Early fall in Cowee Meadows

burying beetles, sweetgale ecology, and dragonfly sex

A trip to Cowee Meadows usually provides a curious naturalist with something to contemplate. It’s also a good idea to keep an eye out for large, brown, sometimes temperamental, mammals with claws or hooves.

A stroll out there in mid-August discovered several things of interest.

A desiccated toad carcass lay in the trail, cause of death unknown. The body was attended by two big, orange and black, sexton beetles, maybe just looking for a meaty snack but possibly foraging for a carcass on which to rear a brood of larvae. Sexton beetles are also called burying beetles; they bury the bodies of small mammals and birds (or chunks of dead salmon), denuding them of fur and feathers, which are used to line a chamber housing the carcass. Eggs are laid near the buried carcass and the larvae crawl into the food-filled chamber. Unusual among insects, both parents feed the larvae on liquefied, partially digested meat, as the larvae also feed for themselves on the stored carcass. The number of larvae feeding on a carcass may be regulated by parental infanticide; if there are too many for the available food pile, the parents reportedly reduce the numbers. If for some reason, a female beetle does not have an active partner, she can raise a brood by herself, fertilizing her eggs with stored sperm. In this case, the question in my head was whether or not a desiccated toad would make good larval meals.

The low wetland before the beach berm is thronged with aromatic sweetgale shrubs. They harbor symbiotic bacteria in the root system; the bacteria take atmospheric nitrogen and ‘fix’ it into a form that plants can use. This species usually (but not always) has male and female flowers on different individuals. Male plants have already set their flower buds for next year, while female plants bear cone-like structures with small one-seeds fruits attached to the core. Some small critter had feasted on the seeds of a few plants, leaving the cone-core and fragments in a heap. A fat green caterpillar grazed steadily along the edge of one leaf, not deterred by the reported insect-repellent properties of this species. I was interested to find out that two field guides and two tomes on the flora of Alaska do not instruct a field naturalist how to tell male from female flowers—but the Trees and Shrubs of Alaska by Viereck and Little does!

Out on the beach, it was time for tea and snacks on a favorite log. The tide was low, and far out on a distant rock there was a black lump, which turned out to be an oystercatcher, able to loaf now that the chicks have been raised.

Instead of hobbling over the cobbles around the point, the return trip came back through the grassy/sedgey meadow, where the trails of trampled vegetation left by wandering horses made easy walking in most places. Sparrows popped up out of the tall grass and quickly dove back into the next dense cover. Closer to the river, the vegetation is shorter and marsh felwort flowers began to show up, not only on gravelly soils (as the books say) but also in deep black muck.

The old trail next to the beaver pond has been abandoned, but the water level was very low; there was not even any water in the stream below the dam that makes the pond. That encouraged a little exploration at the edge of the wet meadow along the old trail, which was apparently built (or rebuilt?) without consideration of beaver activity. In recent years, beavers had raised the pond level so the trail was often flooded well over ankle-deep; water was often trapped between the log rails on the trail margins. Rows of young alders have now sprouted up along the edges of that trail, making most of it rather impassible. But the low water level made it quite easy to tromp through the sedges on a parallel route. The newer, improved trail along the hillside would still be the trail of choice most of the time.

Near the beaver pond, dragonflies zipped to and fro, some of them in copula. Male dragons (and damselflies) chase whatever female flies by. If a female is not interested, she may evade the male by running away or hiding; in some species she just plays dead! A successful male grabs a female behind her head with claspers at the end of his abdomen, and they may fly in tandem for a while. The female, if willing, bends her body under his to bring her genitalia (near the end of her abdomen) next to where he has previously stored his sperm in the anterior part of his abdomen, so sperm can be transferred. Copulating dragons make a circle or ‘wheel’ of their bodies. If the female had mated previously, the present male may try to scrape out the sperm of the first male; the ‘opinion’ of the female with respect to this action apparently has not been recorded.

Northern-Bluets-mating-by-bob-armstrong
Photo by Bob Armstrong

Some days later, I watched a pair of bluet damselflies in tandem, perched on a sedge blade in a mid-elevation muskeg pond. The female bent her body up to touch his, in the copulatory position, several times, but they did not form the mating wheel. Three other bluet males patrolled this pond, sometimes zooming in closely on the pair, and even contacting them, as if to try to steal the female away. This is a behavior I’d not seen before. At the edge of the pond lay a dead female, possibly drowned in the act of laying her eggs in underwater vegetation. Some bluets lay eggs in vegetation near or on the surface, but some species of bluet actually submerge the whole body while egg-laying, and upon occasion need to be pulled out by their partner or perhaps by a nearby unmated male.

Burying beetles and mites (1 of 2)

a beetle with many stories to tell

On a recent junket to watch bears catch salmon, we encountered a beast with a different story to tell. This was a beetle about an inch long, and its entire body, top and bottom, was covered with layers of tiny, squirming mites. Together, the beetle and the mites represent a remarkable and fascinating story—in two parts.

nicrophorus-with-mites-2-bergeson
Photo by Pam Bergeson

First, the beetle. This kind of beetle is called, variously, a carrion beetle, a sexton beetle, or a burying beetle. It belongs to a genus of beetles that is widespread in the world, especially the northern hemisphere, with about 68 species known so far, and three species in Southeast Alaska. The name of the genus is Nicrophorus, but formerly this was spelled Necrophorus, which give a clue to its behavior. ‘Necro’ (as in necropsy and necrophilia) refers to the dead, and ‘phor’ refers to carrying or bearing something –thus, a bearer of the dead. The beetles are typically black, with orange blotches on their wing covers.

These beetles are scavengers, eaters of vertebrate carrion (and one species has recently been found to use snake eggs). Adults can feed on any large or small carrion, but perhaps the most interesting relationship is generally with small mammals or birds.

The entire life history of burying beetles typically centers on finding a dead shrew, mouse, or small bird. There usually aren’t lots of these just lying around, up for grabs, and what few there are, are very attractive to other scavengers, be they ravens, blowflies, worms, or microbes. So it pays to find a carcass quickly; competition for dead mice or birds is intense. One study showed that the size of the population of burying beetles depended on the availability of suitable carcasses, which depended on the size of the populations of small mammals.

In addition to competition with other kinds of organisms, there is competition among the beetles themselves. When a male beetle locates a suitable carcass, he advertises for females, using airborne chemical scents or, in some cases, courtship songs produced by rubbing a rough part of the abdomen against the underside of the wing covers. Large males can outfight smaller males for possession of the carcass, but the subordinate males may hang around and sneak some copulations with arriving females. In addition, because female beetles can store sperm from a previous mating elsewhere, a male parent on a carcass may not be the father of all of her eggs. In at least one species of beetle, females also fight each other for rights to use the carcass, but subordinate females may add some eggs to the brood.

The beetles use the carcass to rear their young. When a burying beetle finds a relatively fresh carcass, it buries its find on the spot, by scratching out a hole under the body, or it moves the prize to a usable spot, by lying on its back under the body and pushing with its legs, and then buries it. These beetles are strong, and can move a carcass over and around obstacles.

Once the body is buried, the beetles chew off the fur or feathers, roll the body into a ball, and anoint it with chemicals that deter the growth of microbes that would decompose the carcass, competing for the flesh and reducing its nutritional value. Adult beetles carve a hole in the top of the ball, chewing some of the flesh into a soup that fills the hole.

Female beetles lay their eggs in a tunnel near the carcass. The eggs hatch in just a few days, and the larvae move to the carcass. There they may be fed by the parents from the soup of chewed up flesh, or they may feed for themselves. The larvae beg from the parent beetles, and larvae that hatch a day or so ahead of others are more successful at begging.

Usually both male and female parents tend the brood of larvae, although either one can also do it alone. The primary role of the male appears to be in defense of the carcass against flies and other beetles but, in addition, the female benefits, because she doesn’t have to work so hard and can save energy for a second brood on a different carcass.

The number of eggs laid by a female beetle depends largely on her body size, her body condition (which depends on how well fed she is), and the size of the carcass. However, if too many eggs are laid (for example, if a second and subordinate female has deposited her eggs in the same area), females may destroy some of the eggs or larvae.

The larvae go through several molts as they grow. In some species, they turn into adults in summer, overwintering as adults that come out the next spring. In other species, the larvae spend the winter as inactive pupae, and adults emerge from the pupae the next summer.

Some of the beetle species, including one of those in our area, may nest communally. That is, several pairs of beetles may share a large carcass (if they can get there before another scavenger gets it!). On the north Pacific coast, salmon carcasses offer a relatively predictable source of carcasses in late summer and fall. Bears and other foragers leave partially eaten carcasses on the floor of the forest, where they become available to burying beetles. A salmon carcass may harbor three or more pairs of beetles, all rearing their broods on different portions of the carcass. In this case, the carcass is not buried; instead, the beetles just move in underneath the body. The larvae are then reared on salmon flesh, instead of the more usual mammal or bird.

The story of the mites riding on the beetles will have to wait until next time.