Territoriality and Inter-Pack Aggression in
Gray Wolves: Shaping a Social Carnivore’s
Rudyard Kipling’s Law of the Jungle Meets
Yellowstone’s 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
NPS PHOTO - D. SMITH
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.
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
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
-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.
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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.
PHOTO © R. DONOVAN
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LEADING THE WAY:
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
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.
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