One may wonder if there is any homing at all—whether the return of one in four thousand to the natal beach is just random chance. Arguing against random chance is the fact that no loggerheads are known to make it to any other beaches, and also that predation is horrific. Most eggs are dug up and eaten by predators. The five-centimeter-long hatchlings must run a gauntlet of vultures and other predators before they make it into the water, where fish predation begins and continues even until they attain large size, when sharks eat them. The turtles now also face human predation, nest raiding, chemical pollution, and disorientation by light pollution from human beach lights. The studies by the Lohmanns, summarized in over thirty technical publications, indicate that loggerhead turtle (as well as salmon and spiny lobster) magnetic sense is especially highly developed, but that they use some half-dozen or more other types of information in homing.
The ocean and the coastlines have, like the land, regionally unique magnetic characteristics. For example, on a large scale the magnetic lines of force have both specific direction and intensity; those at the poles have strong intensity and strong inclination (dipping down in the north, horizontal at the equator, and south angling up), while those at the equator are weak and have little inclination. The superimposed local magnetic anomalies, if sensed, could identify locations. However, turtle hatchlings that have never been in the sea, when placed into magnetic fields characteristic of particular locations in the North Atlantic, still swim in their tank in the appropriate migratory direction.
The Lohmanns speculate that the specific instructions of the turtles’ route are sensed from the magnetic landscape, and that the turtles are genetically programmed to respond to these magnetic cues correctly. The magnetic location of their home, however, is imprinted (learned), and so they know when they are close to home, and then they may be guided by scent, waves, and other cues to the beach of their birth.
Despite our ever-finer understanding of the micro, we still can’t explain all the macro. The satellite-tracking studies of green sea turtles, Chelonia mydas, by Paolo Luschi, Floriano Papi, and colleagues at the University of Pisa in Italy confirm these animals’ abilities to return to pinpoint targets on a straight bearing, presumably over areas where the magnetic information is never a constant. The turtles compensate for wind and current directions in their bearings. Their position fixing can so far not be explained by known navigational mechanisms.
Finally, I now return to the albatross, the bird that began my wondering if and how they know where they are. They are, like sea turtles, long-lived wanderers with fixed home positions within a to-us featureless sea. Suzanne Akesson from Sweden and Henri Weimerkirsch from France have shown, through long-term satellite tracking, that young wandering albatrosses average a flying distance of 184,000 kilometers their first year, have an apparently genetically fixed dispersal direction, and, after reaching a particular ocean zone, may stay there for seven to ten years before returning to their home site to breed. At least in the waved albatross, Phoebastria irrorata, homing is unimpeded by magnetic manipulation. Albatrosses with magnets attached to their heads continued to fly back and forth, like the control animals, the thousand kilometers separating their breeding and feeding sites. So their homing is apparently little or not at all related to magnetic orientation! We have learned much, but it has left us with the mysterious, magical, and miraculous. And the ultimate and perhaps unanswerable question, of whether any animals are conscious of where they are and what they are doing, remains.
Smelling Their Way Home
I miss your fragrance, sometimes I miss it this much that I can clearly smell you in the air.
—Qaisar Iqbal Janjua
THE EIGHTEENTH-CENTURY INSECT BIOLOGIST JEAN-HENRI Fabre raised caterpillars of the great peacock moth, Saturnia pyri, and placed a female immediately after she emerged in the morning from her cocoon “while still damp” in a wire-gauze-covered bell jar. He had no particular plans for her but “incarcerate[d] her from mere habit, the habit of the observer always on the look-out for what may happen.” What happened at nine that evening was a great stir in his household due to what his family at first assumed was bats but was in fact “an invasion of moths” that had entered a study window. They were all great peacock males.
The incarcerated female lived for eight days, but more suitors (each of which was denied physical contact with her and captured) came every night, and each had a life span of only two to three days. In aggregate, 150 males came into the house. Fabre knew that these moths were rare and wondered how the female achieved her fabulous allure. In a series of clever experiments that should be the envy of a Nobel laureate, he disproved three hypotheses of how the male moths might have been attracted and proved that the allure was a species-specific scent (or an “effluvia”), one that is totally imperceptible to us. He later experimented with another giant silk moth, the lesser peacock, Attacus (now Saturnia) pavonia, which emerges more than a month earlier in the season than the great peacock moth, the males of which are attracted to females only near noontime instead of at night. He summed it all up in an exciting narrative that any bright eighth-grader could understand and ended up wondering about the difference in behavior between the two moth species, saying, “Let him who can explain this strange contrast of habits.” At that time, nobody could.
The attractant scents, which we now call “sex pheromones,” of female moths can attract males of the corresponding species from a distance measured in kilometers and can be routinely demonstrated. A freshly emerged female Callosamia promethea moth that I tied to a twig had by late afternoon dozens of males flying around her. Sex pheromones can also be used as a tool to kill moths. The bolas spider, Mastophora hutchinsoni, synthesizes a chemical that mimics the female sex pheromone of the cutworm moth, Lacinipolia renigera, and broadcasts that chemical in the evening when the moth males fly, thus luring them in to catch and eat. After midnight the same spider switches over to dispense the pheromone of another moth species, Tetanolita mynesalis, that is active only late at night. The bolas spider doesn’t build a web but instead dangles a short piece of silk that has a gob of sticky glue at the end from one leg. When a moth comes near, the spider swings its “bola” at the intended victim to entangle and eat it. (We also employ synthetically produced moth sex pheromones to capture moths, but the lazy way—in conjunction with sticky tape rather than relying on tool use and manual dexterity.)
For many animals, scent is the primary window to the world, the cue guiding them to a potential mate, and to food. In fact, odor is an excellent means of “homing in” on a currently emitting source of a volatile chemical. But we don’t generally associate the sense of smell with orienting to a home. The scents of home tend to be various and ambiguous, though, and might mislead more than lead.
Scent concerns the detecting and use of a huge number of different chemicals in an almost infinite variety of combinations, like a vocabulary of signs and signals with a large variety of meaning. The smell of ethanethiol, for example, is to us at very low concentrations (according to the 2000 edition of Guinness World Records) the world’s “smelliest” repellent chemical. Yet, to turkey vultures, ethanethiol is apparently hugely attractive: they use it to home in on their favorite food, rotting carcasses. However, they are sometimes misled by leaking propane lines to which we have added ethanethiol in order to detect this to-us otherwise odorless gas. Isoamyl acetate, on the other hand, which to us has a pleasant banana-like smell, is what honeybees release when they are agitated enough to sting. It excites hive mates to attack, because if one bee is induced to sting, the bees’ home is likely being attacked, and a mass defense is mounted. In the case of the honeybees, that sting results in their sacrificing their lives. Flower scents of various kinds cause insects to search for food. In this case, learning is often involved, as insects associate specific scents with specific flowers.
Nowhere is signaling by scent more important than in coordinating the societies of social insects. In honeybees, for exa
mple, the lack of one specific scent causes the workers to build wax cells to hold queen larvae, another scent emitted by a queen in flight attracts drones to mate, and still another of her scents depresses the ovary development of her workers so that she ends up as sole egg layer of the hive.
Scents are like the words of a private vocabulary; the babble of them in all the millions of species is largely “inaudible” to us, and hence their roles are often mysterious. Perhaps the most famous role for scent in homing occurs with ants that can, when they walk rather than fly, lay down scent, their “trail pheromone,” with which successful foragers can establish a “follow me” communal trail that recruits nest mates to the food, and that can also guide them back.
Sahara Desert ants not only use the sun and landmarks in homing, but after coming close to their home, they orient by the local aromas that they have learned. The authors of a recent study scented desert ant nest entrances with different aromas and allowed the colony members to learn them, and then they moved some of the individuals to a new location and provided sites with the same scents as their parent colony. The ants then searched for their nest entrances at the scented locations, but only if both of their antennae were intact. Apparently, as in vision, they experience a scent gradient, providing them the ability to home in on scent location.
Honeybees also use scent in homing. When a honeybee swarm flies (guided by its scouts) to a new home site in the “cloud” of about ten thousand, most of the members have not before been to the new home where they are going, never mind where its dime-size “door” or entrance might be. But scouts perch at the tree-hole (home) entrance, lift the tips of their abdomens high into the air, open glands that then release a come-hither scent (which to us smells lemony), and beat their wings to broadcast an odor plume that guides the others to them. There may not be much of a breeze to make a scent trail for animals to follow, but the bees that plant themselves at the new home entrance create one by fanning their wings, and they may also align themselves into chains, which creates a directional airflow. Bees follow the scent trail and are led home, and until the bees have thoroughly learned where home is and can navigate to it, there will still usually be some that mark the home entrance with that lemony scent.
The “tubenose” sea birds of the order Procellariiformes (petrels, shearwaters, and albatrosses) use scent in finding food, and petrels additionally use scent to home in on their nests. Petrels are small birds that have probably experienced high selective pressure to avoid nest predation by gulls, and they have reduced predation by going underground. The male digs a burrow up to two meters in length, and the one egg of the clutch is laid at the end of the tunnel. Apparently as an adaptation to avoid gull predation, the birds arrive and depart from their burrows only at night, and both scent and sound are used in homing. Similarly, some experiments have suggested that pigeons may also use scent to aid them in homing in on their loft.
A potential problem for using scent as a marker of home concerns the reliability of signaling. Home can be normally associated with innumerable chemicals that can change on a minute-by-minute or a seasonal basis. How can scent “label” home so that we can reliably return to it by that signal? Which chemical or combination of them creates the smell we perceive to be the relevant one? And how can it end up becoming the label by a strong association in the mind to that place?
Imprinting (rapid irreversible learning) to a specific scent is important to many mammals. Many newborn mammals imprint on the unique chemical signature of their mother, and vice versa. Arguably this is attraction to a place, perhaps even a very specific one, such as mother, or even a teat. This phenomenon is especially important in herd animals, where the young are not restricted to a specific home, and the parents and offspring need to find and recognize each other within hours after birth not by a home or nest where they are located, but as individuals among a crowd of others. Scent also coordinates mother-young interactions during nursing in pigs, sheep, and rabbits, and humans as well. Human infants imprint on the scent of their mother’s breasts. But this scent is not a specific pheromone; in one experiment mothers slathered their breasts with a chamomile concoction while nursing their seven-month-old babies, and fourteen months later when the infants were given a choice to drink out of a chamomile-scented bottle versus one without the scent, they always chose the scented bottle. Those infants who didn’t have the early contact with chamomile scent showed disgust toward it.
In general the main problem with scent for orientation and homing is that, while it is reliable up close, it is much less reliable from a distance. Still, scent can induce yearning, maybe nostalgia or a longing to be near and hence an imprinting to place.
Some mammals, including mice and bears, whose powers of detecting certain odors exceed ours probably by a thousand times, potentially also return home with the aid of scent. People have deliberately displaced both mice and bears (if they were “problem bears” or perhaps “problem mice”) from picnic areas and pantries, to try to discourage them from raiding our food. Eastern deer mice have been reported to return to the same stump where they were caught from at least two kilometers and would presumably return from that distance to a human home with a pantry stocked with Camembert cheese as well. Most animals can’t, like some bees, create their own scent-dispensing air currents. They must wait for a wind, and it must be from the right direction.
Relative to most animals, we seem to have a poor competency for scents, yet that is not certain because we may respond to some scents we are not consciously aware of. Experiments of men smelling sweaty T-shirts suggest that they find those worn by ovulating women more attractive than those of women who are not. That is, the scent may invoke feelings of attraction as well as repulsion. Although most of our feelings of nostalgia that are associated with home depend on conscious memory of what we saw, not smelled, scent is often a powerfully emotional attractant, although it is one not consciously remembered before being encountered. I recall returning one time to and walking up York Hill, my retreat in Maine, and, while coming to the curve of the path through the sugar maples near the top of it, smelling a subtle scent that suddenly altered my mood and made me feel good. The leaves had just fallen from the maples and the birches, and the scent may have come from the senescing or decaying leaves or from the mushrooms growing there. I don’t know what it was, but it reminded me of the woods in Germany where I had lived and been happy as a young child. But being prompted by scent to remember and be attracted to a place is not a unique experience. A vivid description of this was written by Arthur D. Hasler, from the University of Wisconsin in Madison, for whom the experience opened up a lifetime career that led to solving the until-then-enduring biological riddle of how salmon return to the stream of their birth, after years at sea. He writes:
We had driven across the sage country and high desert from Madison, Wisconsin, where I had recently joined the faculty of the Zoology Department, to my parental home in Provo, Utah. Philosophically, this is about as far away from salmon country as possible. As I hiked along a mountain trail in the Wasatch Range of the Rocky Mountains where I grew up, my reflections about the migratory behavior of salmon were soon interrupted by wonderful scents that I had not smelled since I was a boy. Climbing up toward the Alpine zone on the eastern slope of Mt. Timpanogos, I had approached a waterfall which was completely obstructed from view by a cliff; yet, when a cool breeze bearing the fragrance of mosses and columbine swept around the rocky abutment, the details of this waterfall and its setting on the face of the mountain suddenly leapt into my mind’s eye. In fact, so impressive was this odor that it evoked a flood of memories of boyhood chums and deeds long since vanished from conscious memory. The association was so strong that I immediately applied it to the problem of salmon homing.
That salmon swim up streams from the ocean to spawn in fresh water was known for centuries. But it was not known that they were returning to where they were born, or that some of these spawning migrations may be two to four t
housand kilometers long. Given such incredibly long journeys and the commitment to make them—which often cost the fish their lives—ending at just any river or stream is not a good bet. There are adaptive reasons for the fish to come back home, to the place where they were born. The main one is the fact that the fish’s predecessors had found there a proven good place for their offspring to be born and grow. There was, and hence is likely to be, gravel for the female to scrape a “redd” or nest where the water current brings sufficient oxygen, where the temperature is right, where there is food for the hatchlings as they grow, where the risk of predation is not overwhelming, and last where there are no insurmountable obstructions leading to these required stream conditions.
As mentioned, scent nostalgia (or memory) can work as an orienting cue for finding home only if the directions of the wind and water are reliable, and the direction of water flow in a stream is reliable. Fresh water has a different chemical content from the salt water of the ocean, and Hasler formulated the hypothesis that the young salmon can return to their ancestral home in each stream only if each one is flavored by a particular bouquet of fragrances to which the young salmon become imprinted before emigrating to the ocean. Years later when as adults they return from the sea, they could use that same scent as a cue for identifying and then homing to their natal stream.
Hasler’s homing epiphany was first published in 1951 with his PhD student Warren J. Wisby. Their publication would later be followed up by more research, and it translated into a stunning record of science that combined laboratory and field experiments, ranged from chemistry to ecology, and concerned the natural history biology of salmon all over the world.
The Homing Instinct Page 10