Maternal stress increases cooperative behaviour of offspring in wild meerkats

The phenotype of parents can have long-lasting effects on the development of offspring as well as on their behaviour, physiology, and morphology as adults. In some cases, these changes may increase offspring fitness but, in others, they can elevate parental fitness at a cost to the fitness of their offspring. Here, we show that in Kalahari meerkats (Suricata suricatta) the circulating glucocorticoid hormones of pregnant females affect the growth and cooperative behaviour of their offspring. We performed a 3-year experiment in wild meerkats to test the hypothesis that stress-mediated maternal effects reduce the potential for offspring to reproduce directly and therefore cause them to exhibit more cooperative behaviour. Daughters (but not sons) born to mothers treated with cortisol during pregnancy were smaller early in life and exhibited significantly more of two types of cooperative behaviour (pup rearing and feeding) once they were adults compared to offspring from control mothers. They also had lower measures of stress hormones as they aged, which could explain the observed increases in cooperative behaviour. Because early growth is a crucial determinant of fitness in female meerkats, our results indicate that stress-mediated maternal effects may reduce the fitness of offspring but may elevate parental fitness as a consequence of increasing the cooperative behaviour of their daughters. Significance Statement Maternal characteristics can have a profound influence on offspring, which are referred to as maternal effects. It is widely assumed that selfish maternal effects, where mothers modify offspring to benefit their own fitness and decrease the fitness of offspring, are rare but this assumption is rarely tested. We show that a stress-mediated maternal effect in meerkats altered the growth, behaviour, and stress physiology of daughters in such a way that benefitted mothers at the expense of their daughters. Daughters from stressed mothers grew more slowly early in life, which should reduce their future reproductive opportunities, but exhibited more cooperative behaviour that should directly benefit their mother. This highlights the important role of sociality in the evolution of maternal effects.