To judge from now-clichéd chase scenes in wildlife documentaries, you might think predator-prey interactions are simple and predictable: A predator sees, chases, kills, and then eats its prey. End of story.
But don't let such scenes fool you. Recent research indicates that some predator-prey encounters are remarkably intricate, and nuanced interactions can determine their outcomes. This research includes the collection and analysis of video recordings of interactions between rattlesnakes and their prey--including squirrels, chipmunks and lizards—by a research team led by Rulon Clark of San Diego State University.
Snakes: Here, there and everywhere
Clark studies rattlesnakes because these reptiles are key members of many food chains. A case in point: Clark's research has pegged the relatively small rattlesnake as the predator with the greatest overall biomass at Blue Oak Ranch Reserve, located on Mount Hamilton about 10 miles east of San Jose, Calif. The reserve is one of the 38 sites in the University of California's Natural reserve System.
Clark says the rattlesnake's surprising abundance in the reserve is partly due to its cold-blooded biology, which enables individuals to survive by consuming as few as two or three animals per year. Warm-blooded predators such as coyotes and bobcats must eat much more frequently.
Clark's research also indicates that rattlesnake densities in the reserve exceed those documented elsewhere throughout California, including the remote Mojave Desert. Clark attributes this abundance to the fact that the reserve has remained relatively undisturbed by ranchers and hunters for decades. Blue Oak's high rattlesnake density may represent the important influence of these carnivores in certain ecosystems, including those located relatively close to developed areas.
Despite the ecological significance of rattlesnakes, their predatory behavior has not been thoroughly studied. Masters of camouflage, rattlesnakes are furtive, making them difficult to find and follow in the field. What is known is that a successful rattlesnake attack involves three steps:
1) Striking and hitting a prey animal, usually from just 10 inches away.
2) Injecting venom into the prey animal, which may attempt to escape before succumbing to the venom.
3) Relocating the weakened prey animal.
Rattlesnakes usually complete the first and second steps of each attack in less than one second—too fast to be observed in detail with the naked eye. The snakes may also wait weeks or even months before attacking again. That means observing predation by rattlesnakes in the field usually demands more patience and luck than even the most dedicated researchers can muster.
Rattlers ready for close-ups
Funded by the National Science Foundation, Clark is solving many of these problems with high-tech help. He and his colleagues catch wild snakes and insert tiny radio tags into their tissues. The tags help scientists locate snakes hiding under rocks or beneath bushes. When they spot individuals poised in ambush coils, the researchers plant portable mini video cameras one to three meters away to film potential strikes. The film then gets transmitted to reserve headquarters via Blue Oak Ranch's wireless data network.
Video cameras can be employed on snake stakeouts as long as necessary to capture brief squirrel attacks. "Unlike people, they are as patient as a snake," Clark says.
Once a snake strike has been recorded, researchers can simply fast-forward the video until they get to the action. Viewing the film on a frame-by-frame basis can help reveal the exact distances, movements, and timing of the movements of rattlesnakes and their prey. The recordings thus provide more specific and precise information than live, non-filmed observations.
Clark's video recordings show that about 50 percent of strikes by wild rattlesnakes are unsuccessful. This research has also revealed that rattlesnake attacks are most commonly thwarted if the prey makes a rapid, evasive dodge during the fraction of a second after the rattlesnake starts to strike, but before the rattlesnake reaches the prey.
The researchers also discovered that rattlesnakes may strike out during any of the three steps required for a successful attack. For example, the researchers have observed rattlesnakes clearly strike their prey, but then fail to find the struck animal. In these cases, the researchers suspect, the rattlesnake knew it had not successfully envenomated its prey, and chose not to devote energy to a potentially fruitless pursuit.
The researchers have also observed rattlesnakes fatally strike prey, but then fail to relocate their envenomated prey after it succumbed to the venom.
An age-old arms race
Clark's special interests include interactions between rattlesnakes and their favorite prey: California ground squirrels. Young California ground squirrel pups account for up to 69 percent of a rattlesnake's diet, and up to 34 percent of the pups are lost to rattlesnakes.
The predator-prey relationship between rattlesnakes and California ground squirrels is ancient; it originated about 10 million years ago in the ancestors of these species.
As the two species evolved, they developed "a kind of arms race, where an adaptation by one of them triggered the evolution of an adaptation in the other," says Clark. A similar arms race occurs between increasingly strong antibiotics and microbes that are evolving increasing resistance to antibiotics.
Wag the squirrel
The rattlesnake's arsenal isn't limited to toxic venom. It also includes a pair of pit organs on its face that can sense infrared radiation, or heat. The pit organs help the snake find warm-blooded prey.
To counter rattlesnakes, California ground squirrels developed a rare ability to neutralize rattlesnake venom. So armed, California ground squirrels sometimes assertively stand their ground to rattlesnakes instead of quickly fleeing for fear of being bitten. The squirrels may vocalize to warn their pups or kick dirt at the snake while evading the reptile's defensive strikes. On rare occasions, squirrel may even attack and kill rattlesnakes.
In addition, a California ground squirrel may flag its tail, raising and wagging it back and forth, before entering underground burrows or patches of plants likely to conceal rattlesnakes. Tail flagging also increases the temperature of the tail.
Such tail flagging had previously been interpreted solely as warnings to other prey about the presence of rattlesnakes that had been seen by the flagging squirrel. But this interpretation has recently been challenged by observations that California ground squirrels may perform the tail signals whether or not they have actually seen a rattlesnake in the area.
In addition, California ground squirrels that have seen coiled rattlesnakes have been observed to approach, inspect and repeatedly tail flag near the snake for several minutes; this type of signaling appears to be directed at the coiled snake.
Such observations inspired Clark and his team to research the possibility that the squirrels may use tail wagging and tail heating, both of which may be seen by rattlesnakes, to communicate with predatory rattlesnakes in addition to other prey.
Tails you lose
Specifically, the researchers suspect that squirrels may perform two types of tail signaling:
1) "Vigilance advertising" performed before entering areas that are particularly likely to harbor rattlesnakes. The researchers suspect that the squirrel issues this type of signal to generically warn any rattlesnakes that may be nearby whether or not they have actually seen snakes nearby. Squirrels may perform vigilance advertising to demonstrate they are looking out for the predators and prepared to dodge their attacks. The tail flagging might thus inhibit rattlesnake attacks by convincing rattlesnakes that their attacks would be thwarted.
2) "Perception advertising" that is intended: a) to tell a recently discovered rattlesnake that it has been discovered by the flagging squirrel and will therefore be unable to take the squirrel by surprise; and b) to "out" the discovered rattlesnake to other nearby squirrels and other potential prey in order to raise their rattlesnake vigilance.
As increasing numbers of squirrels discover and advertize a rattlesnake's presence at any particular location, the rattlesnake's ability to complete successful surprise attacks there decreases. After rattlesnakes are discovered and "outted" by tail-flagging squirrels, they frequently abandon the area and move someplace where they would still have the advantage of surprise. Snakes often relocate even though flagging squirrels don't usually pose a serious, direct threat to their safety.
When squirrels flag their tails, they may inadvertently announce their presence to other types of predators such as birds of prey. Because other predators are not as dependent on the element of surprise as rattlesnakes, they would not necessarily be inhibited from attacking by tail flagging. So even though tail flagging may help protect squirrels from rattlesnakes, it may increase their risk of being attacked by other predators. For this reason, squirrels probably reserve their tale flagging for certain circumstances that have, as yet, not been specifically identified.
Meet the robotic squirrel
To test Clark's ideas about vigilance advertizing and perception advertizing from squirrels, he and his research team are recording controlled encounters between live rattlesnakes and a lifelike robotic squirrel which can wag its tail. What's more, its body can be heated by copper coils to anatomically correct temperatures, and its tail temperature can be increased above its body temperature during predator-prey interactions.
The robotic squirrel's body is made from a taxidermied skin, and is stored in squirrel bedding when off-duty to give it a realistic smell.
Testing tail signals
Because the robotic squirrel's behavior can be manipulated in ways that a live squirrel's behavior cannot, it can help test the responses of rattlesnakes to certain squirrel behaviors. For example, Clark's team is using the robot to see how live rattlesnakes react to a tail that is motionless, one that is wagging; and one that is wagging and heated.
Clark's goal is to help identify the functions of the tail signals. For example, the experiments could show that when the robotic squirrel's tail is wagging and heated, it inhibits attacks from rattlesnakes and compels rattlesnakes to abandon their ambush sites.
If the experiments show that a motionless tail does not elicit any particular response from rattlesnakes, it suggests tail signaling is designed to inhibit attacks and compel rattlesnakes to abandon their ambush sites.
On the other hand, the experiments could show that the robotic squirrel's tail behavior has no impact on rattlesnake attacks or retreats. Such results would refute the idea that tail flagging is used for perception advertising and vigilance advertising.
The experiments may also reveal that rattlesnake behavior is influenced by related factors such as the distance between the rattlesnake and the signaling robotic squirrel and the amount of time the robotic squirrel spends tail signaling.
The ascent of robots
In addition to creating a robotic squirrel, teams of scientists and engineers have recently created robotic models of bees, fish, lizards, cockroaches, rats, sage grouse, frogs and other organisms. These types of robotic creatures are currently being incorporated into studies addressing diverse topics, including, for example, the design of search-and-rescue robots and the development of methods to save schooling fish from oil spills and other natural disasters. Because of advancements in technology and the decreasing costs of robotics, increasingly sophisticated robotic models will likely soon be incorporated into more types of scientific studies.
—Lily Whiteman, National Science Foundation