Abstract: Humans have a sense of fairness and consequently have an aversion to inequality condition. Recently, animal researchers have suggested that some non-human animals also exhibit an aversion to inequality. This experiment used stress-induced hyperthermia (SIH) to examine inequality aversion in mice. Aversion was analyzed using inequitable restraint stress in Experiment 1. The mice that were restrained, but had free cage mates (disadvantageous inequality) displayed SIH. In contrast, free mice that had restrained cage mates (advantageous inequality) did not show SIH. Inequitable food delivery was used to induce SIH in Experiment 2. Food deprived mice who were surrounded by cage mates who were consuming food (disadvantageous inequality) displayed SIH. The mice that received food and were surrounded by food-deprived cage mates (advantageous inequality) also showed some increase in body temperature, but the increase was not significantly higher than that in the equality condition (where all mice received food). Pre-feeding of the test mice attenuated SIH in the disadvantageous condition suggesting that satisfied mice do not display disadvantageous inequality aversion. These results showed disadvantageous inequality aversion in mice in both negative (restraint) and positive (food) situations but not advantageous inequality aversion.
SIH = stress-induced hyperthermia
BAT = brown adipose tissue
3.2. Results and discussion
Fig. 2 shows the changes in body temperature in the five test conditions. One sample t-tests revealed significant increases in body temperature from baseline in the disadvantageous and the advantageous inequality conditions (t(19)=5.26 and 2.58, P=000004 and P=0.02, respectively). No significant increases were seen in the equality, cheese presentation, disadvantageous inequality after pre-feeding of subjects, or the advantageous inequality with pre-fed cage mates’ conditions (t(19)=1.27, 1.31, 0.24 and 1.66, P=0.22, 0.20, 0.81 and 0.11, respectively). Thus, SIH was observed in both advantageous and disadvantageous inequality conditions without pre-feeding. The disadvantageous inequality condition clearly induced the most profound SIH. This is the first report of inequality aversion in a positive situation in rodents.
A one-way ANOVA revealed a significant effect of the tests (F(5,119)=4.41, P < 0.005). Comparison between the equality condition and cheese presentation condition showed no significant difference (t(19)=0.29, P=0.77) suggesting that the presence of inaccessible cheese did not cause stress/aversion. Comparison between the equality condition and the disadvantageous inequality condition showed a significant difference (paired t-test, t(19)=3.23, P=0.024 after Holm’s correction); however, there were no significant differences between the equality and advantageous inequality conditions (t(19)=1.27, P=0.27). Thus, the mice showed disadvantageous but not advantageous aversion, even though the latter condition caused mild SIH and there was no significant difference between the disadvantageous and advantageous inequality (t(19)=1.44, P=0.17).
The difference between the disadvantageous inequality condition and that after pre-feeding of the subjects was statistically significant (paired t-test, t(20)=3.55, P=0.01 after Holm’s correction); however, there were no significant differences between the advantageous inequality condition and that with the pre-fed cage mates (t(20)=1.44, P=0.17 after correction). Thus, the pre-fed subjects did not display disadvantageous aversion but the pre-fed cage mates did not reduce the advantageous aversion of the subjects. The reduction in SIH by pre-feeding of the subjects in the disadvantageous inequality condition suggests that social comparison of the state of the observer and demonstrators caused the aversion. Cage mates consuming food did not cause aversion when the test subjects were not hungry. Thus, the crucial factor for inequality aversion appears to be the social relationship between one’s own state and that of others. After food deprivation, the mice started eating cheese immediately; however, those who were pre-fed did not. Thus, the subjects easily discriminated states of cage mates, hungry or not, through behavioral expression. In my previous experiment (Watanabe, 2017), the body temperature of mice was measured in a chamber with two compartments, one contained inaccessible cheese and the other a cheese-eating cage-mate. The hungry subject mice displayed significantly higher body temperature in the compartment with the cheese-eating cage mate than in the compartment with inaccessible cheese. Thus, the eating behavior of the cage mate caused SIH in the hungry subjects. The eating behavior of the cage mates had an aversive property for the hungry subjects, because it indicated depletion of food. When the subjects were not hungry in the pre-fed disadvantageous inequality condition, the satiated subjects should not care about the depletion of food; hence, they did not show SIH. Pigeons that previously experienced electric shock displayed suppression of operant behavior when exposed to a pigeon receiving electric shock (Watanabe & Ono, 1986). This phenomenon can be explained in terms of generalization or transfer between their own experience and other’s experiences. Similarly, mice who previously experienced injection with methamphetamine showed conditioned social preference for cage mates injected with the same drug (Watanabe, 2015b). These results suggest that cognition of social comparison, equality or inequality, is possible in animals.
Social comparison of states is common in humans and results in envy (Smith, 2008) and a phenomenon is known as Schadenfreude (Smith, 2013), although Schadenfreude-like behavior has rarely been observed in animals (Watanabe, 2014) (see discussion in Watanabe, 2016). Social comparison is considered a higher-order social cognitive ability in well-developed social animals. The pathway connecting such higher cognition and the thermoregulatory system is unclear. In rats, the stress induced by exposure to an intruder (Mohammed, Ootsuka, & Blessing, 2014) increased BAT temperature. The inhibition of adrenoceptors also reduced the SIH caused by a social intruder (Lkhagvasuren, Nakamura, Oka, Sudo, & Nakamura, 2011). These results suggest that emotional thermoregulation of BAT is controlled by social cognition through the DMH-Raphe system. Given the variety of stressors that result in SIH, it seems likely that many different areas, including higher cognitive areas, project to the DMH; however, the exact circuit controlling the thermoregulatory system remains still unknown.
Check also, about Schadenfreude: The dominant/subordinate relationship between mice modifies the approach behavior toward a cage mate experiencing pain. Shigeru Watanabe. Behavioural Processes, Volume 103, March 2014, Pages 1-4. https://doi.org/10.1016/j.beproc.2013.10.005
Highlights
• This study examined the social preference of mice for a conspecific in pain.
• Subordinate mice preferred a dominant mate in pain to a mid-status mate not in pain.
• Dominant mice did not display a social preference.
• The relationship between observer and demonstrator modifies social preference.
Abstract: Many species display approach behavior to conspecifics. This study evaluated approach behavior exhibited by mice toward a cage mate in pain according to the social relationship between the mice. The relative dominant/subordinate relationship among three cage mates was determined using a competitive food retrieval test. Social preference of the subordinate mouse for the mid-status or dominant cage mate was tested with and without pain induced in the dominant cage mate. Social preference of the dominant mouse was similarly tested with and without pain induced in the subordinate cage mate. Subordinate mice spent more time with the dominant cage mate in pain than with the mid-status cage mate but spent a similar amount of time with dominant and mid-status cage mates that were not in pain. Dominant mice spent a similar amount of time with subordinate and mid-status cage mates regardless of pain. The time that subordinate mice spent with the dominant cage mate in pain inversely correlated with dominancy distance between the two mice. These results demonstrate that social relationship can modify perception of the pain of others.
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