Trigeminal Ganglion and Sensory Nerves Suggest Tactile Specialization of Elephants

Sensory nerves are information bottlenecks giving rise to distinct sensory worlds across animal species. Here, we investigate trigeminal ganglion and sensory nerves of elephants. The elephant trigeminal ganglion is very large. Its maxillary branch, which gives rise to the infraorbital nerve innervating the trunk, has a larger diameter than the animal's spinal cord, i.e., trunk innervation is more substantive than connections of the brain to the rest of the body. Hundreds of satellite cells surround each trigeminal neuron, an indication of exceptional glial support to these large projection neurons. Fiber counts of Asian elephant infraorbital nerves of averaged 4,00,000 axons. The infraorbital nerve consists of axons that are ∼10 μm thick and it has a large diameter of 17 mm, roughly 3 times as thick as the optic and 6 times as thick as the vestibulocochlear nerve. In most mammals (including tactile specialists) optic nerve fibers greatly outnumber infraorbital nerve fibers, but in elephants the infraorbital nerve fiber count is only slightly lower than the optic nerve fiber count. Trunk innervation (nerves and ganglia) weighs ∼1.5 kg in elephant cows. Our findings characterize the elephant trigeminal ganglion as one of the largest known primary sensory structures and point to a high degree of tactile specialization in elephants.