On the water

a whale watch trip stirs up questions

I don’t get to go whale-watching very often, but when I do, I usually see something interesting and new questions frequently get stirred up.

One trip gave us a good view of humpback whales doing their locally famous bubble-net feeding. Most of us here have seen this behavior at one time or another, but somehow it has not become ‘old hat’. How does a group of whales decide which one will create the rising, circular curtain of bubbles that rings a school of little fish? How did they invent this foraging tactic in the first place? Did it really originate here, and get carried to Prince William Sound whales by a wandering Juneau whale? How do they coordinate the upward rush of several whales through the panicking fish and avoid crashing into each other?

Another trip brought a troop of Dall’s porpoises, cavorting around the bow of the ship. The group grew, as additional individuals came zooming in from who knows where. They played there for some time, to the great delight of all on board. Then they disappeared, apparently on some magical signal; suddenly they were simply gone, vanished out of sight. Why did they all gather by our ship? And why did they go?

Then, in early September, we watched a small pod of orcas cruise by Little Island, where hundreds of Steller’s sea lions, of all sizes, had hauled out. Even the few sea lions that happened to be in the water a few feet from their dry confreres on the beach did not seem to be alarmed at the orcas passing nearby. Mammal-eating orcas don’t use their sonar to find prey, because mammals can hear the beeps. So we thought that these must be a resident, fish-eating bunch of orcas, using their sonar to find fish and therefore no threat to the sea lions.

As the orcas went on down along Ralston Island, we learned that this was actually a group of transient, mammal-eating predators, identified by a known mark on one of them. Nevertheless, no prey was visible and they acted (to our eyes) as if they were hunting fish: sudden, brief changes of direction and quick dives. This led to the question of whether mammal-eating orcas might sometimes snack on fish as well.

Then a small band of juvenile sea lions came into view from the opposite direction, seemingly oblivious of the jinking-around orcas—until the two groups of animals were very close. Then the sea lions really freaked out, caught between the rocky shore and the orcas. Much frantic splashing and churning about! As the orcas placidly went on their way, the sea lions calmed down and swam toward Little Island. What was going on here? Mammal-eating, transient orcas with already full stomachs? Just fun and games for the orcas? Mistaken identification by the sea lions?

Later that same afternoon, we encountered a scattered group of humpbacks doing nothing very exciting. But alongside our boat there appeared a solitary sea lion with something in its teeth. That sizable something was tossed and thrashed about until it was just a rag (and a few loose bits for the attending gulls). Finally we got a closer look and saw tentacles with sucker discs, just before the whole thing disappeared down the gullet of the sea lion. The octopus was caught at depth of over four hundred feet, not a very deep dive for a sea lion.

Protective associations among animals

sticking together, for mutual benefit?

As I was reading a research paper about the foraging behaviors of killer whales (orcas), I found a small paragraph about some observations in Prince William Sound that seemed, at first glance, peculiar.

The researchers reported that Dall’s porpoises were often seen in close association with killer whales—swimming with a pod of the whales, popping up in front of the whales’ noses, and even playing with the whales’ calves! One Dall’s porpoise stayed with a pod for over four months. These observations begin to make sense when one recognizes that these killer whales were residents—the type of killer whale that eats only fish. Clearly, it would be folly for the porpoises to hang out with transient killer whales, which gobble up porpoises, seals, sea lions, and even other whales!

Other marine mammals that are prey for transient killer whales were also associated with pods of resident killer whales in the same study. Steller sea lions dove in and out among the whales on several occasions. A minke whale swam with a resident pod for several hours.

These observations were sufficiently frequent and consistent to suggest that the porpoises, sea lions, and minke whale were probably getting protection from predators such as the transient orcas. Transients and residents don’t mix, so by hanging out with residents, the other mammals increased their chances of avoiding the transients.

Those observations got me thinking about other protective associations. Here are various examples: Several kinds of fishes associate with sea anemones. For instance, clownfishes make themselves impervious to the stinging cells of their particular kind of anemone by making tentative approaches, gradually acquiring a covering of the anemone’s mucus—which then prevents the discharge of the stinging cells when the clownfish touches the host anemone’s tentacles. Some fishes that are close associates of anemones even feed their anemone by dropping bits of food into the anemone’s mouth.

The lion’s mane jellyfish of our cold North Pacific waters harbors a variety of small fishes and invertebrates among its tentacles. The portugese man-of- war jellyfish hosts a small fish that color-matches the dangling tentacles of the jellyfish; the fish is partially immune to the stinging cells of the tentacles. Still other small fishes lurk among the spines of sea urchins; their color patterns and behavior make them relatively inconspicuous there.

Lion’s mane jellyfish with silver spotted sculpin. Photo by Annette Smith

The sandwich tern is reported to nest very commonly in colonies of black-headed gulls or arctic terns, where the active nest defenses of those birds help protect the nests of the sandwich tern too. Some neotropical wrens like to nest in acacia trees that are inhabited by protective ants (or wasps), favoring trees with aggressive, active ants, which can deter predation by other birds, monkeys, and snakes.

A most unusual case was reported for the relationship between the giant cowbird and nesting oropendulas and caciques (colonial blackbirds that makes deep, pendant nests) in Panama. The cowbirds are brood parasites, laying their eggs in other birds’ nests, usually to the detriment of the hosts’ chicks. Giant cowbirds favor oropendulas and their relatives as hosts. Normally, the hosts would try to keep the cowbirds from placing eggs in the host nest, but in this case, strong defense against the cowbirds only happened in some colonies—those that were also defended by aggressive wasps and bees. In other colonies, with no protective wasps, the cowbirds were less unwelcome. Why would oropendulas ever allow cowbirds to drop eggs in their nests?

A major enemy of oropendula and cacique chicks is a botfly. Adult botflies lay eggs on the chicks, and the botfly larvae burrow into and feed on the chicks’ bodies. Chicks with botfly infestations often die. Apparently the wasps and bees present in some colonies somehow reduced botfly attacks. In the other colonies, without wasps and bees and where cowbirds were allowed, it turned out that cowbird chicks preened their host nest-mates, removing botflies, so the host chicks actually survived better with cowbirds than without them.

It is not clear how widespread that wonderfully complex situation might be—does it occur in other parts of Latin America, with other oropendulas and caciques, with other wasps or bees and other botflies? Or is it unique to the local circumstances of that study? Then we can ask What made those circumstances so special?

Out and about

bits and pieces from December

I try to get out for a walk every day, whatever the weather, although the weather may determine the length and location of the outing. How much I see of natural history interest varies greatly, depending on many factors, including a perceived need to watch the footing in sloppy mud or on slippery ice or wet rocks, sometimes a wish to be a bit sociable, or even do some serious (or not-so-serious) thinking. But most of the time, I like to keep my eyes and ears open to what is around me. So here are some bits and pieces from December.

As a cold snap settled in, Mendenhall Lake grumbled and growled and muttered in a long-winded soliloquy—the ice, talking to itself as the water froze and expanded. Smaller ponds were less loquacious but still murmured and popped at a lower decibel level. Meanwhile, overhead, large flocks of pine siskins flitted from spruce to spruce, sometimes swooping high over the canopy before disappearing in the crown of another cone-laden spruce.

In between short periods of deep cold, however, we had spells of surprisingly warm temperatures, turning our little bit of snow to slush and sending meltwater down over the existing ice on streams and ponds. Open water formed at inlets and outlets of ponds and along the fringes of Mendenhall Lake. A reliable observer reported seeing a beaver swimming in Mendenhall River in late December, when local beavers are normally snug in their lodges, sleeping or nibbling from their winter cache of twigs. That beaver was not the only one escaping cabin fever: in several locations, I saw very recent tree-cutting and branch-gnawing that had not been there a few days earlier.

On the ground near Moose Lake I found several small wind-broken cottonwood branches, with the upper sides nicely de-barked. Some lucky gnawer had capitalized on this bonanza. But who was it? Not a beaver, although beavers had debarked a cottonwood tree trunk near the lake, leaving the marks of wide incisor teeth. Not a porcupine—the tooth marks were too small. But the marks were too big for a mouse. My best guess was probably a snowshoe hare; hares are generally fairly numerous in the area and the incisor marks were similar in size to the teeth in a hare skull in my collection.

Photo by Pam Bergeson

Along the Treadwell Ditch are many trees, usually hemlocks, that show the marks of porcupine gnawing—tooth-marked, barkless patches, low on the trunk. This is a common sight around here, of course. I was particularly interested to find at least two trees that seemed to have been completely girdled sometime in the past. The bark had been removed all the way around the tree, which would interrupt the flow of water and nutrients between roots and crown, starving the roots of food and the crown of water. Yet these trees sported full crowns of needles and looked healthy. How could that be? The porcupines had removed all the outer bark and eaten most of the nutritious inner bark, but a meager, sketchy, brown network of inner bark was still visible. Could it be that enough strands of inner bark remained to connect the roots and the crowns? Hard to believe that would be enough to support a good-sized tree!

There are other little mysteries about porcupines and hemlocks. Some trees have obviously been visited repeatedly, in different years. Old chewings have partially healed, but new ones are there too. Are these trees particularly tasty? Also, I get an impression (untested, so far) that porcupine gnawing is more common on the uphill side of a hemlock trunk. Is that really so, and if so, why?

A special pleasure was seeing two humpback whales spouting as they cruised near Lena Point. They may have been late-departers for winter in Hawaii or they may have been among the few that overwinter here, feeding on herring (and any other luckless little forage fishes). Not on this day, but sometimes one can see a few sea lions swimming near the corners of a foraging whale’s mouth, trying to catch the fish that slipped away from the whale. The herring equivalent of ‘out of the frying pan, into the fire’!