Individual study: Effects of food supplementation on Western diamond-backed rattlesnakes Crotalus atrox in the Upland Sonoran Desert, Arizona, USA
Taylor E.N, Malawy M.A., Browning D.M., Lemar S.V. & DeNardo D.F. (2005) Effects of food supplementation on the physiological ecology of female Western diamond-backed rattlesnakes (Crotalus atrox). Oecologia, 144, 206-213
Food availability may be a limiting factor that can affect growth, weight change and reproductive success. Supplemental feeding experiments in the wild allow the effects of availability of food on these parameters to be investigated, while animals are free to behave naturally. Most such experiments have been conducted on species with high energy demands (e.g. birds and mammals). This study looked at the affects of supplemental feeding on free-ranging, female Western diamond-backed rattlesnakes Crotalus atrox, a species with low energy demands. Rattlesnakes have extremely low metabolic rates and reproductive frequencies. Additionally, they feed on large meals at infrequent intervals and thus supplemental feeding can have a substantial impact on total food consumption with limited disturbance. The Western diamond-back is an abundant snake and spends much of its time above ground, making a study such as this feasible.
It was predicted that supplementally fed snakes would show higher growth rates, weight gain and reproductive output than unfed (control) snakes. As activity may be influenced by foraging behavior, it was also predicted that food supplementation would reduce the need to forage leading to a reduction in surface activity and home range size.
Study site: The Western diamond-backed rattlesnakes Crotalus atrox supplemental feeding experiment was undertaken in a 1.5 x 1 km area of the Arizona Upland Sonoran Desert, approximately 33 km NNE of Tucson, Arizona. The habitat consists of rocky hillsides and sandy plains. During the snake active season (mid-March to mid-May and mid-September to mid-November) temperatures typically range between 5-30ºC, and in summer (mid-May to mid-September) 20-40ºC.
Experimental design: The study took place from March 2002 to October
2003. Seventeen wild female Western diamond-backs were implanted with 11–13 g radio-transmitters (#SI-2T, Holohil, Carp, ON, Canada). Nine were randomly designated as supplementally 'fed snakes', while the other eight were control snakes. Based on a previous study, most of the snakes (n=13) were known not to have reproduced the previous year (2001), but at least one had (randomly assigned to the fed group). Fed snakes were offered thawed rodents 1–4 times a week. Feeding occurred throughout the active season (but primarily April to October). Snakes sometimes immediately struck and ingested rodents in front of the researcher, at other times the rodent was left by the snake, often but not always returning to see if the rodent was gone. Therefore, exactly how much food each snake consumed was not recorded, but the aim was to provide the fed group with dramatically increased food availability in comparison to the control group (rather than look at the effect of specific amounts of food uptake).
Monitoring: As some snakes died or were lost due to transmitter failure, not all snakes were monitored for the entire 19-month study period. All 17 snakes were monitored for at least 6 months, the average duration was 12.7 months (range: 6–19 months). The average time monitored did not differ between fed and control snakes (fed average 15 months; control average 11.6 months).
During the active season (mid-March to mid-November), each snake was located 1–5 times per week. During the winter (mid-November to mid-March), they were checked 1–2 times per month. Snakes were weighed once a month and examined with a portable ultrasonograph (Concept/MCV, Dynamic Imaging, Livingston, Scotland) to examine reproductive state. Snout-vent lengths (SVL) were measured twice a year while snakes were anesthetized with isoflurane. To collate accurate reproductive data, pregnant snakes were captured and brought into the laboratory one to several weeks prior to giving birth. Within 2 days after giving birth, the offspring were weighed, measured SVL (using a foam padded squeeze box), and the sex determined (by examining tail length and by everting hemipenes in males). Snakes were released at the site of capture within 2 weeks of parturition.
Surface activity & home range size: When radio-tracking, whether snakes were above or below ground was recorded. Surface activity was defined as the proportion of time encountered above versus below ground. Home range estimates were made using global positioning system (GPS) point locations and the fixed kernel method (see original paper). Some snakes traveled long distances to overwinter, others remained within their summer range, resulting in tremendous variation in home range size. As the aim was to examine whether home range size was affected by food supplementation (which only occurred during the active season) active season ranges were calculated for each snake during April to October. Two snakes were removed from the range analyses because they had fewer than the required 30 fixes, therefore analyses include data from seven fed and eight control snakes.
Food intake: All the fed female Western diamond-backs ate at least four extra meals per year, most consumed more. The annual energy budgets of rattlesnakes tend to be very low (e.g. approximately equal to their body weight, or only several meals a year) thus the fed snakes were considered to have consumed considerably more food than the control snakes.
Growth, mass & body condition: Supplementally fed snakes(n=8) had a significantly higher average monthly growth rate than controls (n=8) (fed: 0.39±0.08 cm/ month; control: 0.06±0.10 cm/month). Over the 19-month experimental period, the average total growth of fed females was 3.75 cm (range: 1–7 cm), while that of control females was 0.9 cm (range: -1.5-3.5 cm). Three control females appeared to 'shrink (two by 1.5 cm, and one by 0.5 cm). Although measurements were taken with care, the 'shrinking' up to 1.5 cm. is within the range of measurement error, and careful, repeated measurements will be necessary to confirm that shrinkage is possible.
Fed females had a significantly higher average monthly weight increase than controls (fed: 21.3 g/month, control: 1.4 g/month) and fed snakes were in slightly better body condition at the end of the experiment but the difference was not significant. Similarly, there was no significant difference in body condition when assessed relative to size (fed: -0.44%, control: -2.31%).
Reproduction: Supplemental feeding led to a significantly increased incidence of reproduction. During the experiment, seven fed snakes reproduced (two in 2002 and six in 2003; one of these reproduced in both years as well as in 2001 and 2004), overall producing 37 young (average litter, 4.6 young). Only one control snake reproduced (two young in 2003). A seventh fed snake went underground several weeks before giving birth and avoided capture, thus the actual number of young produced by fed snakes was thought to be infact greater than 37.
Litter characteristics: Fed snakes had higher average total and live clutch weights and average offspring weight than non-fed snakes, but not significantly so. Post reproduction, fed snakes had a significantly higher average body weight and body condition than non-fed snakes. There was no effect of feeding on relative clutch weight, litter size or average
Surface activity & home range size: The proportion of time encountered above ground (fed: 0.45; control: 0.55) and home range size did not differ between fed and control snakes, and SVL did not significantly affect home range size.
Conclusions: Food supplementation of free-ranging female rattlesnakes led to greater weight gain, growth rates, occurrence of reproduction, and post-reproductive body condition relative to females that did not receive supplemental food. However, surface
activity, home range size and litter characteristics were not affected by food supplementation. This study demonstrates that life history characteristics, including growth and reproduction, are limited by low food availability.
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