Female lesser-white-toothed shrews caravanning

not often studied.  Yet shrews really are amazing – for many reasons.  In addition to their unique social behaviors (such as caravanning) and their super-stinky scent glands which while serving to mark territories also make them unpalatable to mammalian predators,  they are the smallest terrestrial vertebrates (the smallest house shrew weighs 2 g – many insects are larger!).  Because of their diminutive size, they exist on the very threshold of life as an endotherm.  Small-bodied organisms, like shrews, have a high surface area to volume ratio, meaning that they lose metabolic heat more quickly than larger-bodied mammals; shrew bodies just aren’t large enough to maintain a thermal inertia of stored heat.  Since shrews are resigned to a life of endothermy, their metabolisms must work overtime to keep their small bodies warm.  To stoke this demanding metabolic fire,  some species of shrew must eat every 1-2 hours or risk death by starvation, and their tiny hearts must beat about 1,0000 times per minute (the average human heart beats just 70 times per minute).  One would think that because of these metabolic constraints, shrews could only survive in warmer climates where they wouldn’t have to expend so much energy staying warm.  Not so!  Shrews are distributed across northern latitudes in Europe and North America and are even active under the snow during winter.  So what gives?  How can shrews possibly survive?

The logic behind Bergmann’s rule illustrated – small bodies have a higher surface area to volume ratio than large bodies

In many mammals, there is a documented pattern of increasing body size with latitude or elevation, known as Bergmann’s Rule.  The logic behind this pattern is that larger body masses tend to have lower surface area to volume ratios and therefore lose heat more slowly and do not expend as much metabolic energy maintaining their core body temperature.  So, is body size in shrews larger at higher latitudes or elevations?

No!  In fact, shrews actually get smaller in the winter.  Yes, even smaller.  August Dehnel

Eurasian shrew

Shrew body mass decreases substantially in the winter

Decreased body mass in winter is associated with a decrease in metabolic rate

observed in 1949 that shrew braincases and internal organs actually shrunk over winter and enlarged again during summer.  This pattern of shrinking body mass (and associated internal organs) in cold conditions is known as Dehnel’s Phenomenon.  While this is really interesting and possibly mind-boggling, does it help shrews survive in the cold?

A recent study by Jan R. E. Taylor and colleagues studied changes in body mass, skull size, and metabolic rates in Polish Eurasian shrews (Sorex araneus) during summer and winter over 4 years.

In addition to documenting substantial decreases in body mass and skull size (19%) during winter, Taylor et al. also show that these reductions in body mass effectively saved energy.  By reducing their body size, shrews are also able to reduce their resting metabolic rates by 18%, resulting in decreased heat loss and reduced demands for food intake during winter.

Shrews have demonstrated a unique mode of coping with the contraints of having very small body sizes while living in colder climates and remaining active during the winter.  Instead of following the “typical” mammalian trend of increasing body mass in colder regions and elevations, shrews, of course came about a solution for saving energy in a totally different way.  Perhaps the advantages of being small and the ability to take advantage of small microhabitats and microclimates were more important over evolutionary time, and have kept shrews small – necessitating a new solution to energy demands for small bodies in the cold.  Maybe we should spend more time thinking about shrews!

References:

Taylor, J. R. E., L. Rychlik, and S. Churchfield. 2012. Winter reduction in body mass in a very small, nonhibernating mammal: consequences for heat loss and metabolic rates. Physiological and Biochemical Zoology 86:9–18.

MacDonald, D. W. 2009. The Princeton Encyclopedia of Mammals. Princeton University Press, Princeton, N.J. 976 pgs.

Attendees of the 6th International Colloquium on Arboreal Squirrels held in Kyoto, Japan

Kyoto Japan.  Besides having an amazing experience and meeting wonderful new colleagues and reconnecting with current ones (also wonderful), I was stimulated to think once again about biodiversity and ecosystem services at a global scale. Here was a conference dedicated solely to a group of small mammals (squirrels), attended by 60 researchers from around the world including countries such as Korea, Taiwan, Italy, United Kingdom, Thailand, India, Finland, Canada, Japan, and the United States.

Why the interest in small mammals, and in this case, squirrels specifically?  To gain insight regarding

Lesser long-nosed bat pollinating a cactus. Photo from USFS

this question, think for a moment about the world without small mammals.  You may be thinking to yourself, “hmmm, I would miss the cute squirrels in the park but otherwise no big deal – I hardly see small mammals anyway”.  Because small mammals (mammals weighing less than 5kg) are mostly cryptic (hard to see or find) due to camouflage, nocturnal activity patterns, and relative shyness, many do not appreciate the services they

Harvest mouse (Reithrodontomys spp) - a seed disperser. Photo by Jean-Louis Klein and Marie-Luce Hubert

A kangaroo rat (Dipodomys spp.) - a seed storer and disperser

Arizona pocket mouse (Perognathus amplus) - a seed diserser. Photo from eNature.com

provide.  Around the world, small mammals act as pollinators, seed dispersers, support forest regeneration and maintain forest health, aerate soil and allow for increased plant diversity, control insects, provide food for people and other carnivores, provide fur and pelts, provide recreational opportunities in the form of hunting and wildlife viewing, and have an aesthetic value to many.  All of these services are known as ecosystem services.  An ecosystem service can be thought of as services provided by an ecosystem and component species therein that are beneficial to humans and to life on earth in general.  For example, healthy wetlands filter and purify water, and forests serve as carbon sinks which reduce the negative impacts of carbon emissions, provide food, habitat, medicines, and even generate their own climate.  Ecosystem services provided by small mammals alone are probably worth billions of dollars, if such services could be accurately quantified.  As just one small example, nearly all of human agriculture and plant production relies on insect, bird, or small mammal pollination!

Ecosystem services from tree squirrels -

Tree squirrels, just one group of small mammals, by themselves provide a multitude of ecosystem services.  Some of their most important services can be attributed to their close association with forests throughout the world.  Squirrels have been around for a long time – roughly 35 – 40 million years – with very little changes in their general morphology, indicating that features squirrels possessed 35 million years ago are still working well for them today.  The evolutionary history between forests and squirrels goes way back, long before humans showed up on the scene (about 6 million years ago), so it is not surprising that forests and tree squirrels have co-evolved some intricate

Eastern gray squirrel caching tree seeds - Photo Mark Baldwin

codependencies such that forests depend on squirrels for a variety of services and tree squirrels are likewise dependent upon forests for food, shelter, reproduction, and survival.  Many tree squirrels exhibit caching behavior, and store tree seeds and cones for later consumption.  Not all of these stored seeds are actually consumed and the squirrels actually help facilitate the germination of many new trees, such as
oaks, chestnut, hickory, and many coniferous species.

Another service tree squirrels perform is via mushroom consumption, or mycophagy.  Mushrooms serve as major food items for many squirrel species.  Squirrels consume the entire mushroom fruiting body, spores and all, in addition to storing

Conifer seedling and associated mycorrhizae

mushrooms in trees to dry.  Either by consumption or storage, squirrels help mushrooms disperse spores in the air and via feces.  Many of these mushroom species form mycorrhizal associations with trees in a relationship known as a mycorrhizal symbiosis, where the tree provides fungi with carbon derived from photosynthesis, while fungi provide trees with improved nutrient uptake and the ability to acquire nutrients not readily available to the tree.  Studies have shown that seedlings grown with mycorrhizal associations grow faster and do better in general compared to seedlings grown without mycorrhizal associations.

Tree squirrels also support food webs within forest

A squirrel being depredated by a hawk

ecosystems by providing food for other species like avian and mammalian predators.  In short, tree squirrels help support forest ecosystem functioning in many ways and forests inhabited by squirrels are healthier and more diverse than those without.

When we think about the diversity of small mammals, including squirrels, around the world, there is hardly an ecosystem that does not benefit from services provided by small mammals, each species with their own suite of services related to the particular landscape it inhabits.  Thus it is a self-perpetuating cycle: small mammals provide services to the ecosystem in which they are intricately a part of, and in turn receive many benefits themselves.

How can we begin to value ecosystem services?

 Humans are a big part of ecosystems world-wide, and yet our economies generally do not recognize ecosystem services as currency.  We tend only to focus on individual ecosystem products (or natural resources) like agriculture, timber, livestock, or wild game instead of valuing the system as a whole.  Because of this, it is hard for us to really value conservation, because it often doesn’t make money “right now”.  But at what future cost?  As landscapes are broken up by development, mining, forestry, and oil exploration, oceans overfished and polluted, climate altered by unprecedented levels of greenhouse gas emissions, species lost to exceedingly high rates of extinction, how will these changes impact living species (including humans), the ecosystems they inhabit, and the services these ecosystems provide?

Japanese giant flying squirrel "Musasabi" (Petaurista leucogenys)

Increasingly we are beginning to try putting value on ecosystem services – and paying for these services proactively is likely to improve and maintain human health and well being into the future.  Scientists study the unique role that various taxa play in ecosystem functioning and are beginning to think more broadly about what these various roles may mean in terms of conservation value, and what might the impacts be should such services be lost.

Small mammals are the most numerous group of mammals on earth (90% of the 5,416 known mammal species) and provide services

Japanese flying squirrel, Pteromys momonga

that are invaluable to ecosystem functioning.  Most species of small mammals are rarely studied and new species are still being discovered.  It is therefore important to focus research efforts on small mammals at a global scale, including tree squirrels which are so important to healthy forest functioning and so at risk from forest clearing and fragmentation.  At every step we should think about what might be lost without services provided by these species, how ecosystems would be impacted, how predatory species would be affected, how plant communities would be affected.  So the next time you ask a squirrel, pocket gopher, woodchuck, pack rat, dormouse, pika, kangaroo rat, or bat “what have you done for me lately?”, the answer is “a lot”.

“A thing is right when it tends to preserve the integrity, stability and beauty of the biotic community. It is wrong when it tends otherwise.”
― Aldo Leopold, A Sand County Almanac: With Other Essays on Conservation from Round River

A stormy winter day in the Pinaleños

of dealing with winter in order to make a living during these cold months.  Some may move to warmer locations, some simply hibernate, or enter a state of torpor, while others remain active all winter long.  Even though we live in the Sonoran desert, several mountain ranges like the Pinaleños tower so high above the desert floor that they are covered in snow all winter.  This means that even here in southern Arizona, animals living on mountain tops will have to deal with the physiological challenges that winter presents.

Tree squirrels don’t hibernate but remain active all winter long.  This presents a physiological challenge because not only is the outside temperature much colder – requiring extra energy expenditure to

Why would I come outside? It's warm in here!

maintain a high mammalian body temperature – food is also scarce.  Although tree squirrels do remain active, active is a relative term.  One of the strategies tree squirrels use to cope with cold temperatures is to do what we all wish we could do in the winter – stay in bed.  Tree squirrels aren’t really very well adapted to live in cold conditions.  Their fur gets thicker but not greatly so, they put on more fat, but not huge amounts, and their resting metabolic rate remains high.  Their strategy for coping then is threefold:

red squirrel out on a snowy day

1.  Put on brown adipose tissue, or fat.  This tissue has more mitochondria, which are a cell’s energy factory.  This tissue is thought to convert food consumed to body heat more quickly.

2. Eat fatty food.  Yum!  Tree seeds are very high in fat, which provides more energy for conversion to body heat.

3. Stay in bed!  Yay!  Studies have shown that nests are a critical resource tree squirrels need to cope with cold temperatures.  Nests can be 20 – 30 degrees warmer than the temperature outside meaning that squirrels in a nest spend very little energy trying to keep warm.  Red squirrels (Tamiasciurus hudsonicus) also drop their body temperature by a couple of degrees when they enter a nest, further reducing their energy

red squirrel feeding sign on the snow

requirements.  In addition to lining a nest with insulating materials such as lichen, grass, and fur, the type of nest a squirrel uses can also conserve energy.  Squirrels can build nests out of grass, twigs, and leaves in the branches of a tree.  This type of nest may not be as warm since it is more exposed to wind and weather.  Squirrels use cavities in trees which are more protected from wind, snow, and sleet. Squirrels will also nest in tunnels they dig under the snow.  At first this may not seem like a very warm option, but it takes advantage of a fact that mice and voles know very well – the layer between the ground and the bottom of the snowpack, or subnivian layer, stays a balmy 30 degrees F no matter what.

Because it is thermally advantageous to stay in the nest, squirrels face a trade off between

red squirrel basking on a winter day

expending energy to find food and saving energy by staying warm and cozy in the nest.  Because the days are shorter in winter, there are also fewer hours available for squirrels to forage.  This is where larder hoarding species like red squirrels have an advantage over scatter hoarding species, like Abert’s squirrels (Sciurus aberti).  Red squirrels invest a great deal of effort during the fall storing conifer cones in one centralized place – either  in the ground or in their scale pile or midden.  This means that red squirrels don’t have to go very far from their nest

An Abert's squirrel foraging in the snow

to find their food.  They can get up, eat,
bask in the sun for awhile, and then go back to bed.  Telemetry studies confirm this decreased activity level; red squirrel winter home ranges are 4-5 times smaller than summer home ranges.  Abert’s squirrels, on the other hand, cache their food in a dispersed manner all over the forest floor. They have to exert more energy searching for food they can find on the ground, which is often covered in snow, or eat less desirable foods like branch tips that they clip from trees.

Engelmann spruce cones

I say potentially because many species don’t produce cones every year, or in some years they produce thousands of cones and in other years just a few cones.  This presents a problem if you depend on cones as a food source as do many tree squirrel species and birds such as crossbills.   Such an unpredictable food resource means that you have to make the most of what you get when you get it.  Red squirrels are really good at this!

Red squirrels (Tamiasciurus hudsonicus) and Douglas squirrels (Tamiasciurus douglasii)   are unique among squirrels because of their food storing behavior known as larder hoarding.  These squirrels feed on cones in centralized locations where the cone scales build up into heaping piles.

red squirrel midden scale pile made primarily of ponderosa pine scales

These centralized scale piles are known as middens and basically act as refrigerators for squirrel food.  Squirrels dig pits into the midden scale pile and ground surrounding the pile and store young cones that they have clipped from surrounding trees in these pits.  The cool, moist microclimate of the scale pile prevents cones from opening up.  This way, the seeds inside each cone are safely stored for later consumption.

Red squirrels are very particular about how each cone is placed into the midden, and this makes sense since squirrels are dependent on their cone stores for overwinter survival because they don’t

cached Douglas-fir cones carefully arranged by a red squirrel

hibernate.   That’s why, throughout their range, you can observe red squirrels busily clipping cones this time of year and dashing off with them to store in their middens.  Watch your head though – some cones, such as those from southwestern white pine trees (Pinus strobiformis), are quite large!  When they crash to the forest floor it sounds like somebody slammed a car door shut.

For young red squirrels setting off on their own, the limited supply of cones both spatially and temporally imposes a limit on how long they can spend searching for a place to call home.  They need to find a place to live in time to store enough cones before

Juvenile male red squirrel has found a good spot - even though the cones here (southwestern white pine) are as big as he is!

the cones dry out on the trees, open up, and lose all of their seeds.  Once a good spot is found, young squirrels get busy clipping and storing.  If they have chosen wisely, they will have enough food to survive the winter and a chance to reproduce the following year.

It appears that yellow jacket populations can increase dramatically following a mild winter, which we had last year.  I also learned something new!  This is probably a “duh” moment for some folks – but I did not know that western yellow jackets nest in the ground.  I began seeing signs such as these – underground wasp nests dug up by bears.  Once I began watching the wasps, I could see them entering underground nests everywhere.  This makes hiking around in the forest treacherous since you have to constantly watch where you step.  Several colleagues inadvertently stepped on underground nest openings with painful results.  Yellow jackets also nest in dead trees.  I found this out while measuring a dead tree, or snag, as part of a red squirrel habitat assessment plot.  I was stung several times, they don’t give up easily!

underground yellow jacket nests dug up by bears

I learned something neat about yellow jacket nesting habits and ecology, and I am very happy about this, although I hope this winter is a bit colder.

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listening for dispersers on West peak

Where do animals go?  How far will they travel to find mates, food, a place to live? How long do they live once they get there? What landscape features are important? How do they avoid predation? Do young animals stay close to mom, or move far, far away?  Do behavioral differences impact these decisions?

These are all questions I find very compelling, questions I have always been curious about.  I study small mammals.  I study how they associate with landscape features that define habitat.  I study movement behavior (or space use) because movements can tell you a lot about what landscape features may be important to an animal.  Conversely movement behavior can also be misleading because it is hard to separate whether an animal is hanging out somewhere because it wants to be there or whether an animal is hanging out somewhere because it doesn’t really have a better option.

juvenile Mt. Graham red squirrel on the move

Think about if someone were to study yours or my movements via radio telemetry.  This observer would notice that I spend about three days each week on campus, either in front of a computer or in a class room.  Does this mean that this is where I would choose to be on these days?  Eh, maybe not.  Does it mean that campus is an important feature in my daily life?  Yes.  This observer would also see that I usually spend the other four days of the week in the Pinaleño mountains (Mt. Graham) studying space use, behavior, and juvenile dispersal in the endangered Mt. Graham red squirrel (Tamiasciurus hudsonicus grahamensis).  The observer would correctly deduce that the Pinaleños are also an important feature in my life based on my movements.

rainbow following a storm in the Pinaleños

The Pinaleños are an important feature for many species, including Mt. Graham red squirrels (which are found nowhere else).  It is also a wonderful place to study animal movements and settlement patterns.  Why?  Because the Pinaleños have experienced several disturbance events (drought, insect defoliation, wildfire) that have resulted in a mosaic of intact forests and shrubland separated by patches of dead trees, burned trees, or no trees at all.  Understanding how animals utilize and navigate this fragmented landscape is important because similar disturbance events are causing large-scale changes throughout the western U.S.  If we can understand how animals are impacted by habitat alteration and fragmentation in the Pinaleños, we can apply this knowledge to other forested landscapes to help predict how animals may respond to current and future changes.  In the process we can also learn how best to mitigate impacts of habitat fragmentation and maintain connectivity.