Megafauna have been seen in the lands and in the seas. The growth of these creatures is fast. The thing has to be. They need to grow quickly. Megafauna have unique challenges that have been addressed in extensive studies. The development of their nervous system may be the greatest obstacle to extreme growth.

Unlike other cell types, neurons do not increase tissue volume by dividing the cells, but by expanding the volume of the cells themselves. Nerve fibers, or axons, that extend from the nucleus of a cell's cell body are grown early in development by using chemical and physical signals. The first phase of axon growth ends when the target is reached.

The axons are not the only ones to grow. The mechanical forces that can extend the fibers are relied on. Stretch growth continues as the distance between the cell bodies and their targets increases.


The axons are in a tug-of-war. The fibers only stretch so far, so tension from pulling on the rope could cause them to break. Extending length to axons is a relief for tetanus. The formation and organization of axon tracts that make up the white matter in the brain and spine is thought to be driven by mechanical stretching of the axons as a body grows. The growth of the spine is stimulated by the growth of the axons. There are many mysteries surrounding axons in both aquatic andterrestrial megafauna.

A Few Centimeters a Day

The blue whale can reach a length of up to 30 meters at maturity. The peak stretch growth rate of axons that span the spinal cord was calculated using recorded measurement of the whale's body length at different times. The rate described in neuroscience textbooks is different to this one. There is a crucial axon building block that moves down the nerve fibers up to a few millimeters per day. This is close to the extension rates of axons growing in culture and regenerating from injury.

The journey down the longest axons in maturing whales takes years, even at this fast pace. Computational modeling analyses suggest that the new proteins for axon stretch growth are pushed out from the neuron cell body so that they don't have to go to the end of the cell to extend axon length

The axon building in the blue whale is difficult to maintain. The volume of the cell body to the axon is doubled every day if the axon length is increased by 3 cm.

The growth rate of cancer cells that double in number each day is comparable to the increase in size. The blue whale'sspinal cord axons can be as long as 24 meters. A long extension of axons results in their volume being more than 1,000 times greater than that of their cells. One of the largest cells by volume for any mammal could be represented by this strange geometry of neurons. It is hard to imagine how a large axon volume can be supplied by the neuronal cell body. We still have a lot to learn about the unusual cellular process that may occur along axons.


Enter the Dinosaurs

Extreme axon stretch growth works well for the enormous blue whale despite its many mysteries. The blue whale has stiff competition from other large mammals. The sauropod is thought to have had high growth rates in the first few years after hatching to avoid being eaten by other dinosaurs. There isn't much fossil evidence to help calculate how large dinosaurs grew. A rough estimation of body growth rates is possible with limited fossil clues from the duck-billed dinosaur Maiasaura. In its first year of life, it grew from a hatchling length of less than 1 m to more than 7 m. In the first year, that's about 3.6 m.

In the first seven to nine months, we knew that Maiasaura had lines of arrested growth in its bones. The daily growth rate would have been between 1.33 and 1.71 cm. The blue whale had its axons grow at a peak rate.

Wait! Dinosaurs may be back in the competition after the discovery of a juvenile sauropod. The missing link between hatchlings and adult specimen of this species has been found. The growth in the body of a juvenile Rapetosaurus is estimated to be up to 2.7 cm per day compared to the growth of a hatchling. The animal's first arrested growth period is believed to have taken place prior to this time period. The Rapetosaurus has a growth rate that is close to that of the blue whale, but still less than that of the spine.

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A new player could be brought into the competition if the focus was on axon growth in the neck. giraffes have a short neck that protects them from injury during birth. Their neck grows up to 2.5 cm each day. The blue whale and Rapetosaurus peak spinal axons have similar rates of growth for their neck axons. The giraffe may be able to surge past the blue whale in the race for the most extreme axon stretch growth if a nerve in the neck is accidentally severed.

The giraffe's left vagus nerve leaves the brain stem and travels down the neck to the left recurrent laryngeal nerve. That goes under the arch and then goes up the neck to the vocal cords. The axons in the animal's left vagus and recurrent laryngeal nerve need to grow twice as fast as its neck. The growth rate of these axons is around 5 cm per day. The giraffe leads the blue whale.


The Winner Is ...

The Rapetosaurus had a long neck and is thought to have had the same odd distribution of the laryngeal nerve as the giraffe. Even if we double the growth rate for the recurrent laryngeal nerve, Rapetosaurus's neck still accounts for only half of its body length. The winner is the giraffe. That is the case at the moment. If Rapetosaurus developed along the same time line as other sauropod species, there could be more spectacular axon growth. Future research on the growth rates of the largest dinosaurs will be the only way to explore that possibility.

The most extreme axon stretch growth could soon be entered by humans. The limits of axon growth in the laboratory are pushed by having people grow supersized necks and bodies. Axon stretch growth of 1 cm a day has already been achieved in a lab dish using bioreactors that mimic nature. This form of axon growth is thought to be able to produce much faster rates of growth. The competition is not over with this promising experimental work and understanding of rapid nervous system development in other species.