Carlsbad Caverns: The Black-throated Sparrow

On the surface, Carlsbad Caverns National Park is a beautiful piece of the Chihuahuan Desert, much like the surrounding area in southeastern New Mexico. Mesas and arroyos are studded with cacti, shrubs, and grasses. The humidity is frequently as low as 10%, and it averages a mere 14 to 15 inches of rainfall, nearly all in the monsoon season of July to September. For most of the year, it's as dry as a bone. Only the hardiest xerocoles (desert-adapted animals) can thrive here. It's a remarkable contrast below ground; there is an expansive world where the humidity sits around 90% to 100% year-round, and water continuously drips from the ceiling. Vast rooms house stalagmites, stalactites, and dripstones in total darkness. It's home to colonies of water-loving troglobites (a.k.a. full-time cave dwellers).

As unexpected and special as any other oasis, the cave and many others like it in the National Park survive in the middle of the desert off rainwater stored in the ground until it percolates all the way through to the caverns. A number of organisms survive above ground in a similar way; agave and cacti store rainwater in their fleshy leaves, sotol in its woody base. The desert tortoise stores it in its specialized bladder for extended periods of time. Others survive by ingesting the water collectors. The Black-tailed jackrabbit, Scaled quail, and Cactus beetles all have their water needs partially or fully met in this way. Then, of course, there are the organisms that eat these organisms. Coyotes, owls, lizards, and wrens all rely on their prey to supplement their hydration. To top off those food and water pyramids are a couple of hardy birds, the Greater roadrunner with its reptile diet and the Turkey vulture surviving on carrion. In addition to its methods to collect water, each organism employs its own methods to make efficient use of and retain the water it collects. One bird, though, takes another tact altogether; the Black-throated sparrow chooses to rely on the water its body makes. This true arid champion can survive indefinitely on dry seeds alone and is our Carlsbad Caverns National Park ambassador.

Through a process known as metabolism, the Black-throated sparrow, and all organisms, including us, break down fats, proteins, and carbohydrates into energy. A byproduct of this process is water, known as metabolic water. The origin of metabolic water is not from the food itself, which does typically contain some amount of moisture, but from the hydrogen atoms from the fat, protein, or carbohydrate combining with the oxygen atoms we inhale. Every body in this way is its own little hydroplant. While metabolic water is far from sufficient to sustain most other organisms, the Black-throated sparrow runs a tight water budget and stretches this meager amount of water to meet its needs to survive.