Hostname: page-component-848d4c4894-sjtt6 Total loading time: 0 Render date: 2024-07-04T17:35:18.131Z Has data issue: false hasContentIssue false
  • English
  • Français

Environmental enrichment for maned wolves (Chrysocyon brachyurus): group and individual effects

Published online by Cambridge University Press:  01 January 2023

AS Vasconcellos ,
MABV Guimarães ,
CA Oliveira ,
CS Pizzutto  and
C Ades
AS Vasconcellos*
Affiliation:
Av Prof Mello Moraes 1721, Institute of Psychology, Cidade Universitária, São Paulo, CEP 05508-030, Brazil
MABV Guimarães
Affiliation:
Av Prof Dr Orlando Marques de Paiva 87, VRA Department of Animal Reproduction, Cidade Universitária, São Paulo, CEP 05508-270, Brazil
CA Oliveira
Affiliation:
Av Prof Dr Orlando Marques de Paiva 87, VRA Department of Animal Reproduction, Cidade Universitária, São Paulo, CEP 05508-270, Brazil
CS Pizzutto
Affiliation:
Av Prof Dr Orlando Marques de Paiva 87, VRA Department of Animal Reproduction, Cidade Universitária, São Paulo, CEP 05508-270, Brazil
C Ades
Affiliation:
Av Prof Mello Moraes 1721, Institute of Psychology, Cidade Universitária, São Paulo, CEP 05508-030, Brazil
*
* Contact for correspondence and requests for reprints: angelv@usp.br
Get access Rights & Permissions [Opens in a new window]

Abstract

Procedures that increase foraging and exploratory behaviours are generally accepted as effective at improving welfare and reducing stereotypies in captive animals. To determine the effect of food and toy enrichment on the behaviour and hormonal levels of maned wolves (Chrysocyon brachyurus), 11 wolves were submitted to a baseline-enrichment-baseline schedule with four enrichment conditions: food scattering, toy presentation, edible toy presentation, and a combination of food and toy conditions. Behaviour and glucocorticoid metabolite concentrations (GCM) were assessed throughout the experiment. Our hypotheses were that enrichment procedures would lead to an increase in activity in the short and long term, a decrease in pacing, and a reduction in GCM concentrations. We also hypothesised that the concentrations of GCM would show a correlation with behaviour. Enrichment conditions increased immediate locomotion but did not produce long-term changes of activity nor reduced pacing. The procedures had a clear effect on individual behaviour and induced, under food enrichment conditions, an increased interest in food. Enrichment did not lead to decreased GCM concentration in the group as a whole, but there was a tendency for male and female animals to differ in hormonal reactions to environmental change. A positive correlation between GCM concentrations and pacing behaviour and a negative correlation between GCM concentrations and social behaviour were found. Results suggest that food scattering as an enrichment procedure may be used with maned wolves to enhance welfare, whilst also revealing the importance of taking individual and gender differences into account when planning enrichment techniques.

Keywords

animal welfareChrysocyon brachyurusenvironmental enrichmentfaecal corticoid metabolitesmaned wolfstress
Type
Research Article
Information
Animal Welfare , Volume 18 , Issue 3 , August 2009 , pp. 289 - 300
Copyright
© 2009 Universities Federation for Animal Welfare

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Ahola, L, Harri, M, Kasanen, S, Mononen, J and Pyykonen, T 2000 Effects of group housing in an enlarged cage system on growth, bite wounds and adrenal cortex function in farmed blue foxes (Alopex lagopus). Animal Welfare 9: 403–412Google Scholar
Albert, FW, Schepina, O, Winter, C, Römpler, H, Teupser, D, Palme, R, Ceglarek, U, Kratzsch, J, Sohr, R, Trut, LN, Thiery, J, Morgenstern, R, Plyusnina, IZ, Schöneberg, T and Pääbo, S 2008 Phenotypic differences in behavior, physiology and neurochemistry between rats selected for tameness and for tameness and for defensive aggression towards humans. Hormones and Behavior 53: 413–421CrossRefGoogle ScholarPubMed
Bandeira de Melo, LF, Lima Sábato, MA, Vaz Magni, EM, Young, RJ and Coelho, CM 2007 Secret lives of maned wolves (Chrysocyon brachyurus Illiger 1815): as revealed by GPS tracking collars. Journal of Zoology 271: 27–36CrossRefGoogle Scholar
Barnard, C 2007 Ethical regulation and animal science: why animal behaviour is special. Animal Behaviour 74: 5–13CrossRefGoogle Scholar
Bashaw, MJ, Bloomsmith, MA, Marr, MJ and Maple, TL 2003 To hunt or not to hunt? A feeding enrichment experiment with captive large felids. Zoo Biology 22: 189–198CrossRefGoogle Scholar
Bean, D, Mason, GJ and Bateson, M 1999 Contrafreeloading in starlings: testing the information hypothesis. Behaviour 136: 1267–1282CrossRefGoogle Scholar
Beattie, VE, O’Connell, NE, Kilpatrick, DJ and Moss, BW 2000 Influence of environmental enrichment on welfare-related behavioural and physiological parameters in growing pigs. Animal Science 70: 443–450CrossRefGoogle Scholar
Beccaceci, MD 1991 The maned wolf (Chrysocyon brachyurus) in Argentina. In: Matern, B (ed) International Studbook for the Maned Wolf (Chrysocyon brachyurus). Frankfurt Zoological Gardens: Frankfurt, GermanyGoogle Scholar
Belz, EE, Kennell, JS, Czambel, RK, Rubin, RT and Rhodes, ME 2003 Environmental enrichment lowers stress-responsive hormones in singly housed male and female rats. Pharmacology Biochemistry and Behavior 76: 481–486CrossRefGoogle ScholarPubMed
Bestelmeyer, SV 1998 Linking zoo research and field research for the conservation of the endangered maned wolf (Chrysocyon brachyurus). American Zoo and Aquarium Association Annual Conference Proceedings pp 48–54. 13-17 September 1998, Tulsa, Oklahoma, USAGoogle Scholar
Boinski, S, Swing, SP, Gross, TS and Davis, JK 1999 Environmental enrichment of brown capuchins (Cebus apella): behavioral and plasma and fecal cortisol measures of effectiveness. American Journal of Primatology 48: 49–683.0.CO;2-6>CrossRefGoogle ScholarPubMed
Bremner-Harrison, S, Prodohl, PA and Elwood, RW 2004 Behavioural trait assessment as a release criterion: boldness predicts early death in a reintroduction programme of captive-bred swift fox (Vulpes velox). Animal Conservation 7: 313–320CrossRefGoogle Scholar
Brown, JL, Wasser, SK, Wildt, DE and Graham, LH 1994 Comparative aspects of steroid hormone metabolism and ovarian activity in felids, measured non-invasively in feces. Biology of Reproduction 51: 776–786CrossRefGoogle Scholar
Cabrera, A 1957 Catalogo de los Mamiferos de America del Sur. Revista del Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’ e Instituto Nacional de Investigacion de las Ciencias Naturales, Ciencias Zoologicas, Buenos Aires 4(1): 107–110. [Title translation: Catalogue of the South American mammals]Google Scholar
Carlstead, K and Brown, JL 2005 Relationships between patterns of fecal corticoid excretion and behavior, reproduction, and environmental factors in captive black (Diceros bicornis) and white (Ceratotherium simum) rhinoceros. Zoo Biology 24: 215–232CrossRefGoogle Scholar
Carlstead, K, Brown, JL and Strawn, W 1993a Behavioral and physiological correlates of stress in laboratory cats. Applied Animal Behaviour Sciences 38: 143–158CrossRefGoogle Scholar
Carlstead, K, Brown, JL and Seidensticker, J 1993b Behavioral and adrenocortical responses to environmental changes in leopard cats (Felis-bengalensis). Zoo Biology 12: 321–331CrossRefGoogle Scholar
Carlstead, K and Kleiman, DG 1998 Behavior profiles of individual animals: a new tool for multi-institutional studies. American Zoo and Aquarium Association Annual Conference Proceedings pp 39–43. 13-17 September 1998, Tulsa, OklahomaGoogle Scholar
Clubb, R and Mason, G 2003 Captivity effects on wide-ranging carnivores. Nature 425: 473–474CrossRefGoogle ScholarPubMed
Cooper, JJ and Mason, GJ 2000 Increasing costs of access to resources cause re-scheduling of behaviour in American mink (Mustela vison): implications for the assessment of behavioural priorities. Applied Animal Behaviour Sciences 66: 135–151CrossRefGoogle Scholar
Cummings, D, Brown, JL, Rodden, MD and Songsasen, N 2007 Behavioral and physiologic responses to environmental enrichment in the maned wolf (Chrysocyon brachyurus). Zoo Biology 26: 331–343CrossRefGoogle Scholar
Daley, B and Lyndaker-Lindsey, S 2000 A survey of maned wolf enrichment practices in North American zoos. American Zoo and Aquarium Association Regional Conference Proceedings pp 17–26. 3-6 May 2000, Toledo, OH, USAGoogle Scholar
De Groot, J, De Jong, IC, Prelle, IT and Koolhaas, JM 2000 Immunity in barren and enriched housed pigs differing in baseline cortisol concentration. Physiology and Behavior 71: 217–223CrossRefGoogle ScholarPubMed
De Jong, IC, Prelle, IT, Van de Burgwal, JA, Lambooij, E, Korte, SM, Blokhuis, HJ and Koolhaas, JM 2000 Effects of environmental enrichment on behavioral responses top novelty, learning, and memory, and the circadian rhythm in cortisol in growing pigs. Physiology and Behavior 68: 571–578CrossRefGoogle Scholar
De Palma, C, Viggiano, E, Barillari, E, Palme, R, Dufour, AB, Fantini, C and Natoli, E 2005 Evaluating the temperament in shelter dogs. Behaviour 142: 1313–1334Google Scholar
Dietz, JM 1984 Ecology and social organization of the maned wolf (Chrysocyon brachyurus). Smithsonian Contributions to Zoology 392: 1–51CrossRefGoogle Scholar
Dobson, H and Smith, RF 1995 Stress and reproduction in farm animals. Journal of Reproduction and Fertility Supplement 49: 451–461Google ScholarPubMed
Fonseca, GAB, Rylands, AB, Costa, CMR, Machado, RB and Leite, YR 1994 Livro Vermelho dos Mamíferos Ameaçados de Extinção. Fundação Biodiversitas: Belo Horizonte, BrazilGoogle Scholar
Forthman, DL, Elder, SD, Bakeman, R, Kurkowski, TW, Noble, CC and Winslow, SW 1992 Effects of feeding enrichment on behaviour of three species of captive bears. Zoo Biology 11: 187–195CrossRefGoogle Scholar
Gorgasser, I, Tichy, A and Palme, R 2007 Faecal cortisol metabolites in Quarter Horses during initial training under field conditions. Wiener Tierarztliche Monatsschrift 94: 226–230Google Scholar
Haemisch, A, Voss, T and Gärtner, K 1994 Effects of environmental enrichment on aggressive behavior, dominance hierarchies, and endocrine states in male DBA/2J mice. Physiology and Behavior 56: 1041–1048CrossRefGoogle ScholarPubMed
Hansen, SW, Malmkvist, J, Palme, R and Damgaard, BM 2007 Do double cages and access to occupational materials improve the welfare of farmed mink? Animal Welfare 16: 63–76Google Scholar
Harri, M, Kasanen, S, Mononen, J and Sepponen, J 2000 Preferences of farmed blue foxes for different floor types. Behavioural Processes 49: 111–119CrossRefGoogle ScholarPubMed
Haubenhofer, D, Möstl, E and Kirchengast, S 2005 Cortisol concentrations in saliva of humans and their dogs during intensive training courses in animal-assisted therapy. Wiener Tierarztliche Monatsschrift 92: 66–73Google Scholar
Hubrecht, RC 1993 A comparision of social and environmental enrichment methods for laboratory housed dogs. Applied Animal Behaviour Science 37: 345–361CrossRefGoogle Scholar
Hughes, B and Duncan, I 1988 The notion of ethological ‘need’, models of motivation, and animal welfare. Animal Behaviour 36: 1696–1707CrossRefGoogle Scholar
Inglis, IR, Forkman, B and Lazarus, J 1997 Free food or earned food? A review and fuzzy model of contra-freeloading. Animal Behaviour 53: 1171–1191CrossRefGoogle ScholarPubMed
Ings, R, Waran, NK and Young, RJ 1997 Effect of wood-pile feeders on the behaviour of captive bush dogs (Speothos venaticus). Animal Welfare 6: 145–152Google Scholar
IUCN 2007 IUCN Red List of Threatened Species. http://www.iucnredlist.org. (Accessed 9 May 2008)Google Scholar
Jensen, GD 1963 Preference for bar pressing over ‘freeloading’ as a function of number of rewarded presses. Journal of Experimental Psychology 65: 451–454CrossRefGoogle ScholarPubMed
Kirkden, RD and Pajor, EA 2006 Using preference, motivation and aversion tests to ask scientific questions about animals’ feelings. Applied Animal Behaviour Science 100: 29–47CrossRefGoogle Scholar
Korhonen, H and Niemela, P 2000 Enrichment value of wooden blocks for farmed blue foxes (Alopex lagopus). Animal Welfare 9: 177–191Google Scholar
Kralj-Fiser, S, Scheiber, IBR, Blejec, A, Moestl, E and Kotrschal, K 2007 Individualities in a flock of free-roaming greylag geese: behavioral and physiological consistency over time and across situations. Hormones and Behavior 51: 239–248CrossRefGoogle Scholar
Krebs, JR, Kacelnik, A and Taylor, P 1978 Test of optimal sampling by sampling by foraging great tits. Nature 275: 27–31CrossRefGoogle Scholar
Lepschy, M, Touma, C, Hruby, R and Palme, R 2007 Noninvasive measurement of adrenocortical activity in male and female rats. Laboratory Animals 41: 372–387CrossRefGoogle Scholar
Lindqvist, C and Jensen, P 2008 Effects of age, sex and social isolation on contra-freeloading in red junglefowl (Gallus gallus) and white leghorn fowl. Applied Animal Behaviour Science 114: 419–428CrossRefGoogle Scholar
Liptrap, RM 1993 Stress and reproduction in domestic animals. Annals of the New York Academy of Sciences 697: 275–284CrossRefGoogle ScholarPubMed
Liu, J, Chen, Y, Guo, L, Gu, B, Liu, H, Hou, A, Liu, X, Sun, L and Liu, D 2006 Stereotypic behavior and fecal cortisol level in captive giant pandas in relation to environmental enrichment. Zoo Biology 25: 445–459CrossRefGoogle Scholar
Maia, OB and Gouveia, AMG 2002 Birth and mortality of maned wolves Chrysocyon brachyurus (Illiger, 1811) in captivity. Brazilian Journal of Biology 62: 25–32CrossRefGoogle ScholarPubMed
Marashi, V, Barnekow, A, Ossendorf, E and Sachser, N 2003 Effects of different forms of environmental enrichment on behavioral, endocrinological, and immunological parameters in male mice. Hormones and Behavior 43: 281–292CrossRefGoogle ScholarPubMed
Mason, GJ, Cooper, J and Clarebrough, C 2001 Frustrations of fur-farmed mink. Nature 410: 35–36CrossRefGoogle ScholarPubMed
McBride, SD and Cuddeford, D 2001 The putative welfare reducing effects of preventing equine stereotypic behaviour. Animal Welfare 10: 173–189Google Scholar
McGreevy, P and Nicol, C 1998 Physiological and behavioral consequences associated with sort-term prevention of crib-biting in horses. Physiology and Behavior 65: 15–23CrossRefGoogle Scholar
Mones, A and Olazarri, J 1990 Confirmación de la existencia de Chrysocyon brachyurus (Illiger) en el Uruguay (Mammalia: Carnivora: Canidae). Comunicaciónes Zoológicas del Museo de Historia Natural de Montevideo XII(174): 1-6. [Title translation: Confirmation of the existence of Chrysocyon brachyurus (Illiger) in Uruguay]Google Scholar
Moreira, N, Brown, JL, Moraes, W, Swanson, WF and Monteiro-Filho, ELA 2007 Effect of housing and environmental enrichment on adrenocortical activity, behavior and reproductive cyclicity in the female tigrina (Leopardus tigrinus) and margay (Leopardus wiedii). Zoo Biology 26: 441–460CrossRefGoogle Scholar
Mülleder, C, Palme, R, Menke, C and Waiblinger, S 2003 Individual differences in behaviour and in adrenocortical activity in beef-suckler cows. Applied Animal Behaviour Science 84: 167–183CrossRefGoogle Scholar
Munck, A, Guyre, PM and Holbrook, NI 1984 Physiological functions of glucocorticoids in stress and their relationship to pharmacological actions. Endocrinology Reviews 5: 25–44CrossRefGoogle Scholar
Natelson, BH, Ottenweller, JE, Cook, JA, Pitman, D, McCarty, R and Tapp, WN 1988 Effect of stressor intensity on habituation of the adrenocortical stress response. Physiology & Behavior 43: 41–46CrossRefGoogle ScholarPubMed
Neuringer, A 1969 Animals respond for food in the presence of free food. Science 166: 339–341CrossRefGoogle ScholarPubMed
Nimon, AJ and Broom, DM 2001 The welfare of farmed foxes Vulpes vulpes and Alopex lagopus in relation to housing and management: A review. Animal Welfare 10: 223–248Google Scholar
Nowak, RM and Paradiso, JL 1985 Mammals of the World, Volume 2. The Johns Hopkins University Press: London, UKGoogle Scholar
Nowak, RM and Paradiso, JL 1991 Mammals of the World, Volume 2. The Johns Hopkins University Press: London, UKGoogle Scholar
Pedersen, V 1996 Combined behavioural and physiological measurements as a basis of the assessment of animal welfare. Acta Agriculturae Scandinavica Section A, Animal Science 27: 69–75Google Scholar
Powell, DM and Svoke, JT 2008 Novel environmental enrichment may provide a tool for rapid assessment of animal personality: a case study with giant pandas (Ailuropoda melanoleuca). Journal of Applied Animal Welfare Science 11: 301–318CrossRefGoogle Scholar
Queirolo, D and Motta-Junior, JC 2007 Prey availability and diet of maned wolf in Serra da Canastra National Park, southeastern Brazil. Acta Theriologica 52: 391–402CrossRefGoogle Scholar
Raffa, KF, Havill, NP and Nordheim, EV 2002 How many choices can your test animal compare effectively? Evaluating a critical assumption of behavioural preference tests. Oecologia 133: 422–429CrossRefGoogle Scholar
Rodden, M, Rodrigues, F and Bestelmeyer, S 2004 Maned wolf (Chrysocyon brachyurus). In: Sillero-Zubiri, C, Hoffmann, M and Macdonald, DW (eds) Status Survey and Conservation Action Plan. Gland: Cambridge, UKGoogle Scholar
Rodden, M and Rosenthal, M 1995 Hand rearing and infant development. In: Fletchall, NB, Rodden, M and Taylor, S (eds) Husbandry Manual for the Maned Molf, Chrysocyon brachyurus. Maned Wolf Species Survival Plan: Virginia, USAGoogle Scholar
Rodrigues, FHG, Hass, A, Lacerda, ACR, Grando, LRSC, Bagno, MA, Bezerra, AMR and Silva, WR 2007 Feeding habits of the maned wolf (Chrysocyon brachyurus) in the Brazilian cerrado. Mastozoologia Neotropical 14: 37–51Google Scholar
Roy, V, Belzung, C, Delarue, C and Chapillon, P 2001 Environmental enrichment in BALB/c mice: effects in classical tests of anxiety and exposure to a predatory odour. Physiology and Behavior 74: 313–320CrossRefGoogle Scholar
Sábato, MAL, Melo, LFB, Magnia, EMV, Young, RJ and Coelho, CM 2006 A note on the effect of the full moon on the activity of wild maned wolves, Chrysocyon brachyurus. Behavioural Processes 73: 228–230CrossRefGoogle ScholarPubMed
Santos, EF, Setz, EZF and Gobbi, N 2003 Diet of the maned wolf (Chrysocyon brachyurus) and its role in seed dispersal on a cattle ranch in Brazil. Journal of Zoology (London) 260: 203–208CrossRefGoogle Scholar
Schapiro, SJ, Bloomsmith, MA, Kessel, AL and Shively, CA 1993 Effects of enrichment and housing on cortisol response in juvenile rhesus-monkeys. Applied Animal Behaviour Science 37: 251–263CrossRefGoogle Scholar
Shepherdson, DJ 1998 Introduction: tracing the path of environmental enrichment in zoos. In: Shepherdson, DJ, Mellen, JD and Hutchins, M (eds) Second Nature: Environmental Enrichment for Captive Animals pp 1–14. Smithsonian Institution: Washington DC, USAGoogle Scholar
Shepherdson, DJ, Carlstead, K, Mellen, JD and Seidensticker, J 1993 The influence on food presentation on the behavior of small cats in confined environments. Zoo Biology 12: 203–216CrossRefGoogle Scholar
Slotta-Bachmayr, L and Schwarzenberger, F 2007 Faecal cortisol metabolites as indicators of stress during training and search missions in avalanche dogs. Wiener Tierarztliche Monatsschrift 94: 110–117Google Scholar
Smith, RF and Dobson, H 2002 Hormonal interactions within the hypothalamus and pituitary with respect to stress and reproduction in sheep. Domestic Animal Endocrinology 23: 75–85CrossRefGoogle ScholarPubMed
Svedsen, PM, Hansen, BK, Malmkvist, J, Hansen, SW, Palme, R and Jeppesen, LL 2007 Selection against stereotypic behaviour may have contradictory consequences for the welfare of farm mink (Mustela vison). Applied Animal Behaviour Science 107: 110–119CrossRefGoogle Scholar
Terio, KS, Citino, SB and Brown, J 1999 Fecal cortisol metabolite analysis for non-invasive monitoring of adrenocortical function in the cheetah (Acinonyx jubatus). Journal of Zoo and Wildlife Medicine 30: 484–491Google Scholar
Timm, NH 1975 Multivariate Analysis with Applications in Education and Psychology. CA Brooks/Cole: Monterrey, CA, USAGoogle Scholar
Touma, C, Sachser, N, Möstl, E and Palme, R 2003 Effects of sex and time of day on metabolism and excretion of corticosterone in urine and feces of mice. General and Comparative Endocrinology 130: 267–278CrossRefGoogle ScholarPubMed
Trolle, M, Noss, AJ, Lima, EDS and Dalponte, JC 2007 Camera-trap studies of maned wolf density in the Cerrado and the Pantanal of Brazil. Biodiversity and Conservation 16: 1197–1204CrossRefGoogle Scholar
Vasconcellos, AS, Guimarães, MABV, Oliveira, CA, Chelini, MO, Adania, CH, Gomes, MS and Ades, C Validation of radioimmunoassay for noninvasive sampling of glucocorticoids in captive maned wolves (Chrysocyon brachyurus), in preparationGoogle Scholar
Velloso, AL, Wasser, SK, Monfort, SL and Dietz, JM 1998 Longitudinal fecal steroid excretion in maned wolves (Chrysocyon brachyurus). General and Comparative Endocrinology 112: 96–107CrossRefGoogle Scholar
Walsh, RN and Cummins, RA 1976 The Open-Field test: a critical review. Psychological Bulletin 83: 482–504CrossRefGoogle ScholarPubMed
Wasser, SK, Hunt, KE, Brown, JL, Cooper, K, Crockett, CM, Bechert, U, Millspaugh, JJ, Larson, S and Monfort, SL 2000 A generalized fecal glucocorticoid assay for use in a diverse array of nondomestic mammalian and avian species. General and Comparative Endocrinology 120: 260–275CrossRefGoogle Scholar
Weiss, D, Helmreich, S, Möstl, E, Dzidic, A and Bruckmaier, RM 2004 Coping capacity of dairy cows during the change from conventional to automatic milking. Journal of Animal Science 82: 563–570CrossRefGoogle ScholarPubMed
Wells, DL 2004 The influence of toys on the behaviour and welfare of kenneled dogs. Animal Welfare 13: 367–373Google Scholar
Wielebnowski, NC, Fletchall, N, Carlstead, K, Busso, JM and Brown, JL 2002a Noninvasive assessment of adrenal activity associated with husbandry and behavioural factors in the North American clouded leopard population. Zoo Biology 21: 77–98CrossRefGoogle Scholar
Wielebnowski, NC, Ziegler, K, Wildt, DE, Lukas, J and Brown, JL 2002b Impact of social management on reproductive, adrenal and behavioural activity in cheetah (Acinonyx jubatus). Animal Conservation 5: 291–301CrossRefGoogle Scholar
Winer, BJ 1971 Statistical Principles in Experimental Design. McGraw-Hill Book Co: New York, USAGoogle Scholar
Young, RJ and Lawrence, AB 2003 Do domestic pigs in controlled environments contrafreeload? Journal of Applied Animal Welfare Science 6: 309–318CrossRefGoogle ScholarPubMed