CSIRO PUBLISHING
Australian Journal of Zoology, 2013, 61, 132–136 http://dx.doi.org/10.1071/ZO13003
Body sizes, activity patterns and habitat relationships of the orange-naped snake (Furina ornata) (Serpentes : Elapidae) in the MacDonnell Ranges, Northern Territory Peter J. McDonald A,B,E, Gary W. Luck C, Chris R. Pavey A,D and Skye Wassens B A
Flora and Fauna Division, Department of Land Resource Management, Northern Territory Government, Alice Springs, NT 0871, Australia. B School of Environmental Sciences, Charles Sturt University, Albury, NSW 2640, Australia. C Institude for Land, Water and Society, Charles Sturt University, Albury, NSW 2640, Australia. D Present address: CSIRO Ecosystem Sciences, PO Box 2111, Alice Springs, NT 0871, Australia. E Corresponding author. Email:
[email protected]
Abstract. Orange-naped snakes (Furina ornata) are small elapids that occur in tropical to arid regions throughout northern and central Australia. We report on the first field-based study of this species, investigating body sizes, activity patterns, and habitat use in the semiarid MacDonnell Ranges bioregion of central Australia. Using systematic road-cruising, we encountered 69 live F. ornata along a 77-km sealed road-transect over a 12-month period from August 2009 to July 2010. Based on measures of snout-to-vent length (SVL), we found that female F. ornata (mean SVL = 459 6.3 (s.e.) mm; n = 16) were larger than males (SVL = 372 25.2 (s.e.) mm; n = 44) (t = 4.7358, P < 0.0001), and that both sexes were larger than previously reported from museum specimens. Despite the extreme weather variability experienced in arid Australia, we found that activity patterns were not significantly related to temperature, rainfall or humidity, and F. ornata was active in all but the single coldest month of the year. The habitat-use analysis found that F. ornata was more likely to be recorded in areas with greater cover of hummock grass (Triodia spp.) and less cover of rocky outcrops or boulders. Hummock grasslands in arid Australia have an abundant and diverse skink fauna, which may attract F. ornata, whose diet consists primarily of diurnal skinks.
Received 8 January 2013, accepted 28 May 2013, published online 17 June 2013
Introduction The genus Furina (Elapidae) consists of five small to mediumsized terrestrial, nocturnal snakes with a widespread distribution throughout mainland Australia. Three of these species (F. barnardi, F. dunmalli and F. tristis) are restricted to eastern Australia, where they occur in association with a range of woodland types (Wilson and Swan 2010). The remaining two species (F. diadema and F. ornata) have more generalist habitat requirements and broader climatic tolerances. The orange-naped snake (Furina ornata) is the most widely distributed of the five species and occurs in arid to tropical regions across Western Australia, the Northern Territory and Queensland (Wilson and Swan 2010). In parts of the southern Northern Territory, it is among the most frequently encountered snake species (McDonald 2012). However, despite its wide range and localised abundance, F. ornata has been the subject of only a single study in which museum specimens were examined to investigate various ecological traits from animals collected across the distribution of the species (referred to in that study as ‘northern Furina’) (Shine 1981). Journal compilation CSIRO 2013
The results of the work by Shine (1981) demonstrated sexual size dimorphism in the species (females larger), dietary specialisation (only skinks (Scincidae) were recovered from snake stomachs), and apparent aseasonal reproductive habits. Larger females and dietary specialisation on skinks are common traits among small Australian elapids (Shine 1980, 1994) and aseasonal reproduction has also been recorded in other genera (Shine 1980). Although Shine’s work was a substantial contribution to our understanding of F. ornata, ecological knowledge of the species remains limited, especially considering that no field studies have been conducted. This is a situation mirrored across most of Australia’s arid snakes which, unlike other vertebrate fauna groups, have largely been overlooked by researchers (McDonald 2012). Here, we present the first field-based study of F. ornata, carried out in the MacDonnell Ranges bioregion in the southern Northern Territory. We examined habitat use by the species and measured body sizes to test for evidence of intraspecific variation and in order to determine whether sexual size dimorphism occurred within the bioregion. Having a distribution that spans www.publish.csiro.au/journals/ajz
Ecology of orange-naped snakes
considerable climatic variation (arid to tropical regions), F. ornata offers a unique opportunity to explore the factors that have enabled this species to penetrate into arid regions. Therefore, we also examined how activity patterns vary in relation to the extreme variation in temperature and rainfall that is characteristic of arid Australia.
Methods Between August 2009 and July 2010, we used systematic roadcruising (Rosen and Lowe 1994) to sample snakes along a 77-km sealed road transect west of Alice Springs in the MacDonnell Ranges bioregion of the southern Northern Territory. One of us (PMcD) drove the transect at night over 12 months from August 2009 to July 2010, with a minimum of four and a maximum of 10 nights driven each month (4 nights in May, June and July; 5 nights in August, March and April; 6 nights in September, October and January; 8 nights in December and February; and 10 nights in November). Field work was limited to night time because F. ornata is nocturnal (Wilson and Swan 2010). Each night, the transect was driven twice (east and west), commencing within 1 h of sunset and ending at variable times depending on the number of snakes encountered. The start point of the transect was alternated between the east and west ends and all work was carried out with a four-wheel-drive vehicle fitted with twin 100-W driving lights and driven at 40–60 km–1 (see McDonald et al. 2011 for further details on the study area and sampling procedure). The position of all F. ornata located on the transect was recorded using a hand-held Global Positioning System. Live snakes were hand-caught and marked with scale clipping (Brown and Parker 1976). All F. ornata were measured for snoutto-vent length (SVL) and their sex was determined by probing. Although the validity of probing for determining sex has not been verified for this species, this method clearly identified distinct differences among individuals, with individuals assigned as males probing 8–15 ventral scales in depth and individuals assigned as females probing no greater than 1 ventral scale in depth. On this basis we are satisfied that probing was an accurate means of determining sex for this species. Individuals were considered sexually mature if they exceeded the minimum size recorded for sexually mature individuals by Shine (1981) (>236 mm for males, >214 mm for females). To assess the influence of weather on F. ornata activity, we recorded variables at monthly intervals and at the time of snake encounter. Monthly weather variables were recorded from a Heavy Weather WS-3610 weather station located at Ormiston Gorge Ranger Headquarters, on the northern edge of the study area. We recorded the mean maximum temperature (C) and the mean minimum temperature (C) calculated for each month from the days of sampling only. Monthly totals of rainfall (mm) were also recorded and calculated from all days in each month. At the time of snake encounter, we recorded air temperature (C) and relative humidity (%) using a Kestrel handheld pocket weather meter. In order to test the influence of weather on the likelihood of F. ornata encounters, air temperature and relative humidity were also recorded in the middle of each hour (so conditions at the point of encounter could be compared with nonencounter times).
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Because F. ornata is strictly terrestrial (Wilson and Swan 2010), we sought to examine relationships between a range of groundcover variables and snake occurrence using logistic regression. The variables measured were as follows: % area of boulders or rock outcropping >200 cm diameter; % hummock (Triodia spp.) grass cover; % tussock grass cover (including native and introduced grass species); % cover of Acacia-derived litter; and number of logs >15 cm diameter. We recorded these variables in a 50-m radius around each F. ornata location on the road transect and around 50 randomly selected ‘pseudoabsence’ locations where the species was not recorded. The pseudoabsence locations were a minimum of 500 m from the nearest presence location. All variables were recorded on the ground using visual estimation. Quantile–quantile plots and frequency histograms of the body size data showed no considerable departures from normality. Accordingly, we compared the mean SVL of male and female F. ornata for sexually mature individuals using the independentsample t-test. To compare with monthly weather variables, encounter rates were standardised to the number of F. ornata per 100 km of transect to account for variation in sampling effort. These encounter rates were graphed against monthly rainfall and mean monthly minimum and maximum air temperature. We also used linear regression to examine whether any of these weather variables were related to snake activity. Variables were included in models singularly and we also included an interaction between minimum temperature and rainfall (in case the influence of rainfall was affected by cooler weather). We compared the mean temperature and humidity at the time of capture with the mean values of these variables averaged across the entire duration of sampling (measured hourly) using one-way ANOVA. We tested for any significant variation in the number of male and female pythons encountered each month using the non-parametric Wilcoxon signed-rank test. In order to examine relations between groundcover variables and the occurrence of F. ornata, we compared the groundcover variables at presence and absence sites using the non-parametric Mann–Whitney U test and univariate logistic regression models. All analyses were run in SPSS ver. 17.0 (PAWS Statistics 17.0). Results We encountered 73 F. ornata on the road transect (69 live animals and four road-kills). Measures of SVL were made for 44 adult male and 16 adult female snakes. Other F. ornata encountered but not measured (n = 13) were either in an agitated state (not relaxing to allow accurate measurement) or too disfigured from road traffic (road kill). Adult males ranged from 286 to 479 mm SVL and adult females from 325 to 630 mm SVL. Females were significantly larger than males, averaging 459 mm (s.e. = 6.3) compared with 372 mm (s.e. = 25.2) SVL for males (t = 4.7358, P < 0.0001). However, because we used a rudimentary method for determining sexual maturity, immature individuals may have inadvertently been included in the adult dataset. Therefore, our values for mean adult SVL may be underestimated. Two distinct peaks in F. ornata activity were observed, the first in September (1.14 snakes per 100 km) and the second, larger peak, in May (1.3 snakes per 100 km) (Fig. 1). Both peaks appear to be independent of the recorded monthly weather
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Fig. 2. Number of male (n = 51) and female (n = 16) F. ornata encountered per month between August 2009 and July 2010. Note that sampling effort was not equal among months; see Fig. 1 for standardised monthly encounter rates.
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The likelihood of encountering F. ornata on the road transect was higher in areas with more hummock grass (Triodia spp.) cover and less cover of rock-outcrops or boulders within a 50-m radius (Table 1). There was also a negative association between F. ornata occurrence and tussock grass cover (Table 1), but this was expected because tussock grass cover was negatively correlated with hummock grass cover (rs = –0.35, P = 0.0001, n = 123); that is, tussock grass and hummock grass do not typically co-occur.
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Fig. 1. Monthly encounter rates of F. ornata (n = 69) and (a) mean minimum and maximum air temperatures, and (b) rainfall totals.
variables and this was confirmed by the results of the linear regression analysis. Neither minimum temperature (R2 = 0.03, F1,12 = 0.26, P = 0.62), maximum temperature (R2 = 0.05, F1,12 = 0.48, P = 0.50), rainfall (R2 = 0.02, F1,12 = 0.24, P = 0.63), or an interaction between minimum temperature and rainfall (R2 = 0.06, F2,12 = 0.29, P = 0.60) were related to F. ornata encounter rates. For weather data at the exact time of snake encounter, there was no significant difference between encounter times and the average recorded for the duration of sampling for temperature (one-way ANOVA: F2,257 = 0.07, P = 0.79) or humidity (one-way ANOVA: F2,257 = 1.44, P = 0.23). The number of male and female F. ornata encountered each month varied significantly (Z = 2.78, P = 0.005). More male than female snakes were encountered in all months except July (when no F. ornata were encountered) with the difference being most pronounced in September (with eight males and a single female encountered) (Fig. 2).
Discussion In the MacDonnell Ranges bioregion, we found that both male and female F. ornata were considerably larger than previously reported from museum specimens, though the degree of sexual size dimorphism was similar (Shine 1981). Despite the extreme weather variability experienced in arid Australia, we found that temperature, rainfall and humidity were not significantly related to F. ornata activity patterns. However, we observed considerable monthly variation in the number of male and female F. ornata encountered. The data on habitat associations suggest some preference for hummock grassland groundcover, which may be related to diet. The body size data from our study supports the results of Shine (1981), with pronounced sexual size dimorphism evident in the MacDonnell Ranges population of F. ornata. On average, adult females were 87 mm larger than males, which is close to the difference reported from museum specimens collected across the distribution of the species (Shine 1981). Larger females in snake species may be the result of a fecundity advantage associated with increased body size (i.e. larger size = more eggs) and usually indicates the absence of male–male combat (Shine 1994). Although the degree of sexual size dimorphism was similar to that reported by Shine (1981), in our study both adult male and female F. ornata were substantially larger than reported from museum specimens: by 28% and 26% on average,
Ecology of orange-naped snakes
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Table 1. Comparison of groundcover variables for sites at which F. ornata was present and absent, and results from the logistic regression analysis The –2(log-likelihood) for the constant only (random) model was 166.19 Variable
% rock outcrop % hummock grass cover % tussock grass cover % shrub litter cover No. of logs
Mean ± s.e. Absent
Present
2.20 ± 0.86 3.70 ± 0.95 8.16 ± 1.22 4.40 ± 0.96 2.00 ± 0.34
0.15 ± 0.08 9.62 ± 1.40 5.29 ± 0.68 3.86 ± 0.73 1.44 ± 0.22
Mann–Whitney U test Z P 2.38 3.22 1.20 1.27 1.50
respectively (Shine 1981). This suggests intraspecific variation in body size across the species’ range, although any patterns in variation will become apparent only with additional field studies in other parts of the species’ distribution. Intraspecific variation in body size has previously been reported among Australian snakes, including for the tropical-dwelling colubrid Boiga irregularis (Trembath and Fearn 2008). Alternatively, it is possible that F. ornata represents a composite of cryptic species, as has been found for other Australian elapids previously assigned to a single taxon (Shea and Scanlon 2007). Our interpretations of activity patterns are limited by sampling in a single year only. Nevertheless, by sampling in each month throughout that year, we are able to report on monthly variation in F. ornata activity patterns. Although our monthly sample size was relatively small, encounter rates fluctuated considerably throughout the year. However, there was no clear association between F. ornata activity and temperature, rainfall or humidity. This is in contrast to a study of the sympatric Stimson’s python (Antaresia stimsoni), conducted in the same region at the same time, which showed that python activity varied as a consequence of rainfall and humidity (McDonald et al. 2011). In that study, it was suggested that increased activity of A. stimsoni in response to rainfall and humidity may have been related to increased activity of frogs, an important food resource for the species. In contrast, F. ornata is not known to prey on frogs (Shine 1981) and this may explain the absence of a correlation with rainfall. Our results for F. ornata are consistent with those of a study from the tropical Top End of the Northern Territory, which demonstrated that standard weather variables were poor at predicting the numbers of various nocturnal snake species (Brown and Shine 2002). However, our results contrast with more recent work on tropical snakes, where rainfall and temperature, or an interaction between these variables, has been demonstrated to predict snake activity (Trembath and Fearn 2008; Trembath et al. 2009). In contrast with tropical Australia, where daily and between-season temperature fluctuations are relatively low and rainfall is more predictable, arid Australia experiences extreme daily fluctuations in temperature throughout the year and rainfall is unpredictable and highly irregular (Fig. 1) (McDonald et al. 2011). Although we recognise the possibility that the variation in encounter rates may be better explained by additional weather variables not recorded in our study (e.g. barometric pressure), it is apparent that the broad tolerance for temperature and humidity allows F. ornata to remain nocturnally active throughout the year while not being restricted to foraging in limited windows of ‘ideal’ conditions. This is in contrast to several sympatric elapid species that are either tightly constrained
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