Food breakdown in an avian herbivore: who needs teeth?

Abstract

The extent to which domestic geese (Anser anser) break down grass, with their fast gut passage rate, limited chemical digestion of cell walls, and the use of a gizzard, rather than a precise cutting apparatus, such as teeth, was investigated. Whether the gizzard is the primary site of grass breakdown, to what extent grass is broken down in the guts of geese and whether the final size is affected by the initial ingested particle size were all factors considered. At two discrete times geese were fed grass that had been cut into different lengths. The resultant particle size in the excreta was measured using an image-analysis system. The extent of grass breakdown in different sections of the gut was also measured using image analysis on the gut contents of geese killed after receiving a further meal of grass. The gizzard was found to be the only site of particle size reduction and the post-gizzard digesta particle size was remarkably consistent. This suggested that there was a critical particle size under which the gizzard retained the grass for further processing. The critical particle size, however, appeared to be overridden when there was a backlog of food caused by ineffective processing. This mechanism, in general, would ensure the achievement of consistent processing regardless of the initial ingested particle size, while also allowing geese to take advantage of abundant food sources when available. It was also found that the geese more effectively processed the larger, rather than the smaller, grass particles. Dissecting the geese after ingestion of a further meal of large grass particles revealed that the latter were aligned within the oesophagus along their long axis as they entered the gizzard. This enabled the gizzard to strip the mesophyll and palisade layers from the vascular and sclerenchyma bundles. The long axis of the small particles was the width of the grass blade, which probably prevented this stripping of the cells, as the vascular and sclerenchyma bundles would have been fed into the gizzard at right angles to the gizzards’ main action. The ineffective processing of plant material that cannot be appropriately aligned in the gizzard has implications as to why there are few folivorous birds, as it is possible that the gizzard cannot effectively mechanically process dicotyledonous plant material with its reticulated veins.