Graduate Thesis Or Dissertation
 

The behavioral ecology of larval and neotonic northwestern salamanders (Ambystoma gracile)

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  • In the summers of 1973, 1974 and 1975, I investigated population age structure and density, and behaviors relating to habitat selection, predator avoidance and intraspecific interaction in a population of northwestern salamanders in a lake in the Oregon Cascade Mountains. The study was carried out by skin- and scuba-diving at night, observing or netting the salamanders directly. Population density and age structure remained stationary from 1973 to mid-1975, when density fell and the age structure shifted toward domination by older age classes. This change in the population was apparently due to the stocking of more fish into the lake. The salamanders had definite depth preferences, with slight but significant differences in preference between age classes. No salamander was found in water deeper than 1.3 m. The salamanders utilized bare mud substrate much less than would be expected on the basis of frequency of occurrence of the substrate, and utilized substrate covered with twigs and wood chips more often than would be expected. When escaping from a simulated predator attack the salamanders avoided bare mud completely. Those animals occupying areas covered with thick grass (Isoetes) tended to move to areas with much sparser grass clumps, when escaping. Preferences based on temperature, oxygen tension or the presence of large pieces of cover such as logs were not evident. Frequencies of predator avoidance behaviors were different in the different age classes. In the case of one-year olds, behavioral frequencies changed after the stocking of new fish. Before the stock ing, the one -year olds tended to avoid orienting in a direction perpendicular to the shore/deep water axis (laterally). If they did orient in this direction, they turned away from it when escaping a simulated predator attack. After the stocking, orientation and direction of escape were essentially random. At the end of the escape movement those one-year olds oriented towards shore and/or in deeper water (>70 cm) burrowed into the substrate; those oriented towards deep water and/or in shallow water (<70 cm) remained on top of the substrate. Two-year olds were essentially random in orientation and direction of escape. At the end of the escape movement those individuals in deeper water burrowed into the substrate; those in shallow water remained on top of the substrate. Three-year old and older salamanders demonstrated quite complex predator avoidance behavior. Animals in this age group tended to avoid orienting laterally, turning towards deep water much more often than would be expected on a random basis. Those animals in deeper water oriented toward deep water; those in shallow water oriented toward shore. These individuals moved straight ahead when escaping a simulated predator. Laterally-oriented animals avoided escaping straight ahead: those in deep water turned towards deep water, while those in shallow water turned towards shore. Most (75%) of the three-year olds burrowed into the substrate at the end of the escape movement. The salamander population was aggregated. This aggregation may have involved some components of habitat selection, but was not produced by a thinning-out of animals as depth increased. Analysis of individual and nearest neighbor ages revealed that young (small) salamanders avoided associating with older (larger) animals. This avoidance behavior began to disappear in the middle of the second year of life, as the one-year olds approached the two-year olds in size. The observed behaviors appeared to be chiefly responses to predation by introduced trout. They apparently constitute the principal mechanism permitting the salamander population to persist under a regime of predation. The behaviors are probably learned; the variation in behavior between age classes was probably due to variation in predation pressure produced by fluctuations in stocking success of the fish.
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