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Small Ruminant Research 85 (2009) 99–104

Contents lists available at ScienceDirect

Small Ruminant Research

journa l homepage: www.e lsev ier .com/ locate /smal l rumres

ffect of individual or paired housing during post-weaning on feedntake, growth rate and behaviour of lambs

ffet du logement, individuel ou en paire, sur la CVMS, les performances deroissance et le comportement des agneaux en post-sevrage

éda Villeneuvea,∗, Hélène Méthota, Dany Cinq-Marsb, Renée Bergeronc

Quebec Expertise Center in Ovine Production, La Pocatière, Quebec, Canada G0R 1Z0Department of Animal Science, Laval University, Ste-Foy, Quebec, Canada G1K 7P4University of Guelph-Alfred Campus, Ontario, Canada K0B 1A0

r t i c l e i n f o

rticle history:eceived 15 January 2009eceived in revised form 10 July 2009ccepted 21 July 2009vailable online 26 August 2009

eywords:heepearingehaviourocial isolation

a b s t r a c t

The aim of this study was to compare the behaviour and weight gain of weaned lambspenned either in pairs or alone, with visual and tactile contacts between pens. A 2 × 2 fac-torial experiment was conducted over seven weeks. Forty-eight Dorset lambs (twenty-fivefemales and twenty-three males) were randomly distributed among pens. All pairs werenon-twin lambs of the same sex. They were fed haylage (48.9% of DM) and a commercial pel-leted feed both provided ad libitum. Weight of lambs was recorded at d0 and then weeklyuntil d49. Consumption of dry matter and average daily gain (ADG) were also recorded.Lamb postures, activities and total bleats were directly observed in the morning and in theafternoon on six different days (d1, d2, d5, d12, d19, d26). On d1, observations were doneonly in the afternoon. For paired lambs, data were averaged by pen. Lambs penned alonewere less active, slept more (P < 0.03) and walked less (P < 0.001) than paired lambs. Totalbleats were not affected by housing treatment or sex, but were significantly higher for the

first two days (P < 0.0001). Lamb growth performance was unaffected by the type of hous-ing (P > 0.05). There was a significant difference in ADG (P < 0.001), total gain (P < 0.001),kg concentrate kg−1 gain (P < 0.05) and final body weight (P < 0.05) in favor of males as com-pared to females. Our results suggest that a housing system in which individually raisedlambs are allowed visual, tactile, and auditive contact with each other, compared to lambs

es not

housed in pairs, do

. Introduction

Numerous studies require a precise measurement ofMI to evaluate the effect of feed or nutrient on animal per-

ormance. Because lambs are typically housed in groups,ndividual recording of DMI is not possible without spe-

∗ Corresponding author. Tel.: +1 418 856 1200x229;ax: +1 418 856 6247.

E-mail address: leda.villeneuve@cepoq.com (L. Villeneuve).

921-4488/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.smallrumres.2009.07.007

impact negatively on their growth and behaviour.© 2009 Elsevier B.V. All rights reserved.

cialised equipment. Under these circumstances, the penis the experimental unit, and a higher number of ani-mals than necessary must be used, which increases thecost of research. Individual penning of lambs would allowresearchers to either reduce the number of animals neededfor research, or increase the number of experimental units

for the same number of animals used. However, it has beenshown that sheep are sensitive to the stress induced by theseparation from the flock (Hashizume et al., 1994). Indeed,Mears and Brown (1997) found that plasma cortisol lev-els of lambs were significantly elevated after 1 h of either

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100 L. Villeneuve et al. / Small Ru

partial (only auditive contacts) or complete isolation, ascompared to lambs kept in groups. No effects of isolationwere found for the next 23 h. Their study revealed that theabsence of visual and tactile contacts is the first cause ofstress in isolated lambs. Porter et al. (1995) used vocalisa-tions as an indicator of stress and found that lambs keptin complete social isolation showed a higher frequencyof bleats compared to lambs kept in the presence of asocial partner, again suggesting that isolation is stressful.In a study that compared two different weaning methods,lambs bleated less when they were completely isolatedfrom their mother at weaning, compared to lambs that stillhad visual contact with their dam (Orgeur et al., 1999).Lambs also bleated less in reaction to final weaning whenthey had been subjected to a daily, temporary separationfrom their mother, as compared to lambs that had not beenhabituated to short term separations (Orgeur et al., 1998).

Sevi et al. (2003) recorded a higher voluntary feedconsumption when lambs were gradually weaned by sepa-rating them from their dam during daytime, as compared tolambs that were suddenly weaned and control lambs. How-ever, no significant difference in growth rate was observedin lambs weaned gradually or suddenly (Orgeur et al., 1998;Caroprese et al., 2006). This indicates that sudden sepa-ration from the dam does not consistently have negativeimpacts on lamb performance.

To our knowledge, no researchers have measuredthe impact of isolation with visual and tactile contactspost-weaning on both growth and behaviour of lambs.Therefore, the aim of this study is to compare post-weaninggrowth, feed intake and behaviour of lambs housed eitherin pair or alone, with visual and tactile contact betweenpens.

2. Materials and methods

The experiment was carried out at the “Centre d’expertise en pro-duction ovine du Québec”, La Pocatière, Qc., Canada (latitude 47◦ 21′00′ ′

North, longitude 70◦ 2′00′ ′ West). The animals were cared for accordingto the guidelines of the Canadian Council on Animal Care (1993).

2.1. Animals

Forty-eight Dorset lambs were randomly distributed between fourpossible treatment combinations and groups were balanced for weaningweight (average: 20.3 ± 0.54 kg) to get similar mean body weight betweencombinations.

2.2. Treatments

The pairs were constituted of non-twin animals of the same sex. Six-teen females and sixteen males were housed in pairs, for a total of 16 pensof two lambs, whereas nine females and seven males were housed alone.Each lamb, whether housed in pair or alone, had the same pen surfacearea of 1.12 m2.

Observations were recorded over seven weeks immediately afterweaning. Lambs in this study were weaned at 60.11 days of age ± 1.59days, on average. Both types of pens allowed for tactile and visual inter-actions between lambs. Space between dividing bars of the pens was7 cm.

2.3. Experimental design

The 2 × 2 factorial arrangement included sex (M and F) and housingtype (Alone A, Pairs P) as factors. The experimental unit (EU) was the pen.Therefore, recorded data for animals housed in pairs was the average of

Research 85 (2009) 99–104

two lambs per pen. The treatment combinations were randomly allottedamong available pens.

2.4. Feeding

Lambs were manually fed first cut haylage (48.9% DM, 15.3% CP,2.15 MCal/kg ME, 34.4% ADF, 52.4% NDF, 0.58% Ca, 0.33% P, 0.14% Mg,2.59% K) and commercial pelleted feed (18% CP, 2.76 MCal/kg ME, 0.90% Ca,0.45% P, 0.30% Na, 17 000 UI/kg Vitamin A, 3000 UI/kg Vitamin D, 35 UI/kgVitamin E and 30 ppm of Deccox®(Alpharma, Inc. Bridgewater, NJ; activesubstance: Decoquinate, an anti-coccidial agent)) both provided ad libi-tum. Water was available ad libitum.

2.5. Data recording

2.5.1. Lamb’s growth performancesLambs were weighed at d0, and then weekly until d49. Forage and

concentrate offered for each pen were weighed daily for the whole dura-tion of the test. Hay consumption could not be measured precisely, sincethe hay feeders allowed some hay wastage that could not be accountedfor.

2.5.2. Behavioural measures in lambsBehaviour was recorded twice daily (morning and afternoon) on six

different days (d1, d2, d5, d12, d19, d26) during a 26-day observationperiod. At d1, lambs were weaned, weighed and distributed among treat-ments in the morning. Therefore, only the afternoon observation was donefor that day.

Lamb behaviour was recorded through direct observation by the sameobserver throughout the study. The observer sat on a ladder in the alley,2 m above ground level. The setup was done before the lambs arrived.The observer climbed the ladder and sat quietly. The lambs did not payattention or react to the observer. For postures and activities, there were10 cycles of observations for each of the two observation periods: from11:00 until 11:30 A.M. and from 3:00 until 3:30 P.M. In each cycle, penswere alternately observed and the posture (lying down or standing) andactivity (sleeping, standing still, walking, eating, drinking, grooming, orother) were instantaneously recorded for each animal. The percentage oftime spent in each posture and activity was then calculated.

Each observation period for postures and activities was preceded andfollowed by a 90-min observation period in which each pen was alter-nately observed for 3 min, and the number of vocalisations recorded.Vocalisations were either high pitched bleats (HPB) (vocalisations of highintensity emitted with the mouth open) or low pitched bleats (LPB) (vocal-isations of low intensity emitted with the mouth closed).

2.6. Statistical analyses

Lamb’s initial and final body weight, gain, ADG and concentrate intakewere analysed according to a 2 × 2 factorial design using the MIXED pro-cedure of SAS (Statistical Analysis System, 2002). The frequency of HPBwas too low for meaningful statistical analysis, so total bleats (HBP + LPB)were analysed. Because the latter variable followed a Poisson distribution,it was analysed with the GLIMMIX procedure of SAS (Statistical AnalysisSystem, 2002), with repeated measures. Lambs only bleated for the firsttwo days in PM, therefore only d1 and d2 were used in the repeated mea-sures analysis. For AM values, measures taken on d2, d5, d12, d19 andd26 were used in the analysis. For posture and activity data, an angulartransformation was performed prior to analysis and results are presentedafter back transforming to the original scale. Since data were transformed,results cannot be presented using SEM. The only possibility for repre-sentation of the variability is to use the back transformed values of theconfidence limits in order to reflect the asymmetric nature of the distri-bution. The significance threshold was set at P < 0.05 for all variables.

3. Results

3.1. Growth performances

There were no significant differences (P > 0.05) infinal body weight, concentrate intake, total gain, ADG orkg concentrate kg−1 gain between lambs housed in pairs

L. Villeneuve et al. / Small Ruminant Research 85 (2009) 99–104 101

Table 1Lamb body weights and growth performances as affected by type of housing (H) and sex (S).

Housing (L) Paired Alone ±SEM P value

Sex (S) Male Female Male Female S H S × H interaction

Experimental units 8 8 7 9Initial body weight (kg) 20.58 19.98 21.10 19.57 0.78 0.16 0.94 0.53Final body weight (kg) 40.63 38.45 41.00 35.91 1.05 <0.05 0.29 0.15

Gain (kg) 20.04 18.47 19.90 16.34 0.62 <0.001 0.06 0.10Average Daily Gain (kg d−1) 0.409 0.377 0.406 0.333 0.013 <0.001 0.06 0.10Concentrate intake (kg DM d−1) 1.41 1.34 1.30 1.28 0.06 0.41 0.13 0.58kg concentrate kg−1 gain 3.46 3.54 3.20 3.87 0.16 <0.05 0.83 0.06

Table 2Percentage of observation time spent in lying down posture (%) by lambs in the morning (AM) and the afternoon (PM) as affected by type of housing (H)and day (d).

Housing (L) Paired Alone P value

Number of EU 32 32 da H d × H interaction

65.59 [578.76 [7

xcept fo

otfatfh

af(

3

odple(bi(lwtdape

awsidt(

meaning that lamb’s activities evolved similarly over timefor both housing systems. Total bleats in AM and in PMwere only significantly affected by day (P < 0.0001), and notby housing type (Fig. 3). In the PM, lambs bleated during the

Lyingdown

AM 48.40 [37–60]PM 68.24 [59–77]

a Observation days were d1, d2, d5, d12, d19 and d26 for AM and PM e

r alone (Table 1). Interactions between sex and housingype were not significant either, meaning that males andemales responded similarly in both types of housing, forll variables reported in Table 1. However, there was a slightendency (P = 0.10) for total gain and ADG to be lower foremale lambs housed individually as compared to thoseoused in pairs (Table 1).

Average daily gain (P < 0.001), total gain (P < 0.001)nd final body weight (P < 0.05) were significantly higheror males than for females, while kg concentrate kg−1 gainP < 0.05) was lower for males.

.2. Behavioural measures

As standing and lying down are mutually exclusive,nly the percentages of observation time spent in lyingown posture are reported. During the morning, lambsenned alone were lying down significantly more than

ambs penned in pairs (P = 0.03). In the afternoon, differ-nces were no longer significant for this variable (P = 0.08)Table 2). Furthermore, there was a significant differenceetween days (P < 0.001) for postures in AM, as well as

n PM (Fig. 1). In AM, differences between treatmentsP = 0.03) appeared at d5 and the percentage of time spentying down was highest at d12, and then decreased after-

ard (P < 0.001). In PM, there were no differences betweenreatments, and after d2, the percentage of time spent lyingown appeared to have stabilized. The absence of an inter-ction between day and type of housing for lying downostures indicates that it evolved similarly over time inach housing system.

Main activities were eating, standing still, sleepingnd walking. Drinking, grooming and other activitiesere performed less than 3% of the time and were not

ubjected to analysis. A significant day effect was observedn AM, for all dominant activities (P < 0.01). There were noifferences between housing types in the AM (Fig. 2a), buthere was one in the PM for sleeping (P < 0.03) and walkingP < 0.001). Lambs housed alone slept more and walked less

5–76] <0.001 0.03 0.901–86] <0.001 0.08 0.74

r d1, where only the afternoon observation was done.

than lambs housed in pairs. In PM, there were significantdifferences between days for dominant activities (sleeping:P < 0.0001; standing still: P < 0.0001; eating: P < 0.03; walk-ing: P < 0.002) (Fig. 2b). Walking was highest on d1, whilesleeping was lowest on d1 and highest on d5 and d26. Therewas no day × housing type interactions in overall activities,

Fig. 1. Effect of housing type on the percentage of time spent lying downby post-weaning lambs for (a) AM and (b) PM. Confidence limits of backtransformed values are presented on the graph and significant differencesare given in the text. ( ) Alone, ( ) Pair.

102 L. Villeneuve et al. / Small Ruminant Research 85 (2009) 99–104

Fig. 2. Effect of penning post-weaning lambs housed alone (A) or lambs housed i(b) PM. Confidence limits of back transformed values are presented on the graph aStanding still, ( ) Eating, ( ) Walking.

Fig. 3. Total number of bleats per 3-min period, for AM and PM, at six dif-ferent days. Confidence limits are presented on the graph and significantdifferences are given in the text. ( ) PM, ( ) AM.

first two days only. Following separation from the mother,lambs bleated more on d1 than d2 (P < 0.0001). In the AM,there was a significant difference in total bleats betweend2 and d5 (P < 0.0001). After d5, total bleats decreasedsignificantly and were almost nonexistent in both AMand PM.

4. Discussion

4.1. Growth performance

Lambs raised alone after weaning were not signif-icantly limited in their growth performance. A similarresponse was also observed before weaning, when lambswere momentarily separated from their dams (Orgeuret al., 1998; Caroprese et al., 2006). Adrichem and Vogt(1993) reported lower digestibility and higher heat pro-

duction when adult sheep were isolated in respiratorychambers. In their study, it took 4 ten-day periods beforethe animals adapted to isolation. However, they used adultsheep that had been continuously kept in groups untilthey were moved to isolated pens without any contact

n pairs (P) on percentage of time spend in major activities for (a) AM andnd significant differences are given in the text. ( ) Sleeping, ( )

with other sheep. In the present study, animals used wereweaned lambs and they were allowed to keep visual, audi-tive and tactile contact with each other, which may havecontributed to reduce the effect of isolation on growth per-formance.

Growth performance of females tended to be nega-tively affected by isolation, while no differences wereobserved in males. Sex differences in reaction to isola-tion or restraint stress have been reported. Tilbrook etal. (1999) found that gonadectomised rams submitted toisolation/restraint stress had a decrease in the amplitudebut not the frequency of LH pulses during stress, whereasgonadectomised ewes showed a decrease in the frequencybut not the amplitude of LH pulses during the same stress.These differences may not be reflected in the circulatingcortisol levels in sheep as reported by Stackpole et al. (2003)following isolation and restraint stress. Westenbroek et al.(2005) reported that female rats were more stressed andreacted differently to stress than male rats. In preweanedlambs, Turner et al. (2006) reported that females had ahigher cortisol concentration than their male lambs coun-terparts following tail docking. Performance results in thepresent study tend to suggest that females were moreaffected by stress than males. However, more researchwith clear significant results is needed to confirm thisassumption.

Our results also suggest that growth performances weresuperior for males than for females. This is consistent withwhat it is generally reported for male and female lambs(Vachon et al., 2007; Hammell and Laforest, 2000).

4.2. Behavioural measures

It has been reported that the absence of visual and tac-tile contacts is the major reason for the stress measuredwhen lambs are isolated (Mears and Brown, 1997). Porter

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t al. (1995) reported that the presence of a social partneresults in a lower frequency of bleats by lambs separatedomentarily from their mother. In their study, the test

en for isolation allowed only auditive contacts. Further-ore, it was shown that a social partner alleviates distress

n a novel situation (Ligout and Porter, 2004). Lambs inhe present study were allowed auditive, visual and tactileontacts with neighbours, which may explain the simi-ar rate of bleating between housing systems. It appearshat the bleating recorded would mostly be due to sepa-ation from the mother. Alexander (1977) demonstratedhat lambs increased their levels of bleating and locomo-ion after such separation. In the present study, lambsleated a lot within the first two days following separa-ion, but their bleating decreased thereafter, and bleatsccurred at a very low frequency for both housing sys-ems after d5. Bleating is normally observed at weaningt the farm level. Vocal communication between mothernd young plays an important role in early attachment inheep (Nowak, 1990, 1996) and a high vocalisation rateight be a consequence of a stressor such as separation

Torres-Hernandez and Hohenboken, 1979; Cockram et al.,993).

The presence of a social partner seems to have had aositive effect on the behavioural activities of lambs follow-

ng separation from their mother. Indeed, lambs housed inairs were generally more active than lambs penned alone.erskin and Jensen (2000) reported that piglets respond

o partial or total isolation by depressed play behaviour,hich is thought to be a passive manifestation of stress.

hey also observed a stronger effect of total isolation asompared to partial isolation. The fact that lambs in theresent study responded to partial isolation by reducingheir activity level may suggests that they experienced aertain level of stress, which was not sufficient however toignificantly affect growth performance. It is also possiblehat the lower activity level observed in the present studyor lambs penned alone was attributable to social inter-ction, since Faerevik et al. (2007) reported that calves inroups of 16 were more active, spent less time standingnactive, but had the same growth performance than calvesn groups of four.

. Conclusion

This study demonstrated that lamb’s growth perfor-ances were unaffected by housing in pairs or alone, when

isual, auditive and tactile contacts were permitted. Therequency of bleats was only high during the first two daysfter weaning for both housing types. The behavioural cri-eria used in the present study did not demonstrate anyvidence that lambs penned alone experienced a high levelf stress, possibly because they had visual, tactile and audi-ive contacts with other lambs.

These results suggest that it is possible to house weanedambs individually, with partial isolation, without compro-

ising their growth potential. Therefore, with the sameumber of lambs housed in pairs, it is possible to doublehe number of experimental units, simply by housing themndividually with visual, auditive and tactile contact.

Research 85 (2009) 99–104 103

Acknowledgements

This study was supported by the Centre d’Expertiseen Production Ovine du Québec (CEPOQ). The authors aregrateful to Sara-Myriam Scantland-Marchand for collectingdata and to Steve Methot for help in the statistical analysisand to the staff of the CEPOQ for the animal managementand technical assistance.

References

Adrichem, v.P.W.M., Vogt, J.E., 1993. The effect of isolation and separationonto the metabolism of sheep. Livest. Prod. Sci. (33), 151–159.

Alexander, G., 1977. Role of auditory and visual cues in mutual recognitionbetween ewes and lambs in merino sheep. Appl. Anim. Behav. Sci. (3),65–81.

Canadian Council on Animal Care, 1993. In: Olfert, E.D., Cross, B.M.,McWilliams, A.A. (Eds.), Guide to the Care and Use of ExperimentalAnimals, vol. 1, 2nd ed, CCAC, Ottawa, On.

Caroprese, M., Napolitano, F., Albenzio, M., Annicchiarico, G., Musto, M.,Sevi, A., 2006. Influence of gentling on lamb immune response andhuman–lamb interactions. Appl. Anim. Behav. Sci. (99), 118–131.

Cockram, M.S., Imlah, P., Goddard, P.J., Harkiss, G.D., Waran, N.K., 1993. Thebehavioural, endocrine and leucocyte response of ewes to repeatedremoval of lambs before the age of natural weaning. Appl. Anim.Behav. Sci. (38), 127–142.

Faerevik, G., Andersen, I.L., Jensen, M.B., Boe, K.E., 2007. Increased groupsize reduces conflicts and strengthens the preference for familiargroup mates after regrouping of weaned dairy calves (Bos Taurus).Appl. Anim. Behav. Sci. (108), 215–228.

Hammell, K.L., Laforest, J.P., 2000. Evaluation of the growth performanceand carcass characteristics of lambs produced in Quebec. Can. J. Anim.Sci. (80), 25–33.

Hashizume, T., Haglof, S.A., Malven, P.V., 1994. Intracerebral methionine-enkephalin, serum cortisol and serum �-endorphin during acuteexposure of sheep to physical or isolation stress. J. Anim. Sci. (72),700–708.

Herskin, M.S., Jensen, K.H., 2000. Effects of different degrees of socialisolation on the behaviour of weaned piglets kept for experimentalpurposes. Anim. Welf. (9), 237–249.

Ligout, S., Porter, R.H., 2004. Effect of maternal presence on the develop-ment of social relationships among lambs. Appl. Anim. Behav. Sci. (88),47–59.

Mears, G.J., Brown, F.A., 1997. Cortisol and �-endorphin responses tophysical and psychological stressors in lambs. Can. J. Anim. Sci. (77),689–694.

Nowak, R., 1990. Lamb bleats: important for the establishment of themother–young bond? Behaviour (115), 14–29.

Nowak, R., 1996. Neonatal survival: contributions from behavioural stud-ies in sheep. Appl. Anim. Behav. Sci. (49), 61–72.

Orgeur, P., Mavric, N., Yvore, P., Bernard, S., Nowak, R., Schaal, B., Levy,F., 1998. Artificial weaning in sheep: consequences on behavioural,hormonal and immuno-pathological indicators of welfare. Appl. Anim.Behav. Sci. (58), 87–103.

Orgeur, P., Bernard, S., Naciri, M., Nowak, R., Schaal, B., Levy, F., 1999.Psychobiological consequences of two different weaning methods insheep. Reprod. Nutr. Dev. (39), 231–244.

Porter, R.H., Nowak, R., Orgeur, P., 1995. Influence of a conspecific agemateon distress bleating by lambs. Appl. Anim. Behav. Sci. (45), 239–244.

Statistical Analysis System, SAS, 2002. Release 9.1. SAS Institute Inc., Cary,NC.

Sevi, A., Caroprese, M., Annicchiarico, G., Albenzio, M., Taibi, L., Muscio,A., 2003. The effect of a gradual separation from the mother on laterbehavioural, immune and endocrine alterations in artificially rearedlambs. Appl. Anim. Behav. Sci. (83), 41–53.

Stackpole, C.A., Turner, A.I., Clarke, I.J., Lambert, G.W., Tilbrook, A.J., 2003.Seasonal differences in the effect of isolation and restraint stress on theluteinizing hormone response to gonadotropin-releasing hormone inhypothalamopituitary disconnected, gonadectomised rams and ewes.Biol. Reprod. (69), 1158–1164.

Tilbrook, A.J., Canny, B.J., Serapi, iglia, M.D., Ambrose, T.J., Clarke, I.J., 1999.Suppression of the secretion of luteinizing hormone due to isola-tion/restraint stress in gonadectomised rams and ewes is influencedby sex steroids. J. Endocrinol. (160), 469–481.

Torres-Hernandez, G., Hohenboken, W., 1979. An attempt to assess traitsof emotionality in crossbred ewes. Appl. Anim. Ethol. (5), 71–83.

minant Research 85 (2009) 99–104

104 L. Villeneuve et al. / Small Ru

Turner, A.I., Hosking, B.J., Parr, R.A., Tilbrook, A.J., 2006. A sex differencein the cortisol response to tail docking and ACTH develops between 1and 8 weeks of age lambs. J. Endocrinol. (188), 443–449.

Vachon, M., Morel, R., Cinq-Mars, D., 2007. Effects of raising lambs in a coldor a warm environment on animal performance and carcass traits. Can.J. Anim. Sci. (87), 29–34.

Westenbroek, C., Snijders, T.A.B., den Boer, J.A., Gerrits, M., Fokkema,D.S., Ter Horst, G.J., 2005. Pair housing of male and female rats dur-ing chronic stress exposure results in gender-specific behaviouralresponses. Hormones Behav. (47), 620–628.