Social status and previous experience in the group as predictors of long-term welfare of sows housed in large semi-static groups

Mixing gestating sows implies hierarchy formation and has detrimental consequences on welfare. The effects of social stress on the most vulnerable individuals may be underestimated and it is therefore important to evaluate welfare between individuals within groups. This study aimed at investigating the impact of social status and previous experience in the group on well-being of sows housed in large semi-static groups (20 groups of 46-91 animals). We assessed aggression (d0 (mixing), d2, d27, d29), body lesions (d1, d26, d84) and feeding order. Social status was based on the proportion of fights won during a 6-hr observation period between d0 and d2. Dominants (29%) were those who won more fights than they lost, Subdominants (25%) won fewer fights than they lost, Losers (23%) never won any fight in which they were involved while Avoiders (23%) were never involved in fights. Resident sows (70%) were already present in the group in the previous gestation while New sows (30%) were newly introduced at mixing. Subdominants and Dominants were highly involved in fights around mixing but this was more detrimental for Subdominants than Dominants, Losers and Avoiders since they had the highest body lesion scores at mixing. Avoiders received less non-reciprocal agonistic acts than Losers on d2 (P=0.0001) and had the lowest body lesion scores after mixing. However, Avoiders and Losers were more at risk in the long-term since they had the highest body lesions scores at d26 and d84. They were followed by Subdominants and then Dominants. New sows fought more (P<0.0001), tended to be involved in longer fights (P=0.075) around mixing and had more body lesions throughout gestation than Resident sows. Feeding order from one-month post-mixing was influenced both by the previous experience in the group and social status (P<0.0001). New sows, especially with a low social status, are more vulnerable throughout gestation and could serve as indicators of non-optimal conditions.


Introduction 52
Due to considerable changes in the pig industry worldwide in favour to groups-housing systems, 53 understanding social behaviour of pigs and its impact on welfare has become increasingly important.

54
The wild counterparts of pigs are highly gregarious and form complex hierarchical structures of GmbH, Prambachkirchen, Austria). The ESF opened at 2200 h and closed once all the sows were fed or 143 one hour after the last visit if one or two sows did not feed. When more than two sows did not feed, the 144 ESF closed at 1800 h. Sows were provided wet gestation diet (with 14.2% min. crude protein, and 4.0% 145 min. crude fat, and 7.5% min. crude fiber as dry matter (DM, 88%) basis) formulated to meet the 146 National Research Council nutritional requirements for gestating sows (34).

147
Pregnancy diagnosis was performed using two methods during the first weeks of gestation. Sows 148 which showed behavioural signs of estrus during the first two weeks after mixing were directly removed 149 from the gestation pen and transferred to the insemination stalls to be re-inseminated and added to the 150 following group of the same genetic line (cohort +1). In addition, pregnancy tests using ultrasound 151 scanning were performed at d23 post-mixing. All sows that tested negative in the pregnancy test were two (N = 318), three (N = 106), four (N = 20) or five (N = 2) replicates, for a total of 1244 observations.

155
Group composition was always different across gestations since new sows from previous cohorts were 156 always added at mixing while others were removed. Hence, previous experience in the group was 157 considered and, within groups, sows were considered as Resident if they were already present in the 158 group in the previous gestation, or New if they were newly introduced in the group at mixing (average 159 of 30.42 ± 2.90 % of New sows in groups).

160
The gestating sows remained in the gestation pen for 102 days after mixing. One week prior to

214
(rump and hind leg). Body lesions for each region and side were scored as follow: 0 = None or 1 215 superficial lesions; 1 = 2 to 5 superficial lesions; 2 = more than 5 superficial lesions and/or 1 slightly 216 deep red lesions of 2 to 5 cm length; 3 = 2 to 5 slightly deep red lesions of 2 to 5 cm length and/or 1 217 very deep red lesion of more than 5 cm length; 4 = more than 5 slightly deep red lesions of 2 to 5 cm 218 length and/or 2 to 5 very deep red lesions of more than 5 cm length; 5 = more than 5 very deep red 219 lesions of more than 5 cm length. The body lesion scoring at day 84 post-mixing of the last replicate 220 that had to be recorded in March 2020 was missing due to prohibited access to the farm, as a safety 221 measure, during the worldwide COVID-19 epidemic. The body lesion scores were recorded by a single 222 trained experimenter and the intra-observer agreement was evaluated using body lesion scoring from 223 304 sows randomly selected at different points of time of the experiment which were scored twice at the 224 same scoring day (intra-observer agreement: 93.2%).

225
Feeding order to the ESF station 226 The order in which each sow went through the ESF was automatically recorded by the ESF 227 monitoring software (FarmManager software, Schauer®, Austria). Daily data was extracted on average extracted for 6 days only due to prohibited access to the farm later during the gestation. Horback

262
The effect of social status on agonistic behaviour was analysed using multivariate linear models. and averaged to create the variable total body lesions. Thereafter, the body lesion scores were re-283 categorised for each body region in four balanced categories to be properly analysed (very low, low, 284 high, very high). Those ordinal categorical variables were then modelled using multinomial models to 285 fit cumulative logit proportional odds model to the data and odds ratio were calculated. The same 286 independent variables as for the behavioural analyses were considered.

287
Pearson correlations were performed between the median FO one-, two-and three-months post-288 mixing to evaluate their consistency across time. Because there was a strong FO consistency between 289 one-, two-, and three-months post-mixing (see results below), the median FO between one-and three-

Dominance and consistency of behaviours over replicates
307 Social groups were characterized by the presence of a high proportion of Subordinate sows (i.e., that 308 did not win any fights) since they represented an average of 45.7 ± 1.5 % of the animals within the 309 group. More particularly, the two sub-categories, Losers (i.e., that lost at least one fight) and Avoiders

392
There was a significant triple interaction effect between social status x previous experience in the 393 group x observation day for all the non-reciprocal agonistic acts delivered except bullying (Fig 2). At 394 mixing and day 2, among the Resident sows, Dominant and Subdominant sows delivered the 395 numerically highest total amount of non-reciprocal acts (i.e., bites, knocks, threats and bullying pooled 396 together), Losers were the next, and Avoiders were the least aggressive. The same effect was observed 397 between the New sows. Behavioural differences between social statuses were weaker one-month post-398 mixing, at days 27 and 29 and pairwise comparisons using Tukey's adjustment were non-significant.

399
Regarding the recipients of non-reciprocal agonistic acts (Fig 3), the social status x observation day 400 interaction was significant for all the non-reciprocal agonistic acts received by the sows except bullying.

401
There was an impact of social status on the reception of non-reciprocal agonistic acts around mixing.
received significantly more non-reciprocal agonistic acts than Dominant sows. Also, the frequency of 406 non-reciprocal agonistic acts received by New sows did not significantly differ from that received by independently of the observation day. In total, Dominants and Subdominants were involved in more 436 fights per hour than Losers (Fig 4a, F 2,939 = 62.0, P < 0.0001). The previous experience in the group also 437 had an impact on fight frequency and New sows were more likely to be involved in fights than Resident 438 sows (Fig 4b, F 1,939 = 25.7, P < 0.0001).

445
There was a triple interaction between the social status, previous experience in the group and 446 observation day on the mean time spent fighting per hour (Fig 5, F 7 Dominants.

482
The previous experience in the group also had an impact on body lesions. Overall, New sows had 483 more total body lesions than Resident sows, independently of the gestation phase (OR = 1.56, F 1,2076 = 484 21.23, P < 0.0001).

485
Feeding order (FO) at the ESF station 486 The feeding order (FO) between one-, two-and three-months post-mixing was highly consistent 487 (0.79 < r < 0.86; P < 0.0001). Thus, the median FO between one-and three-months post-mixing was

500
Animal welfare is about, above all, the extent to which an individual perceives and copes with its post-mixing pooled) and was divided in three categories: the Dominants that won more fights than they 516 lost, the Subdominants that lost more fights than the won, and the Subordinates that never won any fight. proportion. The percentage of agonistic acts won has already been positively correlated with 527 displacement success at the feeding station (26) but it was decided to exclude displacement behaviours 528 since those behaviours might be difficult to detect and may not be reliable indicators in large groups 529 with high density of animals, especially around mixing when there is agitation in the pen. Aggression 530 was also limited at feeding since sows were fed using non-competitive ESF systems (i.e., offering full 531 protection to the sow at feeding, (35)). The authors are thus confident that fight outcomes around mixing 532 was the best indicator of true dominance compared to other variables, including the propensity to initiate 533 a fight that may also be influenced by the aggressive temperament or an internal motivation to dominate.

534
Sows in this study were observed around mixing (i.e., 4-hr at mixing and 2-hr at day 2 post-mixing),

535
including just after mixing when contests are the most intense to establish hierarchy (44). At that time,

537
Among the subordinate sows, half of them were able to avoid fighting around mixing (i.e., the Avoiders). almost twice as high during the first two hours post-mixing and those rates differed according to social 545 status (i.e., based on winning success at mixing) and group size. Indeed, high-ranking sows were 546 involved in approximately 9.5 (i.e., groups of 26 sows) or 11.5 (i.e., groups of 6 sows) agonistic acts per factors than group size may explain differences between studies such as the individual floor space 549 allowance (i.e., 2.21 m 2 /sow or more in this study vs. 1.50 m 2 /sow) or sows' experience and age (i.e., 550 mean parity of 3.2 in this study vs. 2.5). However, social strategies and organisation also differ according

567
Investigating social aspects on the sows' welfare in a commercial setting of semi-static group 568 management offered the opportunity to integrate the previous experience in the group factor since sows 569 with gestation failures were re-inseminated and transferred to the next cohort. In total, groups were and New sows, but behavioural differences between social statuses were numerically greater in New 588 than in Resident sows. In addition, social status and previous experience in the group had dissociated 589 effects on other behaviours (i.e., non-reciprocal agonistic acts received and fight duration) and body 590 lesion scores. Hence, this section will mainly focus on the effects of social status. The effect of previous 591 experience in the group x social status interaction will be addressed only when particularly relevant, but 592 the impact of previous experience in the group on the welfare of sows will be addressed in the next 593 section.

594
Avoider sows, that successfully avoided fights around mixing, were less likely to deliver non-595 reciprocal agonistic acts (i.e., bites, knocks, threats and bullying) and they were also bullied less than 596 Losers and Subdominants. Their lesion scores recorded on the front, middle, rear and total body were 597 the lowest on the day after mixing, confirming that they were successful at avoiding conflicts around 598 mixing. This result contrasts with previous findings from Borberg and Hoy (21) who found a higher may be able to isolate themselves from opponents in larger groups where more space is available.

602
However, although Avoiders' welfare was favourable around mixing, they suffered from more 603 aggression later during gestation. Indeed, it is interesting to note that Avoiders were eventually the main 604 recipients of non-reciprocal agonistic acts one-month later, and they also had the highest body lesion 605 scores one-and three-months post-mixing, as previously found in smaller group size (17). Feeding order

606
(FO) was also associated with social status, as previously demonstrated (22,23) and Avoider sows were 607 the last in the feeder queue while Subdominants and the Dominants had priority access to feed.

608
Subordinate sows are more likely to be subjected to high levels of stress (49), but they may be able to 609 cope by retreating to safe and less preferred areas (50,51) and feeding once Dominants are already fed 610 to avoid conflicts. Avoider sows were characterized by a lower body weight before farrowing than 611 Dominant sows and this body condition asymmetry may explain why those low-ranking sows avoided 612 any fight around mixing. Alternately, Kranendonk et al. (26) claimed that high-ranking status sows may 613 gain more body weight during gestation than low-ranking status sows due to their ability to displace 614 low-ranking sows and steal little amounts of feed from the feeder. However, the ESF used in this study 615 offered a full protection to the sows and prevented higher-ranking sows from displacing conspecifics 616 while feeding.

617
Loser sows were not significantly lighter than Subdominant and Dominant sows but they still lost all 618 the contests in which they were involved around mixing. They gave more non-reciprocal acts than 619 Avoiders but less than higher-ranking sows at mixing and still had an intermediate position on day 2.

620
Also, they were among the principal recipients of non-reciprocal agonistic acts on day 2. They had less 621 total body lesions than higher-ranking sows around mixing, more likely because they were less involved 622 in harmful reciprocal fights both in terms of number and total duration of fights (37). Regardless of their 623 aggressive pattern (i.e., 52 % of them never initiate a fight, but 31 % were predominantly aggressors),

624
Losers may rapidly evaluate their weakness compared to their opponents since the mean fight duration 625 was lower at mixing compared to their higher-ranking counterparts. At day 29, they still gave post-mixing and they tended to have more three-month post-mixing than Dominant sows, but they did 629 not differ from Subdominants and Avoiders both one-and three-months post-mixing. to improve or assert their social status, independently of their familiarity level (i.e., Resident vs. New).

721
The welfare of sows differed between individuals within the social groups according to their social 722 status, based on the fight winning, and their previous experience in the group (i.e., whether they were 723 already Resident in the group in the previous gestation or New). This study draws attention to the social 724 strategies adopted in large groups (up to 91 animals), as sometimes encountered in pig industry, that 725 differ from the ones used in small groups. Here, a quarter of sows were able to avoid fighting around 726 mixing (i.e., the Avoiders), suggesting that subordinate sows are more likely to avoid aggressive 727 individuals when mixed in large pens with higher group size. As a result, those subordinate sows had 728 the lowest lesion scores around mixing. However, subordinate sows (i.e., the Avoiders and Losers) 729 suffered from more body lesions than higher-ranking sows later during gestation. In addition, this study 730 showed that sows newly introduced into the groups paid the cost of being new since they were more deeply the social relationships of sows housed in large groups, including the extent to which pigs can