Armored defenses

hard shells and prickly exteriors

Organisms that can’t run away or hide from would-be predators often defend themselves with some sort of armor that deter access to the soft bodies inside. Clams, cockles, and mussels are enclosed in hard shells. Turtles and armadillos wear hard body-armor.  Butternuts and black walnuts are famously tough nuts to crack. In some cases the armor is not a hard shell but a covering of sharp spines. But for every armored defense, there is a counter-measure that allows some predators to get at the edible parts inside.

The sharp teeth of rodents can gnaw a hole in the side of a seed. Years ago, I sometimes used cherry pits as bait in live traps for mice: the mice could neatly carve a hole in the side of that seed and extract the nutritious nugget inside. Fox squirrels in the eastern deciduous forest harvest and cache black walnuts for later consumption, gnawing open the tough shell (note: these are not the domestic thin-shelled “English” or Persian walnuts we buy in the store). Brazil nuts are notoriously hard-shelled, but agoutis can open the big enclosing fruit and extract the very hard seeds, which they cache, to be opened and eaten later.

The strong bill of a blue jay can hammer open an acorn. Oystercatchers sometimes pound on a mussel shell to break it, then sever the shell-closing muscle, allowing the shell to open. However, our black oystercatcher is said to prefer just to jab its long bill into open shells and then sever that muscle.

Hard shells can be smashed by dropping then onto a hard surface. This seems to be a popular method for a variety of predators. Crows and gulls often do this with shellfish—carrying the prey high above a rocky beach and letting it fall, then coming down to extract the flesh from the broken housing. Sometimes it takes several drops. And there’s often a sneaker nearby who’ll try to snatch the meat before the original bird descends. Some gulls are reported to select hard surfaces for dropping big clams but can use softer mudflats for smaller clams.

Several kinds of eagles are said to carry turtles aloft, then letting them fall onto rocks and smash open. New Caledonian crows drop candlenuts onto anvil stones; then they pry out the edible nut. Coconut crabs eat many things, but when a coconut is on the menu, the crab may climb a tree and drop the large nut to crack it, then finishing the job with its big claws.

Sometimes a tool is used as a hammer to open a food item in a hard shell. Chimpanzees (and a few other primates) are well- known for this behavior, teaching the method to their offspring. Sea otters hold a shelled prey on their chests as the float about, using a rock to beat on and crack the shell. Egyptian vultures may crack ostrich eggs by throwing stones at them.

Or maybe you get somebody else to do it for you. There’s the famous situation in Japan, in which carrion crows have learned to exploit traffic patterns to open hard-shelled walnuts. The crows are reported to wait for a traffic-stopping red light and then place walnuts in the roadway before the light turns green. When the traffic moves on again, the vehicles pass over and crack the nuts, and the crows zip down to grab the kernels. They are also said to drop the nuts in front of moving cars. Similar behavior has been reported for American crows in California, except that there the behavior is not timed to the traffic patterns.

Octopuses have at least two means of opening clam or mussel shells. One method is using the suction cups on the arms to pull the two shells apart. Another method is to drill through the shell (octopuses have two kinds of drills for this purpose), sometimes injecting a venom that relaxes the shell-closing muscle. Different kinds of octopuses have their favorite points of where on the shell to drill. Sometimes an octopus uses its beak to chip off thin parts of a shell, giving access for injection of venom.

Whelks are big, carnivorous snails that drill through shells to get at the meat, using the file-like radula. Lots of insects also can drill through hard seed coverings. Weevils provide good examples; the females of one kind of weevil bore into acorns of various species of oak, chewing channels with mouthparts at the end of the long snout. They then lay eggs in the acorn and eventually the larvae feed on the nutritious material inside (which the oaks intended for their seedlings).

Pinching can do the trick too. The big claws on crabs and lobsters can crack the shells of other crabs and some molluscs. The hefty bill of a grosbeak can crack hard seeds and beetle carapaces.

The ultimate insult to armored or spiny defenses might be just ignoring them and swallowing the prey whole. Gulls and crows gulp down small molluscs and cough up the shells later. These birds can also swallow small sea urchins, somehow sliding that spiny body down their gullets (see photo). Hard, prickly sea stars can be crammed down a gull’s gullet too (photo). Ouch?

Photo by Bob Armstrong
Photo by Bob Armstrong

A few vertebrates are defended by armor plates or spines. How do predators deal with these defenses? Armadillos carry a suit of hardened plates, and one kind of armadillo can roll up into an armored ball. How predators gain access to the vulnerable parts isn’t clear—perhaps a cougar or bear just gives the victim a big swat with a paw to tip it over and disorient it, exposing the vulnerable underparts?

When threatened, European hedgehogs roll up into a ball, erecting their spines to present a predator with a spiky mouthful. European badgers can wedge open the spiny ball and get at the tender belly. Foxes eat hedgehogs too, but they are said to be less successful in attempts to disarm them.

The spines of an American porcupine are a formidable defense—the beast turns its back with raised spines, brandishing its spiny tail. Many an ignorant or over-eager dog can attest to its effectiveness. If it can, the porcupine hides its face between tree roots or its paws. For good reason! A fisher commonly attacks a porcupine by grabbing and injuring its unprotected face, then flipping the damaged victim over to attack the belly and kill it. Other carnivores can use these techniques too.

There is no perfect defense.

Spring medley

progress of a favorite season

Spring is officially here: the vernal equinox has gone by and the days are rapidly lengthening. There are much livelier signs of spring as well. Sapsuckers have arrived in force, rat-atat-tating on rain gutters and stove pipes (and trees). Juncos trill at the forest edge and song sparrows are tuning up in the brush above the beaches. Pacific wrens sound off from invisible lookouts in the understory. Best of all, ruby-crowned kinglets can be heard, high in the conifers, calling ‘peter-peter-peter’ or singing their full, cheerful song. That’s when spring is really here, for me.

A walk on a favorite beach on Douglas Island was focused on finding mermaids’ purses—the egg cases of long-nosed skates. Every year, about this time, we find them washed up in the wrack at the high tide line—there must be a nursery just offshore. On this day, we found sixteen eggs cases, mostly black, dry, and in various stages of decrepitude. Just a few were still mostly whole and khaki-colored, and two had natural openings at one end, where perhaps the young skate had exited. All the egg cases had sizable holes punched into them. I would love to know if marine predators had nabbed the developing embryos or if the holes were made by a tardy, would-be predator just hoping that an embryo was still inside.

A good find in the rolled mats of rockweed at the high tide line was the body of a sea star, entirely eviscerated. All the gonads and digestive parts had been cleanly removed, neatly exposing the calcareous skeleton of the water-vascular system that runs from the center of the star out into each arm. In a living sea star, the canals of this hydraulic system are filled with fluid, mostly sea water. Numerous branches of the main canal lead to the tube feet (often visible in a live star, in rows under each arm) that function in locomotion and in opening clams. When the tube feet are extended, their ends stick to the rocks or the clam shell, and muscles in the feet contract, pulling the animal forward or pulling the clam shell open. We sometimes see a sea star humped up over a partly open clam while the star is having dinner.

A stroll on the Boy Scout/Crow Point trail led to the goose-flat covered with hundreds of crows fossicking in the dead, brown vegetation. Lots of searching and probing. Sometimes half a dozen crows would suddenly converge on another one, everybody poking at something. Apparently, successful hunts were not very common and the gang thought that sharing was appropriate.

Lots of Canada geese were scattered in small groups on the flats, in the river, and in the vegetation by the river. There were mostly head-down, intent on foraging—grubbing for roots and such, and of course talking to each other. Occasionally, two of them would take off and wing around in a wide circle before landing back where they started. One of these duos took off upstream—perhaps a mated pair about to look for a nest site in the forest.

As we often do, out there, we encountered a fellow we call the Raven Man, who carried a big bag of dog biscuits to feed the ravens. He does this from time to time, and the local ravens recognize him. As he passes through each raven territory, the residents come to greet him and cadge some biscuits. We watched some of these ravens carry five biscuits at a time, first stacking them up in a neat pile so they could be held in the bill. A dog, with some hikers, came along later and sniffed out places where ravens had cached their loot, covering it with grass or moss—surprising the hikers who were not expecting to see dog biscuits in the moss.

Most folks in Juneau are glad to see the snow disappear, at least at the lower elevations. But I loved the good snows we had in February, and here are a few flash-back memories.

–Weasels had been very active in the Peterson Creek meadows and Amalga meadows. They bounded over the clean snow, ranging widely. Every so often, the trail dove straight down under the snow and re-appeared a few feet beyond or disappeared under the overhanging edge of a frozen slough. I think they were hunting voles, whose tunnels run under the snow; did they dive down in response to the sound or fresh smell of vole or were the dives just exploratory? Another treat in one meadow were well-defined trails of mice, showing a good tail-drag.

–On the west side of Mendenhall Lake, one day I found a set of tracks running way out onto the snowy ice and right back again. It was clearly a member of the weasel family, probably a mink. What was it doing??

–A snowshoe trek up a creek out the road was a bonanza of tracks (and no recent human tracks). In the woods on the way up the hill, there were tracks of deer, mouse, weasel, squirrel, and maybe a marten. Big excitement of some large tracks that were surely those of a wolverine—the toes and the gait gave it away. The most fun was seeing a set of wolf tracks coursing over a frozen pond that sparkled with sun-struck hoarfrost.

Now the fun in the snow is finished for the year, and the fun of spring begins. Juneau folks typically love to note the progress of spring, as the season unfolds. Skunk cabbage emerging, pussy willows appearing, blueberry buds expanding, the gradual arrival of more kinds of birds, ravens carrying sticks for a nest—they all mark the progress of a favorite season.