September 14, 2011 | 2
The history of penguins in Africa is a history of false starts. The first penguin pioneers that settled Africa millions of years ago all went extinct. But the penguins didn’t give up. They came back, swept there by ocean currents, and repopulated the African coasts. That’s what the palaeontologists Daniel Ksepka and Daniel Thomas conclude in a paper that was published last week.
Africa has always been the most penguin-free continent of the Southern Hemisphere. The first penguins only came to Africa 30 million years after they had first spread to Australia, Antarctica and South America. Today, only the Blackfooted Penguin lives in Africa. But African penguin diversity wasn’t always this low. Palaeontologists have unearthed the fossil remains of four more species of penguin over the past few decades. All of these ancient penguins are extinct today.
There are two possible explanations for the presence of ancient and modern penguins in Africa. Either different species of penguin migrated to Africa on separate occasions, or they all descend from a single group of penguins that arrived in Africa a long time ago.
Biologist call this last scenario an endemic or adaptive radiation. Radiations often happen when a single species colonizes an island or continent. The descendants of this species adapt to different niches and diverge into new lineages. Famous examples of adaptive radiations are Darwin’s finches that colonized the Galapagos and evolved beaks of different shapes, or the lemurs of Madagascar that evolved into creatures the size of mice and gorillas.
Which explanation is closest to actual penguin history, depends on the shape of the penguin family tree. For example, if all African penguins are closer related to each other than they are to other penguins, this would be evidence for an endemic radiation. If, on the other hand, the closest relatives of African penguins all live outside of Africa, it is more likely that they settled Africa multiple times.
Kspeka and Thomas reconstructed the family tree of African penguins by comparing their bones. Of the extinct African penguins, only the skeletons of the large Nucleornis insolitus and smaller Inguza demersus were complete enough to be included in this analysis. Small differences in their bones lead Ksepka and Thomas to conclude that the Blackfooted Penguin, Inguza and Nucleornis are not closely related to each other, indicating that these three penguin species colonized Africa on their own.
But how did they get there? Ksepka and Thomas point out that there are two ocean currents in the South Atlantic that could have swept penguins from South America to the African coast. On his blog, Ksepka aptly describes the currents as a ‘penguin conveyor belt’.
Ocean currents also explain why penguins were unable to spread further north and why they never reached Madagascar (there are exceptions, of course). Both east and west, ocean currents have limited penguin expansion.
Inguza and Nucleornis in Africa arrived in Africa after the current ocean current system was reorganized, 20 million years ago. This would explain why penguins came to Africa as late as they did: the ocean streams that make up the penguin corridor to Africa only opened up relatively recently.
The presence of Blackfooted Penguins in Africa is more recent than that of Inguza and Nucleornis. The oldest fossils of this species are around 300 thousand years old. Few fossils have preserved from the intervening years, so nobody really knows how long Inguza, Nucleornis and the other ancient penguins endured. Ksepka thinks that sea level change played a role in their extinction:
South Africa appears to have had a better environment for penguins in the past than today, because diversity drops from four species 5 million years ago to one today. One possible cause is sea level change. Penguins like to breed in places where land predators can’t reach their eggs, so small rocky islands are ideal. The sea level has dropped over the past few million years in South Africa, and many islands that existed five million years ago are now connected to the mainland. Those areas may have been lost as nesting sites.
The Blackfooted Penguin has made itself quite at home in Africa, despite the low sea levels of today. Yet the future doesn’t look bright for this latest arrival. Commercial fishers and fur seals compete with the Blackfooted Penguin for food. Today, the charismatic flippered bird has been listed as an endangered species. If history teaches us anything, it is that African penguins have been vulnerable to perturbations in the past. We should cherish this lone survivor.