Columbia Spotted Frogs

a glimpse of some little-known local amphibians

A friend told me about a place where frogs were breeding, so I went to look. Not a frog in sight, except for a pair of legs disappearing under an algal mat. But I went back a few hours later, when the day had warmed up, and there they were—at least a dozen of them. The males were singing, if one can call it that: the ‘song’ is a series of grunts, and different males sang on slightly different pitches. Occasionally an eager male approached another frog and tried to grab it from behind, which is the usual position for fertilizing eggs as they emerge from a female. Males have sturdy forearms and strong thumbs for the purpose of holding a female in an embrace called amplexus. But as I watched, the male was kicked off by the presumed female; either she wasn’t ready to mate or ‘she’ was really another ‘he’.

Those singing frogs are probably Columbia spotted frogs, which are native to Southeast Alaska, occurring chiefly in the transboundary river valleys. How they got to Juneau is not known—possibly with help from humans. However, in recent years, they have been seen in several places in the Mendenhall Valley and, a few years ago, specimens were sent to an expert for genetic analysis, which determined the species identity.

Columbia-spotted-frog-Kerry-howard
Photo by Kerry Howard

Columbia spotted frogs hibernate in ponds, springs, beaver dams, and under stream cut-banks where it doesn’t freeze and moisture has adequate dissolved oxygen for them to breathe (through the skin). However, they are not dormant in winter; they can move around, sometimes several meters underwater to a new wintering spot. Come spring, males emerge first; they (unusual for amphibians) then choose an egg-laying site in warm, shallow water. Later-emerging females (up to 100mm long) find the males’ chosen sites. They are larger than males (up to about 70mm) and can lay hundreds of eggs in a globular mass.

Each fertilized egg is surrounded by two jelly layers and takes up to three weeks to hatch; the time is shorter when the water is warm. The tadpoles are about eight millimeters long when they hatch. They can grow up to ninety millimeters (total length) by the time they lose their tails, grow legs, and look like little frogs, but some transform at smaller sizes. If conditions are right, they may transform in their first summer, but otherwise they can hibernate until the next year. The froglets grow but don’t become sexually mature for two to six years, depending on conditions. Males mature at an earlier age than females but have shorter lives, on average. Adults can live for several years: in some regions up to about twelve years for females and ten years for males, but elsewhere just seven years for females and three for males.

The frogs feed primarily on a variety of small insects but also eat snails, worms, and (rarely) a tadpole. Tadpoles are typically herbivorous: they scrape vegetation and filter the fragments; they also filter detritus and occasionally scrape a dying tadpole.

Spotted frogs show a fair degree of site fidelity for breeding and hibernation. They can travel quite long distances overland, from a hibernation site to a breeding site. Then they may move to a summer feeding site and eventually back to a hibernation site. Travels up to about six hundred meters long have been recorded.

This species, along with other amphibians in North America, is at risk from a lethal fungus infection that has decimated other amphibian populations. Spotted frogs (and our western toads, wood frogs, and other native amphibians) are legally protected: one is not allowed to “hold, transport, or release” them without a permit from ADFG.

Hiding in plain sight

camouflage is more than just visual

When the presence of an animal is difficult to detect or it is hard to locate, it is said to be cryptic. Think of a female mallard sitting on her nest; her mottled brown feathers are good camouflage—they make her look like a little heap of old leaves, not readily distinguishable from the background. We may actually be seeing her but be unable to register her presence; we just don’t detect her, much less locate her nest. She is not out of sight, like bunny in a burrow; she is hidden plain sight.

A famous case of background matching was very well documented. The peppered moth (Biston betularia) in the UK likes to rest on lichen-covered tree trunks and branches. The typical form of this moth has white wings speckled with black spots. The wings also reflect UV light, but even that has a speckled pattern. The speckled pattern makes the resting moth almost invisible as it sits on the tree bark.

moth-2-by-bob-armstrong
Photo by Bob Armstrong

During the Industrial Revolution in England, beginning in the mid 1800s or so, pollution from factories killed off many of the lichens and the tree bark became blackened with soot. Then the typical form of the moth was very conspicuous against the dark bark and they were readily picked off by hungry birds. A mutant form with black wings, much less noticeable on the sooty trees, then became more common. Natural selection in action!

More recently, industry has become environmentally somewhat cleaner, the trees are not so dark, and the black form is vulnerable to predation. As a result, the white, typical form of the moth is again becoming common.

Because we humans are so heavily oriented to visual things, we usually think of visual cryptic-ness (or crypsis). But why not think of crypsis that relates to other sensory systems? There are several studies suggesting that there may acoustic or olfactory crypsis, and hints for other sensory modalities.

–Acoustic signals are used by many animals for courtship or other communication among individuals of the same species. For instance, males of a neotropical frog make complex calls to attract females, especially when several males are calling simultaneously. But in the presence of predatory bats, they simplify their calls, even though the females prefer the complex ones. At the price of reduced attractiveness, the simpler call appeared to make the male callers less detectable to the bats. They didn’t completely stop calling—which would be acoustically hiding. They just became less detectable.

Many small birds produce a high-frequency, thin alarm call (“seet”), which does not carry very far. This is thought to be more difficult to locate than the usual contact or distress calls. Experiments with captive hawks indicated that these predators had trouble locating the source of the sound and attacked less often, when the potential prey used the high-frequency calls. The small birds can hear the call and be on the watch for trouble, but they don’t need to know the exact location of the caller.

–The leaf-eating caterpillars of another moth (that’s closely related to the peppered moth) look a lot like little twigs, which might protect them from birds but not from predatory ants that hunt primarily by smell. Yet such ants just walked over the caterpillars as if they weren’t there at all and didn’t attack them. However, if the caterpillars were transferred to a different food plant, the ants attacked them readily. Then, after the caterpillars had fed on the new food plant for a while and molted to the next larval stage, they were again ignored by the ants. In this case, the protection comes from matching the chemical constitution of the food plant.

Some elaborate studies of herbivorous pine bark beetles compared the chemical signals and the responses of predatory beetles in different areas (California and Wisconsin). Pine bark beetles use chemical signals called pheromones to attract others of the same species. Predatory beetles cue in on those signals. As might be expected, California pine bark beetles liked their own pheromones better than the Wisconsin ones, and vice versa. But the California predators homed in better on the Wisconsin pheromones, and the Wisconsin predators homed in on the California pheromones better than on the local ones of each population. In other words, the prey were less apparent to local enemies than distant ones, suggesting they were chemically somewhat cryptic.

Birds have a preen glad just above the tail that produces oils and waxes that the birds use to dress their feathers. A fascinating study revealed that the normal waxes are replaced by less volatile waxes during the nesting season of several ground-nesting shorebirds. Furthermore, if only the female of these species does the incubating, then only the females show the shift in wax composition. Ground-nesting birds are particularly vulnerable to predation by mammals, and experiments with a search dog trained to the smell of these waxes showed that the dog was less able to detect the waxes produced in the nesting season.

–In addition, there are hints—but only hints—that shifts in electrical fields, or in production of wakes by swimming animals, or in substrate vibrations have the potential to be used by prey animals to whom crypsis could be advantageous. There are, potentially, many ways to be hidden in plain ‘sight’. And many investigations are needed. In the meantime, it is useful and interesting to contemplate things beyond our usual ken.

A big thank you to the gracious CBJ librarians who promptly obtained a necessary reference for me!