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Effects of age and experience on reproductive performance of captive red wolves (Canis rufus)

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  • Endangered Wolf Center

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Propagation programs contribute to the conservation of a species by preserving genetic and demographic stock that may be used to reinforce or re-establish wild populations. Identifying traits that affect reproductive success is essential to achieve this goal. Longitudinal reproductive events of the captive population of endangered red wolves (Canis rufus Audubon and Bachman, 1851) were investigated to determine whether parental age, breeding experience, and rearing type were factors in reproduction, litter size, and sex ratio, as well as viability of offspring. Younger wolves were more likely to reproduce and produce larger litters than were older individuals. The age of the female, but not the male, had a negative effect on pup survival. Wolves that had prior experience in offspring production were more likely to reproduce again than were individuals that had no prior reproductive success, but prior sexual experience alone was not a factor in offspring production. Parental breeding experience had a negative effect on pup survival, but no apparent relationships with litter size or sex ratio. Declines in reproduction, fitness, and survival with advancing age suggest the effect is due to senescence, the onset of which occurs at 8 years of age in females. The results are consistent with the breeding-experience hypothesis.
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ARTICLE
Effects of age and experience on reproductive performance of
captive red wolves (Canis rufus)
David R. Rabon, Jr.
Abstract: Propagation programs contribute to the conservation of a species by preserving genetic and demographic stock that
may be used to reinforce or re-establish wild populations. Identifying traits that affect reproductive success is essential to achieve
this goal. Longitudinal reproductive events of the captive population of endangered red wolves (Canis rufus Audubon and
Bachman, 1851) were investigated to determine whether parental age, breeding experience, and rearing type were factors in
reproduction, litter size, and sex ratio, as well as viability of offspring. Younger wolves were more likely to reproduce and
produce larger litters than were older individuals. The age of the female, but not the male, had a negative effect on pup survival.
Wolves that had prior experience in offspring production were more likely to reproduce again than were individuals that had no
prior reproductive success, but prior sexual experience alone was not a factor in offspring production. Parental breeding
experience had a negative effect on pup survival, but no apparent relationships with litter size or sex ratio. Declines in
reproduction, fitness, and survival with advancing age suggest the effect is due to senescence, the onset of which occurs at 8 years
of age in females. The results are consistent with the breeding-experience hypothesis.
Key words: reproductive experience, breeding-experience hypothesis, senescence, age hypothesis, reproduction, red wolf, Canis rufus.
Résumé : Les programmes de propagation contribuent a
`la conservation des espèces en préservant des stocks génétiques et
démographiques pouvant être utilisés pour renforcer ou rétablir les populations sauvages. L’identification des caractères qui ont
une incidence sur le succès de reproduction est essentielle a
`l’atteinte de cet objectif. Les évènements de reproduction longitu-
dinaux de la population captive de loups roux (Canis rufus Audubon et Bachman, 1851) ont été étudiés dans le but de déterminer
si l’âge des parents, leur expérience en matière de reproduction et le type d’élevage avaient une incidence sur le succès de
reproduction, la taille et le rapport de masculinité de la portée, ainsi que la viabilité des louveteaux. Les jeunes loups étaient plus
susceptibles de se reproduire et de produire de grandes portées que les loups plus âgés. Il y avait une corrélation négative entre
l’âge de la femelle, mais pas celui du mâle, et la survie des louveteaux. Les loups ayant déja
`produit une progéniture étaient plus
susceptibles de se reproduire a
`l’avenir que les individus qui ne s’étaient pas encore reproduits avec succès, mais l’expérience
sexuelle seule n’avait pas d’incidence sur la production de petits. Il y avait une corrélation négative entre l’expérience parentale
de reproduction et la survie des louveteaux, mais aucun lien apparent avec la taille ou le rapport de masculinité de la portée. Des
baisses de la reproduction, de l’aptitude et de la survie avec l’âge donnent a
`penser que cet effet est dû a
`la sénescence, qui
commence a
`l’âge de huit ans chez les femelles. Ces résultats sont compatibles avec l’hypothèse de l’expérience de reproduction.
[Traduit par la Rédaction]
Mots-clés : expérience de reproduction, hypothèse de l’expérience de reproduction, sénescence, hypothèse de l’âge, reproduction,
loup roux, Canis rufus.
Introduction
The red wolf (Canis rufus Audubon and Bachman, 1851) is a crit-
ically endangered canid (IUCN 2013) that was once widely distrib-
uted across the eastern and south-central United States (for review
see Hinton et al. 2013). Hunting, aggressive predator-control pro-
grams, and habitat destruction led to the species’ extirpation
throughout most of its range by the early part of the twentieth
century. By the early 1970s, fewer than 300 pure red wolves were
believed to exist (Mech and Rausch 1975). The remaining red
wolves were threatened by hybridization with coyotes (Canis
latrans Say, 1823) and coyote-like animals (e.g., coyote–wolf hybrids,
coyote–dog hybrids) expanding their range into red wolf habitats
(McCarley 1962;Paradiso 1965;Paradiso and Nowak 1971). A plan
to propagate the red wolf in captivity was considered the only
practical means for the species’ preservation when its extinction
appeared inevitable. Because coyotes and coyote-like animals had
replaced many red wolf populations and threatened remaining
populations with hybridization, a comprehensive species verifica-
tion program was implemented; only 14 red wolves were con-
firmed through the program. By 1980, the red wolf was considered
extirpated and biologically extinct in the wild.
A principal objective of propagation programs is to contribute
to the conservation of a species by preserving genetic and demo-
graphic stock that can be used to reinforce or re-establish popu-
lations in the wild (Ballou and Foose 1996). Propagation programs
for imperiled species can achieve this objective by using both
short-term and long-term management strategies (Frankham
et al. 1986). Short-term strategies generally aim to rapidly multiply
the population to ensure the species’ survival in captivity and (or)
for immediate release back into the wild. Long-term strategies
include retaining as much of the founders’ genetic diversity as
Received 20 September 2013. Accepted 10 January 2014.
D.R. Rabon, Jr.* U.S. Fish and Wildlife Service, Division of Ecological Services, Raleigh, NC 27636-3726, USA; North Carolina State University,
Department of Biology, Raleigh, NC 27695, USA.
E-mail for correspondence: david_rabon@fws.gov.
*Present address: U.S. Fish and Wildlife Service, Red Wolf Recovery Program, P.O. Box 1969, Manteo, NC 27954, USA.
251
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possible and maintaining demographic stability within a captive
population. Theoretical analyses suggest that 20–30 founders can
enable a propagation program to retain 90% of the original (i.e.,
founders’) genetic diversity for 200 years (Soulé et al. 1986).
The red wolf propagation program is jointly managed by the
U.S. Fish and Wildlife Service and the Association of Zoos and
Aquariums under the auspices of the Red Wolf Recovery/Species
Survival Plan®(Red Wolf Recovery/SSP; U.S. Fish and Wildlife
Service 1990). With only 14 red wolves as the founding stock, the
Red Wolf Recovery/SSP established a goal to preserve 80%–90% of
the genetic diversity of the species for 150 years, with a target
captive population of 330 individuals. To ensure that the captive
population maintains genetic diversity and demographic stabil-
ity, a Population Analysis and Breeding/Transfer Plan is produced
annually. This plan constructs breeding recommendations for in-
dividuals within the population and sets an objective for the num-
ber of offspring to produce. Each year’s plan is determined using
age, reproductive status, and mean kinship of individuals, poten-
tial offspring inbreeding coefficients, and genetic and demo-
graphic projections that take into consideration the population’s
past and potential growth rates (
), mortality rates, and the
amount of breeding and holding space available at cooperating
institutions.
The captive red wolf population steadily increased from the late
1970s to the mid-1990s when a number of facilities housing red
wolves withdrew from participating in the Red Wolf Recovery/SSP
causing a decline in the number of breeding pairs formed and a
sharp decrease in births (Waddell and Long 2013). The population
decline was further exacerbated by fewer annual breeding recom-
mendations, the pairing of individuals with marginal reproduc-
tive potential, and the use of reproductive inhibitors that may
have compromised future reproductive potential when the inhib-
itors were removed. The distribution of ages within the popu-
lation also became skewed toward a greater number of older
cohorts. However, since the mid-1990s the captive population has
stabilized, and in recent years the population has experienced a
marked increase, in part, from a focused effort to breed a greater
number of younger animals and of older individuals that are ge-
netically desirable (i.e., under-represented founder alleles). As of
December 2013, the captive red wolf population consisted of
193 individuals. Current gene diversity is 89.29%, and is equivalent
to a population descended from about five founders (Waddell and
Long 2013). Under current conditions, gene diversity is expected to
be maintained at or above 85% for at least 16 years (Waddell and
Long 2013). However, inbreeding within the captive population
has resulted in lower reproductive performance, smaller litter
sizes, and reduced survivability of offspring (Rabon and Waddell
2010). Demographic projections to achieve the 5-year target
population size (recently reduced to 200 individuals) require a
3% annual growth rate (
= 1.03), but current spatial limitations
necessitate that the population maintain a 2% growth rate (
=
1.02) and approximately 26–35 births annually to offset the ex-
pected annual mortality rate (Waddell and Long 2013).
Preserving genetic and demographic security while increasing
the captive population can be challenging because genetic diver-
sity is lost during the expansion of the population (Nei et al. 1975).
As the captive population grows, a shift from a short-term strategy
to a long-term strategy may result in the selection of individuals
for breeding with under-represented alleles or older animals that
retain greater representation of the founders’ genetic makeup. If
reproductive output increases with age, then breeding older indi-
viduals can serve to maintain the genetic composition desired by
the propagation program. In contrast, if reproductive success de-
clines with age, then breeding older individuals may diminish the
program’s reproductive output. Similarly, if breeding experience
is correlated with breeding performance, then pairing experi-
enced individuals may increase reproductive output, whereas
breeding inexperienced individuals may require an increase in
the reproductive effort (i.e., more pairs for breeding) to achieve
output goals.
Although there are exceptions (e.g., Sæther 1990;Weimerskirch
1990), most studies suggest that reproductive performance im-
proves with age in mammals and birds (e.g., Nol and Smith 1987;
Festa-Bianchet 1988;Lunn et al. 1994;Côté and Festa-Bianchet
2001;Broussard et al. 2003) during the first few years of reproduc-
tive life before reaching a plateau, or even declining in older,
senescent individuals (e.g., Williams 1957;Clutton-Brock 1988).
Several hypotheses have been proposed to explain age-specific
variation in reproductive performance (e.g., Nol and Smith 1987;
Broussard et al. 2003), including (i) the breeding-experience hy-
pothesis, which predicts that experience in reproduction im-
proves later breeding performance, and (ii) the age hypothesis,
which suggests that breeding performance improves with age,
and that older first-time breeders are more likely to be successful
at reproduction than younger first-time breeders. However, few
studies have separated the effects of age from those of reproduc-
tive experience, in part because longitudinal data on the repro-
ductive histories of known individuals are sparse. In studies that
have evaluated both reproductive experience and age, reproduc-
tive experience contributed to the improvement of reproductive
performance (e.g., Raveling 1981;Harvey et al. 1985;Wooller et al.
1990;Pyle et al. 1991;Sydeman et al. 1991;Forslund and Larsson
1992;Hepp and Kennamer 1993;Lunn et al. 1994; but see also
Pärt 1995).
Maintaining demographic stability is a challenge inherent in
the conservation of the red wolf. Identifying those traits that af-
fect the reproductive success of individuals is essential for achiev-
ing the annual target for population growth rate, understanding
red wolf life history and population dynamics, and managing the
species toward recovery.
Materials and methods
As part of a larger study of traits that affect reproduction (e.g.,
Rabon and Waddell 2010), I examined longitudinal reproductive
events of captive red wolves to determine whether parental age,
reproductive experience (i.e., prior sexual experience and prior
reproductive success), and parental rearing type were factors in
the production of offspring. I also examined longitudinal repro-
ductive events to test the relationships between the parental age
and reproductive experience of adult males and females and the
following measures of reproductive fitness: litter size; litter rear-
ing type; sex ratio of the litter; viability of offspring. I obtained
demographic data for the red wolf population (e.g., pedigrees,
date and place of birth, date of death, reproductive experience,
rearing method, litter size) from the red wolf studbook (Waddell
2007) and from documentation, notes, and general correspon-
dence maintained in the files of the red wolf captive-breeding
program (U.S. Fish and Wildlife Service, unpublished data). Sev-
eral contradictions between the studbook and the file reports on
wolf demography, breeding events, and provenance were discov-
ered. Therefore, I evaluated and reconciled inconsistencies in the
data, and modified the red wolf studbook accordingly before an-
alyzing the data.
I evaluated reproductive success based on 554 recorded pairing
events from 1977 through 2006 (i.e., all pairings). I also evaluated
data on reproductive success with repeated pairing events by the
same male and female excluded. That is, all subsequent pairings
after the initial pairing of a male and female were excluded re-
gardless of the reproductive result of the initial pairing (leaving
351 pairing events). In addition, for those pairings that resulted in
the production of offspring, all subsequent pairings after the first
reproductive event were excluded (leaving 152 pairing events).
The age of wild-caught wolves introduced into the captive-
breeding program were estimated at time of capture, but these
estimates were not likely accurate. Therefore, for the purposes of
252 Can. J. Zool. Vol. 92, 2014
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analyses I considered wild-caught wolves (n= 14) to be at least
1 year of age if captured before 1 June and young of the year (i.e.,
less than 1 year of age) if captured after 1 June. I considered wolves
that were younger than 1 year old when paired for breeding age 0
for the purposes of analyses. I recorded the age of wolves at their
time of pairing in whole-year increments (e.g., 5 years old, not
5.8 years old); wolf pairs maintained for subsequent years are
accordingly age-adjusted.
I categorized the reproductive experiences of individual wolves
according to their participation in, and the results of, pairing
events. If a wolf had not previously been paired with a mate, I
recorded the individual as having no prior sexual experience or
reproductive success. I categorized wolves that had been previ-
ously paired but with no offspring produced as having had prior
sexual experience, but not reproductive success. I classified wild-
caught wolves as unknown and excluded them from analysis of
reproductive experience, unless otherwise noted, because their
sexual and reproductive history was uncertain.
I categorized rearing type of breeding adult males and females
into two groups: the “parent” group if reared by a parent (males,
n= 331; females, n= 321) or the “hand” group if reared by a human
(males, n= 20; females, n= 30). I excluded foster-reared males and
females from the analyses of parental rearing type because there
were too few individuals (males, n= 1; females, n= 3). I included
wild-caught wolves in the “parent” group for analysis because
wild-caught wolves were most likely reared primarily by their
parents.
I calculated the sex ratio of the litter at birth (i.e., number of
males and females produced in a litter and the proportion of
males and females produced in a litter) and at two subsequent
benchmark ages (i.e., pups aged 3 and 12 months). I also calculated
viability of pups when the pups were 3 and 12 months of age. In
calculating survival, I considered pups that were listed as “lost to
follow-up” before 12 months of age in the studbook (n= 10) to have
died.
I categorized the rearing type of offspring into three groups:
“parent” group (n= 169) contained litters in which all pups were
reared by their parents; “hand” group (n= 12) contained litters in
which at least one pup was reared by hand; “foster” group (n= 11)
contained litters in which at least one pup was reared by foster
parents. I excluded fostered litters from most analyses of rearing
type because pups chosen for fostering are based on factors other
than parental age (e.g., number of pups in the donor litter, the
availability of a foster litter, the size of potential foster litters).
I conducted statistical analyses using JMP®version 7.0 (SAS
Institute Inc., Cary, North Carolina, USA). To determine if parental
age influenced the production of offspring, I evaluated reproduc-
tion as a function of the ages of the adult breeding males and
females on actual and predicted litters using nominal logistic
regression. To determine if parental experience was a factor in the
production of offspring, I evaluated reproduction as a function of
(i) prior sexual experience and reproductive success of the adult
breeding male and female and (ii) the rearing type of the adult
breeding male and female on actual and predicted litters using
Pearson’s
2
test. To determine whether parental age and repro-
ductive experience were factors in breeding performance, I eval-
uated performance as a function of the parents’ age and experience
on litter size, litter rearing type, sex ratio of the litter, and viability
of offspring. I evaluated relationships between offspring viability
and parental age and experience using generalized linear model-
ing techniques with a binomial distribution and logit-link model.
I used generalized linear modeling techniques with a Poisson dis-
tribution, identity-link model, and an overdispersion parameter
(cˆ) to test for a relationship between litter size and parental age
and reproductive experience. I used least-squares regression to
evaluate changes in age over time and the effects of parental age
and experience on the sex ratio of litters. I compared the means of
groups using Student’s ttest or Wilcoxon–Mann–Whitney test, as
appropriate. Statistical significance was set at p≤ 0.05.
Results
A total of 192 litters (range = 1–9 pups per litter; litter size = 4.15 ±
0.14, mean ± SE) consisting of 797 offspring (359 males, 406 females,
32 unknowns) were produced between 1977 and 2006. Males as
young as 1.8 years of age sired offspring, and females first whelped at
11 months of age. The oldest captive-born male known to have sired
offspring was 12.8 years of age. Several captive-born female red
wolves whelped litters at 11 years of age. However, the reported
upper-age limits of reproducing individuals may be underesti-
mated as the Red Wolf Recovery/SSP has established a maximum
breeding age of 13 years for males and 11 years for females in
breeding recommendations designed to maximize reproductive
output (Waddell and Long 2013).
To determine whether age was a factor in the production of
offspring, I evaluated reproductive performance as a function of
parental age on actual (n= 192) and predicted (n = 362) litters
(all pairings). The mean (±SE) ages of males and females paired
for breeding were 5.49 ± 0.14 years (range = <1 to 15 years) and
5.62 ± 0.14 years (range = <1 to 14 years), respectively. Younger
males and females were more likely to reproduce than were
older animals, and this pattern continued when repeated pair-
ing events were excluded from the analysis (Table 1a). A similar
pattern was observed when pairing events were restricted to the
Table 1. (a) Unrestricted mean (±SE) ages (years) and (b) Red Wolf Recovery/Species Survival Plan (SSP) age-restricted
mean (±SE) ages (years) of adult breeding male and female red wolves (Canis rufus) that produced offspring and failed
to reproduce for all pairing events and when repeat pairing events of the adult breeding male and female were
excluded.
(a) Unrestricted age range of males (0–15 years) and females (0–14 years).
All pairings (n= 554) No repeat pairings (n= 355)
Reproduction No reproduction pReproduction No reproduction p
Males 4.682±0.193 5.923±0.181 0.0001 4.592±0.234 5.436±0.232 0.0175
Females 4.500±0.178 6.218±0.181 <0.0001 4.208±0.226 5.667±0.229 <0.0001
n192 362 130 225
(b) Red Wolf Recovery/SSP age-restricted range of males (1–12 years) and females (1–10 years).
All pairings (n= 498) No repeat pairings (n= 324)
Reproduction No reproduction pReproduction No reproduction p
Males 4.679±0.195 5.821±0.188 <0.0001 4.500±0.236 5.309±0.237 0.0206
Females 4.547±0.177 5.653±0.171 <0.0001 4.277±0.221 5.335±0.223 0.0014
n190 308 130 194
Rabon 253
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Red Wolf Recovery/SSP recommended age range for breeding
males (i.e., 1–12 years of age) and females (i.e., 1–10 years of age) for
all pairing events and when repeated pairing events were ex-
cluded (Table 1b). The mean ages of breeding males and females
have significantly increased over time (Figs. 1a,1b).
Prior sexual experience of the breeding male (
1
2= 0.419,
p= 0.517) or female (
1
2= 0.588, p= 0.443) was not a factor in the
production of offspring (n= 542, no unknowns). However, prior
reproductive success was important. Both adult breeding males
(
1
2= 25.799, p< 0.0001) and females (
1
2= 7.415, p= 0.007) were
more likely to produce offspring when they had previously pro-
duced a litter (n= 542). When repeated pairings were removed
from the analyses, this pattern continued for males (prior sexual
experience,
1
2= 1.557, p= 0.212; prior reproductive success,
1
2=
11.367, p= 0.0007; n= 343), but not for females (prior sexual experi-
ence,
1
2= 0.355, p= 0.551; prior reproductive success,
1
2= 0.537,
p= 0.464; n= 343).
After statistically accounting for age using nominal logistic re-
gression, the likelihood of producing offspring significantly in-
creased with prior reproductive success, but not prior sexual
experience, both in adult breeding males (
1
2= 22.814, p< 0.0001)
and females (
1
2= 11.184, p= 0.0008). Likewise, after accounting for
prior reproductive success, younger breeding males (
1
2= 20.892,
p< 0.0001) and females (
1
2= 29.555, p< 0.0001) had a significantly
greater likelihood of producing offspring than did older breeding
males and females.
Rearing type (i.e., parent, hand) of adult breeding males (
1
2=
0.666, p= 0.414) and females (
1
2= 1.473, p= 0.225) was not a factor
in the production of offspring (n= 351; no repeat pairings), al-
though there was a slight trend that adult males and females that
were parent-reared were more likely to produce offspring than
were hand-reared adult males and females.
The following results are calculated using only pairs that suc-
cessfully reproduced offspring, unless otherwise noted.
Older breeding males (
1
2= 3.904, cˆ = 0.943, p= 0.048) and
females (
1
2= 4.500, cˆ = 0.946, p= 0.034) produced smaller litters,
but the trend was not significant (males,
1
2= 1.774, cˆ = 0.990,
p= 0.183; females,
1
2= 3.758, cˆ = 0.981, p= 0.053) when repeated
pairings were excluded from the analyses (n= 152 pairing events).
When litter size was measured as a function of parental reproduc-
tive experience (no unknowns), there were no relationships be-
tween litter size and prior sexual experience of breeding males
(
1
2= 0.118, cˆ = 0.974, p= 0.731, n= 186) or females (
1
2= 0.052, cˆ =
0.975, p= 0.819, n= 189). Furthermore, there was no relationship
between litter size and prior reproductive success of breeding
males (
1
2= 0.038, cˆ = 0.977, p= 0.847, n= 186) or females (
1
2=
0.349, cˆ = 0.973, p= 0.555, n= 189).
Parental age (Table 2a) and prior reproductive experience
(Table 3a) had no apparent effect on the proportion of male pups,
female pups, or pups of an undetermined sex in a litter at birth.
Similar trends occurred when repeated pairings were excluded
from the parental age analyses (Table 2a).
There was a significant negative relationship between the
breeding female’s age and pup survival at 3 months of age, but no
other relationships between parental age and pup survival were
apparent (Figs. 2a,2b;Table 2b). When offspring survival was mea-
sured as a function of parental experience (no unknowns), there
was a negative relationship between the male’s prior reproduc-
tive experience and the pup survival at 3 and 12 months of age
(Table 3b). A similar relationship was found between the female’s
prior reproductive experience and the pup survival at 3 and
12 months of age (Table 3b).There was no relationship between
parental age and rearing type of their offspring (Table 4).
Discussion
The Red Wolf Recovery/SSP has contributed to the genetic and
demographic conservation of the red wolf. The restoration of the
wolf to the wild less than 10 years after the species was declared
extinct in the wild is a testament to the effectiveness of the short-
term strategy to rapidly multiply the population in captivity for
the purposes of re-establishing a wild population. Accomplishing
the long-term strategy of retaining the founders’ genetic diversity
and maintaining demographic stability within the captive popu-
lation is realized over time. Current population demographics
suggest that the captive population can persist and accomplish its
genetic-retention goal (Waddell and Long 2013). However, a fun-
damental component to achieving this goal is the ability of the
Red Wolf Recovery/SSP to adjust for the traits, such as parental age
or reproductive experience, that affect the reproductive success of
individuals in the managed population.
The recorded range in age for successful reproduction in cap-
tivity in both male and female red wolves is similar to that of the
wild population. However, reproduction in individuals less than
2 years of age is uncommon in both populations. In males, repro-
duction at less than 2 years of age has only been recorded in the
wild (Phillips et al. 2003; U.S. Fish and Wildlife Service, unpub-
lished data), whereas successful reproduction in females at less
than 1 year of age has only been recorded in captivity (this paper).
The upper age limit for successful reproduction in captivity ex-
Fig. 1. Mean ages of adult (a) males and (b) females at time of
pairing each year for the captive red wolf (Canis rufus) population
(n= 192). Fitted line is based on least-squares linear regression.
254 Can. J. Zool. Vol. 92, 2014
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tends a few years beyond those ages recorded for wild-born
wolves, at least for males. The oldest captive-born, male red wolf
known to have successfully bred was 12.8 years of age when the
pups were sired, while the oldest known wild-born males have
sired pups at 9.8 years of age (U.S. Fish and Wildlife Service, un-
published data). Both captive-born and wild-born female wolves
have whelped litters when they were 11 years of age, though wild-
born females are unlikely to reproduce beyond 10 years of age
(U.S. Fish and Wildlife Service, unpublished data). The differences
in the maximum age for successful reproduction in the red wolf
may be attributed to differential rates of survival between wild
and captive animals (Mech 1988;Phillips et al. 2003; U.S. Fish and
Wildlife Service, unpublished data), thus reducing the opportu-
nity for older wild wolves to reproduce. Captive animals are gen-
erally expected to live longer because they are protected, better
fed, and receive consistent care.
The age range for reproduction in red wolves appears similar to
that of gray wolves (Canis lupus L., 1758) and coyotes. In captivity,
male and female gray wolves have bred as young as 10 months of
age (Medjo and Mech 1976) and as old as 14–15 years of age (Kreeger
2003). Both male and female coyotes are capable of breeding in
their first year, but the likelihood of reproduction appears to be
related to the availability and abundance of food (Gier 1968;
Knowlton 1972). Data on the reproductive activity of very old coy-
otes is sparse. Gese (1990) reported that the behavior of an
estimated-age male coyote associating with an estrus female, and
later observed with pups, suggested that the male was reproduc-
tively active at 11 and 12 years of age. In addition, a few studies
have reported a decline in fecundity in female coyotes older than
9 years of age (e.g., Windberg 1995;Green et al. 2002). The maxi-
mum breeding age of wild gray wolves also is not well known, but
Mech (1988) reported reproduction in a female of at least 10 years
of age.
The effects of parental age and reproductive experience ob-
served in the captive red wolf population are supported by studies
showing a relationship between age and experience and repro-
ductive performance (e.g., Raveling 1981;Harvey et al. 1985;
Wooller et al. 1990;Pyle et al. 1991;Sydeman et al. 1991;Forslund
and Larsson 1992). In the present study, parental age and repro-
ductive experience of male and female red wolves were positive
Table 2. Relationships between parental age of red wolves (Canis rufus) and (a) proportion of pups in a litter at
birth and (b) offspring survival of pups at 3 and 12 months of age for all pairings resulting in reproduction and
when repeat pairing events of a breeding male and female were excluded.
(a) Proportion of pups in a litter at birth.
All pairings No repeat pairings
Sires Dams Sires Dams
Fdf pFdf pFdf pF df p
Males 0.004 1 0.947 0.034 1 0.854 0.428 1 0.514 1.559 1 0.214
Females 1.037 1 0.309 0.079 1 0.306 0.011 1 0.917 0.002 1 0.963
Unknown 2.751 1 0.099 1.652 1 0.200 1.411 1 0.237 3.569 1 0.061
n186 189 152 152
(b) Offspring survival of pups at 3 and 12 months of age.
All pairings No repeat pairings
Sires Dams Sires Dams
2
df p
2
df p
2
df p
2
df p
3 months 0.468 1 0.494 7.776 1 0.005 0.014 1 0.906 2.337 1 0.126
12 months 0.111 1 0.739 0.321 1 0.571 0.559 1 0.455 0.380 1 0.538
n186 189 152 152
Table 3. Relationships between parental reproductive experience (i.e., prior sexual experience only or prior
reproductive success) of red wolves (Canis rufus) and (a) proportion of pups in a litter at birth and (b) offspring
survival of pups at 3 and 12 months of age.
(a) Proportion of pups in a litter at birth.
Sexual experience only Reproductive success
Sires Dams Sires Dams
Fdf pFdf pFdf pFdf p
Males 1.897 1 0.170 0.396 1 0.529 1.717 1 0.192 0.058 1 0.810
Females 1.097 1 0.296 0.587 1 0.445 0.772 1 0.381 0.008 1 0.929
Unknown 0.254 1 0.615 0.059 1 0.809 0.445 1 0.506 0.306 1 0.581
n186 189 186 189
(b) Offspring survival of pups at 3 and 12 months of age.
Sexual experience only Reproductive success
Sires Dams Sires Dams
2
df p
2
df p
2
df p
2
df p
3 months 10.392 1 0.001 10.655 1 0.001 3.881 1 0.049 9.578 1 0.002
12 months 7.711 1 0.006 1.475 1 0.225 7.650 1 0.006 6.963 1 0.008
n186 189 186 189
Rabon 255
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For personal use only.
factors in the production of offspring, and the effects were inde-
pendent of one another in successful reproduction. Younger
males and females were more likely to reproduce than were older
individuals, and litter size tended to decrease with increasing age
of the breeding males and females. There also was a trend for
survival of offspring to be affected by increasing maternal age, but
the effect was limited, in part, by multiple reproductive events.
Conversely, the reproductive experience of the parents appeared
to have a greater negative affect on the survival of offspring. Other
relationships between reproductive experience or parental age
and reproductive performance and fitness were not apparent.
Of the hypotheses put forward to explain positive correlations
of parental age and reproductive experience on reproduction, the
results in the present study are most consistent with the breeding-
experience hypothesis. However, the cumulative benefits of re-
productive experience on reproductive success in the red wolf are
unknown. That is, multiple years of reproductive experience may
offer no advantage over inexperienced individuals. For example,
Sydeman et al. (1991) reported that reproductive experience in
younger female northern elephant seals (Mirounga angustirostris
(Gill, 1866)) was beneficial for weaning success, but that experi-
ence was detrimental to weaning success later in life. Moreover,
the advantages of multiple years of reproductive experience on
reproductive success may be sex-specific. Ollason and Dunnet
(1978) reported that male Northern Fulmars (Fulmarus glacialis (L.,
1761)) show a general improvement in reproductive success with
reproductive experience, whereas female Northern Fulmars had
lower reproductive success than did males with similar reproduc-
tive experience in early and later years.
The effects of age on reproductive performance are more diffi-
cult to explain, as the results presented here do not support the
age hypothesis. Differential effects of parental age and reproduc-
tive experience on reproductive success have been reported in
studies investigating both. In the Antarctic Fulmar (Fulmarus
glacialoides (A. Smith, 1840)), reproductive experience positively
contributed to reproductive success after the first reproduction,
but parental age had no significant effect (Weimerskirch 1990).
Similarly, in a study of three species of European ducks, reproduc-
tive performance varied with parental age and reproductive expe-
rience, but the effects of age were explained more by the
covariates female body mass and nesting date (Blums et al. 1997).
Experimental manipulation of reproductive experience or the
analysis of other age-related improvements in reproduction may
be required to understand age-specific variation in reproductive
success in the red wolf (Pärt 1995). Several additional hypotheses
(e.g., residual reproductive-value hypothesis, selection hypothe-
sis, evolutionary-restraint hypothesis) also have been proposed
that may explain age-specific variation in reproductive perfor-
mance (e.g., Nol and Smith 1987;Broussard et al. 2003). In the
present study, data (e.g., parental survival rates, somatic invest-
ment) were not collected or recorded in a manner permitting
assessment of these hypotheses.
The results presented here are the first investigating the rela-
tionships between parental age, reproductive experience, and re-
production in the red wolf. A previous study investigated changes
in age structure in the captive red wolf population and how those
changes affected the population’s reproductive potential (Lockyear et al.
2009). However, the accuracy of the data set used in the previous
study has been questioned (Rabon and Waddell 2010), and rela-
tively few results of the effects of age on reproduction are similar
between the two studies. For example, Lockyear et al. (2009) re-
ported that reproductive success was negatively correlated only
with female age. They also reported no effect of the age of either
the male or the female on litter size, but found pup survival was
reduced with increasing age of the females. In the present study,
breeding success was negatively affected by age of both males and
females, and litter size significantly declined with increasing age
of the female. Indeed, females older than 7 years of age had sig-
nificantly smaller litter sizes when compared with the litter sizes
of younger females (p= 0.004; Figs. 3a,3b). This result suggests
that female red wolves show signs of reproductive senescence at a
younger age than previously reported. Although the results of
parental age on pup viability are inconclusive, there appear to be
similar trends in their effects between the two studies.
Differences in the results between the two studies may be ex-
plained by differences in the data sets. Lockyear et al. (2009) re-
ported only 165 litters born in captivity compared with 183 litters
identified during the same period (i.e., 1977–2005) in my data set.
Furthermore, Lockyear et al. (2009) conducted some of the analy-
ses using data restricted to breeding events from 1992 through
2005. The failure to include data from all litters born in the prop-
agation program in the analyses may have affected the results.
Unfortunately, a complete comparison of the data sets represen-
tative of the period(s) used by Lockyear et al. (2009) could not be
accomplished. Therefore, it is difficult to determine whether the
differences between these two studies can be solely attributed to
the number of reproducing events considered in the analyses.
In the current study, the effects of parental age and reproduc-
tive experience were evaluated on multiple aspects of fitness in
the captive red wolf population. Understanding the factors influ-
encing reproductive performance is important for managing
and conserving the species both in captivity and in the wild. Nol
Fig. 2. Mean (±SE) age of adult breeding female red wolf (Canis rufus)
plotted against offspring viability at (a) 3 months of age and
(b) 12 months of age. Fitted line is based on least-squares regression.
256 Can. J. Zool. Vol. 92, 2014
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and Smith (1987) cautioned that the detection of effects of age and
reproductive experience could be affected by the relative quality
of environment conditions, including population density and the
availability of resources. Although there are recognizable geo-
graphic and spatial differences among Red Wolf Recovery/SSP
propagation facilities, the general protective environment, food
resources, and animal care are relatively consistent factors for the
captive wolves. In addition, differences in geography and enclo-
sure do not appear to affect the likelihood of successful reproduc-
tion (D. Rabon, unpublished data), although a more complete
analysis of breeding location and reproductive success is required.
The results presented here may prove useful in the preparation
of the annual Population Analysis and Breeding/Transfer Plan for
the red wolf. Additional data on the traits affecting reproductive
performance may allow for an improvement in reproductive suc-
cess while minimizing reproductive effort. For example, an anal-
ysis of the effects of the presence of juveniles from the previous
year’s litter during the breeding season or the proximity of other
breeding pairs may prove important to successful reproduction. If
spatial limitation of propagation facilities persist, improving the
ratio between reproductive success and effort will be necessary to
achieve the estimated growth rates required to maintain demo-
graphic stability and to grow the red wolf population toward
stated recovery goals.
Acknowledgements
Comments provided by H. Heatwole, P. Doerr, N. Haddad,
K. Pollock, B. Bartel, W. Waddell, and two anonymous reviewers
greatly improved the manuscript. This research was supported by
the U.S. Fish and Wildlife Service and The Zoo Society at Point
Defiance Zoo and Aquarium. The findings and conclusions in this
article are those of the author and do not necessarily represent the
views of the U.S. Fish and Wildlife Service. The use of trade names
or mention of a commercial product in this document is not in-
tended to imply endorsement. This manuscript is based on part of
a dissertation by the author for a Ph.D. in Zoology at North Caro-
lina State University, Raleigh.
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... Female age appeared to bear a significant negative relation to birth litter size in our sample of red wolves (Fig. 1a). However, other competing explanatory variables-AFR and hybrid status in particular-appeared to more effectively explain variation in litter size (Fig. 2), suggesting that the low litter sizes at later ages may have been primarily an artifact of higher reproduction in hybrids (likely due to high average litter sizes in coyotessee Rabon 2014;Gese et al. 2016) and females that started reproducing later having smaller litters (perhaps due to poor body condition, or suboptimal conditions) (Fig. 4a). This is not to say that female reproductive senescence does not occur in red wolves-on the contrary, it may occur at later ages, or even commence around 8 years of age, when our sample size becomes restrictive. ...
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