Virtually all animal conflicts occur over access to mates or resources that affect survival, the two key components of fitness. Here we show how predation risk and mate availability jointly affect the outcomes of contests between male sand crabs Scopimera globosa for burrows in which crabs mate and take shelter from predators. Under natural conditions of low predation risk and without the presence of a female the larger crab usually won, whether he was the resident or the intruder. We increased perceived risk to intruders by capturing males and placing them in the opening of other males’ burrows. Accounting for size differences between residents and forced intruders, intruders fought harder and won more fights. We repeated this manipulation but increased the value of the burrow to the resident by giving him a female or allowing him to take one into his burrow. Then, the fights escalated significantly and, correcting for male size differences, residents tended to win. Thus residents fought harder to maintain access to potential mates than did intruders to gain access to shelter. This is the first report of two external factors simultaneously raising resource value, affecting motivation of contestants and altering the outcome of fights.