Love thy neighbor: Social buffering following exposure to an acute thermal stressor in a gregarious fish, the lake sturgeon (Acipenser fulvescens)

Highlights

• Performance of a critical thermal maximum test on groups of conspecific fish.

• Conspecific presence during thermal stress does not affect acute thermal tolerance.

• Conspecific presence during thermal stress does reduce the endocrine and cellular stress response.

• Evidence for social buffering in lake sturgeon, a gregarious acipenseriform fish.

Abstract

Social buffering is a phenomenon where the presence of conspecifics reduces an animal's stress response. Well known in mammals, social buffering was recently described in fishes exhibiting pronounced social hierarchies. Lake sturgeon (Acipenser fulvescens) are a gregarious rather than hierarchical fish. Therefore, we tested their capacity for social buffering following exposure to an acute thermal stress. Isolated or grouped (three or six similarly sized conspecifics) age-0 lake sturgeon were exposed to a critical thermal maximum (CT_max) test. We measured the endocrine and cellular response to acute thermal shock by assessing whole body cortisol concentration and mRNA expression of steroidogenic acute regulatory protein (StAR) and heat shock proteins (hsp90a, hsp90b, and hsp70) during recovery from the CT_max test. Isolation or grouping had no effect on CT_max. Whole body cortisol concentrations in isolated fish were approximately three-fold higher than in grouped fish 1 h post-CT_max and two-fold higher than grouped fish 20 h post-CT_max. Similarly, 1 h post-CT_max, mRNA expression of StAR, hsp90a, hsp90b and hsp70 were three to four-fold higher in isolated fish compared to groups of three and six fish. At 20 h post-CT_max, expression of StAR was approximately two-fold higher in isolated fish, but expression of hsp90a, hsp90b, and hsp70 was not significantly different between isolated and grouped fish. While conspecific presence had no effect on CT_max, the significant reduction of endocrine and cellular stress markers post-CT_max in grouped fish strongly suggests that lake sturgeon may use social buffering to combat potential deleterious effects of exposure to heat stress.

Introduction

Animal social structures, which involve a wide range of behavioral and physiological responses, are linked to how animals may respond to stressful events (Hennessy et al., 2002; LeBlanc et al., 2011; van der Kooij et al., 2018; Culbert et al., 2019). This idea was first illustrated in mammals where, depending on the nature of the social interactions, the presence of conspecifics influenced how an individual responded to stressful situations (Hennessy et al., 2002; Kiyokawa et al., 2013). In mice and primates, aggressive interactions between conspecifics may cause social stress, impairing an individual's ability to tolerate other forms of stress (DeVries et al., 2003; van der Kooij et al., 2018). Conversely, positive or pro-social interactions may improve recovery from stressful encounters, a phenomenon known as social buffering (Bartolomucci et al., 2003; Weiss et al., 2004; Hennessy et al., 2009). Social buffering was first observed in primates with individuals in stable social structures having reduced glucocorticoid responses to stress (Virgin and Sapolsky, 1997; DeVries, 2002; DeVries et al., 2003; Sanchez et al., 2015). The term social buffering was popularized by research on guinea pigs (Cavia porcellus) where the presence of an agonistic conspecific supressed the stress response of individuals (Hennessy et al., 2002). Social buffering has since been observed in multiple mammal species, including humans (Hennessy et al., 2002; DeVries et al., 2003; Kiyokawa et al., 2013). Outside of mammals, social buffering has been observed in the domestic hen (Gallus Gallus domesticus) (Edgar et al., 2015) and recently in two species of fish, the three-spined stickleback (Gasterosteus aculeatus) (Mommer and Bell, 2013; Fürtbauer and Heistermann, 2016) and the daffodil cichlid (Neolamprologus pulcher) (Culbert et al., 2019). In the stickleback, fish exposed to simulated predation in groups showed a reduced cortisol response compared to those in isolation (Mommer and Bell, 2013). Furthermore, pairs of stickleback placed in a stressful environment displayed a positive correlation between each partner's cortisol levels, providing indirect evidence for social buffering (Fürtbauer and Heistermann, 2016). In the daffodil cichlid, air-exposed individuals had lower cortisol concentrations and reduced expression of genes regulating the stress response in the brain when recovering with a familiar social group compared to fish that recovered in isolation (Culbert et al., 2019). Conversely, studies in other social fishes including zebrafish (Danio rerio) and rainbow trout (Oncorhynchus mykiss) did not demonstrate evidence supporting social buffering, indicating that social buffering may be variable in fishes and dependent on their social structures (LeBlanc et al., 2011; Giacomini et al., 2015).

Research examining the relationship between stress and sociality in fishes have largely focused on salmonids and cichlids, both of which are known to establish a dominance hierarchy in their social structures (Alonso et al., 2012; Carpenter et al., 2014; Culbert et al., 2018, Culbert et al., 2019). Dominant individuals in such social constructs may benefit from greater access to resources, increased growth (Sloman et al., 2000a; Gilmour et al., 2005), additional breeding opportunities (Fitzpatrick et al., 2008) and reduced stress (Sloman et al., 2000a; Alonso et al., 2012). While subordinate fish may display the negative effects of social stress, including reduced access to resources (Sloman et al., 2000a), reduced growth (Sloman et al., 2000b), chronically elevated cortisol levels (Gilmour et al., 2005) and increased heat shock protein (HSP) expression (Currie et al., 2010). However, not all social fishes form dominance hierarchies; in gregarious fishes all members of the social group presumably benefit from interaction (Stirling, 1977; Tanaka et al., 2018). These benefits include increased foraging success, reduced predation, and group vigilance (Stirling, 1977; Godin et al., 1988; Tanaka et al., 2018), some social fishes, such as the cichlid, Cichlasoma paranaense, may be adversely affected by isolation, responding with impaired cognition and learning (Brandão et al., 2015). Thus, it is important to understand the role of social context in mediating how fish may respond to stressors.

Thermal tolerance is an important aspect of physiology and can be highly variable in fishes. Differences in acclimation temperature, body size, genotype (Martin et al., 1976), temperature fluctuation (Feminella and Matthews, 1984), and the rate of temperature increase (Becker and Genoway, 1979) have all been shown to influence thermal tolerance. The critical thermal maximum (CT_max), as an estimate of acute thermal tolerance, is a valuable measure of physiological performance that is comparable across species. An animal's CT_max is defined as the temperature at which movement becomes disoriented and the animal loses its ability to maintain physical equilibrium (Cox, 1974; Becker and Genoway, 1979; Beitinger et al., 2000). In fishes, CT_max has typically been assessed using individual fish; however, social context may also influence thermal tolerance. For example, socially subordinate rainbow trout have a lower CT_max compared to their dominant counterparts (LeBlanc et al., 2011). While in the aggressive mangrove rivulus (Kryptolebias marmoratus), social stimulation from a mirror reflection did not affect CT_max but did delay their escape response making them more vulnerable to thermal stress (Currie and Tattersall, 2018). Currently the influence of social structure in gregarious fishes that do no display dominance hierarchies is unknown.

Lake sturgeon (Acipenser fulvescens) are a freshwater acipenseriform fish native to North America. In the wild, juvenile lake sturgeon are known to aggregate in large groups (Barth et al., 2009, Barth et al., 2011) and laboratory studies report no evidence of dominance hierarchies or aggressive behavior between conspecifics (Allen et al., 2009). Indeed, it has been proposed that long-lived species such as the lake sturgeon may be more likely to develop less aggressive social structures as continued repeat encounters between individuals may encourage reciprocal altruism or local dominance (Ridley et al., 2004). To our knowledge lake sturgeon to not defend territory, therefore, we assume that lake sturgeon sociality is based around reciprocal altruism (Ridley et al., 2004). Previous studies examining the influence of social context on the endocrine stress response in two-year-old lake sturgeon have shown that isolation resulted in a prolonged cortisol stress response following air exposure (Allen et al., 2009; Hare et al., 2015). This suggests that the presence of conspecifics may have improved the ability of juvenile lake sturgeon to recover from a stressful event pointing to possible social buffering.

Here, we tested the hypothesis that lake sturgeon will display social buffering when exposed to a stressful thermal event in the presence of conspecifics. The goal of our study was to examine the effects of social buffering in lake sturgeon following recovery from thermal stress, therefore, we selected two time points at 1 h and 20 h post-CT_max for stress measurement. The lake sturgeon endocrine and cellular responses post CT_max were measured by assessing whole body concentrations of cortisol, and mRNA expression of steroidogenic acute regulatory protein (StAR) and the cellular chaperone proteins hsp90a, hsp90b, and hsp70. We selected StAR due to its role as a rate limiting step in steroid synthesis and positive relationship with cortisol concentration in juvenile lake sturgeon (Geslin and Auperin, 2004; Castillo et al., 2008; Earhart, 2018). The selected hsp genes were chosen because of their differing responses to thermal stress in fishes. HSP90a is an inducible protein that is often strongly expressed after heat shock to serve as a cellular chaperone (Krone and Sass, 1994). The isoform hsp90b serves a similar role as hsp90b but is thought to be constitutively expressed and is more weakly affected by heat shock (Krone and Sass, 1994; Taherian et al., 2008). Finally, hsp70, another cellular chaperone is strongly expressed following heat shock in teleosts (Fowler et al., 2009; Clark et al., 2008), but weakly expressed in shortnose sturgeon (A. brevirostrum) (Zhang et al., 2017). We predicted that isolated juvenile lake sturgeon would be less thermally tolerant than those in groups and have concomitant heightened endocrine and cellular stress responses during the recovery period. Furthermore, we predicted that the effect of social buffering would increase with a larger group size and thus tested multiple group sizes alongside the isolated fish.