The cover. A Weddell seal (Leptonychotes weddellii) in the Ross Sea, Antarctica. Weddell seals are one of four species of seals that live in the pack ice and fast ice habitats surrounding the Antarctic Continent. All four species are sexually monomorphic in size and color and males and females may be physically distinguished easily in the field only by the presence or absence of a ventral penile opening, though sex can be determined from tissue samples using the sex-chromosome-specific ZFX and ZFY genes (see Curtis et al. pp 280–285). Photograph by Brent S. Stewart.
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