Fish Grow More Like Their Partners


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

252 Fish Grow More Like Their Partners
When you look at them, you wouldn't think they had many marital differences to sort out. Chloé Laubu

They say opposites attract, but it seems when they do, they don't stay opposites for long. At least, that is, if convict cichlids (Amatitlania siquia) are anything to go by. These monogamous fish prefer to pair up with others of similar personality, but if that doesn't happen, they adapt to find common water.

“Behavioral similarity between partners is likely to promote within-pair compatibility and to result in better reproductive success,” according to a paper in Science Advances. The authors provide evidence from pair-bonded species such as zebra finches and cockatiels. Consequently, species with plenty of opportunity when choosing mates tend to form well-matched pairs. However, the paper confirms what many of us know all too well: “Mate searching is very costly and does not guarantee finding a matching partner.”


Many animals choose instead to mate with a less-than-perfect match. Some, the notoriously fecund convict cichlids included, stay together for the sake of the children, putting aside marital differences to jointly defend against predators and raise the next generation.

First author Ph.D. student Chloé Laubu of the Université Bourgogne Franche-Comté investigated how less-than-ideal pairings work out by putting male and female cichlids with opposing personalities together. She found that with time they became more similar, but didn't see both fish changing equally to meet in the middle.

It's easier to scare off predators if you and your mate are on the same wavelength. F.-X. Dechaume-Moncharmont and C. Laubu

Laubu measured behavior on a proactive-reactive scale, which combines tests for boldness, aggressiveness, and likelihood to explore. “Proactive individuals are better competitors, as they have access to better quality sites and they usually specialize in aggressive activities such as territorial defense,” the paper observes. “Reactive individuals are better at adjusting to unstable environments and invest more in non-aggressive activities such as direct parental care.”


As the paper notes: “Synchronized behaviors can be very important, for instance, for defending their territory against intrusion or for pooling nest visits to reduce the likelihood of the nest being detected by predators.” Laubu put both matched and mismatched pairs together and watched how they behaved.

Both proactive and reactive pairs produced more offspring and had fewer conflicts than pairs that contained one of each personality type. Some of the proactive-reactive pairs proved stuck in their ways, but others adjusted. Those that became quite similar produced as many young ones as the matched pairs, but those pairs who remained incompatible had substantially less mating success.

Where a mismatched pair adjusted, the reactive member usually became more proactive, while the proactive partner's behavior didn't change. This was unaffected by whether it was the male or female fish that was more reactive.

Many may ponder about the implications for people. The paper observes that “studies on humans have yielded inconsistent results.” Drily, the authors add, “For obvious ethical reasons, protocols based on experimentally forced pairs with mismatched partners are complicated to design.”


Perhaps this also explains why it is said people come to resemble their pets.


  • tag
  • mate choice,

  • convict cichlids,

  • domestic harmony,

  • pair formation