Eric W. Jennings1, and Micaela S. Gunther, Department of Wildlife, Humboldt State University, Arcata, California 95521
Effects of High Temperatures and Sun Exposure on Sherman Trap Internal Temperatures
Abstract Trapping during summer months can expose small mammals to high temperatures inside traps. We measured trap temperatures under different ambient conditions and levels of sun exposure. Internal temperatures of Sherman traps exposed to full sun were 14% higher than those exposed to full sun and fitted with a corrugated cardboard cover and tent. Traps with the surrounding cardboard cover and tent experienced up to a 7.6 ºC reduction in internal temperature and could be left out one to two hours longer on average than traps exposed to full sun before internal trap temperatures rose into the minimum lethal range for most small mammal species. Keywords: Hyperthermia, Northern California, Sherman trap, small mammal, sun exposure
Introduction When using Sherman live traps (H.B. Sherman Traps, Tallahassee, Florida) for the purpose of capturing small mammals, timing of trap deployment is usually based on the activity period of target species. Other considerations taken into account are daytime temperatures and the minimization of stress and chance of death of captured individuals. Few studies have tested the effects of environmental temperatures or sun exposure on the internal temperatures of metal box traps. Knowledge of internal trap temperature is important to aid in determination of times to open and check traps and whether closing traps during the hottest part of the day is necessary. Environmental factors such as temperature, solar radiation, and humidity can impose stress on animals (Silanikove 2000). During high daytime temperatures of 26–47 °C, small mammals that are in a metal trap with direct sun exposure may be subjected to temperatures higher than the already high ambient temperature. White-footed mice (Peromyscus leucopus) and deer mice (P. maniculatus) are nocturnal and usually are not exposed to high daytime heat (Johnson 1926), 1Author
to whom correspondence should be addressed. E-mail:
[email protected]
414
Northwest Science, Vol. 89, No. 4, 2015 © 2015 by the Northwest Scientific Association. All rights reserved.
although Behney (1936) observed that during the winter some Peromyscus had periods of activity as early as 1200 to 1800 hrs because of hunger. When traps remain open during the late morning to early afternoon, or when traps are opened late afternoon to early evening, animals that are active may be caught in traps during that time. At 1800 hrs trapped animals may experience high heat and stress in locations that experience high daytime temperatures. Peromyscus spp., Mus musculus, Dipodomys ordii, and Rattus norvegicus exposed to temperatures of 35–40 °C and above can experience rapid impairment of thermoregulation and thermoregulatory breakdown in 30–104 minutes with death in 1–3 hours depending on species (Sealander 1951, 1953; Erskine and Hutchison 1982). Many rodents under thermal stress have been observed spreading saliva on their fur to increase evaporative cooling (Sealander 1953, Murie 1961, Feldhamer et al. 2007). Microtus spp., Dipodomys spp. and Rattus lutreolus have been observed to experience thermoregulatory breakdown at 35 °C and above (Dice 1922, Chew 1951, Dawson 1955, Carpenter 1966, Hudson and Rummel 1966, Collins 1973), and Tamias minimus at 38 °C and above (Verts and Carraway 2001). Heller and Poulson (1972) observed that rates of evaporative water loss for four species of Tamias increased 3- to 4-fold
between 35–45 °C. Some small mammal species are highly susceptible to death at environmental temperatures as low as 36 °C in as little as a few hours, and death at 45 °C in as little as a 30–60 minutes for species such as Peromyscus. Understanding the effects of environmental temperature and sun exposure on the internal trap temperatures of metal Sherman traps is important to minimize stress and chance of death of target species. Methods used to alter the temperature levels inside traps include aluminum tents placed over them to provide shade during warm summer months to reduce sun exposure (Andersen 1994), and corrugated cardboard to minimize heat loss from cages under cold ambient conditions (Sealander 1951). Corrugated cardboard is made with many air pockets distributed within the material and these air pockets make up a significant portion of the total cardboard volume, reducing the thermal conductivity of the material substantially (CeramiBoard 2001). The objective of this study was to quantify the effect that differing levels of sun exposure had on the internal temperatures of Sherman traps and to determine at which ambient temperatures and conditions internal Sherman trap temperatures rise into the lethal range. We tested two hypotheses regarding the effect of sun exposure on trap temperatures. The first hypothesis proposed that intensity of sun exposure would influence the internal temperatures of Sherman traps. If supported, then traps with direct sun exposure would have internal trap temperatures higher than ambient temperatures and higher than traps without direct sun exposure during the mid-day heat period. The second hypothesis proposed that the application of corrugated cardboard around and above the traps would provide some level of thermal insulation. If supported, then traps in full sun encircled with corrugated cardboard covers and tents would have lower internal temperatures than traps in full sun without any corrugated cardboard covers or tents around them. We were particularly interested in the extent to which covers would affect (essentially reduce) the internal temperatures of traps.
Methods Study Area Eight study sites were selected throughout northern California. They were selected such that they would all provide full canopy closure for the shade group of traps, as well as provide a large amount of sun exposure to encompass the full sun and full sun with a cardboard cover experimental groups. These sites varied in their general habitats to obtain a range of environmental temperatures and exposure levels, although these habitat differences were not relevant to the study question or design. Three of the six Modoc County sites—Big Sage reservoir, Tionesta, and Clear Lake —were located in habitat dominated by juniper (Juniperus spp.) rabbitbrush (Chrysothamnus spp.), and sagebrush (Artemisia spp.). The Clear Lake Hills study site habitat consisted of grassy hills with large brush fields in the ravines. The Howard’s Gulch and Reservoir F study sites were both in stands of ponderosa pine (Pinus ponderosa). The single Lassen County site—Bogard —was located in a thinned stand of ponderosa pine (P. ponderosa) with sagebrush (Artemisia spp.) and rabbitbrush (Chrysothamnus spp.). The habitat at the single Shasta County study site—Shasta Lake—was mostly comprised of gray pine (Pinus sabiniana, douglas-fir (Pseudotsuga menziesii), black oak (Quercus kelloggii), poison oak (Toxicodendron diversilobum) and manzanita (Arctostaphylos spp.). The study was conducted from July through August 2010. Data collection occurred 2–3 days a week, from 0800–1200 hrs, and from 1600–2000 hrs at 60-min intervals. Each site was surveyed for at least 1 full day, but no more than 3 days, unless there was a significant change in the weather conditions. This allowed experimentation under a range of daytime high temperature conditions to observe how internal trap temperatures were influenced by a range of ambient temperatures. There were 3 groups of 15 Sherman live traps (7.6 x 8.9 x 22.9 cm) distributed at each site. Traps were arranged such that Group 1 was in full sun, Group 2 in full shade, and Group 3 in full sun but with each trap encircled with both a cardboard cover and cardboard tent over it. The corrugated
Northwest Science Notes: Trap Temperatures and Sun Exposure
415
cardboard cover was wrapped directly over the top panel of the traps and down around the sides where it was attached with duct tape. Each cover consisted of a single piece of cardboard 22.9 x 25.4 cm folded to fit around each trap in Group 3 to provide thermal insulation. An additional piece of cardboard 30.5 x 61.0 cm was folded in half to make a triangular tent placed over each trap in Group 3 to provide partial shade and further thermal insulation. Assembling covers and tents required that cuts be made across the corrugation that exposed the air chambers. To keep the air chambers in the corrugation sealed from the outside air to maintain the insulating properties, 3 layers of duct tape were applied down the edges that were cut and exposed. At each study site, each trap group contained 15 traps arranged in a rectangular pattern of 3 traps long by 5 traps wide with traps spaced 3 m apart. Traps were set out at 0700 hrs and allowed to acclimate to the outside temperatures for 1 hour. At 60 min intervals between 0800–1200 hrs and 1600–2000 hrs the ambient temperature at each group was recorded, followed by the internal trap temperatures for each trap in the 3 groups. Ambient and internal trap temperatures were measured using an Oregon Scientific digital thermometer (model no. THT312, Oregon Scientific Inc, Tualatin, Oregon) with an external lead. This allowed the base unit to measure ambient temperatures at each group, while the lead was placed inside the traps to record internal temperatures. Ambient temperatures were measured at 1.5 m above the ground. The external lead was gently placed in the trap with the door to hold it in place such that the temperature probe on the end was located directly above the treadle and centered within the trap, ensuring that it was not in contact with any of the metal surfaces of the trap. The accuracy of the unit (± 1.0 °C for -40 to +50 °C) was sufficient for the purposes of this study. One digital thermometer unit was used to measure temperatures in all traps for each group of traps. Temperature data were averaged for each hour for each trap and were analyzed using paired t-tests to compare internal and external temperatures
416
Jennings and Gunther
within the full sun group, and internal temperatures of full sun versus full sun with cardboard covers. One-Way ANOVA tests with Tukey’s post-hoc analysis were used to compare internal temperatures among the 3 groups and to compare the difference between internal and external temperatures among the 3 groups. Means are presented ± 1 SE and significance considered at P ≤ 0.05. Results Data were recorded at 10 time intervals per day for 14 days over a 6-week period. Three groups of 15 traps surveyed for 14 days resulted in a sample size of 630 trap days and 6,300 temperature readings. The temperature difference between internal and external trap temperatures differed from 0 °C to 11 °C among groups (F(2,27) = 131.80, P < 0.001, Figure 1). Sherman traps with full sun exposure had average internal temperatures that were 8.8 °C higher than the external ambient temperatures (t9 = 19.96, P < 0.001, 95% CI =7.826, 9.827), Figure 1). The internal trap temperatures differed from 0.5 °C to 17.3 °C among groups (F(2,27) = 38.38, P < 0.001, Figure 2). Average temperatures above the 36 °C lethal temperature for Microtus spp. were observed from 1200–1800 hrs, and temperatures above the 40 °C general lethal temperature were observed from 1600–1700 hrs in traps exposed to full sun. Traps fitted with cardboard covers reached the 36 °C lethal temperature only at 1600 hrs (Figure 2). Average internal temperatures of Sherman traps with full sun exposure were 13.8 °C higher than average internal temperatures of traps in shade (t9 = 16.29, P < 0.001, 95% CI = 11.850, 15.672) and were 4.6 °C higher than average internal temperatures of traps exposed to full sun and fitted with a corrugated cardboard cover and tent (t9 = 6.88, P < 0.001, 95% CI = 3.096, 6.129), Figure 2). Full sun exposure traps reached the 36 °C minimum lethal temperature inside by 1000 hrs on 36% of the survey days, and by 1100 hrs on 79% of the survey days. By contrast full sun exposure traps with the application of corrugated cardboard covers and tents reached 36 °C by 1100 hrs on only 14% of the survey days.
Figure 1. Average temperature difference (± 1 SE) between internal trap and external ambient temperatures of traps placed in full sun exposure, full shade, and full sun exposure with cardboard cover and tent, California, 2010.
Figure 2. Average internal trap temperatures (± 1 SE) of 3 groups of traps placed in full sun, full shade, and full sun with cardboard cover and tent. Lines are minimum lethal temperatures for Microtus spp. (36 °C) and for most other similar sized rodents (40 °C), California, 2010.
Northwest Science Notes: Trap Temperatures and Sun Exposure
417
Discussion The results support the hypotheses that different levels in intensity of sun exposure influences the internal temperatures of Sherman traps, and that the application of a corrugated cardboard cover and tent around the Sherman traps provides some level of thermal insulation. These results emphasize the need to place traps in a shaded location when possible, or to use a method of providing shade directly to the traps to avoid high internal trap temperatures. One of the important findings of this study was that not only did the corrugated cardboard provide thermal insulation, but it provided enough insulation to reduce the internal trap temperatures significantly. This is important to animal welfare, reducing trapped animal exposure to lethal temperatures, and can be achieved with inexpensive and readily available corrugated cardboard. Thus, cardboard insulation can be an effective tool to reduce trap mortality and used on almost any project without significantly increasing project costs. Another important finding of this study is that the application of cardboard covers and tents provided on average one to two extra hours of time before internal trap temperatures rose above the minimum lethal temperature of 36 °C, compared to the full sun traps. Reducing internal trap temperatures or lengthening the amount of time until the trap reaches the minimum lethal temperature inside is important to the health and welfare of small mammals caught in Sherman traps, allowing trappers more time to access all traps in far-ranging grids. An interesting finding of this study was the distinct peak at 1800 hrs (Figure 1). This is the result of several factors. At 1800 hrs the ambient temperature is already starting to drop, while the internal temperatures inside the traps are still very high and dropping at a slower rate than the ambient temperature. This is noteworthy because it illustrates that even though the temperatures were dropping at 1800 hrs, the traps in the sun were still 11 °C warmer, and the traps in the sun fitted with a cardboard cover only 5 °C warmer, than the ambient temperature. This highlights the
418
Jennings and Gunther
concern that if we were conducting an actual trapping study, opening traps at 1800 hrs in full sun could potentially expose small mammals caught in the traps to lethal temperatures at least for a short time, even though the ambient temperature may be dropping to an apparently safe temperature. In addition to the issue of temperatures inside the trap, the act of trapping animals can have an impact on individuals’ survival. Even if the animal being trapped does not die as a result of high temperature inside the trap, the added stress of high heat exposure is just one of many stresses the animal has to endure. It has been suggested that repeated captures can have cumulative effects (Bietz et al. 1977, Korn 1987, Slade 1991) and diminish vigor and fitness of trapped individuals (Pearson et al. 2003). Schon and Korn (1992) and Kaufman and Kaufman (1994) treated trap mortality as a discrete event related to conditions during a single capture period. However Pearson et al. (2003) suggested cumulative effects from trapping probably cause reduced fitness and associated physiological conditions that weaken animals, to the point that they can die in the trap or soon after release. Minimizing unnecessary stress on captured animals will reduce the likelihood of contributing to the mortality of animals after release. Ensuring survival of trapped animals helps to prevent biased results when using live traps to conduct mark-recapture studies or monitor rodent populations (Drickamer and Paine 1992), especially in areas that may reach high daytime temperatures. Management Implications Temperature extremes during trapping periods and their effects on capture success and mortality should always be a consideration for animal welfare. Forecast high daytime temperatures can be an indicator of when it might be too hot to trap, and understanding the implications of those temperature extremes in relation to trap temperatures is very important. Traps should be kept in shaded areas or shielded from direct sunlight by using cardboard covers and tents to minimize the likelihood of mortality inside traps.
Acknowledgments We thank Brett Furnas from the California Department of Fish and Game for providing resources and Literature Cited Andersen, D. C. 1994. Demographics of small mammals using anthropogenic desert riparian habitat in Arizona. Journal of Wildlife Management 58:445-454. Behney, W. H. 1936. Nocturnal explorations of the forest deer mouse. Journal of Mammalogy 17:225-230. Bietz, B. E., P. H. Whitney, and P. K. Anderson. 1977. Weight loss of Microtus pennsylvanicus as a result of trap confinement. Canadian Journal of Zoology 55:426-429. Carpenter, R. E. 1966. A comparison of thermoregulation and water metabolism in the kangaroo rats Dipodomys agilis and Dipodomys merriami. University of California Publications in Zoology 78:1-36. CeramiBoard®. 2001. CeramiBoard® thermal properties. CeramiBoard®, Melbourne, Victoria, Australia. Available online at http://www.ceramiboard.com/ thermal_properties.htm (accessed 31 August 2010). Chew, R. M. 1951. The water exchange of some small mammals. Ecological Monographs 21:215-225. Collins, B. G. 1973. Physiological responses to temperature stress by an Australian murid, Rattus lutreolus. Journal of Mammalogy 54:356-368. Dawson, W. R. 1955. The relation of oxygen consumption to temperature in desert rodents. Journal of Mammalogy 36:543-553. Dice, L. R. 1922. Some factors affecting the distribution of the prairie vole, forest deer mouse, and prairie deer mouse. Ecology 3:29-47. Drickamer, L. C., and C. R. Paine 1992. Sex, age, nest construction and trap mortality for six species of rodents. American Midland Naturalist 128:360-365. Erskine, D. J., and V. H. Hutchison 1982. Critical thermal maxima in small mammals. Journal of Mammalogy 63:267-273. Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, J. F. Merritt, and C. Krajewski 2007. Mammalogy: Adaptation, Diversity, Ecology. John Hopkins University Press, MD. Heller, H. C., and T. Poulson 1972. Altitudinal zonation of chipmunks (Eutamias): adaptations to aridity and
advice with the design of this project. Thank you also to numerous other colleagues for helpful suggestions on previous versions of this manuscript. high temperature. American Midland Naturalist 87:296-313. Hudson, J., and J. Rummel 1966. Water metabolism and temperature regulation of the primitive heteromyids, Liomys salvani and Liomys irroratus. Ecology 47:346-354. Johnson, M. S. 1926. Activity and distribution of certain wild mice in relation to biotic communities. Journal of Mammalogy 7:245-277. Kaufman, G. A., and D. W. Kaufman. 1994. Changes in body mass related to capture in the prairie deer mouse Peromyscus maniculatus. Journal of Mammalogy 75:681-691. Korn, H. 1987. Effects of live-trapping and toe-clipping on body weight of European and African rodent species. Oecologia 71:597-600. Murie, M. 1961. Metabolic characteristics of mountain, desert and coastal populations of Peromyscus. Ecology 42:723-740. Pearson, E., Y. K. Ortega, and L. F. Ruggiero. 2003. Trap-induced mass declines in small mammals: mass as a population index. Journal of Wildlife Management 67:684-691. Schon, I., and H. Korn. 1992. Causes and magnitude of body weight changes in trap-confined bank voles, Clethrionomys glareolus. Journal of Zoology 227:319-325. Sealander, J. A. 1951. Survival of Peromyscus in relation to environmental temperature and acclimation at high and low temperatures. American Midland Naturalist 46:257-311. Sealander, J. A. 1953. Body temperatures of white-footed mice in relation to environmental temperature and heat and cold stress. Biological Bulletin 104:87-99. Silanikove, N. 2000. Effects of heat stress on the welfare of extensively managed domestic ruminants. Livestock Production Science 67:1-18. Slade, N. A. 1991. Loss of body mass associated with capture of Sigmodon and Microtus from northeastern Kansas. Journal of Mammalogy 72:171-176. Verts, B. J. and L. N. Carraway 2001. Tamias minimus. Mammalian Species 653:1-10.
Received 16 May 2014 Accepted for publication 04 December 2015
Northwest Science Notes: Trap Temperatures and Sun Exposure
419
