In our limited way of seeing the world, some behaviors of the animal kingdom, so foreign, become almost alien to us. Of all the forms of behavior, those that have to do with reproduction are always the most striking and surprising.
From cannibalistic females to males that give birth, from fish that change sex, to animals that lose limbs during copulation, we find a great diversity of sexual acts in nature .
sexual cannibalism
When we talk about cannibalism during the sexual act, we immediately think of the praying mantis, although it is not as common a behavior as it is believed. In the natural environment, two out of three sexual encounters between a male and a female praying mantis end with both members alive.
This behavior responds to an advantageous adaptation for the female , especially when food is scarce. Cannibalizing their mate provides the female with quality nutrients that allow them to increase the size of their clutch.
The adaptation for the male is less intuitive , but it goes in the same direction: if in the environment the probability of fertilizing more than one female is low, sacrificing oneself will lead to a greater number of fertilized eggs that will inherit the genetics of the male.
However, it has been observed that there is also a counterproductive effect in cannibalism. When the male is not cannibalized, he has full control of his actions, including intercourse, and this allows him to insert the spermatophore – a small capsule containing sperm – much deeper into the female’s body, achieving greater success in fertilization.
The males that give birth
The case of seahorses represents, together with pipe fish and sea dragons, the only known cases of animals in which pregnancy is a matter of the father .
As in all vertebrates, the female produces eggs and the male produces sperm. However, and unlike what is usually common, during copulation it is not the male who introduces the sperm into the female’s body, but she who penetrates her partner with her ovipositor —a specialized appendage for laying eggs— and deposits the unfertilized eggs inside it, specifically in an organ called the brood sac , where they are fertilized and the egg shell decomposes.
Thanks to a tissue that the male produces and grows around the embryos, analogous to the placenta of mammals , it provides the embryos with the necessary nutrients and oxygen, and maintains salinity levels in optimal conditions.
When the young are ready, they are expelled through the same hole that the female used for laying. The male, in this way, literally gives birth to the young alive.
This reversal of pregnancy roles is also accompanied by a change in other behaviors; while the female is more competitive, the males are more demanding and selective with their partners.
Killer mother and overprotective father
Among aquarium enthusiasts there is a very attractive fish, the male Siamese fighter, which usually exhibits huge fins, bright colors, and a fierce territorial instinct. But it is also an easy fish to breed . This is important, since in its natural state it is threatened —classified as “vulnerable” according to the IUCN—, so captive breeding is the best way to keep these animals in an aquarium while protecting the species.
Although it is a fish, it breathes atmospheric air like us, thanks to an organ called the labyrinth, highly vascularized, analogous to our lungs , which expands through the interior of the skull from the top of the gills. Thanks to this ability, the male can create a very peculiar bubble nest , essential in the courtship process. The female will evaluate the nests of the males and will choose the one that she considers the best.
When the female is ready to spawn, the male strikes her belly, hugging her to fertilize the eggs as they hatch. The male then picks up the eggs with his mouth and deposits them in his bubble nest , where they will be protected. If a bubble breaks and the egg falls out, the male quickly picks it up with his mouth and makes new bubbles in which to deposit it. The dedication to the care of the eggs and then the fry is total . Until a week has passed, the little fish do not develop a swim bladder that allows them to swim freely, and therefore, they are totally dependent on their father’s care.
And this care is essential, because, after laying, the female tries to cannibalize the eggs or the young.
Losing the arm in the attempt
When octopuses and squids go to reproduce, the body of the male undergoes several transformations. The most relevant is the formation of the hectocotyl , a modified arm, at the end of which a structure is arranged with which to collect its spermatophore from inside its body and introduce it into the body of the female.
This form of copulation is peculiar enough, but the Argonauts are on another level. Despite the appearance of the females, with a shell reminiscent of that of a nautilus, the argonauts are octopods, that is, they belong to the octopus group. They have a very marked sexual dimorphism ; the shell of the female can reach 30 centimeters, although her body barely measures 10 centimeters; on the other hand, the male not only lacks a shell, but is five times smaller.
When reproduction occurs, the tiny male mates with the female, and inserts his hectocotyl into her body, like all octopuses. However, in this case the male later detaches from the arm, in a process called autotomy . For a long time, and until this strange behavior was unraveled, it was thought that the hectocotyls abandoned inside the female were some sort of parasite.
changing sex
During the first and sad scene of the movie Finding Nemo , a pair of clown fish, Marlin and Coral, proudly watch the enormous laying of 400 eggs until a terrifying barracuda puts an end to such an idyllic image, devouring the female and all the eggs minus one. The male, sad and lonely, then turns to raising the sole survivor, Nemo .
The beginning of the film is not very different from reality, with the difference that, generally, they do not form pairs but rather small schools . Each group usually occupies an anemone or a group of them, and is made up of a female —always the dominant one—, a reproductive male and several smaller males without the right to reproduce . Apart from that detail, the process is similar. The males carefully clean the anemone and, when the area is ready, the female lays the eggs, which are then watered by the reproductive male’s sperm.
A single clutch can have 300 to 500 eggs. The care of the laying falls on the male, who must clean and guard the eggs against possible intruders. But if a predator approaches, it is the female, larger and more aggressive, who usually defends the nest.
However, if the female dies —as happens to Coral in the film—, the school is left without a leader, and a very curious process begins. The reproductive male begins to accumulate estrogens and transitions to female in a short time . She becomes the dominant of the school, and the next male in the hierarchy becomes the reproductive male. This event is called protandrous hermaphroditism —from the Greek πρωτο-, prōto -, ‘first’ and ἀνδρός, andrós , ‘male’—, that is, although the animal throughout its life can have both sexes, they appear differently. sequentially, first it is male, they change to female only after a certain moment.
REFERENCES:
Goss, R. J. 1992. The evolution of regeneration: Adaptive or inherent? Journal ofTheoretical Biology, 159(2), 241-260. DOI: 10.1016/S0022-5193(05)80704-0
Hinojosa Ruiz, C. 2018. El ABC del pez Betta splendens. Publicación independiente.
Lawrence, S. E. 1992. Sexual cannibalism in the praying mantid, Mantis religiosa: a field study. Animal Behaviour, 43(4), 569-583. DOI: 10.1016/S0003-3472(05)81017-6
Madhu, K. et al. 2006. Protandrous hermaphroditism in the clown fish Amphiprion percula from Andaman and Nicobar islands. Indian Journal of Fisheries, 53(4), 373-382.
Stölting, K. N. et al. 2007. Male pregnancy in seahorses and pipefish: beyond the mammalian model. BioEssays, 29(9), 884-896. DOI: 10.1002/bies.20626
