Behavioral Ecology of a Desert Ambush Predator: Assessing Movement Patterns, Habitat and Microhabitat Use, and the Innate Feeding Response of Eastern Black-Tailed Rattlesnakes (Crotalus ornatus) in the Northern Chihuahuan Desert
Research on the ecology and evolution of rattlesnakes has been sporadic over the past 80 years, but has seen recent expansion into the diverse fields of physiology, physiological ecology, life history, behavioral ecology, ethology, reproductive biology, chemical ecology, venom biochemistry and medicine, conservation, and many other subdisciplines. The development of small, implantable VHF radiotransmitters in the 1980s revolutionized research in the field of behavioral ecology for rattlesnakes, which are uniquely suited for radiotelemetry studies because they possess several morphological, physiological, and behavioral characteristics that are unique among terrestrial vertebrates. The widespread application of radiotelemetry by the mid-1980s advanced the model organism status of rattlesnakes for classes of questions that were difficult to address with other taxa. Radiotelemetry has been responsible for notable insights into several areas of rattlesnake behavioral ecology, including movement and habitat associations, predation and sensory ecology, defensive behavior, and rattlesnake socioecology. Ultimately, research in these areas of rattlesnake behavioral ecology is necessary in order to guide informed conservation plans for this ecologically important group of organisms. Thus, the following research attempts to accomplish the following objectives: 1) I utilized radiotelemetry to determine movement patterns of C. ornatus on the Indio Mountains Research Station, including the estimation of home range sizes, identification of core use areas, and examining seasonal and sexual patterns in movement metrics; 2) I used data collected during radiotelemetry to determine seasonal and sexual patterns of habitat and microhabitat use of C. ornatus; and 3) I presented naive young snakes with aqueous extracts from the integument of known and suspected prey items to determine innate prey preferences of C. ornatus. Recently, accelerated rates of taxonomic change have outpaced the subsequent ecological research that is necessary to inform conservation objectives for newly described or revalidated species. Black-tailed rattlesnakes in the southwestern United States are an example, with Crotalus ornatus, which was recently revalidated, occurring east of the Continental Divide, and C. molossus occupying the range west of the Continental Divide. This created a void in our ecological knowledge about C. ornatus because historical research on black-tailed rattlesnakes in the United States was mostly conducted at study sites west of the Continental Divide. Thus, radiotelemetry was used to monitor C. ornatus on Indio Mountains Research Station (IMRS) in the Trans-Pecos ecoregion of Texas for at least one active season from May 2015 through August 2018. Mean (±1 SE) home range size for all individuals was 22.84 ± 4.49 ha, mean movement frequency was 0.73 ± 0.03, and mean daily movement rate was 9.28 ± 0.93 m/day. Male snakes had larger home range sizes, larger core use areas, higher movement frequencies, and higher movement rates than female snakes. Female movement rates peaked in June, although movement rates were statistically similar for all months. Male movement rates were significantly greater during August, with slight peaks in July and June that were approaching significance. Multinomial logit models were used to analyze habitat use patterns, while controlling for subject, habitat availability, and season. Models predicted that snakes were most likely to be found in arroyo and rocky slope habitats, despite the limited availability of these habitats within snake home ranges. Microhabitats were also used non-randomly, with C. ornatus preferring areas composed primarily of rock and vegetative ground cover and seeking refuge in rocky structure or under dense vegetation. This study presents the first detailed information about habitat and microhabitat use, along with patterns of movement and home range size for C. ornatus. Chemical signals are left behind as organisms move throughout the environment. Chemical eavesdropping is the use of these signals by other organisms to gain information about the animals that they share the ecosystem with. There are two main avenues of chemical eavesdropping: 1) prey species using the signals to avoid predation by gaining information about potential predators, and 2) predators using the signals to gain information about prey species and identify fertile hunting areas. Despite its utility for predators, chemical eavesdropping has usually been examined from the perspective of potential prey items. Thus, the final goal of this study was to determine if C. ornatus differentiates among chemical cues from potential prey items when choosing ambush spots. Naive neonate snakes were raised in captivity, then placed in an arena where they were presented with aquatic extracts from the integument from 10 known and suspected prey items and a tap water control, after which their reactions to the chemical extracts were recorded, and they were given a tongue-flick-ambush score (TFAM) based on their behavior towards the chemical cue. Ten neonate C. ornatus were put through each of the 11 trials, and based on TFAM scores, snakes differentiated among the chemical cues (Fdf=9, df=10 = 19.149, P < 0.001). Snakes showed preference for native small mammal and lizard prey over larger native mammals, native amphibians, native invertebrates, and non-native small mammals. The snakes used in this study have never been exposed to wild prey stimuli in their native ecosystem and have fed exclusively on non-native mammalian prey in captivity, thus their preference for native small mammals and lizards indicates an innate basis for prey preference.
Emerson, James David, "Behavioral Ecology of a Desert Ambush Predator: Assessing Movement Patterns, Habitat and Microhabitat Use, and the Innate Feeding Response of Eastern Black-Tailed Rattlesnakes (Crotalus ornatus) in the Northern Chihuahuan Desert" (2020). ETD Collection for University of Texas, El Paso. AAI27956083.