This post is co-authored by Gordon Sanson.¹
Early dawn light is creeping across a glassy-still wetland, as wreaths of mist curl upwards. A large white egret stands still, poised ready. Nearby an aboriginal man is waiting for kangaroos to venture onto the lush grass near the water’s edge. His spear is balanced in his hand. He sees the sudden movement of the egret, darting its head into the water, and then emerging, a fish writhing wildly in its beak. Rounded grey forms are now appearing out of the mist, into the open. One is in range now. The man throws his spear in a lithe, practised movement, and it flies rapidly. But the roo sees it, and is even quicker. The spear bounces harmlessly along the ground, and the man has to try again.
Perhaps the aboriginal man got tired of hunting kangaroos and bagged the heron instead. And while eating it noticed something odd about its neck. Or maybe he watched herons hunting and wondered about the kink in their necks. Perhaps we will never know exactly how it happened. But at some stage, aboriginal people began using a spear-throwing device, a woomera. Its action is similar to the sixth vertebra of a heron’s neck. Both contribute to the same end: fast and accurate propulsion of a sharp projectile, for the purpose of hunting prey.
But is this similarity a mere coincidence or deliberate design? And was the evolution of the heron’s neck driven primarily by the need to hunt, or were other factors involved?
The sixth vertebra in a heron’s neck² is longer than the others, and pivots forward around its attachment to the seventh neck vertebra. This extra length, and the positioning of an elongated muscle attachment point halfway along the top of the sixth vertebra, is likely to decrease the leverage of the sixth vertebrae, and increase the speed of the heron’s strike. In a similar way, the added length of a woomera, pivoting around the wrist of the holder, adds speed to the flight of a spear.
The kink and the woomera might also improve the accuracy of the strike. Since the action of throwing a woomera involves a wrist flick, rather than an arm rotation, this would reduce the arc through which the tail end of the spear travels. This is likely to lessen deviations in the flight path of the spear, improving accuracy. Could this also be true for the heron? By only extending one vertebra of the neck, the head and bill travels on a shorter arc, possible allowing the heron to focus and be more accurate in its strike.
However, there are always multiple forces operating in the process of evolution, and it is important not to readily accept a ‘just-so’ story when other factors might have led to the evolution of a trait. The heron’s kink also enables its neck to be folded for flight. This would make the heron’s body more compact when airborne, improving its energy efficiency and agility when flying. But many other long-necked birds fly with their necks extended (e.g. ibises, spoonbills, storks and cranes). And if the heron’s kink evolved mainly for folding, why do the neck vertebrae which make up the kink have such large and elongated bony processes for muscle attachment?
Yet if a neck kink is so good for hunting, why don’t cranes and storks have it? Cranes and storks have more sideways movement in their necks, which allows them to strike to the side, which has its advantages. In comparison, the heron’s kink restricts the sideways movement of its head and neck, so lateral movement is sacrificed for increased speed. Herons – when viewed from the front – are far more ‘narrow’ than cranes and storks, and appear to have perfected the ‘sneak up and strike’ method of hunting. This may be particularly effective when stalking in tall, dense reeds and other emergent aquatic plants. The heron’s ability to tuck its head back while hunting would also make its deadly beak harder to see, or at least look further away than it actually is, which would help to fool its prey.
So on balance, the kink in a heron’s neck seems to function primarily to improve its ability to hunt. Was the aboriginal woomera inspired by a heron’s neck? I do not know, but I am no expert on aboriginal culture. Is there an aboriginal story that links the woomera to herons, and how they hunt? If so, I’d love to hear it.
1. Emeritus Professor at Monash University and probably the best teacher I know. I hold Gordon responsible for inspiring my fascination for functional morphology, and for nearly everything I know about teeth, single malt whiskey, and how to make porridge.
2. Counted from the skull.
References: The Unfeathered Bird by Katrina van Grouw.