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Ventilation of badger Meles meles setts
Mammalian Biology
Mamm. biol. 68 (2003) 277±283 ã Urban & Fischer Verlag http://www.urbanfischer.de/journals/mammbiol
Zeitschrift fuÈr SaÈugetierkunde
Original investigation
Ventilation of badger Meles meles setts By T. J. ROPER and J. A. H. MOORE School of Biological Sciences, University of Sussex, Brighton, UK Receipt of Ms. 23. 05. 2002 Acceptance of Ms. 02. 10. 2002
Abstract Air currents were recorded at 44 separate underground locations in 12 badger Meles meles setts (five `main setts' and seven `outliers'). Within-sett air movements were strongly positively correlated with, but were from one to three orders of magnitude slower than, corresponding external wind speeds. Within-sett air movements were significantly weaker in sheltered setts than in open setts, and significantly stronger in nest chambers than in tunnels. Main setts were better ventilated than outliers at sampling locations that were relatively deep within a sett (i. e., more than 1 m from the nearest entrance). We conclude that wind-induced movement of air within badger setts contributes to ventilation of the interior of the sett, and that large setts are better ventilated than small ones. Key words: Meles meles, sett, burrow, ventilation, microclimate
Introduction By comparison with those of most semi-fossorial mammals, the burrows of badgers Meles meles are unusual in terms of their size, structural complexity and architectural variety (Neal and Roper 1991; Roper 1992). Badgers live in pairs or mixed-sex social groups, each of which has, as its place of permanent residence and breeding, a principal burrow system known as a `main sett' (Kruuk 1978; Neal and Cheeseman 1996). Main setts can be continuously inhabited for centuries (Neal 1977) and can grow to remarkable sizes. For example, Wijngaarden and Peppel (1964) refer to a main sett with over 100 entrances covering an area of more than 1 ha, while Roper et al. (1991) describe one with 178 entrances oc1616-5047/03/68/05-277 $ 15.00/0.
cupying 1.75 ha. In addition, besides possessing a main sett that may itself be very large, a badger group may also have access to a number of smaller `outlier' setts that are used as temporary resting places and emergency refuges (Roper et al. 2001 b). Outlier setts usually possess only one or two entrances but can be as large as a small main sett (Roper 1992). Altogether, therefore, badgers provide themselves with what seems to be a disproportionate amount of underground living space. Given that digging is a costly activity (Vleck 1979), it seems unlikely that badgers would dig large main setts unless it were beneficial to them to do so (Neal and Roper 1991). In addition, there is anecdotal
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evidence that badgers only breed successfully in relatively large setts (Likhachev 1956; Roper unpubl. data). Thus, the question arises as to why large main setts are better than small ones. Several hypotheses have been put forward, such as that a large sett enables breeding females to distance themselves from other, potentially aggressive, members of the social group (Neal 1977), or that a sett containing many nest chambers is less likely to become infested with ectoparasites (Butler and Roper 1996; Roper et al. 2001 b, 2002). Here, we test a third hypothesis, namely, that setts that have multiple entrances are better ventilated (Neal 1977). Roper and Kemenes (1997) showed that air currents could be detected within badger setts and that these air movements were largely abolished by blocking the sett entrances. This suggests that the entrances of a sett do contribute to its internal ventilation. In the present study, we measured within-sett air movements more systematically and in a more diverse sample of setts, in order to determine how ventilation of the sett interior is related to number of entrances and to wind speed on the surface.
Material and methods Study area: The study was carried out between 24 January 1995 and 12 March 1996, in a 2-km2 area of private farmland in the South Downs, 3 km south of Lewes, East Sussex, UK. The landscape consisted of chalk hills (the South Downs) running roughly northwards from the south coast of England (see Butler 1995 for details), where the population density of badgers was estimated to be 16.5 badgers/km2 (Moore 1997). Major habitat types were arable crops (68%), permanent pasture (29%) and scrub (2%). Arable crops were mainly winter wheat Triticum aestivum and rape Brassica rapifera, while scrub consisted mainly of hawthorn Crataegus mongyna, blackthorn Prunus spinosa and bramble Rubus fruticosus. Permanent pasture was grazed by sheep and cattle. Setts: Data were recorded from 12 setts in five badger territories. Setts were classified as either `main setts' (large setts that were permanently occupied and used for breeding: n = 5) or `outliers' (small setts that were only occasionally occupied:
n = 7) on the basis of radiotracking, bait-marking and sett surveys that had been carried out regularly in the study area since 1983 (e. g., Roper et al. 1986; Shepherdson et al. 1990; Roper et al. 1993, 1995). Number of entrances per sett varied from 1 to 42 (Tab. 1). All of the setts were located on hillsides, seven of them dug into open pasture and the remaining five within patches of scrub or woodland. Setts in pasture were classified as `open' and those in scrub or woodland as `sheltered' (see Tab. 1). Eleven of the setts faced roughly N (i. e., between NW and NE) while the twelfth (sett 1 in Tab. 1) faced S. Monitoring of air movements within setts: Movement of air within setts was measured by lowering a platinum hot-wire anemometer (129 MS, Solomat Ltd, sensitive to 0.01 m/s in the range 0± 1.0 m/s) into holes bored vertically down into the sett from the soil surface (see Moore 1997 for details). Holes (n = 56) were bored using a hand-operated auger, were lined with a piece of rigid 3-cm diameter plastic tubing, and were then closed by inserting a rubber bung into the protruding end of the plastic tube. The soil around the plastic tube was then tamped down to seal any gaps. Whenever possible, bores were placed so as to be at distances of 1 m, 2 m, 3 m etc. from the nearest sett entrance. The greatest distance of any bore from an entrance was 6 m, but four of the outliers were too short to enable bores to be inserted beyond the 3-m point. Of the 56 bores, 11 penetrated into nest chambers (defined as cavities exceeding 35 cm in height: see Roper and Kemenes 1997) and the remainder into tunnels. Monitoring of wind speed: Wind speed on the surface was measured using a rotating wind vane anemometer (sensitive to 0.1 m/s in the range 0± 20 m/s), positioned at a height of 25 cm above ground level at a distance of 1 m from the relevant sett entrance. Procedure: Prior to data collection, the hot-wire anemometer was lowered into a bore-hole so that the sensor was mid-way between the floor and ceiling of the relevant tunnel or chamber. The rubber bung used to close the bore-hole was then replaced and the system was allowed to equilibrate for 5 min. Recordings of air movements within the sett and on the surface were made by taking simultaneous readings from both anemometers every 10 s for a period of 5 min. Recordings were made in this way at every bore-hole during five separate 5-min sessions that were carried out on different days so as to sample a range of wind conditions. Thus, a total of 150 recordings was made at each of the 56 within-sett sampling locations. To avoid the possibility of air currents being caused by movement of badgers within the
Ventilation of badger Meles meles setts
279
Table 1. Characteristics of the 12 setts. a M = main sett, O = outlier; b O = open sett, S = sheltered sett; c ASR = air speed ratio; d Statistics show the strength of the correlation between speed of within-sett air movements and wind speed on the surface. Sett
Typea
Entrances
O/Sb
ASRc
t (d. f.)d
pd
1 2 3 4 5 6 7 8 9 10 11 12
M M M M M O O O O O O O
42 23 11 9 6 3 2 2 2 1 1 1
O S S O O S S O O S O O
0.100 0.027 0.018 0.034 0.047 0.001 0.006 0.055 0.063 0.003 0.019 0.069
49.3 (1 198) 13.9 (1 048) 12.2 (1 048) 24.9 (748) 24.6 (748) 3.3 (598) 6.1 (448) 14.1 (598) 24.0 (448) 3.6 (448) 11.9 (598) 18.9 (448)
< 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001
target setts, all recordings were made during daylight hours when badgers would be expected to be asleep (Neal and Cheeseman 1996). In addition, simultaneous monitoring of radio-collared badgers while some of the recordings were being made showed that these animals remained immobile throughout the procedure (Moore 1997). When nest chambers were being sampled, recordings were only made when visual inspection, through the bore-hole, showed that the chamber was not occupied by a sleeping badger.
Results Absolute speed of air movement within setts ranged from 0.00 m/s to 1.05 m/s, while wind speed ranged from 0.0 m/s to 8.6 m/s. In all 12 setts, there was a highly significant positive correlation between the speed of air movement within the sett and wind speed on the surface ( p < 0.001 in all cases: see Tab. 1). However, the slope of the regression line relating these two variables was substantially less than one (range 0.001 to 0.1: see Tab. 1) in all setts, indicating that air movements within setts were from one to three orders of magnitude slower than wind speeds on the surface. Henceforth, we use the term `air speed ratio' (ASR) to refer to the slope of the regression line relating speed of within-sett air movement to wind speed on the surface (see Fig. 1 for examples).
ASR was significantly less in sheltered setts than in open setts (Mann-Whitney U-test, n1 = 7, n2 = 5, U = 1, p < 0.01), indicating that sheltered setts were less well ventilated than open setts at any given wind speed. However, there was no significant overall correlation between ASR and number of sett entrances (Spearman test, rs = 0.175, n = 12, p > 0.5), nor was ASR significantly different between main versus outlier setts when readings were pooled from all sampling locations in each sett (Mann-Whitney U-test, n1 = 7, n2 = 5, U = 13, p > 0.5). More detailed analysis, however, revealed that air movements within setts decreased as a function of distance from the nearest entrance, and that the extent of this decrease differed between main and outlier setts. In all setts, ASR became progressively less at sampling points that were further from a sett entrance (see Fig. 1 for an example). However, this decline in ASR was steeper in outliers than in main setts (Fig. 2), showing that main setts were better ventilated than outliers at points that were relatively deep within the tunnel system. Repeated-measures analysis of variance confirmed these conclusions, showing a significant main effect due to sett type (main setts versus outliers: F1.8 = 7.45, p < 0.05), a significant main effect due to distance between sampling point and entrance (from 1 to 4 m: F3.24 = 59.09, p < 0.001, and a significant interaction be-
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Fig. 1. Regression lines relating speed of within-sett air currents to external wind speed, at different distances from the nearest sett entrance, in a single main sett. Filled diamonds: 1 m. Open squares: 2 m. Filled triangles: 3 m. Open circles: 4 m.
Fig. 2. Air speed ratio (ASR: mean and sd) for within-sett measurements taken at different distances from the nearest sett entrance, in main setts and outlier setts.
Ventilation of badger Meles meles setts tween sett type and distance (F3.24 = 5.55, p < 0.01). Note that this analysis only involved sampling points that were up to 4 m from a sett entrance, because none of the outliers contained sampling points at distances of 5 or 6 m from an entrance. In main setts, however, no air movements could be detected at distances of more than 4 m from the nearest entrance. To compare air movements in tunnels with air movements in nest chambers, it was necessary to control for differences in sett type and for the fact that chambers tend to be located relatively far from sett entrances. We therefore compared the ASR for each location that represented a chamber with the mean ASR for all tunnel locations that were from the same type of sett and were the same distance from an entrance. This comparison showed that ASR in chambers was significantly greater than in tunnels (Wilcoxon matched-pairs signed-ranks test, T = 10, n = 11, p < 0.05).
Discussion Burrowing is a widespread habit in mammals (Reichman and Smith 1990; Kinlaw 1999) but relatively little is known about the environmental conditions within mammal burrows and almost nothing about how burrows are ventilated (for a review see Roper et al. 2001 a). Theoretical models suggest that in small mammals, gas exchange between the walls of a burrow and the surrounding soil is sufficient to provide for the respiratory needs of the burrow occupants (Wilson and Kilgore 1978; Withers 1978). Indeed, this must be so for species which occupy closed burrow systems (Reichman and Smith 1990; Nevo 1979). In larger mammals, however, where the surface: volume ratio of the burrow is less, some degree of ventilation via burrow entrances is probably needed (Wilson and Kilgore 1978; Roper and Kemenes 1997). The present study, insofar as it shows that wind-induced air movements penetrate into badger setts to a distance of at least several metres, supports this hypothesis.
281
Our results also show that main setts, which have multiple entrances, are better ventilated than outlier setts, which have fewer entrances. However, it is less clear what mechanism underlies this effect. Vogel and others (Vogel and Bretz 1972; Vogel et al. 1973) suggested that prairie dogs Cynomys ludovicianus achieve passive ventilation of their U-shaped burrows by exploiting the phenomenon of boundary layering, whereby wind speed is reduced close to the ground. By constructing a higher spoil heap at one end of the burrow, so that this burrow entrance is effectively raised off the ground and exposed to a faster air stream, prairie dogs cause air to move through the burrow by viscosity suction. Neal (1977) suggested that a similar effect might operate in badger setts, which, because they are usually dug into slopes, typically have some entrances at a higher level than others. However, Neal's hypothesis misinterprets the nature of boundary layering, the exploitation of which depends on the height of a burrow entrance relative to the surface of the ground surrounding it, not on its absolute altitude. Consequently, if air moves into the entrances of badger setts it must do so by direct penetration of wind currents, not by viscosity suction. Direct penetration of wind currents is consistent with our results since it would predict a positive relationship between the number of entrances and the efficiency with which the interior of the sett is ventilated, given that the tunnel network within a multi-entrance sett is continuously interconnected (Roper 1992). Two other points emerging from our results are that (i) within setts, nest chambers were better ventilated than tunnels, and (ii) comparing different setts, those constructed in open terrain were better ventilated than those situated under cover. The finding that nest chambers tend to be draughtier than tunnels probably results from the facts, demonstrated by sett excavations, that nest chambers are often constructed at the intersection of two or more tunnels, and that they contain a larger air-space (Roper 1992). The difference cannot have been ow-
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ing to convection currents caused by sleeping badgers, because recordings were only made from unoccupied chambers. The finding that sheltered setts are less draughty than open setts is perhaps more significant, because in most landscapes badgers show a strong propensity to site their setts under cover (Neal and Roper 1991). In sheltered setts, ventilation by diffusion via sett entrances, by convection or by movements within the burrow of the animals themselves (the so-called `piston effect') is likely to be more important than wind-induced air
currents. All of these mechanisms, however, still require the presence of open burrow entrances and will be more effective the greater the number of entrances that the burrow possesses.
Acknowledgements We thank Robinson Farms Ltd. for allowing us access to their land, Dr L. Conradt for comments on the manuscript and for preparing the German summary, and the BBSRC for financial support.
Zusammenfassung Ventilation in Bauen des Dachses Meles meles Luftbewegungen wurden an 44 separaten Untergrundpositionen in 12 Bauen (fuÈnf ,Hauptbauen` und sieben ,Auslegerbauen`) des Dachses (Meles meles) gemessen. Luftbewegungen im Bau waren positiv korreliert zur korrespondierenden WindstaÈrke auûerhalb des Baues, aber zwischen ein und drei GroÈûenordnungen langsamer. Luftbewegungen im Bau waren schwaÈcher fuÈr windgeschuÈtzte als fuÈr windungeschuÈtzte Baue, und staÈrker in Schlafkammern als in Tunneln. Luftbewegungen im Bau hingen nicht von der Anzahl der BaueingaÈnge ab, aber Hauptbaue waren tief im Innern besser ventiliert als Auslegerbaue. Es wird gefolgert, daû windbedingte Luftbewegungen in Dachsbauen zur Ventilation des Bauinneren beitragen.
References Butler, J. M. (1995): The ecology of burrowing and burrow use in the European badger Meles meles. Dissthesis., University of Sussex, Brighton. Butler, J. M.; Roper, T. J. (1996): Ectoparasites and sett use in European badgers. Anim. Behav. 52, 621±629. Kinlaw, A. (1999): A review of burrowing by semi-fossorial vertebrates in arid environments. J. Arid Env. 41, 127±145. Kruuk, H. (1978): Spatial organisation and territorial behaviour of the European badger Meles meles. J. Zool. (London) 184, 1±19. Likhachev, G. N. (1956): Some ecological traits of the badger of the Tula Abatis broadleaved forests. In: Studies of Mammals in Government Preserves. Ed. by P. B. Jurgensen. Moscow: Ministry of Agriculture, USSR. Pp. 72± 94. Moore, J. A. H. (1997): Internal environment of badger Meles meles setts. Dissthesis., University of Sussex, Brighton.
Neal, E. G. (1977): Badgers. Poole, Dorset: Blandford Press. Neal, E. G.; Cheeseman, C. (1996): Badgers. London: T. and A. D. Poyser. Neal, E. G.; Roper, T. J. (1991): The environmental impact of badgers (Meles meles) and their setts. Symp. Zool. Soc. Lond. 63, 89±106. Nevo, E. (1979): Adaptive convergence and divergence of subterranean mammals. Ann. Rev. Ecol. Sys. 10, 269±308. Reichman, O. J.; Smith, S. C. (1990): Burrows and burrowing behavior by mammals. In: Current Mammalogy. Vol. 2. Ed. by H. H. Genoways. New York: Plenum. Pp. 197±243. Roper, T. J. (1992): Badger Meles meles setts: architecture, internal environment and function. Mammal Rev. 22, 43±53. Roper, T. J.; Kemenes, I. (1997): Effect of blocking of entrances on the internal environment of badger Meles meles setts. J. Appl. Ecol. 34, 1311±1319. Roper, T. J.; Shepherdson, D. J.; Davies, J. M.
Ventilation of badger Meles meles setts (1986): Scent marking with faeces and anal secretion in the European badger. Behaviour 97, 94±117. Roper, T. J.; Bennett, N. C.; Conradt, L.; Molteno, A. J. (2001 a): Environmental conditions in burrows of two species of African mole-rat, Georhychus capensis and Cryptomys damarensis. J. Zool. London 254, 101±107. Roper, T. J.; Findlay, S. R.; LuÈps, P.; Shepherdson, D. J. (1995): Damage by badgers Meles meles to wheat Triticium vulgare and barley Hordeum sativum crops. J. Appl. Ecol. 32, 720±726. Roper, T. J.; Jackson, T. P.; Conradt, L.; Bennett, N. C. (2002): Burrow use and influence of ectoparasites in Brants' whistling rat Parotomys brantsii. Ethology 108, 1±8. Roper, T. J.; Ostler, J. R.; Schmid, T. K.; Christian, S. F. (2001b): Sett use in European badgers Meles meles. Behaviour 138, 173±187. Roper, T. J.; Tait, A. I.; Fee, D.; Christian, S. F. (1991): Internal structure and contents of three badger (Meles meles) setts. J. Zool. (London) 225, 115±124. Roper, T. J.; Conradt, L.; Butler, J.; Christian, S. F.; Ostler, J.; Schmid, T. K. (1993): Territorial marking with faeces in badgers (Meles meles): a comparison of boundary and hinterland latrine use. Behaviour 127, 289±307.
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Shepherdson, D. J.; Roper, T. J.; LuÈps, P. (1990): Diet, food availability and foraging behaviour of badgers (Meles meles) in southern England. Z. SaÈugetierkunde 55, 81±93. Vleck, D. (1979): The energy cost of burrowing by the pocket gopher Thomomys bottae. Physiol. Zool. 52, 122±136. Vogel, S.; Bretz, W. C. (1972): Interfacial organisms: passive ventilation in the velocity gradient near surfaces. Science 175, 210±211. Vogel, S.; Ellington, C. P.; Kilgore, D. L. (1973): Wind-induced ventilation of the burrow of the prairie dog, Cynomys ludovicianus. J. Comp. Physiol. 85, 1±14. Wijngaarden, A. van; Peppel, J. van de (1964): The badger (Meles meles) in the Netherlands. Lutra 6, 1±60. Wilson, K. J.; Kilgore, D. L. Jnr. (1978): The effects of location and design on the diffusion of respiratory gases in mammal burrows. J. Theor. Biol. 71, 73±101. Withers, P. C. (1978): Models of diffusionmediated gas exchange in animal burrows. Amer. Nat. 112, 1101±1112. Authors' address: Timothy J. Roper and Jude A. J. Moore, School of Biological Sciences, University of Sussex, Brighton BN1 9QG, UK (email: [email protected])
Mamm. biol. 68 (2003) 277±283 ã Urban & Fischer Verlag http://www.urbanfischer.de/journals/mammbiol
Zeitschrift fuÈr SaÈugetierkunde
Original investigation
Ventilation of badger Meles meles setts By T. J. ROPER and J. A. H. MOORE School of Biological Sciences, University of Sussex, Brighton, UK Receipt of Ms. 23. 05. 2002 Acceptance of Ms. 02. 10. 2002
Abstract Air currents were recorded at 44 separate underground locations in 12 badger Meles meles setts (five `main setts' and seven `outliers'). Within-sett air movements were strongly positively correlated with, but were from one to three orders of magnitude slower than, corresponding external wind speeds. Within-sett air movements were significantly weaker in sheltered setts than in open setts, and significantly stronger in nest chambers than in tunnels. Main setts were better ventilated than outliers at sampling locations that were relatively deep within a sett (i. e., more than 1 m from the nearest entrance). We conclude that wind-induced movement of air within badger setts contributes to ventilation of the interior of the sett, and that large setts are better ventilated than small ones. Key words: Meles meles, sett, burrow, ventilation, microclimate
Introduction By comparison with those of most semi-fossorial mammals, the burrows of badgers Meles meles are unusual in terms of their size, structural complexity and architectural variety (Neal and Roper 1991; Roper 1992). Badgers live in pairs or mixed-sex social groups, each of which has, as its place of permanent residence and breeding, a principal burrow system known as a `main sett' (Kruuk 1978; Neal and Cheeseman 1996). Main setts can be continuously inhabited for centuries (Neal 1977) and can grow to remarkable sizes. For example, Wijngaarden and Peppel (1964) refer to a main sett with over 100 entrances covering an area of more than 1 ha, while Roper et al. (1991) describe one with 178 entrances oc1616-5047/03/68/05-277 $ 15.00/0.
cupying 1.75 ha. In addition, besides possessing a main sett that may itself be very large, a badger group may also have access to a number of smaller `outlier' setts that are used as temporary resting places and emergency refuges (Roper et al. 2001 b). Outlier setts usually possess only one or two entrances but can be as large as a small main sett (Roper 1992). Altogether, therefore, badgers provide themselves with what seems to be a disproportionate amount of underground living space. Given that digging is a costly activity (Vleck 1979), it seems unlikely that badgers would dig large main setts unless it were beneficial to them to do so (Neal and Roper 1991). In addition, there is anecdotal
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evidence that badgers only breed successfully in relatively large setts (Likhachev 1956; Roper unpubl. data). Thus, the question arises as to why large main setts are better than small ones. Several hypotheses have been put forward, such as that a large sett enables breeding females to distance themselves from other, potentially aggressive, members of the social group (Neal 1977), or that a sett containing many nest chambers is less likely to become infested with ectoparasites (Butler and Roper 1996; Roper et al. 2001 b, 2002). Here, we test a third hypothesis, namely, that setts that have multiple entrances are better ventilated (Neal 1977). Roper and Kemenes (1997) showed that air currents could be detected within badger setts and that these air movements were largely abolished by blocking the sett entrances. This suggests that the entrances of a sett do contribute to its internal ventilation. In the present study, we measured within-sett air movements more systematically and in a more diverse sample of setts, in order to determine how ventilation of the sett interior is related to number of entrances and to wind speed on the surface.
Material and methods Study area: The study was carried out between 24 January 1995 and 12 March 1996, in a 2-km2 area of private farmland in the South Downs, 3 km south of Lewes, East Sussex, UK. The landscape consisted of chalk hills (the South Downs) running roughly northwards from the south coast of England (see Butler 1995 for details), where the population density of badgers was estimated to be 16.5 badgers/km2 (Moore 1997). Major habitat types were arable crops (68%), permanent pasture (29%) and scrub (2%). Arable crops were mainly winter wheat Triticum aestivum and rape Brassica rapifera, while scrub consisted mainly of hawthorn Crataegus mongyna, blackthorn Prunus spinosa and bramble Rubus fruticosus. Permanent pasture was grazed by sheep and cattle. Setts: Data were recorded from 12 setts in five badger territories. Setts were classified as either `main setts' (large setts that were permanently occupied and used for breeding: n = 5) or `outliers' (small setts that were only occasionally occupied:
n = 7) on the basis of radiotracking, bait-marking and sett surveys that had been carried out regularly in the study area since 1983 (e. g., Roper et al. 1986; Shepherdson et al. 1990; Roper et al. 1993, 1995). Number of entrances per sett varied from 1 to 42 (Tab. 1). All of the setts were located on hillsides, seven of them dug into open pasture and the remaining five within patches of scrub or woodland. Setts in pasture were classified as `open' and those in scrub or woodland as `sheltered' (see Tab. 1). Eleven of the setts faced roughly N (i. e., between NW and NE) while the twelfth (sett 1 in Tab. 1) faced S. Monitoring of air movements within setts: Movement of air within setts was measured by lowering a platinum hot-wire anemometer (129 MS, Solomat Ltd, sensitive to 0.01 m/s in the range 0± 1.0 m/s) into holes bored vertically down into the sett from the soil surface (see Moore 1997 for details). Holes (n = 56) were bored using a hand-operated auger, were lined with a piece of rigid 3-cm diameter plastic tubing, and were then closed by inserting a rubber bung into the protruding end of the plastic tube. The soil around the plastic tube was then tamped down to seal any gaps. Whenever possible, bores were placed so as to be at distances of 1 m, 2 m, 3 m etc. from the nearest sett entrance. The greatest distance of any bore from an entrance was 6 m, but four of the outliers were too short to enable bores to be inserted beyond the 3-m point. Of the 56 bores, 11 penetrated into nest chambers (defined as cavities exceeding 35 cm in height: see Roper and Kemenes 1997) and the remainder into tunnels. Monitoring of wind speed: Wind speed on the surface was measured using a rotating wind vane anemometer (sensitive to 0.1 m/s in the range 0± 20 m/s), positioned at a height of 25 cm above ground level at a distance of 1 m from the relevant sett entrance. Procedure: Prior to data collection, the hot-wire anemometer was lowered into a bore-hole so that the sensor was mid-way between the floor and ceiling of the relevant tunnel or chamber. The rubber bung used to close the bore-hole was then replaced and the system was allowed to equilibrate for 5 min. Recordings of air movements within the sett and on the surface were made by taking simultaneous readings from both anemometers every 10 s for a period of 5 min. Recordings were made in this way at every bore-hole during five separate 5-min sessions that were carried out on different days so as to sample a range of wind conditions. Thus, a total of 150 recordings was made at each of the 56 within-sett sampling locations. To avoid the possibility of air currents being caused by movement of badgers within the
Ventilation of badger Meles meles setts
279
Table 1. Characteristics of the 12 setts. a M = main sett, O = outlier; b O = open sett, S = sheltered sett; c ASR = air speed ratio; d Statistics show the strength of the correlation between speed of within-sett air movements and wind speed on the surface. Sett
Typea
Entrances
O/Sb
ASRc
t (d. f.)d
pd
1 2 3 4 5 6 7 8 9 10 11 12
M M M M M O O O O O O O
42 23 11 9 6 3 2 2 2 1 1 1
O S S O O S S O O S O O
0.100 0.027 0.018 0.034 0.047 0.001 0.006 0.055 0.063 0.003 0.019 0.069
49.3 (1 198) 13.9 (1 048) 12.2 (1 048) 24.9 (748) 24.6 (748) 3.3 (598) 6.1 (448) 14.1 (598) 24.0 (448) 3.6 (448) 11.9 (598) 18.9 (448)
< 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001
target setts, all recordings were made during daylight hours when badgers would be expected to be asleep (Neal and Cheeseman 1996). In addition, simultaneous monitoring of radio-collared badgers while some of the recordings were being made showed that these animals remained immobile throughout the procedure (Moore 1997). When nest chambers were being sampled, recordings were only made when visual inspection, through the bore-hole, showed that the chamber was not occupied by a sleeping badger.
Results Absolute speed of air movement within setts ranged from 0.00 m/s to 1.05 m/s, while wind speed ranged from 0.0 m/s to 8.6 m/s. In all 12 setts, there was a highly significant positive correlation between the speed of air movement within the sett and wind speed on the surface ( p < 0.001 in all cases: see Tab. 1). However, the slope of the regression line relating these two variables was substantially less than one (range 0.001 to 0.1: see Tab. 1) in all setts, indicating that air movements within setts were from one to three orders of magnitude slower than wind speeds on the surface. Henceforth, we use the term `air speed ratio' (ASR) to refer to the slope of the regression line relating speed of within-sett air movement to wind speed on the surface (see Fig. 1 for examples).
ASR was significantly less in sheltered setts than in open setts (Mann-Whitney U-test, n1 = 7, n2 = 5, U = 1, p < 0.01), indicating that sheltered setts were less well ventilated than open setts at any given wind speed. However, there was no significant overall correlation between ASR and number of sett entrances (Spearman test, rs = 0.175, n = 12, p > 0.5), nor was ASR significantly different between main versus outlier setts when readings were pooled from all sampling locations in each sett (Mann-Whitney U-test, n1 = 7, n2 = 5, U = 13, p > 0.5). More detailed analysis, however, revealed that air movements within setts decreased as a function of distance from the nearest entrance, and that the extent of this decrease differed between main and outlier setts. In all setts, ASR became progressively less at sampling points that were further from a sett entrance (see Fig. 1 for an example). However, this decline in ASR was steeper in outliers than in main setts (Fig. 2), showing that main setts were better ventilated than outliers at points that were relatively deep within the tunnel system. Repeated-measures analysis of variance confirmed these conclusions, showing a significant main effect due to sett type (main setts versus outliers: F1.8 = 7.45, p < 0.05), a significant main effect due to distance between sampling point and entrance (from 1 to 4 m: F3.24 = 59.09, p < 0.001, and a significant interaction be-
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Fig. 1. Regression lines relating speed of within-sett air currents to external wind speed, at different distances from the nearest sett entrance, in a single main sett. Filled diamonds: 1 m. Open squares: 2 m. Filled triangles: 3 m. Open circles: 4 m.
Fig. 2. Air speed ratio (ASR: mean and sd) for within-sett measurements taken at different distances from the nearest sett entrance, in main setts and outlier setts.
Ventilation of badger Meles meles setts tween sett type and distance (F3.24 = 5.55, p < 0.01). Note that this analysis only involved sampling points that were up to 4 m from a sett entrance, because none of the outliers contained sampling points at distances of 5 or 6 m from an entrance. In main setts, however, no air movements could be detected at distances of more than 4 m from the nearest entrance. To compare air movements in tunnels with air movements in nest chambers, it was necessary to control for differences in sett type and for the fact that chambers tend to be located relatively far from sett entrances. We therefore compared the ASR for each location that represented a chamber with the mean ASR for all tunnel locations that were from the same type of sett and were the same distance from an entrance. This comparison showed that ASR in chambers was significantly greater than in tunnels (Wilcoxon matched-pairs signed-ranks test, T = 10, n = 11, p < 0.05).
Discussion Burrowing is a widespread habit in mammals (Reichman and Smith 1990; Kinlaw 1999) but relatively little is known about the environmental conditions within mammal burrows and almost nothing about how burrows are ventilated (for a review see Roper et al. 2001 a). Theoretical models suggest that in small mammals, gas exchange between the walls of a burrow and the surrounding soil is sufficient to provide for the respiratory needs of the burrow occupants (Wilson and Kilgore 1978; Withers 1978). Indeed, this must be so for species which occupy closed burrow systems (Reichman and Smith 1990; Nevo 1979). In larger mammals, however, where the surface: volume ratio of the burrow is less, some degree of ventilation via burrow entrances is probably needed (Wilson and Kilgore 1978; Roper and Kemenes 1997). The present study, insofar as it shows that wind-induced air movements penetrate into badger setts to a distance of at least several metres, supports this hypothesis.
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Our results also show that main setts, which have multiple entrances, are better ventilated than outlier setts, which have fewer entrances. However, it is less clear what mechanism underlies this effect. Vogel and others (Vogel and Bretz 1972; Vogel et al. 1973) suggested that prairie dogs Cynomys ludovicianus achieve passive ventilation of their U-shaped burrows by exploiting the phenomenon of boundary layering, whereby wind speed is reduced close to the ground. By constructing a higher spoil heap at one end of the burrow, so that this burrow entrance is effectively raised off the ground and exposed to a faster air stream, prairie dogs cause air to move through the burrow by viscosity suction. Neal (1977) suggested that a similar effect might operate in badger setts, which, because they are usually dug into slopes, typically have some entrances at a higher level than others. However, Neal's hypothesis misinterprets the nature of boundary layering, the exploitation of which depends on the height of a burrow entrance relative to the surface of the ground surrounding it, not on its absolute altitude. Consequently, if air moves into the entrances of badger setts it must do so by direct penetration of wind currents, not by viscosity suction. Direct penetration of wind currents is consistent with our results since it would predict a positive relationship between the number of entrances and the efficiency with which the interior of the sett is ventilated, given that the tunnel network within a multi-entrance sett is continuously interconnected (Roper 1992). Two other points emerging from our results are that (i) within setts, nest chambers were better ventilated than tunnels, and (ii) comparing different setts, those constructed in open terrain were better ventilated than those situated under cover. The finding that nest chambers tend to be draughtier than tunnels probably results from the facts, demonstrated by sett excavations, that nest chambers are often constructed at the intersection of two or more tunnels, and that they contain a larger air-space (Roper 1992). The difference cannot have been ow-
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ing to convection currents caused by sleeping badgers, because recordings were only made from unoccupied chambers. The finding that sheltered setts are less draughty than open setts is perhaps more significant, because in most landscapes badgers show a strong propensity to site their setts under cover (Neal and Roper 1991). In sheltered setts, ventilation by diffusion via sett entrances, by convection or by movements within the burrow of the animals themselves (the so-called `piston effect') is likely to be more important than wind-induced air
currents. All of these mechanisms, however, still require the presence of open burrow entrances and will be more effective the greater the number of entrances that the burrow possesses.
Acknowledgements We thank Robinson Farms Ltd. for allowing us access to their land, Dr L. Conradt for comments on the manuscript and for preparing the German summary, and the BBSRC for financial support.
Zusammenfassung Ventilation in Bauen des Dachses Meles meles Luftbewegungen wurden an 44 separaten Untergrundpositionen in 12 Bauen (fuÈnf ,Hauptbauen` und sieben ,Auslegerbauen`) des Dachses (Meles meles) gemessen. Luftbewegungen im Bau waren positiv korreliert zur korrespondierenden WindstaÈrke auûerhalb des Baues, aber zwischen ein und drei GroÈûenordnungen langsamer. Luftbewegungen im Bau waren schwaÈcher fuÈr windgeschuÈtzte als fuÈr windungeschuÈtzte Baue, und staÈrker in Schlafkammern als in Tunneln. Luftbewegungen im Bau hingen nicht von der Anzahl der BaueingaÈnge ab, aber Hauptbaue waren tief im Innern besser ventiliert als Auslegerbaue. Es wird gefolgert, daû windbedingte Luftbewegungen in Dachsbauen zur Ventilation des Bauinneren beitragen.
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