Male bottlenose dolphins save physical contact for those they feel closest to, but for other weaker – yet still important – relationships they connect through whistling, a study has found.

The animals are known to use gentle petting with strongly bonded allies.

But as the numbers of these relationships grow, they have less time for this kind of touching, and rely on the more time-efficient vocal exchanges.

University of Bristol scientists, with international colleagues, examined nine years of acoustic and behavioural data from a dolphin population in Shark Bay, Western Australia.

Lead author Emma Chereskin, from Bristol’s School of Biological Sciences, said: “Many animals, including humans, use tactile contact, touch, to strengthen and reaffirm important relationships.

“But as the number of close social relationships increases, so too do the demands on the time and space available for relationship maintenance through physical contact.

“Male bottlenose dolphins form strategic, multi-level alliances, and we wanted to know how they maintained multiple alliance relationships in large groups.”

By following groups of males that knew each other, and documenting their physical and acoustic behaviour, researchers identified the different ways these males bonded with each other.

Senior author Dr Stephanie King, also from Bristol, said: “We found that within the core dolphin alliances, strongly bonded allies engaged in more affiliative contact behaviour, such as petting and rubbing, while weakly bonded allies engaged in more whistle exchanges.

“This illustrates that these weaker but still key social relationships can be maintained with vocal exchanges.”

The findings are published in Current Biology.