Much dispersal is natal dispersal, i.e. dispersal by juveniles before they reproduce for the first time. In many taxa this is constitutional: we have already noted that seed dispersal in plants is, by its nature, natal dispersal. Likewise, many marine invertebrates have a sessile adult (reproductive) stage and rely on their larvae (obviously pre-reproductive) for dispersal. On the other hand, most insects have a sessile larval stage and rely on the reproductive adults for dispersal. Here, for iteroparous species, dispersal is most often something that occurs throughout the adult life, before and after the first breeding episode; but for semelparous species, dispersal once again is almost inevitably natal.
Birds and mammals, once they have fledged or been weaned and are independent of their mothers, also have the potential to disperse throughout the rest of their lives. None the less, most dispersal here, too, is natal (Wolff, 1997). Indeed, age-biases and sex-biases in dispersal, and the forces of inbreeding-avoidance, competition-avoidance and philopatry, are all tied intimately together in the patterns of dispersal observed in mammals. Thus, for example, in an experiment with gray-tailed voles, Microtus canicaudus, 87% of juvenile males and 34% of juvenile females dispersed within 4 weeks of initial capture at low densities, but only 16% and 12%, respectively, dispersed at high densities (Wolff et al., 1997). There was massive juvenile dispersal, which was particularly pronounced in the males; and the inverse density dependence, and especially the very high rates at low densities, argue in favor of inbreeding-avoidance as a major force shaping the pattern.
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