[This is my second post of Sex Week]

In the sexual universe, all sorts of things can feel good, even if we human have a hard imagining how they can bring any pleasure. Electricity, for example, may be nothing for us beyond a painful shock. But for some fishes, it is the essence of desire.

The rivers and lakes of central Africa are home to a couple hundred species of fishes called mormyrids. Their tails are packed with special cells that can produce electric discharges, and they use other kinds of cells embedded in their skin to detect the field they produce. If another fish passes by them, the field becomes deformed, and the mormyrid can sense the difference.

What must it be like to be able to sense electricity this way? It’s certainly nothing like the blind pain we feel. Their specialized sensors let them sense subtle changes. They probably get a feeling that’s a bit like touch, a bit like the sensation of heat, and a bit like hearing. It’s like touch in the sense that the fishes can sense electricity across their entire body and they interpret the sensations by creating body-shaped maps in their brains. It’s like the sensation of heat in the way it is diffuse, rather than creating sharp boundaries. And it’s like hearing in the way that fishes can sense fine differences in the frequency of the electric pulses they produce.

Mormyrids are so sophisticated in their sense of electricity that they can use it to hunt in total darkness. They can scan the bottoms of rivers and lakes for hidden prey. In one experiment, German scientists tested the sharpness of mormyrid electrolocation by putting larvae under one of two shapes, a diamond and a pyramid. The fish could quickly learn to head straight for the shape with the food under it.

Mormyrids also use their electric pulses to communicate with each other. In Lake Malawi, for example, Matthew Arnegard of Cornell University and Bruce Carlson of the University of Virginia observed packs of up to ten mormyrids hunt together for smaller fishes for days at a time. The fishes sent out crackles of electricity to each other to stay in close touch as they roamed for prey.

Mormyrids have also borrowed their electric senses for a less deadly purpose: courtship. The fish have tiny eyes, and so they probably don’t care much about how attractive other mormyrids look. In the darkness of their nocturnal world, there’s not much point in putting on a fancy courtship dance or to build an attractive nest on the river bottom. Instead, mormyrids find electricity sexy.

To find a mate, male mormyrid fishes produce electric pulses, which nearby females can detect. Females don’t just rush to the first courtship pulse they detect, though. Several species of mormyrids may live in the same waters, and so if female mormyrids indiscriminately mated with any male that produced a signal nearby, the barriers between the species would soon collapse.

Instead, the females are picky. The males of of each species produce their own unique electric pulse. The photograph shown here illustrates just a few of them. In a paper with the fabulous title, “Electrifying love: electric fish use species-specific discharge for mate recognition,” German scientists demonstrated that females are attracted to the pulses made by males of their own species more than those of other species.

Females don’t just discriminate between species. They also discriminate among the males of their own species. Peter Machnik and Bernd Kramer, two biologists of the University of Regensburg, documented this female preference in a mormyrid species known as the bulldog fish. When a male bulldog fish sings his electric courtship song, an interested female will swim up to him and butt her head into his. The male will float passively, releasing more electric pulses, while the female continues to head butt him dozens of times. Then the female spawns her eggs, which the male fertilizes. (Hey, it’s a mormyrid thing; you wouldn’t understand.)

Machnik and Kramer noticed that different male bulldog fishes produced different pulses. The main variation was their duration: some bulldog fishes produced longer pulses than others. To see of that difference mattered to the females, the scietnists programmed a device to produce pulses like a male bulldog fish and put it in a tank with females. The females were so enamored by the pulses that they head-butted the device. Machnik and Kramer discovered that the females head-butted more when the pulses were longer.

Female bulldog fishes may be attracted to long pulses for the same reason that yeast like strong pheromones: it’s an honest advertisement from desirable males. When bulldog fishes produce courtship pulses, they can put themselves at risk. Catfish, which prey on the bulldog fishes, also sense electricity. If a male makes long signals, he’s putting himself at greater danger. Stronger males can afford that risk better than weaker ones.

Recently, Arnegard and his colleagues studied how these signals have evolved over millions of years. They compared a dozen different species of mormyrids from a small area of jungle in Gabon. The scientists figured out how the species were related to one another, and then they compared the mormyrids in terms of both their anatomy and their electric pulses.

They found a big difference. Even the most closely related species of mormyrids had very different signals. In fact, their signals were about as different from each other than they were from their most distant relatives at the Gabon site. Their bodies, on the other hand, evolved much more slowly. Closely related species looked more like each other than they did to their distant cousins.

This difference may say a lot about how new species evolve. The first thing that changes when two populations of mormyrids start to diverge is their electric song. The fact that individual mormyrids vary in their songs demonstrates that those songs have the potential to evolve. The electric pulses might diverge for any number of reasons. Some fishes might live with dangerous catfish, for example, while others might escape to predator-free waters where their long songs would go unpunished. Once the pulses start changing, females might evolve preferences for the new patterns. Once those preferences take hold, they will keep a population of mormyrids from mating with other fishes. The two populations would start to diverge genetically, until they branched apart into two separate species.

It may be no coincidence that mormyrids have evolved into a vast number of species–200 and probably more–while their closest non-electric relatives have only evolved into 10 species. Their electric sex lives have become an engine of biodiversity.