The Taming of the Shrew

It is a warm night in the northern African veldt. The scene is soothing, with serenades of crickets chirping occasionally interrupted by the calls of nocturnal birds. As a background to the nighttime chorus, a soft wind occasionally blows through the tall grass and shrubs, a gentle hissing flowing through the flora. The half moon provides some light for the scene, casting shadows onto whatever patch of ground is visible through the grass. The sky is clear and the stars are bright.

But life is busy as usual, and soon out of the grass a small figure appears. It is a shrew, medium-sized, running under the foliage and across bare patches in search of something. It stops and listens often, its ears twitching. Does it think one of the nocturnal birds is a predator, perhaps? Does it hear a snake in the grass? Whatever the reason, it does not stop for long, again dashing through the brush, its movement seeming undirected, even random. The moon rises higher as the shrew wanders on.

Suddenly its path becomes linear and its speed increases, its ears zeroed in on a target. Its target gets louder as the shrew gets closer: It's a male cricket, calling for a mate in the darkness, surely unaware of the peril it is in. The shrew seems sure of itself, not making any effort to sneak up on its target as it darts between tufts of grass. As the shrew gets close, the cricket stops singing, but much too late. The shrew walks right out of the grass and into the small divot its target was calling from. The cricket's fate is sealed and it makes no attempt to escape as the shrew closes in.

But the inevitable does not happen. The shrew, easily able to gobble up its find, simply walks right up to the cricket and sits there. The cricket, still alive, doesn't flee. The shrew sniffs the cricket and the cricket reciprocates, tapping the shrew with its long antennae. They seem curious, almost fascinated, investigating the other for several moments. Then the shrew turns around, saunters back into the tall grass, and, miraculously, the cricket follows behind it. The shrew leads, heading in nearly a straight line at a constant speed while its companion follows close behind, occasionally touching the shrew with an antenna. They cover a fair distance, winding through the brush and over rocks, a little faster in clear patches and slower in heavy cover, a steady parade of two. Suddenly, the shrew stops.

It is at the opening of a large hole in the side of a small mound, cleared of debris evidently for easy entry. The cricket feels the ground; something is different here. It scuttles confidently into the hole as if it had been there a hundred times, feeling up and down, while the shrew's mind wanders elsewhere, constantly listening and looking at every sound and sight in its purview, alert as usual. But what the cricket has found is incredible.

It's a horn! A well-formed exponential horn, excellent for singing far and wide, complete with a bulb just behind where the cricket will be compelled to sit. Even the walls are seemingly crafted for this purpose, for they are packed down and hardened, great for resonating sounds. The cricket walks around, scratching here and removing some particles there, doing what little work there is to do to make a perfect home. It then settles into the optimal position and begins to call, prompting the shrew to leave. And what a marvelous call it is, glaring, shrill, booming, and so clear and loud, broadcasting to an enormous swath of grassland, drowning out anything quieter.

This was no coincidence, for both the shrew and cricket knew what they were doing from the very start. The shrew had constructed several of these horns in choice places, working on them by night and retiring to its own home in the day, keeping them in mind as it hunted after sunset. Upon hearing a certain species' chirp in the distance, it had followed the signal with resolution. The cricket it found is chirping not only to attract a female cricket, but for a creature of a certain smell and overall appearance to pop out of the foliage to greet it. As it waits, it sits in the best home it can find, for it can barely dig. Having found its target, the shrew leads it straight to one of its horns, the cricket following instinctively, and if the cricket accepts its new home, it will be rewarded with a sonorous call like no other. But what does the shrew get out of the deal?

The shrew isn't constructing houses for free; it is very much interested in what is attracted to the loud noise. Small frogs and lizards will sometimes show up, sometimes a predatory orthopteran, maybe a smaller shrew, rarely a parasitoid tachinid fly will arrive looking for a host. All these and more are good food for the shrew, who will wait just within earshot and intercept whatever approaches the now-silent cricket. It arrives in the direction directly facing the horn's opening, attacking the distracted prey now cornered by the horn that attracted it. It can take down animals its own size, useful for a relatively small creature. However, what the shrew partakes in at least as often is a twist of cruel irony, for much of its diet is precisely what the cricket wants to attract: female crickets. When it is hungry and a female cricket has found the male, the shrew will intercept the female, eat it, and leave the male to call again just the same.

What kind of deal is this, where your bodyguard will more likely eat your mates than your predators? The male cricket is not fazed, at least; its perfect home gives it plenty of mates even with such a hungry creature nearby. For the females, of course, it is a different story! Female crickets use the chirps of males as a measure of fitness to be a mate, so indeed, having a portion of females killed because they found what appeared to be an ideal mate is bound to cause selection against falling for such traps in female offspring and participating in a deal with the devil in the males. However, the benefits to both sexes are too strong. The majority of male crickets call into the night the normal way, unguarded by shrews, so searching for loud males is still beneficial for females since death to shrews is not a very common cause of mortality. Males are of course still strongly inclined to such a deal, for even just a chance of being found and housed is worth it, but with each death the female population drifts further away from participating further. Thus in each cricket egg a war is being fought, with the males pulling toward one strategy and the females the other, but natural selection has decreed a victory largely for the males. So the female crickets maintain the best method of choosing mates despite the losses, the shrews take care of their cricket tenants, and the male crickets get the best deal of all: plenty of mates and a personal bodyguard.

So who domesticated who in this strange relationship? Did the shrew tame a species of cricket for its food, or did the cricket tame a big shrew bodyguard? Or maybe it was just a fluke? Well, while that question is certainly arguable either way, one fact makes it even more complicated: There certainly is a fluke.

Particularly, this fluke is a type of parasitic flatworm. Most flukes live in snails as their first hosts, as these ones do, and because of this dependence on water they are native not to the veldt but to an area near the western coast of Africa. Here the grasslands give in to a Mediterranean climate, with shrubby trees and plenty of vernal pools. There is a distinct rainy season and dry season, the former occurring during winter and spring and the latter during summer and fall. The prime time for these flatworms was in early spring, with the rains of winter having brought full vernal pools and the warmth of spring returning, providing a suitable habitat for the freshwater snails used as hosts for the young flukes.

As summer came and the pools dried up, the flukes would wait in stasis until a certain species of cricket came by, foraging under the growing foliage. These would eat the dead snails or their waste, unwittingly infecting themselves. Now within the cricket, the fluke needed to find its way into its definitive host, a certain species of shrew, where it could reproduce. To accomplish this it engaged in a craft that flukes, and indeed internal parasites in general, are very adept at: Altering their hosts' behavior. The fluke caused its cricket to lose its fear of larger animals approaching, reducing the chance that it will flee from a predator, and even made them attracted to the pheromones the target shrews used to communicate, causing the cricket to wander through their territories. This inevitably got the fluke into its definitive host, where it could grow into an adult.

However, the fluke has a problem. It is a schistosome, a type of fluke that, very unusual for flukes, has separate male and female sexes instead of being hermaphrodites. Thus, it needs to increase the chance it will find a fluke of the opposite sex in the same shrew it is in, and to do that, it must cause the shrew to eat more crickets. So again it alters its hosts' behavior, causing the shrew to crave the flesh of crickets, tracking them down by their calls. Since infected crickets would likely not run away, this tactic made it more likely for other flukes to enter the shrew. Once two flukes mated in their host's body, they would lay their eggs for the shrew to pass in its feces. These feces would often occur in vernal pools, since in the summer and fall they would be dry, but even if they were deposited elsewhere, the rainy season would come again and wash them in, allowing future snails to eat the feces and start the cycle anew.

The cricket and shrew were quick to adapt to their new situation. Because they were drawn by the smell, male crickets often found themselves in the burrows of the shrew, where their calls were louder due to resonance. The shrews, in turn, learned not to eat these crickets because they would attract many more female crickets to satisfy their cravings, as well as some predators as a hearty bonus. Thus, kickstarted by the fluke, the cricket and shrew began forging their unusual relationship, becoming closer and closer intertwined with each generation. By now it is strong enough that the relationship persists deep into the African veldt where the fluke is completely absent, an odd pair of comrades in a new land.