Suction or filter feeder? Morphology supports a combined feeding mechanism in the Little Auk (Alle alle)

Julia Schroeder, Timo Heinz, Sidsel Iversen, Ane-Louise Ivik

Little Auks (Alle alle) are the most abundant seabird in the Arctic, with their largest colonies in north-west Greenland sustained by foraging nearly exclusively on plankton. Given their high metabolic demands, adults must capture an approximate 60,000 copepods per day, raising long-standing questions about how this can be achieved efficiently, and with this, questions about the anatomical basis of prey capture and retention. Filter feeding has been proposed, but peer-reviewed morphological evidence remains absent. Experimental studies instead point to suction feeding of individual prey items, not sufficient to achieve the required prey consumption rate. Local Inughuit ecological knowledge has described tooth-like structures within the Little Auk's mouth, which aid foraging, a detail rarely, and where so, ambiguously, described in the peer-reviewed literature. We inspected two Little Auks that were taken for Inughuit subsistence use near Savissivik, Thule district. We took the opportunity to report here the first photographic documentation of the oral cavity of Little Auk. Agreeing with Inughit knowledge, we found the upper palate densely covered with posteriorly directed, cornified denticles, while the tongue remained smooth and thick. These observations support a combined suction-and-retention feeding mechanism of likely more than singular prey items, which might be similarly effective as filter feeding. This paper also highlights the value of integrating local ecological knowledge to resolve unanswered questions in Arctic seabird biology.