The Homing Instinct: Meaning and Mystery in Animal Migration

GETTING TO A GOOD PLACE

THE TENT CATERPILLAR MOTH, MALACOSOMA AMERICANUM, is common in North America. It emerges from its light yellow silk cocoon in late summer, and the female is then ready to deposit her batch of over a hundred eggs. She searches for an apple or a cherry tree, and somewhere out on a thin twig of just the right diameter — about a half centimeter — she exudes her eggs along with sticky foam to form her egg mass into a ring that wraps around the twig. The foam dries and hardens, encasing the clutch of eggs and gluing them solidly to the branch where they stay through the coming winter. But the larvae develop inside the eggs during the summer and, while confined in their eggs through the winter, hatch at almost precisely the day, about nine months after the egg-laying, when their tree breaks its buds.

The moth is named for the conspicuous communal homes of silk, called “tents,” that its caterpillars make, and in the spring of 2013 I found a just-made tent on a young black cherry tree next to my Maine cabin. Like nearly everyone else in this part of the country, I was long familiar with these caterpillars but had not deemed them worthy of a closer look. The tents act, I learned, like miniature greenhouses and warm the new caterpillars at a time when nightly frosts are still common. But, despite its advantages, to have any home is to incur costs: it has to be made, and it takes time, energy, and expertise to make, and the wherewithal to travel to and from it. For the time being, I wanted to know where the caterpillars making this home had come from. To my surprise, the ring on the twig with the now-emptied eggs I was looking for was almost a meter from the tent. How had the many hatchling caterpillars “decided” or been able to stay together and then coordinate to make their tent? Squinting against the sun, I could see a glistening trail of fine silk leading from the emptied egg-case ring to their home, so here was at least a hint as to how they crawl together to end up at the same place.

On the second day after I found the tent, May 1, there was still snow on the ground in the woods. There was as yet no sign of fresh green anywhere. But I wrote in my journal, “Black cherry buds ready to pop leaves.” These trees are the first to leaf out, and the caterpillars could not have fed yet. What would they do? An hour after the sun came up, the tiny caterpillars emerged from their tent and massed on its sunny side. An hour later they started milling about, and then a few started crawling, seemingly aimlessly, several centimeters up and down the trunk and branches of the cherry tree.

As I had anticipated, some of the tiny caterpillars started to crawl back onto the same branch they had come from, possibly following their previously made silk trail. But they went only six centimeters before turning back. Others went down the trunk of the tree. Always some would turn back, and then the others followed one behind the other in a line. Finally, by 7:30 a.m., a contingent of about twenty of them had progressed nine centimeters down the tree trunk, although two were coming back up. Then more started to leave the tent, and eventually all were in one long line, going only down the trunk and then angling up another branch. In half an hour the leaders had traveled seventy-three centimeters and reached a bud. The rest were strung out all the way to the tent, but their two other travel-direction options had been abandoned. All were eventually massed at the same cherry bud, three-quarters of a meter from their tent, and in an hour and a half they had all returned to their tent, one following the other in a long train.

The young black cherry tree showing relative locations of a tent caterpillar moth egg cluster (C) from which the clutch of just-hatched caterpillars emerged and traveled to start making their home (H) in a crotch of the tree, and their first travels as a group (T) to feeding places

At noon they came out and crawled onto the outside of their tent, waving their heads back and forth, apparently weaving silk from their salivary glands to enlarge it. Another hour later they were again all massed inside the tent and perched, immobile, tightly against the bark, where they were barely visible through the thin gossamer veil of silk.

The caterpillars stayed in their tent through the night, and I expected them to go at sunup to the same branch where they had been the day before. But instead, this time they all followed an entirely different path, going directly up the tree instead of down as on the previous day, and without taking another side branch. I could not detect any silk on their so-far two different foraging trails, and this time they went even farther — a distance of 130 centimeters. After their one meal the day before, they were already noticeably larger. A few were the same size as the day before, but most had probably doubled in weight. There were many tiny fecal droplets in their web. So they had fed, even though it seemed hardly possible that they had anything to feed on at the barely opening bud.

On the third day the buds had opened and the tree was replete with new small leaves pushing out of the buds. But it had been a cool night — there was again frost on the ground at dawn — and the caterpillars made a slow start.

The pattern soon became clear: the caterpillars spent most of the night and most of the day when they were not feeding in their home. The time spent on tree branches was brief, and it could not have been just to keep warm that they stayed in their home because they went back inside just as quickly after feeding regardless of temperature or time of day.

Having found and watched the caterpillars of one tent, I then observed others for more clues to their homing behavior. One of the surprises to me was that as they grew larger, they foraged independently of one another, no longer going to and from feeding areas in groups. Furthermore, after they were about half grown they left their tents, not to return at all but still to continue feeding before eventually searching for a spot in which to spin their flimsy cocoon. Tent caterpillars usually choose a bark crevice to pupate, although commonly they also choose the cracks in the sides of buildings. But why were the young caterpillars strongly homebound and the older ones not?

I suspect the young ones’ web-making behavior may have evolved in part as an anti-predator response. The tents were visited by red wood ants, Formica rufa, and right after the caterpillars hatched, these ants often loitered alongside them on their trails. I tore a nest open on one side to find out if it served as protection. It must have, because ants entered, though frequently wiping their antennae as though irritated. Nevertheless they tarried inside the damaged nest, and I saw them grab and walk off with caterpillars. No ants entered an intact nest of the several I watched, each of which consisted of several successive layers of silk. Thus, the webbing of the tent acts as a deterrent to predators such as ants. Staying inside the home most of the day and night, as these caterpillars appear to do when they are small, probably reduces mortality from parasitic flies and ichneumon wasps as well. When they are larger, the caterpillars are probably protected from the ants, as well as from most birds, by a layer of fine spines. They pupate without having to bury themselves to escape frost, because the adult emerges long before there is any frost.

Because these caterpillars are protected from predators in the summer homes they build and by the spines they wear, because they mature early enough in the summer for the pupae to avoid the cold of winter (by early emergence of the moth), and because the eggs and young larvae are immune to freezing because of the antifreeze they contain, “everything” in the life of the tent caterpillar moth may be found within a few meters. The adults that emerge in late June are not far from the apple or cherry trees where the parent left her eggs, and their life cycle can be completed without their having to go far from home, unlike some other insects which traverse a continent to be able to satisfy all their needs.

Monarchs. Of all the insects, the travels of the monarch butterfly, Danaus plexippus, are perhaps most famously spectacular in both scale and scope. Dr. Lincoln P. Brower of the University of Florida in Gainesville (now at Greenbriar College), who has studied this butterfly and its migration for over forty years, records the rich history of the emergence of our knowledge of monarch migrations. Early naturalists saw “immense swarms” in the prairie states where the caterpillars fed on the leaves of the many native species of milkweed (Asclepias) and the adults fed on the nectar of their flowers. Monarchs declined when later industrial agriculture destroyed many of their food plants, but in the nineteenth century they resurged in the East due to land clearing and the spread mainly of one milkweed, A. syriaca. Millions of them were seen passing for hours, even in Boston. This was a phenomenon that is hard to imagine now and it ignited much interest then. Charles Valentine Riley, the entomologist who first hypothesized that these butterflies engaged in a birdlike migration, cites people seeing them in the fall in swarms that extended for kilometers and obscured the sun, “blurring day into night.” Huge lines of them passing Boston in 1880 were described as “almost beyond belief.” Now, with reforestation, plowing, and then the use of Roundup and other weed killers that eliminated their food plants in agricultural fields, the monarch is but a shadow of what it was. In the past several years in the East, it seems to have almost disappeared. For the first time, I saw not a single one in late summer of 2013. But our knowledge of the scope of the monarch migration has blossomed.

Monarchs migrate on their own power for thousands of kilometers, and, unlike many other insect migrants, the population (though not the individuals) has a regular two-way migration, although as with the other insect migrants, the individuals that come back are not the same ones that left.

Unlike most of the other North American butterflies and moths, which overwinter in New England as eggs, larvae, pupae, or adults, monarchs cannot survive there through the winter in any stage. The population that normally now graces fields all along eastern North America overwinters at around three thousand meters’ elevation in dense fir groves on the southwest slopes of volcanic mountains thousands of kilometers to the southwest, near Mexico City. The monarchs find shelter in those fir stands from rain, hail, and occasional snow. It is not cold enough for the butterflies to freeze there, but it is cool enough for them to conserve the energy resources that they have accumulated on their way south.

The monarch butterfly adult, caterpillar, and chrysalis

In the summer, the monarchs fly in what look like random zigzag patterns over the New England fields as they stop here and there to sip nectar. Occasionally you see a mated pair, the female doing the work of flying, the male dangling passively with folded wings while attached by his genitals. After the prolonged mating (and/or technically “mate guarding,” since it prevents mating by other males), the female glues her delicately patterned green eggs with gold markings, one at a time, to the undersides of milkweed plants. In a few days, the flashy yellow-black-white larvae hatch and start chomping. After about fifteen days (depending on the temperature), the caterpillars have increased their weight to 1.5 grams (2,780 times the hatchling weight). The caterpillar attaches itself to a support such as the underside of a leaf by a clasping organ at the hind end of its abdomen to hang upside down. It will then molt into the bright green pupa (chrysalis) with the shiny golden spots that is surely familiar to almost all school kids. In a few days, the chrysalis starts to turn dark, and the outlines of the orange-patterned wings are visible through the now-transparent cuticle. When the chrysalis splits, along a predetermined line of weakness in the back, the limp adult slips out and expands its wings, and in two or three hours hormones will have instigated a biochemical process that hardens its body armor and stiffens its wings. The butterfly is ready to fly. Where will its wings take it?

Thanks to the monarch studies initiated in 1935 by Dr. Fred A. Urquhart and his wife, Norah Urquhart, from the Zoology Department of the University of Toronto and continued to the present day with the input and cooperation of thousands of amateur volunteers, there is now an amazing story to tell. The Urquharts noted in the late 1930s that the monarchs they saw in late May and early June in Canada had tattered wings, and they knew that this species would not and could not overwinter in Canada, so they suspected that they may have come a very long way. Monarchs fly in a southwesterly direction in the fall, but nobody had a clue where they ended up. To get some idea of the butterflies’ movements, these researchers in 1937 began gluing paper tags onto monarch wings with this message: “Please send to Zoology University Toronto Canada.” Monarchs weigh almost half a gram and the wing tags only 0.01 gram, so the tags were not likely to hamper the animals’ movements. Similar tags, used today, have pressure-adhesive backing and can be folded in half and glued over the leading edge of the forewing (after the scales are removed).

The idea from the inception of the monarch-marking studies was to try to find out if the butterflies migrated — an idea that at the time, as Urquhart noted, “was considered quite impossible.” But the question of where the butterflies might be going to and coming from grabbed the imagination, and anyone seeing a tagged butterfly would be sure to try to catch it. Sure enough, tags were returned over decades that suggested a migratory pattern. Individual tags were returned from huge distances, up to 1,288 kilometers. One monarch that was tagged in Ontario in 1957 was recovered eighteen days later in Atlanta, Georgia, 1,184 air kilometers distant. Clearly, when the butterflies left Canada in the fall, they headed south.

Still, nobody knew what happened to the mass of butterflies. Then, in January 1975, Cathy and Ken Brugger of Mexico City found them — a dazzling, shimmering, orange display of an estimated 22.5 million monarch butterflies on one 2.2-hectare site (which turned out to be only one of ultimately thirteen overwintering sites in the mountains of Mexico). The millions of monarchs were festooned in the trees in the mountains of Michoacán near Mexico City. The Urquharts excitedly traveled to see the site and on January 18, 1976, listened to “the sound of the fluttering of thousands of wings [that was] like that of a distant waterfall.” As they stood awestruck by this dazzling display, a pine branch broke off from the sheer weight of butterflies attached to it, and it crashed to the ground right in front of them. Fred Urquhart had been posing for a National Geographic photographer surrounded by these just-fallen butterflies when, incredibly, he saw a tagged one among them. When he traced its origin, he learned that it had been tagged on September 6, 1975, by Jim Gilbert, from Chaska, Minnesota. Urquhart, who had encountered countless tagged butterflies in his career, said it was “the most exciting one I have ever experienced.”

The picture that has now emerged from decades of study is that individual butterflies migrate all the way from Ontario to Mexico in the fall, arriving there at their overwintering sites in a torrent during October. They spend most of the winter in Mexico in a cooled low-energy state but soar around on warm days to drink water and replenish on nectar. In early spring, when their sex urge awakens, there is a mating orgy followed by a mass exodus. Most of the females mate before leaving, and their “compasses,” which were set to take them south in late fall, are now “reset” to take them in a northerly direction.

As the tide of butterflies advances northward, the females stop to lay their eggs on milkweed. Some of the butterflies from Mexico make it all the way to the north, and others (their offspring) that grow from the eggs laid along the way arrive later. Those of the first generation have slightly tattered wings when they arrive in the north, while those that arrive later have untattered wings. (However, not all monarch populations migrate, and not all that do, travel in the same directions as the populations of northeastern North America.)

One of the mysteries that puzzled Fred Urquhart was how the butterflies home. In Urquhart’s 1987 book on the monarch, he speculated that the butterflies perhaps use the Earth’s magnetic lines of force, although different populations of the butterfly migrate in different directions, so they could not all be orienting to it in the same way.

A potentially even more puzzling question is the ultimate (evolutionary) one of why these butterflies migrate in the first place. Urquhart simply suggested what he admitted was a “perhaps far-fetched” idea: that “twice each year it [Earth] passes through an area rich in some sort of radiation that could impinge upon animal life [that] might affect in some manner the cells of the body causing reproductive organs to abort in the fall and develop in the spring and initiate the migratory response.” This is an unlikely theory, though, mostly because it depends on a mechanism that is not adaptive in evolutionary terms. Instead, more current thinking about the adaptive reason why the phenomenon has evolved focuses on energy economy and maximization of resource use under the expected evolutionary constraints from the monarch’s having evolved in the tropics, meaning it was not able to survive northern winters. (Monarchs belong to the family Danaidae, an otherwise strictly tropical group.) Migration to the north in the spring opens up the milkweed crop over a major swath of North America as a food base for the larvae. In addition, the journey is probably not costly to the monarchs, either in terms of predation (since they are chemically protected from predation by poisons they sequester from their food plants) or in terms of energy costs, since their energy intake along the way more than makes up for the energy expended for travel. Indeed, unlike most birds that may deplete all their fat reserves on migration, these butterflies instead fatten up on their journey and may consist of about 50 percent body fat by the time they arrive in Mexico, where their overwintering fast begins.

Butterflies and moths experience tremendous selective pressure, and undoubtedly there are constant readjustments of survival strategies. Weather affects the populations, not only through flight activity and flight range as well as growth rates of larvae, but perhaps also indirectly by influencing virus infections. But Urquhart noted that each female monarch butterfly lays up to seven hundred eggs, and he calculated that the “biotic potential” — the number of individuals if there are no deaths — of one female after only four generations (that is, at the end of one summer) is 30,012,500,000 adults. Luckily for the planet, animals’ reproductive potentials are never naturally realized, for long. The limit is quickly reached when the population uses up its food base, in this case milkweed. In some years a virus decimates most of the monarch population over North America, but then several years later it rebounds. But the population cannot rebound from some things: in recent years there have been massive declines of the monarch population that cannot be reversed, because they are due to unnatural causes — the massive conversion of land to crops, and the introduction of genetically modified crops that tolerate herbicides, which have allowed the elimination of milkweed that formerly grew between rows of corn.

The flight performance of monarchs is spectacular, but like the hordes of cluster flies from the surrounding fields and woods that overwinter in my cabin, they are traveling to a specific place for overwintering where they have never been before. Such homing movements are diverse, but common. Robert D. Stevenson and William A. Haber of the University of Massachusetts, Boston, found a regular seasonal migration of about eighty percent (250 species) of butterflies living in the dry lowlands of the Pacific Slope of Costa Rica that migrate to wetter forests of the east. Distances traveled range from ten to a hundred kilometers.

In North America as well as in Europe, the cosmopolitan painted lady, Vanessa cardui, a mostly orange and black butterfly with white spots and pink and blue “eyes” on its under-wings, at times appears in large numbers and then is not seen again for years. Usually the individuals are seen crossing a road, and almost all will be heading in the same direction. The painted lady regularly migrates north from Mexico, from where it originates, after heavy rains in the deserts have created an abundance of food plants, primarily thistles. A friend told me of one migration while he was in Arizona when his windshield wipers “soon became useless” because of the huge numbers of painted ladies plastered onto them as he was driving. I see them regularly in Vermont and Maine, but seldom in large numbers (the summer of 2012 was one of the exceptions).

Red admiral butterfly larva, adult, and chrysalis. The larva makes a shelter for itself by pulling leaves together and holding them with silk, while then feeding on the leaf.

One of the butterflies that not only migrates as an adult but also hibernates in some parts of its range is the red admiral, Vanessa atalanta. It is (as are all butterflies!) beautifully colored. It sports a wide red stripe across each dark forewing ornamented with white spots, and its larvae feed on nettles. I wrote in my journal on May 11, 1985, near my home in Vermont: “In the afternoon from around 2:30 to 4:30 PM, as I was jogging along on an 18-mile circular loop I counted 512 red admiral, crossing the road in front of me. All but 5 of these were flying in a northeasterly direction. At 5:00 PM, after I was home, I take compass readings of butterflies flying over a plowed field where they funnel onto it through a valley. I can see them to take a bearing for at least 50 paces — 250 feet. All 22 that I observed flew in NE direction. At 6:00 PM activity almost stopped. The breeze is slight, from northwest.” In the summer of 2001 and again in the spring of 2010 I saw large numbers of red admirals. They fed on freshly opened apple blossoms, and later all the nettle plants in a neighbors’ sheep pasture had an abundance of their caterpillars.

Moth migrations are perhaps more spectacular than those of butterflies. Jason W. Chapman and colleagues report one recent ten-year study involving radar tracking of about one hundred thousand owlet (Noctuid) moths, primarily the silver Y moth, Autographa gamma, migrating south in the fall from northern Europe, and then north from the Mediterranean in the spring. Like the butterflies, these insects breed along their migration route. Also like the butterflies, the moths partially correct for crosswinds, to maintain specific directions. Most surprising perhaps is the moths’ windsurfing; they choose the most favorable wind currents corresponding to their respective spring or fall migratory directions. If the wind shifts about twenty degrees from the favorable direction, they adjust their flight to accommodate and maintain the correct direction. If the wind shifts ninety degrees, though, they stop and wait for a favorable wind. Millions of them fly together in the dark of night, and, like the monarchs’, their compass directions are likely tuned to the Earth’s magnetic fields. Some studies of radio-tagged green darner dragonflies, Anax junius, suggest that these insects also migrate hundreds to thousands of kilometers from north to south with those that return being a different generation.