SHIVERING BY CAPTIVE MOOSE INFESTED WITH WINTER TICKS

Edward M. Addison1,2 and Robert F. McLaughlin1,3

1Wildlife Research and Development Section, Ontario Ministry of Natural Resources, 300 Water Street,
3rd Floor North, Peterborough, Ontario, Canada K9J 8M5.

ABSTRACT: Occurence and rate of shivering were measured to assess thermoregulatory responses of
captive moose (Alces alces) infested with winter ticks (Dermacentor albipictus). Shivering was
observed on 47 occasions in 5 of 8 infested moose calves from October to April; in contrast, 4 moose
calves not infested with winter ticks did not shiver under identical weather conditions. Only 5 shiver-
ing bouts occurred from October to March, all on a single day. The other 42 shivering bouts occurred
in April with bouts lasting 1–103 min. During the April bouts, ambient temperature was 1– 4 °C (42 of
42), maximum wind speed was ≤12 km/h (38 of 42), and it was raining (30 of 42). Shivering was asso-
ciated with 23–44% hair loss in April, but not during cold weather in mid-winter despite 5–10% hair
loss in March. Maintaining stable core body temperature during late winter-early spring could compro-
mise the energetic balance of wild free-ranging moose with extensive hair loss and abundant ticks, in
conditions equivalent to or worse than measured in this study.

ALCES VOL. 50: 87–92 (2014)

Key words: Alces alces, Dermacentor albipictus, behavior, moose, winter ticks, shivering,
thermoregulation.

There are numerous studies and much
conjecture as to how winter ticks (Derma-
centor albipictus) adversely affect moose
(Alces alces). Effects on growth, food habits,
blood composition, and bioenergetics have
been studied in experimentally infested cap-
tive moose (McLaughlin and Addison
1986, Glines and Samuel 1989, Welch et al.
1990, Addison and McLaughlin 1993, Addi-
son et al. 1994, Addison et al. 1998). Samuel
(2004) and Musante et al. (2007) modeled
the potential blood loss from winter ticks
and concluded that the elevated energy
expenditure and protein imbalance asso-
ciated with blood loss is deleterious to survi-
val of moose calves with heavy infestations.
McLaughlin and Addison (1986) speculated
that heat loss through winter tick-induced
disruption of the hair coat in late winter-early
spring may cause accelerated loss of body fat
as measured in well fed, captive moose. In

contrast, Welch et al. (1990) suggested that
hair loss may impose only nominal thermo-
regulatory costs on free-ranging moose and
may possibly facilitate dissipation of excess
heat in warm, early spring weather.

Shivering in moose is of interest because
it is an involuntary form of thermogenesis
that usually occurs when low ambient
temperatures (Ta) require an increase in
metabolic rate to maintain core body tem-
perature (Tb) (i.e., the lower critical tempera-
ture, Tlc) (Hohtola 2004). Shivering at
or near the Tlc has been observed in other
cervids (Parker and Robbins 1984). Our
objective was to determine under what con-
ditions shivering occurs in captive moose
calves infested with winter ticks. This infor-
mation will increase understanding of the
metabolic impacts during late winter and
early spring weather when moribund and
dead moose with severe winter tick

2Present Address: Ecolink Science, 107 Kennedy Street West, Aurora, Ontario, CANADA L4G 2L8
3Present Address: R.R. #3, Penetanguishene, Ontario, CANADA L0K1P0

87



infestations are most frequently observed in
the wild.

METHODS
The experiments were conducted in

Algonquin Provincial Park, Ontario
(45° 30′ N, 78° 35′ W) where 13 of 18 calves
were captured at <2 weeks of age in May
1982; 5 calves were from other areas in cen-
tral and northeastern Ontario (Addison and
McLaughlin 1993). Male and female calves
were paired in each of 6 adjacent pens
(29.6 × 16.5 m) located within a mixed forest
stand with little undergrowth and a partial
canopy (50% cover in summer) of white
pine (Pinus strobus), white birch (Betula
papyrifera), trembling (Populus tremuloides)
and big tooth aspen (P. grandidentata).
Calves were weaned as described by Addison
et al. (1983) and from late October to the end
of the experiment were fed ad libitum a rumi-
nant ration containing 16% crude protein,
2.5% crude fat, and 6% crude fiber (United
Cooperative of Ontario, Mississauga, Ontario,
Canada).

Three treatment groups of moose were
established: 4 controls without winter ticks,
4 infested with 21,000 winter tick larvae
(moderate), and 4 infested with 42,000 lar-
vae (heavy). Where possible, siblings were
assigned to different treatment groups. Con-
trol moose were sprayed with acaricide
(Dursban M., Dow Chemical of Canada
Ltd., Sarnia, Ontario, Canada) twice in
November and powdered with rotenone in
December, January, and February to control
accidental infestation by larval ticks. All
moose were euthanized at the end of the
experiment when their hair was dissolved
and hides were checked for remaining ticks
(Addison et al. 1979).

Moose were weighed on a calibrated
platform scale (Canadian Scale Company
Model 7045) approximately every 7 days
from 16–325 days of age (Addison et al.
1994). These same 12 moose were used in

concurrent studies of hair loss that was mea-
sured every 4–10 days from 23 January – 15
April 1983 by measuring the area of hair loss
and depth of hair removed (McLaughlin and
Addison 1986).

Shivering and other behaviors were
recorded simultaneously from 3 observation
booths positioned above the back of each
of a pair of 6 adjacent pens. Three observers
recorded observations of 2 moose in a single
pen during a 2.5 h period, after which a sec-
ond group of 3 observers replaced them.
These 2 groups then alternated in successive
2.5 h periods throughout the day; all obser-
vations were in daylight hours. The 6 moose
observed simultaneously included 2 from
each treatment group; subsequently, the
remaining 6 moose (2 per treatment) were
observed the following day and this process
continued until 27–30 h of observations
were recorded monthly (October – April)
for each moose. Behaviour was recorded in
60, 1-min intervals during 1 h observation
periods; 2403 h of observation occurred
from mid-October 1982 to mid-April 1983.
The Ta, wind speed, and precipitation were
recorded at the beginning of each observa-
tion period, and those data were attributed
to each observation min in that period.

RESULTS
Female and male calves weighed 128 ±

6 and 144 ± 15 kg when 4½ months old in
early October and 200 ± 17 and 218 ±
20 kg, respectively, at the end of the experi-
ment at 11 months of age (Addison et al.
1994). The 4 control moose harbored 0, 4,
21, and 85 winter ticks at the end of the
experiment, in contrast to 1179–8290 ticks
recovered from the infested moose. Forty-
two of 47 shivering bouts were observed in
2 individual moose in the moderate treat-
ment group; 29 bouts were by a single moose
(M4) (McLaughlin and Addison 1986).

The Ta was relatively mild on most
observation days particularly in October,

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November, March, and April. In December –
February (the coldest months) there were 59,
18, and 5 h of observations when Ta was −10
to −19, −20 to −29, and −30 to −32 °C,
respectively (Table 1). No wind was mea-
sured in 123 h of observation, winds 20–
40 km/h occurred most frequently in October –
December, and there were 14 h of observa-
tions in February when wind was >20 km/h
(Table 1). Precipitation during observation
periods was lowest in February (Table 1).
Hours of precipitation were generally similar
in September – January and April, with most
April precipitation as rain.

No shivering was observed from Octo-
ber – January and in March. One infested
moose had 5 short shivering bouts within
35 min on a single day in February; Ta was
−10 to −11 °C with wind speed of 12 km/h
during 1 bout, and 28 km/h during the other
4 bouts. These bouts averaged 5 min, ran-
ging 1–10 min in length. The other 42 bouts

occurred in April and averaged 17.8 min,
ranging from 1–103 min in length. The Ta
was 1–4 °C, maximum wind speed was
≤12 km/h (38 of 42), and it rained during
30 of the 42 April bouts.

Shivering occurred when moose were
standing (n = 25) or recumbent (n = 20),
and when both standing and recumbent (n = 2).
Other activities during shivering bouts
included recumbent with head rested on the
ground (n = 9), walking (n = 9), ruminating
(n = 8), feeding (n = 4), defecating (n = 4),
drinking or eating snow (n = 2), urinating
(n = 2), grooming (n = 2), rubbing against
other objects (n = 1), trotting (n = 1), and
galloping (n = 1).

DISCUSSION
Shivering in moose during relatively

warm and often wet April weather is of spe-
cial interest because it seems inconsistent
with results of prior studies. The insulative
properties of moose hair are as high or higher

Table 1. Weather characteristics during monthly observations of captive calf moose infested with winter
ticks, Algonquin Provincial Park, Ontario, 1982–1983. Values represent hours of observation during
specific conditions; total monthly hours per category may differ due to rounding. The proportional
monthly hours of rain are in parentheses.

OCT NOV DEC JAN FEB MAR APR

Ambient Temperature (°C)

>0 64 12 10 0 0 33 50

0 to −9 1 48 26 42 19 26 10
−10 to −19 0 0 19 15 25 0 0
−20 to −29 0 0 5 3 10 0 0
−30 to −32 0 0 0 0 5 0 0

Wind Speed (km/h)

0 7 13 7 28 22 21 25

1–9 5 8 5 5 2 16 3
10–19 24 22 28 25 21 16 21
20–29 24 19 20 2 14 1 9
30–40 1 3 0 0 0 0 0

Precipitation

Rain 15 (56) 10 (27) 10 (28) 7 (19) 0 13 (57) 26 (84)

Snow 17 27 26 29 7 10 5

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than that of any arctic mammal (Scholander
et al. 1950), hence cold weather under most
conditions would not cause thermoregula-
tory stress to moose. Increased metabolic
rate indicative of moose reaching their
winter Tlc was not observed from −25 to
−30 °C (Renecker and Hudson 1986), and
captive, well fed moose calves were almost
as cold tolerant as adults (Renecker et al.
1978). Absence of shivering in December,
January, and most of February, despite very
cold weather, was consistent with metabolic
measurements and predictions (Renecker
and Hudson 1986). In contrast, certain of
our moose had extensive hair loss in April
and shivering occurred in some but not all
tick-infested moose during relatively mild
temperatures in April.

Insulative properties of hair are posi-
tively correlated with the depth of hair for
many arctic mammals and cattle (Scholander
et al. 1950, Bennett 1964). However, wet
pelts and skin substantially increase heat
loss (Scholander et al. 1950, Holmes 1981)
and wind reduces the insulative quality of
hair coats (Scholander et al. 1950, Parker
and Robbins 1985). Hair loss of our moose
was generally ∼5% at the end of February,
but for most infested moose increased to
23–44% by mid-April. Most shivering was
by a single moose with 31% hair loss during
rainy and moderately windy conditions in
April (McLaughlin and Addison 1986).

Distinct individual variation in shivering
was evident as 62 and 17% of shivering
bouts were by 2 moose (moderate). It may
seem anomalous that only 2 moose in the
moderate group shivered the most, whereas
only limited shivering was observed in the
heavy treatment group. It is possible that
the quantity of larval ticks applied in the 2
treatments was not different biologically.
For example, hair loss was not substantially
different between the moderate (23–31%)
and heavy (28–44%) treatment groups.
Further, hair loss in the moderate group

was highly variable (2–24%) at the conclu-
sion of the experiment (McLaughlin and
Addison 1986). Moose also displayed varia-
tion in grooming and rubbing behaviors
within each of the treatment groups.

Little behavioral response was evident in
our tick-infested moose in Alberta during a
relatively warm winter and spring (Welch
et al. 1990). Only 1 trial occurred below
−10 °C, and none below −25 °C, tempera-
tures above the Tlc of moose (Renecker and
Hudson 1986). It has been suggested that
hair loss might have a thermoregulatory
advantage by dissipating excess heat during
warm spring weather (Welch et al. 1990).
Unfortunately, variables affecting thermo‐
regulation such as precipitation, wind, and
radiant energy are seldom documented
(Parker and Robbins 1985), and lack of stan-
dardization in techniques and variable diets
are cause for caution when comparing bio‐
energetic studies of cervids (Renecker and
Hudson 1986). In our study, the duration
and intensity of wind and precipitation were
generally similar in all months except for
less precipitation in February (Table 1). The
only obvious weather differences between
April and the prior 4 months included more
precipitation as rain in April and warmer Ta
in March and April (Table 1). There was
also an exponential increase in hair loss
(McLaughlin and Addison 1986) and rapid
growth by adult winter ticks in April (Addi-
son and McLaughlin 1988), hence higher
blood loss as estimated in previous studies
(see Samuel 2004, Musante et al. 2007). Of
note is that our experimental moose had
>20,000 fewer ticks and much less hair loss
at the end of the experiment than measured,
on average, on free-ranging moose succumb-
ing to heavy winter tick infestation in Alberta
(Samuel and Barker 1979, Samuel 2004).

The greater attrition of fat reservoirs in
infested than non-infested moose, despite
high quality food, presumably occurred
over a period of 1–2 months (McLaughlin

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SHIVERING IN TICK INFESTED MOOSE – ADDISON AND MCLAUGHLIN ALCES VOL. 50, 2014



and Addison 1986). It is unlikely that the
loss of body condition in infested moose
could be attributed only to energy demands
of maintaining core Tb associated with hair
loss. Models developed by Samuel (2004)
and Musante et al. (2007) indicate that the
energy cost associated with replacing direct
blood loss has a substantial metabolic impact
during the 2–3 week peak period of female
adult engorgement. These energy costs and
higher thermoregulatory costs in March and
April likely contributed to the reduced body
condition of the infested calves as described
by McLaughlin and Addison (1986). It is
also possible that due to depleted and less
oxygenated red blood cells, there is less gen-
eral motor activity and reduced movement,
foraging, and production of body heat in
heavily infested free-ranging moose.

Most moose experimentally infested
with up to 42,000 larval winter ticks had lim-
ited hair loss and did not shiver during cold
weather in mid-winter or even with substan-
tial hair loss in spring. When shivering did
occur it was for extended periods (hours) in
spring, at or above freezing temperatures
when moose were wet. The combination of
rainy weather and severe hair loss from
heavy tick infestation likely has more ther-
moregulatory impact than recognized pre-
viously. Calves with severe hair loss and
reduced body fat due to heavy winter tick
infestation likely have an elevated Tlc in
late winter-early spring that could further
compromise their survival.

ACKNOWLEDGEMENTS
We appreciate D. J. H. Fraser for his

coordination of many early aspects of this
study. Special thanks go to A. Rynard,
A. MacMillan, M. Jefferson, V. Ewing, and
D. Bouchard for their steadfast assistance in
collection of data and for moose husbandry
under adverse conditions. Additional assis-
tance was provided by C. Pirie, M. A.
McLaughlin, D. Carlson, D. Joachim and

P. Methner. L. Smith, K. Paterson, K. Long,
A. Jones, S. Gadawski, S. Fraser, D. Fraser,
and L. Berejikian assisted in the earlier care
of calves. We appreciate the assistance of
C. D. MacInnes and G. Smith and staff for
their administrative support. Field work was
conducted at the Wildlife Research Station
in Algonquin Park where R. Keatley, P. C.
Smith, and staff were of great help. Thank
you to R. Moen, B. Samuel, and anonymous
reviewers for providing excellent sugges-
tions for improvements to earlier drafts of
the manuscript.

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	SHIVERING BY CAPTIVE MOOSE INFESTED WITH WINTER TICKS
	METHODS
	RESULTS
	DISCUSSION
	ACKNOWLEDGEMENTS
	REFERENCES