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We are all familiar with scenes where dozens (if not hundreds) of zebras, sheep, or other preys are chased by one lion, one wolf or other predator. Examples are all over youtube, but here is one of lion versus zebras. In that video you can count at least 30 zebras. Even more, zebras (particularly adult ones) are bigger than lions. There is also evidence of prey defending itself against predators, but this seems to be not the norm.

So my question is, why do high numbers of preys always run away from a clearly outnumbered predator?

Surely evolution would have already taught these animals that numbers can make a difference, so that coordinated defense strategies would be part of their natural repertoire, or of their social learning. Why nature seem to be so oddly incapable of making use of relative numbers as defense strategies?

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    $\begingroup$ This definitely doesn't always happen. I've seen videos of groups of larger prey standing their ground against predators, e.g. walruses against polar bears, elephants against lions. $\endgroup$ – gardenhead Aug 28 '18 at 14:22
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Why run?

To some extent, selfishness. Remember that natural selection acts within species.

Combat is dangerous. Very dangerous. Even if you are not immediately killed and successfully fend off a predator, wounds can weaken you, damage your senses, and/or kill over time with infection.

If you are a zebra, or a sheep, and you are in a group of other conspecifics, the safest action for you may be to not be the individual that is attacked. Sometimes, you might achieve this by simply advertising being a tough target. Other times, your best bet is to run away. This concept is sometimes humorously discussed in the context of humans and bears: you don't have to be faster than the bear, you have to be faster than the slowest person with you. This is called selfish herd behavior.

Fleeing predation is likely to be the default behavior

If you imagine an allele that changes an animal's behavior to combat an attacking predator (when other conspecifics predominantly run from predators), it is difficult for this allele to grow in the population. Individuals with the allele will get in fights, and although they may reduce predation of the herd as a whole, they will individually be selected against.

Similarly, imagine an allele that changes an animal's behavior to run from an attacking predator (when other conspecifics predominantly attack predators). Individuals with this allele will avoid fights, stay healthier, and breed better than their conspecifics that are getting beaten up, and eventually the herd will be mostly escapers.

Exceptions to the default

However, as pointed out in a comment by gardenhead, not all prey try to escape.

To avoid the 'default' escape behavior, there must be some other motivation: fighting back has to be special some how. In the context of group selection, fighting back may be selected for if it means improved survival of related conspecifics. This behavior is often seen in defense of offspring, but can expand to larger related groups. However, even defense of offspring can be risky, and it is not uncommon in nature for mothers to abandon offspring they cannot defend. It may be safer and more productive in the long run to survive and produce new offspring next year.

Another alternative would be some sort of social/sexual selection, where strong individuals who defend the group (likely males) have increased likelihood to mate.


References

Fischhoff, I. R., Dushoff, J., Sundaresan, S. R., Cordingley, J. E., & Rubenstein, D. I. (2009). Reproductive status influences group size and persistence of bonds in male plains zebra (Equus burchelli). Behavioral Ecology and Sociobiology, 63(7), 1035-1043.

Hamilton, W. D. (1971). Geometry for the selfish herd. Journal of theoretical Biology, 31(2), 295-311.

King, A. J., Wilson, A. M., Wilshin, S. D., Lowe, J., Haddadi, H., Hailes, S., & Morton, A. J. (2012). Selfish-herd behaviour of sheep under threat. Current Biology, 22(14), R561-R562.

Sjare, B., & Stirling, I. (1996). The breeding behavior of Atlantic walruses, Odobenus rosmarus rosmarus, in the Canadian High Arctic. Canadian Journal of Zoology, 74(5), 897-911.

Smith, W. P. (1987). Maternal defense in Columbian white-tailed deer: when is it worth it?. The American Naturalist, 130(2), 310-316.

Wood, A. J., & Ackland, G. J. (2007). Evolving the selfish herd: emergence of distinct aggregating strategies in an individual-based model. Proceedings of the Royal Society of London B: Biological Sciences, 274(1618), 1637-1642.

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    $\begingroup$ Thanks! A lot to digest. Just to start, I don't see how the phrase "Remember that natural selection acts within species." fits into the picture. Do you mean that individuals only care about the survival of their own genes and have no regard for those of their fellows? So, in this sense, there is no probability of species survival function to maximise, which could help understand why such strategies might gain little traction in selection processes? $\endgroup$ – luchonacho Aug 28 '18 at 16:12
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    $\begingroup$ @luchonacho "Do you mean that individuals only care about the survival of their own genes" - to a primary extent, yes. When you talk about natural selection of alleles, you are talking about variation of allele frequency within a population, which means individuals that can interbreed. Interspecific competition is separate. On some macro scale you might think some of the ideas of natural selection apply across species that compete for the same ecological niche, but really only by analogy. $\endgroup$ – Bryan Krause Aug 28 '18 at 17:19
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    $\begingroup$ @luchonacho You should also be aware of the idea of the "selfish gene" which takes the individual-centered approach to selection even further and says that individual genes are 'selfishly' self-promoting. There are criticisms of that approach, but if you apply it to this particular case like the 'default behavior' paragraph in my answer you will find it works. $\endgroup$ – Bryan Krause Aug 28 '18 at 17:21

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