The uncertain life of a squirrel

19 April 2014 by Malcolm Campbell, posted in Biology

Things as certain as death and taxes, can be more firmly believ’d.” from The Political History of the Devil by Daniel Defoe (1660-1731)

As Benjamin Franklin said, “in this world nothing can be said to be certain, except death and taxes”. For squirrels, taxes are no certainty. Death, on the other hand, is. IMG_2652squirrel1

The chance encounter with the small corpse of a black squirrel was an affecting reminder of the fleeting nature of life – of the certainty of death.

The little corpse lay on the cement – its pristine black coat dampened by the spring melt and rain.

By all accounts the squirrel looked in good shape – the sleek satin coat, with no signs of bite marks or other trauma. Neither thin nor overweight, but muscular. Its tiny, hand-like paws underscored the fine honing of remarkable climbing abilities. It even had startlingly blue eyes, visible as slits beyond its squinted eyelids – they still seemed vibrant, even if the life had gone out behind them. Were it not so still, it would be easy to imagine it jumping up and scampering away.

The only clue suggesting the squirrel’s demise was its location. It sat curbside, near the road, under power lines overhead. One way or another, the squirrel had misstepped – either into oncoming traffic or from the wire above. Either way, something had broken it inside, a hit or a fall, now invisible to the eye. Either way, something had brought an end to a life for this little creature.

In the city, squirrel deaths are fairly commonplace. While a forested city, with tree-lined streets, rooftops, and thick overhead wires can provide urban squirrels with ample habitat, those same streets, houses, and wires also pose a considerable risk. Automobiles, pets, household toxins, and electricity all threaten squirrel existence. squirrel4

While threats to the mortal existence of squirrels abound in urban settings, these do not seem to take a toll any greater than those encountered by squirrels in more “natural” settings. Robert McCleery and colleagues looked at the differences in survival of fox squirrels in urban and “rural” environments. They used radiocollars to track 50 rural squirrels and 78 urban squirrels over an 18 month time period. Remarkably, the survival rates over this time were almost identical – around 94% for both populations.

While they shared similar mortality rates, the mode of the death set the urban squirrels apart from the rural squirrels.

Greater than 60% of rural squirrel fatalities occurred due to predation. That is to say, being eaten was the leading cause of death for rural squirrels. By contrast, less than 5% of urban squirrels died on account of predation. Instead, greater than 60% of the urban squirrel deaths were due to automobiles – being run over.

In urban environments, humans are the major cause of squirrel mortality. Even predation has a strong human element – most is done by companion animals – dogs and cats.

In an urban setting, squirrel death has a face. A human face.

Outside the confines of the city, beyond the suburbs, one might hope that the certainty of squirrel death would not include a human element. Unfortunately, this seems not to be the case.

Matthew Vander Haegen and colleagues from the Washington Department of Fish and Wildlife, examined the cause of death of 81 Western gray squirrels that they had radio-tagged in south-central Washington state. They found that the mortality rate varied from year to year, and that males had a higher likelihood of dying during the breeding season. Surprisingly, there were very little differences in survival between juvenile (5–12 months old) and adult squirrels. Neither were there differences due to winter severity, or the size of the acorn crop upon which the squirrels fed. squirrel2

Like McCleery and colleagues, Vander Haegen and his colleagues found that more than 60% of squirrel deaths were caused by predation. More than 35% of the deaths were caused by disease. Remarkably, the vast majority of deaths due to disease were attributable to notoedric mange.

Notoedric mange, sometimes called cat scabies, is caused by a mite. Mites are arachnids – a class of invertebrate animals that includes spiders and ticks. The mite that causes notoedric mange burrows into the skin of the host animal. This causes a variety of reactions in the skin – a complex of symptoms known as dermatitis. These symptoms can include hair loss. They can also include hyperkeratosis and acanthosis – thickening of the skin – as well as associated lesions in the skin. The infected animals become susceptible to bacterial infections, but also to dehydration, and to extremes of hot and cold.

Nicole Stephenson and her colleagues at University of California Davis have suggested that mange symptoms can result in the squirrels appearing as though they have abnormal neurologic behaviours. This includes involuntary muscle movements, disorientation, and absence of alertness.

In the end, the combination of mange symptoms contribute to an infected squirrel’s death.

Mange is, to say the least, an unpleasant way to die. squirrel5

To their surprise, Vander Haegen and his colleagues found that the occurrence of mange was not correlated with strong winters. Instead, mange occurrence was strongly correlated with mildness of the preceding winter – the number of days when the air temperature was, on average, greater than 0°C.

The researchers hypothesised that mild winters contributed to mange outbreaks due to the way that squirrels make use of their nests, known as dreys. They proposed that the sequential use of dreys by individual squirrels from one year to the next during mild winters contributed to the mange outbreaks. This is because the mild winters had temperatures conducive to survival of the ephemeral, free-living mites. Sequential use of dreys would have provided a long-term survival mechanism for the mites – to their hosts’ detriment.

Death by mange is, on the face of it, a “natural” cause of death as far as squirrels are concerned. Unfortunately, the long reach of human activities is impacting the prevalence of mange.

Habitat fragmentation and loss due to human activities has the immediate effect of increasing squirrel density within the remaining habitat. This brings squirrels into closer proximity with each other, increasing the likelihood of disease transmission. What’s more, habitat fragmentation also means that uninfected animals cannot readily disperse from an infected area.

There is also an increased likelihood of transmission of disease from domesticated animals when human development abuts next to wild habitat. Such locations may also employ wild-animal control measures, such as poisonings and even trap-and-release, that can stress wild animals in a way that compromises their resistance to diseases like mange. squirrel3

Finally, milder winters, brought about by urban “heat island” effects, or by global climate change, also give mange a helping hand. Mites are able to survive the milder winters. They persist in dreys, infecting squirrels throughout the year, increasing the likelihood of mange, and with it, death.

Squirrels remind us that our relationship with nature is a complicated one. When we see squirrels in urban environments it can create the illusion that we are at one with nature – we can co-exist in a harmonious relationship. A closer look at the manner in which squirrels die – even with its various uncertainties – reveals that our impact is far from harmonious, far from benign. We are a species with a profound capacity to nurture and to revel in the wonders of life around us. Let us do better than creating a legacy that is characterised by the death of a squirrel.

Images: All photographs by Malcolm M. Campbell.


Haegen V, Matthew W, Orth GR, & Linders MJ (2013) Survival and causes of mortality in a northern population of western gray squirrels. The Journal of Wildlife Management 77: 1249-1257

McCleery RA, Lopez RR, Silvy NJ, & Gallant DL (2008) Fox squirrel survival in urban and rural environments. The Journal of Wildlife Management 72: 133-137

Stephenson N, Swift P, Villepique JT, Clifford DL, Nyaoke A, De la Mora A, Moore J, & Foley J (2013) Pathologic findings in Western gray squirrels (Sciurus griseus) from a notoedric mange epidemic in the San Bernardino Mountains, California. International Journal for Parasitology: Parasites and Wildlife 2: 266-270


6 Responses to “The uncertain life of a squirrel”

  1. Sean McCann Reply | Permalink

    Do you think your squirrel could have been a young of the year? An adult would not have blue eyes, and from what I recall, Gray Squirrels have a long rearing time.

    • Malcolm Campbell Reply | Permalink

      Great question, Sean! It's possible that it was a juvenile squirrel, but it looked like it was at least a year old - it had an adult-sized body. I have wondered a lot about the blue eyes. It may have been the way the light caught them, or it may be that they had frozen. Alternatively, I wondered if it was the "third eyelid", the nictitating membrane. It didn't look like the nictitating membrane that my dog's have - it looked more like the eye - but, as I had the dogs with me, I didn't get to make the fullest investigation that might have resolved the matter. Still a mystery, I'm afraid! Thanks for your question though. Glad you read the piece!

  2. Jim Woodgett Reply | Permalink

    Feel fairly confident that one positive outcome of this brutal North American winter is that the mites have been knocked for six. The squirrels seem to have survived well although the ice storm damage to trees in Toronto has likely devastated a significant fraction of dreys (which may also cause the mites to have to re-establish). I will also admit that our mini Schnauzers reduced the local population by one last year (to their surprise) as well as a rabbit and a couple of rats. They are not as lethal t the general population as cars, however.

    • Malcolm Campbell Reply | Permalink

      Thanks for the thoughtful comment, Jim! You are undoubtedly correct that both the frigid temperatures and the damage to dreys this past last winter should knock back mange mites this year. Have been keeping a lookout for mange as the squirrels become more active. We're due for a mild winter next year, so it will be interesting to compare year-on-year. A long term project emerges!

  3. M Leybra Reply | Permalink

    Great article, very informative, thanks.

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