Domestication effects on foraging strategies in pigs (Sus scrofa)

Ž .Applied Animal Behaviour Science 62 1999 305–317

Domestication effects on foraging strategies in pigsž /Sus 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 AnimalEnÕironment and Health, Section of Ethology, POB 234, 532 23 Skara, Sweden

b Department of Ecological Agriculture, Wageningen Agricultural UniÕersity, Wageningen, Netherlandsc 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 environmentalchanges in an adaptive fashion. Others have suggested that during domestication, a shift may have

Ž .occurred towards less costly foraging strategies. Eight domestic pigs Sus scrofa and eightŽ .crossbred pigs Holland 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 corridorsbetween them. Pigs obtained the food by manipulating the bucket with the snouts. On everysecond test the cost of moving between patches was increased by inserting 36.5–38.5 cm highwooden barriers between each food patch. Both breeds adapted their foraging pattern to thedepletion of the patches and spent shorter time in each patch on successive visits. Domestic pigsspent longer average time in each patch. Both breeds spent longer time in patches when the mazecontained barriers. The ingested amount of feed was reduced in both breeds when barriers wereintroduced. The domestic pigs passed totally a lower number of barriers compared with thecrossbred pigs. Both domesticated and crossbred pigs visited fewer patches in the maze withbarriers compared with the maze without. Weight of the pigs was not a major factor affecting theresults. We conclude that both crossbred pigs and domestic pigs in general responded as expectedfrom optimal foraging theory. Hence, domestic pigs still posses the ability to adapt their foragingbehaviour 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.Ž .PII: S0168-1591 98 00236-6

( )M. Gustafsson et al.rApplied Animal BehaÕiour Science 62 1999 305–317306

more costly foraging strategy than domestic pigs. q 1999 Elsevier Science B.V. All rightsreserved.

Keywords: Pig; Domestication; Foraging; Genetic influences; Wild boar

1. Introduction

Ethological studies of domestication have mostly involved descriptive comparisons ofŽbehavioural frequencies in domesticated animals and their wild ancestors Desforges and

.Wood Gush, 1976; Price, 1978 . Based on such studies, domesticated species have oftenbeen regarded as having lost adaptive capacities and being less attentive to environment

Žstimuli Ratner 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 relationshipanalogous to other symbiotic relations in nature, carrying specific evolutionary advan-

Ž .tages to both parts in the relationship Bokonyi, 1989; Budiansky, 1994 . According to¨ ¨this theory, domestication can be considered an evolutionary process, similar to thosetaking place in nature.

An important aspect of the domestication process is relaxed pressure from specificnatural selection factors, such as food shortage and predation. Behaviour strategiesimportant for survival and reproduction in nature may therefore lose some of its adaptivesignificance in captivity, such as optimised food- and shelter seeking and predatoravoidance. Through human protection and care, animals that would be less fit in naturewith respect to e.g., foraging and predator avoidance, are allowed to reproduce during

Ž . Ž .the course of domestication Price, 1984 . Jensen and Gustafsson 1997 thereforesuggested that costly behaviour and strategies which are optimal in wild conditions maypay off less in the domesticated situation. The logic of this suggestion is based on theassumption that animals using costly strategies may get a lower fitness benefit from thisstrategy during domestication compared to their wild conspecifics. Hence, costlystrategies will be associated with a fitness penalty during domestication, and thus beselected against.

Optimal foraging theory is based on the idea that animals maximise their net rate ofŽ .energy intake, under the prevailing conditions Stephens and Krebs, 1986 . Probably the

Ž .best known optimal foraging model is the marginal value theorem of Charnov 1976 . Itpredicts, among other things that animals should forage longer in each patch when thecost of moving between patches increases, and adapt their foraging pattern to the foodintake per time unit in a given patch. In accordance with the theory suggested by Jensen

Ž .and Gustafsson 1997 , animals may have gained a fitness advantage during domestica-tion, by using less costly foraging strategies. Domestic animals may therefore be lessinclined to travel between patches and spend longer time feeding in each patchcompared to a wild conspecific in the same situation.

Ž .We therefore decided to study foraging in pigs Sus scrofa in an experimentalenvironment where the cost of moving between patches could be altered. Since wildboars are difficult to handle and to keep for experimental purposes, we compared

( )M. Gustafsson et al.rApplied Animal BehaÕiour Science 62 1999 305–317 307

first-generation crossbred pigs between wild boars and domestic pigs with domesticpigs.

The pig was domesticated about 9000 years ago and originates from the EuropeanŽ .Wild boar Hemmer, 1990 . Studies in semi-natural environments have revealed no

major differences between the behaviour of wild boars, feral pigs or domestic pigsŽ .Gundlach, 1968; Graves, 1984; Jensen, 1986 . Pigs are social, omnivorous animals, thatoccupy variable home ranges of up to 25 km2, when allowed to range freely. Whenforaging, they move between different feeding areas and feed by grazing, browsing or,

Ž .more commonly, by rooting Graves, 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 effortrequired by the pigs to move between patches by forcing them to jump over a barrier toreach the next patch. We had two major aims with this study. Firstly, we wanted toinvestigate whether domestic pigs, in spite of many generations of domestication andartificial selection, still are able to behave in an adaptive fashion in an experimentalforaging situation. Secondly, we wanted to examine whether domestication may havecaused alteration in the foraging strategies used by the pigs.

Ž .Based on the theory suggested by Jensen and Gustafsson 1997 and on the theory ofoptimal 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 patchØ 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 theWageningen Agricultural University, the Netherlands, in March 1996.

Sixteen female pigs, 9 months old, were used. They stemmed altogether from nineŽdifferent mothers four 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 ofthe mother sows were artificially inseminated with semen from Great Yorkshire boar

Ž .and the other four were naturally mated with a wild boar S. scrofa . From the offspring,eight domestic pigs and eight crossbred pigs were selected for the experiment. All pigswere born in farrowing huts outdoors and were weaned between 9.5 and 11 weeks ofage.

The mean weight of the experimental domestic sows at the start of the experimentŽ .was 128 kg range from 108–140 and the mean weight of the crossbred sows was 91 kg


Ž .range from 71–119 . The sows were held from weaning until the start of the experimentŽ 2 .in pairs in indoor standard pens 4.2 m with a concrete floor, where half the pen was

the dunging area. Domestic pigs were kept separated from crossbred pigs. The light wasturned on at 0730 h and off at 2130 h. Each pig was provided with 1 kg standard

Žcommercial feed also used in the experiment digestible energy: 14 MJrkg, raw-protein:. Ž .148 grkg twice a day at 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 anexperimental maze that consisted of six feed compartments and the corridors between

Ž . Ž . Ž 3.them Fig. 1 . In each feed compartment ‘patch’ there was a bucket 5 dm providedwith 50 g of the same feed as the pigs were used to. Each bucket was attached to thewall by a 35 cm long string, which allowed the bucket to be moved freely within therange of the string. Each bucket had a lid which could not be opened by the pigs and

Ž .eight circular holes 2 cm in diameter in the walls of the bucket. The pigs had tomanipulate the food buckets by pushing them around with their snouts in order to get thefeed out onto the floor, where it could be ingested. Rotating the bucket while it was

Ž .lying on the floor caused a gradual decrease in the return of feed per rotation Fig. 2 .Hence, each feed compartment could be regarded as a gradually depleting food patch. Inthe middle of the maze, an area was reserved for the observer.

The cost of moving between patches was increased by inserting barriers between eachfood patch, which the pigs had to jump over in order to move between patches. Thebarriers were constructed by inserting pieces of wood through holes in the walls of thecorridors. 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 foodbucket. The start box and the observation area are also marked. All measures are in meters.


Fig. 2. The cumulative return of feed in relation to number of rotations of the food bucket, measured by meansof manual rotations of the bucket.

Ž .the two others were 38.5 cm high and 1.7 cm thick Fig. 1 . The height of the barrierswas determined after running pilot tests with other pigs than the experimental animalsŽ .of both breeds and was chosen so that it should be possible for all pigs to pass thebarrier, but it would take some apparent effort to do so.

Ž .Each animal was subjected to each experimental treatment with or without barrierstwo times on alternating days. Hence, each pig was tested four times altogether. Eachpig was given only one treatment per day. The order of testing was completely balancedwithin and between breeds and between replicates.

One day prior to testing, the sows were adapted to the test procedures in the maze for15 min without barriers. On these occasions the buckets in the first two patches had 16holes instead of eight to make it easier for the pigs to learn how to manipulate thebuckets. When being tested for the first time, all pigs were able to obtain feed in thepatches, showing that they had learnt successfully.

On the morning of each test day, the feed amounts in the home pens were reduced to0.5 kg per pig. The first animal was tested at about 0830 h and the last animal was testedat about 1230 h. Testing order in relation to last feeding was balanced between breedsand replicates. Testing was conducted in a separate room of the same building in whichthe pigs were housed. For each test, the test pig was taken from the home pen to the start

Ž .pen a 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 wasvideo-recorded and direct observations were also made. Throughout the observation

Ž .period, the observer remained in the observation area Fig. 1 where a video recorder


and a monitor were also placed. The camera was positioned on a 2.5 m high stand onone of the short sides of the maze. The picture obtained allowed a full overview of allthe patches. Feeding behaviour pattern was recorded on a one-zero basis once a minuteby means of direct observations. A pig was scored to be feeding when it ingested at leastone pellet. In addition, the cumulative number of patches visited was recorded as well asthe cumulative number of barriers passed. From the video recordings, frequency of visitsto each patch and time spent in each patch were recorded.

For statistical analysis, we used Anderson–Darling Normality test to test the normaldistribution and a balanced ANOVA model with a repeated measurement design tocompare differences in each variable between the two different treatments in thedomestic pigs and the crossbred pigs. The model tested effects of breed as main effectŽ . Ždfs1 against an error-term with 14 degrees of freedom, and effects of treatment with

. Ž .vs. without barriers as a repeated measures factor dfs1 against an error-term with 14degrees of freedom. We used paired Student’s t-test when testing the difference innumbers of barriers passed and chi-square test when testing the difference betweenbreeds in number of visits to each food patch. Forward stepwise regression was used totest whether the weight of the pigs had a strong influence on the results. The analyses

Ž . Ž .were made with SuperAnova 1.11 and Minitab 10.2, 1994 software packages. Allvariations are given as the standard error of the mean. Data were pooled for eachtreatment.

3. Results

All pigs moved between the patches and manipulated the buckets to obtain feed fromŽ .the first trial Fig. 2 . Apart from occasional short interruptions for defecating andror

Ž .Fig. 3. Mean time for each successive visit to the patches "SEM in the treatment without barriers.


Ž .Fig. 4. Mean time for each successive visit to the patches "SEM in the treatment with barriers.

urinating, the pigs spent all their time either in patches, manipulating food buckets andingesting food, or travelling between patches. In 66% of the tests with crossbred pigsand in 53% of the tests with domestic pigs, all patches were visited and all buckets were

Ž .Fig. 5. Cumulated time spent in each visited patch "SEM for crossbred pigs and domestic pigs in the twodifferent treatments; with barriers and without barriers.


Ž .Fig. 6. Mean percentage of observations "SEM of the behaviour feeding for crossbred pigs and domesticpigs in the two different treatments; with barriers and without barriers.

manipulated. All barriers were passed in 44% of the tests with crossbred pigs and in25% of the tests with domestic ones.

Ž .Fig. 7. Mean amount of ingested feed "SEM in each visited patch for crossbred pigs and domestic pigs inthe two different treatments; with barriers and without barriers.


Ž .Fig. 8. Mean number of passed barriers "SEM for crossbred pigs and domestic pigs.

As shown in Figs. 3 and 4, average time spent in each patch decreased in successivevisits to the same patch. However, there was no significant difference between the

Ž 2breeds in the distribution of visits to different patches without barriers; x s1.053,2 .dfs5, NS, with barriers; x s1.241, dfs5, NS .

Domestic pigs spent significantly more time on average in each food patch comparedŽ .with the crossbred pigs F s5.644, P-0.05 . Both breeds spent longer average time1,14

Ž .in the patches when the maze contained barriers F s14.99, P-0.01 and the1,14Ž .interaction between breed and treatment was not significant Fig. 5 . The total time spent

Žin patches during the entire tests tended to be shorter in crossbred pigs F s3.25,1,14.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 pigsŽ . Ž .ts2.20, P-0.05, two tailed Fig. 6 . Both breeds visited fewer patches when there

Ž .were barriers present than when there were no barriers F s108.54, P-0.001 .1,14

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 .F s3.993, P-0.1 . There was also a tendency indicating that feeding behaviour1,14

Table 1The results from forward stepwise regression analysis of number of barrier crossings on breed, treatment andweight

Variable Coefficient b Standard deviation t P

Intercept 30.250 5.570 5.43 -0.001Breed y7.750 3.523 y2.20 0.045

R2 s25.7%.


Table 2The 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 131.17 19.46 6.74 -0.001Breed 22.96 8.93 2.57 0.016Treatment 29.56 8.93 3.31 0.003

R2 s37.7%.

Ž .was affected by treatment F s3.756, P-0.1 , so that less feeding behaviour was1,14Ž .performed when the maze contained barriers Fig. 7 .

The amount of ingested feed per patch was reduced in both breeds when the mazeŽ .contained barriers F s6.968, P-0.05 but there was no effect of breed and no1,14

Ž .interaction of breed and treatment. Fig. 8 . Total amount of ingested feed wasŽ .significantly lower when there were barriers present F s6.779, P-0.05 but there1,14

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

Ž .patches Tables 1 and 2 . Number of passed barriers was mainly affected by the breed ofthe pig and the time spent in each patch was mainly affected by both breed andtreatment.

4. Discussion

Both crossbred pigs and domestic pigs in general terms responded to a foragingsituation as predicted by optimal foraging: they adapted their foraging pattern to thedepletion of the food source and to the cost of moving between patches. It appeared as ifwild boarrdomestic pig hybrids tended to use a more demanding foraging strategy thandomestic pigs; they spent shorter average time in each patch and passed more barrierscompared to domestic pigs. However, feed intake did not differ significantly betweenthe breeds. Therefore, crossbred pigs obtained a similar amount of food compared todomestic pigs in both maze types, but they spent seemingly more energy in achievingthis. This finding lends support to the theory suggested by Jensen and GustafssonŽ .1997 , that pigs during domestication may have adapted to using less costly foragingstrategies.

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

Ž .situation we experimentally designed: 1 costs associated with manipulating the foodŽ . Ž .bucket and costs of travelling 2 within and 3 between patches. We further assumed

that those costs would be the same to each animal and that our manipulation oftravelling cost by inserting barriers would affect the costrbenefit ratio in the same wayfor both breeds. Since the pigs generally behaved as expected, i.e., they altered betweenfeeding and moving between patches, without doing anything else, we believe our


simplified assumptions may have been valid as a representation of the costs and benefitsinvolved. 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

Ž .major factor in explaining the results see Tables 1 and 2 , which supports theinterpretation that we have recorded mainly genotypic differences in behavioural strat-egy. It is, nevertheless, possible that the crossbred pigs were more physically able tomove between patches and to pass the barriers and that the cost induced by insertingbarriers was not perceived as high to them. However, since the crossbred pigs weregenerally smaller than the domestic pigs, the barriers were relatively higher for thecrossbred pigs, which rather may have increased the perceived costs of the crossbredpigs relative to the domestic pigs. Furthermore, the fact that the domestic pigs stayedlonger in each patch even when there were no barriers present, again suggests thatdifferences in behaviour patterns may be attributed to strategy differences caused bydomestication. We cannot exclude that the perceived benefit of the food obtaineddiffered somewhat between the breeds. We tried to standardise the feeding motivationby using the same deprivation times in both breeds, but the degree of feeding motivationat the start of each test may have been affected by differences in weight and metabolicrates. However, the feeding motivation appears not to affect the performance of oral

Ž .feeding behaviour Haskell et al., 1996 . There is no information available concerningmetabolic rates in wild boar or wild boar crosses, but based only on the weightdifferences between the groups in this study, it is clear that the domestic pigs receivedgenerally lower food rations relative to their body weight.

This might have increased their feeding motivation relative to the crossings andfeeding motivation could perhaps affect the results we obtained. However, if this was animportant aspect, we would have expected weight to be detected as a major factor by thestepwise regression analysis. Since this was not the case, the most likely explanation toour 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 crossbredpigs resembles that of wild boars. In fact, it appears most likely that crossbred pigs

Ž .would represent an intermediate type of strategy Alcock, 1993 , but this does not alterthe interpretation of our results. With our method, we had the advantage that all animalshad the same developmental experience, since all were born and raised by the sametypes of mothers and under the same conditions. We believe this minimised thepossibilities that factors other than genetic differences affected our results.

Previous studies of differences between domestic and wild counterparts have de-Žscribed noticeable behavioural changes Boice, 1972; Boreman and Price, 1972; Des-

forges 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

Ždifferent behaviour patterns that is affected by domestication Hale, 1969; Boice, 1972;.Price, 1984 . This is supported by the present study, which also suggests that evolution-

ary models and costrbenefit analysis can be successfully applied to domesticationchanges. Our results are in accordance with other studies where functional adaptations todomestication have been studied, for example on aggression and schooling in fish


Ž . ŽRuzzante, 1994 and on life-history adaptation in salmon and sea trout Peterson et al.,.1996 .

In conclusion, when exploiting a set of gradually depleting food patches, bothŽ .domestic pigs and crossbred wild boarrdomestic pigs generally behaved as predicted

by optimal foraging, i.e., they adapted the time in a patch to the degree of depletion ofthat patch and to the cost of moving to another patch. This demonstrates that pigs, inspite of many generations of domestication and artificial selection, still possess basiccapacities for behavioural adaptation. However, it appears that domestic pigs may haveused a different, less demanding strategy than wild-boar crossings. This supports thehypothesis, that domestication might have shifted behaviour optima towards less costlystrategies. Since there were obvious physical differences between the breeds, whichmight have interfered with the results, some caution is needed in the interpretation.

Acknowledgements

Financial support was received from the Swedish Council for Forestry and Agricul-tural Research. Maria Gustafsson received travel grants from the Royal SwedishAcademy 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.

Ž .Boice, R., 1973. Domestication. Psychological Bull. 80 3 , 25–230.Ž .Bokonyi, S., 1989. Definition of animal domestication. In: Clutton-Brock, J. Ed. , The Walking Larder.¨ ¨

Unwin Hyman, London, pp. 22–27.Ž .Boreman, J., Price, E., 1972. Social dominance in wild and domestic Norway rats Rattus norÕegicus . Anim.

Behav. 20, 534–542.Budiansky, S., 1994. A special relationship: the coevolution of human beings and domesticated animals. J.

Ž .Am. Vet. Med. Assoc. 204 3 , 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.Ž .Graves, H.B., 1984. Behaviour and ecology of wild and feral swine Sus scrofa . J. Anim. Sci. 58, 482–492.

Gundlach, H., 1968. Brutfursorge, brutpflege, verhaltensontogenese und tagesperiodik beim Europaischen¨Ž .wildschwein Sus scrofa L. . Z. Tierpsychol. 25, 955–995.

Ž .Hale, E.B., 1969. Domestication and the evolution of behaviour. In: Hafez, E.S.E. Ed. , The Behaviour ofDomestic 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 tothe 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.


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 theISAE. 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 ofŽ .predation in juvenile steelhead trout 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¨

Ž .traits of sea trout and Atlantic salmon. J. Fish Biol. 48 4 , 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.

Ž .Ratner, S.C., Boice, R., 1969. Effects of domestication on behaviour. In: Hafez, E.S.E. Ed. , The Behaviourof 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.

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Domestication effects on foraging strategies in pigs (Sus scrofa)