Fig. 1

Scanning electron micrographs of sperm from 10 species. a Human (Homo sapiens). b Rabbit (Oryctolagus cuniculus). c Dog (Canis familiaris). d Pig (Sus scrofa domesticus). e Guinea pig (Cavia porecellus). f Goat (Capra hircus). g Bull (Bos taurus). h Mouse (Mus musculus). i Golden hamster (Mesocricetus auratus). j Rat (Rattus norvegicus). In each panel, the left image shows sperm in whole-view, and the right images sequentially show the head, boundary of the midpiece-principal piece and the terminal of the principal piece in enlarged view. The scale bars are 5 μm in the left image and 2 μm in the right images. Please note the different lengths of the scale bars across the 10 species. The morphological features of the head and flagella are labelled by lines and arrows, respectively (n = 30 sperm/male, n = 3 males/species). The widths of positions 1–9 were measured as the diameters of flagella in these regions