Cryptically coloured eyes

 When we see a tiger's face close up, unnaturally close - we focus on the pupils but not the irides

During a recent rip to Taronga Zoo in Sydney I was attracted to the eyes of the female Sumatran Tiger Panthera tigris Sumatrae and started thinking why they were that colour. But it is obvious that they are adapted as part of the overall colouring of the animal. As a predator, it would be less successful if, as it crept up on prey slowly easing through the grasses and bushes, there were two bright or bold block-coloured irides staring out from the cover. The prey could detect such eyes more quickly and escape. Eyes sometimes need to be camouflaged as much if not more than the rest of an animal's body.    

When seen from a distance, more naturally - a tiger's eyes blend in with the rest of the animal's cryptic colouring

When I watched the large Saltwater Crocodile Crocodylus prosus in its enclosure, I immediately saw the similar adaptation. In this case the crocodile's eyes are coloured like the greens, yellows and greys reflected on the water surface. What prey species ever saw a floating log with big yellow eyes.

A Saltwater Crocodile swimming with its eyes above the waterline - its irides are coloured like the water

Their first glimpse of a crocodile's eyes would be their last.

Even when seen close up, the eyes are cryptically concealed by their colouring
 - but if you do manage to see a wild one this close, you are too late, it also has big teeth and strong jaws

It's not only predators which have cryptically coloured eyes, some potential prey species have also adapted the strategy, and while still in the zoo I was prompted onto the topic of the obvious arms race between predators and prey. If its works for one, the other can use it too.

It's not only predators that have cryptically coloured eyes,
potential prey species have them too - like this Bush Stone-curlew

The best example I found of a prey species with such an adaptation was the Bush Stone-curlew Burhinus magnirostris. These birds live in open woodland with plenty of leaf and bark litter on the ground. They are largely nocturnal birds, so they lie up on the forest floor during the day - literally so. Their defense strategy is to lie out flat on the litter where their camouflage blends with the dappled greys and browns of the substrate. But they need to keep an eye on any potential predator, so when they open their eyes to watch, their mottled irides do not betray their position.

The Bush Stone-curlew is cryptically coloured all over,
 to match the shadowed leaf-litter habitat it lives in

Zoos might not be everyone's favourite method of saving animals, but in the current world they are necessary. They are educational, as shown here, and they help conserve animals. Taronga Zoo runs various conservation programmes, so if you like tigers and can help financially please click here to help conserve tigers.

Bush Stone-Curlews

During the past two weeks I have been in northern Queensland, on a trip to the Iron Range national park where I was part of a group banding birds. On the way north I flew into Cairns and went bird-watching in the local botanic gardens and adjacent cemetry - a well known haunt of Bush Stone-curlews Burhinus magnirostris.

These birds forage and breed quite happily on the short turf and bare patches of earth between the gravestones. Once a familiar bird in southern Australia, the stone-curlews are now rare wherever there are red foxes. These are European predators which were deliberately introduced by man over a hundred years ago. The foxes have not yet colonised the far north-east and the stone-curlews find sanctuary in this cemetery.

 A pair of stone-curlews lead their chick through the tombstones.

A group of six stone-curlews pass the middle of the day under the shade of a tree. They are nocturnal birds and their calls at night have an eerie beauty.