In Ecology, a fundamental niche is the set of environmental conditions such that a population can persist (in other words, the population growth rate is positive). This simple definition ignores many complications. A crucial problem is the one that it is impossible to replicate real “environments” in the laboratory. Therefore, the experiments required to establish the environments leading to a positive growth rate are basically unfeasible. Behavior is almost always ignored, although in real environments it is hugely important. In real life, animals can evade unfavorable environments by getting into holes, by actively “building” their niches, by performing in ways that compensate for unfavorable conditions. I will present a couple of examples of behavior compensating for mere physiological unaffordability, to end briefly talking about humans, the niche builders par excellence.