74 毫不令人讶异地，对社会关系的「欲望」不但古老还具有维持恒定状态的动机。单细胞生物体就展现出这些现象的前身，而我们也可以在鸟类与哺乳动物中找到其他的例子。
在野外，增加的寄生虫传染率与社会动物间的资源竞争，会降低繁殖成功率与减少寿命。这可以经由社会性梳理（social grooming）来消弭，这种适应性行为不仅能将感染寄生虫的机率降至最低，还能在梳理伙伴之间形成社会联系与结盟。在某些灵长类动物中，社会性梳理是社会阶级制度、互惠与资源／服务交换等复杂系统的核心。围绕梳理伙伴关系而形成的社会关系，对个体健康福祉与支持团体的凝聚力至关重要。请参考：Cyril C. Greuter, Annie Bissonnette, Karin Isler, and Carel P. van Schaik, “Grooming and Group Cohesion in Primates: Implications for the Evolution of Language,” Evolution and Human Behavior 34, no. 1 (2013): 61–68; Karen McComb and Stuart Semple, “Coevolution of Vocal Communication and Sociality in Primates,” Biology Letters 1, no. 4 (2005): 381–85; Max Henning, Glenn R. Fox, Jonas Kaplan, Hanna Damasio, and Antonio Damasio, “A Role for mu-Opioids in Mediating the Positive Effects of Gratitude,” in Focused Review: Frontiers in Psychology (forthcoming).
75 社会化游戏行为是由大脑皮质下回路居中传导。研究发现，动物在幼年时期的扭打型游戏，对于学习什么是可接受的社交行为相当重要。家猫若在幼年时期被剥夺社会化游戏，长大后会变成具侵略性的成猫。此外，社会化游戏行为似乎受到鸦片机制的调节，其中μ与κ鸦片受体活化会产生促进或抑制作用。这些鸦片类机制通常与恒定状态的本能需求及情感价值有关；鸦片机制涉及其中的社会性，即表示了利社会行为（pro-social behavior）是受到恒定状态所驱动。Siviy and Panksepp, “In Search of the Neurobiological Substrates for Social Playfulness in Mammalian Brains”; Panksepp, “Cross-Species Affective Neuroscience Decoding of the Primal Affective Experiences of Humans and Related Animals”; Gary W. Guyot, Thomas L. Bennett, and Henry A. Cross, “The Effects of Social Isolation on the Behavior of Juvenile Domestic Cats,” Developmental Psychobiology 13, no. 3 (1980): 317–29; Louk J. M. J. Vanderschuren, Raymond J. M. Niesink, Berry M. Spruijt, and Jan M. Van Ree, “μ-and κ-Opioid Receptor-Mediated Opioid Effects on Social Play in Juvenile Rats,” European Journal of Pharmacology 276, no. 3 (1995): 257–66; Hugo A. Tejeda, Danielle S. Counotte, Eric Oh, Sammanda Ramamoorthy, Kristin N. Schultz-Kuszak, Cristina M. B·ckman, Vladmir Chefer, Patricio O’Donnell, and Toni S. Shippenberg, “Prefrontal Cortical Kappa-Opioid Receptor Modulation of Local Neurotransmission and Conditioned Place Aversion,” Neuropsycho-pharmacology 38, no. 9 (2013): 1770–79; Stephen W. Porges, The Polyvagal Theory (New York and London: W. W. Norton, 2011).
在最近的研究中，具有形成意象所需之神经系统的物种，其正负价值分别与μ及κ鸦片受体持续相关。人体内的四种鸦片受体（δ、μ、κ及NOP）是从下颌脊椎动物身上保留下来的，这些受体在约四亿五千万年前的寒武纪大爆发后首次出现。价值（甚至还有感觉）在动物界中的存在情况，可能比传统所认为的更为普遍，这是个值得思考的有趣情况。Susanne Dreborg, G·rel Sundstr·m, Tomas A. Larsson, and Dan Larhammar, “Evolution of Vertebrate Opioid Receptors,” Proceedings of the National Academy of Sciences 105, no. 40 (2008): 15487–92.
