Auk’s Top Cited, #3: Comparing Digestion Among Ratite Species

Anatomical drawings of ratite digestive tracts, with marker excretion patterns.

This week’s highlighted paper comes from the January 2015 issue of The Auk: Comparative digesta retention patterns in ratites by Samuel Frei et al.

Ostriches, Emus, and Rheas all belong to the group of flightless birds called the ratites, but they have very different digestive systems – Ostriches have a particularly long, voluminous colon and long, paired caeca; Rheas have a short colon with prominent paired caeca; and Emus have neither prominent caeca nor a prominent colon. Frei and his colleagues tested whether the results of the birds’ digestion corresponded with their anatomy, collecting feces from captive birds that were all fed the same alfalfa pellet diet.

Consistent with their shorter digestive tracts, Emus ate more than the other two species relative to their mass and retained food in their digestive system for less time. Ostriches had the longest retention time for digesta, while Rheas were intermediate. Flightlessness is generally considered to be a requirement for herbivory in birds, due to the large (and heavy) digestive system required to digest plant material, and the authors suggest that the diversity among ratite’s digestive systems mean that both herbivory and flightlessness arose multiple times through convergent evolution.

Read the full paper at