Cameras reveal penguins' efficient hunting techniques
Intimate details of Adelie penguin feeding behaviour have been filmed by Japanese scientists.
Using video cameras and accelerometers attached to free-swimming penguins, researchers have gained a unique insight into the birds' hunting techniques.
Adelie penguins adopted different strategies depending on whether they were hunting fish or krill.
The findings are published in the journal PNAS.
Lead scientist Dr Yuuki Watanabe from the National Institute of Polar Research in Tokyo, Japan, told BBC Nature: "Foraging is the most basic activity of animals, but details of foraging behaviour are poorly known, especially in marine animals."
Although previous studies had examined Adelie penguin's (Pygoscelis adeliae) foraging style using video apparatus or sensor technology, results were limited.
"Previously some researchers attached video cameras to marine animals to observe their foraging behaviour, but this was just a few hours."
"In other studies, researchers attached various sensors to marine animals to record indirect signals of prey capture. This method lasted for long periods, but has never been validated in the field," said Dr Watanabe.
To overcome these difficulties, the Japanese scientists decided to use a combination of video footage and indirect signals.
Indirect signals include acceleration of the head, temperature changes in the digestive tract or beak opening movements, all of which indicate that feeding may have occurred.
Modern technologies mean that accelerometers are small enough for the scientists to attach two accelerators to each penguin - one on the head and another on the back.
"We recorded both movies and indirect signals, successfully validating the indirect signals using video footage," explained Dr Watanabe.
"We assumed that penguins move their heads relative to their body when they capture prey; this was confirmed by the footage."
Using these methods the team was able to ascertain when and how the penguins were feeding.Fast food
With the results of the combined technologies, scientists were able to draw further conclusions about the Adelie penguins' feeding strategy.
The penguins' foraging area is largely covered by marine ice and their primary food sources include two species of krill and Pagothenia borchgrevinki - a fish whose blood contains antifreeze proteins.
The Antarctic krill (Euphausia superba) is a shrimp-like crustacean that grows to approximately 6cm long. Antarctic coastal krill (E. crystallorophias) lives farther south than any other species of krill.
Krill is an important part of the Antarctic ecosystem, with around half of its biomass being consumed annually by marine predators such as penguins, squid, whales and fish.
When capturing krill, the penguins swam upward then changed direction at the point of predation, making darting movements with their head.
The team discovered that Adelie penguins can catch krill at a rate of up to two krill per second, despite krill displaying escape behaviours.
Furthermore the researchers found that the camouflage defence of the fish P. borchgrevinki didn't work with foraging Adelie penguins.
The penguins were regularly able to capture the fish from below - the direction from which the fish is camouflaged against the backdrop of marine ice.
Dr Watanabe said: "I was surprised by how the penguins repeatedly captured P. borchgrevinki underneath the sea ice. This fish is known to be well camouflaged."
The technology used to support the findings has a wider application for further study. Dr Watanabe commented, "Our method can be applied to many marine animals to understand the spatial and temporal variability of foraging behaviour."