Seeley broke the home-hunting process down into six basic steps: scouting, reporting, advertising, debating, rousing, and piloting to the agreed-upon home site. He imported swarms to Appledore Island off the coasts of Maine and New Hampshire. On this small deforested island, the bees had no other choice of home sites except the boxes of various dimensions and qualities that he provided them. Not only could he watch what happened at the swarm clusters, as Lindauer had done, but he could here also concentrate his attention on the potential home sites themselves. The scouts, which were marked with tags so he could differentiate individuals, did not dance for some potential home sites they had visited (observers were stationed at the potential home sites to be able to report which individual bees had visited) but did so vigorously for others, and so he could then determine what qualities the bees valued.
Seeley determined that the scouts “paced” out the insides of his boxes, the potential home sites, apparently to assess volume. Home volume, he found out, was only one of several relevant criteria for bees in their home evaluation. The new home must of course be large enough to accommodate all the bees, as well as to provide eventual storage space, as we’ve learned, for at least twenty-five kilograms of honey (mostly for winter heating fuel) and pollen (baby food), and space for rearing young once the queen gets going and lays some thousand to a thousand and a half eggs per day. On the other hand, the home cannot be too large or the entrance too big, or heating it in the winter (by shivering and huddling to retain as much heat as possible) becomes impractical. If the cavity conformed approximately to a usable home site, the scout who found it returned to the swarm cluster and danced on its surface to indicate its quality (by her dance enthusiasm) and its location.
Home site evaluation, though, was just the beginning. The next problem Seeley tackled was how the bees reach a consensus, since the various scouts danced to indicate different home sites they had discovered. The consensus making, he discovered, starts with dancers who have found one potential home site also attending the waggle dances of other scouts advertising other (different) potential home sites. A process of elimination ensues as scouts who found superior home sites dance longer and more vigorously and thus recruit more converts. More and more scouts become converted to a popular new home site, until a “quorum” is reached. That is, the bee is a realist. She is not wedded to her own ideas simply because they are hers.
At that point, the scouts, upon returning to the swarm, stop dancing and instead make “piping” sounds/vibrations (by a shivering-like contraction of their flight muscles), which I presume is a symbolic preflight warm-up behavior. In any case, it is the signal that encourages the other bees to shiver and warm up also, and so the swarm becomes ready to fly to a new home.
Seeley discovered that after the swarm is warmed up and flight ready, the scouts give a liftoff signal. They make mad dashes through the mass of clustered bees in “buzz runs” that signal it’s time for takeoff. The buzz runs look like a symbolic takeoff, and I suspect the piping also comes from flight muscle vibrations arising from preflight warm-up in which the wings are partly engaged with the muscles, as it were, with the clutch no longer all the way in. During this activity, the scouts nudge and bump into other bees, inducing them to take flight. As a result, the swarm dissolves into a diffuse milling cloud of bees, as I saw often over the entire parking lot at the University of California from my upstairs window, and often in some surprise (if not also a tinge of alarm).
After the swarm is airborne, the former scouts, all of whom know where the chosen home site is, “streak” in rapid short flights through the milling crowd. The direction of these streaker flights is toward the previously chosen home site, and their flights induce the other bees to fly in that same direction. But the bees need still one more signal to proceed; they need to be sure the queen is with them.
The streakers (former scouts) have been many times at the intended home, whereas the queen and the rest of the bees have never been there. The queen plays no apparent role in guiding the swarm, except that her scent is required to assure her presence for others to continue their journey to their chosen home.
It was long thought that the streaker bees could be guiding the swarm by their “come here” scent from the Nasonov glands at the tip of the abdomen. But a study by Madeleine Beekman and colleagues, in which these glands were sealed shut in all the scouts, indicates that the bees are directed to the new home site without the need for this scent.
But as already mentioned, scent is the “sign” at the house entrance. Except for the few scouts, none of the thousands of bees can have any notion of where the entrance—a small hole in a large hollow tree—is. On their own, each of the thousands of bees would be unlikely to find it. Nevertheless, it is impressive to see how they quickly enter; indeed, they literally stream into their new home. That is possible because the scouts alight at or in the home entrance, raise their abdomens into the air, and then release the scent from their Nasonov glands. They beat their wings, which creates an air current that spreads the scent, and the swarm follows the scent plumes to its source, the entrance of their new home.
Honeybee real estate like any other depends on supply and demand. When honeybees were first brought to the East Coast of America by the colonists, they had at first many hollow trees to use as home sites. But eventually they had to settle where others had not. They had to occupy hollow trees that had not been already occupied. At first, picking a spot was easy, but the demand created an outward pressure, and America provided an “experiment” on a continent-wide scale showing the effects of only a modest move by offspring from their parental home. There were no honeybees in North America before the colonists came. One jumping-off point for the bees’ expansion was in Virginia in 1622, and another occurred in Massachusetts in 1640. The bees, without any bias to move away from home, spread westward. Their rate of westward expansion exceeded that of the human colonists; wherever the settlers went, the bees, which the Native Americans called “the white man’s flies,” had preceded them.
By 1770 honeybees had spread to the banks of the Mississippi River. Everywhere the colonists went they found “bee trees,” their source of wax for candles, ambrosial sweets, and the means to make intoxicating mead. Sugar was, as it is now, a highly valued commodity, and, as during the previous thousands of years, it was still available only from the nectar of flowers, and only through the labor of bees. Thus, the settlers’ home-making in their westward expansion was intimately associated with, if not facilitated by, the bees’ home-making.
There are thousands of bee species, but since most are tropical and relatively few are social, the honeybee way of evaluating a potential home site is likely unique in scope and complexity. Nevertheless, the specific requirements for bees’ home sites vary enormously. Many tropical social honeybees, for example, build their nests in the open on cliffs, as they have less need for shelter than northern bees. Solitary bees, depending on the species, may choose a sandbank and dig a tunnel for a subterranean home. Others use a narrow tube such as a reed stem or an insect burrow in wood. One, the mason bee, makes a little home of mortar. Each type of nest site is as though inscribed on the animal’s nervous system as a “template,” and the bee then matches any number of specific features of that template in finding a home site and making a nest. As with homing, the criteria for home site selection are tailored by evolution to the animal’s biology in its unique environment.
Choosing a home site is an almost universal problem animals confront, and there are two main possibilities. For birds, one is to search far and wide for the best possible habitat in which to set up a home. The sampling must often be relatively brief because the bird’s life span is short, and potential home sites are infinite. As in insects in a seasonal environment, the right spot must be found quickly, and most birds have an innate “template” of what the habitat looks like that is suited to their behavioral program and their needs. They can also bypass the searching and testing of specific habi
tat type and rely on the experience of others of their kind, taking their cues from them. If others have found an area good because they live there, it “must” be good real estate, according to this “logic.” Colonial sea birds are an obvious example—if two islands look similar but one has a lot of your species nesting on it, it is likely to be the one without foxes, and the one containing other potential life necessities near there as well. Planting artificial models of Atlantic puffins, Fratercula arctica, on islands along the Maine coast has been successful in attracting these birds to breed where they had been absent prior to seeing others there. To test if songbirds are also attracted to habitats where others are present has involved broadcasting the species’ song in an area, and then seeing if others of that species settle there. Some songbirds prior to migration in the fall scout for sites to come back to in the spring and are attracted to the dawn chorus of others of their kind. In that way they have a specific goal and are ready to start nesting soon after they arrive back, so broadcasting after the breeding season plays a role in recruitment for settlement. However, in a study of the black-throated blue warbler, Dendroica caerulescens, broadcasting songs of the species during the part of the season when the birds return to settle into their home territories did not have an effect on recruitment.
In some ways picking a home site is simpler for birds and us than it might be for honeybees, because we act more as pairs or as individuals and so need no elaborate procedures for consensus making for this decision (although I am sure this can be debated). Nevertheless, we still have the necessity of evaluating different possibilities and making a choice. In birds, it is usually the female who is the home site choice maker. She as the egg layer is by default the ultimate decision maker. She prepares the egg-laying place, which may be no more than a scrape on the ground. Almost any site will do, provided it is hidden from predators and offers sufficient support on which to build the nest. A robin’s nest on a branch, for example, will not stay up unless it is built on a sufficient foundation. A robin may repeatedly carry grass and strips of bark onto a slanting limb, from where these items keep slipping off and falling to the ground. I once watched one do this for a while, and she eventually stopped, to try again elsewhere. She was trying to satisfy an urge, which in this case was to place one item on top of the other. If many items had been piled up, she would have sat on them and turned, placing more of them at the edges, as gradually a nest would have been formed. One result sets the stage for the next step. But the first step is always choosing the right spot. The more propitious the original choice of home site, the less time is wasted, and most birds are very good at picking just the right spot. They may go through a process of “seeing” many potential home sites, and then narrowing them down until they come to one that seems “just right.” Of over a half-dozen broad-winged hawk nests I have seen, all were over ten meters in a triple fork of a hardwood tree. Peregrine falcons’ nests are on cliffs, if available. Dozens of winter wrens’ nests I’ve seen in New England were all in the roots of windblown tree tip-ups, every vireo nest that I have ever seen was in a fork of a horizontal twig, and those of brown creepers are always under dead peeling bark.
Homes may be built for safety from the elements and from enemies, especially with respect to the most vulnerable, the young. Consequently, safety is always of critical importance for most animals’ choice of a home site, because eggs and young are easy pickings for almost any predator. How the sites are evaluated depends on a combination of innate preferences, but learning is also involved. The Canada geese that I have observed now for over twenty years in a series of beaver bogs in Vermont show a clear pattern that may be representative of some species. The first time the geese started to nest in the beaver bog by our house they chose a sedge hummock in a large open area of many such hummocks, and they were successful in bringing up goslings. Now, however, there are many geese that each year come to try to nest in the bog, and predators, perhaps coyotes, raccoons, mink, and otters, have learned about the eggs that can be found there. Now every goose that builds its nest on one of these hummocks ends up with its clutch destroyed. There is only one spot where the nest is never (so far) destroyed, and that is on the top of a beaver lodge that is surrounded by water. The geese fight viciously to occupy that safe spot. But it is not islands as such that they recognize as safe. In some other areas Canada geese nest on abandoned raptor nests, on cliffs (as along the Mississippi River), on platforms put onto trees, and I have a report of them nesting, in Cambridge, Ontario, on the third story of an industrial building, and in large flower pots alongside pedestrian paths. When a pair in that area choose to nest on another site, their nest is commonly raided before the eggs hatch.
Safety of the nest site can also involve neighbors. In one study by Harold Greeney and his assistants in the Chiricahua Mountains of Arizona it was found that the nests of black-chinned hummingbirds, Archilochus alexandri, were clumped in the immediate vicinity of accipiter hawk nests. These bird hawks do not hunt hummingbirds, but they do hunt jays and squirrels that prey on hummingbird nests and create a predator-free habitat. The hummingbirds would not need to consciously or deliberately seek hawks to nest near them. They merely need to return to nest where they were successful and move on if they failed. Similarly, in the tropics, some species of birds routinely build their nests next to the homes of aggressive stinging wasps. The wasps attack anything that, like a monkey, would jiggle the branch from which the nest hangs. In a variation of this theme, some small birds in temperate regions make their home directly in the understory of the huge stick nests of eagles and other large raptors that would probably not have access to their “subletters” because they live on the bottom floor, and besides are just “small fry” and not on their hunting menu.
Rules change when the nest site is no longer arbitrary and becomes a unique and valuable commodity. The nest can be a valuable item manufactured at great cost and skill, in which case it can become a nuptial offering of competing males. Thus, in weaverbirds and woodpeckers, the males do most of the site preparation—they build at least a partial nest or hammer a cavity out of solid wood to nest in, and the female inspects it and in choosing it, chooses him.
PART II
HOME-MAKING AND MAINTAINING
The homing mechanisms of other animals are often mysterious to us because they are usually not apparent except through experimentation, and they involve sensory capabilities and neural processing that we lack. But one of the most widespread, diverse, and sometimes spectacular aspects of animal behavior is home-making, and the process is not just inferred. The results of home-making behavior are preserved as discrete artifacts, tangible records of various steps of behavior. Curiously, animal behavior texts seldom mention home-making, even though animal homes and the behaviors contributing to the theme are essential features for survival and reproduction.
Architectures of Home
HOME-MAKING IS PRACTICED BY ANIMALS REGARDLESS OF their position on the evolutionary tree. The home may be a structure that shields both from the elements and from a diversity of enemies, and it may be especially important for rearing offspring until they are ready to face a challenging environment. It is most prominently displayed in insects and birds but is also found in various forms in mammals, spiders, crustaceans, fish, and some reptiles. Home-making is usually species specific and in many cases seemingly idiosyncratic, perhaps because of its various simultaneous and often conflicting functions. It does not invite making other than obvious generalities except for the main one: whether mammal, bird, or insect, home-making functions for security mainly for raising young, and in some cases this has been the crucial step toward an overlapping of generations leading to a truly social lifestyle.
Whether the home is a cavity with solid walls as with honeybees, or consists of underground tunnels and cavities as with naked mole rats or of “castles of clay” as with many termites, making the home may require excavating; piling up sticks, rocks, fibers, and a nearly endless variety of oth
er materials; and assembling and cementing them together with silk, mud, feces, or saliva into any of a huge variety of structures that are often stunningly complex and beautiful. If they were not for specific uses, some of these creations would be considered art of the most exquisite sort. Nevertheless, some are indeed “for show” also.
Home-making may involve using existing shelters. But it can also involve making the very material that is used to build with, such as a bird that makes its nest entirely from its own spittle, frogs that make a nest from foam created with their cloacae, cicadas that make homes from abdominal secretions whipped into a foam by a kind of breathing-apparatus-cum-air-pump, or caterpillars that make a communal nest from their silk, to trap solar heat on cold mornings.
As elegantly described and illustrated in Karl von Frisch’s book Animal Architecture, home-making is most “advanced” (changed from its ancestral form) in some of the bees, wasps, ants, and termites, the latter recycling their own feces as binder to construct their homes which may house millions of occupants. Termites’ homes may in some cases be up to seven meters tall and are reminiscent of cities in miniature. They are constantly repaired and may last decades. Homes such as those of the African Macrotermes bellicosus attain air conditioning by a system where hot air, which rises, sucks in cool air from the bottom. Millions of inhabitants live in a temperature-controlled environment created by this thermo-siphon temperature control system, without ever venturing outside. The compass termites, Amitermes meridionalis, of the scorching treeless Australian steppes achieve an efficient kind of temperature control by building homes up to five meters tall and aligned so that their broad flat surfaces receive the rays of the low morning and evening sun, whereas the tops of the mounds are sharp and pointed and present the least reflective surface area to the searing midday sun. In areas where heat is a problem but flooding is not, ants, termites, and many rodents build subterranean homes where active cooling can be dispensed with. The huge communal homes may contain (as in some ants) gardens that grow food crops and “cows” in the form of aphids that are “milked” for food secretions. Ants’ homes, depending on the species, may be no more than a hollowed-out acorn, leaves silked together to create a cavity, or in the case of leafcutter ants, Atta, nests so huge that they would dwarf a person who might enter one. These underground spaces contain fungus gardens, where the leaves the ants harvest to grow fungus that they feed on are kept, and also contain spaces for refuse heaps, nurseries, and a chamber for the one female in millions that is allowed to be sexual. The variations are endless, and the mechanisms of how they evolved from just seeking any available shelter to making “castles of clay” are often inscrutable.
The Homing Instinct Page 12
