Certain groups of animals show a remarkable capacity for quickly evolving into new species to seize control of unexploited niches in the environment. And among these ecological opportunists, there are few better examples than the cichlids, a group of freshwater fishes that are one of the most varied group of back-boned animals on the planet.
In the words of Edward O. Wilson, the entire lineage seems “poised to expand.” The Great Lakes of Africa – Tanganyika, Malawi and Victoria – swarm with a multitude of different species; Lake Malawi alone houses over 500 that live nowhere else in the world.
All of these forms arose from a common ancestor in a remarkably short span of time. Now, a new study suggests that this explosive burst of diversity has been partly fuelled by rivalry between hostile males.
Michael Pauers of the Medical College of Wisconsin found that male cichlids have no time for other males that look like them and will bite, butt and threaten those who bear the same colour scheme. In doing so, they encourage diversity in the lake since mutant males with different tints are less likely to be set upon by territorial defenders.
This process is just part of the cichlids’ tale. Their rise to dominance in the African lakes probably happened in several stages and were driven by different evolutionary forces.
In the beginning, different habitats almost certainly played a role. The Great Lakes may be single bodies of water but they are nonetheless huge, and contain several different habitats from shallow inlets to deep basins. As the ancestral cichlids spread through these, they became isolated from groups elsewhere.
From there, they started to exploit different food sources. Today, some graze on algae, others crush snails with powerful teeth and yet others have turned into pike-like hunters. The result was different lineages of cichlids, each with their own home and lifestyles. The mbuna or rock-dwellers, for example, are a group of cichlids that all live among piles of rocks and are all mostly vegetarians.
But even among this single group, there is great diversity and until now, it was mostly credited to choosy females. A female cichlid chooses a mate based on physical traits like colour and pattern. Often this initial fancy is the result of arbitrary genetic changes but it has far-reaching consequences.
Consider a green female cichlid that develops a liking for blue. There are plenty of fish in the lake and she chooses a male on the bluer side of green. Their offspring inherit genes for both the sexy blue colour of their father and the preference for blue of their mother. Over time these two traits become linked and become stronger with each passing generation. This process was first proposed by Charles Darwin himself, who called it ‘sexual selection‘.
Meanwhile, another female of the same species develops a liking for yellow and her offspring head off on that evolutionary tangent. Cut to several generations later and you have two new lineages, a yellow one that likes yellow and a blue one that likes blue. Neither is attractive to the other and they do not cross-breed. Et voila, two new species.
Male vs. male
According to Pauers, this model is only half the story. By focusing on female choice, it sidelines the males and it turns out that they too have an important part to play. He was inspired by earlier studies which found that male cichlids bearing different colours were rarely seen in the same locations.
To find out why, Pauers recorded the behaviour of males from a single species (Metriaclima mbenjii) when they were confronted by other males. The rivals could see each other, display and attack but the separated containers ensured that no cichlids were harmed in the making of these results.
Pauers found that males were much more aggressive towards rivals that looked the same. Given a choice, M.mbenjii, a blue fish with a red top fin, was much more likely to threaten and attack another M.mbenjii male than an M.zebra male, whose black and blue body is topped with a blue fin. It was also more hostile towards males from another species, Laebotropheus fuelleborni, that also sport red fins.
The results suggest that male cichlids use colour as a badge to single out rivals who could compete for the limited number of breeding sites. This fierce competition creates an evolutionary vacuum that could be filled by males bearing slightly different colours. These mutants would be rare at first but by evading the violence wrought upon other males, they would soon gain a foothold in the population and become more common.
In this way, the hostile males and the picky females work fin-in-fin. Subtle changes in female preferences lead to male fish with slightly different colour schemes, which have a good chance of breeding successfully as other males squabble around them.
Reference: Pauers, M.J., Kapfer, J.M., Fendos, C.E., Berg, C.S. (2008). Aggressive biases towards similarly coloured males in Lake Malawi cichlid fishes. Biology Letters, -1(-1), -1–1. DOI: 10.1098/rsbl.2007.0581