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Contributions of the rostral vs. middle conchae in temporal countercurrent exchange in the domestic chicken

The nasal conchae in extant endothermic vertebrates have been hypothesized to contribute to heat conservation during breathing at low ambient temperatures and heat dissipation at high ambient temperatures. In birds, the rostral and middle conchae in the nasal cavity increase surface area of the respiratory epithelium while decreasing the distance between the epithelium and the respiratory airstream. This improves the efficiency of heat and water transfer between them during temporal countercurrent exchange. To determine the role of each concha, we used surgical ablation to remove either the rostral conchae or both rostral and middle conchae in juveniles of the domestic chicken, as well as performing sham surgery on the control group (n=8, n=6, n=8, respectively). Using fast‐response thermocouples, we measured exhaled air temperature measurements at the external nares and glottis to determine heat and water savings in different portions of the upper respiratory tract. Measurements were taken in adult chickens under mild sedation within the ambient temperature (Ta) range of 10‐40°C and relative humidity of 30%. We expected that minimum exhaled air temperatures (Te) in animals which had undergone conchaectomies would be higher than those of the control group, especially in individuals with both the rostral and middle conchae removed. As expected, Ta had a significant effect (MANOVA, p<0.0001) on the efficiency of heat exchange in the nasal cavity, with the most pronounced (Ta‐Te) difference at the lowest Ta (10°C), and the least at the highest Ta (40°C). We also found surgical treatment had a significant effect, with double conchaectomised animals having higher Te’s than either the sham (p<0.05) or single conchaetomised (p<0.01) animals. Surprisingly, there was no significant difference between the latter two treatment groups. We found a significant interaction between Ta and surgical treatment (p<0.001), Ta and animal sex (p<0.01), and treatment and sex (p<0.05), Hens had slightly higher Te’s than roosters. Whether this was an effect of body mass, breathing rate, or cranial thermal windows (comb, wattle) remains to be determined. Overall, our data suggest that the middle conchae makes a significant contribution to the efficiency of the temporal countercurrent exchange in the nasal cavity, but the rostral conchae does not. Which begs the question – why did the rostral concha evolve?