Do the Young Naturally Try to Kill the Old and Sick?

A while back, we had a case that involved a younger dog attacking and seriously injuring her older housemate when the older dog went into a seizure. The dogs had lived together without conflict for years, until the older dog began having the seizures. The owner thought that the attack happened because the younger dog recognized the older dog as sick and weak and it was just part of nature’s way for the young to try to kill the old and infirm.
Over the years we have had several similar cases involving younger dogs, and also cats, attacking and sometimes injuring younger older ones. While there can be a number of different causes for such attacks, some clients have believed that the attacks occurred because killing the sick and the old is nature’s way to purify the gene pool.
We’re not sure where these ideas come from. Perhaps it’s from not-too-scientific nature shows or a twisting of Darwinian ideas of natural selection. First of all, there is no known mechanism for nature to "purify the gene pool". Killing the old and the sick doesn’t have a direct benefit to the species as a whole.   In fact, animals which are healthy and robust enough to make it to old age have already made their contribution to the gene pool.   And it’s not as if the youngsters all get together and decide that killing the old will benefit everybody.
Second, the idea of "Nature Red in Tooth and Claw" (Alfred Lord Tennyson) – that natural selection operates through direct conflicts among animals and the strong always kill their weak competitors is not accurate.  While direct conflicts sometimes occur, and sometimes one (or both) competitors die, direct fighting is a risky business and can result in injury to the stronger as well as the weaker competitors.  In natural selection, all that really needs to happen for an animal to out-compete her competitors and to have her genes predominate in a particular environment is to leave just more one offspring than her competitors. And that can often be achieved without any direct confrontations.
Finally, focusing on evolutionary explanations for behavior doesn’t tell us anything about the immediate cause of why a younger dog or cat attacks an older one at any given moment.   It’s through understanding those immediate causes that we can create plans to resolve, manage and prevent behavior problems.   Evolutionary explanations have their place in helping us put the behavior in the larger context and explain differences between breeds or species. But we need to be careful not to get side-tracked by evolutionary explanations when our goal is to stop fighting between two dogs or cats.
So why did the younger dog attack when the older dog had a seizure?  We aren’t sure. One hypothesis is that during the seizure the older dog looked, acted, sounded and probably smelled different than she normally did and the younger dog was reacting to that strange behavior.  Some dogs seem to become aggressively aroused when exposed to unusual behavior by people or other animals. The immediate cause of behavior is one of the four types of “why” questions originally proposed by classic ethologist Niko Tinbergen which can be revisited in Robert Hinde’s text  Ethology.   Many of Hinde’s ethology books from the 1980s are out of print, but a few can still be found on by searching on the author’s name.
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