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Cactus spines are an adaptation to desert life. (Photo by Megan Wilde)

By James Saunders

February 12, 2009 was the 200th birthday of Charles Darwin, the English naturalist who first proposed that species evolve through the process of natural selection. 2009 also marked the 150th anniversary of the publication of his most famous work, On the Origin of Species, in which he explained his groundbreaking ideas. Darwin first began to conceive of his revolutionary idea while on a five-year, around-the-world ocean voyage, during which he visited Africa, South America, Australia and numerous islands and observed first-hand the diversity and interrelatedness of life.

One place he did not visit while on his voyage was the Chihuahuan Desert region of West Texas and Northern Mexico. If he had come around here, however, he certainly would have been fascinated by the diversity of the cacti native to this area. He undoubtedly would have realized that the features that make cacti unique are merely adaptations that have evolved in response to living in such an arid and inhospitable place as the Chihuahuan Desert.

The most obvious of these adaptations is the cacti’s spines. What is the evolutionary origin of cactus spines? How do the spines help cactus plants in their struggle for survival?

In his book, Darwin explained that in each generation there are more individuals produced than can possibly be supported by their environment. Furthermore, the individuals that make up each generation show a natural variation of characteristics. Individuals that have characteristics that give them an advantage over the rest of their generation would have more success reproducing. Therefore, those characteristics would be passed along more frequently and become more common in later generations.

Thus, the species would slowly change, or evolve, over time. Although biologists still debate the details of Darwin’s theory, it has become the unifying, central principal of biology, accepted by the vast majority of biologists. Evolutionary ideas pervade every biological discipline or endeavor.

In the case of cactus evolution, the harsh, arid environment in which they evolved played the role of the natural selective agent.

A plant loses most of its water through its leaves. Many desert plants’ leaves have adaptated to slow this water loss. In the case of cacti, individual plants with smaller leaves lost less water, and so smaller and smaller leaves were selected. In addition, leaves with sharp points gave cacti an advantage, so the leaves, as they got smaller also became stiffer and more needle-like, eventually becoming the spines we see today. But even the spines themselves demonstrate an amazing diversity of size, shape, number and function.

To anybody who has ever had an unpleasant encounter with a cactus, the obvious benefit to having spines is the protection they give the cactus from animals. While this is undoubtedly true to a certain extent, deer, javelina, rabbits, and even cattle happily eat prickly pear, and some rodents dine without injury on the cholla cactus.

Cactus spines offer cacti so much more than mere physical protection. A cholla stem sticks to your skin not to protect itself, but to hitch a ride to a place where it can possibly take root and grow into a new plant.
The curved spines on a fishhook barrel cactus are designed to hook into the skin or fur of a passing animal. As the animal pulls away, the cactus, if its roots are weak, will be uprooted and hopefully dragged to a new location that offers a better place to grow. If, however, the roots are strong, and the cactus is doing fine where it is, the spines will give way, and the cactus will not be uprooted.

The roots and lower stems of a prickly pear can die and rot away, but the pads higher up the plant will remain alive. Again, if the pads can hitch a ride on a passing animal, they will easily break off from the dead stem and perhaps end up in a more hospitable area.

Spines also provide protection from the sun. A cactus living in hot desert areas can produce its own shade by covering itself in spines. The feather cactus is so-named because its spines hook together like the barbules on a bird’s feather, providing a complete, almost impenetrable covering. Like a bird’s feathers or an animal’s fur, this layer probably also provides insulation when the temperature drops below freezing. The appropriately named hair-covered cactus has spines that look like bristly hair, also providing shade and perhaps protection from the cold.

Spines also provide a place where moisture can condense, allowing the cactus to harvest water even when it doesn’t rain.

The next time you are wondering about something living, try and think about it from an evolutionary viewpoint. Any question you can think of about a living thing can usually be answered using Darwin’s Theory of Evolution by Natural Selection.

Have a question or comment about this episode? Contact Nature Notes Coordinator Megan Wilde at mwilde [at] cdri [dot] org. Or discuss this episode on Nature Notes’ Facebook page. This episode originally aired on February 19, 2009.

References & Resources for Educators

• Understanding Evolution for Teachers: University of California Museum of Paleontology
• “Cactus Spines” by James D. Mauseth, cactus expert at The University of Texas at Austin

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