How Is Information Transmitted in Large Animals?

Being able to react to stimuli allows organisms to survive. Be it identifying a visual threat and evading it or sensing pain when an extremity finds the edge of furniture and retracting that limb, humans and animals can rapidly recognize and respond to changes in their environment.

However, all of these actions rely upon neurons to send, receive, and interpret signals, which takes time. Saltatory conduction, or the process of sending electrical signals through action potentials down the axon, has an average information transmission speed of 100 meters per second in motor neurons, the cells that control muscle movements. Since most human neurons are less than two meters long, this means signals reach their target destination and can initiate appropriate responses in a matter of milliseconds.

Yet across life on Earth, some animals tower above humans. Given the physical limitations of electrical signaling in neurons, how is information transmitted in large animals, like giraffes and whales, and what are the effects of signaling over these long distances?

Neuron Signaling Always Has a Delay

In 2008, Samuel Wang, a neuroscientist at Princeton University, evaluated the differences in neuron morphology and signaling speed in animals of various sizes to study how these factors scaled with one another. Neuron size influences signaling speed: Smaller diameter cells send slower signals but take up less space, while a larger axon diameter expedites signaling transmission but also rapidly increases the cell volume. Wang and his team showed that there was a tradeoff between this fast axon signaling and the extra volume required for the increased size in animals, ranging from shrews to humpback whales, to balance cell signaling with brain size.¹

Considering the impacts of relaying information beyond the brain in exceptionally large animals, Wang said, “One answer is that there are just delays.”

Even in humans, information isn’t transmitted instantaneously. Besides the speed of axon signaling, Wang said that delays also come in during synaptic transmission and in the initial process of sensing a stimulus, such as converting photons into electrical information. He said that the brain is constantly accounting for these small discrepancies.

“Everything we do—like our whole experience of the world—is through some little time machine where we never do learn about things until tens to hundreds of milliseconds after they occur. And so, a lot of what our brains struggle with is basically running a time machine to undo that,” Wang said.

Spinal Cord Nodes May Expedite Responses in Large Animals

Another adaptation is in specialized neurons, called Betz cells, which produce extra wide axons for fine motor control.² While in humans and primates these cells become larger the farther they must travel from the brain, studies in giraffes didn’t find this type of morphology.³ Indeed, a different team showed that giraffes also don’t have accommodations to improve their neuron sensitivity, suggesting that these animals may have come to adapt to not having the same sensory and rapid responses.⁴

However, one more potential adaptation for overcoming communication delays in large animals may lie between the sensory nerve and the brain. “Perhaps these…little control centers are down the spinal cord in periodic intervals, and we call those central pattern generators,” said Douglas Smith, a neuroscientist at the University of Pennsylvania. Smith studies how mechanical forces physically stretch neurons and previously showed that this process can extend the neuron of a blue whale at a rate of three centimeters a day.⁵

Although central pattern generators have not been identified in whales due to limited samples, researchers have observed these nodes, which appear to store a short, rhythmic behavior in the spinal cord, in animals from mice and rats to turtles.^6,7 Instead of sensory information having to travel from the stimulus point to the brain and back again, specific senses can trigger certain automated responses kept in neurons in the dorsal root ganglion. “It could be that these, what we call megafauna, the very large animals, have central pattern generators distant or distal to the brain that can help them overcome this lag time,” Smith said.