Adult Snakes Research Articles

The fitness correlates of scalation asymmetry in garter snakesThamnophis sirtalis parietalis

SummaryIn a field population in Manitoba, about 16% of Red‐Sided Garter Snakes (Thamnophis sirtalis parietalis) have a different number of ribs on either side of the body, as indicated by anomalous ventral scales. A study on congenerics shows that this asymmetry reflects disruption of embryogenesis by environmental factors, including suboptimal maternal thermoregulation during pregnancy.Correlates of this scalation asymmetry in adult snakes can clarify the degree to which (and pathways by which) the thermal environment that an individual experiences during embryogenesis affects its later fitness.Data on >4000 free‐ranging snakes reveal no significant association between asymmetry and fitness‐related traits such as body size, body condition, locomotor speeds or female ability to resist harassment from males.However, adult male snakes with scale asymmetry dispersed from the den‐based mating aggregations sooner than did symmetric males. In outdoor arenas, asymmetric males were less able to obtain matings than were symmetric males; and if asymmetric males did mate, they consistently used the hemipenis on the side of the body opposite to their extra rib.Our data suggest that careful thermoregulation by a female garter snake during pregnancy provides a fitness benefit to her sons after they mature, via sexual selection.

Does intraspecific niche partitioning in a native predator influence its response to an invasion by a toxic prey species?

Abstract The introduced and highly toxic cane toad (Bufo marinus) is rapidly spreading across northern Australia where it may affect populations of large terrestrial vertebrate predators. The ecological impact of cane toads will depend upon the diets, foraging modes and habitat use of native predators, and their feeding responses to cane toads. However, intraspecific niche partitioning may influence the degree of vulnerability of predators to toxic prey, as well as the time course of the impact of alien invaders on native species. We studied the diet of the northern death adder Acanthophis praelongus and their feeding responses to cane toads. In the laboratory, death adders from all size classes and sexes readily consumed frogs and cane toads. Diets of free ranging A. praelongus from the Adelaide River floodplain were more heterogeneous. Juvenile snakes ate mainly frogs (39% of prey items) and small scincid lizards (43%). Both sexes displayed an ontogenetic dietary shift from lizards to mammals, but adult males fed on frogs (49%) and mammals (39%) whereas adult females (which grew larger than males) fed mainly on mammals (91%) and occasionally, frogs (9%). Feeding rates and body condition of adult snakes varied temporally and tracked fluctuations in prey availability. These results suggest that cane toads may negatively affect populations of northern death adders in the Darwin region. However, we predict that different size and sex classes of A. praelongus will experience differential mortality rates over different timescales. The initial invasion of large toads may affect adult males, but juveniles may be unaffected until juvenile toads appear the following year, and major affects on adult female death adders may be delayed until annual rainfall fluctuations reduce the availability of alternative (rodent) prey.

A fish prey found in the coral snake Micrurus alleni (Serpentes: Elapidae) in Costa Rica

A fish prey found in the coral snake Micrurus alleni (Serpentes: Elapidae) in Costa Rica. The presence of a small specimen of the swamp eel Synbranchus marmoratus (84 mm total length) in the stomach contents of an adult coral snake Micrurus alleni with 692 mm total length from the Caribbean versant of Costa Rica is reported. This eel was swallowed headfirst.

Population Structure, Body Size, and Seasonal Activity of Black Swamp Snakes (Seminatrix pygaea)

We examined the ecology of a Black Swamp Snake (Seminatrix pygaea) population inhabiting an isolated wetland in the upper Coastal Plain of South Carolina. The observed population structure was skewed towards mature individuals, with neonates and juveniles underrepresented, perhaps due to trapping bias. The sex ratio was biased during May and June, with females outnumbering males, but no sex ratio bias was evident at other times of the year. Seminatrix pygaea showed sexual dimorphism in body size, with females being longer and heavier than males and males having relatively longer tails than females, but there was no difference in monthly growth rates. Approximately 76.3% of the mature females captured in May and June 1998 were pregnant. Therefore, most mature females in this population probably exhibit annual reproduction, while some undergo biennial reproduction. The majority of females gave birth in July or August, with a few births occurring in September and October, and perhaps even as early as May or June. Our mark-recapture estimates suggest a population density greater than 60 adult snakes per hectare, which indicates this small aquatic snake, endemic to the Southeast, may play a large role as both predator and prey within isolated wetland ecosystems.

Gastric Impaction in a Milk Snake, Lampropeltis triangulum, caused by Kalicephalus sp.

An adult male milk snake, Lampropeltis triangulum, presented with a history of anorexia of two months and regurgitation of a partially digested mouse that was ingested three months previously. Radiographs and ultrasound demonstrated several fluid-filled coelomic masses and suggested gastrointestinal impaction. The snake died during hospitalization. Gross necropsy findings revealed esophageal nematodes and gastric impaction due to a 10 cm × 2 cm × 2 cm firm solid white inspissated mass. Histopathologic findings included severe, diffuse chronic gastritis with ulceration, Kalicephalus sp. eggs within the mass, and chronic granulomatous coelomitis secondary to intralesional eggs.

Reproductive biology and stress of captivity in male brown treesnakes (Boiga irregularis) on Guam

This study was designed to describe the reproductive biology of a sample of wild-caught brown treesnakes and to determine the effect of increased food intake (nutrition) on the development of the testis and sexual segment of the kidney, and on sperm storage in the vas deferens in captive males. The wild snakes were trapped and preserved within 24 h, in December 1999, from a snake trap-line in northern Guam. In the nutrition experiment adult snakes were matched for snout–vent length and separated into three groups. Groups 1 and 2 were fed a high-calorie diet for 8 and 4 weeks, respectively. Group 3 males, the control group, received a low-calorie diet and were preserved at 7 weeks. In the wild sample all snakes above 1035 mm snout–vent length were undergoing spermatogenesis, had a well developed sexual segment of the kidney, and had sperm stored in the vas deferens. In all experimental groups, spermatogenesis and development of the sexual segment of the kidney was terminated or inhibited, and in most snakes sperm were absent from the vas deferens. Blood levels of stress hormones (i.e. corticosterone) and reproductive hormones (androgens) were not measured. However, a previous study reported that the stress of captivity causes an increase in plasma levels of corticosterone in brown treesnakes, and other studies on squamates indicate that high corticosterone levels can disrupt reproductive hormones and shut down the reproductive cycle.

Predatory Attacks to the Head vs. Body Modify Behavioral Responses of Garter Snakes

AbstractAn animal's response to predatory attack may depend upon which part of its body is the focus of that attack, because of differential vulnerability to injury. Many avian and mammalian predators direct attacks preferentially toward the prey's head, so simulated attacks that do not have this focus may elicit non‐natural responses. We ‘pecked’ 152 free‐ranging adult male garter snakes (Thamnophis sirtalis parietalis) in Manitoba either on the head or the midbody, and recorded their responses. The snakes’ antipredator tactics were affected not only by body size (larger snakes performed threat displays more often) and body temperature (hotter snakes were more likely to flee), but also by location of the attack. Pecks to the head generally resulted in snakes coiling and hiding their heads, often simultaneously elevating and wriggling their tails in an apparent distraction display. In contrast, pecks to the midbody stimulated either escape responses, or (in snakes that did not flee) open‐mouthed threat displays. More generally, antipredator tactics may respond in flexible ways to details of the predator–prey encounter (including attributes of the habitat as well as the morphology and behavior of both participants) and hence, experimental studies need to carefully simulate such details. The part of the body under attack may be an important factor in this respect.

Sexual dimorphism in head shape and diet in the cottonmouth snake (Agkistrodon piscivorus)

AbstractSexual dimorphism is a common phenomenon among animals. The usual cause cited for sexual dimorphism in animals is sexual selection acting through female choice or male–male combat. Natural selection acting to reduce resource competition between the sexes, however, is an important alternative evolutionary scenario, but this possibility has received little empirical study. Here this issue is addressed by examining the relationships among body size, head shape and the functional aspects of diet in the adult male and female cottonmouth snake Agkistrodon piscivorus. In this species, males are larger in overall body and head size. Whereas an analysis of gross head measurements (simple linear head dimensions) shows little dimorphism in head shape, a more detailed analysis of head shape (using digital images of the snakes’ heads) revealed some subtle, yet functionally significant, differences in head shape between adult male and female cottonmouths. Specifically, male cottonmouths have longer quadrate bones, and have greater lateral surface areas than females. Male cottonmouths also consumed relatively taller prey (prey size relative to snake body size) than conspecific females, and the sexes consumed significantly different proportions of prey. Because the size of the quadrate bone is a strong determinant of maximum gape in snakes, we suggest that the observed shape differences may reflect functional differences in maximum gape between similarly sized male and female cottonmouths. In turn, such differences in maximum gape width may explain why males consume taller prey than similarly sized females.

Sexual abstinence and the cost of reproduction in adult male water snakes, Nerodia sipedon

Low frequency of reproduction among iteroparous organisms is most often observed among female ectothermic vertebrates and is thought to be a strategy used to defer reproductive costs. We assessed reproductive costs of male water snakes (Nerodia sipedon) to determine why half of adult males abstain from reproduction each year. There was no evidence of a short‐term energetic cost of reproduction. Change in mass did not differ between reproductive and non‐reproductive males during the one‐month mating season or during the entire four‐month activity season. Changes in mass of reproductive males were similar at two sites in which the spatial distribution of females differed. However, there were size‐specific differences in growth and survival between reproductive and non‐reproductive males. Among reproductive males growth rate decreased with body size at a lower rate than among non‐reproductive males. Survival increased with body size for reproductive males, but decreased with body size among non‐reproductive males. Most of the differential survival between reproductive and non‐reproductive males did not occur during the mating season but rather during hibernation. Size‐related differences between reproductive and non‐reproductive males may reflect selection having eliminated low quality males from the larger size classes. Overall our results appear most consistent with there being high variance in male quality, such that the best males can bear the cost of reproducing and still grow and survive as well or better than low quality males that abstain from reproduction.

Aquatic locomotion and behaviour in two disjunct populations of Western Australian tiger snakes, Notechis ater occidentalis

Adaptive phenotypic plasticity can be viewed as an evolutionary strategy that enables organisms to match their phenotypes to local conditions. I studied two neighbouring populations of amphibious fresh-water tiger snakes, Notechis ater occidentalis, from Western Australia. One mainland population occurs around a lake, feeds primarily on frogs, and is under strong predation pressure, whereas the second population inhabits a small offshore island with no standing water and no known predators, and feeds primarily on chicks. The availability of water and its use as a habitat differ considerably between the two populations. I compared experimentally the locomotor capacities and behaviour of individuals from both populations in water, treating neonates and adults separately. Sex did not significantly influence any trait in both populations, in either adults or neonates. At birth, the two populations exhibited very little difference in swimming speed, stamina, apnoea capacities or behaviour. However, adult mainland snakes had greater apnoea capacities and faster burst swimming speed than island snakes. Mainland adult snakes also spontaneously swam underwater for longer, and a higher proportion filled their lungs while surface swimming compared with island specimens. This study suggests strong behavioural flexibility in tiger snakes depending on habitats.

Body size plasticity and local variation of relative head and body size sexual dimorphism in garter snakes (Thamnophis sirtalis)

AbstractFor snakes, prey size and sex are two possible determinants of head and body size. In garter snakes Thamnophis sirtalis, females are generally longer and have greater body weights than males, and also have larger relative head sizes, which may facilitate foraging success. The selective pressures that account for sexual size dimorphism in garter snakes have not been unequivocally demonstrated. In this study, the body (length and mass) and head (head length, head width, jaw length, inter‐ocular distance) sizes of garter snakes inhabiting two nearby but ecologically dissimilar sites with different types of available prey were compared. Overall, the adult female snakes were larger and had greater relative head sizes than males. Males from the two sites did not differ significantly in body or head sizes. However, the mean body length and mass of females from a site where vertebrates are included in the diet were greater than that of females feeding almost exclusively on earthworms. There were also significant site differences in all four head measurements in females, although the direction of the difference varied by site. Diet‐induced morphological plasticity is well documented and was evident in this study, although the relative roles of genotype, ontogeny, and competing selective forces in the expression of such plasticity can only be ascertained through future experimental studies.

SURVEY AND MONITORING OF THE EASTERN INDIGO SNAKE IN GEORGIA

We studied the federally threatened eastern indigo snake (Drymarchon couperi) from 1992 to 2002 in southeastern Georgia, including a 4-year mark-recapture study conducted on the Fort Stewart Military Reserva- tion. Indigo snakes in this region are sexually dimorphic in size, with males attaining greater maximum lengths. Subadult and small adult snakes grow more rapidly than larger adults. Georgia specimens prey on a variety of vertebrates, including juvenile gopher tortoises (Gopherus polyphemus). The return of adult indigo snakes to the same sandhills in multiple years has conservation and management significance. Long-term population monitoring of indigo snakes is feasible and may yield valuable information.

Intraspecific habitat partitioning by the sea snake Emydocephalus annulatus (Serpentes, Hydrophiidae): the effects of sex, body size, and colour pattern

An individual's sex, body size and colour pattern can influence its habitat use, and such partitioning can have important ecological and evolutionary consequences. We studied a system very different from those that have attracted previous research on this topic: sea snakes (Emydocephalus annulatus) in shallow-water coral-reef areas of New Caledonia. The snakes used habitats non-randomly in terms of substrate types and water depths, with frequent use of coral-rubble areas that also contained the nests of fish (damselfish and blennies) whose eggs are eaten by these snakes. Mate-searching adult male snakes were found across a broader range of habitat types than were foraging females and juveniles. Smaller snakes were found in shallower water. Colour polymorphism was evident (melanism increased with body size, and was more common in males than in females) but did not affect habitat use. The effects of colour morph on operative temperatures of physical models (evident in terrestrial situations) disappeared under water. Habitat use in this population is affected by a snake's body size and sex, but not by colour. Studies of terrestrial snakes have emphasized thermal or camouflage benefits of colour polymorphism, but the superficially similar polymorphism in E. annulatus is not consistent with either of these hypotheses and thus challenges their generality. Similarly, there was no dietary difference between age or sex groups and thus dietary partitioning cannot explain the observed intraspecific habitat partitioning.

SMALL-SCALE GEOGRAPHIC VARIATION IN ANTIPREDATOR TACTICS OF GARTER SNAKES

Garter snakes (Thamnophis sirtalis parietalis) in Manitoba court and mate at communal overwintering dens and then disperse to feeding ranges in summer. The restriction of mating activities to den areas may reduce gene flow between populations from adjacent dens, in turn allowing divergence in traits such as antipredator behavior. We quantified responses of adult male snakes to the approach of human observers at four dens, prior to dispersal in late spring. Larger snakes bit us more frequently than did smaller individuals, at all four dens. Although some of the dens were <20 km apart (and hence, individuals from each would likely encounter each other during summer), we found strong geographic differences in antipredator tactics (approach distance and propensity to bite). These differences may reflect genetic isolation of den populations, due in turn to den-based mating and philopatry. However, to rule out alternative explanations such as learned responses to different predation exposure, studies are needed on naïve snakes.

Non-invasive measure of body composition of snakes using dual-energy X-ray absorptiometry

Non-invasive techniques to measure body composition are critical for longitudinal studies of energetics and life histories and for investigating the link between body condition and physiology. Previous attempts to determine, non-invasively, the body composition of snakes have proven problematic. Therefore, we explored whether dual-energy X-ray absorptiometry (DXA) could be used to determine the body composition of snakes. We analyzed 20 adult diamondback water snakes ( Nerodia rhombifer) with a DXA instrument and subsequently quantified their body composition by gravimetric and chemical extraction methods. Body composition components scaled with body mass with mass exponents between 0.88 and 1.53. DXA values for lean tissue mass, fat mass and total-body bone mineral mass were significantly correlated with observed masses of lean tissue, fat and ash from chemical analysis. Using regression models incorporating DXA values we predicted the fat-free tissue mass, lean tissue mass, fat mass, ash mass and total body water content for this sample of water snakes. A cross-validation procedure demonstrated that these models estimated fat-free tissue mass, lean tissue mass, fat mass, ash mass and total-body water content with respective errors of 2.2%, 2.3%, 16.0%, 6.6% and 3.5%. Compared to other non-invasive techniques, include body condition indices, total body electrical conductivity (TOBEC) and cyclopropane absorption, DXA can more easily and accurately be used to determine the body composition of snakes.

A Comparison of Migration Rates of Radio‐ and PIT‐Tagged Adult Snake River Chinook Salmon through the Columbia River Hydropower System

Documentation of adult salmonid migration behavior in the Columbia River drainage is critically needed to assess the effects of dams on travel time and passage. In 2000, we compared the upstream travel times of passive integrated transponder (PIT)-tagged and radio-tagged adult chinook salmon Oncorhynchus tshawytscha from Bonneville Dam on the lower Columbia River to Lower Granite Dam on the Snake River. We found no evidence that radio tagging negatively affected chinook salmon behavior. In fact, their median travel time (14.1 d, range = 7.8-44.4 d, N = 113) was actually faster (statistically significant at P = 0.005) than that for PIT-tagged fish (median = 15.9 d, range = 8.9-67.4 d, N = 164) after accounting for temporal effects (i.e., Bonneville Dam passage date). However, we concluded this difference was not biologically significant and was probably related to differences in study design or data complications. This study indicated that radio transmitters weighing less than 2% of chinook salmon body weight and the handling associated with implanting those tags did not negatively affect temporal rates of fish movement from dam to dam.

A test of the thermal coadaptation hypothesis with black rat snakes ( Elaphe obsoleta) and northern water snakes ( Nerodia sipedon)

1. The thermal coadaptation hypothesis predicts that (1) ectotherms experiencing a narrow range of body temperatures in the wild will evolve to perform well over a narrow range of body temperatures and that (2) the optimal temperature for performance will be equal to the preferred body temperature of the species. 2. We tested the predictions of the thermal coadaptation hypothesis with black rat snakes ( Elaphe obsoleta) and northern water snakes ( Nerodia sipedon) because black rat snakes experience lower and more variable body temperatures than northern water snakes at our study site. 3. We measured swimming speed, tongue-flicking speed, and striking speed in black rat snakes, and swimming speed and tongue-flicking speed in northern water snakes. 4. Adult water snakes generally had narrower performance breadths and higher optimum performance temperatures than adult black rat snakes. 5. Performance breadths were the same for swimming, tongue flicking, and striking within adult black rat snakes, but performance optima for these behaviours differed significantly. Performance breadths differed and performance optima were the same for swimming and tongue flicking within adult northern water snakes. 6. The relative swimming performance of neonates of the two species was similar in breadth to that of adults, but the thermal optimum for neonate black rat snakes was higher than that of adults. 7. Overall, our results provided support for the thermal coadaptation hypothesis.

Food Habits of the Snake Psammophis phillipsi from the Continuous Rain-Forest Region of Southern Nigeria (West Africa)

The food habits of the snake Psammophis phillipsi were studied in the rain-forest region of southeastern Nigeria, where this is the most common snake species of the area. A total of 120 prey items were recorded from 327 specimens (73 juveniles; 55 gravid females); many gravid females contained prey. Lizards (mainly Agama agama and Mabuya skinks) were the most common prey type for adult snakes, both gravid and nongravid, and for subadults as well; small mammals were the second most common prey type for all snake categories. Insects and small snakes (including conspecifics) were occasionally consumed. Diet composition did not differ significantly between gravid and nongravid specimens, but gravid specimens more frequently contained small mammals and fewer lizards. Ontogenetic dietary change was relatively minor. Prey and predator mass were significantly related in both adults and subadults.

A radiotelemetric study of movements and thermal biology of insular Chinese pit‐vipers (Gloydiusshedaoensis, Viperidae)

Because of their low metabolic costs, ectothermic predators can specialise on prey resources that are available for only a brief period each year. Endemic pit‐vipers (Gloydius shedaoensis) on a small island in north‐eastern China offer an extreme example of this phenomenon, with adult snakes feeding only on seasonally migrating birds that are available for a few weeks in spring and autumn. We surgically implanted radiotransmitters in 16 pit‐vipers, and located the snakes at weekly intervals for the next 12 months to record activity, movements, home ranges, habitat use, body temperatures and associated environmental temperatures. The snakes were extremely sedentary, with daily displacements averaging &lt;2 m and home ranges &lt;0.3 ha. The snakes maintained diurnal body temperatures around 25°C throughout the active season. Diurnal activity was reduced in summer, when birds were unavailable. Males and females were similar in most aspects of their ecology, except that females were often found in trees in autumn (feeding) while males were more often on the ground at this time (probably mate‐searching). Males also tended to shelter under vegetation rather than under rocks. The low vagility, relatively low body temperatures and summer inactivity of these snakes may reduce energy expenditure, and thus facilitate survival in a habitat where food is available only in brief periods several months apart.

A RADIOTELEMETRIC STUDY OF MOVEMENTS AND SHELTER-SITE SELECTION BY FREE-RANGING BROWNSNAKES (PSEUDONAJA TEXTILIS, ELAPIDAE)

Radio tracking of 40 free-ranging eastern brownsnakes (Pseudonaja textilis) in an agricultural landscape in southeastern Australia clarified the spatial ecology of these highly venomous animals. Most snakes over-wintered in burrows within a small area on the bank of an irrigation canal, dispersing into agricultural land during the warmer months. The snakes sheltered overnight in burrows or soil cracks, reusing the same retreat on successive nights and moving about to forage during the day. Successive shelter-sites averaged 152 m apart, and the snakes moved between them on average every six days. Home ranges of adult snakes were small (average MCP = 5.8 ha), and did not differ significantly between years with varying prey abundance. Movement patterns depended upon the snakes' sex, reproductive condition and body size. Adult males moved earlier in spring than did the (smaller) adult females, moved further and more often, and had larger home ranges. Home range size increased with body size in males, but not in females. Shelter-site selection was influenced by the location of potential prey (House Mice, Mus domesticus) and the location of other snakes. When radio-tracked snakes moved, they generally traveled from areas of lower to higher prey abundance (as determined by mammal-trapping). Throughout most of the year, adult male snakes were avoided by females and by other males. Adult males rarely cohabited with other snakes, and their arrival at an occupied burrow generally induced the resident snake to depart. Our study thus provides the first strong evidence that agonistic interactions can influence the spatial ecology of snakes.