Programmed for Sacrifice

Many animals freeze when they are caught. They sacrifice their lives, and the health of their community is preserved.

Cattle, sheep, and most other social herbivores often react to a threat by freezing, especially when neither fleeing nor fighting is possible. That’s what happens when you put a cow in a “squeeze chute,” a contraption that helps us doctor sick animals. Essentially, the cow moves down a chute controlled by a lever. When you pull the handle, the walls of the chute close tightly around the animal, forming a v-shape that lifts some of the weight off the animal’s legs. When that happens, surprisingly, cows relax and grow still.

If an adult cow continued to struggle in the squeeze chute, the situation would be unmanageable. It’s her autonomic surrender response that makes it possible to give her the medicine she needs, or even do superficial surgeries.

Once when one of our cows was struggling hopelessly in a patch of quicksand, Carolyn and I managed to rescue her by passing a strap under her body (not the easiest task, as you can probably imagine) and lifting her out with the hydraulic bucket on our tractor. As long as her body was suspended in the strap, she just hung there, placid.

Sheep, horses, and other social herbivores react similarly. To handle a sheep, we first catch its legs and turn it over on its back. When they are firmly held, they freeze.

At first glance, the freeze response may seem counterintuitive. If the individual continued struggling, there would be, of course, some chance of escape. So why do some animals freeze when they are caught? How did that instinct evolve? How could it possibly provide an evolutionary advantage to a creature to freeze in the grips of a predator?

I’ve not found any science to support my theory about this, but it seems logical to me that social animals—especially herbivores that are subject to constant threats from predators —may have developed their freeze response through evolution, to protect their communities. If every animal struggled continuously while being attacked by a predator, many individuals would logically escape, wounded. The herds and flocks would include many maimed individuals, whose reproductive potential would be limited by their injuries. Furthermore, the predators would still need to eat, so it would be necessary to continue further attacks until a kill was made. So, day in and day out, many more of the prey animals would be subject to attack.

When the individual freezes in surrender, he virtually ensures his own death. But in so doing, he temporarily forestalls any further attacks on his herd. In effect, the animals are programmed to sacrifice themselves for the common good.