Got you under my skin

Notice anything unusual about this squirrel?

Yeah, I've noticed that it's a male too, but that's not unusual. (Arrow "x" is there only for reference.)

What I'm talking about is the dark thing hanging down from the squirrel's belly? Can you see it? Look again below the scrotum. Arrow "y" points to it.

The luckless rodent seems to be carrying an alien from the underworld of the Arthropoda. What you probably see is a repugnant swelling with an oozing sore, a warble. It's the domicile of a bot, the parasitic larva of the bot fly.

As flies go, bot flies are rather pleasing. They are hairy, somewhat blocky in build, and resemble bees. They have small or vestigial mouthparts. The raison d'etre of their brief adult existence is reproduction, and during their quiet interlude under the skin of squirrels they accumulate the energy stores needed to go forth and procreate.



There are 34 species of the bot fly genus Cuterebra in the US, and their larvae, the bots, must feed on small mammals, mainly rodents and rabbits in order to reach maturity. Each species of Cuterebra has its preferred host. In other words, gophers, wood rats, white-footed mice, and squirrels have their own dedicated species of bot fly, but they are not totally loyal to one host. Occasionally they meet up with the wrong host. Forty species of insects, including bot flies have been known to "accidentally" parasitize people in the US.

Finding a bot on your own body or in something you intend to eat is not a pleasant experience. In the southern US, only 2% of squirrel hunters were reported to eat bot-infected squirrels. The rest of them got really turned off. So a lot of hunted squirrels cycle back into the food chain without benefit of human digestion.

The fly bot is the closest thing to the slimy xenomorph (read Alien) from Planet LV-426 that exploded from the chests of its human hosts. Surely you remember Sigourney Weaver and aliens that got scarier with each new movie.

The difference of course is that aliens are fictional. True, they are all pretty much based on biological themes, though embellished and recombined to be extremely scary. Bot flies and their parasitic larvae however are REAL. So let's concentrate on the kinds of zoological reality that gave the producers of Alien its wildest ideas. There's a good reason for this, because our youth often confuse reality and fantasy, and in the age of intelligent design people seem to be losing their curiosity about biology and the natural world.

You will recall that the infective phase of the Alien life cycle is the Facehugger, which leaps onto the head of the human host and shoves an egg down its throat. This is extremely disconcerting for the human host. Cuterebra have far more finesse.
The female uses olfactory cues to home in on the nests of rodents, and simply lays her eggs on the vegetation nearby. When the eggs hatch the tiny bullet-shaped larvae (the first instar) zero in on body heat and attach themselves to the passing rodent.

There is no need for forced entry. The tiny larva crawls undetected into the body through the eye, nose, or mouth -- for that matter any opening they find, natural or otherwise.

The Alien's embryo (the "crawler") settles in the thoracic cavity and matures in a remarkable 24 hrs. Our baby bot (the first instar) on the other hand, takes about a week to migrate from the point of entry to a comfortable space beneath the skin, often in the squirrel's hindquarters. There it prepares its lodging for the next several months. The first order of business is to rasp a small breathing hole to the outside word. It also uses this porthole as a latrine, periodically expelling its liquid brownish excreta on the surface, which the squirrel periodically grooms away.

For the next month and a half the bot is the squirrel's constant companion. The squirrel's body reacts to it by encapsulating it in a fibrous sac, not unlike a mummy bag which seals it off from the rest of the body. The host's white blood cells and antibodies also increase. but this has little effect on the bot. It is basically a couch potato. Its daily routine is to lounge in its mummy bag, feeding continually on tissue fluids and cellular debris.

When it reaches the stage known as the third instar, it's almost as big as the end of your thumb. But unlike the Alien, it doesn't explode through the body wall, killing its host and scaring the hell out of everyone. On the contrary, it goes gently into that good night of pupation by squeezing through the breathing pore and dropping to the ground. There it burrows and undergoes its final transformation. It may emerge as an adult in another month or pass into a diapause and emerge next year.

It is hard to say how squirrels and other bot-infested rodents feel about all of this. They seem to carry on normally and ignore the bot, but in the laboratory at least, rodents become inactive about the time the bots emerge, and we may infer from this that harboring a bot has its discomforts.

What else? Well, if you are a nestling squirrel and unfortunate enough to be selected by several bots, you may not live long. If you are an adult female and the bots settle in your back side (as they often do) you may develop a pseudo-scrotum, and fool all the other squirrels. Male's with a bot-infested scrotum on the other hand may sport an appendage that resembles a duffel bag, but this renders them reproductively useless for the time being.

The affinity of bots for the scrotum accounted for the name Cuterebra emasculator, but Drs Bob Timm and Robert E. Lee Jr showed that scrotal bots don't affect subsequent reproduction. Unlike a lot of other parasites, bot flies don't practice "parasitic castration" upon their hosts.

If that wasn't so, a lot of southern squirrel hunters might be wearing camou codpieces made of fly screen.


References

Baudoin, M. 1975. Host castration as a parasitic strategy. Evolution 29:335-52.

Bennett, G.F. 1973. Some effects of Cuterebra emasculator Fitch (Cuterebridae, Diptera) on the blood and activity of its host, the Eastern chipmunk. Journal of Wildlife Diseases 9:85-93.

Jacobson, H.A., D.C. Guynn, and A. Hackett. 1979. Impact of the bot fly on squirrel hunting in Mississippi. Wildlife Society Bulletin 7(1):46-48.

Scott, H.D. 1964. Human myiasis in North America (1952-1962 inclusive). The Florida Entomologist 47(4):255-261.

Slansky, F. 2006. Cuterebra bot flies (Diptera: Oestridae) and the indigenous hosts and potential hosts in Florida. Florida Entomologist 89(2):152-160.

Timm, R.M. and R.E. Lee Jr. 1982. Is host castration an evolutionary strategy of bot flies? Evolution 36(2):416-417.

Still keeping tabs

You will recall that since the puma slobbered, rubbed, and urinated on the mossy rock on May 24th, the spot became a point of interest to several passing creatures. Since then, only two black-tailed deer walked by, and both stopped to check out "the spot". The doe visited on June 5th, and the one-antlered buck visited on June 22. That's 12 and 29 days since the cat left her scent mark.

I can't prove they were smelling the cat's scent, but it's the most likely explanation. The stuff seems to have staying power.

I find the attraction of these prey species to the scent of the puma just fascinating, and now that I've established a record here, I won't beat the dead horse any longer.

A beak for dead meat

Ol' timey naturalists maintained that weasels commonly use creek beds as thoroughfares. I've seen them following small creeks in the Rockies, so I decided to camera trap a weasel in a dry creek choked with vegetation.

I baited a fallen log with a couple of mouse carcases, and doused it with predator scent -- a commercial concoction that would gag a maggot.

It was wishful thinking. There was no weasel in the neighborhood, but two days later a turkey vulture arrived. It was a remarkable feat of detection. I don't know if the vulture's cue was the sight or smell (or both), but the bird polished off the meager meal, and the camera captured the gourmet moment.

Afterwards, the vulture spent 5 minutes studying the ground beneath the perch. Or perhaps it was admiring its feet. I'm not sure.




Vultures are remarkable birds. Drs. David Houston and Kenneth Stager showed conclusively that turkey vultures have good noses. Until then it was assumed that birds in general do not have the gift of olfaction. The stench of decomposing animals however is a powerful attractant to mammalian scavengers, and the ancestral turkey vulture must have developed this sense early in its evolution to be competitive. The evolutionary scenario is believed to have been forested habitats where vision alone was not sufficient for vultures to 'bring home the bacon'.

My own experience with vultures is limited to the black vulture, which is less gifted than the turkey vulture as a glider and sniffer of decomposing flesh. Vernon appeared as an unidentified egg in a large aviary at the National Zoo in Washington, D.C. in the mid-1970s. He was hand-reared and transferred to the barracks building at the zoo's Conservation & Research Center, where most of the bird collection was held during renovation of the zoo's bird house.

Vernon's intelligence quickly earned him liberties that would never be granted to other birds in the collection. He had a quiet confidence. There was just something about the way he cocked his head and studied you that set him apart. He also had a foot fetish -- a beguiling way of shaking his beak at your shoes before he untied the laces. There was an unbird-like intelligence in the wrinkled head that resembled a charred bratwurst.

Before long the keepers let him out of the building, which immediately caused a stir in the administration. But it wasn't a problem. He flew around the buildings and studied the staff's activities from the rooftops, but he spent most of his time hanging around the barracks building, perched on the railing near the back steps, waiting for his daily pan of thawed white mice.

Vernon's boldness grew with his familiarity of the surroundings, and though his offenses drew criticism, his popularity always outweighed his misdeeds. As parents ignore a nose-picking child, we weren't blinded to his virtues by his bad breath and habit of crapping on his legs.

The daily Vernon report became a source of great entertainment, and his food-snatching technique in particular became legendary. He would show up at a Saturday afternoon barbecue like a weary traveler, and amble about with an air of boredom. When the chef was no longer paying attention he would flap up to the grill, snatch the steak, and disappear into the wild blue yonder. He was also seen to eat balloons and drink paint, worrisome feats that we learned had no physiological side effects.

When wild vultures started to show up, we learned that Vernon was a leader among his kind. One morning as I walked to work, I met Vernon sitting on the lawn. I extended my foot with kind words of encouragement, and he 'did his thing'. When I reported the story to Guy Greenwell, our bird curator, he informed me that it couldn't have been Vernon. Vernon had been locked up for bad behavior. A wild vulture had diddled my shoes, and only Vernon could have been its teacher. I believe it was about this time that Vernon revealed a long-kept secret. He laid an egg!

For all of her faults Vernon performed a final service to the institution. There was this traveling salesmen, one of our vendors, who was smitten with our adminstrative officer. From the era of blue-suede shoes, he was one of those smarmy country boys who wore plaid pants and mustard-yellow sports jackets, and he drove a humongous road hog with pleated and rolled upholstery. Got the picture?

It was one of those still summer days. The air conditioners were humming, the cicadas were buzzing, and Mr Slick had long overstayed his welcome in the office. Vernon had been casting about looking for stimulation when she found the plush upholstery in the road hog. The windows were open. She made herself at home and started to disembowel the front seat.

She has made respectable progress when Mr Slick finally went to his car and witnessed Vernon's clamorous escape.

What you need to know is that a startled vulture jettisons all extra baggage before take-off. That's right . . . it defecates and regurgitates.

Mr Slick couldn't believe his bad luck, and his insurance agent laughed him out of the office when he tried to collect for damages.

It was a long time before he came back, and when he did, his office visits were brief.

Vultures are remarkable birds.

Yosemite's scrofulous cadgers

That's him, next to the squirt gun. We're talking about the California ground squirrel, Yosemite's scrofulous cadger.

His life seems to be an obsession with junk food. If his cute squirrelly presence doesn't seduce you into tossing him a handout, he has no qualms about taking it from your plate, car, backpack or sleeping bag. When he needs a fix of corn chips and bean dip, he'll go to any length to get it. Like junkies anywhere his personal hygiene leaves something to be desired. If you think all ground squirrels look slightly raunchy, just hike into the high country and check out their svelte cousins there. A diet of seeds, gooseberries, bark and the occasional bug makes a world of difference.

Before continuing, however, let me commend the National Park Service. Despite modest increases in federal funding in past years, the park service has carried out its mission exceedingly well. And it has done an excellent job of notifying us of the consequences of feeding wildlife. It has appealed to our common sense ("feeding wildlife can be dangerous"), to our sympathy ("feeding wildlife is bad for their heath"), and to our fear ("feeding wildlife can expose you to disease"). But the task is overwhelming, because most campers can't help themselves. They seem to leave a trail of food wherever they go. So, the park service invests its greatest effort in protecting visitors from bears, and protecting bears from visitors.



If you don't believe it, just leave your salt shaker, an empty pop bottle, or a bag of chips on your camp table. Mr. Park-Ranger-Man makes his rounds after you've turned in, and it doesn't matter what you are doing in there. He'll provide the lighting and wait patiently for you to put the food in the bear-proof locker or trash can.

But rodents are a different matter. The park service simply warns you in advance not to feed them, and then lets the rodents put you to the test.

We had our test last year. Our group came prepared to fend off the cadging rodents. We already knew that it was impossible NOT to feed them. There was always the occasional fallen chip, or the misplaced dish of snacks to give them hope.

So a few anonymous members of the group decided that a squirt gun or two would not only discourage the rodents, but also provide them with a much needed bath. Mind you, this is frowned upon by park management, and I certainly don't endorse or recommend it, but a drama of man and beast inevitably plays itself out in the campground. (And let me add that I use the term "man" advisedly, because few women in campgrounds do as many stupid things as men.)

Well, no sooner than the snacks and libations started to appear so did the squirrels, and with a little practice the marksmen were spot-on target. It was great sport, and a highly entertaining diversion. But the squirrels were really pumped from eating the junk food. Even as the stream of water drew closer, they kept eating at high speed, and when the water hit its mark, they merely scampered beyond the circle of people, and took a hurried dust bath.



Then the muddy rodents made a diversionary circuit behind the tents, and appeared unexpectedly in our midst ready for the next round.

Well, when happy hour came to a close, a few human casualties were almost dry, and the squirrels seemed to have gone to bed. The ladies laid out a splendid assortment of salads, pasta, baked beans, assorted vegies, and grilled meat. A couple more bottles of two-buck Chuck appeared, and we filled our plates. Then we settled down to enjoy the repast. By now, as they say, we weren't feeling any pain. The hikers' aches were gone, the conversation was flowing, and we dined in a spirit of conviviality.

Then one of the ladies got up to refill the wine glasses, and we heard the anguished cry. A scrofulous cadger was on the table, sitting in the casserole, enjoying the pasta. There was a great hue and cry, the men took up their arms, and the food was sprayed with water.

The squirrel seemed to know we'd lost our edge. A few minutes later he was discovered once again in the casserole dish. He had obviously been there for a while and looked rather bloated, but he made his exit unscathed.

Five months of Mossy Rock visitors

It's been seven months since I cabled the camera trap to a bay tree next to the mossy rock. Why have I left it there so long? The pumas are the reason. I thought they might return, and they did. But a lot of other animals passed by too.

Before I go any further, however, I have to confess that this was not a scientific experiment. If it was, I would have controlled my own impulses to attract animals to the site. On the contrary, my own flights of fancy introduced a lot of confounding variables -- like the different baits and lures I used to attract photographic subjects.

The camera has been in operation most of the time--164 days (or 5.4 months out of 7 months)--and it's still there. I lost a couple weeks of data due to downloading errors, and a few interruptions were due to battery failures and temporary dementia resulting from over-excitement. (The latter happens when I get pictures of pumas, and I forget to switch the mode from "View" to "Photo". You should see me cursing myself when I return a week later and find that the camera was in view mode.)

Back to the data. During the sampling period the camera took 588 pictures, 79% of which were images of 17 species of birds and mammals. Not a bad success rate.

Now let's start the drum roll and focus on the graph at the top of the page . . .

As you can see, the winner of the most commonly photographed animal is . . . . THE DEER MOUSE (huzzaaah)!

However, if I hadn't scattered sunflower on the rock I am not sure mice would have accounted for 45% of the photos. Sometime in February I got the bright idea that baiting deer mice would attract the screech owl, and I started to fantasize about photos of owls gripping deer mice.

Spotted skunk (29), wood rat (26), puma (24), and squirrel (24) were the next most commonly photographed species, followed by gray fox (19). All other species, including all the birds, were photographed from one to 8 times.

Camera traps are commonly used to inventory wildlife, and especially mammals. Plot a species accumulation curve and often you find that the number of species photographed rapidly increases at first, and then decreases.



In this case, 70% of the 17 species showed up in the first 58 days. But a long time must pass before you stop getting the occasional new species. You may think you know what's there, but sooner or later something new shows up. I know that opossums, coyotes, raccoons, long-tailed weasels, and otters occur in the area, but not one visited the mossy rock.

Camera traps are a useful inventory method, but they have their limitations. Some species are highly selective in their use of habitat, avoid strange objects such as cameras, or may only be attracted to special baits and lures. To photograph these species you have to know where and when to look, and how to attract them. If you are working in a unknown area, and don't know what to expect, you have to explore all habitats thoroughly.

Even then, you need to rely on a number of different methods, and consult with local people. They usually have a very good idea of what's around. I now consider myself one of them, and can tell you with confdence who visits the mossy rock.

Is this a death wish?

I know what you are thinking. "Has this guy run out of material?

Well, the answer is: "Photographically, yes (and let's hope the action picks up soon). But as fodder for armchair theorization, I find it fascinating that only two days since its last visit, the bear cub is back again checking out the area where the puma slobbered last week.

Why? Does this scruffy little bear have a death wish? I mean, why revisit a site where you know the predator has lingered?

To the contrary. The cub is gathering intelligence.

I would bet dollars to doughnuts that the bear . . .

a) knows that the scent on that rock is not its own scent, and

b) recognizes it as unusual or out-of-the-ordinary. In relation to the kaleidoscope of plant and animal smells it encounters in the course of a normal working day, it probably files this smell in the "suspicious scent category". [Scientists call the fear of the new (or unknown) neophobia.]

In addition. I would wager that scruffy the bear might temporarily avoid areas where the strange-smelling scent is frequent and fresh.

We know that carnivores in general live by their noses, and have remarkable powers of olfactory discrimination. While it might seem foolhardy to revisit the mossy rock, the bear cub can probably guage the age of the scent. A fresh mark by the mountain lion may well tell it that it is time to move on to another place.

In due course, this cub may live to learn that encounters with mountain lions are frightening and injurious. Then again, it may not live to learn that lesson.

To my knowledge, the definitive experiment has not been done to give credence to any of this rambling discourse. That's why it's fun being an armchair theorist.

The scent of the cat

The puma's lovefest on the rock didn't go unnoticed by three species that no doubt appear on the puma's menu. You saw the black bear sniffing the rock on the previous post. That was 26 hours after the puma's visit. Two more days passed, and then came the gray fox at 3:45 in the afternoon. It looks to me like his coat is bristling a bit -- the old autonomic response of pilo-erection. It could well be that just the scent of the cat made his hair stand up.



The next morning at 7:45 the jackrabbit hopped up on the rock and checked the scent too. In the past 6 months I've gotten perhaps a half dozen pictures of jackrabbits at this camera trap set, but this is the only rabbit that took its picture on the rock.

All of these mammals are sniffing the very same area where the cat rubbed it neck. I'd wager they're tuned in to an important cue -- a predator was here.