Early Life Stress Strengthens Trait Covariance: A Plastic Response That Results in Reduced Flexibility

Abstract

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 fledging stages as well as 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 stress hormone exposure in early life 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.

Keywords: corticosterone; development; morphometrics; mortality; phenotypic programming; zebra finch.

Figure 1:

Mean trait associations by corticosterone (CORT) treatment (A, B) and mortality (C). Triangles and circles represent the mean absolute value of the z-transformed correlation coefficient for a given trait, and the line connects the same trait for CORT and control birds (A, B) and for birds that died prior to the median age of survival (Y) versus those that survived to at least the median age of death (N; C). Black triangles are within morphological trait associations (A, C), gray triangles are within physiological trait associations (A), black circles are among morphological trait associations (B, C), and gray circles are among physiological trait associations (B).

Figure 2:

Absolute z-transformed correlation coefficient values for corresponding trait associations. Triangles are within-trait associations, circles are among-trait associations, black shapes are morphological traits, and gray shapes are physiological traits. The line represents a 1∶1 relationship, and values above the line reflect associations that were stronger for birds treated with corticosterone (CORT) during development.