These Baby Fish Are Born Knowing How to Kill

By Elizabeth Preston | August 4, 2016 9:15 am


Some babies are born totally useless (I’m looking at you, Homo sapiens). Others can wobble upright shortly after birth and start teetering around. And still other animals are almost frighteningly precocious.

For example, the metallic livebearer, a little golden fish native to Cuba, hatches from an egg while still inside its mother. That means the mom gives birth to live young. The more traditional fish-y way is to lay eggs. But some other fish also bear live young, including guppies and most kinds of sharks.

Metallic livebearers (Girardinus metallicus) are hunters very with precocious babies. As soon as a newborn G. metallicus swims into the world, it’s ready to murder.

Physiologist Martin Lankheet and his colleagues at Wageningen University in the Netherlands wanted to know how much of a baby livebearer’s skill is inborn. Which deadly skills develop while it’s still in the womb, and which does it learn in its first days of life?

They studied 28 G. metallicus to learn more. The fish were born at night, and early the next morning, researchers placed each one in its own Petri dish. They set up a high-speed camera to observe the fish hunting. Then, one at a time, they dropped tiny prey animals into the dishes and waited.

The prey were brine shrimp larvae—babies that, unfortunately for them, aren’t born with any special skill for escaping. Researchers tested the little livebearers on their first, second, and third days of life. Each day, they fed the fish until they were full. The video camera captured more than 2,000 prey captures in all, and from these videos the researchers analyzed the hunters’ every movement.

Metallic livebearers hunt by swimming close to a prey animal and slurping it up like a vacuum cleaner. Hours-old fish were already proficient hunters, succeeding in about 80 percent of their attempts to catch prey. And they got even better as time went on. On their second day of life, they succeeded about 90 percent of the time. By day three they were close to perfect. (The fish in the video above is two days old. Its target is the oblong speck near 10:00 in the Petri dish.)

Video analysis showed that the babies had honed some skills while they were still in the womb. The way they tracked prey with their eyes, for example, didn’t change much as they got older. Neither did their pattern of speeding up just before striking. They knew how to do these things despite not having room to exercise their muscles before birth, or light to see by, or prey to practice capturing.

Other skills improved with time. “The most surprising finding for me was that the newborn fish cannot use their tail for directed swimming during prey capture,” Lankheet says. They can flick their tails to escape a threat, but rely on their pectoral (side) fins to aim themselves at prey. Once the fish get a couple of days older, their mastery of their tails improves. This lets them whip their bodies toward prey more quickly.

Adult livebearers are adept and eager hunters. In fact, they’ll even eat their own babies if it’s convenient—apparently that time inside the mother’s body doesn’t create much affection.

Image: by Michael Göllnitz (via Wikimedia Commons)

Lankheet, M., Stoffers, T., van Leeuwen, J., & Pollux, B. (2016). Acquired versus innate prey capturing skills in super-precocial live-bearing fish Proceedings of the Royal Society B: Biological Sciences, 283 (1834) DOI: 10.1098/rspb.2016.0972

  • John Do’h

    Live bearers (Poelicidae) have their eggs hatch inside (like mammals) and the fry pop out when the eggsack is absorbed and they are ready to swim… and wow, they learn how to swim and eat just like all outside of the womb egg hatching fish that absorb their eggsacks and are able to swim.

    What is the new research here? Am I missing something? What did they expect? When a fish is able to swim it learns to eat, which involves murdering living organisms. If fish are slow to learn to eat they won’t survive.

    • CarolAST

      My newborn guppies were immediately able to suck in their fish food.

    • Balint

      I’m not even close to being able to claim that I know what I’m talking about but I thought the research was interesting because it showed that hunting and killing behavior and skills didn’t need to be taught? I guess the newborns had no chance to observe other members and learn from them. So maybe that’s a new thing, seeing that skills and knowledge can be inborn rather than learned. I am out of my league so I could be wrong.

    • JohnnyMorales

      When it comes to science it’s assumed vs. demonstrated/proved.

      While you, I and anyone who’s had pet livebearers as fish and who have observed them closely would know this, the wider scientific world doesn’t decide what is a fact that way, nor can you assume that someone who’s never been interested in fish beyond eating them would have any idea or thought about this subject.

      For something to become a scientific fact, the observed behavior must be documented, and the study published so other researchers can try and repeat the results or see the same observations or not.

      On something like this I imagine since their claim is not absurd, and as you said sounds like it would be an obvious thing, it will likely see little opposition and be readily accepted and deemed “scientific fact” instead of a widespread assumption ;).

    • Prefabsprout

      Most Livebearers do not have their eggs hatch inside them, the females have an egg which is fertilized and then grows by reproducing cells. The egg does not hatch inside them. Also, most livebearer fry still have a small yolk sack when born and that is all they need to feed on to survive the first few hours/day. They then start to eat anything small enough to fit into their mouths.

  • Nicole

    Fish do not murder. people murder. Fish just need to eat and if they have no parents to feed them as in birds, they must hunt for themselves.



Like the wily and many-armed cephalopod, Inkfish reaches into the far corners of science news and brings you back surprises (and the occasional sea creature). The ink is virtual but the research is real.

About Elizabeth Preston

Elizabeth Preston is a science writer whose articles have appeared in publications including Slate, Nautilus, and National Geographic. She's also the former editor of the children's science magazine Muse, where she still writes in the voice of a know-it-all bovine. She lives in Massachusetts. Read more and see her other writing here.


See More

@Inkfish on Twitter


Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

Collapse bottom bar