Post-hatch heat warms adult beaks: irreversible physiological plasticity in Japanese quail

Abstract

Across taxa, the early rearing environment contributes to adult morphological and physiological variation. For example, in birds, environmental temperature plays a key role in shaping bill size and clinal trends across latitudinal/thermal gradients. Such patterns support the role of the bill as a thermal window and in thermal balance. It remains unknown whether bill size and thermal function are reversibly plastic. We raised Japanese quail in warm (30°C) or cold (15°C) environments and then at a common intermediate temperature. We predicted that birds raised in cold temperatures would develop smaller bills than warm-reared individuals, and that regulation of blood flow to the bill in response to changing temperatures would parallel the bill's role in thermal balance. Cold-reared birds developed shorter bills, although bill size exhibited 'catch-up' growth once adults were placed at a common temperature. Despite having lived in a common thermal environment as adults, individuals that were initially reared in the warmth had higher bill surface temperatures than cold-reared individuals, particularly under cold conditions. This suggests that blood vessel density and/or the control over blood flow in the bill retained a memory of early thermal ontogeny. We conclude that post-hatch temperature reversibly affects adult bill morphology but irreversibly influences the thermal physiological role of bills and may play an underappreciated role in avian energetics.

Keywords: Allen's rule; ontogeny; phenotypic plasticity; thermal window.

Figure 1.

Summary of morphometric measurements from quail raised at warm (30°C) and cold (15°C) temperatures from 5 to 51 days post-hatch (grey shaded area). (a) Log₁₀ (body mass), (b) bill size (PC1 score) and (c) log₁₀ (tarsus length). Plotted values are least-squares means ± 1 s.e.m.

Figure 2.

Bill surface temperatures of adult Japanese quail raised at warm (30°C; n = 8) and cold (15°C; n = 8) conditions, tested at 15°C, 22°C and 30°C measured at 120–130 days of age. (a) Data arranged by developmental temperature, (b) data arranged by sex. Plotted values are least-squares means ± 1 s.e.m. from LMMs.