Kent A. Prior

Genetic structure of populations of the threatened eastern massasauga rattlesnake, Sistrurus c. catenatus: evidence from microsatellite DNA markers

Throughout its distribution in North America, the threatened eastern massasauga rattlesnake (Sistrurus c. catenatus) persists in a series of habitat‐isolated disjunct populations of varying size. Here, we use six microsatellite DNA loci to generate information on the degree of genetic differentiation between, and the levels of inbreeding within populations to understand how evolutionary processes operate in these populations and aid the development of conservation plans for this species. Samples were collected from 199 individuals from five populations in Ontario, New York and Ohio. Our results show that all sampled populations: (i) differ significantly in allele frequencies even though some populations are < 50 km apart, and may contain genetically distinct subpopulations < 2 km apart; (ii) have an average of 23% of alleles that are population specific; and (iii) have significant FIS values (mean overall FIS= 0.194) probably due to a combination of Wahlund effects resulting from fine‐scale genetic differentiation within populations and the presence of null alleles. Our results imply that massasauga populations may be genetically structured on an extremely fine scale even within continuous populations, possibly due to limited dispersal. Additional information is needed to determine if dispersal and mating behaviour within populations can account for this structure and whether the observed differentiation is due to random processes such as drift or to local adaptation. From a conservation perspective, our results imply that these massasauga populations should be managed as demographically independent units and that each has high conservation value in terms of containing unique genetic variation.

Comparative demography of black rat snakes (Elaphe obsoleta) in Ontario and Maryland

This study investigated how demographic characteristics of black rat snakes Elaphe obsoleta are affected by the length of the active season, and also used the resulting demographic data to determine the proximate factors responsible for male-biased sexual size dimorphism (SSD) in the species. Demographic data collected from 1981 to 1998 in Ontario (ON; 583 males and 588 females) and from 1942 to 1976 in Maryland (MD; 180 males and 150 females) were used to develop growth models with the von Bertalanffy growth equation. Instantaneous growth rates declined significantly with increasing snout–vent length (SVL) in males and females of each population. The growth models predicted age using SVL satisfactorily, but the accuracy of the model decreased significantly with increasing SVL. As predicted, based on the brevity of their active season (ON [ape ] 135 days, MD [ape ] 190 days), rat snakes of both sexes from Ontario had lower and more variable instantaneous growth rates and matured at a more advanced age (ON [ape ] 9 years, MD [ape ] 4 years) than snakes from Maryland. However, the rapid growth and early maturation in Maryland snakes occurred at the expense of longevity (maximum: ON [ape ] 30 years, MD [ape ] 20 years). Slower growth and later maturation will make rat snakes in Ontario less capable of recovering from population declines. Within each population, males grew faster than females. Survivorship for Ontario snakes did not vary by sex, but increased significantly with increasing SVL. The demographic consequence of sex differences in growth was a male-biased sex ratio among larger snakes. Because SSD is a function of males growing faster than females, SSD in black rat snakes is probably a product of sexual selection, suggesting that large size confers a mating advantage in males.

Genetic analysis of populations of threatened snake species using RAPD markers

Snakes are a particularly threatened vertebrate taxon, with distributions of many species and populations becoming increasingly fragmented. At present, little is known about the degree of genetic differentiation that exists between isolated populations even though such information may be critical to their survival and conservation. As an example of how recently developed RAPD genetic markers can be used in conservation genetics, we present preliminary results from a study which used these DNA‐based markers to assess population divergence in two threatened Canadian snakes, the black rat snake (Elaphe o. obsoleta) and the eastern massasauga rattlesnake (Sistrurus c. catenatus). We present information on the levels of variation and reliability of amplification for fragments generated from five primers. We then use a recently developed analytical technique to estimate levels of nucleotide diversity within populations and sequence divergence between populations. Our results show that intrapopulation levels of divergence as estimated by the methods of Clark & Lanigan (Molecular Biology and Evolution 1993, 10, 1096–1111) approximate those found for mtDNA in vertebrates and that diversity between snake populations is small and non‐significant when tested using randomization procedures. Thus, our study provides an example of how RAPDs can be applied to conservation genetic studies of vertebrates and suggest that the snake populations we examined have only recently become isolated and maybe considered genetically equivalent from a conservation perspective, although this conclusion needs to be confirmed with other DNA‐based markers.

Preliminary observations of habitat use and movements of the eastern Massasauga rattlesnake (Sistrurus c. catenatus)

We used radiotelemetry to monitor 12 eastern massasauga rattlesnakes, Sistrurus c. catenatus, within Bruce Peninsula National Park (Ontario) for 419 days. The snakes exhibited non-random use of habitat, strongly associating with wetlands and coniferous forest, and avoiding open areas (roads, trails), open water, and mixed forest. Hibernation sites also occurred in wetland and coniferous forest habitat. By contrast, snakes were disproportionately captured in open areas, presumably because they are much more easily detected there

Habitat Features of Black Rat Snake Hibernacula in Ontario

Suitable over-wintering habitat is critical to the survival of snake populations at higher latitudes. The identification and protection of traditional, communal hibernation sites (hibernacula) is important for the conservation of threatened species, while the assessment of hibernacula availability may help determine the extent to which population distributions are limited by habitat suitability. In this paper, we quantified surface habitat characteristics of 10 hibernacula and the composition of basking trees used by a threatened population of black rat snakes (Elaphe o. obsoleta) at the northern limit of the species range. Hibernacula were typically situated on relatively rocky, south-facing slopes. The co-occurrence of these features was sufficiently unique as to distinguish hibernacula from (1) a series of random sites, but not from (2) a set of intuitively identified potential hibernacula in the surrounding landscape. This implies that additional requisite elements (e.g., underground structure and micro-climatic conditions), which we were unable to quantify, set actual hibernacula apart from sites that appear to be otherwise suitable (i.e., potential hibernacula). Basking trees found at hibernacula tended to be relatively large and decayed or dying with numerous cavities. Unused trees exhibiting these characteristics were also available at both random sites and potential hibernacula indicating the suitability of those sites for basking. Our results suggest that rat snake hibernacula cannot be predictably located by simply searching for key surface habitat features in the landscape. The current data are equivocal as to whether or not this most northern population is limited by the availability of suitable over-winter habitat since the possibility that essential subterranean features of hibernacula are limiting remains to be tested. We recommend the continued use of radio-telemetry to identify and protect additional hibernacula, the preservation of basking trees at known hibernacula, and further research to determine the internal structure and micro-environ- ments of hibernacula.

HIERARCHICAL PATTERNS OF GENETIC POPULATION STRUCTURE IN BLACK RAT SNAKES (ELAPHE OBSOLETA OBSOLETA) AS REVEALED BY MICROSATELLITE DNA ANALYSIS

We investigated the distribution of variation at six microsatellite loci in the black rat snake (Elaphe obsoleta obsoleta). Sampling occurred at three hierarchical scales ranging from communal hibernacula to regional populations, with most locales situated within the Frontenac Axis region of eastern Ontario. We detected no statistically significant pairwise differentiation (FST and RST) between hibernacula within the same subpopulations (interhibernaculum distance <6 km). However, isolation‐by‐distance was evident among locales within the Frontenac Axis (maximum of 50 km) and among regional populations (maximum of 1500 km). Conservative estimates of Nc derived from heterozygosity values ranged from approximately 600 to 2000. These values suggest relatively large genetic neighborhoods encompassing many communal hibernacula. Our results considered together suggest viscosity of gene flow over relatively short distances (tens of kilometers), but substantial genetic exchange among local hibernacula.

Turkey vultures foraging at experimental food patches: a test of information transfer at communal roosts

SummaryExperimental food patches were used to assess the importance of food information transfer in communally roosting turkey vultures (Cathartes aura) in southern Ontario. Feeding trials failed to provide evidence of recruitment due to information transfer. Overall, fewer birds arrived at novel food patches on the days following discovery than was expected, had information transfer been operating. Earlier arrivals on second days were more likely due to local enhancement rather than information transfer since the size of groups arriving at food patches did not differ between the 2 days. These results indicate that turkey vulture roosts in Ontario do not operate as centers for food information transfer. Intraspecific competition, preferential selection of small-sized carrion, and a low degree of kin association make the proposed benefits of information transfer much less applicable to turkey vultures than has been found for other scavenging species.

Do cattle egrets gain information from conspecifics when foraging

SummaryWe examined whether individual cattle egrets (Bubulcus ibis) base their decisions of where to forage, and how long to stay in a patch, on the behavior of other flock members. Cattle egrets commonly forage in flocks associated with cattle and capture prey at higher rates when they do not share a cow with another egret. Foraging egrets provide cues of the location of prey and their success in capturing prey. Therefore, there is the possibility of information transfer between egrets in a flock. We predicted that egrets should only move to occupied patches when the resident was capturing enough prey that it is profitable for the invader to share the patch or take over the patch. However, egrets did not seem to decide where to forage based on neighbors rates of energy intake, but rather on the presence or absence of conspecifics in a patch. We also predicted that an egret should remain in a patch until its rate of energy intake dropped to or below the average rate for other egrets within the flock. However, egrets that were foraging more efficiently than the average rate for the flock switched patches sooner than less efficient foragers. Egrets did not appear to increase foraging success by gaining information on patch quality from neighbors.