Image copyright Jamie Norris / University of York

A debate is unfolding among primatologists about a study, published in February, which reported that chimpanzees can adapt their grunts to communicate with new neighbours.

It was based on a group of chimps that moved from a Dutch safari park to Edinburgh Zoo.

Now, three researchers have written to the journal Current Biology suggesting the results don't stack up.

The original team has responded, and stands by its findings and conclusions.

"There are a number of problems with the original study," said Dr James Higham, from New York University. "Some of these relate to the methods used, while others are fundamentally a misrepresentation of what the data actually show."

Warwick University's Dr Simon Townsend, who co-wrote the original paper with colleagues in York and St Andrews, told the BBC: "We think that we've addressed the points that they bring up. It's an interesting critique of our research - and this is exactly how science works."

In the study, Dr Townsend and his colleagues observed the behaviour and vocalisations of a group of Dutch chimpanzees, after they moved in with an existing colony in Edinburgh. Over several years, they described a change in the call that the Dutch chimps used for apples - a common food for both groups.

After three years in their new home, the Dutch group had shifted from calling for apples with a high-pitched, excited grunt, to a low-pitched one that more closely matched the rather unenthusiastic "apples" call used by the Edinburgh chimps.

This was noteworthy because the Dutch animals, before and after the move, really liked apples - and such calls were generally thought to be closely fixed to the emotional value of the food concerned.

But the study's critics argue that the two groups' calls were not, in fact, so different in the first place.

Image copyright Sophie Pearson / University of York

"Closer inspection of the data reveals that both groups largely overlapped in the range of calls they were originally giving in response to the apples," said Dr Brandon Wheeler from the University of Kent.

The original authors shared their data on the chimp calls, and their critics have re-plotted it in a way that emphasises this similarity. Dr Wheeler describes the change in calls over time as "statistically significant but biologically weak".

Dr Townsend and colleagues, meanwhile, have also re-plotted the data, tracking changes in individual chimps over time - emphasising the original difference, and convergence towards lower-pitched calls across the three years.

It is a notable illustration of how scientists can draw different conclusions from the same data.

"There is a lot of variation in the data; there are clearly some individuals who are changing more than others," Dr Townsend said. He also pointed to some of the team's additional data, which suggests that greater social cohesion produced a bigger shift in the calls.

"If you've integrated more over the three-year period, then your calls tend to change more over time," he said.

Trying to fit in?

Also at the heart of the debate is a disagreement over what precisely constitutes "vocal learning".

The critics suggest that, even if they concede that the Dutch chimps' calls did indeed shift, it amounts to "social modulation": more of an accent change, than the learning of a new "word".

But the study's authors counter that they were open about these two possibilities - and agree that they are difficult to tease apart.

"It could be that they're actively changing this call to improve understanding. Or it could be that they are adjusting their calls to fit in," Dr Townsend explained. The key point, he added, was that these calls - which referred to objects (apples), rather than being purely social - showed a previously unseen flexibility.

Finally, the critics argue that the higher-pitched calls could simply reflect greater excitement in the Dutch chimps, following their move, which settled down over time.

But Dr Townsend and his colleagues suggest that, if this were the case, you might expect a parallel change in the Edinburgh chimps' behaviour, as they adjust to their new companions - which does not seem to be the case.

Dr Higham, one of the critics, told the BBC it was important to thrash out this argument, especially since the original findings were "widely publicised (and over-interpreted)" - adding that it wasn't a case of "trying to tear down other people's research".

"There are some genuine and very strong disagreements among primate communication researchers about what these types of data show, and mean," he said in an email.

And although the correspondents are far from reaching any agreement, Dr Townsend also described the to-and-fro as "a useful exchange" and said he hoped others in the field would find the discussion valuable.