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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-Leitao, 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.