April 18, 2014 Desert Tortoise a Master of Adaptation
With a motto of slowly but surely, the desert tortoise is like a miniature four-wheel-drive vehicle clambering slowly along, negotiating steep rocky slopes by simply remaining in first gear.
Found in the creosote flats palo verde-saguro forests and hilly terrains of arid southwest America, even though its part of the turtle family, the desert tortoise is so specialized to living on land that if it falls into deep water, it will drown.
It takes about 15 to 20 years for the diminutive desert tortoise to reach its full length of 12 inches. By that time its upper – or carapace – and lower – or plastron – shells become fully hardened. With its armament in place it can live for another 80 years.
Some truly remarkable adaptations have evolved over the past 37 million years to enable this critter to live the equivalent of a human lifespan.
The southwest desert receive less than 10 inches of precipitation yearly and experience a 113 degrees Fahrenheit range in temperatures from winter to summer.
The desert tortoise relies on an abundance of spring and early summer new plant growth for both its nutritional and fresh water sustenance. Three months of feeding on fresh growth will tide a turtle over for the rest of the year. Moisture from that new growth is just enough to enable them to survive during the furnace-like temperatures of May to September. If they encounter free water they will drink it. They will also dig shallow depressions to capture any incoming rainfall.
If drinking water is available then the desert tortoises are able to digest woody twigs. They have bacteria in their intestines that break down otherwise indigestible wood into proteins. They obtain this bacteria as juveniles by eating adult feces.
Usually, however, water is in short supply and so the desert tortoise stores water in its body. They also conserve water by allowing waste products to build up in their system and urinate infrequently in times of drought.
The desert tortoise is the largest tortoise in North America. And its ability to excavate is legendary with their stout claws functioning like teeth on a backhoe bucket. Burrows measuring nearly 33 feet long and four meters deep are not uncommon. The labyrinth of tunnels provides necessary shelter for brumation (a hibernation-like state), which can last as long as five months. Most tortoises awaken at least once during brumation and may emerge to bask and even feed on a warm winter day.
Male desert tortoises are slightly larger than females. Both have a horn called called a gular that protrudes from the lower shell beneath their neck and head, but the male gular if in tact, is marginally longer than the female’s.
Males are pugnacious and their horns are used to ram and flip over other males during bloody fight. Once on its back the tortoise is very vulnerable to dehydration and predators.
Males bite females and use their gular horns to batter them into submission during an aggressive courtship. The female’s lower shell is flat-shaped and the male’s is concave, designed to fit over top of the female’s upper shell and assist the male with balancing during copulation.
Females are able to reproduce at about the age of 12 years. They use their powerful hind legs to dig a hole roughly 8 inches deep and lay between four and 12 ping-pong-ball-sized eggs in the chamber before covering them. Before departing the female urinates on the covered hole to mask the scent of the eggs and to humidify the chamber site. About three months later the eggs hatch.
Many animals prey on the desert tortoise’s eggs and hatchlings. Kits foxes dig up and eat the eggs and hatchlings. Ravens are notorious predators of the soft-shelled hatchlings – a single pair can dispatch a dozen hatchlings. Gila monsters also dig up and devour tortoise eggs. Coyotes, badgers and golden eagles and mountain lions will attack, crack the shell and kill adult tortoises.
The best form of offense is defense, and the tortoise shape and concealing coloration are big advantages. When not moving, the desert tortoise is hard to spot. Particularly in rocky areas when it looks like another stone.
They often freeze when danger approaches, pulling their heads and legs into their shell as tightly as possible. They can last in this position for more than an hour.
A final defensive action for the tortoise is to expel the contents of its bladder making it as unappetizing as possible.
The desert tortoise is the most loved yet most persecuted critter in the Mojave and Sonoran Deserts. Their numbers have rapidly declined because of disease, senseless vandalism, illegal collecting, off-road vehicles, military operations, attacks by domesticated dogs and habitat destruction from residential and commercial development.
They have been officially recognized as a threatened species in the southwest United States and their road to recovery for this prehistoric-looking survivor has slowly begun.
Earth Dr Reese Halter is an award-winning broadcaster and distinguished conservation biologist. His latest book with Chris Maser is Life, The Wonder of it All
Text © by Dr Reese Halter 2014. All rights reserved.