Sociality, glucocorticoids and direct fitness in the communally rearing rodent, Octodon degus

► Per capita direct fitness decreases with group size (sociality) in the communally rearing Octodon degus. ► Baseline cortisol levels increased with direct fitness estimates, but not so with group size. ► Glucocorticoid hormone levels were more responsive to reproductive challenge than to the costs of group living. ► Our study that focused on group living supported reproductive effort is a major determinant of cort-fitness covariation.