It is not difficult to see the video of a wary shark wandering in the oceans, hunting for food. It is more unusual to observe the same behavior within the body of a female: in 2016, it happened to Kiyomi Murakumo, biologist and researcher of the Okinawa Churaumi Aquarium, in Japan.

While examining a tawny nurse shark pregnant female (Nebrius ferrugineus) with an underwater ultrasound machine, Kiyomi noticed the outline of a shark embryo that was not simply turning over in the mother's body, but seemed to swim from one uterus to another ( mothers this and other species have two, both functional).

Serious pregnancies. Over the next two years, the aquarium scientific team observed this type of migration in three other female nurse sharks. Their scientific observations came together in a scientific article entitled: Locomotion is not a privilege after birth ("locomotion is not a post-birth privilege").

In some cases, an embryo appeared in a uterus during an ultrasound, and in the other in the next; in others, the four embryos were distributed in one or the other uterus, sometimes fairly and sometimes crowding in only one; in one case a small shark was seen swimming from one uterus to another at a speed of 7.5 cm per second.

Where will he have hidden them? But what are these movements for? According to ethologists, to procure food. In another species of shark, the bull shark (Carcharias taurus), the first embryos to come out of the fertilized eggs, still inside the maternal uterus, devour all the other eggs present in the same uterus, even before they can hatch. The resulting embryo thus eliminates competition in the management of nutritious resources (non-fertilized eggs), and at birth it is already in force, perfectly able to defend itself from most predators.

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Go away, intruder! These unborn sharks are so active that Stewart Springer, the first biologist to discover their survival tactics, was bitten by one of them, as he introduced his hand into the birth canal of a pregnant female. The nurse shark embryos do not eat the brothers but still feed on the non-fertilized eggs released by the mother: this could be the reason for the swims seen with the ultrasound.

A preview. However, shark behavior before birth is still a rather mysterious matter. In one of the females studied by the Japanese team, pregnant with four specimens, an embryo briefly faced the canal of the maternal birth, as it was observing the outside world. Biologists have managed to look at their faces, almost as if they were at the window, but the reason for this behavior remains unknown.