Biosonar capabilities of large-brained dolphins and small-brained bats: Size does matter

Abstract

The large encephalized brain of the bottlenose dolphin, with a small corpus callosum, functions to maintain continuous, 24-hour biosonar signal processing. Bats are not faced with the dolphin’s challenge of maintaining constant vigilance, as bats are able to retreat to roosts for safety from predators. Although both bats and dolphins echolocate, dolphins face greater challenges in the aquatic environment. Dolphins must locate prey of a similar density to that of the environment, which itself has a high speed of sound transmission. Bats can readily distinguish between prey from the surrounding air because of a strong impedance mismatch, and the speed of sound is much slower. Perhaps stemming from these challenges, dolphins have developed a highly sophisticated echolocation system in which they are able to emit signals and receive echoes, decide, and respond accordingly at a remarkably rapid speed. Dolphin brains may also be specially equipped for early auditory development in utero. The dolphin body has a close impedance match to water, exposing the fetus to environmental sounds. Furthermore, auditory fibers develop myelin early during gestation. Thus, although dolphins and bats both echolocate, the dolphin’s experience during life in the ocean promoted the development of a large and specialized brain.