Thermal tachypnea in avian embryos [RESEARCH ARTICLE]

Kenia C. Bicego and Jacopo P. Mortola

Many adult mammals and birds respond to heat with thermal tachypnea, an increase in breathing frequency with shallow tidal volume, with minimal increase in oxygen consumption (VO2). This pattern favors heat dissipation by evaporative water loss (EWL) through the respiratory tract. We asked to what extent this response was apparent at the earliest stages of development, when pulmonary ventilation (VE) initiates. Measurements of VE(barometric technique), VO2(open-flow methodology) and EWL (water scrubbers) were performed on chicken embryos at the earliest appearance of VE, during the internal pipping stage. Data were collected, first, at the normal incubation temperature (37.5°C); then, ambient and egg temperatures were increased to approximately 44°C over a 2 h period. Other embryos of the same developmental stage (controls) were maintained in normothermia for the whole duration of the experiment. During heat exposure, the embryo's VO2 and carbon dioxide production increased little. In contrast, VE more than doubled (~128% increase) entirely because of the large rise in breathing frequency (~132% increase), with no change in tidal volume. EWL did not change significantly, probably because within the egg the thermal and water vapor gradients are almost non-existent. We conclude that chicken embryos respond to a major heat load with tachypnea, like many adult mammals and birds do. The appearance so early in development, although ineffective for heat loss, signifies that thermal tachypnea represents an important breathing response necessary to be functional since hatching.

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Source: Journal of Experimental Biology recent issues
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