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I recently watched a clip on Discovery Channel, where I saw deers crossing a river full of crocodiles, ignoring the fact that some of them would have been killed doing so.

What could be an evolutionary reason for this behaviour?

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What's with the down-votes? At least give a reason! –  Luke Jul 13 '12 at 10:03
    
I agree, this is a perfectly reasonable question. If you're going to downvote, have the courtesy to leave a comment explaining why. –  Richard Smith Jul 13 '12 at 13:40
    
Have not downvoted nor upvoted the question, but I surely would say that the formulation is quite "rough" as it stands. It could, however, be a good question if better rephrased. –  nico Jul 13 '12 at 13:41

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up vote 6 down vote accepted

The reason for their behavior is that exactly the fact that even though some of them do indeed get killed, most of them survive -- what you have said. The deer cross the river because that allows for the better continuation of the species rather than if they had stayed on the original side of the river. You could think of it this way: If there was a crowd of 5000 people crossing a field of grass and aliens were abducting them one by one, the actual chances of them abducting you are rather little.

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The deer do not live for the continuation of the species, but otherwise this answer is correct. The genes which control the urge to cross the river proliferate despite a small amount of predation by crocodiles. Presumably this is because there is a great reproductive or food payoff of crossing the river to better breedinbg/feeding grounds. –  Richard Smith Jul 13 '12 at 13:41
    
@Richard Yes, I agree I could've worded that better. –  Chase Yuan Jul 13 '12 at 13:43
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@Chase Yuan: feel free to edit your question to improve it so to include Richard's suggestions! –  nico Jul 13 '12 at 13:45

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