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UBC Theses and Dissertations

The effects of sperm competition on testes size and intromittent organ morphology in waterfowl Coker, Chris R.


Waterfowl are one of very few avian taxa that possesses an intromittent organ (10). This thesis examines the adaptive significance of the 10 in waterfowl by determining the relationships between 10 morphology and the intensity of sperm competition (as reflected by frequency of extra-pair copulations (EPCs)). Intromittent organ morphological characteristics, including length and circumference (adjusted for body size), number of ridges and/or knobs (per unit area), ridge/knob height, ridge/knob length, and % area covered by ridges/knobs were measured from scaled museum drawings of freshly killed, sexually mature, specimens of 57 waterfowl species (across 33 genera). Thirty of which were ranked by frequency of EPC (1= monogamous, 2= rare EPC, 3= frequent EPC, 4= promiscuous). Testes sizes were also investigated in relation to EPCs, where testes masses (adjusted for body size) from 47 species (across 24 genera) were obtained (32 species with mating strategies). The size of the testes, the size (length) of the 10, the size (height) of the 10 ridges/knobs and the % area covered by ridges/knobs increased significantly with the frequency of EPC. These relationships exist even after the removal of phylogenetic constraints. These results are consistent with the hypothesis that waterfowl 10s are involved in sperm competition. Further research into the actual mechanism by which the 10 is involved with sperm competition would be worthwile.

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