6 Things About Squirrels

6 Things About Squirrels

Tree Squirrels

Tree squirrels have notable adaptations for navigation, being more accentuated than ground squirrels, considering their correspondence to an arboreal environment.

For example, they are lighter and have robust forelimbs plus foot padding to cushion landing impact. They also have longer, bushier tails since leverage for balance is a necessity - where larger tree squirrels have the longest tails proportionately and absolutely compared to body length.

Furthermore, they are hypermobile, somehow capable of rotating their ankles 180 degrees (the sharp claws act as brakes when facing backwards), which enhances grip - for descending trees head first. For running up trees, they have long, muscly and powerful hindlimbs. Some can even leap 10x their body length and/or run 20mph - such as the grey tree squirrel.

In general, squirrels can survive falls of terminal velocity, which is a low speed compared to other animals because they have a large surface area to mass ratio - implying less gravitational impact with more aerodynamic resistance.

The Arctic Ground Squirrel

Ground squirrels have the firmest body and are the only type that hibernates through winter. The arctic ground squirrel is unique because they're the only warm-blooded animal that can withstand temperatures below freezing when hibernating. They're usually solitary but can occasionally form arranged sleep groups for heat transfer.

Their body temperature drops a little below 0 degrees celsius, plus the heart rate falls to 1 bpm. It's speculated that the blood can remain liquid as the animal somehow cleanses its body of 'ice nucleating particles' - needed to form crystals (so the blood doesn't freeze).

Flying Squirrels

On the other hand, flying squirrels live almost like birds, with specialised anatomy that most prominently pertains to their skin attached to the limbs ('patagia'), functioning like a glider that lets them cross the length of a football pitch from a tree.

They are more awkward when moving on the ground; but they mostly never leave trees unless burying food or something. In the air, they have deft manoeuvrability and a long tail that functions as a parachute for braking. Bigger flying squirrels are less manoeuvrable but resist air turbulence (sudden airflow changes) better than smaller ones.

An interesting extra gliding structure is their protruding, upturned wing tips, similar to aircraft, controlled via extension of wrist cartilage. They reduce drag while creating a little lift force because of the added surface area, saving energy and gliding further.

They work because the added height of the wing tips disperses the air flow more, regarding the swirling vortices surrounding the wings (created by pressure differences below and above them). The air distribution results in a larger vortex area - suggesting it's harder for the airflow to encompass the entire structure, meaning a weaker resistance.

Overall, skin flap adaptations enable enough lift force, combined with their light weight (compared to other squirrels of similar size), high accelerative thrust force generated from a powerful jump, and lowered drag - a glide is feasible to maintain for a little while.

Caching

One thing squirrels are infamous for is 'caching'. They bury food, such as acorns, after digging little storage holes, which usually get made in the autumn; then they return in winter. The digging helps soil aeration, allowing water, air and nutrients to reach the roots easier.

They likely use both spatial and visual memory cues to return to the location - with their smell and recognition of any disturbed soil to help them remember further. Their sense of smell is strong enough to detect food buried under a foot of snow, alongside if nuts are ripe or not.

Even so, squirrels still miss roughly over a quarter of their buried acorns (which helps trees grow), and it's about the same with their food caches in general because of squirrel thieves - not even including thieves of other species. It can be a lot of food, as squirrels can eat through their body mass each week.

Regarding squirrel competition, lots will steal instead of forage to let others do the work and save energy. However, they can pretend to hide their food after digging a hole and covering it to mislead onlooking thieves, while instead carrying it in their mouth to eat or bury again secretly when the thief is distracted searching for the non-existent food. It's called 'deceptive caching', which is probably best to cover their backs first just in case or when they're aware of being watched.

Seed Perishability and Dormancy

Squirrels typically eat 'high perishability' seeds, which cannot remain stored until winter as they will germinate in autumn, unlike 'low perishability' ones (depending on the time until their germination). These types likely get differentiated by smell.

At the same time, there are 'dormant' and 'non-dormant' seeds. Unlike dormant seeds, non-dormant ones germinate within a certain period when exposed to adequate environmental conditions - involving light and water, for instance. So, non-dormant seeds can probably coincide with high perishability.

The learned squirrel behaviour of eating high-perishability seeds first is a countermeasure, which combats their autumn germination - presumably, the seed's adaptation to avoid predation. Otherwise, they can damage/'notch' the growing point of the seed - then save it for later (dormant/low-perishability seeds are mostly cached undamaged).

Notching the growing point will prevent up to about half of the nutritional value from getting transferred to the seedling inside the tough outer casing for germination. Therefore, the squirrel can retrieve it in winter with its nourishment intact.

Pilferage vs Predation Risk Conflict

When foraging, squirrels are understandably close to shelter, so a safe retreat option is available. Except caches are out in the open, further away from safe cover - meaning there's more predation risk yet less chance of other squirrels pilfering because of said predation risk.

Pilferage risk would be more of a 'constant' in terms of the open opportunity for attempts (unmoving, defenceless cache left alone for months), compared to the squirrel's predation - which is less consistent of a risk, especially considering squirrels' careful vigilance, navigational efficiency and agility.

So perhaps it makes sense to use a vulnerable space as a potential deterrent to squirrel thieves, even if predation would be more consequential in theory compared to stolen food. However, that may still detriment their winter survival - which is why they can 'scatterhoard' with enough food - having multiple caches so their eggs aren't all in one basket.

That's how they balance the dilemma of pilferage (concerning risk frequency) vs predation (involving risk severity).