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Let species A be prey to another species B. Assume that predators B only attack isolated individuals A (because they are afraid of larger groups of A's). So it is good for species A to be on the way in rather dense herds, flocks, or swarms. But individuals A which find themselves isolated (which happens every now and then) are well adviced to flee when a predator is in sight. So there are two antagonstic tendencies in individuals A: to flee or not to flee when a predator comes into sight. To flee as a herd, flock, or swarm should keep it together, so it's best to flee into the same direction as one's neighbours. Nevertheless single individuals will become isolated and be easier catch for the predator, so for the species (herd, flock, or swarm) it is best not to flee at all.

This can be advanced when the individuals with the least tendency to flee (the most fearless ones) are at the outside border of the herd. So it would be an evolutionary advantage of the species when the most fearless individuals come to be at the border of the herd - by own initiative or being pushed by the others.

I wonder if there have been investigations of this: Are there species of which it is known that (and how) the most fearless individuals are at the border of their herds, flocks, or swarms?


This is how the investigation could go:

  1. Are there the same individuals at the border of the herd more often than not?
  2. Do they start fleeing at a smaller rate than other indivuduals which happen to be at the border?
  3. Why are they at the border? Is it because the others tend to go away from the border, into the center of the herd?

Note that my question is not for the strongest individuals but for the most fearless. (I know the case of the muskox and its typical defense formation.) There may be species with no extraordinally strong, but with extraordinally fearless individuals - which is another case.

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Species evolve, but individuals are selected. Simplifying a bit, a species evolves as relative allele frequencies change in the population. When natural selection is at play, these changes in allele frequencies come about because individuals are more effective at transmitting their genes to offspring compared to others of the species. If individuals with a particular allele P reproduce more than individuals with a different allele Q, there will be a bigger ratio of P:Q alleles in subsequent generations.

So it would be an evolutionary advantage of the species

is not the sort of question you should ask, because advantages of the species don't affect allele ratios within the species. If there is a "fearless allele" F that causes individuals with that allele to occupy the outside of the herd, and a fearful allele G that causes individuals with the allele to group up and occupy the inside of the herd, and this reduces predation of the herd equally, then the ratio of F:G stays the same. If, on the other hand, those fearless animals on the outside of the herd are at higher risk for being on the outside (remember that fighting in nature is very dangerous; you don't have to lose the fight to die, as an injury that makes you slower, more susceptible to infection, interferes with eating, etc, still prevents you from reproducing in the future, even if you kill the predator and save the day), you can expect that the ratio of F:G will decrease: there will be more G alleles in the future generations. This is the basis of selfish herd theory

Herd behavior is fairly well-studied and doesn't really result from the antagonism you mention (flee vs not flee/stay as a group) but rather from aggregating sets of individual selfish rules like trying to be at the center of a group of conspecifics.

The exception to this is group selection, which is when selection acts on whole groups. In this case, instead of thinking of selection by individuals you might think of different herds. If herds with a higher percentage of F alleles reproduce better than herds with a higher percentage of G alleles, then there could be selection in favor of F across herds. However, the features that promote selection of this type can still be thought of at an individual level and this helps to reveal the features of groups that promote selection. For example, if the F allele promotes aggression against predators specifically in adults, or perhaps specifically in adult males, and the animals live in small groups of related individuals, then individuals with the F allele may have better reproductive success due to improved survival of their offspring, and those offspring are also more likely to have the F allele than offspring of parents without F, so the frequency of F increases.

In summary, think of the advantages that a trait like this provides the individuals with the trait, rather than the group as a whole. If you're looking for species where certain individuals defend the borders of their groups, you want to look for species that live in familial groups, where related individuals benefit the most from this behavior. Examples where you might find this would be in species like elephants (smaller family groups) rather than species like wildebeest (very large groups of only distantly related individuals).

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  • $\begingroup$ Thanks a lot for this instructive answer! But what about alleles that allow individuals to become or better: to learn to become more fearless? Not all individuals that have them will necessarily become fearless, but due to circumstances some of them do. (The predecessor species without the allel may have had only fearful individuals.) $\endgroup$ Jan 5 at 21:02
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    $\begingroup$ @Hans-PeterStricker You'd think about it exactly the same way: if an allele "allowing an individual to become more fearless" causes there to be more instances of that allele in the next generation relative to other alleles, it is being selected for and you can expect its frequency to increase in the population. If that allele causes some individuals possessing it to behave in riskier ways for that individual and therefore not reproduce as well on average (for example, because they are more likely to die of injury) you'd expect it to be selected against. $\endgroup$
    – Bryan Krause
    Jan 5 at 21:16
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    $\begingroup$ The only difference is that you'd expect the rate of change in the population (in either direction) to be slower if only some individuals with the gene express the trait. $\endgroup$
    – Bryan Krause
    Jan 5 at 21:17

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