Multiple Annual Litters in Glaucomys sabrinus (Northern Flying
Squirrel)
Jesse E.H. Patterson1,* and Stephen J. Patterson2
Abstract - We provide positive documentation of multiple litters in a single year born to an individual
Glaucomys sabrinus (Northern Flying Squirrel) residing in southern Ontario, Canada.
The first litter, consisting of four young, was observed on May 18, 2003. The second litter,
consisting of five young, was observed 15 weeks later on September 2, 2003. The individual
female, identified by a unique ear-tag, established natal nests in different nearby nest boxes.
Our finding suggests the Northern Flying Squirrel possesses greater population growth potential
than previously thought.
Debate surrounding the number of litters produced annually by Glaucomys
sabrinus Shaw (Northern Flying Squirrel) was initiated when Seton (1929) suggested
that, like Glaucomys volans L. (Southern Flying Squirrel), Northern Flying
Squirrels might produce two or even three litters of young in a single year. Soon
after, Cowan (1936) observed that, although multiple litters may be common in
Southern Flying Squirrels, there was no evidence for this behavior in Northern Flying
Squirrels. Accordingly, mammalogists have traditionally believed the Northern
Flying Squirrel to be capable of producing only one litter per year (Wells-Gosling
and Heaney 1984). The peak breeding season for Northern Flying Squirrels typically
occurs from March to April throughout most of the range (Davis 1963), with
litters born from May to July. In some regions, such as the Pacific Northwest forest
zone (northern California through southern British Columbia), the breeding season
has been observed to extend into early July (Carey 1991). Reproductively active females
in late autumn and winter have been reported in some studies (Forsman et al.
1994, Ransome and Sullivan 2003, Reynolds et al. 1999, Witt 1991), as has a single
winter litter (Raphael 1984). Ransome and Sullivan (2003) stated that breeding in
autumn and winter may represent second litters; however, they failed to differentiate
between multiple litters and late litters. Multiple litters require autumn or winter
parturition to be preceded by spring parturition in the same individual, a behavior
that has never before been explicitly documented for the Northern Flying Squirrel.
Also of note, all previous observations of autumn and winter breeding activity have
been from the Pacific Northwest forest zone, where winter snowfall typically does
not either exceed 350 mm (Ransome and Sullivan 2003) or remain on the ground
longer than a week (Raphael 1984).
Northern Flying Squirrels preferentially construct their natal nests in tree cavities
(Bakker and Hastings 2002, Holloway and Malcolm 2007). However, despite their
importance to Northern Flying Squirrels, nests within tree cavities remain poorly
studied due to logistical constraints. Nest boxes have often been employed to study
reproductive parameters of several species of small arboreal mammals, including
Northern Flying Squirrels and Southern Flying Squirrels (Althoff and Althoff 2001,
Carey 2002, Fokidis and Risch 2005). Between 2002 and 2004, we deployed 266
nest boxes throughout Bruce and Grey counties in southern Ontario, Canada, as
part of a larger study on Northern Flying Squirrels (Patterson 2008). This region of
Canada receives an average of 4100 mm of snow annually from October through to
the end of March and the average winter snow depth over the same period is 174 mm
(Environment Canada 2000). The nest box design used in this study followed Sonenshine
et al. (1973), except that we did not include a hinged door or wire screen at the
Notes of the Northeastern Nat u ral ist, Issue 17/1, 2010
167
168 Northeastern Naturalist Notes Vol. 17, No. 1
front of the shelter. Instead, our boxes were constructed such that the top lid could
be flipped up to remove animals. All nest boxes were checked twice per year, once
in the spring and once again in autumn, and cursory visual nest-box inspections were
conducted monthly to monitor the condition and status of the boxes. All adult females
retrieved from nest boxes were uniquely marked with Monel No. 1 ear-tags (National
Band and Tag Co., Newport, KY) at time of first capture.
On May 18, 2003, a female Northern Flying Squirrel and four (three female, one
male) approximately nine-day-old pups (following Muul 1969) were initially found
occupying a nest box near Shallow Lake, ON (44o38'47"N, 81o03'47"W; elevation:
230 m). This same ear-tagged female was found again in a different, nearby nest box
fifteen weeks later, on September 2, with five young (four females, one male). The
young found in autumn had fully open eyes, lower incisors present, and were fully
furred; thus, their estimated age was 28 days. The gestation period for the Northern
Flying Squirrel is between 37 and 42 days (Muul 1969), placing the date of first
copulation at about March 28 and the date of second copulation at about June 24.
While previous findings have only been able to confirm late breeding activity, our
observation provides the first positive documentation of multiple litters in a single
year by a Northern Flying Squirrel, demonstrating that Northern Flying Squirrels are
indeed capable of polyestry. Additionally, while late breeding has been reported in
the subspecies G. s. oregonensis and G. s. fuliginosis, we provide the first evidence
of late breeding for G. s. macrotis, the subspecies found in our study area.
We have little reason, based solely on this single observation, to believe that
polyestrous behaviour is prevalent in the Northern Flying Squirrels. Nonetheless,
when integrated with evidence provided by Ransome and Sullivan (2003), Raphael
(1984), and Witt (1991), late-autumn/winter parturition appears to be relatively
common in Northern Flying Squirrels throughout their range. The possibility exists
that our supplemental nest boxes acted to improve the fitness of the Northern Flying
Squirrels; however, previous work has suggested that Northern Flying Squirrel populations
are not limited by den-site availability, nor is population growth enhanced by
nest-box supplementation (Ransome and Sullivan 2004). Our finding suggests that
the Northern Flying Squirrel possesses a capacity for greater population growth potential
than previously thought. We are unable to confirm the frequency of polyestry
in Northern Flying Squirrels based on this single observation. Future studies are
warranted into the extrinsic (i.e., ambient temperature, food resources) and intrinsic
(i.e., chemosensory stimuli, hormone levels) cues of polyestry in Northern Flying
Squirrels, as well as geographic variability in their reproductive parameters.
Acknowledgments. We thank the Grey-Sauble Conservation Authority for allowing
us access to their property, as well as Thistlewood Timber Frame Homes in
Markdale, ON, for donating the lumber used to construct the nest boxes. We would
also like to acknowledge the support, assistance, and editorial comments provided
by J.R. Malcolm, M. Patterson, and H.C. Maddin. Further helpful criticisms were
received from three anonymous reviewers and manuscript editor Thomas J. Maier.
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1Department of Biological Sciences, University of Calgary, 2500 University Drive, Calgary,
AB, Canada, T2N 1N4. 2251 Queen Street South, Suite 523, Streetsville, ON, Canada, L5M
1L7. *Corresponding author - jesse.patterson@ucalgary.ca.