Evaluating the phenotype-linked fertility hypothesis: Do exaggerated male phenotypes advertise potential fecundity benefits to females?
When choosing mates, females may evaluate whether males can provide them with indirect (i.e., genetic) or direct (i.e., resources, parental care, high quality ejaculates) benefits. The phenotype-linked fertility hypothesis (PLFH) predicts that ejaculate characteristics correlate with male phenotype, and that females select for exaggerated phenotypes to guarantee high fertilization success. Research is hampered by incomplete testing of the PLFH, as well as by the limited techniques available to quantify ejaculate quality. Support for the PLFH should require testing three prerequisites: 1) female preference for the male phenotype, 2) male phenotype correlates positively with at least one ejaculate characteristic, and 3) females should obtain fecundity benefits by mating with males with more exaggerated phenotypes. I review the literature, investigate the strength of the evidence for the PLFH, and make several recommendations for future work, including a) testing the PLFH in species in which females are most likely to gain fecundity benefits, b) considering the static and dynamic nature of male phenotypic signals, and c) transitioning from correlational to experimental testing. I then test the prerequisites associated with the PLFH using gray tree frogs (Hyla versicolor). Female gray tree frogs prefer males with longer duration calls and faster call rates. I found no evidence that sperm number, sperm viability, or fertilization success correlated with male call duration or rate. I did, however, find that males became significantly sperm depleted after a single mating, and that fertilization success decreased as the breeding season progressed. Future studies should consider the fitness trade-offs that males and females make when breeding late in the season. Lastly, I describe new techniques to estimate amphibian sperm number using the spotted salamander, Ambystoma maculatum. I found that both RT-qPCR and spectrophotometry could be used to estimate sperm number, providing a viable alternative to counting sperm directly. Collectively, my research contributes to the scientific community by providing useful techniques, testing the PLFH in an appropriate species, and considering how future PLFH studies could be improved.
Lucas, Purdue University.
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