Most species have what we call sexual dimorphism, which is the physiological difference between females and males. This difference affects, among other aspects, the reproductive system. That of the females is the one that is prepared to give birth or lay eggs and that of the males to fertilize the oocytes. This means that males cannot give birth or lay eggs because their organisms do not have the necessary organs to do so. What does happen in some species is that males are much more involved in reproduction than the mere fertilization of females. Two clear examples are those of seahorses and some species of fish.
In the case of seahorses, what happens is that the females transfer the eggs to an incubator bag carried by the males and in that bag, first the male fertilizes them internally and then incubates them, that is, keeps them safe to allow the embryos to develop.
Once the eggs have developed and the young are ready to hatch, it is the males who expel the eggs. And they do it through contractions similar to those of childbirth, even in their bodies hormones are released that are very similar to those released in the bodies of females during pregnancy. Pipefish have a system very similar to that of seahorses for the fertilization and incubation of their eggs. In some other species of fish what happens is that the males keep the eggs in their mouths until they hatch.
There are also cases of incubation of eggs by males in some species of birds. For example, in the case of the emperor penguin, it is the males that incubate the only egg that the females deposit in each clutch. Surely you have seen photographs or illustrations showing how these animals keep the egg warm between their paws while the female brings them food. In the case of rheas, which are South American birds similar to ostriches, it is the males that build a nest in which the females lay the eggs. But both the incubation and the care of the young once the eggs hatch and until approximately six months later are carried out exclusively by the males who are also, like the females of many species, especially aggressive towards other animals that approach the eggs or the young.
There are various evolutionary explanations for this fact, which, as I have explained, does exist in nature, although it is not common. The most accepted of these evolutionary explanations is that this type of shared parenting has an evident lower energy cost for females. As soon as the females have gotten rid of the eggs, since they do not have to incubate them, they are ready again to lay more eggs, so the survival of the species has a greater chance of success, since it will be able to increase the number of offspring.
There is another aspect that may also have an evolutionary explanation. In these cases of fertilization within the male’s own body, they ensure that the offspring will be his descendants.
But what I am explaining to you cannot occur in mammals because, as I told you at the beginning, mammals have sexual dimorphism: the reproductive systems of females and males are different and have different functions. Males lack structures such as ovaries, fallopian tubes, uterus or birth canal, necessary for gestation and giving birth.
Ana Josefa Soler Valls She is a professor of Animal Production at the University of Castilla-La Mancha, director of the Higher Technical School of Agronomic and Forest Engineering and Biotechnology and an expert in animal reproduction.
Coordination and writing:Victoria Toro.
Question sent via email byPaula García Pásaro.
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