![]() There is limited but contradictory information on the association between sperm competition intensity and sperm energetics Tourmente et al. ![]() Studies in a range of taxa have demonstrated that species with high sperm competition intensity produce sperm that are longer (but see ), faster and show greater morphological uniformity. Morphological traits that confer enhanced motility and/or higher energetic capacity should therefore be favoured. To reach and fertilize the ovum, sperm must travel through the challenging environment of the female tract, often competing with sperm from other males. Sperm are one of the most morphologically diverse cell types in internal fertilizers, evolving in response to strong selection within the female reproductive tract. Contrary to the assumption that longer mid-pieces are larger and therefore produce or contain a greater amount of energy, our results indicate that the amount of mitochondrial material is consistent across mid-pieces of varying lengths, and longer mid-pieces are simply proportionately ‘thinner’. We used selective plane illumination microscopy to study mid-piece structure from three-dimensional images of zebra finch sperm and used high-resolution transmission electron microscopy to quantify mitochondrial density. Using a multi-dimensional approach to study mid-piece structure, we tested whether this unexpected relationship can be explained by a trade-off between mid-piece length and mid-piece thickness and/or cristae density inside the mitochondrial helix. However, in the zebra finch Taeniopygia guttata, a recent study showed a surprising negative relationship between mid-piece length and sperm energy content. ![]() Larger mid-pieces have therefore been assumed to provide greater energetic capacity. ![]() The sperm mid-piece has traditionally been considered to be the engine that powers sperm. ![]()
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