Fish sand castles and pits reveal rapid courtship evolution in Lake Malawi cichlids

Ryan York from Stanford University has looked at the various courtship rituals of cichlid fish at different depths in Lake Malawi. His findings, published in the journal Frontiers in Ecology and Evolution, shows the species evolved different behaviour rituals very quickly according to the depth at which they live and other environmental factors.

Lake Malawi is about five million years old, meaning behavioural evolutionary changes in the cichlids' happened within this short period.

The researchers made family trees for 75 species of Lake Malawi cichlids, looking at which courtship technique each employed. Two core types of bower (fish nest) were identified: in the shallow waters, where light is good, males build sand castles to attract mates; in darker, deeper waters, they dig pits and put on elaborate swimming displays to entice females.

However, findings showed that among the disparate species, the closet relatives of sand castle builders often dig pits and vice versa. This suggests the two core bower-building techniques evolved repeatedly across the various species according to shifts in ecology and biology, such as the average depth at which each feeds.

"At the start of the study we did know that some species built castles and some dug pits but were not aware of the extent to which these behaviours occur in Lake Malawi," York told IBTimes UK. "We were furthermore very surprised to find that similar bower types have evolved over and over again, seemingly at least in part due to the same environmental and biological variables."

Castle building is time consuming and is more striking, but this technique does not pay off when there is little light. Digging pits takes less time and effort, so these fish use the extra energy to put on swimming displays – spending twice as long performing a special dance than the castle builders.

"We found that both species shared a characteristic 'dance' in which males court females by circling around them while shaking their bodies. Interestingly the differences we observe seem to have more to do with how much the males rely on these dances to attract females compared to how much they rely on their bowers for attracting mates."

Castles and pits are only used during courtship. The female lays eggs in the castle or pit to be fertilised by the male. She then keeps them in her mouth for several weeks until they hatch – never eating in this time.

The findings, York said, show courtship behaviours can evolve very quickly: "While we do know of many cases in which things like body shape and colouration have evolved rapidly we know of many fewer in which behaviours have done so.

"Our study suggests that, under the right conditions, behaviours too can diversify very rapidly and in some cases may evolve convergently where two distantly related species come upon the same behaviour independent of each other.

"Given the very high rate of evolutionary change observed in Lake Malawi, especially for traits related to sexual reproduction such as bower-building, we expect that these behaviours will continue to diversify and change as long as fish can survive in the lake."