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A hemipenis (plural hemipenes) is one of a pair of intromittent organs of male squamates (snakes, lizards and worm lizards). Hemipenes are usually held inverted, within the body, and are everted for reproduction via erectile tissue, much like that in the human penis. Only one is used at a time, and evidence indicates males alternate use between copulations as a behavioral means of increasing sperm transfer when the interval between copulations is relatively brief.
The hemipenis itself has a variety of shapes, depending on species. Often, the hemipenis bears spines or hooks, in order to anchor the male within the female. Some species even have forked hemipenes (each hemipenis has two tips). Sexual conflict over copulation duration may have shaped the evolution of hemipenis morphology, favoring more elaborate organs in species in which a long duration of copulation is especially beneficial to males, despite the associated costs to females. Due to being everted and inverted, hemipenes do not have a completely enclosed channel for the conduction of sperm, but rather a seminal groove which seals as the erectile tissue expands.
The word hemipenis (plural "hemipenes") comes from the word "hemi", meaning half, and "penis".
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