"A Mathematical Biologist Spends His Vacation Hiking In The Scottish Highlands" joke

A mathematical biologist spends his vacation hiking in the Scottish highlands. One day, he encounters a shepherd with a large herd of sheep. One of these cuddly, woolly animals would make a great pet, he thinks...
"How much for one of your sheep?" he asks the shepherd.
"They aren't for sale", the shepherd replies.
The math biologist ponders for a moment and then says: "I will give you the precise number of sheep in your herd without counting. If I'm right, don't you think that I deserve one of them as a reward?"
The shepherd nods.
The math biologist says: "387".
The shepherd is silent for a while and then says: "You're right. I hate to loose any of my sheep, but I promised: One of them is yours. Have your pick!"
The math biologist grabs one of the animals, puts it on his shoulders, and is about to march on, when the shepherd says: "Wait! I will tell you what your profession is, and if I'm right I'll get the animal back."
"That's fair enough."
"You must be a mathematical biologist."
The man is stunned. "You're right. But how could you know?"
"That's easy: You gave me the precise number of sheep without counting - and then you picked my dog..."