Five hundred feet above the dry, rolling woodlands, the bat had begun the hunt through its last dusk. Though it had left its roost late, the animal already felt the urge to return to shelter, rest, and recover from the infection it carried. But the bat had waited too long to forage – now it had to eat or starve. Its light weight, coupled with its hyperactive metabolism, left it with only a thin margin of survival.
Its heart was firing at five hundred beats per minute, driving the search for flying insect prey, and also sustaining a battle within, against an inflammation in its kidneys. The bat was succeeding in its hunt, but not against the virus it carried. Its blood was growing more acidic in the absence of full kidney function – a condition that would not sustain its muscles much longer.
Soon the creature found itself struggling to breathe. It went into a glide to catch its breath – coasting with its mouth agape, gasping for air – but it found no relief. Its laboring heart had fallen into an irregular, non-productive rhythm, then finally seized. The bat lost consciousness the same second.
It was dead and cold when it hit the ground – a formless lump settling among fallen leaves – barely recognizable as the predator it had been only a minute before. Its body was transforming into something else – a resource – the object of the searches of a broad guild of other animals.
The dead mammal had become a cache of protein – far superior to anything else that scavengers might find among the sticks and dead grass. Its discovery would be a bonanza for a foraging vulture, or crow, coyote, skunk, badger, raccoon, opossum, shrew, or for ants – for whichever of these hunters had senses keen enough to find it first.
The microbes that had lived on the bat’s skin and in its gut were acclimated to a warm, oxygenated environment. The sudden change threw their fine-tuned symbiosis wildly out of balance. Within minutes they began to lyse, releasing volatile components of their own metabolism to diffuse out through the body and into the air. Within the hour, a mile away, a pair of antennae perked up.
* * *
A beetle, smooth black with carmine red chevrons paired across its wing-covers, spun around to face into the breeze and lifted off into the night. He flew low to the ground, scrambling through the thickets he collided with, running across dry leaves and back into the air. He lost the scent he was pursuing, so he weaved a search pattern back and forth across the direction of the breeze and recovered it.
It took him an hour to find the fallen bat. He ran up and back across its skin, antennae working constantly, then he climbed to the top of the body, raised his abdomen and released his own scent – a signal that carried even farther than the scent of death. Then he waited, biding time he did not have to waste in a race against the coming of day.
An hour later, two miles away, a second pair of antennae perked up. A female beetle opened wing covers and ascended straight up into the air, gauged the direction of the breeze, and turned into it. She flew high – above the obstructions hidden in the darkness – to follow a strengthening pheromone trail through the sky. The scent built in intensity, then abruptly disappeared. At that point she doubled back, and began a cross-wind search pattern. She recovered the scent, and descended to the ground to complete the journey on foot.
She met the male beetle standing on his windfall two hours after he himself had found it. She walked up and back across its skin, antennae working constantly, then she climbed to the top of the body and mated with the male.
A short while later they climbed down the lee side of the fallen animal, pressed and scraped their way under the edge where the body met the dirt, and crawled beneath it. Then they twisted themselves over onto their backs, and pushed up with their legs, holding the dead mammal above them to judge its weight.
Still on their backs, still holding their prize above them, they reached out to the earth to push themselves forward, side by side. The animal began to move – sliding off the hardpan where it had fallen, toward a bush under which the soil was less compacted. The two beetles struggled against the ground for purchase, but when they start to pull out divots as they moved, they stopped their advance. They had found soil soft enough to dig in.
* * *
The beetles flipped over again, still underneath the animal they held up on their backs, and began to burrow into the ground. Digging steadily, taking no breaks, working to bury their find while they were still hidden in darkness. They needed to conceal their cache before its scent grew too obvious. They pushed soil out from beneath it, then they shoved the loose earth up into mounds on both sides. As the hours passed, the fallen animal began to settle toward ground level.
The burial hit a snag several inches down – when the beetles encountered a root directly below them. Time was lost as they chewed through it. One of the dead bat’s wings came open, and the beetles chewed through the bone to prevent the wing from holding on to the excavation wall. Pebbles were dug out one by one – moved to the side as soon as they were unearthed – but the work was slow and darkness was running out on the beetles.
First light expanded along the eastern horizon to reveal the diggings to predators, but the dawn was silent and the beetles kept working. Soon the day would warm, and diurnal insect scavengers would become active. But the beetles pressed on and before the sun shone directly on the burial site, the last bit of gray fur disappeared into the mound of churned earth.
As they bury their prize, the beetles bury themselves. They push soil out through a tunnel that deepens – following the body down into the ground. Their work slows as they are cooled by the subterranean chill, and as their metabolism adjusts to the low oxygen levels in the close quarters.
They push the carcass into a ball, and then, when it has settled deep enough – so that the earth above it can be compressed into a stable ceiling – the beetles finally pause. The female moves off to the side and lays eggs in the soil. Then the excavations continue, enlarging the underground crawl spaces in which these beetles will raise their brood.
* * *
As the sun climbs, a fox enters the clearing, pursuing the faint scent of carrion. It searches every nook and cranny – nothing is to be found. The vibrations of its footfalls alert the male beetle, who is near the entrance to the excavation tunnel. He stops in place, folds down his antennae, pulls his feet beneath him, and waits for the commotion to pass.
The fox stops to taste the air repeatedly, but the scent is fading. Is this a strong scent carried from farther away, or a faint one near by? The predator begins to search more avidly, as if looking for prey that might escape – she cannot afford to spend too much time in fruitless pursuit.
As she runs through the area, the fox treads squarely on the burial site, collapsing the tunnel on top of the male beetle. The fox takes no notice of the soft spot into which her foot has sunken, but continues her frenetic search pattern. She explores in ever wider circles, eventually moving so far upwind that she looses the scent, then looses interest – and abandons the search.
The buried beetle can barely move his joints and feet, but through the faintest of motions, he compresses the loose soil into a thin cavity just beneath him. The open space gives him room to move his forelegs and head, and he works on the wall of earth, enlarging the cavity that will give him the freedom of motion to dig.
He advances slowly upward, undistracted by the suffocating closeness of his confines. Then he finds that his back legs are free – the female has dug up behind him. With his legs straightened, he has the leverage to dig again; he emerges through the surface in a matter of minutes. He and the female pause with the open air circulating back through their bodies, then they turn around and return to their buried chamber, reinforcing the tunnel as they go.
* * *
Before the eggs hatch, the beetles prepare the buried meal for their young. They remove the hair, and cover the exposed surface with antimicrobial secretions. This will preserve the food ball and mitigate the generation of scents that might otherwise lead scavengers to come and dig them up.
The food ball is a rich cache of nutrients. The hatchling larval beetles will grow rapidly upon it, but for them to succeed, their parents will have to nurture and defend them with great care.
The day the eggs hatch, the food ball is attacked by a fungus. The beetles set to work patrolling its surface, trimming away the white strands. They work around the tiny pale larvae crawling on the surface – and they fail to notice the blue bottle fly exploring the tunnel entrance.
The iridescent fly walks into the chamber, stepping light-footed across the food ball and the diminutive white larvae upon it. The beetles come alert to its presence, running over the curved surface, but the fly – buzzing its wings to speed its evasions – stays ahead of them, playing a game of tag. Whenever it gains a second’s pause from the pursuit, the deposits an egg on the carrion ball. Eventually the beetles corner the fly, leaving its only escape back through the tunnel. The fly scurries out and is gone.
Then the beetles resume their perpetual inspection of their brood chamber. When they encounter a fly egg, they eat it – as they will any maggots they may find later. Competition with flies leads to brood failure in burying beetles, but it is more of a problem when the flies had found the food source before the beetles could bury it.
When the food ball is settled within its crypt, then it, and the rest of the brood chamber become protected – by an infestation of mites. The beetles carry these microscopic creatures as symbionts. The service the mites provide, in exchange for the sustenance the beetles bring them to, is fly control. The mites swarm the fly eggs that escaped the notice of the parents, piercing the shells and consuming the contents.
* * *
The hatchling grubs are too small to feed themselves, and soon, like a clutch of baby birds, they approach the parents and ask for help. They arch their heads and push their faces against a parent’s mouth, then they stroke its mouth-parts with their tiny legs. The parents respond by feeding the hungry nestlings, regurgitating food just as a parent dove would respond to begging chicks. The larvae grow quickly with this attention, and soon are large enough to feed on their own.
Such nurturing care is more easily seen in the hives of the social insects – the bees, wasps, ants, and the termites. But the burying beetles are no less attentive as parents. The pair of them will stay with the brood and defend the food resource until the larvae are fully grown. Both parents work continuously in the chamber, cleaning the food ball for their young, and spreading their antimicrobial secretions across its surfaces.
The female has produced too many larval beetles, which assures that the entire food ball will be consumed. If she had produced too few eggs, a portion of it would go to waste. But if the parents attempt to raise too many young, they may all run out of food before the larvae mature, allowing only a few to pupate – and then to emerge later as underweight adults. As the days pass, the parents realize that the dwindling food ball will not support them and all the growing grubs full term. They solve this problem by seeking out the smallest of the hatchlings and eating them.
* * *
The light at the end of the tunnel is eclipsed on the sixth day by the appearance of a large burying beetle – a different species that has caught the scent emanating from under ground. The intruder enters and attacks the female directly, pushing her back with its greater leverage. This larger beetle is also a female; her intent is to cannibalizing the grubs on the food ball, and then lay her own eggs there.
As she pushes the resident female farther back, the aggressor is struck from the flank by the male who grabs one of her legs in his mandibles. The smaller female shifts under the weight of the larger, sliding to the other side where she attempts to bite down on another leg. The two defenders stop the intruder’s advance, and the larger beetle reverses, rolling the smaller female over. But the smaller one still attempts to grasp the larger, even wedged between the food ball and the crypt wall, and the intruder steps backward.
The male looses his grip – having bitten the foot off of the intruder. The larger beetle continues to back down the tunnel, in constant contact with the two occupants. Soon she is back on the surface, running with undiminished speed, but with a noticeable jerkiness – a consequence of the missing digit on one leg.
The resident parents appear above ground and face off against the attacker. She hesitates for a moment, then raises her wing-covers and lifts off, flying straight up. The breeze directly above the burial site does not carry the scent that she had earlier followed, so with no impetus to guide her foray, the intruder banks off across-wind and flies away.
A cuckoo in the crown of a nearby bush watches the departing beetle – too strong a flier to pursue, but then notices the red and black insects walking on the ground below. The bird descends through the branches, only to find from the base of the bush that the two beetles have disappeared.
* * *
On ensuing days, other marauding burying beetles will notice the faint scent emerging from the crypt. But this is a scent that changes constantly, and they recognize the nuance. At later stages, not enough of the food ball remains to support a new brood to term. So potential intruders forego the lower value resource and move on.
In a week, the food ball is completely consumed and the larval beetles fully grown. They cease foraging across the skeletal remains, crawl off to burrow into the wall of the brood chamber and pupate. Before the last larva does that, the parents will have flown – each in its own separate direction.
Without the continuous maintenance, the entrance tunnel collapses, burying the burying beetle pupae. The plowed soil, salted with a few unconsumed teeth, claws, and bones will serve to enrich the earth over the years. It will stimulate the growth of plants nearby; those plants will in turn be grazed by insects; those will eventually take wing; some of them will be eaten by bats.
The burying beetle larvae have lived an airless life from the moment they hatched from eggs underground, until the day they finally eclose from their buried pupal cases. Their subterranean metabolism is adapted to low oxygen conditions – in which they pursue only a slow-motion, sedentary lifestyle – creeping along on little legs one tenth the length of their body.
But when they finally dig their way upward through the earth and emerge under the wide-open sky as adults, they encounter the sensation of fresh air moving freely throughout their bodies. All that oxygen will bring them a vitality they had never known – allowing them to use their high oxygen-demand muscles. They will find the energy to run – on legs that now span twice their length, and to fly – on wings they find folded beneath their shells. For the first time they will see the light, and sense the flavors of the world through newly unfurled antennae. And when they finally notice the faint suggestion of a burying beetle bonanza floating toward them on the evening breeze, those antennae will perk up.
Undertakers; notes A Mexican Free-tailed bat, weighing 15 grams, is large enough for one of the smaller species of burying beetles (Nicrophorus) to raise a brood of young. Those young are altricial (Smiseth & Moore, 2002; Smiseth et al, 2007) a term more commonly associated with baby birds that are born featherless, blind, and dependent upon their parents for their initial survival. Beetles remove hair or feathers from the food ball, and work the carcass into a ball (Scott, 1998). The phorid mites the beetles carry have not been conclusively shown to improve brood success in the competition with the carrion flies – whose eggs they destroy.
Scott, M. P. (1998) The ecology and behavior of burying beetles. Annual Review of Entomology 43:595-618
Smiseth, P. T. et al (2007) Parents influence asymetric sibling competition: Experimental evidence with partially dependent young. Ecology 88:3174-82
Smiseth, P. T. & Moore, A. J. (2002) Does resource availability affect offspring begging and parental provisioning in a partially begging species? Animal Behavior 63:577-85