An ethological explanation seeks to understand the innate behaviour of animals (including humans) by studying them in their natural environment. The focus of an ethological explanation is to try and account for behaviour in terms of its adaptive value to the specific species. With regard to aggression, which is seen in all animal species, it is believed to be an innate behaviour that has an adaptive function. Aggression can aid survival, as it can be used to protect resources such as land and food. Aggression can establish dominance hierarchies, and these are vital to allow access to other resources, such as females. The ethological explanation proposes that aggression can be the result of an evolved automatic biological response in the brain; it is believed that animals have a built-in neural structure (a network of neurons) which, when exposed to specific stimuli (signs or releasers), such as facial expressions, will cause the release of an automatic behavioural response.