Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

What's for Dinner?

Tuesday, September 6, 2011

“Here. Smell it”.beetle on hand

As I held the black and red beetle up to her nose, she made a face.
“Yuck. It stinks!”

“What does it smell like?” I asked.

“It smells like a dead animal”.

“Exactly!”

This was her first encounter with a burying beetle, a type of carrion beetle. As the name implies, it, and it’s larvae, feed on carrion, or dead animals; hence the smell. And while some people turn up their nose at the thought of eating dead rotting animals, it’s an important job and somebody’s got to do it.

Consider this: the deer mouse, native to my back yard, can have up to 6 young in one litter. She may have 4 litters in a year, making a total of 24 young per year. Each one of these young mice may also breed in this year. If they each have only one litter of 6 young this year, then the total number of offspring from that first mouse is 144 deer mice per year. Of course, not all of the mice will survive to breeding age, but you get the idea that each year there may be a lot of deer mice running around my back yard. Add to that the white-footed mice, field mice, chipmunks, red squirrels and a whole host of other mammals, birds, reptiles and amphibians - all contributing to the small wildlife in my back yard - there are a lot of animals living in my back yard. And since all living things must die one day, there must be a lot of dead animals in my back yard as well. Strange, but I almost never see them.

While the thought of dining on dead animals may not appeal to us humans, it is an important food source for many creatures. In fact, scientists have done studies to chronicle the sequence of animals that will visit a carcass to feed. It seems the competition for this food source is quite intense. From large animals such as foxes and crows to a host of insects like beetles and flies down to worms and even bacteria - you had better come to the table prepared to fight for your share. The burying beetle - the yucky insect that smells like dead animals - has quite a few weapons in it’s arsenal, making it an insect well worth investigating.

The trick to getting a good meal is to arrive early and fight for a good seat.
As soon as an animal dies, it starts to release an odor. One hour after death sets in, we humans may not be able to detect the odor, but it is like a dinner bell to a burying beetle, sometimes noticeable up to two miles away. If the male locates the carcass before the female, he may stand upon it, raise his abdomen in the air and release a pheromone that will help guide her to it.
Once the male and female arrive at the carcass, they must assess it’s size to see if it is suitable for raising a family. This they do by trying to move it. If it checks out, they want to get it buried quickly, before other diners come along to share the meal. First, they must find a suitable burying spot. If the ground the animal lies on is not suitable, they will crawl under it and move it, sometimes up to a dozen feet away. This activity has occasionally surprised some people - seeing an obviously dead animal moving along the ground.
Once a suitable burying spot has been located, the beetles begin to dig a hole by pushing out the dirt below the carcass, eventually burying it a few inches under the soil.

Now that the carcass is safely buried, the competition for it becomes less. However, some of the competitors may have already sat at the table by the time the beetles arrived. There are many species of flies that will lay their eggs on the carcass. When they hatch, these maggots, if left to their own devices, can make short work of the food, leaving little for the burying beetles. These unwanted diners must be removed from the table if there is to be anything left for the expected beetle young. So the adult beetles set about eating the fly eggs and larvae.
But they can’t do this work all by themselves, so they brought some help. Certain mites beetle with miteshave hitched a ride on the beetles. Once on the carcass, the mites leave the beetles and start to feed on the fly eggs. These mites then mate, lay their eggs on the carcass and climb back aboard the beetles to be carried to the next feast when the beetles move on.
Other unwanted dinner guests are bacteria and fungi. While there is not much danger in them devouring the whole carcass, they can produce toxins that make the food unpalatable to the beetle young, so out they must go. As the beetles are burying and preparing the carcass, they are constantly applying secretions from their mouth and anus onto the meat. These secretions are naturally occurring antimicrobial agents, reducing the amount of bacteria and fungus.

Now that unwanted diners have been taken care of, the beetles can get down to the serious business of preparing the feast - for themselves and the family to come. Once the carcass is buried, the beetles set about removing the outer covering - stripping it of fur or feathers, and working the remains into a compact ball. A chamber is then constructed above this for the eggs.
After building this chamber, the female returns to the carcass and prepares a depression on it. Both parents will regurgitate partly digested food into it. This will serve as the first food of the newly hatched larvae. In some species, parents have also been seen to feed begging larvae directly. Soon, however, the larvae are big enough to feed from the carcass itself..

After about a week, the carcass is pretty much finished with only the bones remaining. At this time, the parents, along with their assortment of mites, will break out of the chamber and fly away, perhaps to find another carcass to raise another brood, while the larvae pupate in the soil and emerge as adults about a month later.

The level of parental care shown by burying beetles is very unusual for insects other than the social insects, like bees, ants and wasps. Most insects just lay a lot of eggs and leave. Chances are that with enough eggs laid, some will survive to adulthood. But with the care given to the young by the burying beetles, they don’t need to lay as many eggs, perhaps up to 50. The amount of eggs laid is determined by the size of the female, not the size of the food supply. So, if the female is large and the food carcass is small, there may not be enough food to go around, even after all the unwanted diners have been eliminated.

Rather than equally distribute the food to all the young and end up with undernourished larvae with little chance to survive, the beetles have a tough choice to make. Two options are available. First, the male can leave the table, leaving more food for the young. But this runs the risk of leaving the nest undefended in case of predators. The second option is to cull the brood, killing off some number of the larvae. This will insure an ample food supply for the survivors.

The next time you see a large black and red beetle, give it a smell. If it smells like a dead animal, you have found a burying beetle. Chances are it is on it’s way to dinner. Or more likely, it has just finnished dinner.

 

To learn more about carrion beetles, download Carrion Beetles of Nebraska