What Organ System in Frogs Is the Most Complex?

Frog brains are mostly nose and eyes -- or rather, structures devoted to processing input related to smell and sight. Frogs' squat, flattish, neckless bodies mean their skulls are fairly narrow, compressed dorsoventrally -- top to bottom -- and extend into their backs, where they lead into a short spinal cord encased by the vertebral column. Frogs have 10 pairs of cranial nerves, which branch into the brain itself, and 10 pairs of of spinal nerves, which branch out from the spinal cord into each organ and tissue of the body.

About half of the frog's forebrain is made up of the olfactory lobe. Sitting approximately where the frontal lobes do in humans and taking up nearly as much space, this structure begins with the olfactory tract that runs from the frog's principal olfactory organ, the structure inside the snout that senses smells, and vomeronasal organ, the one that senses pheromones from other frogs. Depending on the species of frog, these olfactory senses can be critical for finding prey, avoiding predators, locating egg-laying sites, finding mates and avoiding same-species competitors.

Immediately behind the olfactory lobe is the cerebellum. It's split into two hemispheres, just like in humans. This is the area associated with learning and behavior, including communication, planning and problem solving. The frog's pineal gland is located between the hemispheres. They sit in front of one portion of the diencephalon, or mid-brain, and on top an even larger portion of it. The diencephalon includes the thalamus and hypothalamus, which allow communication between different portions of the brain and regulate the body's physiological states.

The optic lobes comprise approximately another third of the frog's brain. They are located posterior to the diencephalon and are about as large as the cerebellum. The eyes are actually outgrowths of this brain region and connect directly to it via the optic nerves. Frogs' incredible nearly 360 degree vision and ability to react to stimuli with lightning speed -- be it grabbing a tasty insect on the fly, or dashing into a pond at the first glimpse of a predator -- are attributable to this incredible neurological adaptation for sight.

Though they take up the largest brain space, sight and smell aren't the only highly developed senses in frogs. Frogs also have incredibly acute hearing, which they rely on to find mates and defend territories. Frogs hear through their tympanic membranes -- exposed eardrum-like organs on the sides of their heads -- and directly through their skin. Sound input is fed into the brain stem's auditory center, where the laminar nucleus translates awareness of sounds into behavioral responses.