Nerodia Sipedon Research Articles

Habitat requirements of two common aquatic snakes, Regina septemvittata and Nerodia sipedon sipedon

Regina septemvittata (Queen Snakes)and Nerodia sipedon sipedon (Common Watersnake) are semi-aquatic colubrid snakes that utilize streams for their primary source of prey. Both species are currently listed by the International Union for Conservation of Nature as stable. The goal of this study was to determine which specific habitat aspects affect the presence or absence of R. septemvittata and N. sipedon sipedon. Snakes were collected in the summer and fall of 2017 from August to October in North Fork of Short Creek, a 3rd order stream located just south of West Liberty, WV. Using the Ohio QHEI form to quantify stream quality and program PRESENCE to generate Logistic Regression Models, stream sub-habitat types were compared with occurrence data. Resulting models revealed which specific sub-habitat types were associated with positive occurrence. Univariate models selected for riparian zone determined by the width and predominant cover present with an AIC weight of 0.499. Multivariate models selected riparian zone with an AIC of 0.372 and riparian plus total stream health with AIC of 0.185. Our preliminary data indicates that snake presence is related to both riparian zone and water quality. This suggests that sufficient cover quality and stream health are required to maintain populations of Regina septemvittata and Nerodia sipedon sipedon.

Demographic effects of prolonged drought on a nascent introduction of a semi-aquatic snake

Faced with a nascent introduction of a non-native species, conservationists need to quickly determine how a population performs in its new environment. Although correlative models can predict environmental suitability at a coarse scale, they often neglect short-term climatic variability, instead relying on long-term averages. Accurately projecting the fate of any particular introduction requires demographic data on how a population responds to a novel environment. The recent introduction of watersnakes (genus Nerodia) poses a risk to California’s already imperiled aquatic vertebrate fauna. Despite inhabiting a seemingly suitable climate, a non-native Nerodia sipedon population in central California is likely to have been affected by a prolonged extreme drought from 2012 to 2015. We studied the only known population of N. sipedon in California for 3 years from 2013 to 2015, and estimated its abundance and annual survival. Its abundance declined from a peak of 218 (95% Credible Interval 149–313) in August 2013 to 97 (80–119) in July 2015. Annual survival of N. sipedon from 2013 to 2014 (0.23, 0.13–0.39) and from 2014 to 2015 (0.29, 0.18–0.41) was lower than survival estimates reported from native populations. Snake body condition, the abundance of large adult females, and prey availability all declined throughout the study. We conclude that the population of N. sipedon declined from 2013 to 2015, likely due to decreasing habitat and prey availability from the prolonged regional drought. This study highlights the importance of the effect of climatic extremes on the trajectory of introduced populations in a novel environment.

Effects of Moderate Food Deprivation on Plasma Corticosterone and Blood Metabolites in Common Watersnakes (Nerodia sipedon)

Abstract Lack of food is one of the most common natural stressors that animals face, yet the physiological response to food restriction in most nonmammalian species is poorly understood. Food restriction can elicit an elevation of plasma glucocorticoid hormones and changes in blood metabolites in several vertebrates, but this has not been shown in snakes, despite their remarkable ability to tolerate food shortages. The purpose of this study was to determine the physiological response to moderate food deprivation in Common Watersnakes (Nerodia sipedon). When food was withheld for 15 d, snakes lost body mass, had elevated baseline plasma corticosterone concentrations, lower hematocrit, and depressed levels of triglycerides and uric acid. Food deprivation had no effect on blood glucose or lactate. Elevation of corticosterone levels could help snakes mobilize stored energy to allow them to survive periods of restricted feeding. Depressed triglycerides likely indicate increased utilization of lipid stores. Bec...

Scanning Snakes to Measure Condition: A Validation of Quantitative Magnetic Resonance

Abstract Body composition is a measure of an animal's energetic state that can inform many research fields, yet the analysis traditionally requires individuals to be killed, and chemical analysis is labor intensive. Quantitative magnetic resonance (QMR) measures body composition noninvasively in live and nonanesthetized animals. Our aim was to validate QMR analysis for snakes by comparing it with gravimetric chemical analysis. We collected Northern Watersnakes (Nerodia sipedon sipedon) and Eastern Massasaugas (Sistrurus catenatus catenatus) that were found dead on roads, analyzed their body composition using the QMR scanner, and then by gravimetric chemical analysis. We compared fat mass, wet lean mass, and total water mass between the two methods, and then calculated bias, absolute error (g), and relative error (%) of the QMR analysis. Body composition values from the QMR analyses were highly correlated with the values obtained by gravimetric chemical analysis. Bias and errors were reasonable for wet lea...

Maternal investment and delayed feeding in neonatal Lake Erie watersnakes: a life‐history strategy

AbstractNeonatal growth can have lasting consequences on survival and reproduction. In many taxa, larger, faster growing neonates experience higher survival, reach sexual maturity more rapidly, and achieve higher lifetime fecundity. In contrast to their closest relatives, Lake Erie watersnakes grow slower, mature later, and may delay feeding until after their first hibernation. To determine if Lake Erie watersnakes do indeed delay feeding until after their first hibernation, we compared age class 0 (between birth and first hibernation) and age class 1 (emergence from hibernation and through the first full season) snakes in the field and in the laboratory. In the field, only 0.6% of pre‐hibernation neonates were found to contain prey, while 11.9% of post‐hibernation neonates contained prey. During captive feeding experiments, the probability of eating was positively correlated with age (binary logistic regression, Wald = 25.354, P < 0.001). To clarify the underlying mechanism for delayed feeding, we compared neonatal yolk and fat reserves among species with delayed feeding (Lake Erie watersnakes, Graham's crayfish‐eating snakes), species that commence feeding immediately after birth (common gartersnakes, Dekay's brown snakes) and a species falling between these extremes (queen snakes). Like Lake Erie watersnakes, Graham's crayfish‐eating snakes contained large energy reserves (yolk + fat body mass to carcass mass ratio = 0.39 and 0.24, respectively), common gartersnakes and Dekay's brown snakes lacked measurable reserves (0.00), and queen snakes contained intermediate reserves (0.17). Taken together, this evidence suggests that neonatal Lake Erie watersnakes do delay feeding until after their first hibernation and contain large energy reserves at birth to facilitate this behavior. While most snake life‐history studies have focused on the trade‐off between offspring size and number, Lake Erie watersnake females invest large amounts of energy in offspring condition.

Habituation of adult sea lamprey repeatedly exposed to damage-released alarm and predator cues

Predation is an unforgiving selective pressure affecting the life history, morphology and behaviour of prey organisms. Selection should favour organisms that have the ability to correctly assess the information content of alarm cues. This study investigated whether adult sea lamprey Petromyzon marinus habituate to conspecific damage-released alarm cues (fresh and decayed sea lamprey extract), a heterospecific damage-released alarm cue (white sucker Catostomus commersonii extract), predator cues (Northern water snake Nerodia sipedon washing, human saliva and 2-phenylethylamine hydrochloride (PEA HCl)) and a conspecific damage-released alarm cue and predator cue combination (fresh sea lamprey extract and human saliva) after they were pre-exposed 4 times or 8 times, respectively, to a given stimulus the previous night. Consistent with our prediction, adult sea lamprey maintained an avoidance response to conspecific damage-released alarm cues (fresh and decayed sea lamprey extract), a predator cue presented at high relative concentration (PEA HCl) and a conspecific damage-released alarm cue and predator cue combination (fresh sea lamprey extract plus human saliva), irrespective of previous exposure level. As expected, adult sea lamprey habituated to a sympatric heterospecific damage-released alarm cue (white sucker extract) and a predator cue presented at lower relative concentration (human saliva). Adult sea lamprey did not show any avoidance of the Northern water snake washing and the Amazon sailfin catfish extract (heterospecific control). This study suggests that conspecific damage-released alarm cues and PEA HCl present the best options as natural repellents in an integrated management program aimed at controlling the abundance of sea lamprey in the Laurentian Great Lakes.

The relationship of head morphology and diet among three sympatric watersnake species

Abstract Investigating dietary resource utilization and head morphology of gape-limited predators can provide a basis for understanding interspecific competition and species coexistence. For sympatric species, convergence of head morphology can indicate similar prey or foraging strategies while divergence can suggest competition, resource partitioning or expansion into new habitats. Sexual dimorphism can further complicate the head morphology-diet relationship by potentially reducing intraspecific competition. To understand the relationship between head morphology and diet, we studied three sympatric species, the plain-bellied (Nerodia erythrogaster), diamondback (N. rhombifer), and northern (N. sipedon) watersnakes in western Kentucky. All three species inhabit similar wetlands and feed on amphibians and fishes. The anurophagous plain-bellied watersnakes had longer, narrower heads that likely allow snakes to capture anuran metamorphs and froglets while facilitating movement through dense vegetation. The piscivorous diamondback watersnake had a wider head, which would enhance contact with prey in this open-mouth forager, and smaller interocular distance, resulting in dorsally placed eyes that facilitate seeing fish prey from below. The mostly piscivorous northern watersnake had an intermediate, generalized head shape, which may reflect the typically wide diet range of this species. Head shape did not differ between snake sexes but diamondback and northern females had larger heads than males and fed more frequently on fishes. The relationship between head morphology and diet highlights potential interspecific and intersexual competition and aids in the understanding of species coexistence patterns.

Effects of Maternally-Transferred Methylmercury on Stress Physiology in Northern Water Snake (Nerodia sipedon) Neonates.

Biomagnification of methylmercury in aquatic systems can cause elevated tissue mercury (Hg) and physiological stress in top predators. Mercury is known to affect stress hormone levels in mammals, birds and fish. In this study, the effects of maternally-transferred methylmercury on the stress physiology of Northern Water Snake (Nerodia sipedon) neonates were tested. Gravid females were dosed via force-fed capsules during late gestation with 0, 0.01, or 10µg methylmercury per gram of body mass. Plasma corticosterone levels and leukocyte differentials were analyzed in baseline and confinement-stressed neonates from all dose levels. Neither Hg nor confinement stress had a significant effect on leukocyte differentials nor was Hg related to corticosterone levels. However, stress group neonates showed lower heterophil/lymphocyte ratios and this study was the first to show that neonate N. sipedon can upregulate CORT in response to stress. These results indicate that N. sipedon may be somewhat tolerant to Hg contamination.

Only fear the fatal foe: predation risk assessment by eastern newts (Notophthalmus viridescens) in response to common snapping turtles and other potential predators

Many organisms utilize toxic or noxious compounds as a means of deterring predation. Eastern newts (Notopthalmus viridescens), along with other species in the family Salamandridae, possess a potent neurotoxin called tetrodotoxin (TTX). Although TTX can serve as an effective antipredator mechanism in species of newts with high concentrations (e.g., Taricha), eastern newts have relatively low levels of toxicity in comparison to those species, and it may not serve as an effective antipredator mechanism against all threats. In this case, they may benefit rather by utilizing behavioral changes to avoid initial contact with predators. We tested for predator-avoidance behavior in newts by exposing individuals to kairomones from various predators. We recorded activity patterns of newts when they were exposed to cues from potential predators including bullfrogs (Lithobates catesbeiana), water snakes (Nerodia sipedon) and snapping turtles (Chelydra serpentina), as well as a non-predator (bullfrog tadpoles), and a control (deionized water). Newts reduced activity when exposed to snapping turtle stimuli, but did not change activity when exposed to any other chemical cues. We verified that newts interact with this predator by trapping snapping turtles found in ponds from which newts were collected. Finally, we used turtles caught during this sampling to test whether they are an actual predator of newts and whether newts shift microhabitat use when exposed to this predator. In each replicate, turtles consumed newts, and newts spatially avoided the snapping turtle, relative to a control. The results of these experiments indicate newts rely on predator-avoidance behavior to reduce the probability of being consumed by snapping turtles, but do not reduce activity in response to other potential predators that may only consume them rarely.

Size Matters: Individual Variation in Ectotherm Growth and Asymptotic Size.

Body size, and, by extension, growth has impacts on physiology, survival, attainment of sexual maturity, fecundity, generation time, and population dynamics, especially in ectotherm animals that often exhibit extensive growth following attainment of sexual maturity. Frequently, growth is analyzed at the population level, providing useful population mean growth parameters but ignoring individual variation that is also of ecological and evolutionary significance. Our long-term study of Lake Erie Watersnakes, Nerodia sipedon insularum, provides data sufficient for a detailed analysis of population and individual growth. We describe population mean growth separately for males and females based on size of known age individuals (847 captures of 769 males, 748 captures of 684 females) and annual growth increments of individuals of unknown age (1,152 males, 730 females). We characterize individual variation in asymptotic size based on repeated measurements of 69 males and 71 females that were each captured in five to nine different years. The most striking result of our analyses is that asymptotic size varies dramatically among individuals, ranging from 631–820 mm snout-vent length in males and from 835–1125 mm in females. Because female fecundity increases with increasing body size, we explore the impact of individual variation in asymptotic size on lifetime reproductive success using a range of realistic estimates of annual survival. When all females commence reproduction at the same age, lifetime reproductive success is greatest for females with greater asymptotic size regardless of annual survival. But when reproduction is delayed in females with greater asymptotic size, lifetime reproductive success is greatest for females with lower asymptotic size when annual survival is low. Possible causes of individual variation in asymptotic size, including individual- and cohort-specific variation in size at birth and early growth, warrant further investigation.

Helminths of Liophis miliaris (Squamata, Dipsadidae): a list of species and new records

Summary In order to have better knowledge of the parasites of the common water snake Liophis miliaris (Linnaeus, 1758), a checklist of its helminths was produced based on a review of the literature and new records of worms identified during the course of a parasitological survey combining data from stool analysis (n = 22) and necropsies (n = 8) of specimens of this snake from Muriaé, state of Minas Gerais, Brazil. Thirty-one helminth species (two acanthocephalans, one cestode, 11 nematodes and 17 trematodes) were so far reported in L. miliaris in the Neotropical region, already including the records in the present study of Acanthorhabdias acanthorhabdias Pereira, 1927, Paracapillaria (Ophidiocapillaria) cesarpintoi (Freitas & Lent, 1934) and Strongyloides ophidiae Pereira, 1929. Taxonomic comments on these nematode species are given, and areas of occurrence of A. acanthorhabdias and P. cesarpintoi are expanded in southeastern Brazil. In addition, factors related to parasite richness of L. miliaris, which is likely related to its aquatic habits, are discussed.

Projecting invasion risk of non-native watersnakes (Nerodia fasciata and Nerodia sipedon) in the western United States.

Species distribution models (SDMs) are increasingly used to project the potential distribution of introduced species outside their native range. Such studies rarely explicitly evaluate potential conflicts with native species should the range of introduced species expand. Two snake species native to eastern North America, Nerodia fasciata and Nerodia sipedon, have been introduced to California where they represent a new stressor to declining native amphibians, fish, and reptiles. To project the potential distributions of these non-native watersnakes in western North America, we built ensemble SDMs using MaxEnt, Boosted Regression Trees, and Random Forests and habitat and climatic variables. We then compared the overlap between the projected distribution of invasive watersnakes and the distributions of imperiled native amphibians, fish, and reptiles that can serve as prey or competitors for the invaders, to estimate the risk to native species posed by non-native watersnakes. Large areas of western North America were projected to be climatically suitable for both species of Nerodia according to our ensemble SDMs, including much of central California. The potential distributions of both N. fasciata and N. sipedon overlap extensively with the federally threatened Giant Gartersnake, Thamnophis gigas, which inhabits a similar ecological niche. N. fasciata also poses risk to the federally threatened California Tiger Salamander, Ambystoma californiense, whereas N. sipedon poses risk to some amphibians of conservation concern, including the Foothill Yellow-legged Frog, Rana boylii. We conclude that non-native watersnakes in California can likely inhabit ranges of several native species of conservation concern that are expected to suffer as prey or competing species for these invaders. Action should be taken now to eradicate or control these invasions before detrimental impacts on native species are widespread. Our methods can be applied broadly to quantify the risk posed by incipient invasions to native biodiversity.

Operating Body Temperatures in a Snake Community of Northern Virginia

Thermal data were collected from 15 of 16 species of snakes found at the Mason Neck National Wildlife Refuge, Fairfax County, VA. Data recorded at each capture included the date, 24-hour military time, body temperature (BT), air temperature (AT), ground-surface temperature (ST), water temperature (WT) if in water, and the snake's activity (under cover, moving on land, basking, foraging, climbing, swimming, courting/mating). The purpose of this study was to determine the potential range of operating body temperature (OBT) of the individual species. The range of OBT is interpreted as the snake's operating temperature at its current environmental temperatures (ET), which can be used in comparisons with similar data from other North American regions, and represents the first such report from the Mid-Atlantic Region. The mean and ranges of BT, AT, ST, and WT are presented for the eight snakes with 20 or more records: Carphophis amoenus (n = 238), Coluber constrictor (204), Nerodia sipedon (67), Thamnophis sirtalis (55), Diadophis punctatus (54), Pantherophis alleghaniensis (43), Thamnophis sauritus (26), and Agkistrodon contortrix (24). New thermal records are reported for several of these species. The ranges of BT during activities are also reported. New temperature records are also reported for Virginia valeriae (n = 16 encounters), Storeria dekayi (12), Opheodrys aestivus (6), Lampropeltis calligaster (6), and Regina septemvittata (2).

The behavioural response of adult Petromyzon marinus to damage-released alarm and predator cues.

Using semi-natural enclosures, this study investigated (1) whether adult sea lamprey Petromyzon marinus show avoidance of damage-released conspecific cues, damage-released heterospecific cues and predator cues and (2) whether this is a general response to injured heterospecific fishes or a specific response to injured P. marinus. Ten replicate groups of 10 adult P. marinus, separated by sex, were exposed to one of the following nine stimuli: deionized water (control), extracts prepared from adult P. marinus, decayed adult P. marinus (conspecific stimuli), sympatric white sucker Catostomus commersonii, Amazon sailfin catfish Pterygoplichthys pardalis (heterospecific stimuli), 2-phenylethylamine (PEA HCl) solution, northern water snake Nerodia sipedon washing, human saliva (predator cues) and an adult P. marinus extract and human saliva combination (a damage-released conspecific cue and a predator cue). Adult P. marinus showed a significant avoidance response to the adult P. marinus extract as well as to C. commersonii, human saliva, PEA and the adult P. marinus extract and human saliva combination. For mobile P. marinus, the N. sipedon washing induced behaviour consistent with predator inspection. Exposure to the P. pardalis extract did not induce a significant avoidance response during the stimulus release period. Mobile adult female P. marinus showed a stronger avoidance behaviour than mobile adult male P. marinus in response to the adult P. marinus extract and the adult P. marinus extract and human saliva combination. The findings support the continued investigation of natural damage-released alarm cue and predator-based repellents for the behavioural manipulation of P. marinus populations in the Laurentian Great Lakes.

Oestrus in a secretive species: endogenous estradiol varies throughout the shed cycle and influences attractiveness in wild northern watersnakes (Nerodia sipedon)

Oestrogen (e.g., 17β-estradiol, E2) stimulates vitellogenesis, female sexual behaviour, and induces sex pheromone production throughout vertebrates. Therefore, the quantification of its role in any one these may prove challenging; particularly in taxa such as snakes where mating coincides with vitellogenesis. Studies examining steroid hormones in snakes are further confounded by the typical sampling interval (monthly) which is likely insufficient to observe the brief hormone fluctuations associated with an oestrus period. Thus, the relationship between oestrus and endogenous sex steroids in snakes remains equivocal. We sampled plasma E2 of 12 radio-equipped free-ranging adult female northern watersnakes (Nerodia sipedon) twice weekly during the 2008–2009 mating periods. Reproductive females experienced a large E2 surge coincident with shedding, movement, and male accompaniment indicating that endogenous E2 is involved in oestrus, a phenomenon that has previously not been documented in snakes.

It takes two to tango: female movement facilitates male mate location in wild northern watersnakes (Nerodia sipedon)

To facilitate location of mating partners, females of many taxa emit chemical signals (i.e., sex pheromones) to inform male conspecifics of their location and reproductive status. Males subsequently alter their movements to increase their likelihood of encountering females and this movement has been historically viewed as a primary determinant of mate location. However, because of the method of female sex pheromone release, particularly via terrestrial trails, female movement likely contributes to mate location; however, information on this topic is lacking. We monitored the movements of 27 free-ranging radio-equipped adult female northern watersnakes (Nerodia sipedon) during the 2007–2009 mating seasons to determine if females employed movement tactics to facilitate location by males. For a limited period following shedding, female movement increased. During this period, they were approximately five times more likely to be located by a male than during the remainder of the mating period. Further, females experienced maximum male mate location following shedding. Because increased movement is associated with increased costs, females may minimise these costs by restricting this risky behavior to a limited period of time when their attractiveness and/or receptiveness is presumed to peak.

Differential habitat use by Common Watersnakes (Nerodia sipedon)


 
 
 
 Understanding intraspecific variation in habitat use is important for the management of any species. In many studies of reptiles, habitat use by juveniles is poorly understood when compared to their adult conspecifics because of capture biases and logistical constraints. We compared habitat use between sexes and age classes of Common Watersnakes (Nerodia sipedon) at a reservoir in central Illinois. Juvenile N. sipedon occurred more frequently in habitat with high conopy cover, whereas adults N. sipedon, especially reproductive females, were found exclusively in habitat with no canopy cover. Adult males used both locations equally. We emphasize the need to investigate ontogenetic variation in habitat use to better understand how reptiles utilize diverse anthropogenically altered landscapes.
 
 
 

Snake lower jaw skin: Extension and recovery of a hyperextensible keratinized integument

The skin of squamates consists of a keratinized epidermis divided into thick scale and thinner, folded interscale regions underlain by a dermis containing a complex array of fibrous connective tissues. We examined the skin of the lower jaw of watersnakes (Nerodia sipedon) to determine how skin morphology changes when highly stretched during ingestion of large prey. Video records of skin behavior in the lower jaws of watersnakes feeding on fish or anesthetized watersnakes being stretched on an Instron machine showed that most skin extension involves the interscale skin. The largest intermandibular separation recorded during feeding was 7.7× resting distance, but intermandibular separation reached 10× without tissue failure during mechanical testing. Histological and anatomical analyses of lower jaws fixed in resting, moderately or highly stretched conditions showed that stretching had little effect on scale regions of the epidermis. However, stretching flattened folds of interscale regions at both gross and cellular levels and imposed changes in epidermal cell shape. Stretching of the dermis is primarily limited to realignment of collagen and stretching of elastin in the deep dermis. The configuration of dermal elastin suggests a model for passive recovery of epidermal folding following release of tension.

Intraspecific and interspecific variation of female genitalia in two species of watersnake

The morphological differences in female genitalia within and between species are little studied and poorly understood, yet understanding patterns of variation in female genitalia can provide insights into mechanisms of genital evolution. The present study aimed to explore the patterns of intraspecific and interspecific variation in female genitalia in two sister taxa of watersnake (Nerodia sipedon and Nerodia fasciata) that have similar genital shape. We used a geometric morphometric (GM) approach to study variation in shape of the vagina between and within two sister species. We examined genital shape in female watersnakes ranging from small, sexually immature females to large reproductive females that had recently given birth. We found that shape variation of genitalia is strongly correlated with body size, where larger but not smaller females have a bifurcation in the vagina. However, we also found significant shape variation in the structure of the vagina between the two species, where N. fasciata has narrower genitalia with more prominent bifurcation, whereas N.sipedon has wider genitalia with less marked bifurcation. Using GM allowed us to detect significant differences in genital shape that were not apparent upon visual examination alone, suggesting that shape variation in female genitalia may be greater than previously assumed. Additional study of morphological differences in male reproductive organs for these species would help to determine whether there has been genital co-evolution, and potentially mechanical reproductive isolation, in these two closely-related and occasionally sympatric species. © 2013 The Linnean Society of London.

Trapping Efficiency, Demography, and Density of an Introduced Population of Northern Watersnakes,Nerodia sipedon, in California

Northern Watersnakes, Nerodia sipedon, have been introduced into California's Central Valley and pose an important new challenge for the management of biodiversity in the state's already greatly distressed freshwater ecosystems. Nonnative watersnakes will likely compete with federally threatened Giant Gartersnakes, Thamnophis gigas, and prey on native amphibians and fish, including young salmonids, many of which are imperiled. We used three types of aquatic funnel traps and three different methods to estimate the abundance and density of N. sipedon in a small wetland in Roseville, California. Capture rates did not differ significantly among the three trap types but snakes captured in large box funnel traps were nearly 300 mm longer on average than those captured in minnow traps. Our estimates of the abundance of N. sipedon in our 2-ha trapping area were similar for the mark–recapture model, Leslie depletion curve, and the actual number of snakes removed over 57 days (112.4–119 individuals; approximately 56.2 snakes/ha). Extrapolating to the entire 6.2-ha aquatic area, the population likely numbered approximately 348 individuals. Several females were gravid, demonstrating successful reproduction by this species outside its native range. We caught more small N. sipedon compared with studies in its native range. This may be due to a sampling bias in our trapping methods but more likely reflects a population growing rapidly from a few initial founders with relatively fewer large adults. We recommend immediate action to prevent the spread and broader establishment of N. sipedon across the Central Valley of California.