Unborn baby sharks swim between uteruses to eat their siblings

Sibling rivalry is common in most species. Human twins will jostle in the womb, as shown in a viral video here (don’t mind the Joshua and Esau comparisons):

Some species take it to a new level, though. It’s well known that sharks eat their siblings in utero – but we had no idea how far it goes. Researcher from Okinawa Churaumi Aquarium in Motobu, Japan designed and created a portable ultrasound machine which can be brought underwater and can withstand high pressure.

Portable ultrasound machines have been in use for a while, but the team in Motobu found that pulling a shark out of the water to scan them was well…inconvenient. Science Alert tell us more:

[Researchers] were surprised to find the unborn pups of captive tawny nurse sharks (Nebrius ferrugineus) not only moving around their own uterus, but moving house altogether.

“Our data shows frequent embryonic migration between the right and left uteri, which is contradictory to the “sedentary” mammalian fetus,” the team write in their report.

The researchers studied 3 tawny nurse sharks in the aquarium’s exhibition tank over a period of a few years and got over 40 ultrasound clips in that time period.

In these time-lapsed clips, you can see the embryos swap from the left uterus to the right uterus. In one of the mothers, the embryos swapped sides three times. In another, the movement was accelerated – a total of 24 migrations were recorded.

Science Alert continues:

The pup swam at a speedy eight centimeters per second.

One of the female sharks started her pregnancy with two pups in each uterus. After some back and forth, four soon became three. Just under two months later, there were two.

By the end of the gestation just a single, no doubt smugly satisfied, winner remained.

The act of migrating between uteruses has been a known possibility since 1993, when there was footage caught of a sand tiger shark embryo migrating to the opposite uterus. However, since this evidence was caught during a high-stress operation, it was unclear if it could be recreated under other circumstances.

This cannibalistic migration behavior has only been seen in nurse and sand tiger sharks. Other species of orectolobiform sharks have much calmer gestations, with only mouth movement being detected in embryos. The reason for this, researchers speculate, is the fact that these types of sharks are not sustained by a placenta, but rather surplus eggs and the occasional sibling.

The ocean can be a strange place, filled with brutality. For a much cuter and less violent sibling rivalry, check out these chubby corgis: