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The role of vocalizations in spacing out and mate selection in Pacific tree frogs Whitney, Carl Linn


This thesis investigates the role of male vocalizations in two aspects of Pacific Tree Frog breeding behavior: Spacing out of males on breeding areas and selection of mates by females. Spacing out: An analysis of nearest neighbor distances of calling frogs showed that males space out more than if distributed randomly on the available calling sites. An addition experiment provided corroborative evidence that males tend not to call too close together; of equal numbers of frogs added to an empty (control) enclosure and an enclosure occupied by calling frogs, more subsequently called in the control. The species' "mating" call (D call), as well as attracting females, functions in spacing out; in an addition experiment, fewer frogs called in an enclosure occupied only by loudspeakers playing back D calls than in the control. Another vocalization (S call) seems to function only in spacing out. If two calling frogs come close together (less than ca. 50 cm), both usually begin making the S call. One frog may then move away; if not, physical combat may follow. I hypothesized that the S call is a stronger warning to nearby males than the D call. A playback experiment partially supported this hypothesis. I consider possible functions of spacing out, and suggest that males which maintain spacing may attract more females than they would otherwise, perhaps because females can more easily locate them. Mate selection: I attempted to test two hypotheses: First, females select large males on the basis of their low-pitched vocalizations (there is an inverse correlation between call pitch and body length). A comparison of body lengths and call pitches of males found in amplexus with a sample of males from the calling population did not support this hypothesis. Second, females choose males which initiate bouts of calling (chorus leaders). Both systematic field observations and a laboratory experiment support this hypothesis. Chorus leaders also end choruses, call at a faster rate during choruses, call more during periods not defined as choruses, and call louder than other frogs. In nature, females may also use these differences as cues in mate selection. The advantages of being a chorus leader should select for frogs calling all of the time, yet frogs show frequent lulls in calling. A playback experiment showed that frogs are less responsive (measured by latency to call) to stimulus calls shortly after having stopped calling than later in the lull period. I hypothesized that fatigue is responsible for this short-term waning of calling tendency. A respirometry experiment suggested (but quite inconclusively) that the fatigue hypothesis is feasible.

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