Climate change and urbanisation of watercourses affect water temperatures and current flow velocities in river systems on a global scale. This represents a particularly critical issue for migratory fish species with complex life histories that use rivers to reproduce. Salmonids are migratory keystone species that provide substantial economical value to ecosystems and human societies. Consequently, a comprehensive understanding of the effects of environmental stressors on their reproductive success is critical in order to ensure their continued abundance during future climatic change. Salmonids are capital breeders, relying entirely on endogenous energy stores to fuel return migration to their natal spawning sites and reproduction upon arrival. Metabolic rates and cost of transport en-route increase with temperature and at extreme temperatures, swimming is increasingly fuelled anaerobically, resulting in an oxygen debt and reduced capacity to recover from exhaustive exercise. Thermally challenged salmonids also produce less viable gametes, which themselves are affected by water temperature after release. Passage through hydrological barriers and temperature changes both affect energy expenditure. As a result, important energetic tradeoffs emerge between extra energy used during migration and that available for other facets of the reproductive cycle, such as reproductive competition and gamete production. However, studies identifying these tradeoffs are extremely sparse. This review focuses on the specific locomotor responses of salmonids to thermal and hydrological challenges, identifying gaps in our knowledge and highlighting the potential implications for key aspects of their reproduction.
Climate change alters the thermal habitat of aquatic species on a global scale, generating novel environmental challenges during all life stages, including reproduction. Changes in water temperature profoundly influence the performance of ectothermic aquatic organisms. This is an especially crucial issue for migratory fish, because they traverse multiple environments in order to reproduce. In externally fertilizing migratory fish, gametes are affected by water temperature indirectly, within the reproductive organ in which they are produced during migration, as well as directly, upon release into the surrounding medium at the spawning grounds. Both direct (after release) and indirect (during production) thermal impacts on gamete quality have been investigated, but never in conjunction. Here, we assessed the cumulative influence of temperature on brown trout, Salmo trutta, sperm quality during sperm production (male acclimation temperature) as well as upon release (sperm activation water temperature) on two consecutive dates during the brown trout spawning season. Early in the season, warm acclimation of males reduced their fertilization probability (lower sperm velocity) when compared with cold-acclimated males, especially when the activation water temperature was also increased beyond the thermal optimum (resulting in a lower proportion of motile sperm with lower velocity). Later in the season, sperm quality was unaffected by acclimation temperature and thermal sensitivity of sperm was reduced. These results give novel insights into the complex impacts of climate change on fish sperm, with implications for the reproduction and management of hatchery and wild trout populations in future climate scenarios.
In male vertebrates, androgens are inextricably linked to reproduction, social 28 dominance, and aggression, often at the cost of paternal investment or prosociality. 29Testosterone is invoked to explain rank-related reproductive differences, but its role 30 within a status class, particularly among subordinates, is underappreciated. Recent 31 evidence, especially for monogamous and cooperatively breeding species, suggests 32 broader androgenic mediation of adult social interaction. We explored the actions of 33 androgens in subordinate, male members of a cooperatively breeding species, the 34 meerkat (Suricata suricatta). Although male meerkats show no rank-related testosterone 35 differences, subordinate helpers rarely reproduce. We blocked androgen receptors, in that T relates to rates of pup provisioning (Carlson et al., 2006a). Yet, because 91 behavioral endocrinologists tend to focus on understanding dominance or the 92 differences between social ranks, little is known about the role of T in regulating 93 subordinate male interaction in this or other species (although see: Virgin and Sapolsky, 94 1997). Given that dominant and subordinate animals may respond differently to the 95 same T treatment (Fuxjager et al., 2015) or that T-associated variation in behavioral 96 'style' may exist within the same class (Virgin and Sapolsky, 1997), it is increasingly 97 relevant to understand how the different social classes respond to endocrine challenges. 98Meerkats are an appropriate model in which to test the proposition that androgens 99 may regulate social behavior beyond aggression: Firstly, subordinates are far more 100 numerous than are dominant animals and necessarily account for a large proportion of 101 social interaction; secondly, these 'helper' males rarely reproduce, but curiously 102 6 maintain androgen concentrations commensurate with those of dominant males; thirdly, 103 access to an exceptional wild population allows us to consider social and ecological 104 relevance, while overcoming logistical challenges that typically preclude field 105 neuroendocrine studies (see Fusani et al., 2005). 106With relatively few exceptions, typically involving avian species (e.g., Hegner and 107 , 1987;Schwabl and Kriner, 1991), hormones or their actions are rarely 108 experimentally manipulated in the field (see Fusani et al., 2005), particularly to explore 109 their relationship to the broad social repertoire. Instead, androgen-manipulation studies 110 in laboratory animals, particularly rodents and birds, aim to improve our mechanistic 111 understanding of isolated traits (either e.g., reproduction: Södersten et al., 1975; 112 aggression: Searcy and Wingfield, 1980; play: Meaney et al., 1983; scent marking: 113 Fuxjager et al., 2015; or song: Grisham et al., 2007). This historical focus can occur at 114 the expense of gaining comparative, ecological, and evolutionary understanding of 115 hormone action: detecting tradeoffs and constraints, for instance, requires an integrated 116 approach (Wingfield et al., ...
In cooperatively breeding species with high reproductive skew, a single breeding female is dominant to all other group members, but it is not yet known if there are consistent dominance relationships among subordinates. In this study on meerkats (Suricata suricatta), we used naturally observed dominance assertions and submissive interactions within dyads of subordinate females to investigate: (i) whether or not a dominance structure exists among them and what factors influence dominance relationships; and (ii) how dominance may influence the future reproductive success of subordinate females. Our study indicates that superiority in age and weight provide a competitive advantage during conflicts among subordinate females and that females who consistently dominate in these contests are subsequently more likely to attain a dominant breeding position. This provides a starting point for further investigations into dominance structure among subordinates in meerkat societies and other cooperative breeders.
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