They may have a fearsome reputation, but when it comes to parenting, Nile crocodiles are some of the most devoted reptiles on the planet.
The mother will find a suitable site near water to dig a hole and lay her eggs in. Then she settles in for a three-month incubation period, during which she rarely leaves the nest. Here, she guards her nest against hippos, leopards, hyenas and monitor lizards.
When her hatchlings are ready to emerge, they’ll start to call from underground. Now, a camera trap set up by researchers in the Okavango Delta has filmed potentially groundbreaking behaviour.
- Camera hidden inside crocodile nest – this is what it filmed
- Camera strapped to jaws of Nile crocodile. This is what it filmed
Sven Bourquin (an ecologist who leads the wildlife monitoring efforts of the Great Plains Foundation) and Vince Shacks (the Group Head of Impact at Wilderness) have been studying the Okavango Delta's Nile crocodile population for more than 20 years, conducting nest counts and assessments of hatching success. But the addition of camera traps now means they can passively monitor some of the mature nesting females over the nesting season.
And one of the cameras captured unusual behaviour.
“It was the behaviour leading up to the digging that was most interesting to us,” explains Shacks. “Dr Bourquin was managing this trap on a nesting female in the Duba area of the Okavango, and on viewing the footage, we noticed what appeared to be a head tilt from the female, holding her head in this position for a while.”
“We do see crocodiles tilt their heads to pick up items from the ground, but this is usually a precise and swift movement. This behaviour was soon followed by the opening of the nest chamber by the female.
“Looking back at the head tilt, it was then also clear that she had her ear opening pressed flat to the ground. It’s our view that this female was optimising her position to hear these calls from underground by physically placing her ear directly to the ground.”
Shacks also notes that crocodiles have thousands of integumentary sensory organs along their bodies, including around their jaws and inside their mouths.
A 2002 study revealed that these organs could detect ripples from a single drop of water. And in 2012, researchers found that these organs, which appear as bumps, contained touch receptors that are specifically tuned to pressure and vibration, as well as lots of raw nerve endings. These, Shacks suggests, could therefore be used to detect movement or calls from underground.
“We don’t think about hearing in an aquatic animal,” says Bourquin. “The abilities of crocodiles go beyond just aquatic adaptations. We forget that they need to be out of the water to bask, nest and breathe.”
Top image: the Nile crocodile digging hatchlings out of her nest shortly after tilting her head to the ground. Credit: Steve Bourquin
