A critical mass of human and animal studies has turned up links between parenting and the hormones prolactin and cortisol. Cortisol is a stress hormone that has also been implicated in mother-infant bonding. Human mothers with high levels of cortisol are more sympathetic to infant cries, better able to recognize their own infants' odors, and more attracted to these odors than mothers with lower levels. Prolactin stimulates a mother's breasts to fill with milk. In female mammals, levels of prolactin increase in response to suckling and to hearing offspring's cries. Suppressing prolactin, on the other hand, disrupts maternal behavior, prompting a lactating meadow vole to spend more time away from her nest and a female golden hamster to scatter, attack, or even kill her young. In males, high levels of prolactin have been linked to a meerkat's decision to babysit the colony's youngsters instead of going out to forage. And human fathers with high levels of the hormone have been shown to react more positively and with more concern to infant cries.