Standing Variation and the Capacity for Change: Are Endocrine Phenotypes More Variable Than Other Traits?

Area of Science:

• Endocrinology
• Evolutionary Biology
• Animal Behavior

Background:

• Circulating steroid hormone levels show significant individual variation, prompting hypotheses about their evolutionary implications.
• Few studies have compared endocrine variation to other phenotypic traits or examined population-level differences.

Purpose of the Study:

• To compare the variability of corticosterone (CORT) and testosterone (T) levels with morphological, physiological, and behavioral traits in tree swallows and Anolis lizards.
• To investigate the macroevolutionary patterns of steroid hormone variation.

Main Methods:

• Measured baseline, stress-induced, and post-dexamethasone CORT in tree swallows, alongside morphology, physiology (d-ROMs, antioxidant capacity), and behavior (provisioning rate).
• Analyzed variation in baseline testosterone (T) and non-endocrine traits (body length, mass, social display, locomotion) across 17 Anolis lizard species.

Main Results:

• Post-dexamethasone CORT was more variable than all other measured traits in tree swallows.
• Baseline and stress-induced CORT were more variable than antioxidant capacity and provisioning rate.
• Elevated variation in post-dex CORT and d-ROMs observed in a higher-latitude tree swallow population.
• Circulating T levels and social display rate showed higher variation at the macroevolutionary level in Anolis lizards.

Conclusions:

• Within-population variability in steroid hormone levels is substantial but generally comparable to other animal phenotypic traits.
• Circulating steroid levels in free-living animals can be considered traits with similar individual-to-individual variability within a population.