Photo of Bear Grylls’ 18-year-old son, Marmaduke, stuns the internet

Bear Grylls has shared a sweet tribute for his son’s 18th birthday – but there’s one detail that fans can’t get over.

Fans were amazed by how tall Bear's son is. Picture: Instagram/Bear Grylls OBE

Survival expert and TV star, Bear Grylls, posted a sweet birthday tribute for his son’s 18th birthday, and fans online were stunned by his kid’s impressive height and build.

Grylls, who has amassed over 7 million followers on Instagram, posted a photo of his son, Marmaduke, and his wife, Shara Grylls.

“18 today! We love you so much, Marmaduke … keep shining bright,” he wrote.

The photo featured the three dressed up for a fancy occasion, with Marmaduke beaming while sandwiched between his parents.

It was also evident that the teenager was much taller than his father, who is already six feet tall.

He also towers over his mum to a comical extent, who looks tiny in comparison.

Fans were amazed by how tall Bear's son is. Picture: Instagram/Bear Grylls OBE

He has been taller than his parents for years. Picture: Instagram/Bear Grylls OBE

Online, fans were delighted to see the explorer’s son all grown-up and also couldn’t help but remark on his impressive size.

“What you been feeding him? 18 and already looks incredibly strong,” one commented.

“Bear Junior is turning out to be a Grizzly,” one quipped.

“You must look up to him a lot,” another joked.

“I think drinking piss somehow created a super human,” someone remarked, referring to Grylls drinking his own urine on multiple occasions in his survival show, Man Vs Wild.

“Get that kid in rugby gear,” another demanded.

Grylls is an impressive adventurer, famous for his shows about survival, like Running Wild, where he takes famous stars on a rigorous journey in the wild. Guests have included A-listers like Bradley Cooper and Zac Efron.

One of Grylls’ other son’s, Jesse, is an artist. Picture: Instagram/Picture: Instagram/JesseGrylls

He is a famous adventurer. Picture: Instagram/Bear Grylls OBE

Despite his huge profile, he has always been relatively private about his family life and children.

He has another son Jesse Grylls, 20, who is a musician and artist and has amassed 23,000 followers on Instagram. He has also embraced his adrenaline junkie Dad’s approach to life.

He even shared a clip of him on social media jumping out of a helicopter into snowy mountains.

“You got your dad’s No Fear genes,” someone remarked.

Meanwhile, Grylls’ middle but tallest son, Marmaduke, has all his social media accounts set to private and doesn’t share his life publicly. The adventure man’s youngest son, Huckleberry, is 15 and also avoids the spotlight.

The adventurer opened up to The Times UKabout fatherhood and his philosophy of letting his boys take risks.

“With kids, if you strip risk out of their lives you do them a disservice, because the truth is, the world is full of risk — in business, relationships, everything. You empower your kids when you teach them how to manage risk, how to cope with It.,” he said.