Figure 4.

Within-individual plasticity in CORT and clutch size generates an among-individual correlation between CORT and a fitness metric. In this hypothetical example, the plastic response of CORT (a) and clutch size (b) to varying nutritional state is shown (black lines showing mean plastic responses to nutritional state, with grey shading representing variation in the responses). Assuming that individuals vary in their nutritional states at any given time, a random sample of individuals (coloured circles; each colour represents a sample from one individual) will produce a negative correlation between CORT and clutch size (a fitness metric) (c) that might be incorrectly interpreted as directional selection favouring lower CORT. Instead, selection has favoured the plastic responses whereby individuals optimize their CORT and clutch size to their given nutritional states, creating the among-individual hormone–fitness correlation (c).