Infanticide in the name of a new life

When Kronos swallowed his children, it never occurred to anyone that it was he from hunger. However, this is how they think about animals eating their own cubs (eggs, eggs). In biology, such behavior is called branch cannibalism. Moreover, the main threat is precisely the father, who was entrusted with caring for the offspring. And the males of some fish can consume all the masonry cleaning.

Eating their own offspring is now not considered a deviation from the norm, but a common adaptive strategy. The father, who protects the masonry, unable to leave, swallows part of the eggs, so that, having gained strength, how to take care of the remaining ones. But when animals destroy the masonry completely, then such an explanation does not pass. Who then to guard, whom to take care of?

Japanese scientists became interested in the reason for the total branch cannibalism: an employee of the National Research Institute of Fisheries in Tokhoku Yukio Matsumoto and Associate Professor of the University of Nagasaki Takesi Takegaki (Takeshi Takegaki) [1].

The object of their research was the Marine Dog Rhabdoblennius nitidus . This A small fish 5-6 cm long, which lives off the rocky shores of the western part of the Pacific Ocean. In the spawning season, which lasts from the end of June before the beginning of October, males occupy suitable places for masonry: holes that make bipped mollusks, or free shells of the belly -legged.

Females choose a nest and lay caviar on its inner surface in one layer. In a day or two in the nest, several trends of different ages gather in the nest, and the male cares about them until they will be spent (Fig. 1). During this period, males usually eat some of the eggs, but if there are less than a thousand (which approximately corresponds to three masonry), everyone can break.

Scientists staged a laboratory in small coastal backwaters. In a natural nest behind eggs, it is impossible to observe, so the males were offered nests from acrylic tubes stuck into the ground. Inside the tube is a translucent insert sheet on which caviar is laid. It can be pulled out, count the eggs, remove them or put them again. Earlier, researchers found that males do not distinguish between their own and other people's eggs.

All natural nests in the experimental site were laid with stones, and the males were measured, weighed, marked and released into their native waters, where they entered into the ownership of acrylic nests.

If we assume that eggs are eaten to reinforce the forces, then his physical condition must influence the male branch cannibalism. When the masonry is large and sea dogs eat part of it, it really is: less fed fish in two days destroy more eggs than large and well -fed. But if you leave less than 400 eggs in the nest, then the sea dogs eat everything on the first day. It turns out that partial cannibalism is really caused by a lack of energy, and the total should have other reasons.

Researchers have put forward a hypothesis that the matter is the male size of masonry. If it is too small, it makes no sense to spend time and effort on it. It is better to destroy the eggs and start all over again. Indeed, Kannibal fathers get rid of objectionable offspring on average 8–9 hours after they have taken it to guard, and the next day they renew the courtship and acquire new masonry. But what prevents them from combining the attachment with courtship, luring to the mink of other females, which the presence of someone else's caviar not only does not scare away, but even attracts [2]. It turned out that it is impossible to combine - hormones do not allow.

In the spawning season, the life of a male strip: the courtship phase and the parent phase alternate in it (Fig. 2). The period of concern for laying begins two days after the first egg and ends when the offspring begins to curse. These phases are distinguished by the concentration of androgens: testosterone and 11-ketotestosterone. During courtship, it is high, during the period of parental care, it is low, but by the end of the incubation period it increases again.

The success of mating is affected by the intensity of courtship, which requires a high level of androgens. Males at this time are quite aggressive: behind the female who leaves the nest, without laying off the caviar, they chase, pinch her, sometimes hold it by force until she leaves the masonry. In the parental period, the lack of androgens does not allow the male to effectively eliminate females, only 16% continue on courtship on the third day. And the sea dogs have to force as many eggs as they could collect in two days.

The only way to increase the level of androgens is to completely destroy the masonry and leave the parent phase. Scientists suggested that the secretion of androgens depends on the presence of caviar in the nest. To check this hypothesis, they removed all the eggs from the nest, and after a day the concentration of hormones from the robbed male jumped to the state of courtship. And vice versa, if you fill up with an empty nest of caviar, the level of androgens will become as low as the sea dogs protecting their own masonry. The fish becomes a father, bypassing the stages of care and fertilization.

Getting rid of the masonry, the Kannibals fathers not only eat caviar, but often spit it out of the nest. Perhaps the fact is that the stomach of the sea dog does not hold more than 300 eggs. And wait for everything to be digested, they have no time. They hurry to multiply. Such behavior, according to researchers, confirms the hypothesis that males destroy the masonry not from hunger. They just need to clean the nest and go to another phase of the reproductive cycle.

Total branch cannibalism turned out to be a way of hormonal self -transshipment. It is not clear why the sea dogs developed such a strange cycle. Theoretically, with a high concentration of androgens, they could guard the masonry, without stopping the females.

Most likely, the fact is that the high level of sex hormones reduces the immunocompetence of males. If this is so, fish, combining courtship and incubation, run the risk of getting infected themselves and infecting eggs. Another possible explanation is that the high level of androgens is poorly combined with parental duties. In some animals, including fish, fathers care about offspring, without stopping courtship and having an increased level of androgens. At the same time, they behave very aggressively, they monitor the masonry poorly and can even abandon them completely.

But from the point of view of courtship, the full branch cannibalism is unprofitable. Females choose nests in which there are already eggs, and they try to escape from empty nests. Males, starting a new cycle, completely clean the mink.

Nevertheless, sea dogs are not the only ones who behave in this way. It is known that males of two species of fish - thin -intelligent Etheostoma Etheostoma Flabellare and the ordinary bull -calf Pomatoschistus Microps - can also devour the whole, regardless of the presence of other food and their own exhaustion. Perhaps there are other species that bring themselves in this way to a state of reproductive readiness. And the term "cannibalism" in this case is not entirely successful. After all, fathers do not want to do, but to change fate. Like Kronos.

Natalia Reznik

1. Matsumoto Y., Tateishi T., Trada R., Soyano K., Takegaki T. Filial Cannibalism by Male Fish as Anfanticide to Restart Couurtship by Self-Regulating Androgen Levels // Curr. Biol., 2018, 28, 2831–2836, DOI: 10.1016/J.Cub 2018.06.056

2. Matsumoto Y., Tawa A., Takegaki T. Female Mate Choice in a Paternal Brooding Blenny: The Process and Benefits of Mating with Males Tending Eggs // Ethology, 2011, 2011 117, 227E235, DOI: 10.1111/J.1439–0310.2010.01868.x