Breeding/adult population estimates
In this study, breeders and adults were defined as bears that produced
≥1 offspring between 1998 and 2020, and bears that had the potential
ability to breed, respectively. For females, adults included both
breeders and bears ≥4 years old (the youngest age of the first mating in
this population, reported by Shimozuru, et al. (2017). For males,
the youngest age of the first mating in this population was 6 years of
age (Shimozuru, et al. , 2020), whereas males potentially reach
sexual maturation at 3.5 years of age in the Scandinavian population
(Zedrosser, et al. , 2007). In the current analysis, bears ≥4
years old and sexually experienced males (indicated by a parentage
analysis) were included in the adult population, which allowed us to
compare the breeding/adult population size between sexes. In this study,
we estimated the breeding/adult population size as of 2019, the first
year of the intensive genetic sampling period. This was because 2019 was
expected to be the year when the highest number of breeders/adults would
be identified as alive. For example, females identified with cubs for
the first time in 2020 could be counted as breeders in 2019. Likewise,
all females and males identified in 2019 and/or 2020 were confirmed as
breeders if their offspring were sampled during 1998–2020. In addition
to breeding experience, some bears were confirmed to be ≥4 years old
based on their appearance when identified in an ongoing bear monitoring
survey that has been continuously conducted in recent decades in the
area between Rusha and Utoro (Shimozuru, et al. , 2020;
Shimozuru, et al. , 2017). Some bears were confirmed to be ≥4
years based on the year of first genetic identification, or on the year
when their parent was dead (e.g., if the father was dead in 2014, his
offspring could potentially be born in 2015, suggesting they were ≥4
years old as of 2019).
First, we calculated the minimum number of breeders, including the
existing bears and hypothetical parents. The former included bears
identified in 2019 and/or 2020 that were confirmed as sexually
experienced based on a parentage analysis. The later included
hypothetical parents generated by the COLONY software, which were
estimated to produce cubs during 2018–2020. For example, if a
1-year-old bear, killed in 2019, was not assigned a mother from the list
of candidate mothers, it was reasonable to assume that his/her mother,
although not genetically identified, gave birth in 2018 and was alive
until the timing of mother–offspring separation in 2019. Similarly, if
a cub-of-the-year, sampled in 2020, was not assigned a father from the
list of candidate fathers, it could be assumed that his/her father was
alive and mated with the mother in 2019.
Second, we estimated the maximum breeding population size by a pedigree
reconstruction approach, based on the simple assumption that the number
of breeders would not exceed the total number of parents that produced
bears identified in 2019–2020. We estimated the number of breeders that
produced bears identified during 2019–2020, but whose mother and/or
father (genetically identified during 1998–2018) were not sampled
during 2019–2020, or whose mother and/or father were missing from the
list of existing candidates sampled during 1998–2020. One of the
problems of this approach is that the assumption that each missing
parent constitutes a new individual would most likely cause an
overestimation (Spitzer, et al. , 2016). In this study, COLONY
allowed hypothetical parents to produce multiple offspring, which
reduced the likelihood of overestimation due to this issue. Another
problem is that it is difficult to know how many of the parents that
were identified as alive until 2018, but not sampled during 2019–2020,
were still alive as of 2019. To account for mortality among those
individuals, we calculated the period between the year of the last
identification and 2019, and multiplied it by the survival rate to
estimate his/her survival probability. Because the adult survival rate
was not investigated in this population, we applied the median value of
the survival rates (0.94 for females and 0.89 for males) among the other
brown bear populations (0.89–0.96 for females, 0.62–0.94 for males;
reviewed in Schwartz 2003). For example, a mother identified as alive in
2017 was counted as 0.88 of an individual (i.e., 0.94 × 0.94). In
addition, we assumed that adult bears who had not been sampled for the
last 5 years (2016–2020) in the area where successive surveys were
conducted were no longer alive. Continuous genetic samples from
hair-traps, biopsy darts, and fecal collection have been conducted in
the area between Rusha and Utoro (Figure 2; Shimozuru, et al. ,
2020; Shimozuru, et al. , 2017), enabling us to assume that those
individuals were less likely to be alive.
A similar but more serious concern, reported by Creel and Rosenblatt
(2013), was that there is no way to ascertain how many of the
hypothetical parents are actually alive. To avoid overestimation, we
made several assumptions. First, females ≥30 years old and males ≥28
years old were not counted as breeding individuals. This assumption was
based on previous studies regarding reproductive senescence in brown
bears (Schwartz, et al. , 2003; Van Daele, et al. , 2001;
Zedrosser, et al. , 2007). Upon pedigree reconstruction, the age
of each hypothetical parent was estimated based on the age of the oldest
offspring and generation intervals. The generation interval between
mother and offspring was set at 7.3 years based on our bear monitoring
survey in the Rusha area. We calculated the first age when females gave
birth to cubs that survived the first year (for eight females = 5–9
years, average of 7.25; Shimozuru, et al. , 2017), and used it as
the minimum interval between generations. This value was more realistic
than their primiparity age (for 15 females = 5–6 years, average of 5.3;
Shimozuru, et al. , 2017), which was more likely to induce
overestimation in the current analysis. The generation intervals between
father and offspring, i.e., the first age when males sired cubs that
survived the first year, was not well investigated in this population.
Males become sexually mature at 3.5 years old (Zedrosser, et al. ,
2007), but it is rare to gain a reproductive opportunity until physical
maturation at around 9–11 years of age (Moriwaki, et al. , 2018;
Shimozuru, et al. , 2020; Shirane, et al. , 2020).
Therefore, we set the same value (7.3 years) as for females, based on
the assumption that the generation intervals between father and
offspring were not less than those between mother and offspring. The
second assumption was that more than four matrilineal generations do not
exist at the same time, which was also based on our bear monitoring
survey conducted in the Rusha area. In this area, four generations
(offspring, mother, grandmother, and great-grandmother) exist at the
same time, but a great-great-grandmother has never been identified
(Shimozuru, et al. , 2017). Similarly, our previous pedigree
reconstruction conducted in the same population revealed that more than
three paternal generations (offspring-father-grandfather) do not exist
at the same time (Shimozuru, et al. , 2019). Hypothetical mothers
and fathers that correspond to great-great-grandmother and
great-grandfather, respectively, were assumed to be dead, and were not
counted as breeding individuals in the current analysis. Finally, we
calculated the minimum and maximum number of adults, by adding the
number of bears confirmed to be ≥4 years old in 2019 (based on the
criteria described above) to the minimum and maximum number of breeders.