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Endogenous T elevation with an individual's reactive scope does not always lead to enhanced aggression, and social challenges do not necessarily lead to further T elevation, even in systems where exogenous T enhances aggression. Unique shapes represent unique individuals, each with a different level of aggression, represented by the intensity of shading. Throughout these multiple ways of exploring Prediction 3, we have not yet evaluated the degree to which different individuals respond to challenges with varying effects on T production. Notably, we found no immediate or lasting effects of competition on ovarian aromatase gene expression, suggesting that aggressive interactions do not lower T levels via conversion to E2, at least in the ovary at the level of mRNA. We also found that the amount of time females spent behaving aggressively negatively correlated with T levels observed 30–60 min after the start of the simulated challenge, suggesting to us that the act of behaving aggressively caused T to go down.Indeed, expression of steroid receptors and steroid-binding enzymes in socially relevant brain regions has been found to correlate with aggression (Delville et al. 1984a;Trainor et al. 2006; Rosvall et al. 2012;Horton et al. 2014). This result has been taken to indicate that important behavioral variation lies in other aspects of the androgen signaling system, beyond T levels in circulation. Within the androgen signaling system, many factors can contribute to behavioral differences—for example, binding globulins (Breuner and Orchinik 2002), tissue sensitivity (Ball and Balthazart 2008), or metabolism (Soma et al. 2015).
We have been systematically examining the relationship between T and aggression in female tree swallows (Tachycineta bicolor), an obligate secondary cavity-nesting songbird species that readily breeds in artificial cavities (i.e., nestboxes) across North America. Presumably individual condition also may change over time, and further investigation of these within-individual perspectives will advance our understanding of the causal relationship between T and aggression, and how these relationships affect performance in nature. Along these lines, a recent meta-analysis found some indication that fitness effects of hormone implants, including T, depend upon starting hormone levels (Bonier and Cox 2020). Applying this phenomenon to the context of social challenges, the observation that T often decreases after a simulated territorial challenge could in fact be evidence for, rather than against, T's involvement in mediating the behavioral response. Similarly, if T is metabolized into inactive forms after hormone-receptor binding, we might expect to see lower T among animals in which T has recently activated behavioral changes. Collectively, these processes may affect our ability as researchers to see socially induced changes in T secretion when analyzed among individuals. It is also reasonable, if not parsimonious, to expect animals of different quality or condition to have different degrees of social responsiveness, such that some individuals’ T secretion changes more quickly or with greater magnitude (i.e., animals differ in their reactive scope).
This subsection scrutinizes the impact of testosterone on neurotransmitter systems, including serotonin, dopamine, and gamma-aminobutyric acid (GABA). Recent research suggests that testosterone plays a role in modulating cognitive functions, including spatial ability, memory, and attention. Additionally, the discussion encompasses testosterone’s impact on bone density, red blood cell production, and overall metabolic activity. This section provides a detailed overview of the hormonal pathways involved in the production and release of testosterone, emphasizing the hormonal cascade that orchestrates its synthesis.
Treatment with exogenous T, on the other hand, may override such self-regulation to elicit a behavioral change that would not otherwise be induced. We believe this apparent contradiction lies in the fact that GnRH injections limit T production to an individual's own physiological capabilities—which are likely to be both condition-dependent and highly variable among individuals. These case studies make it clear that behavior does not necessarily respond to endogenous T elevation in the same way that behavior responds to exogenous treatment. In European ground squirrels (Spermophilus citellus), for example, males exhibit more aggression on the day after treatment with GnRH (Millesi et al. 2002). Finally, the GnRH challenge is another way to induce brief fluctuations in T, though this approach also affects other hormones too, both via the pleotropic effects of GnRH (Adkins-Regan 2005) as well as any stress responses induced by handling and restraint before the final blood draw (George et al. 2021).
We find qualitative support for all three predictions, though there are also many studies that do not support Predictions 1 and 3 in particular. In particular, we find that Predictions 1–3 have been tested frequently, especially using an among-individual approach. Then, we discuss how the results of these studies can be interpreted in neurobiological terms. Testosterone interacts with various neurotransmitter systems to modulate aggression.