07.03.2023: Neolamprologus pulcher (N. pulcher) is a species of cichlid found on rocky coasts in East Africa. They are one of only a handful of highly social fish species in the world. Instead of dispersing, they often prefer to stay at home. A research team from the Konrad Lorenz Institute of Ethology at the University of Veterinary Medicine, Vienna, has now investigated the reasons for this unusual social behaviour – and thus complements scientific theories explanations for the evolution of complex sociality in fishes. The long-term study has just been published in the top journal "Science Advances".

Extreme pressure from predators has previously been seen by scientists as the main reason for the unusual social behavior of N. pulcher: This is because a fish that wants to leave its home range is likely to be eaten. So it pays to stay at home.

Only particularly strong and fit individuals in good condition can overcome these “ecological constraints” (the name of the corresponding well-known theory). Weaker animals, however, are forced to guard the home and have very little room for self-realization “under their parents´ fins” - the chance of having their own offspring is thus extremely low.

First such long-term study on the cichlid species N. pulcher.

To confirm this common hypothesis, however, measurements of actual reproductive success have been lacking until now. In the world's first long-term study of this kind, a research team from Vetmeduni therefore examined N. pulcher under natural conditions. First author Arne Jungwirth of the Konrad Lorenz Institute of Ethology at the Vetmeduni said: "We measured the lifespan, reproductive success and social status of nearly 500 tagged fish over a period of up to five years.

Both sexes benefit from sedentariness

It turned out that both sexes benefit from staying at home, as survival probability and reproductive success increase. This is not consistent with the predictions of "ecological constraints," but rather with another classic theory - the benefits of sedentariness (benefits of philopatry).

"That both sexes benefit equally from sedentariness is surprising in that they differ both in dispersal behavior - males move around more - and in other aspects of their life history strategies. Females, for example, grow more slowly and to a smaller maximum size, but then live much longer," says Arne Jungwirth.

Polygyny with consequences: Males fight more and therefore have to relocate

According to Arne Jungwirth, the scientists found the following explanation for the fact that males have to move more often: "Competition between males prevents them from settling down more often than females - male cichlids fight more because there are fewer territories for them: There is only about one breeding male for every two breeding females, because the species practices polygyny."         

 

Photos of Neolamprologus pulcher (N. pulcher), all taken by Arne Jungwirth/Vetmeduni