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Rattlesnakes!
Amphibians and Reptiles
in the Headwaters of the San Diego River

There are few pristine places remaining in San Diego. Undisturbed environments are in short supply. The San Diego River, the prominent watercourse running through Mission Valley, is a mixture of both. A continuum from disturbed to undisturbed habitats runs toward the east. The lowland sections, nearer to the coast, run through urban centers that have had a long history of human disturbance. In comparison, the headwaters of the river, located to the east of El Capitan Reservoir, contain long sections of pristine areas with clear-water creeks and tree-lined streams. A good indicator of the relative health of these watercourses is the species diversity and abundance of amphibians and reptiles living there. Salamanders, frogs, gartersnakes, and freshwater turtles rely on aquatic habitats for their livelihood. The less disturbed, the more likely they will thrive.

Boulder Creek, photo by Brad Hollingsworth

The remoteness of these headwater tributaries has contributed to their preservation. Many are surrounded by dense stands of chaparral, and roads are scarce. Boulder Creek, running from Cuyamaca Reservoir to the San Diego River, is a good example. Getting to remote sections of Boulder Creek requires a great deal of effort, but its relatively untarnished ecosystem provides a fabulous insight into the evolutionary diversity of this region and is worth the undertaking. Boulder Creek flows year-round and cuts through steep canyons. Granite basins have been eroded to form ponds and pools, some over 30 feet deep, but now filled with sediment. Stretches are choked with willows, while other sections are open and lined with live oaks and sycamores. Amphibians and reptiles flourish here, living in great abundance.

Taricha torosa, California newt,  photo by Brad Hollingsworth

The California Newt (Taricha torosa) is one of the more prominent members of Boulder Creek's aquatic community. This is the species southernmost population, its last outpost. The California Newt lives a dual life as either a terrestrial eft or an aquatic newt. During the late summer and fall months, this species has a terrestrial existence, hiding under logs and in rock crevices. After the first winter rains, the terrestrial efts will migrate to the water and transform into an aquatic newt. The body of the male makes a dramatic transformation. Its rough, glandular skin becomes smooth, the tail becomes fin-like, and the cloacal vent becomes engorged in preparation for breeding. Males also develop nuptial pads on their hands and feet so they can hold females during courtship. Their ability to breed is closely tied to their aquatic existence. The males lack a copulatory organ but deposit a package of sperm, the spermatophore, in the water for the female to pick up with her cloacal lips for internal fertilization.

In Boulder Creek, newts can be found walking and swimming in small and large pools, both in the daytime and at night. They are somewhat immune to predation because they produce potent toxins (tetrodotoxin and tarichatoxin) in their skin and tissues. These toxins are strong enough to kill large vertebrates, so eating newts is selected against in native predators. Newts are brown above and orange below. They are known to display their bright orange coloration when harassed by arching their back and turning their legs upward. Their coloration allows them to be cryptic when viewed from above, but also gives them a chance to warn potential predators.

Unfortunately, in less pristine areas, newts are susceptible to introduced predators, such as mosquitofish (Gambusia affinis) and crayfish (Procambarus clarkii). These exotics either pay no regard to newt toxins, or eat the newt's larvae. The presence of exotics is also known to disrupt courtship and drive newts from the water. Fortunately, Boulder Creek appears to be free from exotics, and the abundance of newts reflects it.

Pseudacris cadaverina, California Treefrog, photo by Brad Hollingsworth

Other amphibians in Boulder Creek are treefrogs. Both the California Treefrog (Pseudacris cadaverina) and Pacific Treefrog (P. regilla) can be found in abundance. Few places exist where both treefrog species occur in exactly the same place. In Boulder Creek, both can be found within a few feet from one another. The Pacific Treefrog has slightly longer legs and a prominent eye-stripe. In addition, males have a different call, helping the females of the two species discern "us" verses "them." During spring and summer nights, males will set up territories and call by resonating sound in their vocal sacs. Against the canyon walls, the chorus of frogs is maddening, but a trained ear can distinguish one species from the other, the alpha males, and clamorous arguments (likely over space or females).

The California Treefrog is more abundant in areas with granite boulders, while the Pacific Treefrog prefers aquatic vegetation, algal mats, and cattails. These habitats are intermixed in Boulder Creek. California Treefrogs are light gray with a darker mottled pattern, similar to the color of the granite stone. This makes them difficult to see against the granite rocks in a classic example of substrate matching. Natural selection has the ability to remove color pattern variants, because predators are better able to see the standout patterns. Eventually, only difficult-to-see color patterns remain in the population. At night, when these frogs are active, having a cryptic color pattern is puzzling. Nocturnal predators hunt mostly by sound and smell, so camouflage is of little use. The mystery is quickly solved when watching California Treefrogs during the day. During a casual stroll down the creek, one can see dozens of individuals basking on rocks in full sunlight. They keep their legs tucked along the sides of their bodies and only spring to life when you get too close. The selecting pressure for matching the granite stone likely comes from diurnal predators, such as birds.

Thamnophis hammondii, Two-striped Gartersnake, photo by Brad Hollingsworth

Natural selection doesn't just change color patterns. The behavior of basking on rocks in the open is unusual for nocturnal amphibians. Frogs, in general, have problems with water loss and many have devised adaptations to compensate. The California Treefrog is peculiar because it doesn't possess any remarkable strategy for conserving water. Sitting on rocks in direct sunlight, sometimes yards from the water, dries their skin. Why this species sits in the open, under direct sunlight, when the Pacific Treefrog hides during the day is unknown. One possible explanation is that they are thermoregulating, perhaps to increase their body temperature for better digestion. Another possibility is that they are escaping from predators. Instead of birds from above, predators lurk in the water below. One such culprit is the Two-striped Gartersnake (Thamnophis hammondii), a diurnal predator known for eating frogs and other aquatic prey. These snakes are common in Boulder Creek and can be seen sitting along the shore and patrolling the shallows. Newt toxins would be enough to deter gartersnakes, but treefrogs do not have this luxury. It is possible that basking in the sun is naturally selected, tied to the evolution of substrate matching. Avoiding gartersnakes during the day, sitting out of the water, exposed these treefrogs to the sun and aerial predators from above. They jumped out of the pot and into the sunlight.

Clemmys marmorata, pond turtle, photo by Brad Hollingsworth

The headwaters of the San Diego River are rich with evolutionary diversity. The Pacific Pond Turtle (Clemmys marmorata) lives in small populations in the larger basins. These turtles no longer occur in many parts of the river, but they hold out here. Other species haven't been so lucky. According to the Museum's collections records, the Red-legged Frog (Rana aurora) has not been seen in San Diego County since the mid-1970s, but lived in Boulder Creek at one time. Their loss has never been fully explained. Still, the headwaters of the river contain pristine pockets of riparian habitat where amphibians and reptiles thrive. Their survival strategies evolved to handle challenges of the native ecosystem. These adaptations, the interaction of species, and the natural history of the river are preserved in tributaries like Boulder Creek. They remain snapshots of the entire river from a time that has now gone by.

Text and photos by Brad Hollingsworth, Ph.D., Curator of Herpetology