- Email: [email protected]
Domestication effects on foraging strategies in pigs (Sus scrofa)
Applied Animal Behaviour Science 62 Ž1999. 305–317
Domestication effects on foraging strategies in pigs žSus scrofa / Maria Gustafsson
a,)
, Per Jensen a , Francien H. de Jonge b, Teun Schuurman c
a
Swedish UniÕersity of Agricultural Sciences, Faculty of Veterinary Medicine, Department of Animal EnÕironment and Health, Section of Ethology, POB 234, 532 23 Skara, Sweden b Department of Ecological Agriculture, Wageningen Agricultural UniÕersity, Wageningen, Netherlands c Department of Physiology of Humans and Animals, Wageningen Agricultural UniÕersity, Wageningen, Netherlands Accepted 10 November 1998
Abstract It has been suggested that domestic animals have lost the ability to respond to environmental changes in an adaptive fashion. Others have suggested that during domestication, a shift may have occurred towards less costly foraging strategies. Eight domestic pigs Ž Sus scrofa. and eight crossbred pigs ŽHolland Landrace= Wild boar. were allowed to forage alone in a maze for 30 min on four successive days. The maze contained six gradually depleting food patches and corridors between them. Pigs obtained the food by manipulating the bucket with the snouts. On every second test the cost of moving between patches was increased by inserting 36.5–38.5 cm high wooden barriers between each food patch. Both breeds adapted their foraging pattern to the depletion of the patches and spent shorter time in each patch on successive visits. Domestic pigs spent longer average time in each patch. Both breeds spent longer time in patches when the maze contained barriers. The ingested amount of feed was reduced in both breeds when barriers were introduced. The domestic pigs passed totally a lower number of barriers compared with the crossbred pigs. Both domesticated and crossbred pigs visited fewer patches in the maze with barriers compared with the maze without. Weight of the pigs was not a major factor affecting the results. We conclude that both crossbred pigs and domestic pigs in general responded as expected from optimal foraging theory. Hence, domestic pigs still posses the ability to adapt their foraging behaviour in an adaptive fashion to the prevailing conditions. Crossbred pigs seemed to use a
)
Corresponding author. E-mail: [email protected]
0168-1591r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 5 9 1 Ž 9 8 . 0 0 2 3 6 - 6
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M. Gustafsson et al.r Applied Animal BehaÕiour Science 62 (1999) 305–317
more costly foraging strategy than domestic pigs. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Pig; Domestication; Foraging; Genetic influences; Wild boar
1. Introduction Ethological studies of domestication have mostly involved descriptive comparisons of behavioural frequencies in domesticated animals and their wild ancestors ŽDesforges and Wood Gush, 1976; Price, 1978.. Based on such studies, domesticated species have often been regarded as having lost adaptive capacities and being less attentive to environment stimuli ŽRatner and Boice, 1969; Boice, 1973; Coppinger, 1983; Hemmer, 1990; Budiansky, 1994.. Recent thinking has partly departed from this view and it has been suggested that domestication rather resembles a coevolved mutualistic relationship analogous to other symbiotic relations in nature, carrying specific evolutionary advantages to both parts in the relationship ŽBokonyi, 1989; Budiansky, 1994.. According to ¨ ¨ this theory, domestication can be considered an evolutionary process, similar to those taking place in nature. An important aspect of the domestication process is relaxed pressure from specific natural selection factors, such as food shortage and predation. Behaviour strategies important for survival and reproduction in nature may therefore lose some of its adaptive significance in captivity, such as optimised food- and shelter seeking and predator avoidance. Through human protection and care, animals that would be less fit in nature with respect to e.g., foraging and predator avoidance, are allowed to reproduce during the course of domestication ŽPrice, 1984.. Jensen and Gustafsson Ž1997. therefore suggested that costly behaviour and strategies which are optimal in wild conditions may pay off less in the domesticated situation. The logic of this suggestion is based on the assumption that animals using costly strategies may get a lower fitness benefit from this strategy during domestication compared to their wild conspecifics. Hence, costly strategies will be associated with a fitness penalty during domestication, and thus be selected against. Optimal foraging theory is based on the idea that animals maximise their net rate of energy intake, under the prevailing conditions ŽStephens and Krebs, 1986.. Probably the best known optimal foraging model is the marginal value theorem of Charnov Ž1976.. It predicts, among other things that animals should forage longer in each patch when the cost of moving between patches increases, and adapt their foraging pattern to the food intake per time unit in a given patch. In accordance with the theory suggested by Jensen and Gustafsson Ž1997., animals may have gained a fitness advantage during domestication, by using less costly foraging strategies. Domestic animals may therefore be less inclined to travel between patches and spend longer time feeding in each patch compared to a wild conspecific in the same situation. We therefore decided to study foraging in pigs Ž Sus scrofa. in an experimental environment where the cost of moving between patches could be altered. Since wild boars are difficult to handle and to keep for experimental purposes, we compared
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first-generation crossbred pigs between wild boars and domestic pigs with domestic pigs. The pig was domesticated about 9000 years ago and originates from the European Wild boar ŽHemmer, 1990.. Studies in semi-natural environments have revealed no major differences between the behaviour of wild boars, feral pigs or domestic pigs ŽGundlach, 1968; Graves, 1984; Jensen, 1986.. Pigs are social, omnivorous animals, that occupy variable home ranges of up to 25 km2 , when allowed to range freely. When foraging, they move between different feeding areas and feed by grazing, browsing or, more commonly, by rooting ŽGraves, 1984.. We studied the foraging pattern of pigs in an experimental set-up where pigs could move freely between gradually depleting food patches. We were able to alter the effort required by the pigs to move between patches by forcing them to jump over a barrier to reach the next patch. We had two major aims with this study. Firstly, we wanted to investigate whether domestic pigs, in spite of many generations of domestication and artificial selection, still are able to behave in an adaptive fashion in an experimental foraging situation. Secondly, we wanted to examine whether domestication may have caused alteration in the foraging strategies used by the pigs. Based on the theory suggested by Jensen and Gustafsson Ž1997. and on the theory of optimal foraging, we made the following predictions: Ø Pigs will adapt their foraging pattern to a gradual depletion of food patches and stay shorter time in a more depleted patch Ø When the cost of moving between patches is higher, pigs will stay longer in each patch. Ø Wild boar hybrids will use a more costly strategy, i.e., stay shorter time in each patch and move more between patches. To our knowledge this is the first attempt to experimentally investigate functional adaptation during domestication.
2. Material and methods 2.1. Animals and management This study was carried out at the Wageningen Institute of Animal Sciences at the Wageningen Agricultural University, the Netherlands, in March 1996. Sixteen female pigs, 9 months old, were used. They stemmed altogether from nine different mothers Žfour from one sow, one from each of four sows and two from the rest of the sows.. All mothers of the experimental pigs were Holland Landrace and four of the mother sows were artificially inseminated with semen from Great Yorkshire boar and the other four were naturally mated with a wild boar Ž S. scrofa.. From the offspring, eight domestic pigs and eight crossbred pigs were selected for the experiment. All pigs were born in farrowing huts outdoors and were weaned between 9.5 and 11 weeks of age. The mean weight of the experimental domestic sows at the start of the experiment was 128 kg Žrange from 108–140. and the mean weight of the crossbred sows was 91 kg
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Žrange from 71–119.. The sows were held from weaning until the start of the experiment in pairs in indoor standard pens Ž4.2 m2 . with a concrete floor, where half the pen was the dunging area. Domestic pigs were kept separated from crossbred pigs. The light was turned on at 0730 h and off at 2130 h. Each pig was provided with 1 kg standard commercial feed also used in the experiment Ždigestible energy: 14 MJrkg, raw-protein: 148 grkg. twice a day Žat 0800 and 1600 h. and had free access to water. 2.2. Experimental procedure Each animal was allowed to forage alone for 30 min on four different occasions in an experimental maze that consisted of six feed compartments and the corridors between them ŽFig. 1.. In each feed compartment Ž‘patch’. there was a bucket Ž5 dm3 . provided with 50 g of the same feed as the pigs were used to. Each bucket was attached to the wall by a 35 cm long string, which allowed the bucket to be moved freely within the range of the string. Each bucket had a lid which could not be opened by the pigs and eight circular holes Ž2 cm in diameter. in the walls of the bucket. The pigs had to manipulate the food buckets by pushing them around with their snouts in order to get the feed out onto the floor, where it could be ingested. Rotating the bucket while it was lying on the floor caused a gradual decrease in the return of feed per rotation ŽFig. 2.. Hence, each feed compartment could be regarded as a gradually depleting food patch. In the middle of the maze, an area was reserved for the observer. The cost of moving between patches was increased by inserting barriers between each food patch, which the pigs had to jump over in order to move between patches. The barriers were constructed by inserting pieces of wood through holes in the walls of the corridors. The three barriers between patches 1–4 were 36.5 cm high and 7 cm thick and
Fig. 1. The maze with its different parts. Number 1–6 represents the food patches, each provided with a food bucket. The start box and the observation area are also marked. All measures are in meters.
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Fig. 2. The cumulative return of feed in relation to number of rotations of the food bucket, measured by means of manual rotations of the bucket.
the two others were 38.5 cm high and 1.7 cm thick ŽFig. 1.. The height of the barriers was determined after running pilot tests with other pigs than the experimental animals Žof both breeds. and was chosen so that it should be possible for all pigs to pass the barrier, but it would take some apparent effort to do so. Each animal was subjected to each experimental treatment Žwith or without barriers. two times on alternating days. Hence, each pig was tested four times altogether. Each pig was given only one treatment per day. The order of testing was completely balanced within and between breeds and between replicates. One day prior to testing, the sows were adapted to the test procedures in the maze for 15 min without barriers. On these occasions the buckets in the first two patches had 16 holes instead of eight to make it easier for the pigs to learn how to manipulate the buckets. When being tested for the first time, all pigs were able to obtain feed in the patches, showing that they had learnt successfully. On the morning of each test day, the feed amounts in the home pens were reduced to 0.5 kg per pig. The first animal was tested at about 0830 h and the last animal was tested at about 1230 h. Testing order in relation to last feeding was balanced between breeds and replicates. Testing was conducted in a separate room of the same building in which the pigs were housed. For each test, the test pig was taken from the home pen to the start pen Ža walk of about 15 m. and was then let into the maze. After each test the animals were returned directly to their home pens. Each food bucket was weighed and the amount of food ingested in each food patch was calculated. The mazes were also cleaned after each tested animal. 2.3. ObserÕations and statistical analysis The behaviour of the pigs was recorded for 30 min in each session. Each session was video-recorded and direct observations were also made. Throughout the observation period, the observer remained in the observation area ŽFig. 1. where a video recorder
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and a monitor were also placed. The camera was positioned on a 2.5 m high stand on one of the short sides of the maze. The picture obtained allowed a full overview of all the patches. Feeding behaviour pattern was recorded on a one-zero basis once a minute by means of direct observations. A pig was scored to be feeding when it ingested at least one pellet. In addition, the cumulative number of patches visited was recorded as well as the cumulative number of barriers passed. From the video recordings, frequency of visits to each patch and time spent in each patch were recorded. For statistical analysis, we used Anderson–Darling Normality test to test the normal distribution and a balanced ANOVA model with a repeated measurement design to compare differences in each variable between the two different treatments in the domestic pigs and the crossbred pigs. The model tested effects of breed as main effect Ž df s 1. against an error-term with 14 degrees of freedom, and effects of treatment Žwith vs. without barriers. as a repeated measures factor Ž df s 1. against an error-term with 14 degrees of freedom. We used paired Student’s t-test when testing the difference in numbers of barriers passed and chi-square test when testing the difference between breeds in number of visits to each food patch. Forward stepwise regression was used to test whether the weight of the pigs had a strong influence on the results. The analyses were made with SuperAnova Ž1.11. and Minitab Ž10.2, 1994. software packages. All variations are given as the standard error of the mean. Data were pooled for each treatment. 3. Results All pigs moved between the patches and manipulated the buckets to obtain feed from the first trial ŽFig. 2.. Apart from occasional short interruptions for defecating andror
Fig. 3. Mean time for each successive visit to the patches Ž"SEM. in the treatment without barriers.
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Fig. 4. Mean time for each successive visit to the patches Ž"SEM. in the treatment with barriers.
urinating, the pigs spent all their time either in patches, manipulating food buckets and ingesting food, or travelling between patches. In 66% of the tests with crossbred pigs and in 53% of the tests with domestic pigs, all patches were visited and all buckets were
Fig. 5. Cumulated time spent in each visited patch Ž"SEM. for crossbred pigs and domestic pigs in the two different treatments; with barriers and without barriers.
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Fig. 6. Mean percentage of observations Ž"SEM. of the behaviour feeding for crossbred pigs and domestic pigs in the two different treatments; with barriers and without barriers.
manipulated. All barriers were passed in 44% of the tests with crossbred pigs and in 25% of the tests with domestic ones.
Fig. 7. Mean amount of ingested feed Ž"SEM. in each visited patch for crossbred pigs and domestic pigs in the two different treatments; with barriers and without barriers.
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Fig. 8. Mean number of passed barriers Ž"SEM. for crossbred pigs and domestic pigs.
As shown in Figs. 3 and 4, average time spent in each patch decreased in successive visits to the same patch. However, there was no significant difference between the breeds in the distribution of visits to different patches Žwithout barriers; x 2 s 1.053, df s 5, NS, with barriers; x 2 s 1.241, df s 5, NS.. Domestic pigs spent significantly more time on average in each food patch compared with the crossbred pigs Ž F1,14 s 5.644, P - 0.05.. Both breeds spent longer average time in the patches when the maze contained barriers Ž F1,14 s 14.99, P - 0.01. and the interaction between breed and treatment was not significant ŽFig. 5.. The total time spent in patches during the entire tests tended to be shorter in crossbred pigs Ž F1,14 s 3.25, P - 0.1., with no effect of treatment or of interaction between breed and treatment. The domestic pigs on average passed fewer barriers per test than the crossbred pigs Ž t s 2.20, P - 0.05, two tailed. ŽFig. 6.. Both breeds visited fewer patches when there were barriers present than when there were no barriers Ž F1,14 s 108.54, P - 0.001.. There was no effect of breed and no interaction of breed and treatment. Domestic pigs tended to perform more feeding behaviour than crossbred pigs Ž F1,14 s 3.993, P - 0.1.. There was also a tendency indicating that feeding behaviour Table 1 The results from forward stepwise regression analysis of number of barrier crossings on breed, treatment and weight Variable
Coefficient b
Standard deviation
t
P
Intercept Breed
30.250 y7.750
5.570 3.523
5.43 y2.20
- 0.001 0.045
R 2 s 25.7%.
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Table 2 The results from forward stepwise regression analysis of average cumulative time per patch on breed, treatment and weight Variable
Coefficient b
Standard deviation
t
P
Intercept Breed Treatment
131.17 22.96 29.56
19.46 8.93 8.93
6.74 2.57 3.31
- 0.001 0.016 0.003
R 2 s 37.7%.
was affected by treatment Ž F1,14 s 3.756, P - 0.1., so that less feeding behaviour was performed when the maze contained barriers ŽFig. 7.. The amount of ingested feed per patch was reduced in both breeds when the maze contained barriers Ž F1,14 s 6.968, P - 0.05. but there was no effect of breed and no interaction of breed and treatment. ŽFig. 8.. Total amount of ingested feed was significantly lower when there were barriers present Ž F1,14 s 6.779, P - 0.05. but there was no effect of breed and no interaction between breed and treatment. Weight was not a major factor in affecting the number of passed barriers or time in patches ŽTables 1 and 2.. Number of passed barriers was mainly affected by the breed of the pig and the time spent in each patch was mainly affected by both breed and treatment.
4. Discussion Both crossbred pigs and domestic pigs in general terms responded to a foraging situation as predicted by optimal foraging: they adapted their foraging pattern to the depletion of the food source and to the cost of moving between patches. It appeared as if wild boarrdomestic pig hybrids tended to use a more demanding foraging strategy than domestic pigs; they spent shorter average time in each patch and passed more barriers compared to domestic pigs. However, feed intake did not differ significantly between the breeds. Therefore, crossbred pigs obtained a similar amount of food compared to domestic pigs in both maze types, but they spent seemingly more energy in achieving this. This finding lends support to the theory suggested by Jensen and Gustafsson Ž1997., that pigs during domestication may have adapted to using less costly foraging strategies. There are alternative possible explanations to our results which need consideration. Our set-up assumes that there were only three types of costs involved in the foraging situation we experimentally designed: Ž1. costs associated with manipulating the food bucket and costs of travelling Ž2. within and Ž3. between patches. We further assumed that those costs would be the same to each animal and that our manipulation of travelling cost by inserting barriers would affect the costrbenefit ratio in the same way for both breeds. Since the pigs generally behaved as expected, i.e., they altered between feeding and moving between patches, without doing anything else, we believe our
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simplified assumptions may have been valid as a representation of the costs and benefits involved. However, it was clear that the costs were not identical to all animals. Domestic pigs and wild boars differ in both physical appearance and physical ability, which consequently might have interfered with the results. However, weight was not a major factor in explaining the results Žsee Tables 1 and 2., which supports the interpretation that we have recorded mainly genotypic differences in behavioural strategy. It is, nevertheless, possible that the crossbred pigs were more physically able to move between patches and to pass the barriers and that the cost induced by inserting barriers was not perceived as high to them. However, since the crossbred pigs were generally smaller than the domestic pigs, the barriers were relatively higher for the crossbred pigs, which rather may have increased the perceived costs of the crossbred pigs relative to the domestic pigs. Furthermore, the fact that the domestic pigs stayed longer in each patch even when there were no barriers present, again suggests that differences in behaviour patterns may be attributed to strategy differences caused by domestication. We cannot exclude that the perceived benefit of the food obtained differed somewhat between the breeds. We tried to standardise the feeding motivation by using the same deprivation times in both breeds, but the degree of feeding motivation at the start of each test may have been affected by differences in weight and metabolic rates. However, the feeding motivation appears not to affect the performance of oral feeding behaviour ŽHaskell et al., 1996.. There is no information available concerning metabolic rates in wild boar or wild boar crosses, but based only on the weight differences between the groups in this study, it is clear that the domestic pigs received generally lower food rations relative to their body weight. This might have increased their feeding motivation relative to the crossings and feeding motivation could perhaps affect the results we obtained. However, if this was an important aspect, we would have expected weight to be detected as a major factor by the stepwise regression analysis. Since this was not the case, the most likely explanation to our results is a genotypic difference in foraging strategy. The fact that we did not use pure wild boar could have made the results less clear. The method we used relies on the assumption that the behavioural strategy of crossbred pigs resembles that of wild boars. In fact, it appears most likely that crossbred pigs would represent an intermediate type of strategy ŽAlcock, 1993., but this does not alter the interpretation of our results. With our method, we had the advantage that all animals had the same developmental experience, since all were born and raised by the same types of mothers and under the same conditions. We believe this minimised the possibilities that factors other than genetic differences affected our results. Previous studies of differences between domestic and wild counterparts have described noticeable behavioural changes ŽBoice, 1972; Boreman and Price, 1972; Desforges and Wood Gush, 1975; Desforges and Wood Gush, 1976; Price, 1978; Johnson and Abrahams, 1991.. Most studies conclude that it is the frequency and intensity of different behaviour patterns that is affected by domestication ŽHale, 1969; Boice, 1972; Price, 1984.. This is supported by the present study, which also suggests that evolutionary models and costrbenefit analysis can be successfully applied to domestication changes. Our results are in accordance with other studies where functional adaptations to domestication have been studied, for example on aggression and schooling in fish
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ŽRuzzante, 1994. and on life-history adaptation in salmon and sea trout ŽPeterson et al., 1996.. In conclusion, when exploiting a set of gradually depleting food patches, both domestic pigs and crossbred Žwild boarrdomestic. pigs generally behaved as predicted by optimal foraging, i.e., they adapted the time in a patch to the degree of depletion of that patch and to the cost of moving to another patch. This demonstrates that pigs, in spite of many generations of domestication and artificial selection, still possess basic capacities for behavioural adaptation. However, it appears that domestic pigs may have used a different, less demanding strategy than wild-boar crossings. This supports the hypothesis, that domestication might have shifted behaviour optima towards less costly strategies. Since there were obvious physical differences between the breeds, which might have interfered with the results, some caution is needed in the interpretation.
Acknowledgements Financial support was received from the Swedish Council for Forestry and Agricultural Research. Maria Gustafsson received travel grants from the Royal Swedish Academy of Agriculture and Forestry and the Royal Swedish Academy of Sciences. Thanks also to Dr. Rick D’Eath and professor Tommy Radesater ¨ for constructive comments on the manuscript.
References Alcock, J., 1993. Animal Behaviour—An Evolutionary Approach, 5th edn. Sinauer Associates, Sunderland. Boice, R., 1972. Some behavioural tests of domestication in Norway rats. Behaviour 42, 199–231. Boice, R., 1973. Domestication. Psychological Bull. 80 Ž3., 25–230. Bokonyi, S., 1989. Definition of animal domestication. In: Clutton-Brock, J. ŽEd.., The Walking Larder. ¨ ¨ Unwin Hyman, London, pp. 22–27. Boreman, J., Price, E., 1972. Social dominance in wild and domestic Norway rats Ž Rattus norÕegicus.. Anim. Behav. 20, 534–542. Budiansky, S., 1994. A special relationship: the coevolution of human beings and domesticated animals. J. Am. Vet. Med. Assoc. 204 Ž3., 365–368. Charnov, 1976. Coppinger, R.P., 1983. The domestication of evolution. Environ. Cons. 10, 283–292. Desforges, M.F., Wood Gush, D.G.M., 1975. A behavioural comparison of domestic and mallard ducks. Habituation and flight reactions. Anim. Behav. 23, 692–697. Desforges, M.F., Wood Gush, D.G.M., 1976. A behavioural comparison of Aylesbury and mallard ducks: sexual behaviour. Anim. Behav. 24, 391–397. Graves, H.B., 1984. Behaviour and ecology of wild and feral swine Ž Sus scrofa.. J. Anim. Sci. 58, 482–492. Gundlach, H., 1968. Brutfursorge, brutpflege, verhaltensontogenese und tagesperiodik beim Europaischen ¨ wildschwein Ž Sus scrofa L... Z. Tierpsychol. 25, 955–995. Hale, E.B., 1969. Domestication and the evolution of behaviour. In: Hafez, E.S.E. ŽEd.., The Behaviour of Domestic Animals. Baillieire, Tindall and Cassell, London, pp. 22–42. Haskell, M.J., Terlouw, E.M.C., Lawrence, A.B., Deans, L.A., 1996. The post-feeding responses of sows to the daily presentation of food rewards in a test arena. Appl. Anim. Behav. Sci. 49, 125–135. Hemmer, H., 1990. Domestication—The Decline of Environmental Appreciation. Cambridge Univ. Press, Cambridge.
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Jensen, P., 1986. Observations on the maternal behaviour of free ranging domestic pigs. Appl. Anim. Behav. Sci. 16, 131–142. Jensen, P., Gustafsson, M., 1997. Towards a functional view on domestication. Proc. 31 st Int. Congress of the ISAE. 13–16 of August, Prague, Polygravia SAV, Czech Republik, pp. 43–44. Johnson, J.I., Abrahams, M.V., 1991. Interbreeding with domestic strain increases foraging under threat of predation in juvenile steelhead trout Ž Oncorhynchus mykiss .: an experimental study. Can. J. Aquat. Sci. 48, 243–247. Peterson, W., Jarvi, T., Steffner, G., Ragnarsson, B., 1996. The effect of domestication on some life history ¨ traits of sea trout and Atlantic salmon. J. Fish Biol. 48 Ž4., 776–791. Price, E.O., 1978. Genotype versus experience effects on aggression in wild and domestic Norway rats. Behaviour 64, 340–353. Price, E.O., 1984. Behavioral aspects of animal domestication. Q. Rev. Biol. 59, 1–32. Ratner, S.C., Boice, R., 1969. Effects of domestication on behaviour. In: Hafez, E.S.E. ŽEd.., The Behaviour of Domesticated Animals. Williams and Wilkins, Baltimore, pp. 3–18. Ruzzante, D.E., 1994. Domestication effects on aggressive and schooling behavior in fish. Aquaculture 120, 1–24. Stephens, D.W., Krebs, J.R., 1986. Foraging Theory: Monographs in Behaviour and Ecology. Princeton Univ. Press, NJ.
Domestication effects on foraging strategies in pigs žSus scrofa / Maria Gustafsson
a,)
, Per Jensen a , Francien H. de Jonge b, Teun Schuurman c
a
Swedish UniÕersity of Agricultural Sciences, Faculty of Veterinary Medicine, Department of Animal EnÕironment and Health, Section of Ethology, POB 234, 532 23 Skara, Sweden b Department of Ecological Agriculture, Wageningen Agricultural UniÕersity, Wageningen, Netherlands c Department of Physiology of Humans and Animals, Wageningen Agricultural UniÕersity, Wageningen, Netherlands Accepted 10 November 1998
Abstract It has been suggested that domestic animals have lost the ability to respond to environmental changes in an adaptive fashion. Others have suggested that during domestication, a shift may have occurred towards less costly foraging strategies. Eight domestic pigs Ž Sus scrofa. and eight crossbred pigs ŽHolland Landrace= Wild boar. were allowed to forage alone in a maze for 30 min on four successive days. The maze contained six gradually depleting food patches and corridors between them. Pigs obtained the food by manipulating the bucket with the snouts. On every second test the cost of moving between patches was increased by inserting 36.5–38.5 cm high wooden barriers between each food patch. Both breeds adapted their foraging pattern to the depletion of the patches and spent shorter time in each patch on successive visits. Domestic pigs spent longer average time in each patch. Both breeds spent longer time in patches when the maze contained barriers. The ingested amount of feed was reduced in both breeds when barriers were introduced. The domestic pigs passed totally a lower number of barriers compared with the crossbred pigs. Both domesticated and crossbred pigs visited fewer patches in the maze with barriers compared with the maze without. Weight of the pigs was not a major factor affecting the results. We conclude that both crossbred pigs and domestic pigs in general responded as expected from optimal foraging theory. Hence, domestic pigs still posses the ability to adapt their foraging behaviour in an adaptive fashion to the prevailing conditions. Crossbred pigs seemed to use a
)
Corresponding author. E-mail: [email protected]
0168-1591r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 5 9 1 Ž 9 8 . 0 0 2 3 6 - 6
306
M. Gustafsson et al.r Applied Animal BehaÕiour Science 62 (1999) 305–317
more costly foraging strategy than domestic pigs. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Pig; Domestication; Foraging; Genetic influences; Wild boar
1. Introduction Ethological studies of domestication have mostly involved descriptive comparisons of behavioural frequencies in domesticated animals and their wild ancestors ŽDesforges and Wood Gush, 1976; Price, 1978.. Based on such studies, domesticated species have often been regarded as having lost adaptive capacities and being less attentive to environment stimuli ŽRatner and Boice, 1969; Boice, 1973; Coppinger, 1983; Hemmer, 1990; Budiansky, 1994.. Recent thinking has partly departed from this view and it has been suggested that domestication rather resembles a coevolved mutualistic relationship analogous to other symbiotic relations in nature, carrying specific evolutionary advantages to both parts in the relationship ŽBokonyi, 1989; Budiansky, 1994.. According to ¨ ¨ this theory, domestication can be considered an evolutionary process, similar to those taking place in nature. An important aspect of the domestication process is relaxed pressure from specific natural selection factors, such as food shortage and predation. Behaviour strategies important for survival and reproduction in nature may therefore lose some of its adaptive significance in captivity, such as optimised food- and shelter seeking and predator avoidance. Through human protection and care, animals that would be less fit in nature with respect to e.g., foraging and predator avoidance, are allowed to reproduce during the course of domestication ŽPrice, 1984.. Jensen and Gustafsson Ž1997. therefore suggested that costly behaviour and strategies which are optimal in wild conditions may pay off less in the domesticated situation. The logic of this suggestion is based on the assumption that animals using costly strategies may get a lower fitness benefit from this strategy during domestication compared to their wild conspecifics. Hence, costly strategies will be associated with a fitness penalty during domestication, and thus be selected against. Optimal foraging theory is based on the idea that animals maximise their net rate of energy intake, under the prevailing conditions ŽStephens and Krebs, 1986.. Probably the best known optimal foraging model is the marginal value theorem of Charnov Ž1976.. It predicts, among other things that animals should forage longer in each patch when the cost of moving between patches increases, and adapt their foraging pattern to the food intake per time unit in a given patch. In accordance with the theory suggested by Jensen and Gustafsson Ž1997., animals may have gained a fitness advantage during domestication, by using less costly foraging strategies. Domestic animals may therefore be less inclined to travel between patches and spend longer time feeding in each patch compared to a wild conspecific in the same situation. We therefore decided to study foraging in pigs Ž Sus scrofa. in an experimental environment where the cost of moving between patches could be altered. Since wild boars are difficult to handle and to keep for experimental purposes, we compared
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first-generation crossbred pigs between wild boars and domestic pigs with domestic pigs. The pig was domesticated about 9000 years ago and originates from the European Wild boar ŽHemmer, 1990.. Studies in semi-natural environments have revealed no major differences between the behaviour of wild boars, feral pigs or domestic pigs ŽGundlach, 1968; Graves, 1984; Jensen, 1986.. Pigs are social, omnivorous animals, that occupy variable home ranges of up to 25 km2 , when allowed to range freely. When foraging, they move between different feeding areas and feed by grazing, browsing or, more commonly, by rooting ŽGraves, 1984.. We studied the foraging pattern of pigs in an experimental set-up where pigs could move freely between gradually depleting food patches. We were able to alter the effort required by the pigs to move between patches by forcing them to jump over a barrier to reach the next patch. We had two major aims with this study. Firstly, we wanted to investigate whether domestic pigs, in spite of many generations of domestication and artificial selection, still are able to behave in an adaptive fashion in an experimental foraging situation. Secondly, we wanted to examine whether domestication may have caused alteration in the foraging strategies used by the pigs. Based on the theory suggested by Jensen and Gustafsson Ž1997. and on the theory of optimal foraging, we made the following predictions: Ø Pigs will adapt their foraging pattern to a gradual depletion of food patches and stay shorter time in a more depleted patch Ø When the cost of moving between patches is higher, pigs will stay longer in each patch. Ø Wild boar hybrids will use a more costly strategy, i.e., stay shorter time in each patch and move more between patches. To our knowledge this is the first attempt to experimentally investigate functional adaptation during domestication.
2. Material and methods 2.1. Animals and management This study was carried out at the Wageningen Institute of Animal Sciences at the Wageningen Agricultural University, the Netherlands, in March 1996. Sixteen female pigs, 9 months old, were used. They stemmed altogether from nine different mothers Žfour from one sow, one from each of four sows and two from the rest of the sows.. All mothers of the experimental pigs were Holland Landrace and four of the mother sows were artificially inseminated with semen from Great Yorkshire boar and the other four were naturally mated with a wild boar Ž S. scrofa.. From the offspring, eight domestic pigs and eight crossbred pigs were selected for the experiment. All pigs were born in farrowing huts outdoors and were weaned between 9.5 and 11 weeks of age. The mean weight of the experimental domestic sows at the start of the experiment was 128 kg Žrange from 108–140. and the mean weight of the crossbred sows was 91 kg
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Žrange from 71–119.. The sows were held from weaning until the start of the experiment in pairs in indoor standard pens Ž4.2 m2 . with a concrete floor, where half the pen was the dunging area. Domestic pigs were kept separated from crossbred pigs. The light was turned on at 0730 h and off at 2130 h. Each pig was provided with 1 kg standard commercial feed also used in the experiment Ždigestible energy: 14 MJrkg, raw-protein: 148 grkg. twice a day Žat 0800 and 1600 h. and had free access to water. 2.2. Experimental procedure Each animal was allowed to forage alone for 30 min on four different occasions in an experimental maze that consisted of six feed compartments and the corridors between them ŽFig. 1.. In each feed compartment Ž‘patch’. there was a bucket Ž5 dm3 . provided with 50 g of the same feed as the pigs were used to. Each bucket was attached to the wall by a 35 cm long string, which allowed the bucket to be moved freely within the range of the string. Each bucket had a lid which could not be opened by the pigs and eight circular holes Ž2 cm in diameter. in the walls of the bucket. The pigs had to manipulate the food buckets by pushing them around with their snouts in order to get the feed out onto the floor, where it could be ingested. Rotating the bucket while it was lying on the floor caused a gradual decrease in the return of feed per rotation ŽFig. 2.. Hence, each feed compartment could be regarded as a gradually depleting food patch. In the middle of the maze, an area was reserved for the observer. The cost of moving between patches was increased by inserting barriers between each food patch, which the pigs had to jump over in order to move between patches. The barriers were constructed by inserting pieces of wood through holes in the walls of the corridors. The three barriers between patches 1–4 were 36.5 cm high and 7 cm thick and
Fig. 1. The maze with its different parts. Number 1–6 represents the food patches, each provided with a food bucket. The start box and the observation area are also marked. All measures are in meters.
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Fig. 2. The cumulative return of feed in relation to number of rotations of the food bucket, measured by means of manual rotations of the bucket.
the two others were 38.5 cm high and 1.7 cm thick ŽFig. 1.. The height of the barriers was determined after running pilot tests with other pigs than the experimental animals Žof both breeds. and was chosen so that it should be possible for all pigs to pass the barrier, but it would take some apparent effort to do so. Each animal was subjected to each experimental treatment Žwith or without barriers. two times on alternating days. Hence, each pig was tested four times altogether. Each pig was given only one treatment per day. The order of testing was completely balanced within and between breeds and between replicates. One day prior to testing, the sows were adapted to the test procedures in the maze for 15 min without barriers. On these occasions the buckets in the first two patches had 16 holes instead of eight to make it easier for the pigs to learn how to manipulate the buckets. When being tested for the first time, all pigs were able to obtain feed in the patches, showing that they had learnt successfully. On the morning of each test day, the feed amounts in the home pens were reduced to 0.5 kg per pig. The first animal was tested at about 0830 h and the last animal was tested at about 1230 h. Testing order in relation to last feeding was balanced between breeds and replicates. Testing was conducted in a separate room of the same building in which the pigs were housed. For each test, the test pig was taken from the home pen to the start pen Ža walk of about 15 m. and was then let into the maze. After each test the animals were returned directly to their home pens. Each food bucket was weighed and the amount of food ingested in each food patch was calculated. The mazes were also cleaned after each tested animal. 2.3. ObserÕations and statistical analysis The behaviour of the pigs was recorded for 30 min in each session. Each session was video-recorded and direct observations were also made. Throughout the observation period, the observer remained in the observation area ŽFig. 1. where a video recorder
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and a monitor were also placed. The camera was positioned on a 2.5 m high stand on one of the short sides of the maze. The picture obtained allowed a full overview of all the patches. Feeding behaviour pattern was recorded on a one-zero basis once a minute by means of direct observations. A pig was scored to be feeding when it ingested at least one pellet. In addition, the cumulative number of patches visited was recorded as well as the cumulative number of barriers passed. From the video recordings, frequency of visits to each patch and time spent in each patch were recorded. For statistical analysis, we used Anderson–Darling Normality test to test the normal distribution and a balanced ANOVA model with a repeated measurement design to compare differences in each variable between the two different treatments in the domestic pigs and the crossbred pigs. The model tested effects of breed as main effect Ž df s 1. against an error-term with 14 degrees of freedom, and effects of treatment Žwith vs. without barriers. as a repeated measures factor Ž df s 1. against an error-term with 14 degrees of freedom. We used paired Student’s t-test when testing the difference in numbers of barriers passed and chi-square test when testing the difference between breeds in number of visits to each food patch. Forward stepwise regression was used to test whether the weight of the pigs had a strong influence on the results. The analyses were made with SuperAnova Ž1.11. and Minitab Ž10.2, 1994. software packages. All variations are given as the standard error of the mean. Data were pooled for each treatment. 3. Results All pigs moved between the patches and manipulated the buckets to obtain feed from the first trial ŽFig. 2.. Apart from occasional short interruptions for defecating andror
Fig. 3. Mean time for each successive visit to the patches Ž"SEM. in the treatment without barriers.
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Fig. 4. Mean time for each successive visit to the patches Ž"SEM. in the treatment with barriers.
urinating, the pigs spent all their time either in patches, manipulating food buckets and ingesting food, or travelling between patches. In 66% of the tests with crossbred pigs and in 53% of the tests with domestic pigs, all patches were visited and all buckets were
Fig. 5. Cumulated time spent in each visited patch Ž"SEM. for crossbred pigs and domestic pigs in the two different treatments; with barriers and without barriers.
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Fig. 6. Mean percentage of observations Ž"SEM. of the behaviour feeding for crossbred pigs and domestic pigs in the two different treatments; with barriers and without barriers.
manipulated. All barriers were passed in 44% of the tests with crossbred pigs and in 25% of the tests with domestic ones.
Fig. 7. Mean amount of ingested feed Ž"SEM. in each visited patch for crossbred pigs and domestic pigs in the two different treatments; with barriers and without barriers.
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Fig. 8. Mean number of passed barriers Ž"SEM. for crossbred pigs and domestic pigs.
As shown in Figs. 3 and 4, average time spent in each patch decreased in successive visits to the same patch. However, there was no significant difference between the breeds in the distribution of visits to different patches Žwithout barriers; x 2 s 1.053, df s 5, NS, with barriers; x 2 s 1.241, df s 5, NS.. Domestic pigs spent significantly more time on average in each food patch compared with the crossbred pigs Ž F1,14 s 5.644, P - 0.05.. Both breeds spent longer average time in the patches when the maze contained barriers Ž F1,14 s 14.99, P - 0.01. and the interaction between breed and treatment was not significant ŽFig. 5.. The total time spent in patches during the entire tests tended to be shorter in crossbred pigs Ž F1,14 s 3.25, P - 0.1., with no effect of treatment or of interaction between breed and treatment. The domestic pigs on average passed fewer barriers per test than the crossbred pigs Ž t s 2.20, P - 0.05, two tailed. ŽFig. 6.. Both breeds visited fewer patches when there were barriers present than when there were no barriers Ž F1,14 s 108.54, P - 0.001.. There was no effect of breed and no interaction of breed and treatment. Domestic pigs tended to perform more feeding behaviour than crossbred pigs Ž F1,14 s 3.993, P - 0.1.. There was also a tendency indicating that feeding behaviour Table 1 The results from forward stepwise regression analysis of number of barrier crossings on breed, treatment and weight Variable
Coefficient b
Standard deviation
t
P
Intercept Breed
30.250 y7.750
5.570 3.523
5.43 y2.20
- 0.001 0.045
R 2 s 25.7%.
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Table 2 The results from forward stepwise regression analysis of average cumulative time per patch on breed, treatment and weight Variable
Coefficient b
Standard deviation
t
P
Intercept Breed Treatment
131.17 22.96 29.56
19.46 8.93 8.93
6.74 2.57 3.31
- 0.001 0.016 0.003
R 2 s 37.7%.
was affected by treatment Ž F1,14 s 3.756, P - 0.1., so that less feeding behaviour was performed when the maze contained barriers ŽFig. 7.. The amount of ingested feed per patch was reduced in both breeds when the maze contained barriers Ž F1,14 s 6.968, P - 0.05. but there was no effect of breed and no interaction of breed and treatment. ŽFig. 8.. Total amount of ingested feed was significantly lower when there were barriers present Ž F1,14 s 6.779, P - 0.05. but there was no effect of breed and no interaction between breed and treatment. Weight was not a major factor in affecting the number of passed barriers or time in patches ŽTables 1 and 2.. Number of passed barriers was mainly affected by the breed of the pig and the time spent in each patch was mainly affected by both breed and treatment.
4. Discussion Both crossbred pigs and domestic pigs in general terms responded to a foraging situation as predicted by optimal foraging: they adapted their foraging pattern to the depletion of the food source and to the cost of moving between patches. It appeared as if wild boarrdomestic pig hybrids tended to use a more demanding foraging strategy than domestic pigs; they spent shorter average time in each patch and passed more barriers compared to domestic pigs. However, feed intake did not differ significantly between the breeds. Therefore, crossbred pigs obtained a similar amount of food compared to domestic pigs in both maze types, but they spent seemingly more energy in achieving this. This finding lends support to the theory suggested by Jensen and Gustafsson Ž1997., that pigs during domestication may have adapted to using less costly foraging strategies. There are alternative possible explanations to our results which need consideration. Our set-up assumes that there were only three types of costs involved in the foraging situation we experimentally designed: Ž1. costs associated with manipulating the food bucket and costs of travelling Ž2. within and Ž3. between patches. We further assumed that those costs would be the same to each animal and that our manipulation of travelling cost by inserting barriers would affect the costrbenefit ratio in the same way for both breeds. Since the pigs generally behaved as expected, i.e., they altered between feeding and moving between patches, without doing anything else, we believe our
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simplified assumptions may have been valid as a representation of the costs and benefits involved. However, it was clear that the costs were not identical to all animals. Domestic pigs and wild boars differ in both physical appearance and physical ability, which consequently might have interfered with the results. However, weight was not a major factor in explaining the results Žsee Tables 1 and 2., which supports the interpretation that we have recorded mainly genotypic differences in behavioural strategy. It is, nevertheless, possible that the crossbred pigs were more physically able to move between patches and to pass the barriers and that the cost induced by inserting barriers was not perceived as high to them. However, since the crossbred pigs were generally smaller than the domestic pigs, the barriers were relatively higher for the crossbred pigs, which rather may have increased the perceived costs of the crossbred pigs relative to the domestic pigs. Furthermore, the fact that the domestic pigs stayed longer in each patch even when there were no barriers present, again suggests that differences in behaviour patterns may be attributed to strategy differences caused by domestication. We cannot exclude that the perceived benefit of the food obtained differed somewhat between the breeds. We tried to standardise the feeding motivation by using the same deprivation times in both breeds, but the degree of feeding motivation at the start of each test may have been affected by differences in weight and metabolic rates. However, the feeding motivation appears not to affect the performance of oral feeding behaviour ŽHaskell et al., 1996.. There is no information available concerning metabolic rates in wild boar or wild boar crosses, but based only on the weight differences between the groups in this study, it is clear that the domestic pigs received generally lower food rations relative to their body weight. This might have increased their feeding motivation relative to the crossings and feeding motivation could perhaps affect the results we obtained. However, if this was an important aspect, we would have expected weight to be detected as a major factor by the stepwise regression analysis. Since this was not the case, the most likely explanation to our results is a genotypic difference in foraging strategy. The fact that we did not use pure wild boar could have made the results less clear. The method we used relies on the assumption that the behavioural strategy of crossbred pigs resembles that of wild boars. In fact, it appears most likely that crossbred pigs would represent an intermediate type of strategy ŽAlcock, 1993., but this does not alter the interpretation of our results. With our method, we had the advantage that all animals had the same developmental experience, since all were born and raised by the same types of mothers and under the same conditions. We believe this minimised the possibilities that factors other than genetic differences affected our results. Previous studies of differences between domestic and wild counterparts have described noticeable behavioural changes ŽBoice, 1972; Boreman and Price, 1972; Desforges and Wood Gush, 1975; Desforges and Wood Gush, 1976; Price, 1978; Johnson and Abrahams, 1991.. Most studies conclude that it is the frequency and intensity of different behaviour patterns that is affected by domestication ŽHale, 1969; Boice, 1972; Price, 1984.. This is supported by the present study, which also suggests that evolutionary models and costrbenefit analysis can be successfully applied to domestication changes. Our results are in accordance with other studies where functional adaptations to domestication have been studied, for example on aggression and schooling in fish
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ŽRuzzante, 1994. and on life-history adaptation in salmon and sea trout ŽPeterson et al., 1996.. In conclusion, when exploiting a set of gradually depleting food patches, both domestic pigs and crossbred Žwild boarrdomestic. pigs generally behaved as predicted by optimal foraging, i.e., they adapted the time in a patch to the degree of depletion of that patch and to the cost of moving to another patch. This demonstrates that pigs, in spite of many generations of domestication and artificial selection, still possess basic capacities for behavioural adaptation. However, it appears that domestic pigs may have used a different, less demanding strategy than wild-boar crossings. This supports the hypothesis, that domestication might have shifted behaviour optima towards less costly strategies. Since there were obvious physical differences between the breeds, which might have interfered with the results, some caution is needed in the interpretation.
Acknowledgements Financial support was received from the Swedish Council for Forestry and Agricultural Research. Maria Gustafsson received travel grants from the Royal Swedish Academy of Agriculture and Forestry and the Royal Swedish Academy of Sciences. Thanks also to Dr. Rick D’Eath and professor Tommy Radesater ¨ for constructive comments on the manuscript.
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