• Acutisoma;
  • costs of reproduction;
  • Gonyleptidae;
  • Opiliones;
  • parental investment


  • 1
    Few studies have experimentally quantified the costs and benefits of female egg-guarding behaviour in arthropods under field conditions. Moreover, there is also a lack of studies assessing separately the survival and fecundity costs associated with this behavioural trait.
  • 2
    Here we employ field experimental manipulations and capture–mark–recapture methods to identify and quantify the costs and benefits of egg-guarding behaviour for females of the harvestman Acutisoma proximum Mello-Leitão, a maternal species from south-eastern Brazil.
  • 3
    In a female removal experiment that lasted 14 days, eggs left unattended under natural conditions survived 75·6% less than guarded eggs, revealing the importance of female presence preventing egg predation.
  • 4
    By monitoring females’ reproductive success for 2 years, we show that females experimentally prevented from guarding their eggs produced new clutches more frequently and had mean lifetime fecundity 18% higher than that of control guarding females.
  • 5
    Regarding survival, our capture–mark–recapture study does not show any difference between the survival rates of females prevented from caring and that of control guarding females.
  • 6
    We found that experimentally females prevented from guarding their eggs have a greater probability to produce another clutch (0·41) than females that cared for the offspring (0·34), regardless of their probability of surviving long enough to do that.
  • 7
    Our approach isolates the ecological costs of egg-guarding that would affect survival, such as increased risk of predation, and suggests that maternal egg-guarding also constrains fecundity through physiological costs of egg production.
  • 8
    Weighting costs and benefits of egg-guarding we demonstrate that the female's decision to desert would imply an average reduction of 73·3% in their lifetime fitness. Despite the verified fecundity costs of egg-guarding, this behaviour increases female fitness due to the crucial importance of female presence aimed to prevent egg predation.