Every spring, off New Zealand, the seasonal bounty draws in rare visitors. False killer whales.

They are relatives of the orca, six metres long and weighing over a tonne.

They appear to be searching for dolphins. And there are many in these coastal waters.

Here, bottlenose dolphins stick together, constantly chattering with whistles and clicks. Such a din carries for miles underwater.

The false killers have detected them. Travelling at ten knots, the killers quickly close in on them.

But then, something truly extraordinary happens. The dolphins turn, as if to greet their pursuers.

They seem to change their course. Could it be that they are attempting to communicate?

Scientists studying this annual encounter now think that individuals may recognise one another.

Almost unbelievably, it seems that these different species appear to be old friends.

Together they are gathering as one unified army, up to 1,000-strong.