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A study has shed light on how certain alarm cries made by meerkats are more effective than others at alerting the group to possible dangers.

Researchers from Switzerland and South Africa suggest "non-linearities" make the cries "unpredictable", distinguishing them from other calls.

However, it is uncertain how the meerkats produce the "non-linear" vocal sounds, the team adds.

The findings appear in the Royal Society journal Biology Letters.

"You have normal calls, which in we refer to as 'linear' calls," explained co-author Simon Townsend from the University of Zurich.

"However, from time to time - about 20% of these types of calls - you get this intrusion of complexity, which we call 'non-linear'."

He added that the difference between the two types of calls was very hard for humans to hear, but the meerkats were able to distinguish between the two.

Uncertain origins

The researchers used recordings of a series of nine "medium-urgency aerial alarm calls", the sort that would be produced by a meerkat when it spotted a potential predator in the sky above.

Four of the nine recorded calls possessed non-linear properties, Dr Townsend explained.

"We focused on eight different dominant males from eight different groups," he told ����ý News.

Once the researchers had ensured the animals were not stressed, they began measuring responses to the recorded calls.

"What we found was that they always responded more strongly to the non-linear alarm calls," Dr Townsend recalled.

"Five of the eight males ran to a bolt hole straight away - the strongest behavioural response. The remaining three males also responded more strongly to the non-linear calls."

In the minute after the researchers played the non-linear recording, the animals also showed a reluctance to start foraging again.

Dr Townsend said the study did not focus on what caused the animals to produce these non-linear calls.

"This is something that we are not sure about at this stage; why some of these calls have the non-linearities and others don't is something we want to probe a little bit further," he said.

Previous research has suggested that animals spotting predators make non-linear calls because the stress causes irregular vibrations of the vocal cords.

However, Dr Townsend would like to carry out more research before his team could support or rule out that hypothesis.

"You'd expect these non-linearities to be there if the call signalled urgency, but they are not - they are only there in about 20% of these types of calls," he said.