Abstract

Cognitive ability is defined as a mechanism that animals acquire, process, store, and act based on the information they have received from the environment. The process includes perception, acquisition, memorization, and decision making. In other words, the brain is the most important center of cognitive processing, which has undergone many changes in structure, size, neuronal pathways, effective hormones, etc. In this paper, we reviewed data on the role and effect of these factors on different groups of invertebrates and vertebrates. Also, it has been reviewed how they are involved in cognitive processes. The brain size and the density of the neurons vary noticeably across invertebrates and vertebrates clades. It seems the evolution of brain size is firmly correlated to the evolution of cognition. Generally, species with larger brains and more neurons demonstrated more complex cognitive behaviors except in certain taxa. Besides, hormonal-mediated events have a crucial role in the central nervous system. It shows the changes in cognitive function correlate with different kinds of hormones and chemicals. We provided an insight into the interrelationship between different telencephalon regions and cognitive functions. We compared how different areas in the brain change the cognitive abilities in different clades of animals, including invertebrates and vertebrates taxa. Also, we reviewed invertebrates' nervous systems in a separate section because of the substantial discrepancy.