Posted tagged ‘coevolution honey locust and mastodon’

Mastodons in Manhattan: How the Honey Locust Tree Got Its Spikes

March 29, 2010

Walking south in Riverside Park, somewhere around 91st Street, the ground is littered with long brown  seed pods, some with visible bumps inside, like giant pea pods.

The tree trunks sport spikes, sharp and menacing.

“Aha,” I think, reaching for a nugget of knowledge given to me many years ago by my 5th grade best friend, Janet. “Carob trees. People make fake chocolate with the pods.”

Back at home with my trusty tree guide, I discover the tree is actually a honey locust tree (Gleditsia triacanthos), a relative of the carob.  Both are members of the legume family, and the pods are indeed giant pea pods. The honey locust derives the “honey” in its name from the sweet taste of a gummy edible paste that surrounds the protein-packed peas inside the pods.  The pods are eaten by livestock, including horses, cattle and pigs, as well as squirrels, rabbits, deer and birds.

Now about those stegosaurus-like spikes. The spikes are an arboreal defense mechanism to keep herbivorous predators from browsing the tasty trees.

Fascinating to realize that herbivores, prey for carnivorous predators, are themselves  predators of plants. Since trees can’t run away or attack with tooth and claw, their defenses are stationary, usually involving tough bark, leathery leaves, a variety of sharp thorns and spikes, and even bad smells. Some acacia trees have evolved a symbiotic relationship with fierce stinging ants. The trees provide special food and shelter for the ants, which bite any animal that tries to browse the leaves and shoots.

Which way to the honey locusts?

The honey locust co-evolved with giant herbivores, its impressive spikes serving to protect it from the browsing mastodons and woolly mammoths that roamed North America, including  Manhattan, until somewhere between 6,000 and 11,000 years ago. The tree has not yet lost the adaptation, still sprouting spikes sharp and tough enough to puncture a truck tire. Or a mastodon tongue. They’ve been used as nails and blowgun darts, and Civil War soldiers used them to pin together torn garments.

The honey locust withstands pollution, drought and poor soil. According to New York City Trees: A Field Guide for the Metropolitan Area, the honey locust is the most common street tree in Manhattan. The trees in full leaf are said to provide a lovely dappled shade. I look forward to walking beneath one in midsummer.

For more on plants and evolution at Out Walking the Dog, read The Burry Man, The Burry Dog and Burdock. For a monthly round-up of up-to-the-minute evolutionary science blogs, check out The Carnival of Evolution.

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