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[00:00.44] The sound of a very close bond.

[00:03.62] A killer whale mother and son surfacing together, because in orca family life, offspring stay by their mother's side into adulthood.

[00:11.07] But sons are particularly dependent on their mums.

[00:14.53] Well into adulthood, males will demand to be fed fish by their mothers, even though they're much larger than females.

[00:21.43] But that close family bond comes at a cost.

[00:23.82] This new study showed that having a son cut by half a mother's chance of reproducing again in the future.

[00:29.66] The scientists believe that mothers invest so much of their energy and effort in their male offspring because the biggest, oldest ones tend to father most of the new calves in a killer whale pod.

[00:40.03] As well as providing an insight into the complex, close-knit lives of these marine mammals, the findings could help us protect them. Understanding how much it costs to raise the next generation of orcas is an insight into what these mammals need to survive.