Investigating Reproductive Success in the Declining Moose Population of
Northeastern Minnesota
Background
There are two important drivers of the long-term performance and dynamics of moose (Alces
americanus) populations, 1) adult survival and 2) reproductive success. That is why the two
aggressive MN DNR companion moose studies are focusing on these two aspects of their
population ecology. Enhancing the agency’s knowledge of specific causes of mortality of adults
and calves is absolutely essential to assessing the quantitative impact these two drivers have on
the population long-term trend and to ensuring the most effective management responses for
stabilizing or reversing the present declining trend.
The study of reproductive success is focusing on fertility (pregnancy rates), calf production,
survival, cause-specific mortality and annual recruitment (calf survival to one year). Calves are
considered recruited at one year of age, because at that age their probability for survival
increases dramatically compared to young calves and old adults.
There is a habitat component to this study which involves examination of moose dam (mother)
selection for specific forest characteristics that best afford forage, and hiding and thermal cover
on calving sites and peak lactation sites to fulfill nutritional requirements, and at the same time,
allow dams and calves to avoid predators and to thermoregulate. Habitat is also being studied on
calf mortality sites to determine if causes of mortality can be related to habitat deficiencies.
Importantly, the multiple years of this study are improving our understanding of the annual
variability of calf production, the timing, location, and habitat associated with calving activity, as
well as of survival and cause-specific mortality rates in northeastern Minnesota.
Results
Adult Female Fertility and Calving Seasons, 2013 to 2015
During 2013–2015, the average pregnancy rate of all adult females with functioning GPS collars
was 81.4%, which is similar to average (“normal”) pregnancy rates for moose across North
America (83%). Annually, pregnancy rates ranged from 74.3% to 88.3%. Pregnancy rates were
based on computer-monitoring of the hourly movements of 74, 70, and 60 GPS-collared adult
females from 2013 to 2015. Specifically, calving activity was indicated by an adult female
making a long distance “calving movement” then localizing (see Figures 1 and 2). Most females
captured in each year also had their blood analyzed for serum progesterone. It is noteworthy that
these normal pregnancy rates indicate adequate nutritional condition of moose coming into the
fall rut, which in turn reflects positively on seasonal habitat and the ability of adult moose to
recover their body condition subsequent to the previous winter’s nutritional bottleneck.
Figure 1. Calving activity of adult female moose was indicated by a long distance calving
movement, followed by localization for 7–10 days in northeastern Minnesota.
Figure 2. Calving sites of moose determined by monitoring GPS-collared adult females for a
variable, long distance calving movement, followed by localization in northeastern Minnesota,
2013 to 2015.
Average calving dates in 2013 to 2015 were 14 May (range = 2 May–2 June), 19 May (range = 5
May–16 June), and 11 May (range = 29 April–14 June), respectively. In 2015, 76% of births
occurred during 3–15 May.
Estimated Calf Production, 2005 to 2015
Along with the decline of the overall population and its adult female component, estimated calf
production decreased approximately 59% from 2006 to 2015 despite the relatively normal annual
pregnancy rates (see Figure 3).
Figure 3. Estimated annual moose calf production in northeastern Minnesota, 2006 to 2015.
*This particularly low estimate of calf production was associated with atypically lower quality
overall survey conditions during the 2013 aerial moose survey, which may have resulted an
underestimation of the overall population.
Survival and Cause-Specific Mortality, 2013 to 2015
During 2013 to 2015, 40% of the calves (whether or not GPS-collared) were lost to natural
causes of mortality (60% survival) within the first 30 days of their lives (see Figure 4). During
2015, the average age at death was 13.2 days old (range = 2–23 days old).
Figure 4. Survival rates of moose calves to 30 days of age in northeastern Minnesota,
2013 to 2015.
During 2013 and 2014 combined (due to a small sample size in 2014), the natural mortality rate
of our 40 GPS-collared calves (see Figure 5) was 77.5% (31 of 40) by late November, which
translates to only a 22.5% survival rate by that time.
Figure 5. A newborn moose calf captured, GPS-collared, examined, and released
within 60 seconds in northeastern Minnesota, spring 2014.
During 2013 and 2014, most of the calf mortalities were due to wolf (Canis lupus) and black
bear (Ursus americanus) predation; other causes had less of an impact (see Figure 6).
Figure 6. Proportion of moose calf mortalities due to specific causes in northeastern Minnesota,
2013 and 2014.
During late-October to early-December 2015, we completed our fall helicopter survey of calf
survival. We observed 40 of the 50 GPS-collared adult females that had calved in spring,
accompanied by 15 singleton and 8 twin calves (4 sets) for a total of 23 calves. Assuming a 30%
twinning rate at birth for these 40 cows, a total of 52 calves would have originally been born, and
the 23 calves we observed in the fall represented a 44% survival rate since birth. This is about
twice the natural survival rate by the same time in 2013 and 2014 combined.
In 2015, without GPS collars on the neonate calves, we used the movement patterns of GPScollared cows and field investigation to determine calf mortalities (see Figure 7). Eleven calf
mortalities occurred during 3 May to 2 June which we could confirm with a field investigation.
The proportional mortality breakdown is presented in Figure 8. Without collars on the calves we
could not determine the specific causes of mortality of calves beyond 30 days of age or assess
their cause-specific impacts on the dynamics of the population.
Figure 7. Moose movements (top) and field investigations (bottom) used to identify and
confirm cause-specific moose calf mortalities up to 30 days of age in northeastern Minnesota,
2015.
Figure 8. Proportion of moose calf mortalities due to specific causes up to 30 days
of age in northeastern Minnesota, 2015.
Habitat Analyses of Pre-calving, Calving, Peak Lactation, and Mortality Sites, 2013 to 2015
Habitat analyses have been conducted at 65 locations of GPS-collared cows prior to their calving
movement (pre-calving sites), 91 calving sites, 22 peak lactation sites, and 34 calf mortality
sites. Analyses will focus on assessments of hiding and thermal cover and forage availability at
these sites to better understand habitat selection by adult females to balance their increased
nutritional, predator avoidance, and thermoregulation requirements (see Figure 9). Nutritional
requirements are particularly important at this time of year as dams attempt to recover from the
previous winter and spring calving, support lactation, and ensure that quality natural nutrition is
available for their calves’ rapid growth. Computer analyses are in progress.
(*Please note, the first three years of these data comprise the substance of University of
Minnesota, Ph.D. graduate student, William J. Severud’s dissertation research.)
Figure 9. Analyzing habitat within plots on moose calving sites to assess forage availability,
hiding cover for predator avoidance, and cover for thermoregulation in northeastern Minnesota,
2013 to 2015.