More strange knowledge: not all squirrels are vegetarians!

Produced by: Science Popularization China

Author: Su Chengyu

Producer: China Science Expo

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Most people imagine squirrels as small farmers in the forest, filling their stomachs with nuts, seeds and sprouts.

California Ground Squirrel

(Image credit: Sonja Wild, UC Davis.)

But can you imagine a California ground squirrel suddenly tiptoeing up to a vole, then pouncing on it, killing it and carrying it back to its burrow? This scene is no longer a special case in animal documentaries, but a real scene in real life.

Ground squirrel enjoying a meal of vole

(Image credit: Sonja Wild, UC Davis.)

An unusual summer

A 2024 study systematically documented for the first time the hunting of voles by California ground squirrels, a discovery that has more ecological stories worth exploring.

The study took place in Briones Regional Park in California, U.S. The research team began tracking the local California ground squirrel population in 2013, capturing and marking them every summer to observe long-term behavioral changes.

California Vole

(Photo credit: Sonja Wild)

However, the summer of 2024 was very different from any other: researchers noticed that the number of voles suddenly surged, reaching more than seven times the average level of the previous decade.

From 2016 to 2024, there was a clear upward trend in the number of reports of California voles.

(Image source: Reference [1])

Voles move in groups on the grass, providing a "meat feast" for ground squirrels.

Ground squirrel capturing prey

(Image credit: Sonja Wild, UC Davis.)

Ground squirrels live primarily on the ground, digging complex burrow systems as their habitat, while tree squirrels live in trees and are skilled climbers and jumpers.

"This is a typical 'resource pulse' phenomenon," explained the head of the study. " When a certain food resource appears in large quantities in the environment in a short period of time, the behavior patterns of animals will change significantly ." In other words, the hunting behavior of California ground squirrels is not a "new skill" in their genes, but a natural response to the sudden increase in the number of voles.

From June to July, the research team arranged a total of 146 hours of observation on the park's grass, from 8 a.m. to 12 p.m. The observation method was very particular. The researchers sat at least 20 meters away from the squirrels and used binoculars to record their every move. The results showed that in just two months, the team recorded 74 interactions with voles, including 31 scenes of ground squirrels actively hunting.

A ground squirrel escapes with a vole

(Photo credit: Sonja Wild)

The hunting scenes are very exciting. In most cases, ground squirrels will keep a low profile and approach voles cautiously. They have even learned the technique of "lurking" - lowering their bodies, reducing movements and sounds. After approaching the target, they will quickly start chasing, firmly hold the voles with their front paws, and then bite the neck of the voles. The whole process is full of tension, just like a miniature lion hunting.

Although ground squirrels appear skilled at hunting, their success rate is not high. Only 17 of 31 hunts were successful (a success rate of about 55%). Most of the failures were caused by voles escaping during the chase or fighting back to break free after being briefly captured. The researchers also observed several cases of voles biting squirrels, which clearly shows that hunting is not completely risk-free for ground squirrels.

Hunting and eating incidents were almost nonexistent from early to mid-June, but increased rapidly from the end of June and peaked in early July. The number of incidents then gradually decreased and was close to zero by mid-July.

Ground squirrels display strong opportunistic characteristics in their behavior, actively hunting when vole populations are high.

(Image source: Reference [1])

Peel the skin first and then eat the meat. It’s so delicious!

After successfully capturing a vole, ground squirrels usually carry their prey to a secret place for processing. The research team found that squirrels have a fixed "dining procedure": they first bite off the vole's head, then peel off the fur, and then eat the exposed muscles and internal organs. The order and method of eating show that squirrels' use of meat resources is far more complicated than we imagined.

It is worth noting that these hunting and eating behaviors are almost not restricted by gender and age. Both adult and juvenile ground squirrels, male and female, have shown the willingness and ability to hunt. This phenomenon shows that hunting behavior is likely to be an opportunistic response of ground squirrels, rather than a unique skill of certain specific groups.

Successful hunting does not mean that the "prey" belongs to the hunter himself. The research team also recorded 8 competitive behaviors between squirrels around the corpses of voles. These competitions included direct snatching, pushing and even chasing, but there were also several rare "social tolerance" phenomena observed - a squirrel allowed other squirrels to eat the remnants of the prey it left behind.

Ground squirrels eating captured voles

(Image source: Reference [1])

These scenes show that voles are a relatively scarce but high-energy food resource for ground squirrels. Compared with seeds or sprouts, the meat of voles is obviously more worthy of competition. The scenes of competition highlight the impact of meat resources on the internal relations of ground squirrel populations and provide new clues for our understanding of the social behavior of ground squirrels.

Squirrels are not vegan, but they are not carnivores either

This study reveals an important fact: California ground squirrels are not strict vegetarians. Studies decades ago have shown that some ground squirrels occasionally prey on insects or bird eggs, and even eat their own young. However, these behaviors are mostly accidental and have not been systematically studied. This study, through the surge in the number of voles, helps us more clearly understand the "opportunistic omnivorous" characteristics of ground squirrels.

In fact, the behavioral flexibility of ground squirrels is an ecological adaptation. Plant food is still their main source of food, and meat is more of a supplement under environmental pressure or opportunity conditions. The surge in the number of voles in 2024 has reduced the difficulty of hunting, allowing more squirrels to try this new food source.

Simply put, there is too much vole meat, and it is hard not to eat it... For them, whether to eat meat or vegetables depends more on "where there is food and how to eat it easiest"!

References:

1. Smith JE, Ingbretson JE, Miner MM, et al. Vole hunting: novel predatory and carnivorous behavior by California ground squirrelsJ. Journal of Ethology, 2024: 1-10.