David Duvall

Prairie rattlesnake seasonal migrations: episodes of movement, vernal foraging and sex differences

Abstract Functional study of animal migration in nature requires both description and empirical analysis of component episodes of movement dedicated to the potential solution of problems in survival. Good study systems are those exhibiting larger patterns of migration that can be partitioned empirically into functionally analysable, component episodes. This facilitates analysis of alternative explanations both for larger cycles of migration and the component episodes that comprise them. Free-ranging prairie rattlesnakes, Crotalus viridis viridis, are good subjects in this respect. Results of a long-term field study of radiotagged individuals indicate that while both males and females search for small mammal prey in the first half of the brief, 35-month summer in Wyoming, males but not females search for mates in the second half of the season. Females continue to forage for the duration of the season. Accordingly, males must search for and locate two stations in time and space (food and mates) each season, while females need search only for one (food). Given this difference, males and females exhibit predicted differences in key patterns of seasonal activity. Males exhibit significantly greater spatial searching efficiency during the vernal foraging episode than females, allowing males to concentrate foraging activities into the first half of the season, so that the second can be largely dedicated to a search for females. The mating system of prairie rattlesnakes in Wyoming is best described as prolonged mate searching polygyny.

Strike-Induced Chemosensory Searching by Rattlesnakes: The Role of Envenomation-Related Chemical Cues in the Post-Strike Environment

Rattlesnakes and many other viperids typically strike and release adult rodent prey (Gans, 1966; O’Connell et al., 1982; Radcliffe et al., 1980), allowing the envenomated rodent to wander up to 600 cm before succumbing to the venom (Estep et al., 1981). The snakes then follow the chemical trail left by the envenomated prey. Although this predatory strategy risks losing the prey, it avoids tissue damage that could result from rodent teeth and claws if the snake attempted to hold the struggling prey after the strike. Even some of the deadliest elapids exhibit this strategy when they prey upon rodents (Chiszar et al., under review; Radcliffe et al., 1982; Shine & Covacevich, 1982), indicating that rodents are formidable prey and that the strike-release-trail system probably appeared very early in the evolution of venomous snakes (see Marx & Rabb, 1965, for a discussion of viperid evolution).

The West Indian manatee (Trichechus manatus) lacks a vomeronasal organ

Completely aquatic marine mammals of the order Cetacea such as whales and dolphins have a reduced or absent olfactory system and neither a vomeronasal organ nor an accessory olfactory bulb. In comparison, seals, which are only partially aquatic, have olfactory and accessory olfactory systems including the vomeronasal organ. Thus, there seems to be a strong relation between the degree of adaptation to an aquatic environment and the degree of reduction in olfactory structures. Sirenians, such as manatees and dugongs, are another family of marine mammals which have secondarily adapted to a fully aquatic existence, yet there is dispute about the status of their olfactory structures. In the present study there was no evidence for a vomeronasal organ in the adult West Indian Manatee, Trichechus manatus. Additionally, the main olfactory system appeared quite rudimentary. These observations support the hypothesis that, in mammals, secondary adaptation to an aquatic environment leads to the reduction or loss of the olfactory senses.

Seasonal variations in spermatogenesis, testicular weights, vasa deferentia, and androgen levels in neotenic male tiger salamanders, Ambystoma tigrinum.

Significant seasonal changes were observed in spermatogenesis, testicular weights, vasa deferentia, and cloacal development as well as in plasma androgen levels (testosterone and dihydrotestosterone). Androgen levels were highest in the fall and lowest during the period of spermatogenesis. Low levels of androgens were also observed during the breeding season. Diameter of the vasa deferentia corresponded to presence of stored spermatozoa and not to plasma androgen levels. Testicular weights were independent of androgen levels and stage of spermatogenesis. Maximal testicular weight occurred at the onset of spermiation. Hypertrophy of the epithelial lining of the vasa deferentia and the condition of the cloacal gland complex were correlated closely to plasma androgen levels. Comparison of spermatogenetic events, androgen-dependent sex accessory structures, and plasma androgen levels suggests either differential release of pituitary gonadotropins on a seasonal basis or variable testicular responsiveness to gonadotropins.

Ontogenetic Variation in Growth and Sexual Size Dimorphism in a Central Arizona Population of the Western Diamondback Rattlesnake (Crotalus atrox)

The western diamondback rattlesnake (Crotalus atrox) is sexually dimorphic (adult males larger than females) throughout its range. A comprehensive analysis of the ecological and evolutionary sources of sex differences in growth and size requires a detailed understanding of growth trajectories during ontogeny. We studied a population of C. atrox in the upper Sonoran Desert of central Arizona. Specifically, we asked at what point during ontogeny males and females diverge in size: at birth, during juvenile growth, or as mature adults? We used mark-recapture data and a novel analysis of rattle characteristics to test for sex differences during each of these three ontogenetic stages. Our analyses suggest that male and female neonates are similar in size. Juvenile growth rates also appear to be uniform between sexes. However, males and females begin to diverge in size beyond sexual maturity, when female but not male growth rates become undetectable given our recapture intervals. Our results indicate that, when partitioned into its lifecycle components, sexual size dimorphism can be expressed at different stages and may be affected by a variety of behavioral, bioenergetic, and selective forces. Accordingly, studies of proximate and ultimate causation in sexual size dimorphism may benefit from consideration of the ontogeny of size separately in males and females.

Chemical cues used by prairie rattlesnakes (Crotalus viridis) to follow trails of rodent prey

Each of 10 prairie rattlesnakes (Crotalus viridis) was exposed to three types of trails after striking rodent prey (Mus musculus). One trail was made with mouse urine, another was made with tap water, and the third consisted of materials from mouse integument. The snakes exhibited trailing behavior only when integumentary trails were available. It was concluded that prairie rattlesnakes do not utilize urinary cues; instead they attend to materials associated with rodent skin and fur.

Initial Den Location by Neonatal Prairie Rattlesnakes: Functions, Causes, and Natural History in Chemical Ecology

The concept of adaptation has long been a central idea in the study of the survival and evolution of life on earth (e.g., Williams, 1966). The potential existence of biological adaptation is best thought of as a hypothesis to be tested or an idea to be approached, and not a phenomenon that is automatically inherent in each and every characteristic of a living system1s biology. To be considered an adaptation, sensu stricto, potentially important mechanisms and processes should be rigorously studied to determine if they do in fact figure significantly into an organism’s “fit” with its environment. Once such features of phenotype are determined to be genuinely critical to survival, subsequent analysis of the trait’s mechanisms can lead to much more full explanations of how things actually work, and why they may have evolved into the structural and functional mosaics that we now see.

Chemical and behavioral ecology of foraging in prairie rattlesnakes (Crotalus viridis viridis)

Free-ranging prairie rattlesnakes (Crotalus viridis viridis) exhibit lengthy vernal migrations upon emergence from winter hibernation. A series of laboratory experiments was designed to test hypotheses regarding the function and causation of vernal movements. Rattlesnakes obtained from Wyoming and Colorado populations were used. First, we hypothesized that the function of vernal movements is to locate small mammal prey. Second, we predicted that activeC. v. viridis use prey chemicals, as well as other cues, to decide whether or not rodents are present in an area. Third, we hypothesized that vernally active males would be more responsive to rodent prey and their odors than females, given observed differences in behavior in the field. Fourth, we predicted that rattlesnakes captured in Colorado would be more sensitive to prey odors than those obtained in Wyoming, because of disparate community structure and, hence, small mammal spatial distributions. As expected, snakes exhibited reduced activity, as well as certain other dependent measures reflecting predatory investigation, in arena zones containing either live rodents or their chemicals. However, responses to the latter were reduced in Wyoming rattlesnakes tested with chemicals from deer mice (Peromyscus maniculatus), relative to Colorado animals tested with chemicals obtained from house mice (Mus musculus). In contrast to patterns observed in nature, males and females exhibited almost no differences in overall responsiveness. Results are discussed in the context of simulation modeling and ongoing studies of prairie rattlesnake behavior.

Characteristics and non-rhythmicity of retinal hydroxyindole-O-methyltransferase activity in trout (Salmo gairdneri)

The validation and optimization of an hydroxyindole-O-methyltransferase (HIOMT) activity assay system for trout ocular enzyme was undertaken. High-performance liquid chromatographic analysis, cross-validated with a standard thin-layer chromatographic technique verified that HIOMT activity was present in trout ocular tissue by identifying the 3H-melatonin end product. This activity was observed in the neural retina and was restricted to those fractions containing the cytosolic component. The enzyme displayed bimodal pH optima, one at pH 7.6, the other at 8.4. N-Acetylserotonin was O-methylated preferentially and a Km of 4.5 X 10(-7) M was calculated for this substrate. The S-[3H]adenosylmethionine Km was 4.6 X 10(-6) M. Time of incubation and enzyme concentration studies were conducted. The analysis of retinal HIOMT activity in groups of trout killed over a day--night period did not detect a rhythm in enzyme activity. Results of these investigations are discussed in terms of the evolution of melatonin synthesis and functions.

Fossil and Comparative Evidence for Possible Chemical Signaling in the Mammal-Like Reptiles

The mammal-like reptiles, or the Synapsida, hold an important position in the study of tetrapod vertebrate evolution for several reasons. First, the pelycosaurs and their therapsid derivatives, the two groups that comprise the synapsids, probably evolved from the cotylosaurs (the “stem reptiles”) (Carroll, 1969), and gave rise much later to the mammals (Fig. 1). Indeed, the mammal-like reptiles were the dominant terrestrial tetrapods throughout the Permian and the Triassic, the period from roughly 300 to 190 million years ago (Romer, 1966). Given this position in phylogeny, these extinct members of the Class Reptilia have always held the interest of those interested in the origins and derivation of mammals (e.g., see Simpson, 1959; Crompton and Jenkins, 1979).