Imagine a bird taking flight. Feathers, hollow bones, and relatively small organs help them overcome gravity and reduce the high costs of flying. Now imagine an herbivorous bird in a cold, snowy winter where thermoregulation and larger guts needed to digest fibrous plants add to energetic costs. Our team of Americans, Swedes, Icelanders, and Norwegians wanted to understand how non-migratory, herbivorous grouse maximize digestion when consuming woody and chemically defended plants during the winter compared to grazing geese and more omnivorous birds. We compared the activity of enzymes responsible for digesting sugars (maltase and sucrase) and protein (aminopeptidase-N) in the small intestinal and cecal tissues of Black Grouse (Tetrao tetrix), Capercaillie (T. urogallus), and Willow Ptarmigan (Lagopus lagopus) from Sweden and Rock Ptarmigan (Lagopus muta) from Iceland collected between February and April 2017. We found that enzyme activity in the small intestine was higher than in the ceca indicating the relatively larger role of host digestion in the upper gut and larger role of the microbiome in digestion in the ceca. In addition, we found that grouse had relatively lower activities of sugar-digesting enzymes when compared to geese and omnivores and that both grouse and geese had lower activities of protein-digesting enzymes than omnivores. Within the grouse, Black Grouse had relatively higher enzyme activity for sugars in the small intestine than Ptarmigan which may reflect greater access to sugar-containing foods than Ptarmigan. Results indicate that relatively longer guts observed in grouse in winter serve to increase retention time to maximize yield of digestion and may not require high activity of digestive enzymes. Results also underscore the need to investigate links among diet quality, morphology, and physiology relative to season, geographic location, and taxonomy to better understand the behavior, ecology, and evolution of avian herbivores.
This research has been accepted for publication in the Journal of Ornithology and has recently been published on-line here: https://doi.org/10.1007/s10336-020-01835-z.
The research was made possible through the support of Henrik Andrén and staff at the Grimsö Wildlife Research Station as well as Geir Rune Rauset, Þorkell Lindberg Þórarinsson, Friðrik Jónasson, and Onni – the barking bird dog. Funding was provided by the Fulbright-Hays Grant and the National Science Foundation [DEB-1146194 and OIA-1826801].
Reference: Newman, J., Maurer, M., Forbey, J.S., Brittas, R., Johansson, Ö., Nielsen, Ó.K., Willebrand, T. and Kohl, K.D., 2020. Low activities of digestive enzymes in the guts of herbivorous grouse (Aves: Tetraoninae). Journal of Ornithology, pp.1-9.
Text: Jennifer Forbey, Boise State University
Picture: Willow ptarmigan (photo credit: Lawrence Hislop, Norwegian Polar Institute)