With hungry chicks to feed, the pressure is on for African penguin parents to find food. But how do they locate sparsely distributed shoals of their preferred fish, anchovies and pilchards, when they hunt predominately by vision? Lorien Pichegru and her colleagues Kyran Wright and Peter Ryan from the University of Cape Town, South Africa, explain that the penguin's close relatives – petrels – track tasty shoals by sniffing out odours such as dimethyl sulphide (DMS) – released as the fish feast on zooplankton – and fish oil – produced by other predators devouring the fish. The team realised that penguins would benefit enormously if they could follow tell-tale DMS trails at sea. Knowing that African penguins are able to detect DMS odours on land, Pichegru and her colleagues decided to spike vegetable oil slicks in the sea with either DMS or cod liver oil, to find out whether African penguins follow their nostrils to locate lunch (p. 2509).
Releasing small vegetable oil slicks just off the shore of St Croix and Bird Islands in Nelson Mandela Bay, South Africa, while the birds foraged to feed their young, the team counted the number of penguins that visited each slick and the duration of their stay. Comparing the number of penguin visits to the DMS-scented slick with the number of penguins visiting an odour-free slick, the team found that the scented slick attracted three times as many penguins as the unscented slick and the individuals spent longer in the vicinity. The scientists also calculated that the penguins could travel as far as 2 km to locate a promising DMS slick, ‘Which is particularly important given their slow commuting speed relative to flying seabirds,’ they add.
So, African penguins appear to be guided, at least in part, by the odour of DMS and they probably use it to locate shoals of fish when foraging. The team also showed that the penguins pay no heed to fishoil-scented slicks; which isn't too surprising as penguins swallow their prey whole without releasing oil, unlike petrels, which are content to scavenge dead and damaged fish.