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Territoriality and Inter-Pack Aggression in Gray Wolves: Shaping a Social Carnivore's Life History Rudyard Kipling's Law of the Jungle Meets Yellowstone's Law of the Mountains</i

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When Rudyard Kipling wrote The Jungle Book in 1894 and included the famous line "For the strength of the Wolf is the Pack, and the strength of the Pack is the Wolf," he would have had no idea that over a century later, scientific research would back up his poetic phrase. Recent studies in Yellowstone have found that both the individual wolf and the collective pack rely on each other and play important roles in territoriality. At a time when most fairy tales and fables were portraying wolves as demonic killers or, at best, slapstick gluttons, Kipling seemed to have a respect or even reverence for the wolf. Wolves in The Jungle Book raise and mentor the main character Mowgli, with the pack's leader eventually dying to save the "man-cub" from a pack of wolves. Kipling may have extended intra-pack benevolence to a human boy for literary sake, but he was clearly enthralled with how pack members treat each other. As wolf packs are almost always family units, most commonly comprised of a breeding pair and their offspring from several years, amiable behavior within the pack is unsurprising. By contrast, wolf packs are fiercely intolerant of their neighbors, their rivals. And this competition is proving to be an important facet in the life of a wolf and its pack.
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Territoriality and Inter-Pack Aggression in
Gray Wolves: Shaping a Social Carnivores
Life History
Rudyard Kiplings Law of the Jungle Meets
Yellowstones Law of the Mountains
Kira A. Cassidy, Douglas W. Smith, L. David Mech, Daniel R. MacNulty,
Daniel R. Stahler, & Matthew C. Metz
24(1) • 2016 Yellowstone Science
When Rudyard Kipling wrote The Jungle Book
in 1894 and included the famous line “For
the strength of the Wolf is the Pack, and the
strength of the Pack is the Wolf,” he would have had no
idea that over a century later, scientific research would
back up his poetic phrase. Recent studies in Yellowstone
have found that both the individual wolf and the collec-
tive pack rely on each other and play important roles in
territoriality. At a time when most fairy tales and fables
were portraying wolves as demonic killers or, at best,
slapstick gluttons, Kipling seemed to have a respect or
even reverence for the wolf. Wolves in The Jungle Book
raise and mentor the main character Mowgli, with the
pack’s leader eventually dying to save the “man-cub”
from a pack of wolves. Kipling may have extended in-
tra-pack benevolence to a human boy for literary sake,
but he was clearly enthralled with how pack members
treat each other. As wolf packs are almost always fam-
ily units, most commonly comprised of a breeding pair
and their offspring from several years, amiable behavior
within the pack is unsurprising. By contrast, wolf packs
are fiercely intolerant of their neighbors, their rivals.
And this competition is proving to be an important facet
in the life of a wolf and its pack.
Although many animals live in groups, only some are
considered territorial (willing to fight other groups or
invading individuals to protect their territory). African
lions, meerkats, chimpanzees, and mongooses regular-
ly attack and even kill non-group members (Heinsohn
and Packer 1995, Doolan and MacDonald 1996, Wilson
et al. 2001, Cant et al. 2002). Even nomadic hunter-gath-
erer human groups fought; the often lethal conflicts
ranged from primitive to complex warfare (Wrangham
and Glowacki 2012). For this behavior to evolve, it must
afford group members a survival advantage. Wolves
likely evolved to be territorial because it benefits them
in several ways: repelling intruders makes it easier to
protect vulnerable pups at the pack’s den, and securing
territory with abundant prey ensures an uncontested
Illustration by Charles Maurice Detmold from The Jungle Book
by Rudyard Kipling, Macmillan & Co., London, UK, 1894.
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Yellowstone Science 24(1) • 2016
food source (Kittle et al. 2015). Success in both these
aspects of life—reproducing and eating—perpetuates
the genes of high-performing individuals. And in the
case of the wolf, the ones best at reproducing and eating
are aggressive with their rivals. In fact, of all the dead
Natural Other
Control Human Other Unknown (a)
Figure 1. Causes of mortality for Yellowstone National Park collared wolves (1995-2015). (a) All causes of mortality; (b) Natural,
known causes of mortality.
Natural Other
The Agate Creek pack, led by several adult females, chases the Oxbow Creek pack (out of frame). Within a few minutes, the Agate
Creek pack caught and killed a female from the Oxbow pack, effectively reducing that pack to only two wolves.
wolves recovered in Yellowstone, intraspecific (wolf vs.
wolf) strife accounts for two-thirds of natural mortality
(figure 1).
Although inter-pack conflict is not rare, wolves display
a variety of nonaggressive territorial behaviors that di-
All known mortality Natural mortality
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24(1) • 2016 Yellowstone Science
minish the risk of confrontation. They scent-mark with-
in territories and along boundaries, and these scents can
be detected by other wolves for 2-3 weeks(Peters and
Mech 1975). They also howl, to signal their location and
strength to neighboring packs (Harrington and Mech
1983). When these behaviors fail to separate neighbor-
ing packs or one pack decides to engage another, the
ensuing confrontations are almost always aggressive. In
these cases, each pack tries to displace the other and, if
possible, catch and kill an adversary.
But what makes one pack better or more successful
at aggressive encounters with another group? Is it sim-
ply a numbers game? Does the larger pack always win?
If so, that would fit well with the first line of Kipling’s
writings: “The strength of the Wolf is the Pack.” Using
data gathered during direct observations of 121 aggres-
sive encounters between packs from 1995-2011, we were
able to test these questions. As expected, pack size was
important to successful conflicts. The larger group was
more likely to win (Cassidy et al. 2015), as seen in groups
of African lions, chimpanzees, and hyenas (Mosser and
Packer 2009, Wilson et al. 2002, Benson-Amram et al.
2011). And just one wolf can make quite a difference; a
pack with one more wolf than its opponent has 140%
higher odds of winning (or 2.4 to 1). If a pack of 10 fought
a pack of nine 100 times, the pack of 10 would win about
71 of the encounters.
If the strength of the wolf is the pack, it makes sense
that wolves have evolved to live in large groups. Between
1995 and 2015, Yellowstone packs averaged 9.8 wolves
and frequently grew to 20, with the largest pack record-
ed at 37 members. But living in such a large family isn’t
always beneficial to other aspects of wolf life. The most
efficient pack size for successful elk hunting is only four
wolves (MacNulty et al. 2012) and eight for reproduction
(Stahler et al. 2013). Living in a large group often means
each individual wolf gets less to eat (Schmidt and Mech
1997). The largest packs tend to exhibit more fission-fu-
sion behavior (Metz et al. 2011), much like chimpan-
zees and hyenas (Lehmann and Boesch 2004, Smith et
al. 2008). They may be able to get away with being less
cohesive because when they break into smaller groups,
each wolf gets more food; and as long as each group is
larger than its neighbor’s full size, it is still likely to win
in a territorial contest.
Wolves do several things to indicate that on some lev-
el, they might realize pack numbers give them an ad-
vantage. They will often disperse in same-sex cohorts.
These pack mates, typically siblings, look to join an op-
posite sex individual or, even better, a cohort of oppo-
site sex wolves. Most packs in Yellowstone have formed
this way. Becoming an immediately-sizeable pack is crit-
ical to establishment and persistence on the wolf-dense
northern range (wolf density in Yellowstone’s northern
range has ranged from 20.1 to 98.5 wolves/1000km2 and
averages 52.9, almost double the average wolf density in
northeastern Minnesota and 10 times higher than De-
nali National Park [Fuller et al. 2003]). While each year
new wolf pairs form, since 1995 only two simple packs—
packs made up of one male and one female—have suc-
cessfully raised pups and established a territory in the
hyper-competitive northern range (Leopold, which
formed early on in 1996; and Swan Lake, which formed
at the western edge of high-wolf density territories).
Although infanticide, the killing of pups, has been re-
corded in gray wolves (Latham and Boutin 2012, Smith
et al. 2015), it is less common than in bears and wild fe-
lids, and occurs when one pack attacks the wolves at
the den site of another pack. Spring is the most effective
time for one pack to impact another; den-attacks are
more likely to result in adult and pup mortality, some-
times even wiping out an entire litter (Smith et al. 2015).
Unlike wolves, female bears and felids become sexu-
ally receptive after they stop lactating, thus motivating
males to kill nursing juveniles and mate with the female,
replacing a rival’s offspring with their own (Hausfater
and Hrdy 2008). By contrast, female wolves come into
estrus only once per year for about a week (Asa et al.
1986). So although mating competition is intense for a
short time, there is no immediate advantage for outside
males to kill dependent young. In fact, the evidence sug-
gests that newly established breeding males attend the
pups as if they were their own. There are several cases
of a new dominant male joining a pack, either when the
dominant female is pregnant with the previous male’s
pups (e.g., the Lamar Canyon pack in 2015) or after the
pups were born. This suggests the new male realizes the
value in raising unrelated pups; it ensures his pack size
increases and remains competitive against neighbor-
ing packs. He can then breed with the female the next
mating season—an incredibly long-vision for individu-
als that, in Yellowstone, only live an average of 4.6 years
(MacNulty et al. 2009a).
During 121 aggressive interactions recorded in Yellow-
stone, 71 escalated to a physical attack and 12 resulted in
mortality. We also recorded seven cases of apparently
altruistic behavior, where one wolf was being attacked
by a rival pack and its pack mate disrupted the attack
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Figure 2. Predicted values for the probability of a wolf pack winning an aggressive inter-pack interaction based on relative pack size
(RPS) and old adults. Red lines indicate probability of winning while having relatively fewer (-1, -2, -3) old adults than an opponent.
Blue lines indicate probability of winning while having relatively more old adults than an opponent. Data collected from 1995-2011
in Yellowstone National Park.
by running close by or even jumping into the middle of
the group of wolves. In four cases the victim escaped.
Kipling penned a similar scenario wherein Mowgli was
saved from a rival pack of wolves by his lead male wolf,
who was injured and eventually died—effectively giving
his life for his pseudo-offspring. The risky behavior ex-
hibited by the altruist is difficult to explain; but if suc-
cessful, it enjoys the benefits of maintaining a packmate,
who usually shares its genes (kin selection [Hamilton
1964]) and may reciprocate or aid them in the future (re-
ciprocal altruism [Trivers 1971]). Whether it is through
rescuing a pack mate, raising unrelated offspring, or
traveling in a large pack to defeat rivals, “The strength of
the Wolf is the Pack” rings true.
But there is the second part: “The strength of the Pack
is the Wolf.” Could Kipling be right? Could there be
some pack composition influence: that one individual
has a disproportionate effect, maybe helping its pack
beat an opponent in an aggressive encounter, even when
outnumbered? While statistically holding pack size
fixed, we tested for effects from all age and sex catego-
ries. We also tested to see if residents were more likely to
defeat intruders. This home-field-advantage hypothesis
was not supported; even intruders were likely to win if
they were larger. But Kipling would be happy to know
that some types of wolves have a significant and positive
effect on their pack’s success: adult males and old adults
(6 years or older; Cassidy et al. 2015). Adult males are the
most aggressive wolves in the pack, and having one more
than a rival meant 65% higher odds of winning (1.65 to 1).
Males are 20% larger and more muscular than females
(Morris and Brandt 2014), though this actually hinders
males during some stages of prey hunting, as their bulk
makes them slower (MacNulty et al. 2009b). This sexual
dimorphism probably evolved as an adaptive response
to intense inter-pack competition and protection of the
family unit through fighting. A male wolf’s aggressive-
ness actually increases throughout his entire lifespan,
even as hunting ability and body size diminish into old
age (MacNulty et al. 2009a, b).
Perhaps related to the value of adult males to territo-
riality, we have recorded several cases of an unrelated
male joining an already established pack as a subor-
dinate member. Even though the new male could be
viewed as competition for breeding rights with the fe-
males, he is accepted, perhaps for the positive influence
he has on pack success when encountering a neighbor.
Conversely, in 20 years we have never recorded an un-
related female joining an already-established group. Fe-
males did not have an effect on conflict success. Their
+1 -1
+2 -2
+3 -3
-8 -6 -4 -2 02468
Probability of winning
Relative pack size
old adults (- or +)
fewer old adultsmore old adults
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24(1) • 2016 Yellowstone Science
aggression stays approximately constant throughout
their entire lifespan and may drop slightly during their
most reproductively-active years, likely a product of
But the most influential factor in whether or not a
pack defeated an opponent was the presence of an old
wolf. A pack with one old wolf more than the opposi-
tion has 150% greater odds of winning, making age more
important than having a numerical advantage (figure 2).
But why? Old wolves are past their physical prime, par-
ticipating less and less in hunts as they age, instead rely-
ing on the younger, faster, stronger wolves to risk bison
and elk hooves and antlers to provide food for the entire
pack (MacNulty et al. 2009b). Even the lead wolf in The
Jungle Book eventually became so old that he rarely left
his lair yet was still the leader, as Kipling wrote in one of
the last lines of wolf code or “The Law of the Jungle”:
“Because of his age and his cunning,
because of his grip and his paw,
in all that the law leaveth open,
the word of the head wolf is law.”
What old wolves possess is experience. They’ve en-
countered competitors many times, seen pack mates
killed, participated in killing rivals. They may avoid a
conflict they figure they can’t win, upping their chance
of survival. Having an experienced wolf allows a pack
to draw from past knowledge, increasing the odds that
even a small pack can defeat a larger pack.
As death by rival pack is by far the most common cause
of natural mortality, the packs that can reduce this risk
by being larger than their neighbors, having more adult
males, or having old adult pack members are the ones
most likely to acquire and maintain productive territory.
Those territories include safe places to raise pups, lots
of prey, and separation from humans and roads. One
pack in Yellowstone, the Mollie’s pack (originally called
the Crystal Creek pack) has persisted for over 21 years,
likely because it has traditionally been one of the largest
packs with many adult males and long-term, old mem-
bers. This pack has had only six dominant males and five
dominant females in their entire history—reigns that
help explain the pack’s success and longevity.
The loss of an old adult or an adult male, through
natural- or human-causes, reduces the competitive
strength of the pack, likely affecting the remaining pack
members’ long-term survival, reproduction, ability
to hold productive territory, and ultimately the entire
pack’s persistence. Over 100 years ago, when Kipling
wrote “For the strength of the Wolf is the Pack, and the
strength of the Pack is the Wolf,” he couldn’t know his
creative writings would someday be interwoven with
wolf research. But maybe that is why The Jungle Book is
still such a classic; although Kipling’s premise of wolves
raising a human boy is obviously fictitious, the way he
describes the heart of the wolf pack and the ways the
pack treats its family versus rivals is based in truth and,
now, supported with science.
Literature Cited
Asa, C.S., U.S. Seal, E.D. Plotka, M.A. Letellier, and L.D. Mech.
1986. Effect of anosmia on reproduction in male and female
wolves (Canis lupus). Behavioral and Neural Biology 46:272-
Benson-Amram S., V.K. Heinen, S.L. Dryer, and K.E. Holekamp.
2011. Numerical assessment and individual call discrimination
by wild spotted hyaenas (Crocuta crocuta). Animal Behavior
Cant, M., E. Otali, and F. Mwanguhya. 2002. Fighting and mat-
ing between groups in a cooperatively breeding mammal, the
banded mongoose. Ethology 108:541–555.
Cassidy, K. A, D. R. MacNulty, D. R. Stahler, D. W. Smith, and L.
D. Mech. 2015. Group composition effects on interpack
aggressive interactions of gray wolves in Yellowstone National
Park. Behavioral Ecology. doi: 10.1093/beheco/arv081
Doolan, S.P., and D.W. Macdonald. 1996. Dispersal and ex-
tra-territorial prospecting by slender-tailed meerkats (Suricata
suricatta) in the south-western Kalahari. Journal of Zoology
Fuller, T. K., L. D. Mech, and J. Fitts-Cochran. 2003. Population
dynamics. pp. 161-191 in L. D. Mech and L. Boitani, (eds.).
Wolves: behavior, ecology, and conservation. University of Chi-
cago Press, Chicago, Illinois, USA. 405 pp.
Hamilton, W.D. 1964. The genetical evolution of social behavior.
Journal of Theoretical Biology 7:17-52.
Harrington, F.H., and L.D. Mech. 1983. Wolf pack spacing:
howling as a territory-independent spacing mechanism in a
territorial population. Behavioral Ecology and Sociobiology
Hausfater, G., and S.B. Hrdy. 2008. Infanticide: comparative and
evolutionary perspectives. Transaction Publishers. Piscatawa,
New Jersey, USA.
Heinsohn, R., and C. Packer. 1995. Complex cooperative strate-
gies in group-territorial African lions. Science 269:1260-1262.
Kittle, A. M., M. Anderson, T. Avgar, J.A. Baker, G.S. Brown, J.
Hagens, E. Iwachewski, S. Moffatt, A. Mosser, B.R. Patterson,
D.E.B. Reid, A.R. Rodgers, J. Shuter, G.M. Street, I.D. Thomp-
son, L.M. Vander Vennen, and J.M. Fryxell. 2015. Wolves adapt
territory size, not pack size to local habitat quality. Journal of
Animal Ecology 84:1177-1186
Kipling, R. 1894. The Jungle Book. Macmillan & Co., London,
Latham, A.D.M., and S. Boutin. 2012. Wolf, Canis lupus, pup
mortality: interspecific predation or non-parental infanticide?
Canadian Field Naturalist 125:158-161.
Lehmann, J., and C. Boesch. 2004. To fission or to fusion: ef-
fects of community size on wild chimpanzee (Pan troglodytes
verus) social organisation. Behavioral Ecology and Sociobiology
WOLF ISSUE.indb 41 6/7/2016 6:23:56 PM
Yellowstone Science 24(1) • 2016
MacNulty D.R., D.W. Smith, L.D. Mech, J.A. Vucetich, and C.
Packer. 2012. Nonlinear effects of group size on the success of
wolves hunting elk. Behavioral Ecology 23:75–82.
MacNulty, D.R., D.W. Smith, J.A. Vucetich, L.D. Mech, D.R.
Stahler, and C. Packer. 2009a. Predatory senescence in ageing
wolves. Ecology Letters 12: 1347-1356.
MacNulty, D. R., D.W. Smith, L.D. Mech, and L.E. Eberly. 2009b.
Body size and predatory performance in wolves: is bigger bet-
ter? Journal of Animal Ecology 78: 532-539.
Metz, M. C., J. A. Vucetich, D. W. Smith, D. R. Stahler, and R.
O. Peterson. 2011. Effect of sociality and season on gray wolf
(Canis lupus) foraging behavior: implications for estimating
summer kill rate. Plos One 6: e17332.
Morris, J.S, and E.K. Brandt. 2014. Specialization for aggression
in sexually dimorphic skeletal morphology in gray wolves (Ca-
nis lupus). Journal of Anatomy 225: 1-11.
Mosser, A., and C. Packer. 2009. Group territoriality and the
benefits of sociality in the African lion, Panthera leo. Animal
Behavior 78:359-370.
Peters, R.P., and L.D. Mech. 1975. Scent-marking in wolves: ra-
dio-tracking of wolf packs has provided definite evidence that
olfactory sign is used for territory maintenance and may serve
for other forms of communication within the pack as well.
American Scientist 63:628-637.
Schmidt, P.A., and L.D. Mech. 1997. Wolf pack size and food
acquisition. The American Naturalist 150: 513-517.
Smith, J. E., J.M. Kolowski, K.E. Graham, S.E. Dawes, and K.E.
Holekamp. 2008. Social and ecological determinants of fis-
sion–fusion dynamics in the spotted hyena. Animal Behavior,
Smith, D.W., M. Metz, K.A. Cassidy, E.E. Stahler, R.T. McIntyre,
E.S. Almberg, and D.R. Stahler. 2015. Infanticide in wolves:
seasonality of mortalities and attacks at dens support evolu-
tion of territoriality. Journal of Mammalogy.
Stahler, D.R., D.R. MacNulty, R.K. Wayne, B.M. vonHoldt, and
D.W. Smith. 2013. The adaptive value of morphological, be-
havioral, and life history traits in reproductive female wolves.
Journal of Animal Ecology 82:222-234.
Trivers, R.L. 1971. The evolution of reciprocal altruism. Quarterly
review of biology 46: 35-57.
Wilson, M.L., M.D. Hauser, and R.W. Wrangham. 2001. Does
participation in intergroup conflict depend on numerical as-
sessment, range location, or rank for wild chimpanzees? Ani-
mal Behavior 61:1203-1216.
Wilson, M.L., N.F. Britton, and N.R. Franks. 2002. Chimpanzees
and the mathematics of battle. Proceedings of the Royal Soci-
ety of London B: Biological Sciences 269:1107-1112.
Wrangham, R. W., and L. Glowacki. 2012. Intergroup aggres-
sion in chimpanzees and war in nomadic hunter-gatherers.
Human Nature 23:5-29.
Kira Cassidy is a research associate with the
Yellowstone Wolf Project. After graduating from Southern
Illinois University in 2007, Kira started as a biological
technician with the Wolf Project in 2008. For two years
Kira worked on the Druid Road Management crew and
participated in six winter studies, all but one following
the famous Druid Peak pack. In 2013 she completed a
MS degree at the University of Minnesota, advised by
wolf biologist Dr. L. David Mech. Her projects focused on
territoriality and aggression between wolf packs. In 2014
Kira accompanied a film crew to Ellesmere Island, Canada,
to document arctic wolves. Living next to a wolf den for
six weeks fostered Kira’s desire to help communicate
science through media, art, and writings for the public.
Kira’s current projects focus on wolf pack behavior and
sociality. Results from some of these projects highlight
the importance of old adults in a wolf pack and led Kira
to consider connections to other social species, including
humans. This was the topic of Kira’s TEDx talk in Bozeman,
Montana, in April 2016.
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24(1) • 2016 Yellowstone Science
Women in Science
Lisa Koitzsch, Kira Cassidy, Erin Stahler, & Brenna Cassidy
Early on, almost all people who studied wolves were
men (with the notable exception of Lois Crisler who
wrote Arctic Wild in the 1950s). Whether or not this in-
fluenced the science being done is debatable, and per-
haps unknowable; but men and women often approach
the same situation or problem differently. This may be
especially evident in research concerning who was the
“leader of the pack.” Arguably, the very first wolf bi-
ologist, Adolph Murie, who studied wolves in Mount
McKinley National Park (now Denali National Park
and Preserve) in the 1930s and 40s set the stage for years
to come in this area of behavioral study. Murie closely
observed wolf behavior in the park and at one point in
his book The Wolves of Mount McKinley wrote, “He [the
alpha male] seemed more solemn than the others, but
perhaps that was partly imagined by me, knowing as I
did that many of the family cares rested on his shoul-
ders.” More recent research in Yellowstone and Elles-
mere Island indicates it may be the alpha (now called the
dominant breeder) female who runs the show.
- Doug Smith
Lisa Koitzsch currently works as a technician for the Yellowstone Wolf Project. She graduated from Johns Hopkins
University with a BA in Humanities and French Literature and worked for several years in publishing and administra-
tion. During the two intensive months of winter study, her main focus is downloading location data from GPS-collared
wolves, creating maps of clustered wolf locations, and coordinating searches of these clusters, which typically represent
feeding and resting locations, in order to estimate wolf-pack predation rates. Lisa has worked with the wolf project
every winter since 2008, when she and her husband, Ky, were hired as a two-person crew to conduct necropsies on
wolf-killed prey. In addition to her current work with the Yellowstone Wolf Project, Lisa and Ky are working on a three-
year noninvasive study estimating winter population size and vital statistics of moose in Yellowstone National Park’s
Northern Range.
Kira Cassidy (see page 42)
Erin Stahler (see page 54)
Brenna Cassidy is a Biological Technician with the Yellowstone Wolf Project. She graduated from University of
Wisconsin-Stevens Point as a Wildlife Ecology major in 2012 and moved to Yellowstone National Park shortly after to
participate in her first winter study. Since then, she has done six winter studies and has spent most of that time with the
Junction Butte pack. Brenna has worked on a number of projects in Yellowstone including the Raptor Initiative, the Core
Bird Program, and the Yellowstone Cougar Project. Studying multiple species has allowed Brenna to travel throughout
the park by plane, foot, canoe, and skis. Seeing the park through the eyes of multiple species has shown her that each
has a important role in the interconnected ecosystem of Yellowstone.
WOLF ISSUE.indb 43 6/7/2016 6:23:58 PM
... A primary cause of nonanthropogenic adult wolf (Canis lupus) mortality is other wolves (summarized by Mech and Boitani 2003), and much new information has been documented about such mortality (Cassidy et al. 2015(Cassidy et al. , 2016(Cassidy et al. , 2017Smith et al. 2015a). Explanations for intraspecific strife among wolves have included territorial defense (Murie 1944;Mech 1970;Mech and Boitani 2003;Stahler et al. 2013), because most wolf killing by other wolves both in northeastern Minnesota and Denali National Park, Alaska, has occurred along edges of wolf-pack territories or in adjacent territories (Mech 1994;Mech et al. 1998). ...
... Similarly, wolf-wolf killings of adults in YNP generally track these trends except for April. Although males are more aggressive and are especially important to winning interpack fights (Cassidy et al. 2015(Cassidy et al. , 2016(Cassidy et al. , 2017, similar numbers of both sexes were killed by conspecifics in the SNF and Denali. That testosterone and aggression peak around the breeding season accords with our findings and those of Mech and Boitani (2003) that wolf-wolf killings of adults generally are concentrated around the breeding season. ...
Of 41 adult wolf-killed gray wolves (Canis lupus) and 10 probably or possibly killed by wolves from 1968 through 2014 in the Superior National Forest (SNF) in northeastern Minnesota, most were killed in months leading up to and immediately following the breeding season, which was primarily February. This finding is similar to a published sample from Denali National Park, and the seasonality of intraspecific mortality generally parallels the known seasonality of testosterone levels, scent-marking, howling frequency, and general interpack aggression. Males and females were killed in the same proportion as in the population of radiocollared wolves. The annual rate of wolf-killed wolves was not related to the annual wolf density. Our findings tend to support intraspecific mortality of adult wolves as a means to reduce breeding competition and to maintain territories.
... Parameters like equilibrium density, territory size, number of migrants and proportion of emigration that need to be defined by the user give one way to account for spatial constraints given by a particular environment, and these parameters can be changed to best represent the study area of interest. That said, we acknowledge that an explicit spatial mechanism would be very interesting to implement as wolf pack occupancy is mainly driven by exclusive territoriality (Cassidy et al., 2016), but the population division and affiliation into packs approximated spatiality in our model. To add more spatial constraints without changing the model structure, a new individual characteristic could be defined to represent distances between individuals based on their pack affiliation. ...
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The occurrence of wolf populations in human-dominated landscapes is challenging worldwide because of conflicts with human activities. Modeling is an important tool to project wolf dynamics and expansion, and help in decision making concerning management and conservation. However, some individual behaviors and pack dynamics of the wolf life cycle are still unclear to ecologists. Here we present an individual-based model (IBM) to project wolf populations while exploring the lesser-known processes of the wolf life cycle. IBMs are bottom-up models that simulate the fate of individuals interacting with each other, with population-level properties emerging from the individual-level simulations. IBMs are particularly adapted to represent social species such as the wolf that exhibits complex individual interactions. Our IBM projects wolf demography including fine-scale individual behavior and pack dynamics based on up-to-date scientific literature. We explore four processes of the wolf life cycle whose consequences on population dynamics are still poorly understood: the pack dissolution following the loss of a breeder, the adoption of young dispersers by packs, the establishment of new packs through budding, and the different breeder replacement strategies. While running different versions of the IBM to explore these processes, we also illustrate the modularity and flexibility of our model, an asset to model wolf populations experiencing different ecological and demographic conditions. The different parameterization of pack dissolution, territory establishment by budding, and breeder replacement processes influence the projections of wolf populations. As such, these processes require further field investigation to be better understood. The adoption process has a lesser impact on model projections. Being coded in R to facilitate its understanding, we expect that our model will be used and further adapted by ecologists for their own specific applications.
... The closest reports seem to be a description of a lone female red fox being harassed by males who intruded on her den and eventually the entire litter died (Zabel, 1986;Zabel and Taggart, 1989) and Latham and Boutin (2011) suggested that the death of a gray wolf pup may have been infanticide by a male, though the evidence was inconclusive and might best be explained by intergroup resource competition. Furthermore, in contrast to infanticidal male takeovers in other taxa (e.g., Loveridge et al., 2007), quite the opposite has been reported in gray wolves: when a new and unrelated alpha wolf takes over a pre-existing pack, he provisions and cares for pups that are not his own, which may increase his acceptance by the pack (Cassidy et al., 2016). Thus, paternity confusion to prevent infanticide is an unconvincing explanation for extra-pair mating in canids. ...
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The Canidae are successful, being a widespread, abundant, speciose, and adaptable family. Several canids in particular have recently experienced rapid expansions in range and abundance, with similar situations mirrored on several continents by different species. Despite extreme behavioral diversity between and within species, monogamy is a common denominator in canid societies. In this review, we ask why canids are monogamous and how monogamy is related to their success. We begin with an overview of canid social monogamy, describing the pair bonding, paternal care, and often alloparental care that is characteristic of the family, and discuss theories on the evolution of mammalian social monogamy. We discuss why and how monogamy is maintained in canids, either voluntarily or enforced, and how ecological conditions influence either the functional advantages of monogamy or ability for enforcement and thus whether social monogamy is maintained. Social monogamy does not necessitate exclusive mating and many canids exhibit extra-pair paternity. We consider the costs and benefits of extra-pair mating for male and female canids and how ecological conditions can shift this cost/benefit balance and thus affect its prevalence. Monogamy may be responsible for many of the unusual canid reproductive characteristics through facilitating alloparental care and monogamy enforcement, and the domestic dogs' departure from monogamy supports our interpretation that it is an adaptation to resource availability. In asking whether monogamy is responsible, at least in part, for their success, we propose the monogamy as pro-cooperative hypothesis, suggesting four characteristics have contributed to canid success: (1) ecological flexibility, (2) high mobility, (3) high reproductive rates, and (4) sociality/cooperation, with the latter two being consequences of monogamy. These four interconnected traits enhance one another and it is their combination, with monogamy at its foundation enabling cooperative sociality and thereby enhanced reproduction and survival, that together comprise the formula of canid success.
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Evidence for territoriality is usually correlative or post hoc as we observe the results of past selection that are challenging to detect. Wolves (Canis lupus) are considered territorial because of competition for food (resource defense), yet they exhibit classic intrinsic behaviors of social regulation (protection against infanticide). This emphasis on prey and infrequent opportunity to observe wild wolf behavior has led to little investigation into the causes of or competitive underpinnings in the evolution of wolf territoriality. We report 6 cases of territorial wolf packs attacking neighboring packs at or near their den; 2 attacks were observed in detail. In all cases, except perhaps one, the attacking pack killed adult wolves either at the den or near it; in 4 cases, pups were probably lost. Loss of pups led to future loss of territory and in one case pack cessation. Intraspecific killing (measured in collared adults only) peaked in April, the month when pups were born and helpless in dens, even though aggressive interactions were at their seasonal low. Twelve of 13 (92%) of the wolves killed during the denning season (March, April, May) were reproductive (males and females), and 8 of 12 were dominant individuals (highest ranking wolf for that sex in the pack). Wolf-wolf killings were also high in October and December, the beginning and middle of the nomadic season, respectively. Aggressive interactions were more frequent during the nomadic season when wolves were roaming their territory as a group compared to the denning season when wolf activity was centered on the den and pack members less cohesive. We conclude that attacks on dens are a more effective form of interpack competition than interference during the breeding season, the current best-supported hypothesis, and that protected pup-rearing space is the primary cause of wolf territoriality.
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Knowledge of characteristics that promote group success during intraspecific encounters is key to understanding the adaptive advantages of sociality for many group-living species. In addition, some individuals in a group may be more likely than others to influence intergroup conflicts, a relatively neglected idea in research on social animals. Here we use observations of aggressive interactions between wolf (Canis lupus) packs over an extended period and use pack characteristics to determine which groups had an advantage over their opponents. During 16 years of observation in Yellowstone National Park from 1995 to 2010, we documented 121 interpack aggressive interactions. We recorded pack sizes, compositions, and spatial orientation related to residency to determine their effects on the outcomes of interactions between packs. Relative pack size (RPS) improved the odds of a pack displacing its opponent. However, pack composition moderated the effect of RPS as packs with relatively more old members (>6.0 years old) or adult males had higher odds of winning despite a numerical disadvantage. The location of the interaction with respect to pack territories had no effect on the outcome of interpack interactions. Although the importance of RPS in successful territorial defense suggests the evolution and maintenance of group living may be at least partly due to larger packs’ success during interpack interactions, group composition is also an important factor, highlighting that some individuals are more valuable than others during interpack conflicts.
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1.Although local variation in territorial predator density is often correlated with habitat quality, the causal mechanism underlying this frequently observed association is poorly understood and could stem from facultative adjustment in either group size or territory size. 2.To test between these alternative hypotheses we used a novel statistical framework to construct a winter population-level utilization distribution for wolves (Canis lupus) in northern Ontario, which we then linked to a suite of environmental variables to determine factors influencing wolf space use. Next we compared habitat quality metrics emerging from this analysis as well as an independent measure of prey abundance, with pack size and territory size to investigate which hypothesis was most supported by the data. 3.We show that wolf space use patterns were concentrated near deciduous, mixed deciduous/coniferous and disturbed forest stands favoured by moose (Alces alces), the predominant prey species in the diet of wolves in northern Ontario, and in proximity to linear corridors, including shorelines and road networks remaining from commercial forestry activities. 4.We then demonstrate that landscape metrics of wolf habitat quality - projected wolf use, probability of moose occupancy and proportion of preferred land cover classes - were inversely related to territory size but unrelated to pack size. 5.These results suggest that wolves in boreal ecosystems alter territory size, but not pack size, in response to local variation in habitat quality. This could be an adaptive strategy to balance tradeoffs between territorial defense costs and energetic gains due to resource acquisition. That pack size was not responsive to habitat quality suggests that variation in group size is influenced by other factors such as intra-specific competition between wolf packs. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.
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A breeding male Gray Wolf, Canis lupus, equipped with a GPS collar was documented going to the den site of another Gray Wolf pack. This trip was coincident with an attack on the den of the other pack and the occurrence of a dead and partially con - sumed Gray Wolf pup at the same location. We present two possible explanations-interspecific predation and non-parental infanticide-to account for this observation. Because the Gray Wolf with the GPS collar and his mate were first-time breeders and were attempting to establish a territory space of their own, we speculate that, based on the available evidence, this observation most likely represents a case of non-parental infanticide that fits the predictions of the resource competition hypothesis.
Aggressive behaviour is important in the life history of many animals. In grey wolves (Canis lupus), territory defence through direct competition with conspecifics is severe and often lethal. Thus, performance in aggressive encounters may be under strong selection. Additionally, grey wolves frequently kill large dangerous prey species. Because both sexes actively participate in aggressive activities and prey capture, wolves are expected to exhibit a low level of musculoskeletal sexual dimorphism. However, male wolves more often lead in agonistic encounters with conspecifics and must provision the nursing female during the pup-rearing period of the breeding season. These behaviours may select for males that exhibit a higher degree of morphological adaptation associated with aggression and prey capture performance. To test this prediction, we assessed skeletal sexual dimorphism in three subspecies of grey wolves using functional indices reflecting morphological specialization for aggression. As expected, sexual dimorphism in skeletal shape was limited. However, in two of three subspecies, we found sexually dimorphic traits in the skull, forelimbs and hindlimbs that are consistent with the hypothesis that males are more specialized for aggression. These characters may also be associated with selection for improved prey capture performance by males. Thus, the sexually dimorphic functional traits identified by our analysis may be adaptive in the contexts of both natural and sexual selection. Several of these traits may conflict with locomotor economy, indicating the importance of aggression in the life history of male grey wolves. The presence of functional specialization for aggression in a generally monogamous species indicates that sexual dimorphism in specific musculoskeletal traits may be widespread among mammals.
Reproduction in social organisms is shaped by numerous morphological, behavioural and life-history traits such as body size, cooperative breeding and age of reproduction, respectively. Little is known, however, about the relative influence of these different types of traits on reproduction, particularly in the context of environmental conditions that determine their adaptive value. Here, we use 14 years of data from a long-term study of wolves (Canis lupus) in Yellowstone National Park, USA, to evaluate the relative effects of different traits and ecological factors on the reproductive performance (litter size and survival) of breeding females. At the individual level, litter size and survival improved with body mass and declined with age (c. 4-5 years). Grey-coloured females had more surviving pups than black females, which likely contributed to the maintenance of coat colour polymorphism in this system. The effect of pack size on reproductive performance was nonlinear as litter size peaked at eight wolves and then declined, and litter survival increased rapidly up to three wolves, beyond which it increased more gradually. At the population level, litter size and survival decreased with increasing wolf population size and canine distemper outbreaks. The relative influence of these different-level factors on wolf reproductive success followed individual > group > population. Body mass was the primary determinant of litter size, followed by pack size and population size. Body mass was also the main driver of litter survival, followed by pack size and disease. Reproductive gains because of larger body size and cooperative breeding may mitigate reproductive losses because of negative density dependence and disease. These findings highlight the adaptive value of large body size and sociality in promoting individual fitness in stochastic and competitive environments.