Ce

Normal

Stumps

Release Distance point to nest

Figure 2 Compass cues and odometry. Insects are known to use the sun and the pattern of polarized skylight as a compass cue. (a) An experiment by Santschi (modified after Schone, 1983), in which he followed an ant on a straight path (line with arrow heads) and periodically screened off the Sun on the left and reflected it back onto the animal with a mirror on the right during those parts of the ant's path that are marked with a gray patch. The ant always moves in such a way as to keep the Sun to its left; making a U-turn whenever the apparent position of the Sun has changed. This is an example of a class of directed orientation reactions called 'taxes' (Fraenkel and Gunn, 1961). (b) Many insects possess a specialized part of their eye, the dorsal rim, which carries photoreceptors that are particularly sensitive to the plane of polarization of light (left). Because the pattern of polarized skylight has an invariant structure with respect to the position of the Sun, animals that are sensitive to the direction of polarization of light can infer the position of the Sun from the direction of polarization of a few patches of blue sky: the sun must lie on the intersection of great circles perpendicular to the direction of polarization of these patches. (c) Bees integrate optic flow to measure the distance they have to fly to a food source. Bees trained to find food about halfway into a small tunnel carrying a pattern of vertical stripes at its walls concentrate their search at a location where they had found the food source before. If the pattern in the tunnel is changed to horizontal stripes (which do not produce optic flow), the bees are unable to pinpoint a location in the tunnel and search throughout the tunnel. (d) Desert ants (Cataglyphis) determine the distance they have walked by counting and measuring the size of their steps: when they walk on stilts on their home journey, they overestimate, and when they walk on shortened legs, they underestimate home distance. (b) Modified from Wehner R (1982) Himmelsnavigation bei Insekten. Neurophysiologie und Verhalten. Neujahrsblatt der Naturforschenden Gesellschaft in Zurich 184: 1-132, with permission. (c) Modified from Srinivasan MV, Zhang SW, Lehrer M, and Collett TS (1996) Honeybee navigation en route to the goal: Visual flight control and odometry. Journal of Experimental Biology 199: 237-244. (d) Modified from Wittlinger M, Wehner R, and Wolf H (2006) The ant odometer: Stepping on stilts and stumps. Science 312: 1965-1967.

Navigational mechanisms used by animals for pinpointing goals

Path integration

Record of 'distances' and 'directions' traveled

Landmark guidance

Record of 'distances' and 'directions' from objects

Proprioceptive

Proprioceptive

information

information

(counting steps)

(vestibular, leg receptors)

External distance

External compass

information

information

(integrating optic flow)

(vision, magnetic compass)

Test: Displace animal

Goal landmarks Route landmarks (distance, identity) (distance, identity) Visual memories & image matching

Distance Direction

Angular size, Retinal position motion parallax relative to compass

Test:

Displace landmarks

Constraints: spatial layout, spatial resolution

Constraints: spatial layout, spatial resolution

Choice frequency

100%

Choice frequency

100%

Figure 3 Path integration and landmark guidance. (a) The two navigational mechanisms, the information required and examples of the cues, animals are known to employ. (b) To test whether an animal employs path integration the animal is passively displaced before it heads home (dashed arrow). If the animal can compensate for the displacement (thin black arrow) it must have external (geocentric) information on its release site or on the nest location. An animal that relies exclusively on path integration information, like desert ants on featureless saltpans, runs into the direction and for the appropriate distance after displacement (open arrow) in which the nest would be, had it not been displaced (gray arrow). It then executes systematic search movements for the nest (see Figure 8). (c) The demonstration that animals use the appearance of landmarks in order to pinpoint a location, like their nest, involves displacing the landmarks in the animal's absence and asking where it would search for the nest on its return. In many ground-nesting insects, the nest location is defined by the surrounding landmark panorama, as this famous experiment by Tinbergen clearly demonstrates. The sand wasp Philanthus searches for her nest not at its true location (marked by a star), but in the center of a ring of three-dimensional landmarks (black) rather than a ring of flat disks (gray) that had surrounded the nest during training. (b) Modified from Wehner R (1982) Himmelsnavigation bei Insekten. Neurophysiologie und Verhalten. Neujahrsblatt der Naturforschenden Gesellschaft in Zurich 184:1-132, with permission. (c) Modified from Collett TS and Zeil J (1997) Selection and use of landmarks by insects. In: Lehrer M (ed.) Orientation and Communication in Arthropods, pp. 41-65. Basel: Birkhauser Verlag, with permission.

navigational mechanism of path integration can therefore be identified by shifting an animal just before it heads home. If path integration is the only information an animal has about the location of its goal, like its nest, then after displacement it will move in the direction and the distance where it would find the goal, had it not been displaced (Figure 3b). Path integration does not allow an animal to compensate for such passive displacements. Many details of the neural computations involved in this process are still unknown, but one aspect of path integration has been well established: the process accumulates errors, because directions and distances cannot be measured with infinite accuracy, so that the information about the home position becomes less and less trustworthy, the longer the journey takes. There seem to be two solutions to this very basic problem. First, animals use visual, olfactory, or even magnetic landmarks, which is the most robust way of pinpointing a goal. Landmarks in this context are any sensory cues that uniquely specify a location in space. Second, animals apply efficient search strategies to help them find the goal at the end of the home vector.

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