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Relative Efficacy of 9-mm and 12-mm PIT Tags for Studying Little Brown Myotis (Myotis
lucifugus): a Cautionary Note
Al Sandilands1 and Derek Morningstar2
1Gray Owl Environmental Inc., 1356 Lockie Road, Branchton, ON N0B 1L0, Canada, and
2Myotistar, 51 Silverthorne Drive, Cambridge, ON N3C 0B4, Canada
Passive integrated transponders (PIT) are
commonly used to monitor bat populations
(Garroway and Broders, 2007; O’Shea et al.,
2011) and are more effective than
conventional capture-recapture studies in
determining survival rates (Ellison et al.,
2007). We studied little brown myotis
(Myotis lucifugus) at a roost in a stone shed,
with two openings, near Cambridge, in
southwestern Ontario. Our initial purpose
was to study the demography and roosting
ecology of a population exposed to white-
nose syndrome, but our project provided an
unexpected opportunity to test results
obtained using two sizes of PIT tags.
We implanted bats with tags on seven
dates from 2015 to 2017. In May 2015, we
used 12-mm tags (HPT12; 12.5 by 2.1 mm,
0.12 g, BioMark, Boise, Idaho), but upon
recommendation of the Ontario Ministry of
Natural Resources and Forestry (OMNRF),
we applied 9-mm transponders (HPT9; 9 by
2.1 mm, 0.08 g) in July and August 2015.
During August 2015, we observed that some
bats had lost their 9-mm tags or that the
transponder did not register when the animal
was recaptured. After further consultation
with OMNRF, we again implanted 12-mm
tags in May, July, and August 2016 and July
2017. In all years, we positioned two cord-
type PIT readers (BioMark IS1001) around
each opening without restricting access by
the bats. The readers ran continuously,
except during power outages, collecting data
from 2015 until the end of 2020.
We marked individuals of all age and sex
classes, but only the sample of adult females
(89) was large enough for statistical analyses.
We implanted 24 adult females with 9-mm
transponders, 54 with 12-mm units, and 11
with both sizes. The double-tagged bats
initially were implanted with 9-mm tags in
2015, and the 12-mm versions were inserted
when the animals were recaptured in 2016 or
The readers detected 11 of 35 (31.4%) 9-
mm tags at least once in the year of placement
and/or a subsequent year (2015–2020),
compared with 50 of 65 (76.9%) 12-mm tags
(Χ12 = 17.93; P < 0.001). Number of animals
detected in a year subsequent to the year of
capture was 7 of 35 (20%) for those carrying
9-mm units and 40 of 65 (61.5%) for bats
with 12-mm tags (Χ12 = 14.14, d.f. = 1,
P < 0.001). For 9-mm transponders, mean
number of days between first and last dates of
detection within a year was 16.5 ± 7.7 (SE)
days compared with 69.3 ± 3.6 days for 12-
mm tags (β = 51.22; t75.8 = 4.32; P < 0.001).
Mean number of days detected within a year
was 2.2 ± 0.5 days versus 24.3 ± 1.9 days for
9-mm and 12-mm tags, respectively (β =
25.69; t65.1 = 4.50, P < 0.001).
Our results suggest that 9-mm tags do not
perform as well as 12-mm tags and may lead
to inaccurate conclusions about movements
of bats and underestimates of return rates.
This was probably due to a combination of
tag loss and lower detectability of 9-mm tags.
We documented loss of some 9-mm
transponders but no loss of 12-mm units. The
signal from the 9-mm tags appeared weaker
and occasionally could not be detected even
when the bat was in hand. Ousterhout and
Bat Research News Volume 62: No. 3
Semlitsch (2014) reached similar conclusions
in a study of movements by juvenile ringed
salamanders (Ambystoma annulatum). Those
authors determined that detection and
recapture rates increased with increasing size
of PIT tag, due to the longer detection
distances of larger units, and that detection
rates decreased when the long axis of the tag
was perpendicular, rather than parallel, to a
This study was partly funded by the
Canadian Wildlife Service and the Rapid
Response Fund of the National Speleological
Society. The OMNRF issued the necessary
permits. We thank A. Alic, G. Buck, A.
Ceballos-Vasquez, R. Del Giudice, S. Fraser,
L. Greville, L. Haines, K. Hoo, S. Labrie, N.
Leava, O. Morningstar, L. Owens, R.
Perdiao, C. Risley, P. Ronald, M. Russel, B.
Talbot, and R. Valdizon for helping with field
work and decontamination of equipment. G.
Bettini conducted the statistical analyses.
Ellison, L. E., T. J. O’Shea, D. J. Newbaum,
M. A. Neubaum, R. D. Pearce, and R. A.
Bowen. 2007. A comparison of
conventional capture versus PIT reader
techniques for estimating survival and
capture probabilities of big brown bats
(Eptesicus fuscus). Acta
Garroway, C. J., and H. G. Broders. 2007.
Nonrandom association patterns at
northern long-eared bat maternity roosts.
Canadian Journal of Zoology, 85:956–
O’Shea, T. J., L. E. Ellison, and T. R.
Stanley. 2011. Adult survival and
population growth rate in Colorado big
brown bats (Eptesicus fuscus). Journal of
Ousterhout, B. H., and R. D Semlitsch.
2014. Measuring terrestrial movement
behavior using passive integrated
transponder (PIT) tags: effects of tag
size in detection, movement, survival,
and growth. Behavioral Ecology and