Its explicit scenes of mating molluscs have been called “slug porn”.
Now the producers of Sir David Attenborough’s Wild Isles have revealed that the series features extra lashings of sex to compensate for a shortage of dramatic hunting footage.
Penises have loomed large in the hit BBC One natural history series. Sunday night’s “Grassland episode” will give a starring role to mating adders in Northumberland.
In footage captured for the first time, viewers see how male adders are dragged around by their penises by the females. The mates stay bound together by dozens of “penis barbs” that have evolved for this purpose.
The unusual sex lives of Dartmoor’s ash-back slugs featured in last week’s edition. Sir David told viewers how the hermaphrodite molluscs have both male and female sexual organs and mate by hanging from a branch and twisting together.
The veteran broadcaster described a “magical pendulum of love”, as two penises protruded, each as long as its owner’s 30cm body, corkscrewing around each other as sperm is passed between them.
Viewers called the pre-watershed scenes “slug porn”.
The Wild Isles team denied that the series – which in its opening episode showed banded demoiselle damselflies unloading sperm – had a “penis obsession.”
But sex has substituted for scenes of predators closing in on their kill in Wild Isles, which puts a spotlight on the wide variety of flora and fauna to be found in the UK and Ireland.
Chris Howard, the series’ producer, told i: “Apart from the orca, we don’t have so many of those large predators – lynx, bears and wolves.
“So it turns out the sex sequences are coming to the fore a bit more in this series there’s because we have slightly less of the domestic hunting.”
Mr Howard acknowledged that “the adders sequence is pretty full-on I have to say.” He hoped families with young children would still be able to sit down and watch Wild Isles, which airs pre-watershed at 7pm on Sundays, despite the explicit mating scenes.
“We put a lot of thought in how to pitch it. It’s important to engage a young audience. We need to serve an audience from age five to 85 – and older than that, because David is 96 – so it is difficult to toe the line and keep everyone happy.”
It was important that Wild Isles didn’t sanitise a natural world fixated on sex, he added: “Once animals find enough food to survive, their sole object is to find a mate and pass on your genes.”
“So the two stepping stones for most animal behaviour stories are eating or having sex. It is inevitable that we have to go there.”
He added: “There’s always an evolutionary reason behind the most fascinating behaviour. The male adder has that penis so it can stay with the female and stop others getting in.”
Considerations of taste equally apply to scenes of predators catching prey. “With the hunting sequences, we take great care that they are not too explicit as well. We’re always making choices about how far we push hunting scenes.”
The adder sex sequence also delivered a conservation message. “Quite often people let dogs out where adders are and that stresses the adders out,” Howard said.
“The females will be dragging the males along all over the place just at the point where they need to be as still as possible for obvious reasons. It’s crucial that we think about our effect.
“Please keep your dogs on a lead in April, it can make a huge difference to an adder.”
There will be some respite from animal copulation, Mr Howard promised. “The next episode is freshwater and I don’t there is any sex. There are salmon spawning but that is essentially laying eggs so that’s OK, I think.”
Nick Gates, another Wild Isles producer, who oversaw the adder shoot, added: “While filming this sequence, we routinely had male adders slithering over our feet as they sought out females.
“The mating balls that feature in the sequence are the first time this behaviour has ever filmed, and the courtship we captured was the most detailed this has ever been filmed.”
He warned that adders are declining across almost their entire range in Britain due to “ongoing persecution, habitat fragmentation and disturbance by people and dogs, amongst other reasons.”
Mr Gates added: “Adders have been negatively portrayed in the media for decades, and this sequence is an opportunity to highlight the remarkable side of these extremely shy animals.
“Adders are extremely reluctant to use their venom, which is only ever used as a defence or last resort.”