In the world of capuchins grapes are luxury goods (and much preferable to cucumbers) So when one monkey was handed a grape in exchange for her token, the second was reluctant to hand hers over for a mere piece of cucumber.

参考译文：在卷尾猴的世界里，葡萄是奢侈品（比黄瓜受欢迎得多）。所以当一只猴子用一个代币换回一颗葡萄时，第二只猴子就不愿意用自己的代币换回一片黄瓜。

over为副词，和动词hand构成搭配，形成短语动词。hers=her token

hand over sth for sth为某物交出某物

(1)vt. hand over交给;递给

He also handed over a letter of apology from the Prime Minister 

他还递交了一封首相写的道歉信。

'I've got his card.' Judith said, handing it over.  

“我有他的名片，”朱迪丝边说边把名片递了过来。

(2)hand over sth to sb（把…）移交（给…）处理

They would just catch the robbers and hand them over to the police.

他们会抓住抢劫犯，并把他们交给警方。

This morning the American was formally handed over to the ambassador.  

今天上午这名美国人被正式移交给了大使。

The present leaders have to decide whether to stand down and hand over to a younger generation.

现任领导层必须决定是否应退居二线，把重任移交给更为年轻的一代。