## 2 Materials and Methods  
### Animals, Pre-Slaughter and Slaughter Conditions  
Data were collected in a commercial processing plant located in Northern Italy from a total of 1680 crossbred heavy pigs (Duroc \(\times\) (Landrace \(\times\) Large White)). The pigs were reared in the same farm until a final live weight of about 160 kg (164.9 \(\pm\) 5.6 kg) at 9 months of age. Then, the animals were transported to the same slaughterhouse on 16 dates, between January and June 2014. Pigs were transported in the same truck (Carrozzeria Pezzaioli, Montichari, Italy), consisting of a lorry and a trailer, each one equipped with three hydraulic decks, as described in [PERSON] et al. [38]. Each deck was loaded with 23/24 pigs (a batch), for a total of about 140 pigs being transported on each date. After a fasting period of 12 h, pigs were loaded into trucks and transported to the commercial abattoir. The loading procedures started at 6:00 a.m. and lasted about 45 min; during this time, pigs from the same pen were loaded on the truck using a mobile ramp (width 0.7 m, length 6.0 m, with 1.0 m solid side walls and adjustable height) avoiding mixing between unfamiliar animals of different batches. All decks had almost the same dimensions (bottom deck = 17.40 m\({}^{2}\); middle deck = 15.60 m\({}^{2}\); upper deck = 15.20 m\({}^{2}\)). The vehicles traveled on secondary roads at an average speed of 30 km/h, taking approximately 31 \(\pm\) 5 min to arrive from the farm to the abattoir. At the plant, in some cases, the vehicle was made to wait before starting the unloading phases. This waiting time before unloading was measured by the operators. The unloading procedures lasted about 12 min for all vehicles. After unloading, pigs were weighed and driven to lairage pens, where they were allowed to rest for about 30 min. The 23/24 pigs loaded on each deck were kept together in the same linkage pens, avoiding mixing unfamiliar animals of different batches. The abattoir had 17 lairage pens, located in the same area, at the entrance of the abattoir, in the dirty area. The size of the 17 lairage pens was measured with a laser meter (DTAPE DT50, [PERSON], Shenzhen Hanmer Precision Technology Co.,Ltd., Shenzen, China). Lairage pens had different sizes, while the pen door had the same dimension for all pens. In addition to lairage pens dimensions, the distance between each lairage pen door and the entrance of the passageway toward the stunning area was also measured with the same laser meter. Corridors and lairage pens had a concrete floor. After the rest (lasting about 30 min), the pigs were showered and moved through a single passageway toward the stunning area. The animals were handled and slaughtered in compliance with European rules on the protection of animals during transport and at slaughter (Council Regulation (EC) No. 1/2005 and Council Regulation (EC) No. 1099/2009). All slaughter procedures were monitored by the veterinary team appointed by the Italian Ministry of Health. A trained operator observed and noted the animal behaviors and events at two different checkpoints: at unloading (T1, from the truck ramp to the lairage pens), and when the pigs were moved towards the stunning area (T2, from the lairage pen door to the entrance of the stunning area). The events and type of observations were adapted from [36; 40], following the protocol in Table 1. The data sheet used to annotate events is reported as File S1 in the Supplementary Material. A graphical representation of the experimental design and of the variables annotated at the two checkpoints is reported in Figure 1.  
Electric stunning was performed with head-only electrodes applied manually between the eyes and the base of the ears, with an intensity of 1.3 A and a voltage of 170 V. Pigs were then horizontally exsanguinated for 3 min, hanged up by the left leg for 10 min, and then were immersed in a scaling tank for 5 min at 62 \({}^{\circ}\)C. Hairs were then manually removed and carcasses were evaluated for skin damage by a trained operator.