The mountains of the Sonoran Desert region have often been described as "islands" reaching toward the sky from a sea of surrounding grassland and desert.
This analogy is useful in understanding the ecology and origins of the plant communities and wildlife found in these ranges.
In southern Arizona, the Rocky Mountains reach their southern limits, while the Sierra Madre Occidental of western Mexico reaches its northern limits here.
The uplift of these great formations occurred during the Miocene, giving rise to rich biotic communities which now overlap in southern Arizona.
Changes in climate began early in the Miocene and continued intermittently through the glacial periods of the Pleistocene.
These influences probably enabled northward movement of Sierra Madrean animals and southward dispersal of Rocky Mountains-Great Plains species which were adapted to the cooler climate at higher altitudes and latitudes.
When hotter, drier climates returned to the lowlands, many of these species became isolated in the mountains where suitable habitats remained.
A number of species dispersed northward from Mexico during interglacial periods.
Madrean alligator lizard (Elgaria kingi)
Sonoran mountain kingsnake (Lampropeltis pyromelana)
northern blacktailed rattlesnake (Crotalus molossus)
As a result, such generalists are more widely distributed in our region than specialists like the rock rattlesnake (Crotalus lepidus) and the twin-spotted rattlesnake (Crotalus pricei).
Particularly adapted to rocky canyons and talus rock slides at moderate to high elevations, these species probably crossed lower land bridges during glacials.
Now, wider dispersal is not possible due to the warmer, drier conditions in the lowlands.
HOW SPECIES EVOLVE
ISOLATION AND NATURAL SELECTION
Physical isolation is a major influence in the evolution of living organisms.
Once separated from ancestral lineages, changes in form, color, physiology or behavior often occur in response to selective pressures in the environment.
Depending upon the length of isolation and the extent and nature of other selective forces, such changes may be subtle or more obvious.
Relatively short periods of isolation under similar conditions usually result in minor variations of color, pattern or other physical features.
These are sometimes scientifically described as geographic races, or subspecies.
When isolation has occurred for longer periods of geologic time, the changes can be dramatic, producing distinctive species which often differ from their nearest relatives not only in appearance, but also in physiology and behavior.
A number of interesting and unique amphibians inhabit the cool, seasonally-moist montane habitats in the Sierra Madre Occidental.
The mountain treefrog (Hyla eximia) is found above 5000 feet, usually in coniferous woodland, although it may be found in mesquite grassland scrub in Mexico.
Populations are usually situated near permanent water, where breeding congregations form during summer rains. Substantial populations occur north of the Mogollon Rim.
Further south in Arizona, it is known only from the Huachuca Mountains. It ranges extensively through the Sierra Madre Occidental of western Mexico.
The barking frog (Hylactophryne augusti) is named for its dog-like call. Toad-like in appearance, the skin is relatively smooth and a fold of skin is apparent behind the head.
A circular fold of skin on the belly may aid the frog in adhering to rocky retreats. The barking frog presents an unusual posture as it walks on its toes with the body elevated well off the ground.
The western race (H. a. cactorum) reaches its northern range limits in the Pajarito and Santa Rita Mountains where it is more often heard than seen. It calls sporadically, making it difficult to locate.
Only three specimens from Arizona are known. Because of the paucity of records, its distribution is unclear. Considered endangered in Arizona, its secretive nature may present an inaccurate picture of rarity.
The barking frog is a member of the tropical frog Family Leptodactylidae, and is unique among regional anurans in reproducing entirely on land, without dependence on standing water for metamorphosis.
The large-yolked eggs are laid during rainy periods, often in moist seepages of limestone caves and fissures. Tadpoles complete metamorphosis inside the eggs and emerge as tiny froglets.
Two true frogs (Family Ranidae) are prominent in the Madrean mountains. The Chiricahua leopard frog (Rana chiricahuensis) is a member of the wide-ranging leopard frog (Rana pipiens) complex. A number of distinct species are now recognized to exist within this group, in spite of their superficially similar appearance.
The Chiricahua leopard frog is the upland representative, thriving in the vicinity of permanent rocky mountain streams in pine and oak woodlands. It establishes quickly in local artificial ponds and tanks.
This frog is considered threatened in Arizona because of habitat destruction, pollution, and the introduction of the bullfrog which is both predator and competitor. Further studies are needed to accurately determine the status of this formerly common upland riparian inhabitant.
The Tarahumara frog (Rana tarahumarae) no longer exists in Arizona. The last individual was found dead in the Santa Rita Mountains in 1983.
Fortunately, sound populations occur further south in Sonora and points beyond in the Sierra Madre.
The probable cause of its demise in Arizona is exposure to toxic metals leached from rock and soil by acid rain caused by copper smelter effluents, particularly sulfur dioxide.
This theory is plausible because of the proximity of extirpated populations to the copper smelters in Douglas, Arizona and Cananea, Sonora.
The highly aquatic Tarahumara frog is always found a hop or two from water, usually a rocky stream with sporadic "plunge pools". These bedrock depressions hold water during the summer period preceding the monsoons.
During such dry periods, Tarahumara frogs may abandon their preference for moving water and gather around these temporary oases.
Although Tarahumara frogs resemble their introduced nemesis, the bullfrog, they are readily distinguished by the presence of dorsolateral folds and indistinct eardrums.
Bullfrogs lack dorsolateral folds and have very distinct eardrums.
As with leopard frogs, bullfrogs are considered deleterious to Tarahumara frog populations. Tarahumara frogs are unusual among true frogs in apparently lacking a distinctive call.
Salamanders are not well-represented in the Sonoran Desert region. Most are members of the mole salamander Family Ambystomatidae, the best-known of which is the tiger salamander (Ambystoma tigrinum).
In the mountains, the prominent salamander is the Tarahumara salamander (Ambystoma rosaceum)(Fig. 15b). It inhabits most of the same rocky permanent Madrean streams occupied by the Tarahumara frog. It does not occur in the United States, but reaches the borderlands in the Sierra de los Pinos just south of Nogales.
Like others in its family, the Tarahumara salamander often demonstrates NEOTENY, in which sexual maturity is reached in the gilled larval form.
Water temperature seems to be the principal stimulus affecting metamorphosis. Transformation often occurs when water temperatures rise above about 15ø C.(60øF.). The permanently cold streams over much of its range may account for the high incidence of neoteny observed.
The most unusual salamander in the Sonoran Desert region is found in the Sierra Madre of Sonora, near Yecora.
The Yecora salamander (Pseudoeurycea belli sierraoccidentalis), is the northernmost form of a species ranging widely through the Sierra Madre Occidental and Sur of western and southern Mexico.
It is a member of the Family Plethodontidae, the lungless salamanders, so named because they lack functional lungs, respiring entirely through their skin. Such salamanders require cool, moist micro-habitats in which to live.
The Yecora salamander is restricted to two known localities in Sonora and adjacent Chihuahua where sufficient moisture is retained year-round in the cool forest substrate.
Moist soil beneath fallen, often saturated pine and oak logs seems to be the preferred micro-habitat. Subadults have been found inside rain- saturated logs.
The Yecora salamander is unique among the salamanders of western Mexico in laying eggs on land. The larvae complete metamorphosis inside the egg, emerging as tiny terrestrial salamanders.
Most snakes and lizards rely heavily on camouflage for protection from predators.
Natural selection reduces conspicuous individuals over time, leaving those less obvious to predators to pass along their genes for concealment.
Eventually, the population achieves a degree of background matching sufficient to insure its survival, although this does not protect every individual.
Rattlesnakes are masters of camouflage in their natural environments. Their color, pattern, and behavior are clearly influenced by the specific habitats in which they have evolved.
Spiny lizards (Sceloporus sp.) and horned lizards (Phrynosoma sp.) are good examples of physiological evolution in response to climatic conditions.
Species which occur in lower desert habitats reproduce by laying eggs.
The short-horned lizard (P. douglassi), is adapted to higher elevations (up to 11,300 ft.) and latitudes (southern Canada). It is more cold-tolerant than most other horned lizards and has evolved a viviparous (live-bearing) mode of reproduction. The cooler, shorter summers are less favorable for external incubation of eggs.
The mountain spiny lizard (Sceloporus jarrovii) occurs as high as 11,000 feet. It also bears live young, in contrast to other spiny lizards of the region. It is typically seen on rocks and logs in wooded habitats.
The males sport a black collar lined with white and are further distinguished by reflective patches of rich blue on the throat and belly. These conspicuous colors serve to announce territories to other males and may attract the more drably-colored females.
The Madrean alligator lizard (Elgaria kingi) lives primarily in or near montane riparian canyons.
It closely resembles the ridgenosed rattlesnake in color and pattern. This is an example of evolutionary convergence in background matching with the woodland leaf litter in a similar micro-habitat.
Prowling among the leaf litter in search of arthropods, it can be quite elusive when pursued. One is as likely to catch a section of writhing tail as the lizard.
This function, called TAIL AUTOTOMY, is found to varying degrees in many lizard groups. Tiny fracture planes in the vertebrae of the tail allow easy breakage and blood vessels constrict quickly to reduce blood loss. The detached tail continues to move, attracting the predator's attention as the lizard (often) escapes.
The Madrean alligator lizard is sometimes found as low as 2400 feet in riparian corridors from the higher mountains.
The northern twin-spotted rattlesnake (Crotalus pricei), associated mainly with the Sierra Madre Occidental, is smaller than most desert rattlesnakes.
The gray ground color conceals these diminutive pit vipers from detection among the rhyolite talus. They are often noticed only when their tiny buzz, resembling the sound of a flying grasshopper, is heard.
The dark gray color also aids in regulation of body temperature by absorbing solar warmth rather than reflecting it. The small body mass reaches a preferred activity temperature quickly.
These features are clearly advantageous to an ectothermic animal living as high as 10,000 feet. This ability to be active at lower ambient temperatures, using solar energy as the key, enables them to succeed in the rocky highlands, even during the winter.
The banded rock rattlesnake (Crotalus lepidus), as its name implies, is almost always associated with rocks.
It is often found near canyon stream beds where it favors the crevices of granitic outcrops and the talus fallen from them.
The rock rattlesnake is a master of crypsis in this micro-habitat, the bands resembling the cracks and fissures of the rocks.
Males in many populations have a greenish sheen along the back which closely matches the lichens which decorate the relatively moist, rocky canyons they favor.
Rock rattlesnakes are quite catholic in their diet, accepting small rodents, lizards, other snakes, and even centipedes as prey.
The ridge-nosed rattlesnake (Crotalus willardi) is a Sierra Madrean pine-oak woodland generalist which reaches its northern range limits in southern Arizona, where it is recorded from the Santa Rita, Huachuca, and Patagonia Mountains. Its range extends southward through the mountains of eastern and central Sonora.
The brown color and broken pattern effectively conceal it as it moves among the leaves and needles of the woodland floor. The white lines on the face disrupt the outline of the head, blending especially well with bunch grass and pine needles in its primary habitat.
Some have suggested that the Apache Indians who inhabited some of the same mountain ranges may have modeled their war paint after the facial markings of the ridgenosed rattlesnake!
Its diet includes lizards, small mice, birds, other snakes, and even invertebrates such as scorpions and centipedes.
A small species, rarely reaching 2 feet in length, it is most often encountered in montane deciduous hardwood canyons among rocks and leaf litter, although it inhabits the wooded slopes as well.
The Sonoran mountain kingsnake (Lampropeltis pyromelana) is the most colorful snake in the mountains of the Sonoran region. It is tri-colored, with red, black, and cream colors often calling the western coral snake to mind.
But the colors of the Sonoran mountain kingsnake are not as neatly arranged in rings as in coral snakes; and the red and black colors are always in contact, separating red from cream. The Sonoran mountain kingsnake has a cream-colored nose, in contrast to the black head and nose of the western coral snake.
Sonoran mountain kingsnakes are generalists in mountain habitats, but are often found in the vicinity of permanent or intermittent streams.
They feed chiefly on mountain spiny lizards, but small rodents and snakes are also taken.
Sonoran mountain kingsnakes are usually found on the ground, hiding in rockpiles, logs and underground.
They are, however, exceptional climbers, capable of negotiating the irregularities of pine bark on a vertical tree without the benefit of branches.
SIDE BAR STORY: Arizona's Protected Rattlesnakes
Like all wildlife, rattlesnakes are important components of their natural environments and should not be needlessly destroyed or removed from the wild.
In Arizona, the ridge-nosed rattlesnake, banded rock rattlesnake, twin-spotted rattlesnake, and desert-grassland massasauga are protected under state wildlife regulations.
They may be legally collected only under a scientific collecting permit issued by the Arizona Game and Fish Department.
While not endangered, these species are highly prized by reptile collectors all over the world. This demand has created commercial incentives which, combined with their well-known specialized habitat preferences, make them particularly vulnerable to unscrupulous commercial collectors.
Insects and other arthropods abound in the mountain habitats. Some are particularly conspicuous because of their color or behavior.
In the mountain canyons of the Sonoran region, countless numbers of the colorful convergent ladybird beetle (Hippodamia convergens) often decorate grass clumps and shrubs in warm areas before hibernating for the winter.
Ladybird beetles, or "ladybugs", are beneficial insects which feed on aphids and the eggs and larvae of other potentially harmful insects.
Another gaudy group of beetles in these mountains are the shining leaf chafers (Plusiotis gloriosa; P. beyeri and others).
These extraordinarily beautiful scarabs are light metallic green in color. Some species have gilded outlines along the edges of the body.
Feeding on oak and juniper, the adults are well-concealed by day. At night, they fly about, often attracted to black lights set up by intrepid entomologists.
Some authorities believe these beetles are becoming endangered due to over collecting.
The large, beautiful moths of the Lepidopteran Family Saturniidae are the delight of insect collectors the world over.
The Sonoran region mountains are home to a variety of species, most of which feed on oak and other tree leaves as larvae.
The often elaborately adorned caterpillars can consume a remarkable volume of plant material prior to pupation.
Scorpions occupy nearly all natural habitats in the Sonoran region. One of these, sometimes called the Pe¤a Blanca scorpion (Diplocentrus spitzeri), is commonly encountered in the mountains south of Tucson.
It reaches about 50 mm (2 inches), and is light to medium brown in color, providing good protection on its leaf-covered woodland substrate. Like all scorpions, it seeks moist micro-habitats and is most readily found during the summer monsoons.
The venom of this scorpion effectively subdues its arthropod prey. It was once thought to possess a venom dangerous to people, but this has not been confirmed. However, defensive stings are locally and temporarily painful.
Like other scorpions, it is viviparous, producing relatively small litters of about 9 babies. Centipedes (Scolopendra sp.) are also prevalent in montane environments, hiding under rocks or beneath the soil and emerging when conditions are cool and moist.
Ferocious predators, they are fed upon in turn by other predators, including the ridge-nosed and banded rock rattlesnakes, which must risk their own fates when tackling these awesome arthropods.
Arachnids are well represented by spiders, scorpions, and the unusual whipscorpion(Mastigoproctus giganteus) of the order Uropygi.
It can be found under logs and rocks where it waits for moist evenings to venture forth in search of invertebrate prey which it captures with its formidable pincers.
Also known as the vinegaroon or vinegarone, it secretes acetic and caprylic acid which is dispersed by the whiplike abdomenal appendage when confronted by a predator. It usually is found near grassy arroyos with scattered oak and juniper.