Figure 2. Homing experiments distinguish between an animal’s ability to orient from its ability to navigate, based upon how the animals behave when released in an unfamiliar area. The red circles are “home” (the capture site); the black hexagons are unfamiliar release sites. Arrows show what direction the released animal selects. An animal that can orient chooses the same direction, regardless of whether that path actually takes it toward home. An animal that navigates, however, first determines its location relative to home and then chooses a direction that will return it to home.
The offshore orientation of hatchlings, described above, took many experiments, done over several years, to fully understand and appreciate. How was it possible for most hatchlings to migrate successfully, given the variation in conditions they encounter in nature? For example, on some evenings hatchlings crawled from the nest to sea when the ocean was glassy calm. Yet, even in the absence of waves hatchlings still swam directly offshore in spite of other experiments showing that when waves were present, hatchlings used them as a guidepost.
The answer came from other experiments showing that the beach crawl could also be used to establish an offshore directional preference, transferred immediately in the absence of waves to the turtle’s magnetic compass. Similarly, if a female placed her nest too close to the surf zone, hatchlings experience an abbreviated crawl that was insufficient to establish an offshore directional preference. Under those conditions, swimming into oncoming waves served that purpose. Collectively, then, these experiments established that hatchlings possessed an amazingly effective set of flexible orientation mechanisms characterized by built-in redundant features. These virtually guarantee that under most circumstances, turtles successfully complete their offshore migration. It turns out that redundancy, the use of alternative mechanisms to orient migrations, is a general feature of these systems that applies to a wide variety of species, from birds to fishes and of course, to marine turtles!
But even an impressive system based exclusively on orientation pales in comparison with an ability to navigate. Navigation underlies some (but not all) migratory movements and is defined as an ability to reach a goal from any unfamiliar location. One can’t distinguish between orientation and navigation on the basis of how well animals maintain a direction; instead, homing experiments are done in which an animal is displaced to an unfamiliar location, released, and its ability to return observed (Figure 2). These tests show that orientation is a one-step process (a similar response to a guidepost) but navigation requires two steps. First, the animal must determine its location relative to a goal; this is known as the “map step”. Second, it uses that information to orient on an appropriate homeward path (the “compass step”; Figure 2). The guidepost(s) used for the second step are often identical to those used by animals only capable of orientation, but that’s where the similarities end. It’s the map sense that makes navigation such an interesting and more complicated process.
Think for a moment about what such a capacity means to a migrating animal! Maps only work if each location within the range of places an animal might frequent has a unique characteristic, one that defines and distinguishes it from any other location within that range. Humans create abstract maps of the entire world using a labelling system of orthogonal (fixed at a 90° to one another) bi-coordinates we call latitude and longitude. Animal maps must also consist of bi-coordinates though they do not necessarily have to be
Figure 3. Left: The earth’s magnetic field consists of lines of force (small arrows) that produce a symmetric array of gradually changing dip angles. These range between 0° (lines parallel to the earth’s surface) at the magnetic equator that gradually increase to 90° at the magnetic poles. An animal that can detect dip angle can use that information to determine its North-South (N-S), or latitudinal, location on the earth’s surface. Magnetic field strength also varies along a N-S axis. It is weakest at the magnetic equator and strongest at the magnetic poles. Right: Hatchling loggerheads orient in different directions (see circle diagrams) when exposed in the laboratory to the magnetic field present on the west, northeast, and south side of the circular current (the North Atlantic Gyre) that is their nursery. Since their orientation varies with N-S location, the turtles must be able to determine their latitude. That determination can be based upon magnetic field dip angle, magnetic field intensity, or both qualities. However, their ability to determine their longitudinal (E-W) position is not conclusively proven by these observations. (Modified from Lohmann Lab. Webpage, University of North Carolina, Chapel Hill, N.C.).