Devoted fathers or selfish lovers? Conflict between mating effort and parental care in a harem-defending arachnid

When there is a temporal trade-off between mating effort and parental care, theoretical models predict that intense sexual selection on males leads to reduced paternal care. Thus, high-quality males should invest more in mating effort because they have higher chances of acquiring mates, whereas low-quality males should bias their investment towards parental care. Once paternal care has evolved, offspring value should also influence males' decisions to invest in offspring attendance. Here, we performed a manipulation under field conditions to investigate the factors that influence male allocation in either mating effort or parental care. We predicted that facultative paternal care in the harem-holding harvestman Serracutisoma proximum would be negatively influenced by male attractiveness and positively influenced by offspring value. We found that attractive males were less likely to engage in egg attendance and that the higher the perceived paternity, the higher the caring frequency. Finally, egg mortality was not related to caring frequency by males, but predation pressure was much lower than that recorded in previous studies with the same population. Thus, the benefits of facultative male care may be conditional to temporal variation in the intensity of egg predation. In conclusion, males adjust their investment in either territory defence or egg attendance according to their recent mating history and perceived paternity. Our findings suggest that exclusive paternal care can evolve from facultative paternal care only if the trade-off between mating effort and parental care is circumvented.