How Do Shell-dwelling Cichlids Build Perfect Nest?

The individual steps of nest-building are instinctive, but only through practice do the fish become true masters

To the Point

• Nest-building in fish: Shell-dwelling cichlids build nests from empty snail shells, using them for shelter and to raise their young.
• Stereotypical behavioral sequence: The fish follow a set sequence of actions to bury the shell in the sand. However, they can respond flexibly to variables, such as the shape of a snail shell.
• Innate behavior: Even fish with no prior exposure to snail shells know how to transform one into a nest. However, it takes them longer the first time and they become more skilled with practice.
• Cognitive components: Alongside instinctive behaviors, the ability to adapt and learn also plays a role in nest-building.

We associate nests with shelter, warmth, and a safe retreat - and usually picture a bird's nest made out of twigs, grass and feathers. Yet many other animals take advantage of such refuges, with nests being built by a diversity of species ranging from termites to great apes, which impress with their hugely varied forms and the wide array of materials used to construct them.

For fish, nest-building comes with an added challenge as they must put together their underwater nests equipped with 'only' their fins. Yet fish too have developed a remarkable variety of nest-building innovations, burrowing into sandy lake beds, creating masses of floating bubbles on the water's surface, or setting up camp in abandoned snail shells repurposed as nests - as is the case with the shell-dwelling cichlid Lamprologus ocellatus.

Endemic to Lake Tanganyika in Africa, these cichlids use empty snail shells for shelter and to raise their young. To do so, the snail shell is positioned and covered in sand in a very specific way, leaving just the opening exposed - only then does it become the perfect home.

And this is precisely where fascinating questions arise from a biological perspective: How do cichlids know what the nest should look like and how to build one? Is this knowledge innate, or must it first be learned? A team from Herwig Baier's department at the Max Planck Institute for Biological Intelligence has now investigated these questions in a new study.

Sequence of different behavioral motifs

Working with 3D-printed shells, the team showed that nest-building follows a set sequence of different behavioral motifs and takes the fish around three hours on average. After encountering a snail shell, the fish dig a pit using their body and mouth. Then they grasp the shell with their mouth and maneuver it clockwise into the pit, repeating this until the shell sits tip-down in the sand, with its opening protruding upward. Finally, rapid body movements are used to flick sand over the shell, covering the nest.

To test whether this behavior is innate, the team raised cichlid fish in aquaria without snail shells. When these fish were later introduced to shells as adults, they instinctively began displaying typical nest-building behaviors. At first the unpracticed animals were rather clumsy about it, taking an average of around 12 hours to finish making a nest out of a shell.

However, this clumsiness didn't last. When exposed to shells in two more sessions (each separated by ten days), they created their nests noticeably faster and with more skill, completing their third round of nest building in around four and a half hours.

This shows that the individual steps in nest building are innate, but cichlids only become master nest builders through practice. Remarkably, fish that were given another chance at nest building after a whole year without access to snail shells still displayed the skills of practiced architects and did not have to start from scratch - they were able to remember what they had learned even after this long period.

Flexible behavior

The researchers also explored whether the fish could adapt to unexpected challenges to their nest-building program. They exposed the fish to 3D-printed, sinistral snail shells. These shells are extremely rare in nature and are a mirrored version of the natural shell of right chirality. Amazingly, after just a short time the fish learned to simply change direction, rotating the shell counterclockwise rather than clockwise into the sand as they do for dextral shells.

"For a long time, it was assumed that nest building consisted of purely innate behavioral patterns. But studies in birds and our own research show that cognitive abilities such as learning, remembering and adapting also play important roles," says Swantje Grätsch, project leader at the Max Planck Institute for Biological Intelligence and first author of the study. "Accordingly, we were able to show that during nest building in cichlids, brain regions homologous to the mammalian hippocampus are active, which is known to be responsible for precisely these abilities. We are only just beginning to understand how complex this goal-directed behavior and the underlying processes in the brain really are."