I’ve heard of “carrion beetles” but this is more like a “carry-on beetle”:
Amanda and I were outside the cabin in Cass County, Minnesota last week, cutting pieces of plywood for sub flooring, and we saw this creature among the debris. At first I thought it was some kind of wasp covered with tiny spiders, but on further investigation it turned out to be a beetle covered with mites. When we first saw it, there were many more mites than in this photo, and they were virtually roiling on the insect’s surface. It looked almost as though the insect was foaming.
We captured it for later identification, and Amanda ran it down. This is a carrion beetle. There are several species of carrion beetles, all in the family Silphidae. There are over 20,000 species of beetle in North America. Beetles are incredibly diverse. Therefore, it did not surprise me to learn that there are several dozen species of carrion beetle in Minnesota. This is probably in the genus Nicrophorus.
So, what are the mites doing there?
Carrion beetles face a special problem that we see here and there in the world of animals, and when it occurs there is often an interesting adaptive result. (This is how we know adaptation is a real thing and the diversity we see in nature is often the result of adaptatoin.) Carrion beetles lay their eggs in or on the body of dead mammals or birds. But dead mammals and birds are somewhat rare, especially the relatively undisturbed ones, the ones that are not about to be digested by something. Therefore, a carrion beetle that has a trait that maximizes the use of this rare resource will have a selective advantage over carrion beetles that don’t.
Lots of things lay their eggs in dead critters, or simply find the dead critter’s body and eat it, but many of those species are less discriminating. Think of a simple world in which there is the occasional dead mouse, carrion beetles, house flies, and a large amount of poop. Never mind where the poop comes from, just assume it is there. In this imaginary simplified ecology, house flies will lay their eggs on poop and dead mice, but the carrion beetle, with larva that can only grow on a meat diet, will lay their eggs only on the dead mice. Selection that might lead to the development of a costly trait to monopolize the resource will not be strong for the house fly. If the house fly fails now and then because the carrion beetle out competes it on the mouse carcass, no big deal, there is always poop. But selection that might lead to the development of a costly trait in the beetle to make use of the mouse carcass may be strongly selected for. From the carrion beetle’s perspective, there is a strong possibility that there are already fly larva (maggots) on the carcass eating it when it arrives to lay its eggs, or soon after. This situation is probably made worse by the fact that house flies are quick and seem to be able to travel long distances, while beetles are slower and travel less far than flies in a given period of time. The only way carrion beetles will ever be able to raise their young in this scenario is to do a better job of accessing or using the mouse carcass than the fly does, so any variation that arises in carrion beetles that facilitates this will be strongly selected for.
This is known as the life/lunch dichotomy. In the competition for the use of the meaty carcass of a dead mouse, if the fly loses out it gives up the equivalent of lunch … there are still other opportunities, in this case, poop, for it’s young to eat. The carrion beetle, however, may be giving up its life (or the life of its offspring, really) because mouse carcasses are very rare, so if the one carcass it manages to locate is eaten up by fly maggots, it’s offspring will not survive.
This is where the mites come in.
The carrion beetle pays a huge cost carrying the mites around wherever it goes, because they are heavy and affect its ability to move and fly. But otherwise, the mites do nothing …. they just hang on for the ride, waiting for the beetle to locate a dead mouse. Then, when the beetle does located a dead mouse, the mites do not eat it. Rather, they eat the maggots, the fly eggs, and larva of anything that is not a carrion beetle. They clean the carcass of the potential competitors of the carrion beetle’s larva.
Now, if I can only find a dead mouse somewhere to feed to my new pet and her friends….