Reproduction in the Rattlesnake, Sistrurus miliarius barbouri, in Central Florida

-We studied reproduction in female pigmy rattlesnakes in a subtropical area. Twenty-six females were either captured gravid and held until parturition or found in the field with their litters. Over 92% of the snakes gave birth in August. Individuals with large clutch masses gave birth earlier in the season than individuals with small clutch masses. Mean clutch size was 5.88 (range 2-11). Litter size and clutch mass were significantly correlated with female SVL. There was significant variation in offspring size among females, but this variation was not a result of larger females having larger young, or the young being smaller in larger litters. Of 19 females whose reproductive condition was determined in the summers of both 1992 and 1993, 32% were not gravid in either year, 42% were gravid in one of the two years, and 26% were gravid in both years. We hypothesize that annual reproduction in some females is a result of the year-round activity of S. miliarius in central Florida. Journal of Herpetology, Vol. 29, No. 1, p . 21-27, 1995 Copyright 1995 Society for the Study of Amphibians and Reptiles Reproduction in the Rattlesnake, Sistrurus miliarius barbouri, in Viperids typically exhibit a suite of life history characteristics, including slow growth, which results in delayed maturation and low frequency of reproduction (LFR) by adult females (Dunham and Miles, 1985). In many viperids most adult females reproduce every other year (reviewed by Seigel and Ford, 1987; Dunham et al., 1988) and in some species females reproduce only every third or fourth year (Macartney and Gregory, 1988; Brown, 1991; Martin, 1993). It is not clear why viperids lack t e resources for frequent reproduction (Seigel and Ford, 1987). Shine (1991) suggests that sitand-wait predators, including most viperids, will tend to have low foraging success compared to snakes that actively search for prey. Some costs of reproduction resulting from viviparity may be independent of fecundity. For example, gravid females may need to move to basking sites with low prey availability to accelerate development of young. These fecundity-independent costs of reproduction may re21 This content downloaded from 207.46.13.169 on Sat, 01 Oct 2016 05:31:40 UTC All use subject to http://about.jstor.org/terms T. M. FARRELL ET AL. sult in LFR (Bull and Shine, 1979). Shine (1980) also suggested that sit-and-wait predators are less likely to be detected by their own predators and therefore have the high survivorship rates that are needed to make delayed maturity and infrequent reproduction a potentially successful life history. Another factor that may result in LFR is short activity periods imposed by climatic factors. Thoroughly studied viperid populations have been most frequently found in high latitude or high altitude sites (e.g., Macartney and Gregory, 1988; King and Duvall, 1990; Brown, 1991; Madsen and Shine, 1993; Martin, 1993), where cold weather results in prolonged periods of hibernation. Martin (1993) reviewed evidence that indicates the length of the yearly activity period in Crotalus horridus influences both the age of first reproduction and reproductive frequency. Similarly, Larsen et al. (1993) found that Thamnophis sirtalis, an active foraging colubrid, exhibited LFR near its northern distributional limit. If viviparity and foraging mode are the major determinants of life history characteristics in viperids, then subtropical and tropical species should often exhibit LFR, just as in temperate species. If, however, the length of the activity season is an important factor then we expect that viperid populations in warm regions will exhibit LFR less often. Unfortunately, we know very little about reproductive frequency and other life history traits of viperids in subtropical and tropical regions. The pigmy rattlesnake has a broad range, including subtropical areas, and is fairly common in much of its range. However, few studies have addressed its life history (Ernst, 1992). In this study we present a detailed examination of reproductive parameters of Sistrurus miliarius barbouri, including (a) data from a large sample of females encompassing the full range of sizes at which these snakes reproduce, and (b) data on the frequency of reproduction in females. In central Florida, S. miliarius can be found in a variety of habitats, ranging from drier pine uplands to seasonally flooded hardwood forests. In this subtropical region rattlesnakes are active above ground throughout the year. The highest density populations (>40 rattlesnakes/ ha, unpubl. data) are found in mesic habitats located near freshwater marshes where they feed primarily on lizards (Scincella laterale, Eumeces inexpectatus, Anolis carolinensis) and frogs (Hyla spp. and Rana utricularia). MATERIALS AND METHODS Our study sites were four tracts of mesic-hydric forest dominated by live oak, sabal palmetto, red maple, and scattered slash pines. The sites were along the floodplain of the St. Johns River in Volusia County, Florida and were seprated by <4 km. We visually searched for S. m. barbouri in our study sites from 17 February 1992 to 31 August 1993. We individually marked snakes when they were first captured by scale clips (February-July 1992) or PIT-tags (after August 1992). For each snake, we measured weight using spring-loaded Pesola scales, and snoutvent length (SVL) in a squeeze box (Quinn and Jones, 1974). The reproductive condition of each snake was determined by palpation. Data were collected in the field and the snakes were rel ased at the site of capture within approximately 15 min of capture. To study reproduction more fully we held some gravid females captive until they gave birth. They were collected by visually searching the study sites in the morning or late afternoon. We placed cages in one hammock to hold gravid females until birth. The cages measured 0.6 m x 0.6 m x 0.25 m (1 x w x h) and were made of wood framing surrounded by aluminum screen (mesh size = 1 mm x 1 mm). A layer of dirt and leaf litter was placed in the cages along with a length of PVC pipe (5 cm diameter, 30 cm long) and a piece of oak or pine bark to provide shelter. The cages were exposed to direct sunlight for much of the day to allow females to bask. A palm frond was placed on each cage to provide shade. Once placed in a cage, each gravid snake was hecked daily until it gave birth. Snakes had access to water but were not fed. After parturition, the female and young were brought into the laboratory for measurement of mass and length. The data were analyzed using Systat (Wilkinson, 1988). Since many correlations were carried out in this analysis, we used a sequential Bonferroni technique (Rice, 1989) to ensure the probability of Type I error was less than 0.05 for the correlation matrix involving reproductive variables (Table 1).