Figure 5.

Hypothetical model by which an individual’s mean song length, its song environment, and other potential intrinsic and extrinsic factors may influence singing effort (see Fig. 1 for abbreviations). Norepinephrine secretion in Area X and serotonin secretion in the CMM may, in part, mediate the influence of male quality and the quality of the perceived social environment, respectively, on singing effort (forward solid arrows). Alternatively, the variation in singing effort may elicit the observed changes in norepinephrine secretion in Area X and serotonin secretion in the CMM (reverse solid arrows). Male quality and the quality of the perceived social environment also likely influence singing effort through alternate neural pathways (dashed arrows). Other intrinsic and extrinsic factors may also affect secretion of norepinephrine in Area X and serotonin in the CMM (dotted arrows), through which they may ultimately influence singing effort. The unexplained variation in singing effort can be attributed to alternative intrinsic and extrinsic modulatory factors acting on additional neural pathways to regulate plasticity in singing effort (dotted-dashed arrow).