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Not all animals have neurons; Trichoplax and sponges lack nerve cells altogether. Neurons may be packed to form structures such as the brain of vertebrates or the neural ganglions of insects. The number of neurons and their relative abundance in different parts of the brain is a determinant of neural function and, consequently, of behavior.
Some animal phyla have gone through major brain enlargement through evolution (e.g. vertebrates and cephalopods both contain many lineages in which brains have grown through evolution) but most animal groups are composed only of species with extremely small brains.
Animal ability to process and respond to stimuli is correlated with brain size. Small-brain animals tend to show simple behaviors that are less dependent on learning than those of large-brained animals. Vertebrates, particularly mammals, have larger brains and complex behavior that changes with experience.
An ancient human cousin may have buried its dead and carved symbols into cave walls, surprising findings for a creature with a small brain. Fossil remains of the species — named Homo naledi ...
The fundamental bilateral body form is a tube with a hollow gut cavity running from the mouth to the anus, and a nerve cord with an enlargement (a ganglion) for each body segment, with an especially large ganglion at the front, called the brain. The brain is small and simple in some species, such as nematode worms; in other species, such as ...
The size of the brain is a frequent topic of study within the fields of anatomy, biological anthropology, animal science and evolution.Measuring brain size and cranial capacity is relevant both to humans and other animals, and can be done by weight or volume via MRI scans, by skull volume, or by neuroimaging intelligence testing.
Fish typically have quite small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal. [10] However, some fish have relatively large brains, most notably mormyrids and sharks, which have brains about as massive relative to body weight as birds and marsupials. [11]
The brains of rhinoceroses are relatively small compared to body size, [18] [19] around 531 grams (18.7 oz) in an adult black rhinoceros. [19] The limb bones tend to be robust (proportionally thick and stocky). All living and the vast majority of extinct rhinoceroses have three toes on each foot. [14]