This observation was apparently presented at the Evolution 2011 meetings in Oklahoma, but I missed it. Via Alex Wild at Myrmecos comes a report from New Scientist of the discovery of only second species (besides H. sapiens) that raises animals for meat.
It’s an ant! To be precise, it’s a species (it’s not clear if it’s more than one) in the genus Melissotarsus. And the work is apparently that of Scott Schneider, a graduate student in biology at the University of Massachusetts at Amherst. (All quotes below from the New Scientist piece.)
Lots of ants practise a rudimentary form of agriculture. Some are gardeners, gathering leaf fragments on which they cultivate a crop of tasty fungus. Others are dairymaids, “milking” the sweet excretion known as honeydew from aphids, scale insects and other related insects.
But the Melissotarsus ants of continental Africa and Madagascar are special. If biologists’ best guess proves correct, these ants raise their insect herds for meat, not milk – the first example of meat farmers other than humans. And that’s not all. The insects they cultivate may be the best example of true domestication outside of our crop plants.
You have to know what you’re looking for to even see Melissotarsus. The ants – barely 3 millimetres long – live most of their lives within the intricate gallery systems they excavate in and under the bark of trees. They’re such committed burrowers that their second pair of legs points up, not down, so they can get a foothold in the tunnel roof as well as the floor. They share their galleries with several species of armoured scale insects, so-called because most species secrete a tough, waxy scale that covers and protects them.
Note the position of the second pair of legs in this photo from Myrmecos and the following one from the Encyclopedia of Life:
If you have house plants, you’ve probably been plagued by scale insects (order Hemiptera: true bugs!), which often have a hard shell or cottony outer covering to protect them from predation. Here’s another photo, by the indefatigable Alex Wild, which he captions as “Camponotus rosariensis ant tending scale insects for honeydew. Notice that the younger scale insects have legs. These first instar scale are more mobile than the older instars, which settle down once they find a good spot for feeding.”:
Most ant/scale insect/honeydew systems are evolved mutualisms (or “symbiosis”), in which the ants tend the scale insects, protecting them from predation, while they eat the honeydew (the not-completely-digested sap exuded by the scale insects). Both species benefit, and some of these associations are at least 15 million years old. It’s clear that the ants have evolved special behaviors to “farm” these insects for the exudate, but I’m not sure whether the scale insects themselves have evolved to make them more attractive to the ants (if both species evolved, it would also be a case of “coevolution,” but I’m not sure about that here).
In the case of the Melissotarsus species, the scale insects “tended” by the ants don’t produce honeydew, nor do they have an edible wax exudate. The idea is that the ants are raising the scale insects as livestock, completely analogous to the way humans raise cows:
So what do the ants get from all their work housing and protecting the scale insects? Almost the only remaining possibility is that the ants sometimes make a meal of the insects themselves, Schneider reported at a recent meeting of the Society for the Study of Evolution in Norman, Oklahoma. No one has yet caught Melissotarsus in mid-munch, partly because the ants like their privacy and quickly seal off any peepholes into their galleries. Next year, however, Schneider will measure stable isotopes in the ants’ bodies, which will indicate whether their diet is mostly plant or animal in origin.
Now they haven’t yet seen the ants eating the scale insects, nor has this observation yet been published in the scientific literature, so we have to take it as provisional. (Presentations at meetings don’t count, at least in evolutionary biology, as “publications” because they’re not peer-reviewed). That’s why the “farming idea” is characterized by New Scientist as “biologists’ best guess.” The authors really do need to observe the scale insects being nommed, which is hard to do because the ants + scales are secretive.
But if the “meat farming” idea is true, you might well ask why haven’t the scale insects, who seem to gain nothing but death from this interaction, evolved defenses against the ants? I can think of two answers. First, maybe the scale insects really do benefit from being farmed. Perhaps, though kin selection, a “herd” of scale insects is actually a related group, with the eaten individuals “sacrificing themselves” for the good of their kin, who are protected from predation by species other than the “farmer” ants. This makes the testable prediction that the “herd” consists of related individuals, and the harder-to-test prediction that the genes of those individuals being “farmed” leave more copies of themselves than the genes of scale insects who aren’t farmed. (It’s not clear if the scale insects occurs in groups that aren’t farmed by ants.) That would also suggest that the scale insects may have some yet-undescribed adaptations for being farmed.
Alternatively, it may simply not have been possible for the scale insects to evolve defenses. After all, evolution isn’t perfect: every successful parasite represents a failure of adaptation by its host. If the genetic variation for protection hasn’t arisen (or can’t arise) through mutation, then there’s no counter-evolution.
h/t: Matthew Cobb