A Tree Might Forgive, But It Never Forgets

I recently started doing a program on plant adaptations for 4th grade classrooms.  One of the things that we discuss in this program is the idea that plants can defend themselves.  They do this either through chemical means (such as being toxic or tasting bad) or physical means (such as prickly leaves, thick bark, prickers, etc.).

I have a tree in my backyard that has taken physical defense to an extreme level.

Honey Locust thorns

Those long three-pronged thorns belong to a Honey Locust  (Gleditsia triacanthos).  The Honey Locust is a deciduous tree that native to the central United States from the west slope of the Appalachians westward to a line running from central Texas north to central Kansas and eastern Nebraska.  It ranges south to the Gulf Coast and north as far as Minnesota and Wisconsin.  In Michigan it is native only to the southern tier of counties.  The Honey Locust can grow to heights of 80 feet and commonly lives 120 to 150 years.  It has compound leaves and as a member of the Bean Family (Fabaceae) it produces long seed pods with large seeds.

Honey Locust seed pod and leaves

The Honey Locust developed its impressive thorns to deter herbivores from browsing its leaves and branches.  This seems like a sensible approach, many other species have developed similar defenses (roses, raspberries, prickly ash, etc.).  Let's take a closer look at those thorns to see if they would be a good defense.

Honey Locust branch with leaves removed to show thorns

The thorns are certainly impressive and would seem to be enough to deter the average herbivore, but lets add a ruler for scale.

Honey Locust branch with 12-inch rule for scale

Those thorns seem to rather far apart to deter an herbivore like a White-tailed Deer (Odocoileus virginianus).  The thorns spiral around the branch and are all at least three inches apart.  The distance between any two thorns on the same side of branch may be greater than eight inches.  It would not be too difficult  for a deer to avoid.  Also the length of the thorns seems a little more than is needed to deter a deer.  Many of the thorns are over 4 inches long! 

This Honey Locust thorn measures more than four inches long!

Just for fun here is picture of one of the thorns with my hand for scale.  I have repeatedly stepped on fallen limbs in my yard and stuck those small thorns in the soles of my shoes.  I definitely do not want to close my hand around them.

A Honey Locust thorn in hand

So these thorns are a little big to be defense against herbivores like White-tailed Deer.  Just what is the Honey Locust trying to protect itself against?

The Honey Locust, like most plants, has a long memory.  A very long memory.  While each individual tree might only reach an age of 150 years, the memory of the species goes back much further.  When the tree species was developing those thorns there were much larger herbivores walking around North America.  

How much larger?

How about the size of an elephant?

Mural depicting American Mastodons (Mammut americanum) -  Museum of Natural and Cultural History at Central Michigan University

One of the species that the Honey Locust had to defend against throughout much of its evolutionary history was the American Mastodon (Mammut americanum).  This relative of elephants was found throughout North America for thousands of years before going extinct about 8,000 years ago.  (There is some evidence to suggest some populations may have existed even later going extinct only 4,000 years ago.)  The American Mastodon could grow to a height of 10 feet and weighed up to 12,000 pounds.  Each adult mastodon could consume up to 400 pounds of vegetation daily. When you add in other herbivores such as Columbian Mammoths, camels, giant ground sloths, horses, and other species of extinct megafauna it is easy to see why the Honey Locust would have developed such large thorns to protect itself.

So would the long thorns have been effective at defending the Honey Locust from Mastodons and other large herbivores?  We can assume that Mastodons (and mammoths) fed much like their modern relatives the African and Indian Elephant.  Elephants use their trunks to gather vegetation either from the ground or from trees and then use massive molars to grind the vegetation.  The large thorns of the Honey Locust may have deterred the Mastodon from using its trunk to strip leaves and branches from the tree.

Fossil American Mastodon Jaw -Museum of Cultural and Natural History at Central Michigan University

Close-up of fossil Mastodon molar

If the mastodon did manage to remove the branches from the tree without injuring its trunk, would the thorns have prevented the animal from chewing the branches?  Let's take a closer look at one of the mastodon's molars.

Replica mastodon molar

This replica of a mastodon molar is massive.  It features rows of prominent cusps that would grind leaves, bark, and branches into a pulp that could be swallowed and digested by the mastodon.  Over time the grinding action of chewing would wear away the cusps (as seen in the fossil jaw above) making the teeth less effective.  Fortunately for the mastodon as a set of teeth wore out it would be replaced by a new set.  This would happen several times over the animal's lifetime.

Just how big are these teeth?

American Mastodon molar  (replica) - measures 8 inches long

American Mastodon molar (replica) - 4 inches wide

Looking at what the Honey Locust had to contend with, it becomes easier to see why it would have developed such over-sized defensive thorns.  Even if the mastodon succeeded in stuffing its mouth with Locust branches, chewing would have been extremely painful.  The thorns would have punctured the animal's gums, cheeks, tongue, and the roof of its mouth.  The Mastodon probably would not have tried this meal unless it was desperate.

Honey Locust branch and American Mastodon molar (replica)

Its thorns might be over-sized for any herbivorous threat it will encounter today, but the Honey Locust is unconcerned.  It took thousands of years to develop its thorns, it is not about to give them up easily.  If the Mastodon ever returns the Honey Locust will be ready for it.  A tree doesn't forget easily.