现在，剑桥大学的富兰克·顾尚了原因。
Now Franck Courchamp of Cambridge University has found a reason why.
顾尚通过模拟随着狗群的减小看护狗的代价的变化研究这种两难境地。
Courchamp investigated this awkward trade-off by modelling how the costs of a babysitter change with decreasing pack size.
顾尚修说，多数用户很乐意收到信息，费用很低，每条信息才两块人民币，就一瓶水的价格；
'For the most part, users are just happy to receive messages, ' said Mr. Koo, the finance chief. 'The payment is very low, two renminbi per message. It's a bottle of water.
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