Theodor Seuss Geisel, the wonderful mind behind children's classics like The Cat in the Hat and Green Eggs and Ham, would have turned 115 on Saturday (March 2). Since 1998, the National Education Association has used the date to encourage and celebrate reading across the United States with their "Read Across America" programme. And what better books for young noses to be buried in than those that tell tales of "Bar-ba-loots" and the "Zizzer-Zazzer-Zuzz". At the top of the pile for many Seuss fans is The Lorax – Geisel's cautionary tale about the dangers of environmental exploitation. While most of Dr Seuss's fantastic characters were products of his seemingly endless imagination, the Lorax may actually have been inspired by a real-world moustachioed monkey from East Africa.

Dr Seuss's Lorax may have been based on the patas monkey. Image © Geoffrey Chandler (left), Eric Kilby (right)

In the early 1970s, Geisel was suffering from writer's block. The Environmental Protection Agency had just been created in the US and amid a surging wave of eco-consciousness, the author was eager to pen a conservation-themed tale that wasn't boring or preachy. But he was stuck. His wife suggested he take a trip to Kenya’s Laikipia plateau in the hopes that it might lift him out of his funk. It was on this East African safari that inspiration struck. Geisel scrawled much of the now-famous story on the back of a laundry list, possibly while watching the very primate that is possibly the Lorax's real-life counterpart.

Fast forward almost five decades to a Dartmouth College faculty dinner where anthropologist and evolutionary ecologist Dr Nathaniel Dominy is trying to spark up a conversation with acclaimed Seuss expert Donald E. Pease. Dominy tells the Seussian scholar about a monkey he'd encountered during his fieldwork in Kenya - an orange-capped creature with a distinctive moustache a bit like the "shortish and oldish and mossy" titular creature from The Lorax. The patas monkey "just looks like a Seuss character," said Dominy. "The resemblance is striking."

Pease was skeptical at first. Geisel hardly needed real-world inspiration to give rise to his marvellous creations. “But then, I thought about what it might add to the established understanding of the Lorax,” Pease told Popular Science. In The Lorax, a primate-like creature who "speaks for the trees" becomes enraged when the Once-ler moves in and starts hacking down all the Truffula trees to knit "theeds". There is a prevailing belief in literary circles that the Lorax is a grouchy eco-crusader with as much right to demand the Truffula trees remain standing as the Once-ler has to chop them down.

Unless, of course, the Lorax is a part of this fictional ecosystem he so desperately wants to protect. The patas monkey derives much of its nutrients from a knobbly tree called the whistling thorn acacia – a species that bears a resemblance to the barren trees that feature in the book. The coincidences were too juicy not to explore further, so Pease and Dominy teamed up and went high-tech. Using a machine-learning algorithm they compared the face of the whiskered Lorax to five Kenyan monkeys and a second Seuss creature from one of his other works.

A comparative analysis revealed that the Lorax bears more of a resemblance to the patas monkey (and to two other real-world monkeys) than it does to the other Seuss character. “Even the voice of the Lorax (a ‘sawdusty sneeze’) resembles the ‘whoo-wherr’ vocalization of patas monkeys,” the researchers point out.

We may never know for certain if Geisel had the patas monkey in mind when he mustered up the Lorax, but simply acknowledging the comparison teaches us about the dangers of removing keystone species and how this can lead to ecosystem collapse. “Ecosystems are communities, they’re assemblages of species that are interacting, and if you affect one, you affect all of them," says Dominy. "That’s what’s important, and he nailed that beautifully."