By Kevin Fitzgerald
Certain species of insects, along with species of vertebrate scavengers, fungi, and bacteria, are members of nature’s clean-up crew. These entities break down organic matter from complexity to simplicity, converting it into nutrients for plants, thus completing a cycle of life, death, and decay.
Ways of reducing dead matter in nature vary, but one of the most fascinating is the elaborate method employed by burying beetles, also known as sexton beetles, in the genus Nicrophorus. There are about 70 different species, and they are found throughout the Americas, northern Africa, Europe and Asia.
These are big beetles. The largest, the American burying beetle (Nicrophorus americanus) can be up to 1.5 inches (4 cm) long. Most burying beetles are shiny-black with bright spots of red, orange, and yellow on the elytra (wing cases), thorax, face, and the tips of the antennae.
A burying beetle can detect the scent of a dead animal by means of chemosensors in its antennae. If one or more burying beetles find a small carcass, they prepare it for food for their young.
Once at the carcass, a beetle waits for more beetles to arrive. Males emit a pheromone to attract females. A single fertilized female can set up housekeeping on her own or with a male. If several beetles are attracted to the carcass, they fight it out, the largest male and female beetles usually being the winners. They will be the parents of a new generation. Such male-female cooperation, outside of the communal insects, is extremely rare in the invertebrate world. Most invertebrates produce large numbers of offspring in hopes that a few will survive to adulthood. By guarding a secure food source, the carcass, the parents ensure that many offspring will survive.
Once established, the beetles prepare the carcass for larval feeding. They snip off all of the fur or feathers, which they’ll use to line a pit, then they roll the carcass into a neat little ball, while simultaneously digging underneath it, slowly sinking the carcass into the earth. The entire operation, including removing fur or feathers and burying the carcass, takes about 12-18 hours.
In the crypt, the beetles cover the carcass with secretions from oral and anal glands, including antimicrobial peptides and microbes. These slow the growth of soil molds and bacteria that may develop on the carcass, which slows decay. It also diminishes the scent, so other scavengers and burying beetles are not attracted to the carcass.
Copulation occurs during burying activity. The female excavates a small chamber underground and near the carcass and lays about 30 eggs there. These hatch in about four days, after which they wriggle to the corpse and begin feeding. Both parents also feed the larvae regurgitated food that they’ve eaten from the carcass. The larvae beg for this food by stroking the female’s jaws. They larvae are thought to carry antimicrobial substances in their feces.
The adult burying beetles monitor the population of young, and may kill off any they deem superfluous. This assures that the larvae get enough food.
The larvae take eight to nine days to mature, by which time the carcass is reduced to bones. Then they burrow into the soil around the nest and pupate, emerging in the adult form after 48 to 60 days. After emerging, the young beetles feed on insects and carrion, and become inactive in winter.
Field and laboratory studies have shown that in some cases the principle function of the male parent is to protect the brood and carcass from intruding members of its own species. A male intruder may kill the nest male and the larvae, then mate with the female to produce his own offspring. Conversely, an intruder female may kill the nest female and brood, then mate with the nest male.
Dr. Stephen Trumbo is an animal behaviorist at the University of Connecticut’s Department of Ecology and Evolutionary Biology. He and his team study burying beetles in the field and in the lab. In the latter, the beetles thrive on pieces of raw chicken liver.
“Burying beetles are easy to study,” he said. “We have 100 pairs of several species in the lab. We can’t do that with other species. We have questions about physiology and hormones.”
Dr. Trumbo uses the dynamics of burying beetle populations to study the aging process in burying beetles; juvenile hormones, which have many functions in the beetles’ lifestyles and breeding behavior; the microbiology of carcass burying by the beetles; their parental care; and the habits of Nicrophorus pustulatus, which is an exception to the carcass-feeding method, since it preys on the eggs of the black rat snake (Elaphe obsoleta).
“Aging affects reproductive effort in many organisms,” Dr. Trumbo said. “In burying beetles, we found that younger adults were less likely to initiate breeding or fight for a breeding opportunity, but once they had larvae to care for, they were just as able as older parents to care for their young and would take just as many risks to defend their young.”
Juvenile hormones, which are common among insect species, peak in burying beetles just before laying eggs, and again during the early parental stage.
“Juvenile hormone has been an enigma in burying beetles,” Trumbo said. “The second peak in juvenile hormone corresponds to the early stage of parental care when parents actively feed their young and maintain a high metabolic rate. These correlations are intriguing but we still do not know the primary role for JH during care. In both males and females there is an initial peak in JH after a carcass is discovered. In pairs, there is a second, higher peak in JH in the female parent at the beginning of active parental care. In the first 48 hours after hatching, larvae are on the carcass and beginning to be fed by the parents. The male parent does not show so much of a second peak. However, if you remove the female parent so the male parent cares by himself, he does more feeding of young, will stay longer with the brood and will have a much more noticeable parental peak in juvenile hormone. Parental care is a complex topic. We used to look at parental care in insects as something straightforward, with a primary function. Work on burying beetles and other caregiving insects has revealed that care often has many mechanisms that address many functions, and the investment in these different mechanisms can be subtly adjusted to the needs of the moment.”
This article may have grossed you out a little, but somebody must clean up the messes in nature, including carrion. Otherwise, there would be heaps of dead creatures everywhere. We should be thankful for the valuable services rendered by burying beetles.
Kevin Fitzgerald is a freelance science writer living in Connecticut. He has published in newspapers, encyclopedias, and online.