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Foraging and food hoarding of wild Rattus norvegicus in an urban environment
BEHAVIORAL AND NEURAL BIOLOGY 29, 527--531 (1980)
BRIEF REPORT Foraging and Food Hoarding of Wild Rattus norvegicus in an Urban Environment LOREY K . TAKAHASHI AND RICHARD K . LORE 1
Psychology Department, Douglass College, Rutgers University, New Brunswick, New Jersey 08903 The foraging activities of a free-living population of wild Rattus norvegicus was confined to the nighttime hours. Peak feeding occurred during the early evening hours. Subsequent excavation of the burrow systems of these animals revealed no evidence that food had either been consumed or stored in the burrows.
According to Calhoun's (1962) observations, wild Rattus norvegicus reared in a large outdoor pen forage throughout the night. In Calhoun's study, peak activity occurred during the 3-hr period following sunset. Thereafter, foraging slowly declined until 2-3 hr prior to sunrise when another moderate increase in activity was observed. It is possible, however, that captivity distorts the typical foraging pattern of free-living populations of rats in unknown ways. For example, in most urban locations human activity remains high during the early evening hours when Calhoun's captive animals were also most active. Thus, rats might adjust their diurnal feeding cycle to avoid human contact during the early evening hours. Surprisingly, no information on the feeding activity of a free-living population of wild Rattus norvegicus is available. In the current study, we recorded the foraging activity of an undisturbed population of wild rats in a typical urban environment. After completion of the foraging observations, all burrows within a radius of 100 m of the food source were excavated and the contents of the burrows examined in order to determine if stores of food were present. The study site was located in the rear of a supermarket in central New 1 To whom requests for reprints should be addressed: Department of Psychology, Douglass College, Rutgers University. New Brunswick, N. J. 08903. This study was supported by grants from the Rutgers Research Council and the Busch Fund, Rutgers University. We thank William Carr for the use of the infrared telescope and Kevin Flannelly for his help in excavating the burrows. 527 0163 - 1047/80/080527-05502.00/0 Copyright© 1980by AcademmPress, Inc. All rightsof reproduettonm any form reserved
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TAKAHASHIAND LORE
Jersey. The only food source was a large trash compactor (6.8 x 2.3 x 2.7 m) attached to the store via a delivery chute. A survey of the entire area indicated that signs of rat activity (burrow entrances, fecal pellets, trails) were confined to a single densely burrowed earthen bank separated from the compactor by an asphalt driveway approximately 12.8 m wide. Prior to our observations, we counted 114 burrow entrances along the bank. Virtually all of these were within 17 m of the compactor. In order to reach the compactor from their burrows, rats had to cross the asphalt driveway which was regularly used by vehicles during the daylight and early evening hours (0700-2100 hr). Human activity was minimal from 1800 to 0600 hr but the trash compactor operated until 2030 hr. Water was available from drainage along the bank. Condensation from the store's air conditioners provided a continuous water supply during the dry summer months. According to store employees, rats had occupied the site for more than 12 years. Observations were conducted during September to mid-November, 1978. Temperatures ranged from 6.2 to 19.5°C and sunset and sunrise were approximately 1830 and 0600 hr, respectively. Behavioral observations were made from a vehicle parked in a location that enabled the observation of rats crossing the driveway to either enter or leave the compactor. In addition, foraging activities in the immediate vicinity of the two sides of the compactor that could be observed were recorded. The latter measure represents animals that appeared from one of the many holes in the compactor and either entered the compactor at another location, disappeared under it, or fed on garbage spilled along its sides. The animals could be observed easily during the twilight hours with 7 x 50 binoculars and night observations were done with an infrared telescope (Varo, Inc., Garland, Tex.). The site was observed for 60 hr at irregular intervals (no observations were made during cloudy weather) with all hourly periods represented by 2-3 hr of observation time. After completion of the observational phase of the study, eight burrow systems that appeared to have been recently used because entrance holes were in good repair were excavated according to a procedure described by Lore & Flannelly (1978) and the presence of food and nesting material was recorded. Entries, exits, and sightings near the trash compactor were recorded separately but since each measure revealed essentially similar results, they were combined to yield a single index of foraging activity (Table 1). A total of 235 sightings were made between 1900 and 0800 hr and no rats were observed at the site before 1900 or after 0800 hr. Rats were most active during the period 2000-2200 hr and almost 70% of all sightings occurred during the 5-hr period from 1900 to 2400 hr. A slight increase in rats entering the compactor began again at 0300 hr with the majority leaving by 0500 hr.
FORAGING AND FOOD HOARDING IN WILD RATS
529
TABLE 1 Distribution of the Forage Activities of an Established Population of Wild Rats in a Typical Urban Setting a Time (hr)
Foraging activity index
1900-2000 2000-2100 2100-2200 2200-2300 2300-2400 2400-0100 0100-0200 0200-0300 0300-0400 0400-0500 0500-0600 0600-0700 0700-0800
7.00 20.33 13.00 11.00 6.33 2.33 2.67 4.00 9.00 5.00 2.50 1.33 0.89
The foraging activity index was derived by dividing the total number of sightings during each hourly interval by the number of hours observed at that interval.
Figure 1 illustrates the major portion of one excavated burrow. The eight burrows we examined contained a total of 18 chambers. In 16 chambers nesting material consisting of grass, paper, remnants of plastic bags, and bits of cloth was present. However, no stored food nor any evidence that food had been eaten (e.g., gnawed bones) in any of the burrows was found. Our results indicate that an established population of free-living rats inhabiting an environment that is typical for the species in the urban United States exhibits a feeding pattern similar to that reported for a captive population (Calhoun, 1962). In both studies, foraging activity was highest 2-3 hr following sunset and then declined slowly throughout the night until another slight increase in foraging occurred 2-3 hr prior to sunrise. But, there are differences. Calhoun's captive animals did feed occasionally just prior to sunset and after sunrise whereas we never observed foraging activity during the daylight hours. Wild rats will feed during the day when food is more readily available at this time (Chitty & Shorten, 1946; Cottam, 1948) or when humans are absent (Pisano & Storer, 1948) but at our site it seems likely that the presence of considerable pedestrian and vehicular traffic confined foraging activity to the nighttime hours. It should be noted, however, that peak feeding was not disrupted at our site during the early evening hours by either the intermittent operation of the trash compactor or passing vehicles. Thus, the feeding pattern was not modified in order to minimize contact with humans. Rattus norvegicus is considered to be a notorious hoarder of food
530
TAKAHASHI AND LORE
FIG. 1. This excavated burrow system was one of eight located near the refuse bin behind the supermarket. Tunnels (strips of white plastic), three entrances (small circles), and two chambers (large circles) are visible in the photograph. The meter stick in the middle provides size perspective. One tunnel segment and entrance, on the other side of the fence, are not visible. This burrow was gassed with chloropicrin prior to excavation and the body of one adult female weighing 216.7 g was recovered. despite the fact that the species lacks the specialized cheek pouches of other small rodents that are p r e s u m e d to a m a s s large stores of food (e.g., the Eastern Chipmunk, Tamias striatus, or any of the species of w o o d rats belonging to the genus, Neotoma)~ There is, of course, a long tradition and a large literature on " h o a r d i n g " by domestic strains of Rattus norvegicus in the laboratory (Hunt, 1941; Marx, 1950; Wolfe, 1939), but very little field data to support the assumption that free-living Rattus norvegicus consistently hoards or stores large amounts of food in specialized c h a m b e r s or " l a r d e r s " (Ewer, 1968) within its burrow. N o n e of the e x c a v a t e d c h a m b e r s at this site contained either stored supplies of food or any evidence that food had b e e n c o n s u m e d in the burrows. Perhaps these rats were not hoarding food because it was plentiful, consistently available, or susceptible to spoilage. Alternately, rats m a y not normally hoard food at all or hoard only under limited conditions such as pregnancy or the onset of cold weather. Both captive (Barnett & Spencer, 1951; Calhoun, 1962) and free-living populations (Southern, Watson, & Chitty, 1946; T h o m p s o n , 1948) will transport large quantities of food f r o m a relatively e x p o s e d source to either a nearby
FORAGING AND FOOD HOARDING IN WILD RATS
531
p r o t e c t e d a r e a o r to t h e i r b u r r o w s p r i o r to c o n s u m p t i o n . P i s a n o a n d S t o r e r (1948) c a l l e d t h e s e i m m e d i a t e l y c o n v e n i e n t p r o t e c t e d sites " s h u c k ing s t a t i o n s " a n d L o r e a n d F l a n n e l l y (1978) f o u n d a b a n d o n e d t i r e s a d j a c e n t t o a l a r g e r e f u s e m o u n d to c o n t a i n l a r g e c a c h e s o f t h o r o u g h l y g n a w e d b o n e s . I n t h e l a t t e r s t u d y , no f o o d w a s f o u n d in t h e m o r e d i s t a n t b u r r o w s y s t e m s o f t h e a n i m a l s . C l e a r l y , t h e r a t s ' r e p u t a t i o n as a h o a r d e r is b a s e d a l m o s t e n t i r e l y u p o n its t e n d e n c y t o t r a n s p o r t f o o d ( b o t h in t h e l a b o r a t o r y a n d in the field) a n d w e n e e d m u c h m o r e i n f o r m a t i o n o n t h e c o n t e n t s o f b u r r o w s c o n s t r u c t e d b y f r e e - l i v i n g p o p u l a t i o n s o f wild r a t s to r e s o l v e this important issue.
REFERENCES Barnett, S. A., & Spencer, M. M. (1951). Feeding, social behaviour and interspecific competition in wild rats. Behaviour, 3, 229-242. Calhoun, J. B. (1962). The Ecology and Sociology of the Norway Rat. Bethesda. Md.: U. S. Department of Health, Education, and Welfare. Chitty, D., & Shorten, M. (1946). Techniques for the study of the Norway rat (Rattus norvegicus). Journal of Mammalogy, 27, 63-78. Cottam, C. (1948). Aquatic habits of the Norway rat. Journal of Mammalogy, 29, 299. Ewer, R. F. (1968). Ethology of Mammals. London: Logos Press. Hunt, J. McV. (1941). The effects of infantile feeding-frustration upon adult hoarding in the rat. Journal of Abnormal and Social Psychology, 36, 338-360. Lore, R. K., & Flannelly, K. (1978). Habitat selection and burrow construction by wild Rattus norvegicus in a landfll. Journal of Comparative and Physiological Psychology. 92, 888-896. Marx, M. H. (1950). A stimulus-response analysis of the hoarding habit in the rat. Psychological Review, 57, 80-93. Pisano, R. G., & Storer. T. I. (1948). Burrows and feeding of the Norway rat. Journal of Mammalogy, 29, 374-383. Southern, H. N., Watson, J. S., & Chitty, D. (1946). Watching nocturnal animals by infra-red radiation. Journal of Animal Ecology, 15, 198-202. Thompson, H. V. (1948). Studies of the behavior of the common brown rat. Bulletin of Animal Behaviour, 6, 26-40. Wolfe, J. B. (1939). An exploratory study of food-storing in rats. Journal of Comparative , Psychology, 28, 97-108.
BRIEF REPORT Foraging and Food Hoarding of Wild Rattus norvegicus in an Urban Environment LOREY K . TAKAHASHI AND RICHARD K . LORE 1
Psychology Department, Douglass College, Rutgers University, New Brunswick, New Jersey 08903 The foraging activities of a free-living population of wild Rattus norvegicus was confined to the nighttime hours. Peak feeding occurred during the early evening hours. Subsequent excavation of the burrow systems of these animals revealed no evidence that food had either been consumed or stored in the burrows.
According to Calhoun's (1962) observations, wild Rattus norvegicus reared in a large outdoor pen forage throughout the night. In Calhoun's study, peak activity occurred during the 3-hr period following sunset. Thereafter, foraging slowly declined until 2-3 hr prior to sunrise when another moderate increase in activity was observed. It is possible, however, that captivity distorts the typical foraging pattern of free-living populations of rats in unknown ways. For example, in most urban locations human activity remains high during the early evening hours when Calhoun's captive animals were also most active. Thus, rats might adjust their diurnal feeding cycle to avoid human contact during the early evening hours. Surprisingly, no information on the feeding activity of a free-living population of wild Rattus norvegicus is available. In the current study, we recorded the foraging activity of an undisturbed population of wild rats in a typical urban environment. After completion of the foraging observations, all burrows within a radius of 100 m of the food source were excavated and the contents of the burrows examined in order to determine if stores of food were present. The study site was located in the rear of a supermarket in central New 1 To whom requests for reprints should be addressed: Department of Psychology, Douglass College, Rutgers University. New Brunswick, N. J. 08903. This study was supported by grants from the Rutgers Research Council and the Busch Fund, Rutgers University. We thank William Carr for the use of the infrared telescope and Kevin Flannelly for his help in excavating the burrows. 527 0163 - 1047/80/080527-05502.00/0 Copyright© 1980by AcademmPress, Inc. All rightsof reproduettonm any form reserved
528
TAKAHASHIAND LORE
Jersey. The only food source was a large trash compactor (6.8 x 2.3 x 2.7 m) attached to the store via a delivery chute. A survey of the entire area indicated that signs of rat activity (burrow entrances, fecal pellets, trails) were confined to a single densely burrowed earthen bank separated from the compactor by an asphalt driveway approximately 12.8 m wide. Prior to our observations, we counted 114 burrow entrances along the bank. Virtually all of these were within 17 m of the compactor. In order to reach the compactor from their burrows, rats had to cross the asphalt driveway which was regularly used by vehicles during the daylight and early evening hours (0700-2100 hr). Human activity was minimal from 1800 to 0600 hr but the trash compactor operated until 2030 hr. Water was available from drainage along the bank. Condensation from the store's air conditioners provided a continuous water supply during the dry summer months. According to store employees, rats had occupied the site for more than 12 years. Observations were conducted during September to mid-November, 1978. Temperatures ranged from 6.2 to 19.5°C and sunset and sunrise were approximately 1830 and 0600 hr, respectively. Behavioral observations were made from a vehicle parked in a location that enabled the observation of rats crossing the driveway to either enter or leave the compactor. In addition, foraging activities in the immediate vicinity of the two sides of the compactor that could be observed were recorded. The latter measure represents animals that appeared from one of the many holes in the compactor and either entered the compactor at another location, disappeared under it, or fed on garbage spilled along its sides. The animals could be observed easily during the twilight hours with 7 x 50 binoculars and night observations were done with an infrared telescope (Varo, Inc., Garland, Tex.). The site was observed for 60 hr at irregular intervals (no observations were made during cloudy weather) with all hourly periods represented by 2-3 hr of observation time. After completion of the observational phase of the study, eight burrow systems that appeared to have been recently used because entrance holes were in good repair were excavated according to a procedure described by Lore & Flannelly (1978) and the presence of food and nesting material was recorded. Entries, exits, and sightings near the trash compactor were recorded separately but since each measure revealed essentially similar results, they were combined to yield a single index of foraging activity (Table 1). A total of 235 sightings were made between 1900 and 0800 hr and no rats were observed at the site before 1900 or after 0800 hr. Rats were most active during the period 2000-2200 hr and almost 70% of all sightings occurred during the 5-hr period from 1900 to 2400 hr. A slight increase in rats entering the compactor began again at 0300 hr with the majority leaving by 0500 hr.
FORAGING AND FOOD HOARDING IN WILD RATS
529
TABLE 1 Distribution of the Forage Activities of an Established Population of Wild Rats in a Typical Urban Setting a Time (hr)
Foraging activity index
1900-2000 2000-2100 2100-2200 2200-2300 2300-2400 2400-0100 0100-0200 0200-0300 0300-0400 0400-0500 0500-0600 0600-0700 0700-0800
7.00 20.33 13.00 11.00 6.33 2.33 2.67 4.00 9.00 5.00 2.50 1.33 0.89
The foraging activity index was derived by dividing the total number of sightings during each hourly interval by the number of hours observed at that interval.
Figure 1 illustrates the major portion of one excavated burrow. The eight burrows we examined contained a total of 18 chambers. In 16 chambers nesting material consisting of grass, paper, remnants of plastic bags, and bits of cloth was present. However, no stored food nor any evidence that food had been eaten (e.g., gnawed bones) in any of the burrows was found. Our results indicate that an established population of free-living rats inhabiting an environment that is typical for the species in the urban United States exhibits a feeding pattern similar to that reported for a captive population (Calhoun, 1962). In both studies, foraging activity was highest 2-3 hr following sunset and then declined slowly throughout the night until another slight increase in foraging occurred 2-3 hr prior to sunrise. But, there are differences. Calhoun's captive animals did feed occasionally just prior to sunset and after sunrise whereas we never observed foraging activity during the daylight hours. Wild rats will feed during the day when food is more readily available at this time (Chitty & Shorten, 1946; Cottam, 1948) or when humans are absent (Pisano & Storer, 1948) but at our site it seems likely that the presence of considerable pedestrian and vehicular traffic confined foraging activity to the nighttime hours. It should be noted, however, that peak feeding was not disrupted at our site during the early evening hours by either the intermittent operation of the trash compactor or passing vehicles. Thus, the feeding pattern was not modified in order to minimize contact with humans. Rattus norvegicus is considered to be a notorious hoarder of food
530
TAKAHASHI AND LORE
FIG. 1. This excavated burrow system was one of eight located near the refuse bin behind the supermarket. Tunnels (strips of white plastic), three entrances (small circles), and two chambers (large circles) are visible in the photograph. The meter stick in the middle provides size perspective. One tunnel segment and entrance, on the other side of the fence, are not visible. This burrow was gassed with chloropicrin prior to excavation and the body of one adult female weighing 216.7 g was recovered. despite the fact that the species lacks the specialized cheek pouches of other small rodents that are p r e s u m e d to a m a s s large stores of food (e.g., the Eastern Chipmunk, Tamias striatus, or any of the species of w o o d rats belonging to the genus, Neotoma)~ There is, of course, a long tradition and a large literature on " h o a r d i n g " by domestic strains of Rattus norvegicus in the laboratory (Hunt, 1941; Marx, 1950; Wolfe, 1939), but very little field data to support the assumption that free-living Rattus norvegicus consistently hoards or stores large amounts of food in specialized c h a m b e r s or " l a r d e r s " (Ewer, 1968) within its burrow. N o n e of the e x c a v a t e d c h a m b e r s at this site contained either stored supplies of food or any evidence that food had b e e n c o n s u m e d in the burrows. Perhaps these rats were not hoarding food because it was plentiful, consistently available, or susceptible to spoilage. Alternately, rats m a y not normally hoard food at all or hoard only under limited conditions such as pregnancy or the onset of cold weather. Both captive (Barnett & Spencer, 1951; Calhoun, 1962) and free-living populations (Southern, Watson, & Chitty, 1946; T h o m p s o n , 1948) will transport large quantities of food f r o m a relatively e x p o s e d source to either a nearby
FORAGING AND FOOD HOARDING IN WILD RATS
531
p r o t e c t e d a r e a o r to t h e i r b u r r o w s p r i o r to c o n s u m p t i o n . P i s a n o a n d S t o r e r (1948) c a l l e d t h e s e i m m e d i a t e l y c o n v e n i e n t p r o t e c t e d sites " s h u c k ing s t a t i o n s " a n d L o r e a n d F l a n n e l l y (1978) f o u n d a b a n d o n e d t i r e s a d j a c e n t t o a l a r g e r e f u s e m o u n d to c o n t a i n l a r g e c a c h e s o f t h o r o u g h l y g n a w e d b o n e s . I n t h e l a t t e r s t u d y , no f o o d w a s f o u n d in t h e m o r e d i s t a n t b u r r o w s y s t e m s o f t h e a n i m a l s . C l e a r l y , t h e r a t s ' r e p u t a t i o n as a h o a r d e r is b a s e d a l m o s t e n t i r e l y u p o n its t e n d e n c y t o t r a n s p o r t f o o d ( b o t h in t h e l a b o r a t o r y a n d in the field) a n d w e n e e d m u c h m o r e i n f o r m a t i o n o n t h e c o n t e n t s o f b u r r o w s c o n s t r u c t e d b y f r e e - l i v i n g p o p u l a t i o n s o f wild r a t s to r e s o l v e this important issue.
REFERENCES Barnett, S. A., & Spencer, M. M. (1951). Feeding, social behaviour and interspecific competition in wild rats. Behaviour, 3, 229-242. Calhoun, J. B. (1962). The Ecology and Sociology of the Norway Rat. Bethesda. Md.: U. S. Department of Health, Education, and Welfare. Chitty, D., & Shorten, M. (1946). Techniques for the study of the Norway rat (Rattus norvegicus). Journal of Mammalogy, 27, 63-78. Cottam, C. (1948). Aquatic habits of the Norway rat. Journal of Mammalogy, 29, 299. Ewer, R. F. (1968). Ethology of Mammals. London: Logos Press. Hunt, J. McV. (1941). The effects of infantile feeding-frustration upon adult hoarding in the rat. Journal of Abnormal and Social Psychology, 36, 338-360. Lore, R. K., & Flannelly, K. (1978). Habitat selection and burrow construction by wild Rattus norvegicus in a landfll. Journal of Comparative and Physiological Psychology. 92, 888-896. Marx, M. H. (1950). A stimulus-response analysis of the hoarding habit in the rat. Psychological Review, 57, 80-93. Pisano, R. G., & Storer. T. I. (1948). Burrows and feeding of the Norway rat. Journal of Mammalogy, 29, 374-383. Southern, H. N., Watson, J. S., & Chitty, D. (1946). Watching nocturnal animals by infra-red radiation. Journal of Animal Ecology, 15, 198-202. Thompson, H. V. (1948). Studies of the behavior of the common brown rat. Bulletin of Animal Behaviour, 6, 26-40. Wolfe, J. B. (1939). An exploratory study of food-storing in rats. Journal of Comparative , Psychology, 28, 97-108.