the fungus to survive in harsh conditions. The termites treat the cashew balls just like eggs: they place them in the special rooms for eggs, clean them and wrap them in an antibiotic mucus that protects them from bacteria as well as from dryness. The fungi do not grow inside the egg pile, but over time the ball becomes shriveled and deformed, and then the termites throw it from the pile into the corner of the nest as garbage, where the fungus grows and thrives and produces new termite balls, which the termites recognize as eggs and return to the pile. In this way, the fungus creates for itself a living area free of competitors, since the termites make sure to remove every other fungus from the nest, but apparently do not notice the "cheats" of the fibulorizoctonia.

And of course the main question: how do the termites fall into this trap? After all, to our eyes the round, brown eggs look so different from the eggs, there is no similarity between them. What, they don't see?

The simple answer is: no, they don't see - since the working termites are blind, so the issue of color is irrelevant. The shape might have given them a clue, but the termites consistently grab the eggs from the rounded end, and thus do not differentiate between an elongated body and a round one. But this does not mean that the termites will treat any rounded body as an egg. Experiments in which beads of varying size and weight were inserted into a termite nest showed that the insects would only handle bodies whose diameter was very similar to that of an egg. Weight was also important, but less so than size. Not surprisingly, the fungi that live in termite nests produce cassions of a constant size, which exactly mimics the size of the eggs (on their rounded side), unlike other species of fungi, where the size of the reproductive bodies varies. The surface area is also important for identification: the eggs are very smooth, and in order to imitate the touch of the mushrooms, they gave up the upper, hard layer, which characterizes the callus in other mushroom species. This makes them much more vulnerable, and they have to rely on the termites for protection. But it seems that this barter deal is paying off for them: studies have shown that almost 100% of the mushrooms that started to grow produce new cassions.

Apart from size and surface area, fungi have one more trick they use to fool termites: chemical warfare. In a study published a year ago by researchers from Okayama University in Japan, the substance released by the mushrooms to mislead their hosts was revealed, an enzyme called beta-glucosidase. It is an enzyme whose function is to break down cellulose, a hard plant material, and is quite common in fungi as well as in the saliva of termites. On top of that, the enzyme is also secreted by termite eggs and marks them as such. The fibulorhizoctonia use it to get termites to treat them, and experiments have even shown that they secrete it only when they feel they are near the termites. Apparently, the fact that both termites and fungi digest cellulose with this enzyme allowed the development of the egg mimicry of Fibulorhizoctonia.

The social parasites seem especially sophisticated to us, because we are used to seeing cheating as an action that requires thought, planning in advance and maybe even malice. But one should not conclude from this that the parasites are consciously trying to deceive their surrogates - they are nothing more than performing the actions that allowed their parents and their parents' parents to survive and reproduce, and to pass on the genes responsible for these actions. Social parasitism, like any other parasitism, is nothing more than an evolutionary adaptation: from this point of view there is no real difference between the exploitation of the host's resources through the behavior of attacking nests or laying eggs in the body of another fish, and between exploiting them with the help of special body structures, such as that of the tick that is able to stick to its skin of the dog and suck his blood.