A Nile crocodile enters the water.

Nile crocodiles react more strongly to babies that are more in distress based on sound cues in their cries. Image © diegooscar01/Shutterstock.

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Crocodiles might understand how upset ape babies are

Crocodiles may be better at understanding a baby’s cry than humans are.

A new paper suggests that listening in on the crying of human and ape babies could help the reptiles to decide whether or not to make the juveniles their next meal.

Crocodiles may have a limited ability to understand our emotions.

While the animals are renowned for their crocodile tears, new research suggests they’re more interested in our own. Experiments on Nile crocodiles suggest they react more strongly to the cries of young humans, bonobos and chimpanzees in greater distress by focusing on certain cues.

Though it’s not entirely clear why the crocodiles are interested in crying children, it’s thought that the reptiles could use the ability to find easy prey.

The findings of the study were published in the journal Proceedings of the Royal Society B.

A red-breasted nuthatch stands on a tree branch.

The red-breasted nuthatch listens to the calls of other birds to get advance warning of danger. Image © Paul Reeves Photography/Shutterstock.

Cross-species communication

While the idea of cross-species conversations is not entirely unheard of, what evidence exists for it is limited. It’s very difficult to prove whether an animal is just repeating something they’ve previously seen, let alone show that they understand it.

That said, animals have something of an idea of what another species is getting at. A variety of species eavesdrop on each other’s distress calls to get advance warning of the threats in their environment.

The red-breasted nuthatch, for instance, listens in on the calls of black-capped chickadees, which can provide information about the size and risk of a predator. As these birds often live together in mixed-species flocks, this means they can mob any high-risk predator together as a group.

The birds’ ability to understand alarm calls from another species may be due to similarities in the calls structure such as jumps in frequency. Some researchers have even suggested that these shared characteristics have been inherited by a wide range of animals from a distant common ancestor.

While infant alarm calls certainly stand out and grab their parents’ attention, the piercing noises can also draw unwanted attention. Predators listen in on alarm calls to find new prey, with juveniles being a tempting target.

Crocodiles are among a variety of animals which do this, but the new research suggests they can take it a step further. The researchers suggest that the crocodiles can perceive just how distressed a juvenile is to help decide whether or not to attack.

A young chimpanzee sits among grass.

Sharp changes in frequency and jitters are examples of nonlinear acoustics, which the researchers believe the crocodiles listen out for in baby cries. Image © Ronnie Howard/Shutterstock.

Tuning into a bittersweet symphony

The researchers used a group of Nile crocodiles held in a Moroccan zoo to test their theory. They placed speakers into the enclosure, where they played back the cries of young humans, bonobos and chimpanzees.

These three mammals are closely related, but their cries are slightly different. Bonobos, for instance, cry at a much higher pitch than humans but otherwise have relatively similar distress calls.

Pitch is used to judge distress by humans, so people asked to judge the distress of a young bonobo from its call generally overestimate just how upset it is. The team found that crocodiles, however, didn’t respond any more to higher pitched calls than lower ones.

Instead, they were drawn towards the speakers when calls exhibited more of certain characteristics, such as chaotic sounds and jitters. These are more common in the cries of a very distressed juvenile, meaning Nile crocodiles are probably a better judge of how upset a baby is than a human.

The researchers suggest that crocodiles may have evolved to focus on factors other than pitch because this changes a lot between species, so isn’t as useful for the reptiles which target a large range of prey.

While the results seem promising, the experiment didn’t include a control sound to judge how the crocodiles reacted to non-distress calls. The scientists argue that they were unable to do this because the crocodiles might have associated the speakers with unusual sounds and started ignoring them.

They add that their previous research demonstrates that crocodiles are capable of selectively responding to certain calls from not just the reptiles’ own species, but others too.

In the future, the researchers hope to find out whether crocodiles respond equally to juveniles and adults in distress, or if they have a preference for one over the other.