Linear Hierarchy Provides Context for Evolution of Social Cognition in Pinyon Jays

1
Linear Hierarchy Provides Context for Evolution
of Social Cognition in Pinyon Jays
Erin H. Strasser, Russell Benford, and Russell P.
Balda
Avian Cognition Laboratory, Department of
Biological Sciences, Northern Arizona University,
Flagstaff, AZ 86011
Abstract
Results
Discussion

The social complexity hypothesis suggests that
social cognition evolved in response to selective
pressures favoring skills that allow individuals
to identify and track the social status of
conspecifics.  This hypothesis predicts that
social cognition should evolve in species that
are socially complex, but not in species that are
socially simple.  Social cognition is evident in
the Pinyon Jay.  This species is social, yet the
organization of Pinyon Jay societies is not
entirely understood.  To determine if Pinyon Jay
societies are organized hierarchically, food was
provided to 9 captive jays, (5 males, 4 females)
and agonistic interactions among foraging
individuals were observed.  Winners and losers of
interactions were determined, and rank status was
calculated by comparing winloss ratios among
individuals.  Results indicate a linear hierarchy
involving all birds (h 0.75, p 0.04).  Mated
birds are ranked independent of their partners. 
To investigate factors that might affect an
individuals rank, numerous physical and
behavioral characteristics were measured.  Two
variables lower mandible length ( P 0.87, p
0.00), and body mass ( P 0.73, p
0.02)-correlated with rank.  Combined, these
results suggest that Pinyon Jay societies contain
linear hierarchies, and dominant birds achieve
social status with large size.  Linear
hierarchies in Pinyon Jay societies provide a
context for social cognition to evolve.
Pinyon Jay societies appear to be hierarchically
organized. The social rank of an individual
appears to be correlated with both body mass and
mandible length. Since both these features are
directly related to overall body size, it is
likely that body size is the morphological factor
most relevant to the establishment of social rank
in Pinyon Jays. Since the establishment of
social rank in this species often involves
competitive interactions such as pecking and
supplanting, this is not surprising. These
results confirm observations made by Marzluff and
Balda (1992). The hierarchy in this Pinyon Jay
flock is strongly linear. The linearity observed
in this study is comparable to linearities
observed in societies of social primates, such as
Hamadryas baboons14, chimpanzees27, and sooty
mangabeys19. Such species are capable of tasks
generally associated with social cognition. For
example, Hamadryas baboons are capable of social
manipulation12, assessing pair-quality2,
recognition of others intentions and inference
of target vocalizations11, and deception22.
Chimpanzees are capable of using gestural
signals25, individual recognition, revenge10,
transitive inference8, and deception6. And sooty
mangabeys form affiliative relationships and
coalitions19, use complex vocalizations1, and
have knowledge of third-party relationships18.
Many of these behaviors, including the use of
complex vocalizations, deception, individual
recognition, and transitive inference, have also
been observed in Pinyon Jays. Thus, results of
this study provide some support for the
hypothesis that similar social configurations
among Pinyon Jay and some social primate
societies have provided evolutionary contexts in
which socially cognitive skills have evolved.
If this hypothesis were correct, one might
expect social cognition to have evolved in a
majority of species in which strong social
hierarchies are present. However, this does not
appear to be the case. In species that exhibit
more strongly linear social structures than do
Pinyon Jays, such as northern pintails16,
mallards16, Eurasian tree sparrows26, freshwater
crabs23, and American bison21, social cognition
does not appear to have evolved. The apparent
absence of social cognition in these species does
not weaken the conclusions of this study.
Conditions that encouraged the evolution of
social cognition in Pinyon Jays and social
primates might not have been present in species
that exhibit social structure but do not exhibit
social cognition. For example, it is possible
that the fundamental neurophysiological
mechanisms required for social cognition were not
present in lineages where social cognition did
not evolve. Alternatively, it is possible that
the capacity to track temporal changes in social
structure is selectively relevant in species such
as Pinyon Jays and social primates, but not in
species such as freshwater crabs or American
bison. Additional research that investigates the
relevance of phylogenetic, physiological, and
ecological factors to the evolution of social
cognition is therefore required.
The social structure of the flock was strongly
linear (h 0.75, p 0.04).  Two tied
relationships and two observational zeros in
which there were no interactions between a dyad
occurred (Figure 1). Two morphometric
characteristics, each directly related to body
size, correlated with rank (lower mandible length
? 0.87, p 0.00 body mass ? 0.73, p
0.02) (Figures 2 and 3). Mated birds were ranked
independently of their partners (p 0 .41).
Dominant
Subordinate
Introduction
The social complexity hypothesis suggests that
social cognition evolves in response to selective
pressures favoring skills that allow individuals
to identify and track the social status of
conspecifics 7,5.  This hypothesis predicts that
social cognition should evolve in species that
are socially complex, but not in species that are
socially simple.  Pinyon Jays (Gymnorhinus
cyanocephalus) show evidence of social cognition
4,5. It is possible that social cognition
evolved in this species because of the complexity
of Pinyon Jay societies. Pinyon Jays are highly
social 15, yet the structural complexity of
Pinyon Jay societies is not entirely understood.
What type of social structure exists within
Pinyon Jay societies? One possibility is that
Pinyon Jay societies are linearly hierarchical. A
second possibility is that they are hierarchical,
but that the hierarchy is non-linear (consisting
of triads, etc.). A third possibility is that
Pinyon Jay societies are simple aggregations
without a measurable social structure. If
complex social structure in Pinyon Jay societies
is present, what factors are associated the
establishment and maintenance of that structure,
and/or the social ranks of individuals within
flocks? One possibility is body size. Body size
is associated with rank in African elephants 1
and in blue jays 24. Another possibility is
mandible length. Mandible length is believed to
be associated with rank in Pinyon Jays 15. These
factors, as well as plumage characteristics and
the rank of an individual's mate, were considered
in this study.
Figure 1. Matrix of dominance relationships
between 9 jays. A value of 1 indicates row
individual is dominant to column individual 0
column individual is dominant to row individual
0.5 tied dominance relationship 0 unknown
relationship.



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have h values most similar to this flock of
Pinyon Jays Those in blue have h values lower
than a flock of Pinyon Jays.
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Methods
To determine what, if any, social structure
exists in a Pinyon Jay population, individuals in
a captive flock of 9 Pinyon Jays (4 females, 5
males) were observed for 44 days in Dec 2005 -
Feb 2006 in a 7m x 5m x 3m enclosed aviary. At
the beginning of each observation period, the
flock was given 27 pinyon pine seeds in a 1m
square inside the aviary. Individuals were then
permitted to forage freely within and outside of
the square for 10 min. During that period, data
on agonistic interactions between a
pseudorandomly selected focal individual and that
individual's flock mates were recorded.
Dominance relationships between dyads were
determined by comparing winloss ratios of all
interactions among all pairs of individuals.
Social rank was determined by constructing a
matrix of dominance relationships among all
individuals. The degree of linearity (h') of the
social hierarchy in the flock was calculated
using deVries' (1995) improved linearity test
(1000 iterations). This test was used because
ties and observational zeros existed between some
dyads. To investigate possible relationships
between rank and morphometric characteristics,
right and left primaries 9 and 10, right and left
outer tail feathers, lower mandible length, bill
depth, bill width, bib length, bib width, and
body mass were measured. Spearman's rank tests
were used to determine if any of these
measurements correlated with social rank. To
investigate possible correlations between ranks
of mated individuals, differences in rank between
mates were calculated. A randomization test
(1000 iterations) was used to determine if
differences in these differences deviated
significantly from what would be expected from
chance.
Acknowledgements
Figure 2. The relationship between lower mandible
length and social rank in a flock of Pinyon Jays.
Spearmans rank coefficient ? 0.87, p 0.00
Figure 3. The relationship between body mass and
social rank in a flock of Pinyon Jays. Spearmans
rank coefficient P 0.73, p 0.02
This work was generously funded by a Hooper
Undergraduate Research Grant and Northern Arizona
University. We would like to thank Lauren Harter,
Christian Nunes, and Michael Barber for their
help with data collection and project
organization.