The World of the Burying Beetles
Natural Selections (appeared December 3, 2004 in the Daily Camera)
The fields and hills abound with mice, voles, gophers, ground squirrels, and birds, and all of them die, sooner or later. So why are carcasses so rarely seen? Carrion feeders, such as vultures, ravens, crows, foxes and coyotes certainly dispatch some of them. But in addition, when burying beetles find a carcass, it disappears overnight.
Burying beetles, in the genus Nicrophagus, are large black beetles with bright red-orange markings (see photo). North America has 19 species of Nicrophagus. The larger species, almost 1.5 inches long, specialize on large carcasses, such as ring-necked pheasants, while the smaller species seek mice and voles.
Burying beetles search for the dead at night, by smell. When a male finds a carcass, he lifts the tip of his abdomen into the air, and releases a pheromone, or chemical signal, to attract a female.
The beetles excavate the soil beneath the body, and the carcass slowly sinks into the ground. At the same time, they remove the fur or feathers from the body, and chew and manipulate the putrefying mass, shaping it into a ball. Within a few hours, the beetles cover the ball of carrion with dirt, and excavate a chamber around it. When the sun comes up, the cadaver is gone, and the only evidence of the crypt is a patch of soft soil.
Entombed with the rotting ball, the couple begins a family. The female lays eggs in the soil, and one day later the eggs hatch into larvae. The adults make audible clicks that help the larvae find the carrion. But the larvae can not yet feed themselves.
Burrowing beetles provide parental care to their offspring, and that includes feeding the young larvae. An adult makes a chirping sound, and the small larvae lift their heads and open their mouths. The parent then regurgitates food into the mouths of the larvae. Within a few days, the larvae are big enough to feed themselves. But the parents stay with them to protect the carrion and the larvae from predators.
The parents and the larvae remain until the carrion is completely consumed. Even the bones are consumed. The adults tunnel to the surface, and search for another carcass. The larvae, abandoned and out of food, burrow into the soil, and transform into pupae. They emerge the following summer as adults, and start hunting for a carcass.
Burying beetles have a mutualistic relationship with mites. The beetles are covered with orange mites that jump off the beetle and onto the carcass, where they feed. But they do not eat carrion, they eat the eggs and larvae of carrion flies, a competitor of the burying beetles. In effect, they eat the competition. The mites mate on the carcass, and when the carcass dwindles, they climb back onto the adult beetles to catch a ride to the next carcass.
Burying beetles are extraordinary in that they literally practice family planning. When they first discover a carcass, they wriggle underneath it, and while lying on their backs, they heft the carcass to weigh it. They might lay as few as 2 eggs in a small mouse, or as many as 40 in a large pocket gopher. If the larvae have ample food, they will turn into large adults. Large adults always win the fights among buying beetles when disputes arise over the ownership of a carcass. Small adults, unable to defend a carcass, may not be able to breed.
Each evening burying beetles armed with mites hide the cadavers, and initiate parental care.
Burying beetlse and larvae in a carcass.
Photo by Rosemary Smith
Adult burying beetles and larvae in a carcass.
Photo by Rosemary Smith
When one or several burying beetles come across a small mammal, they set to
work, and have it underground in less than 24 hours.
Photo by Rosemary Smith