Pollen-feeding in a late Oligocene giant pelobatid tadpole from Lake Enspel (Westerwald Mountains, Germany)

Examination of the gut content from a fossil giant tadpole (> 15 cm) displays that it predominantly derived from airborne pollen of spruce (Picea) trees. Gut analyses of extant tadpoles demonstrate a broad spectrum of food, including algae, protista, protozoa, macrophytes, rotifers, crustaceans, and pollen. Therefore, living tadpoles seem to be indiscriminate and opportunistic feeders able to adapt and change their feeding behavior according to food source. The hitherto only observation of active pollen surface feeding by tadpoles in the wild shows that Hyla regilla alter their submerged feeding behavior in the presence of a dense pollen scum on the water surface. Pollen studies in conjunction with changes in feeding behaviour also show that H. regilla tadpoles can discriminate between fresh and leached pollen, suggesting an advanced interaction with pollen as food source. The time of origin and evolution of pollen feeding in tadpoles is obscure. Our unique find shows that tadpoles were already feeding on pollen in the late Oligocene, and the amount and purity of pollen filling the gut suggests intentional feeding. Still, in tadpoles of the genus Pelobates, as at Enspel, neustophagia has been previously described. This inertial feeding mechanism for the ingestion of fine particles from water surfaces is at present not confined to pollen. Therefore, it remains open whether the giant Oligocene tadpole selectively fed only on pollen or whether  pollen was just one course from a rich menu during opportunistic surface feeding.