The discovery is a surprise because the sounds most mammals make were thought to be too primitive to allow subtle variations to emerge or be learned.
They made the discovery using 23 newborn kids. To reduce the effect of genetics, all were born to the same father, but from several mothers, so the kids were a mixture of full siblings plus their half-brothers and sisters.
The researchers allowed the kids to stay close to their mothers, and recorded their bleats at the age of 1 week. Then, the 23 kids were split randomly into four separate "gangs" ranging from five to seven animals. When all the kids reached 5 weeks, their bleats were recorded again.
Some of the calls are clearly different to the human ear, but the full analysis picked out more subtle variations, based on 23 acoustic parameters. (Source)
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