Suzana Herculano-Houzel

Behavior is any observable action


All behavior has an agent, who need neither be alive nor have a brain

Behavior does not require a nervous system – a set of organs and tissues made up of excitable cells that transmit signals rapidly throughout the body. If behavior is defined simply as observable actions in a given context, then plants have behaviors, such as orienting themselves in relation to sunlight; single-celled organisms, such as bacteria, have behaviors, such as moving towards food and away from toxins. Even inanimate objects, such as spinning fans, dancing robot dogs or the moon and tides, have behavior: their actions of spinning back and forth, dancing, orbiting the Earth or filling and waning.

Whoever is performing an action, living or not, is the agent of the behavior. And if life is not necessary for there to be behavior, then a nervous system cannot be necessary – and behavior cannot be defined as “what the nervous system does”. Accepting that behavior can occur without a nervous system provides a solid basis for understanding what changes in animal behavior once they have a nervous system.

Rather than adopting a circular definition of behavior as “what the nervous system does”, it is much more useful to work with a broader, more comprehensive definition of behavior and then examine what the nervous system contributes to this. This author considers behavior as any observable action of an agent. The key word here is action, rather than just change or difference, as in a game of 7 wrongs. The concept of behavior presupposes an agent, or “owner”, of the action being observed: the agent of the behavior.

We describe many observable changes in the world as the actions of an agent: the sun sets, the moon rises, the dog chases the squirrel, we fall in love. It’s no coincidence that actions presuppose agents, the entity that is acting, because this is how the human brain, which created the language in use, works. Given that the brain is always generating actions, making predictions about the changes in the body and the world that will be the consequences of these actions, checking that the results of the actions match expectations, identifying itself (or not) as the source of the change – and again, and again – then, for this brain, any change in the state of the world “must” logically also be attributed to an agent: if not ourselves, then something else whose change in its state could have caused the observed change in state.

For example: stating that “the sun sets” or “the moon rises” establishes the sun and the moon as agents of the behaviors of setting and rising. Note that this does not imply or require that the sun or moon are conscious in any way, let alone that they know what they are doing. Claiming that the sun and moon behave in certain ways doesn’t even require a purpose, let alone a purpose. This statement simply provides us with a description of a change – the behavior of a certain entity – whose characteristics and patterns we can now examine and whose cause in the agent we can even look for, if we wish.

A fundamental mission of neuroscience is to understand what makes animals behave as they do. Since animals are defined by having a nervous system, and there are many behaviors that only animals among living creatures perform – walking, flying, eating this but not that or choosing who to be close to – then it is a reasonable expectation that much of animal behavior must stem from how their nervous systems are built and operate. It may seem counterintuitive that the best way to understand how this happens is to first assess how behaviors can occur without a nervous system, but once this is done, it becomes much easier to examine what the true contribution of nervous systems to behavior is.

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