Prior to the invention of GPS, my family’s vacations were always bound to include a few hours of driving aimlessly through a new city while trying to decipher a map. If it was supposed to take an hour to get from the airport to the hotel, we could be sure it would take at least two – and more if there were detours or construction on the way. Now that handheld GPS is common, things have changed. Instead of staring at maps, vacations include a few hours of acquiring satellites, and listening to a pleasant, if slightly monotone, British woman encourage us to ‘turn left here’.
Animals don’t have GPS . They just never bothered to figure out how to put satellites in orbit. But they still need to able to navigate their environments: whether to migrate across the globe like the Arctic tern, plumb the depths of the ocean like Pacific salmon, or navigate pitch-black subterranean tunnels like the blind mole rat. These are significant challenges, but animals have evolved a creative array of navigational tools to insure they get lost far less frequently than we do.
Let’s start in the skies.
Arctic terns hate winter. Why else would they try so hard to avoid it? The medium-sized seabirds travel 70,900 km every year just to make sure they enjoy two summers. They’re the avian equivalent of Canadians who keep a winter home in Arizona. Arctic terns breed in the Arctic during the northern summer, before winging their way across the globe to spend the southern summer foraging in the Southern Ocean, just north of Antarctica. Their 70,900 km annual round-trip makes them the champion long distance migrants of the animal kingdom.
But the first migration for an Arctic tern is a stressful time. Over the course of years, migratory birds acquire knowledge about their migration route – the locations of good stopping points, reliable food supplies, and coastal landmarks to aid navigation. But a rookie tern doesn’t have this knowledge base, so they have to fallback on their biology. First-time migrants rely on a “clock-and-compass” method of navigation.
The mechanism is simple, in theory. Migratory birds have a genetically engrained sense of direction. They ‘know’ that they must migrate south, and generally know what direction south is. That’s the basic compass part. The clock part comes from using the sun as a cue to inclination of their southward journey. By tracking the position of the sun, they can measure the angle of their trip to make sure they are heading towards the correct goal. This “clock-and-compass” method, combined with sticking to the tail-feathers of smarter birds, allows a first-time migrant to successfully complete his or her journey. Along the way, they learn the route so that when migrating the second time they can rely on geographic knowledge in concert with the angle of the sun. On average, an Arctic tern will travel nearly 1.5 million kilometres over its lifetime.
Navigation in the skies isn’t easy, but you’re still aided by a number of features: landmarks and coastlines, as well as the sun and stars. But underwater these features disappear. The light penetrating through the surface is dim and inconsistent, and the ocean floor is too deep to use as a map. Instead you float in a dimensionless blue bubble. Divers know the disorienting feeling of being in open ocean, unable to see the surface or the bottom. You rapidly become disoriented – unless you’ve evolved for it. Instead of relying on sight, like terrestrial animals might do, oceanic long-distance travellers need to be sensitive to other sources of information. One of these, strangely enough, is smell.
The Pacific salmon, genus Onchorhynchus, is a group of 15 species of salmon and trout known for their ugly, hooked visages and the delicious way they taste when grilled on a cedar plank. But the true salmon species, eight of them, are also known for another remarkable feat – navigating the open ocean by scent. Pacific salmon are anadromous: they spend most of their lives in saltwater, but return to freshwater to breed. Young salmon are born in quiet pools of shallow inland rivers on the Pacific coast. After birth, they travel down the river and enter the ocean, where they spend the majority of their lives feeding on small fish and trying not to be eaten by everything larger than them. When they reach sexual maturity, 1-5 years later, they return to the fresh-water stream to breed and die.
But they don’t return to just any stream – all salmon return to the exact stream they were born in. This type of navigation requires more complexity than the “clock-and-compass” method of first-time bird migrations. The salmon need a map too, and they create one out of smell.
Where saltwater and freshwater meet at estuaries and deltas the water separates into layers based on its physical and chemical properties. Saltwater is denser than freshwater and settles onto the bottom. On top of this salty base, layers of freshwater arrange themselves according to the weight of particulate matter dissolved in them: minerals and ions and organic detritus. This layering is unique to every stream, depending on the composition of the inland river. When young salmon leave the river for the first time, they travel through each of these layers and imprint on them, forever remembering the particular smell and taste of their home river. Years later, when returning to breed, they track this smell.
To do this, they use “map and compass” reasoning. The compass tells them direction, and the map gives them a simple frame of reference. For example, a Pacific salmon feeding off the coast of California, but breeding in British Columbia might follow the rule: “travel east until I reach the coast, then travel along the coast. If water is getting warmer, I’m going the wrong way.” Then as it travels up the coast, into colder water, it detects the olfactory clues emerging from each river and stream until it finds the one that smells like home.
The salmon uses fairly simple sensory mechanisms to complete its complex migration – detecting differences in pressure and temperature to determine direction, and using its sense of smell to find its natal river. But what if you live in an environment where even these methods are unavailable to you? That’s the challenge faced by the blind mole rat.
Blind mole rats (not to be confused with their uglier naked cousins) are solitary living, subterranean rodents that have lost all use of their eyes. They spend virtually their entire lives underground, emerging only briefly as juveniles to leave their mother’s territory and establish their own – a brief, and probably terrifying, period of time. Then it’s back down into the darkness. Navigating underground has its own set of challenges. No sight means no landmarks. The temperature is a fairly consistent “chilly and damp”, and the smell is mostly of dirt. Luckily for the blind mole rat, they’ve evolved their own way of navigating. They use the Earth’s magnetic field.
Physics time. The Earth generates a magnetic field that extends from the core into outer space, where it interacts with the solar wind emanating from the sun. Imagine placing a bar magnet over the Earth, from pole-to-pole. The actually field is a little off-centre, but that’s the gist of it. The field plays the important role of deflecting charge particles from the sun (keeping our ozone layer intact, until we destroy it from below), and it also makes compasses useful. At random intervals every few thousand years, the field reverses (north becomes south), which probably makes mapmakers angry, and almost certainly screws with mole rats.
Aside from this occasional change, the magnetic field is stable, which makes it a useful way to navigate. In the same way that honeybees judge direction by calculating the angle of deviation from the sun, mole rats navigate underground by judging deviation from the N-S line of the magnetic field. In an ingenious, but kind of mean, experiment, Tali Kimchi and her colleagues put mole rats through mazes of varying complexity with both normal and altered magnetic fields. The goal for the mole rat was to find its way home.
They found that over short distances, altering the magnetic field had little effect on navigation – mole rats could trial-and-error their way home. But as the mole rats were moved further and further away from home, they relied more on magnetic fields. In the trials were the magnetic field was altered, the mole rats didn’t get home – they went searching in completely the wrong direction.
Navigating by magnetic fields is used in birds and sea turtles too, but usually as a fallback when another mechanism fails, generally if it’s too overcast for using the sun or stars. The subterranean mole rats are one of the only species that relies almost completely on magnetism for their sense of direction.
So have patience, next time your GPS fails in a confusing city. Be glad you’re not a salmon: using your sense of smell to navigate New York would be an unpleasant experience.
Burda H, Marhold S, Westenberger T, Wiltschko R and W Wiltschko. 1990. Magnetic compass orientation in the subterranean rodent Cryptomys hottentotus (Bathyergidae). Experientia 46: 528-530.
Kimchi T, AS Etienne, and J Terkel. 2004. A subterranean mammal uses the magnetic compass for path integration. PNAS 101: 1105-1109. Available http://www.pnas.org/content/101/4/1105.full.pdf
Lohmann KJ, Lohmann CMF, and CS Endres. 2008. The sensory ecology of ocean navigation. Journal of Experimental Biology 211: 1719-1728.
Quinn TP. 2005. The Behavior and Ecology of Pacific Salmon and Trout. Seattle: University of Washington Press.
Quinn TP, Volk EC and AP Hendry. 1999. Natural otolith microstructure patterns reveal precise homing to natal incubation sites by sockeye salmon (Onchorhynchus nerka). Canadian Journal of Zoology 77: 766-775.
Wiltschko R and W Wiltschko. 2003. Avian navigation: from historical to modern concepts. Animal Behaviour 65: 257-272.