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A Wilder View: A ghost of predators past

deer winter
Posted at 8:14 AM, Sep 09, 2021
and last updated 2021-09-09 10:14:57-04

MISSOULA — Sometimes prey animals get to spend their lives where not a single predator exists, but the ghost of predators past keeps them vigilant even when no harm is at play.

A deer feeds in a field and hears a faint crack -- could this be a predator? Maybe it’s just a bird or maybe it’s something that wasn’t even there.

In response to predation, prey have developed various physiological and behavioral traits. Of these, vigilance is a key way to avoid being eaten. By being alert the animals will likely notice a predator before being attacked.

In situations where predators aren’t around for a large time frame like years, you would expect prey to be much less vigilant. But a new study used an innovative approach to study vigilance of black-tailed deer on two predator-free islands.

Surprisingly, deer used a significant amount of time being vigilant. Vigilance levels during the day were similar to those observed in studies where predators were actually present.

These levels were higher than those observed in elk in Yellowstone when wolves were not present. Yellowstone is of course the place where wolves were reintroduced after becoming extinct.

Despite the fact these deer are in an area of no predators the fear of predators is still in their genes. And therefore, they are being vigilant for something that’s not even there -- a ghost of predators past.

This vigilance could be caused by human disturbance, much like most wildlife behaviors, we must account for the roles people play in our ecosystem. Although there may not be any real risk, this can provoke increased vigilance in some animals.

This behavior of being more cautious is likely under strong selection pressure. Meaning that the genes attributed with vigilance stay within the population for generations and doesn’t get weaned out quickly.

This is because the cost of carrying that trait is relatively low. It takes little effort to be alert, just in case, so the trait has stayed. This study showed that the deer at the study site may abandon clear feeding opportunities when exposed to wolf urine.

Yet, these deer have never even seen or interacted with a wolf in their life. Indicating that instinctive alertness while feeding has persisted for several generations.

Wolves became locally extinct in the 20th Century in Europe and North America, but are currently recolonizing certain areas, many of which have seen a recent and dramatic increase in abundance of deer and elk, together with associated vegetation and biodiversity changes.

Studies are therefore critical for understanding how the foraging and antipredator behavior of animals might have changed in the absence of wolves.

The environmental factors driving vigilance levels might override the absence of significant risk from large predators.