Te Papa’s curator of terrestrial vertebrates Dr Colin Miskelly tells the fifth part of the unfolding story of the emperor penguin that went where none had gone before (at least in the age of digital media). Colin accompanied Department of Conservation staff to Peka Peka Beach on the morning of 21 June, and identified the bird just before the first journalists and media photographers arrived. He is also a member of the committee advising on the care and rehabilitation of the bird, and told the first four parts of its story in Te Papa blogs posted between 23 June and 11 July.
It is four weeks since the emperor penguin was taken into care due to concern at his deteriorating condition and the large quantity of non-food items that he had consumed. In addition to beak fulls of sand (believed to have been mistaken for ice, and therefore consumed in an attempt to cool down and rehydrate), the bird had also been seen swallowing driftwood.
The veterinary team at Wellington did a great job of removing the sand and small bits of driftwood. It is assumed that he regurgitated the larger bits of driftwood himself on the beach, as these were not found in his system after arrival at the zoo. But once the x-rays revealed that his alimentary tract was sand-free, they revealed another surprise – a large mass of small stones. But Peka Peka is a sandy beach, so where did the stones come from, and why were they there?
Penguins as a group are well known for often having pebbles in their tummies (or, in science-speak, ‘gastroliths in their proventriculi’), but the reasons why are poorly understood. At least five theories have been proposed:
1. The ballast theory
Penguins catch food by diving, and the added weight may improve their energy efficiency by making them neutrally-buoyant at a shallower depth. The catch is that they would then need to actively swim back to the surface (expending energy), rather than floating up, and a bird the size of an emperor penguin would need to swallow several kilograms of stones to make any difference.
2. The food-crusher theory
Penguins don’t have teeth (another reminder that they are not human), and may use stones to aid the physical break-up of food in the gizzard.
3. The “I’m-not-really-hungry” theory
All penguins are capable of surviving fasts of weeks, or even months, in duration – up to 4 months for an incubating male emperor penguin. Having a gut part-full with stones may assuage hunger pangs, by triggering stretch receptors in the gut wall, thereby ‘tricking’ the brain into believing that the stomach is not empty.
4. The gut-cleansing theory
Many penguins have large numbers of gut parasites, particularly nematode worms. The stones may create a harsh physical environment to either kill parasites or make the stomach a less pleasant place to live.
5. The accidental ingestion theory
Maybe penguins swallow stones because they are already inside their fish prey. An intriguing variation on this is that they may mistake sinking stones for diving fish, if stones fall out of the bottom of melting icebergs formed from glaciers that have picked up rocks from the Antarctic continent.
The jury is still out on why penguins swallow stones, but there is ample scope for some nifty experiments to test each of these theories.
If it is assumed that emperor penguins deliberately swallow stones, another intriguing question is where do they get them from? Emperor penguins breed on fast-ice, a term used to describe floating ice that is anchored to the Antarctic coastline (as opposed to the free-floating pack-ice). With the exception of the occasional ablated meteorite, fast-ice is notably free of rocks, adding to the intrigue of where the stones came from.
Emperor penguins do occasionally come ashore on ice-free sections of the Antarctic coast, but it is more likely that they pick up stones from the sea-floor – they are capable of diving to over 500 metres. Most of the ocean between Antarctica and New Zealand is thousands of metres deep, and so the stones inside the penguin that came ashore at Peka Peka may yet be able to tell us part of the story of where he came from.
But the story could be more complicated than that. The stones might tell us where his parents used to forage, as emperor penguin chicks are known to get stones along with regurgitated food from their parents. Again, we do not know whether this is incidental, or whether the adults are deliberately passing on a geomorphological mouthful that will benefit their chick.
Previous blogs on this topic:
The global penguin – Part 1. How a lone emperor ventured into superstardom
For later blogs on this bird: