The Horns of Africa

Eating the inedible

Safari camps in southern Africa often display the skulls of various mammals for visitors to admire.  These might be hippos (wow!) or lions (scary!), but most commonly they are the skulls of various bovids (two-toed ruminants such as buffalo, antelopes, sheep, goats, and bison).  These are my Found Objects for this essay, although all I have of them are photos because packing such a skull in my carry-on luggage was simply too challenging.  All male and many female bovids have horns, that is, bony projections of the skull covered with a tough sheath secreted by the dermis.  Almost invariably, the horns of these deceased creatures appear to be melting, forming what look like stalactites dripping from the horny sheaths.  These are the visible signs that horn moths (or their caterpillars, to be precise) are dining on the horn sheaths, their long cases protruding in dangling bunches, looking for all the world like roots seeking soil.  The conspicuous tubes in which they live are made of fecal pellets bound together with silk.

Horn moths belong to the genus Ceratophagus (Greek: horn eater) and are among the few creatures that can thrive by eating the hard sheaths of ungulate horns.  There are about a dozen species in Africa and three in Asia.  C. vastella in the image below is the most common one in Africa.  An African entomologist once told me that there is a species of horn moth that eats only the foot pads of dead elephants.  He said they do so by forming tubular burrows in the soil under the pad and eating it from below.  Sadly, I have searched for references to this behavior, and have found none.  Such a great story, but it appears to be word of mouth only, a sort of minstrel song making the rounds among biologists. 

(Left) A skull of a cape buffalo with an abundant population of horn moth cases. (Right) Detail of the horn moth cases. They are constructed of fecal matter cemented with silk. The caterpillars burrow in the horn sheath.

Given enough time, these caterpillars can completely strip the sheath from the bony horn within.  When the skull once belonged to a cape buffalo (below), the huge bony forehead under the sheath reveals the weight-saving honeycomb openings in the skull that allow the animal to hold its head high.

Skull of a cape buffalo whose horn sheaths have been largely eaten by horn moth caterpillars. Note the weight-saving fenestration of the skull.

The African megafauna with all its diversity, both alive and dead, has created an amazing diversity of “employment” for other creatures.   On the spectacular end are the diverse mammalian and avian scavengers that can reduce an elephant carcass to hide and bones in 36 hours. These scavengers of dead mammals basically eat mostly the same parts that we eat--- muscles and organs.  Some, such as hyenas go farther and reduce the bones to dust. 

But when all these large scavengers are finished and gone, and only dry skin, hair, hooves, and horn sheaths are left, a whole array of insect scavengers move in--- the indigestible remnants left behind by the first waves of scavengers are breakfast, lunch, and dinner for these late-coming insects.  Most of these dry remains are made of keratin, a tough, indigestible protein that, at the molecular level, is built of bundles of very long, super-coiled, rope-like molecules in which adjacent molecules are cross-linked by a lot of sulfur-containing amino acids, like bunches of ropes glued together with tar.  The more cross-linking, the harder the keratin, with horn, hooves, and nails harder than hair, for example.  This high frequency of cross-linking with sulfur makes keratin very impervious to chemical and enzymatic attack, much like vulcanization with sulfur toughens rubber.  The high sulfur content is also why both keratin and rubber smell so bad when burned, and why the process of straightening or curling of human hair is accompanied by the smell of sulfur.  During this latter process, the sulfur bonds are temporarily broken to allow the shape of the hair keratin to be changed, followed by reforming the sulfur bonds to cement the changes in place.  This is what “having a head for chemistry” means.

In keeping with hair chemistry as just described, insects that can digest keratin (and only insects can do this) must also first break the sulfur bonds that cross-link the keratin molecules, after which more humdrum, workaday proteases can do their digestive tricks.  Insects that can do this include the caterpillars of the clothes moths (Tineinae), the skin beetles (Dermestidae) and trogid beetles (Scarabidae).  

We in North America also have keratin-loving insects, though it is a less spectacular array than that in Africa.  The species you are most likely to become aware of (unless you collect dried-out roadkill), are the clothes moths. Clothes moths, like horn moths, belong to the family Tineidae, a family with well over 3000 species in 300 genera.  Of these, very few eat living plants like “normal” caterpillars.  Rather, most feed on fungi, lichens, and detritus.  A handful of species in one subfamily (Tineinae) eat the hair, wool, and fur that remains at the end of the decay of a mammal or bird carcass.  This is why clothes moths (actually, their caterpillars) love that fur hat on the top shelf of your closet or that cashmere sweater in your drawer just as much as you do, but for different reasons.  You probably love the warmth and style, but to the clothes moths these items are just all that remains of a long-dead roadkill after it has been reduced to a sack of desiccated fur and bones. When you finally discover the clear-cut patches in your fur hat and the holes in your sweater, it’s just evidence that dinner is over, and a clothes moth caterpillar is burping contentedly somewhere in your closet.  

Here in the South, we contend mostly with several species of Tinea, the case-bearer cloths moths.  The caterpillars construct a tube of hair, fur, and detritus, reminiscent of the horn moth, and drag this case around by projecting their leg-bearing anterior end from the case while holding themselves in with the prolegs at their hind end.  When they are done snacking on your sweater, they climb up your walls, attach themselves to the ceiling and pupate.  Their empty cases and their shed pupal skins remain, ready to make you worry about the well-being of your woolen goods. 

(Left) Cases of Tinea with protruding pupal skins after emergence of the adult moths. (Right) Detail of the case showing the construction from fibers, hair and detritus.

Adult case-bearers are small, brownish moths. The males make zig-zag flights searching for females to mate with.  At cocktail time in our house, you might sometimes think that we suffer from some neurological disorder that makes us suddenly make wild grabs into the air, but it is only an attempt to catch and smash a male moth cruising around in our living room airspace.  Even under good conditions, these erratic moths are hard to nab, but with a martini in one hand, it is almost impossible.  We have always had a small population of these case-bearers that made our house their home, so we maintain a watchful eye for increases in their abundance because they have eaten holes in many a sweater and even a rug or two.  One rug that we left unattended in an apartment for several months had the pile clear-cut in patches reminiscent of what the St. Joe Paper Company does in the piney flatwoods.  The warp and weft being cotton, not keratin, were left untouched.

(Left) Case-bearer clothes moth cases after emergence of the adult moth, a sure sign that you need to check your woolens. (Right) A carpet that we didn’t check often enough. The bare patches are clearcuts by the case-bearer moth caterpillars.

The presence in North America of a single species of horn moth (Ceratophagus vicinella) invites (perhaps unwarranted) speculation.  Here in Florida this horn moth caterpillar dines on the hard keratinous plates (scutes) covering the shell of the gopher tortoise (Gopherus polyphemus), as in the image below.  The moth is rare and probably getting rarer as gopher tortoises become rarer.  Compared to its African horn moth relatives this is a pitiful showing and suggests a dim future. 

Scutes of a gopher tortoise shell partly eaten by the native American horn moth caterpillar, Ceratophagus vicinella. Inset: A gopher tortoise in the entrance to its burrow.

But not very long ago, North America was home to a megafauna that was easily the equal of that in Africa today.  There were 9-foot tall ground sloths, mastodons, mammoths, camels, and several types of elephants, not to mention a tailored-to-size predator fauna and a generous selection of hoofed animals.  All of those had vanished by about 13 millennia ago.  For any horn moths that depended on the desiccated remains of these giants, their livelihood would have disappeared practically overnight.  Perhaps they could have survived on the remains of buffalo, wood bison, and other lesser ungulates, but these too became scarce with European colonization of the continent.  The question thus hangs in the air--- was the North American megafauna associated with an abundant fauna of keratin-eating insects, including the horn moths?  Is the horn moth that ekes out a precarious existence on gopher tortoise scutes a mere survivor of these abundant days? Did these scavengers seal their fates by specializing on a source of livelihood that suddenly vanished?  Could this be a reminder that the extinction of a focal species or a group of species may also mean the extinction of a whole host of other, less conspicuous creatures?

Bonus: A sable antelope skull whose horn sheaths are being consumed by horn moth caterpillars, like postmortem hair extensions.