Compasses and Maps
Animals capable of navigating long distances to specific destinations, and of compensating for experimental displacements into unfamiliar territory, are thought to possess both a directional or compass sense for maintaining headings and a positional or "map" sense for determining location relative to a goal. Both of these abilities are crucial to navigation.
|Compasses: A compass provides a navigator with directional information. Among humans, the best-known compasses are magnetic; a compass needle aligns with the north-south magnetic axis and thus provides a reliable reference that can be used for maintaining a consistent heading. Many animals are known to possess compasses based on various environmental features. For example, some birds and fish are known to have a time-compensated sun compass. Additional animal compasses are based on stars, patterns of skylight polarization, and the Earth's magnetic field.||
Maps: A map provides a navigator with positional information -- in other words, it tells an individual something about where it is relative to a goal. Although compasses are vital for maintaining a consistent heading, a compass by itself is seldom sufficient to guide an animal to a particular destination. To understand why, consider a juvenile sea turtle that has been captured in the feeding grounds where it lives and released by researchers at an unknown location in the ocean. If the turtle wants to return home, it needs more than just a compass. It also needs to know where it is with respect to the goal (for example, whether it is north or southeast of the target in the diagram below). Only then can it use its compass to set an appropriate heading for home.
An animal with the ability to determine its position relative to a goal is said to have a map (or a map sense). It is important to understand that when researchers say that an animal has a "map", this does not necessarily mean that the animal has a detailed mental representation in its head, similar to a roadmap that humans might use. The information contained in an animal map may be minimal and very general; it might, for example, simple tell the animal that it is too far north or west. Thus, in the field of animal navigation, an animal with a map is one that can figure out the appropriate direction to travel to reach a goal, even when the animal is in unfamiliar territory and too far from the goal to see or sense it directly.
What is known about sea turtle compasses and maps?
Considerable progress has been made toward characterizing the compass cues that enable sea turtles in general, and hatchling turtles in particular, to maintain consistent headings through the sea. For example, hatchling and juvenile turtles are known to have a magnetic compass sense. Hatchlings can also maintain headings by swimming into ocean waves, and recent results suggest that juvenile turtles can maintain headings using celestial cues (possibly a sun compass or skylight polarization patterns). Thus, some information has been acquired on the compasses of sea turtles. Until recently, however, little was known about the basis of the sea turtle map sense.
In principle, sea turtles might determine where they are relative to a goal in several different ways. One interesting possibility, however, is that turtles might be able to derive helpful positional information from the Earth's magnetic field.
last edited 04/28/2004