Meet the male fish who builds sand castles just to attract females

Lake Malawi. Mark Mainz/Getty Images for HELP Malawai Foundation

Lake Malawi, an African great lake, has a curious fish called the cichlid which changes its mating habits depending on the depth of the water.

In the shallows where the light is good, males build sand castles to attract females.

Those who live in deeper water dig less elaborate pits and compensate with longer swimming displays.

The results of a study into the fish are published in the journal Frontiers in Ecology and Evolution.

Here’s the fish at work:

“Lake Malawi cichlids are famous for the diversity and fast evolution of their feeding habits, body form, and sex determination system,” says Ryan York, a graduate student at Stanford University and lead author of the study. “Here we show for the first time that their courtship rituals also evolve exceptionally fast.”

The evolution of cichlid courtship seems to be driven by shifts in the average depth at which each species feeds.

The theory is that the castles require more effort to build but are more striking to females in clear, shallow waters. At greater depth where light is scarce, castle building doesn’t pay off.

The researchers say castle-building species live at an average depth of 15 metres in Lake Malawi, compared to 30 metres for pit-digging species.

Pits and castles are only used during courtship and mating and have no other function.

If a female likes what she sees, she lays her eggs inside the pit or castle, to be fertilised by the male.

She then keeps them in her mouth for several weeks, never eating until they hatch.

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