Thyroid hormone determines energy efficiency of locomotion in zebrafish ( Danio rerio ) in a temperature-sensitive manner

The energetic cost of animal movement has important ecological ramifications because it can comprise a large proportion of energy budgets. Understanding the mechanisms that determine energetic efficiency of locomotion (cost of transport; ∫CoT) is important to estimate environmentally induced changes in movement patterns and energy allocation. We aimed to determine whether thyroid hormone and differences in morphology underlie individual differences in ∫CoT in zebrafish (Danio rerio). We found that pharmacologically induced hypothyroidism increased ∫CoT particularly at cooler temperatures (24°C cf. 30°C). Hypothyroid fish also had a lower critical swimming speed (Ucrit) and a reduced tailbeat amplitude overall, but increased amplitude at higher swimming speed. Tailbeat frequency increased with increasing speed, but hypothyroidism modulated these responses. Fineness ratio and cross-sectional area were unrelated to ∫CoT. Our data indicate that ∫CoT is embedded within the broader thyroid regulatory network, which renders efficiency of movement sensitive to environmental temperature changes and endocrine-disrupting pollution.