“Early-life stress strengthens trait covariance: a plastic response that results in reduced flexibility”
Loren Merrill and Jennifer L. Grindstaff (Nov 2018)
Early-life stress hormone exposure can result in reduced developmental flexibility, with potential fitness ramifications
Nobody likes stress; it can result in suppressed immunity, increased levels of free radicals, and elevated energy expenditure, among others. But for most organisms, once the stressor has passed, things revert to normal. For developing organisms, however, stressful experiences can have permanent effects. Individuals stressed in early life may be smaller, have less impressive sexual displays, develop altered stress responses later in life, and have modified immune function. In addition to impacting the development of these individual traits, early-life stress could also alter the relationships among different traits, or within traits over time by more broadly impacting an organism’s developmental machinery. Merrill and Grindstaff investigated this idea and found that zebra finches exposed to simulated early-life stress in the form of daily oral doses of the “stress hormone” corticosterone, developed along more rigid trajectories.
As metrics of developmental flexibility, Merrill and Grindstaff measured morphological and physiological traits in control and corticosterone-treated birds prior to, during, and after corticosterone administration, which occurred during the nestling and fledgling stages. They then examined the strength of association within traits over time (e.g. body mass when the birds were 10 and 60 days old) and among different traits at the same age (e.g. body mass and wing length when the birds were 60 days old) to determine whether early-life stress resulted in tighter or weaker trait correlations. All of the within-trait correlations, and the vast majority of the among-trait correlations, were tighter for corticosterone-treated birds, providing strong evidence that sustained exposure to elevated levels of this stress hormone in early-life broadly alters developmental patterns. This is important because independent of early-life treatment, birds with tighter within and among trait correlations died earlier. If developmental flexibility is constrained by early-life stress, then this alteration may have profound impacts on the organism and could limit capacity to respond to changing environmental conditions.
Stress exposure during development can impact both the expression of individual traits and associations between traits, but whether stress results in stronger or weaker associations between traits is unclear. In this study, we examined within and among-trait associations for morphological and physiological traits in zebra finches (Taeniopygia guttata) exposed to corticosterone (CORT) during the nestling and fledgling stages, and in control birds. Birds exposed to CORT exhibited stronger within-trait correlations over time and stronger associations among traits. We found preliminary evidence that birds that died before the median age of death had stronger within and among-trait correlations independent of treatment, and among CORT-treated birds, smaller birds were more likely to survive beyond the median age than larger birds. These findings suggest that early-life stress hormone exposure can result in reduced developmental flexibility, with potential fitness ramifications, and that these costs may be greater for larger offspring. Furthermore, our results provide experimental evidence for pleiotropic effects of hormones during development through altered patterns of phenotypic correlation.