Navigation

Animal navigation involves a gamut of sensory acuities to various environmental signals. As a consequence, there are countless ways to avoid noncircadian clocks: birds, insects, and fish use celestial and magnetic compasses; honeybees use polarized light; wasps have memory for landmarks; salmon can find their breeding grounds by smell; and amphipod crustaceans use the slope of the ground (Daan, 1981; Dyer, 1998; Gould, 1998). In fact, the only evidence for an event timer in navigation is that some animals appear to know how far they have gone.

This is best exemplified by the waggle dance of honeybees. Remarkably, honeybees returning to the nest can inform other workers of the exact whereabouts of a forage site. The waggle dance transmits direction by establishing an angular relationship between the sun and the forage site in the form of a linear waggle movement at the same angle from the vertical axis of the hive. The distance to the site is transmitted by the distance of the linear waggle. The worker increases her waggle distance by about 75 msec per 100 m of foraging distance (Seeley, 1995). A decay timer would be useful in transmitting this signal, but a kind of event timer would be required for measuring the initial distance and for other bees receiving the signal at the hive. Time based on metabolic costs is probably far more prevalent than time based on the observation of events. The energy expenditure in flight could be used as the assay of flight duration. Bees may leave the hive or return to it with tuned energy stores so as to accomplish this. That another bee watches or follows the dance and then knows the distance to the forage site implies a more complicated mechanism.

The behavior of honeybees in the wild makes the FI data for honeybees particularly cumbersome (Grossman, 1973). It suggests that despite Grossman's effort to simulate a naturalistic environment in order to measure honeybee event timing, he may have been asking the question in the wrong way. There is no evidence that honeybees are confronted with anything that replenishes itself at 1-min intervals in the wild (Seeley, 1995). A honeybee may return to the hive to communicate the exact location of a new nectar source and still be unable to learn that a flower takes 20 sec to replenish its nectar stores. This entire behavior pattern appears to follow the logic of the specificity of decay timers. In general, if one expects to find event timers in insects, then one needs to accept the possibility that these event timers are very phylogenetically derived and highly specified.

The role of chemotaxis or navigation by thermal gradients is typically overlooked in the standard navigation literature. Gradient navigation requires either spatially or temporally separated samples of the environment. Which of these to choose depends critically on the size of the organism. For Escherichia coli, and similarly sized animals, the primary difficulty lies in the signal-to-noise ratio introduced by Brown-ian motion (Berg, 1983). Temporally spaced samples are of limited utility if animals are unable to find their way back to previous positions. It is for this reason that as size decreases, spatial mechanisms become more informative (Dusenberry, 1998). While there is support of spatially distributed processing at the opposite ends of E. coli (Grebe and Stock, 1998; Dusenberry, 1998), there is also evidence that they weight temporal experiences over time (Segall et al., 1983) (also see Section 4.4).

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