If you've ever been apple bobbing, you'll know just how hard aquatic feeding can be; every time you get close to grasping that tantalising apple, it is pushed away again by the bow wave created by your jaw entering the water. Underwater diners have overcome this problem, as Egon Heiss, a post-doc at the University of Antwerp, Belgium, explains: ‘They use a technique called suction feeding where they very rapidly expand their mouth and throat cavity through a series of very coordinated and fast movements. This then drives the prey and the surrounding water to flow into the mouth. But the whole thing changes on land, you cannot use suction feeding on land because the air is not dense enough, so most animals use a prey-capture mode based on jaw movements or tongue movements.’ This is all well and good if you decide to stick to living on land or solely underwater, but what about animals that regularly transition between the two? For example, Alpine newts spend half of the year as aquatic animals before transitioning to a terrestrial phase during the autumn and winter months. To find out whether, and how quickly, these newts changed their feeding techniques between phases, Heiss teamed up with Peter Aerts and Sam Van Wassenbergh (p. ).
Heiss began by capturing several newts during their aquatic phase before settling them into an aquarium in the lab. Tempting them with maggots, he could then use high-speed cameras to capture their underwater feeding dynamics. As expected, during their aquatic phase they used suction feeding to capture the tasty meal. When the newts switched to their terrestrial phase, the team found they had also switched feeding tactics, using their tongues to capture the maggots.
But how quickly could they switch between the two feeding modes? During their aquatic phase, Alpine newts are known to wander from pond to pond and might decide to have a mid-trip meal on land. Similarly, during their terrestrial phase, these newts can temporarily enter and feed underwater. ‘I thought that the neuromotor control that controls these movement patterns also changes when the animals change their phase and so I thought that when they're in the terrestrial phase they would use lingual prehension on land and underwater, because they would not be able to change that fast’, says Heiss. ‘And vice versa I thought that when they're in the aquatic phase they would use suction feeding movements on land.’ However, after patiently luring his newts to feed on land during the aquatic phase and in water during the terrestrial phase, Heiss and his colleagues found the exact opposite. These newts have learnt that using terrestrial feeding mechanisms underwater is not an effective way to ensure hunting success, and they had easily switched to suction feeding. Feeding on land during the aquatic phase was a little harder, with newts adopting an intermediate approach between suction feeding and terrestrial feeding.
When the team went back and re-analysed the tongue-dependent terrestrial feeding method used during the terrestrial phase, the team could also see elements of aquatic feeding. ‘We're pretty convinced that aquatic feeding is the more ancestral form of feeding, and that terrestrial prey capture has been achieved by step-wise modifications of this ancestral pattern’, concludes Heiss. What's more, this bit-by-bit modulation of the aquatic feeding behaviour not only has helped newts successfully transition between land and water but it may also represent what happened in our evolutionary history in the transition from water to land.
