Animal models are a critical aspect of neurobiological research because, for obvious ethical reasons, scientists can perform much more invasive procedures on animal brains than on human brains. In fact the ethics of animal research is a controversial and difficult area. Studies of animal brains shed important light on the workings of the human brain but they also highlight the ways that brains differ across species. When the brains of various animal species are compared, we can generate hypotheses about how our own brains developed across evolution. For example, the frontal lobe, which is associated with planning and other complex cognitive functions, is proportionately larger and more convoluted (providing more surface area) in animals with higher intelligence. This suggests that the frontal lobe grew in size across human evolution as intelligence became an increasingly important evolutionary strategy.