1. Introduction
Sows are frequently confined in crates to maximize space utilization in intensive pig farming; thus, behaviors such as turning, exploring, and learning in pregnant sows are considerably limited in restricted and barren environments [
1]. The restriction of these behaviors can generate increased stress [
2]. Compared with sows housed in individual stalls, sows housed in a group housing system display more exploratory behavior, less vacuum chewing, and less sitting behavior [
3]. To avoid undesirable stimuli, animals may change their behaviors in terms of pattern, frequency, and intensity, compared to normal behaviors, which may result in stereotypies [
4]. Stereotypic behaviors are mechanisms by which animals respond to unfavorable environmental conditions such as long-term spatial restriction [
5,
6]. In pregnant sows, long-term space restriction increases oral behaviors, which may develop into stereotypic behaviors such as vacuum chewing, bar-biting, and trough-biting [
7].
The expression of stereotypic behaviors in confined sows increases with confinement time, and multiparous sows show more stereotypic behaviors than gilt sows [
8]. Novak et al. [
9] observed that mice with higher levels of stereotypy displayed a negative cognitive bias, which was influenced by the form of stereotypy. Similar results were observed in captive tufted capuchins (Cebus apella) [
10]. Zhang et al. [
11] found that the stereotypic behaviors of sows at parity 5 were higher than at parity 0. However, Liu [
12] found no significant difference in the stereotypic behaviors of sows at parity 2 and 3. Therefore, assessing the affective state of animals based only on the frequency of stereotypic behaviors would be inadequate.
PLR characteristics are considered as sensitive indicators to assess psychological states. PLR is controlled by sympathetic and parasympathetic nerves of the autonomic nervous system [
13]. Bao et al. [
14] found that PLR might be a better way to monitor the psychological status of animals. Abnormal PLR characteristics indicate a disorder of the autonomic nervous system, which may be caused by an individual psychological or neurological disorder [
15]. Human patients with neurological disorders, including traumatic neurological disorders, anxiety disorders, specific phobias and depression, exhibit abnormal PLR characteristics [
16,
17]. Compared with group-housed sows, the PLR latencies and duration of stall-housed sows were longer [
14]. Additionally, the PLR of sows is affected by age [
18]. Our colleague previously found that there were differences in the psychophysiological states of sows with different PLRs and the differences increased with the increase in parity [
19]. Therefore, we supposed that there were differences in the behaviors and affective states of sows with strong/weak PLR characteristics and different parity.
Affective states are the subjective experiences, feelings, or emotions of animals and are considered to be an external projection of multiple subjective aspects of neurological, physiological, behavioral, and cognitive states, and consciousness [
20]. Animals can experience a variety of advanced emotions and they also have negative, neutral or positive subjective experiences [
21,
22]. Emotional states are classified as an important component of animal welfare [
23]. Abnormal emotional states not only cause the animal’s physiological state to fluctuate, but also lead to deviations in their subjective consciousness such as preferences and motivations [
24]. Currently, there are two main ways to assess the affective state of animals. One way is to allow animals to control their environment and observe the choices and decisions they make, using preference and motivation tests (including aversion tests). The other is to look for signs of deprivation, frustration or distress when the animal is confined in an environment or subjected to a treatment without any means of control [
24].
Testing the reward stimulus response of sows can verify the state of anhedonia or lack of motivation in individuals with affective disorder, which helps to reveal their affective disorder types. Many animals have a natural reward circuit for sweetness [
25], and the response to aversive stimuli is also an important test of individual motivation control [
26]. Chronic Mild Stress (CMS) rats are insensitive to sweeteners, and their consumption of sucrose solution is greatly reduced, which is considered an indicator of anhedonia [
27,
28]. Depressed model and alcohol-dependent model monkeys are insensitive to bitterness [
29]. Swine naturally prefer sweetness over bitterness, so testing sows’ response to sweeteners can assess anhedonia, and testing sows’ response to bitterness can test motivation. Pigs naturally love sweetness and have a natural reward circuit [
25,
30]. Sweetener tests have been widely used in studies of reward response [
31]. Reduced sucrose preference is considered to be reflective of an anhedonia-like state [
32].
Animals may respond to novel stimuli with neophilic (explorative) and/or neophobic (cautious) behavior. Variations in responsiveness to environmental change within and across species is associated with cognitive, physiological, and social propensities that reflect ontogenetic variance and the natural lifestyles of a species [
33,
34]. Testing reactions to novel stimuli in animals has been used in fear and anxiety studies [
35,
36]. Compared with sows in poor environments, sows in enriched environments showed greater diversity in behavior, greater motivation to explore novel objects, less fear and lower anxiety in novel object stimuli tests. The exploring behavior of sows decreased with an increase in parity, and the number of contacts and contact duration in regard to a novel object decreased [
11].
The objective of this study was to investigate the behaviors and affective states of sows with strong/weak PLR and different parity, and clarify the applicability of PLR tests for assessing the affective state of confined sows. Our results may have important implications regarding advances in psychophysiological research on sows so as to improve animal welfare.
4. Discussion
PLR is a part of visual function tests, and is used in clinical situations as an important indicator to evaluate factors such as potential pathologies of the central nervous system, the depth of anesthesia, and severity of certain diseases [
38]. The PLR test is also a novel approach for the evaluation of neuropsychiatric disorders such as depression and anxiety [
25]. From a psychophysiological perspective, pupil response is modulated by emotionally evocative stimuli [
39]. Our results showed significant differences in behaviors and affective states, including postures and novel object contact, between WR and SR sows.
The behavioral expression of sows partly reflects their biological needs and adaptability to their environment. Stereotypic behaviors of pigs are adaptive responses to adverse environmental stimuli, and stereotypies of sows may include postural behaviors and oral behaviors such as ventral lying, lateral lying, sitting, rooting, trough-biting, and vacuum-chewing [
40]. In human patients with depression and anxiety disorders, decreased sympathetic nerve function results in reduced physical activity [
41], and patients with schizophrenia and depression show reduced activity and symptoms of negative volition, decreased motivation, and anhedonia [
42]. The results of the present study suggested significant differences in behavior of same-parity sows in PLR groups, as the standing and lateral lying behaviors of WR sows were less frequent than SR sows, and the ventral lying and sitting behaviors of WR sows were more frequent than SR sows. Interestingly, our results showed that standing behavior in P2 sows was significantly more frequent than in P0 or P5 sows in the WR group, rather than in the SR group. In our behavioral observations, we found that with the increase in parity, sows showed a similar stutter-like behavior, but we did not accurately record it. Sows were able to adapt within each environment through behavioral mechanisms [
43]. We believe that the change in sows’ standing behavior with the increase in parity is influenced by many factors, such as affective state, body weight, and movement space in the crates. We observed no significant differences in oral behaviors in different PLR groups at the same parity; however, bar-biting, rooting, and trough-biting behaviors in P5 sows were significantly lower than in P0 sows, whereas vacuum-chewing behavior was significantly higher in P5 sows than in P2 and P0. This indicates that long-term space restrictions in sows results in reduced activity and symptoms of negative volition, decreased motivation, and anhedonia. We previously found that compared with the SR group, the WR group showed lower 5-hydroxytryptamine levels and higher cortisol, interleukin-6, and beta-endorphin levels [
19]. 5-hydroxytryptamine is an important neurotransmitter in depression, and haloperidol can inhibit dopamine receptors and enhance dopamine conversion in the brain [
44]. 5-Hydroxytryptamine is an inhibitory neurotransmitter that affects the emotional state, and low levels can aggravate depression. Inducers or antagonists of serotonin can increase or decrease stereotypic behavior in animals [
45]. We found that the changes in behavior in different PLR groups sows might be the manifestation of their psychological disorder [
19].
The sucrose response test has been used to assess motivation, depression, anhedonia, and related affective states in rodents [
46,
47]. Low levels of aversion to foods such as quinine can imply a predisposition to certain psychological disorders [
48]. The research results of Zhu et al. [
49] showed that mice in the depressed group had a lower preference for sucrose than mice in the healthy control group. Chronic stress can lead to changes in the glutamate energy system and cause excessive release of glutamate, which is a potential mechanism to trigger depression [
50]. Rats subjected to chronic stress of different duration can display depression-like behavior, and their preference for sucrose will also be reduced [
51]. Sucrose solution can cause enjoyment in pigs, and a decrease in the response to sucrose indicates that sows have anhedonia. Chronic stress leads to neurobehavioral alterations [
46]. We observed that the response to sucrose in WR sows was slightly lower than that of SR sows, even though there was no significant difference between the PLR groups. Olney et al. [
52] found that mice subjected to long-term stress showed depression-like behavior, increased quinine consumption and quinine preference. Scinska et al. [
53] found that the taste preference of mice in the depressed group changed and the highly-depressed group had a higher preference for quinine. We found that the response for quinine in WR sows was slightly higher than SR sows; even though there was no significant difference between the PLR groups. Therefore, we deem that WR sows may experience affective states such as depression and anhedonia. The response to sucrose was significantly lower and the response to quinine was significantly higher in sows of P5 than in sows of other parities, suggesting anhedonia in long-term space-restricted sows. In mice, the preference for sucrose in depressed individuals is significantly lower than normal mice [
49]. Paul et al. [
29] found that maternally-deprived rhesus monkeys showed reduced sucrose intake and increased quinine consumption than the control group. They suggested that maternally-deprived rhesus monkeys do not display gustatory signs of anhedonia, but rather of insensitivity to gustatory stimuli.
Animals in prolonged stressful environment develop a psychological response of fear. Fear is arguably the most commonly investigated emotion in domestic animals. The novel object test is a common type of fear test [
54]. Our results revealed that the number of contacts and contact duration in WR sows were significantly lower, and the response latency time in WR sows was significantly higher than SR sows. This indicates that PLRs in confined sows are correlated with behavioral responses to novel objects, and WR sows may thus suffer from severe stress. Under the same work pressure, healthy people and depressed people appear in the same group [
55]. A study used behavioral cognitive tests on major depressive disorder (MDD) patients. They found that MDD patients were more sluggish in responding to novel objects, showed a lack of focus, difficulty with divided attention, difficulty with decision making, difficulty thinking quickly, and difficulty learning new things [
56]. Dickson et al. [
57] found that depressed participants reported fewer approach goals and their reaction to novel objects took longer than control participants. This indicates that animals with depression will avoid the possibility of higher contact with novel objects. We suggest that the WR sows were more depressed than SR sows. With increasing confinement time, the number of novel object contacts and contact duration in P5 sows were significantly lower than in those of other parities, and the response latency time in P5 sows was significantly higher. These results suggest that an increase in confinement time may aggravate fear, reduce the number of novel object contacts numbers and duration, and delay the novel object latency time of sows. Stress responses are more severe, and exploratory behaviors as well as novel object contact numbers and duration decrease with increasing parity in sows [
11]. In human patients with severe depression, novel object tests showed that most depressed patients were less responsive to novel objects [
56,
58]. Thus, novel object responses can be expected to be reduced in animals with depression, which is consistent with the results of our study.