Defending offspring provides fitness benefits to parents, but is costly. To moderate costs, parents should adjust aggressive responses to the threat posed by different species entering their territory. However, few studies have experimentally tested behavioral adjustments in response to the threat posed by different types of intruders, particularly in the field, and in environments with an array of heterospecific intruders. Here, using a biparental cichlid, the poor man’s tropheus (Hypsophrys nematopus), we investigated whether males and females in the wild invest equally into offspring defense and what impact the absence of a partner might have on the quality of offspring defense provided by a solitary parent. In a separate experiment, we assessed responses of breeding pairs to three common heterospecific intruders that pose different levels of threat to the breeding pair and their brood. We found that both paired and unpaired females invested significantly more into territorial aggression than males. However, unpaired females were unable to fully compensate for the absence of their partner, with intruders left to venture closer to their offspring. Furthermore, we show that breeding pairs can readily discriminate between heterospecific intruders, with pairs responding quicker and more aggressively towards species that posed a greater potential threat. Our results demonstrate that biparental species can have extensive behavioral flexibility in their aggressive responses towards intruders, even in environments with a high frequency of territory incursion. The quality of territorial defense can nevertheless be compromised if one parent is left to defend the brood alone
