The variability of life history traits is affected by domestication. As gestation length is an important life history trait for production management, its variability is hypothesized to be lower in highly controlled production animals. Furthermore, some authors claim that horses have a particularly variable gestation length compared to other domesticated mammalian species. To test this, we compared 192 gestation lengths from the literature for eight different mammalian species. In this sample, gestation length does not contain a phylogenetic signal. Instead, production animals display lower variation than non-production animals. Horses fall well within the range of variation of gestation length in other domesticated companion animals.