Evolution of facultative sperm allocation and mating behavior in a group -breeding salamander, Ambystoma texanum
Abstract
Allocation of time and energy to reproduction influences the physiology, morphology and behavior of both sexes. In this research, I investigated the causes and consequences of patterns of sperm allocation in males. Historically, research has focused on differential energetic investment on reproduction between females and males. Reproductive output for females is relatively large per offspring compared to males. Thus, females are predicted to maximize mate quality, while males are predicted to maximize mate quantity. Recent research has revealed that reproductive resources for gamete production can be limited for males as well as for females in many species. Critical to patterns of sperm investment by males is the degree to which females mate with multiple males. Such multiple mating sets up intense competition among sperm of different males to fertilize the same ova. One consequence of such sperm competition is the production of large ejaculates. I modeled spermatophore production in the salamander, Ambystoma texanum using an economic approach. This species is highly suitable for such a study because within any breeding aggregation, each male may deposit 60+ spermatophores each containing many thousand sperm, and each female may induct 20+ spermatophores into their reproductive tract prior to egg deposition. Patterns of male sperm allocation and mating behavior in this species show that sperm production is constrained by male body size but is not influenced by sperm competition intensity. However, males behaviorally adjust spermatophore deposition behavior in group-breeding aggregations to increase the chance their spermatophores are found by females by covering spermatophores of other males with their own spermatophores. Finally, I reviewed the literature to synthesize a general hypothesis for the evolution of sperm allocation, for which both positive and negative evidence exists, and to assess evidence for sperm limitation costs to males. I propose that selection for sperm allocation should be driven by the amount of variation in two factors: sperm competition risk and the latency to mate. While sperm competition risk has been explored both theoretically and empirically and a growing literature supporting it exists, no formal analysis of the effect of mating latency of males on sperm allocation has been made. I tested the mating latency portion of my model using data from the literature and found some support for my predictions. These results may explain why some males exhibit little variation in ejaculates, even though they are subject to strong sperm competition.
Degree
Ph.D.
Advisors
Howard, Purdue University.
Subject Area
Ecology|Zoology
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