Behavioral Ecology
and Sociobiology
Behav Ecol Sociobiol (1989) 25:141-145
9 Springer-Verlag 1989
Agonistic interactions in siskin flocks:
Why are dominants sometimes subordinate ?
J.C. Senar 1, M. Camerino 1, and N.B. Metealfe 2
1 Museu de Zoologia, Ap. 593, 08003 Barcelona, Spain
2 Department of Zoology, University of Glasgow, Glasgow G12 8QQ, UK
Received July 15, 1988 / Accepted April 18, 1989
Summary. Dominance interactions among captive
siskins were examined to see if the behavior of
dominants reduced the risk of subordinates leaving
the flock. The outcome of aggressive encounters
was related to the possession status of the two
birds (i.e., which bird was first to arrive at the
contested resource) and the type of aggression used
(i.e., display or attack). More dominant birds were
successful whether they were possessors or intruders, and whether they attacked or displayed.
When possessors, they tended to display, presumably because of the greater cost of attack and the
lack of substantial benefits associated with it.
When intruding, they tended to attack, possibly
because attack is slightly more successful than display. When initiating encounters against dominants, subordinates were more successful if they
were possessors than if they were intruders. Subordinates tended to use displays whether they were
possessors or intruders, even though when the
birds were intruding, displays were less successful
than attacks. Subordinates may use display when
intruding because attack holds a higher risk of retaliation. The fact that siskins can repel more dominant intruders merely by using displays suggests
that dominants, by respecting possession and allowing reversals, are able to reduce the likelihood
that subordinates will leave the flock. This may
be to the dominants' long-term advantage, since
they gain benefits from being in stable flocks.
Introduction
An animal that consistently wins agonistic encounters with another is said to be dominant over
Offprint requests to: J.C. Senar
that individual (Wilson 1975). Early descriptions
of dominance relationships stressed consistency
and predictability of encounters between group
companions and related it to fighting ability (see
Schein 1975). However, there is increasing evidence
that many other factors can be important in determining the outcome of a fight; the outcome cannot
always be predicted accurately on the basis of the
relative status of the opponents (Huntingford and
Turner 1987).
Social species that live in groups with stable
membership are highly relevant to this topic since
they are potentially faced with the greatest conflict
between aggression and gregariousness (Balph
1977). Living in permanent groups increases the
risk of localized competition for resources and,
hence, increases agonistic encounters. However,
despotism that leads to the driving off of subordinates may not be advantageous to dominants (Ekman 1987) since it can lead to group disruption,
the loss of the benefits of grouping, and as Hogstad
(1987) has pointed out, to the breakdown of the
entire social system. Therefore, the possibility arises for the evolution of mechanisms that reduce
despotism and increase tolerance in dominants (see
Rohwer and Ewald 1981; Hogstad 1987; Ekman
1987).
Cardueline finches are very good subjects for
the study of these topics. They flock all year around and show a high level of integration among
the members of their stable groups (Glfick 1980;
Senar 1984; Senar and Metcalfe 1988). Dominance
relationships in these birds have been widely described, and these relationships show frequent "reversals" in which dominants are defeated by lowerranking birds (i.e., Tordoff 1954; Coutlee 1967;
Thompson 1960; Senar 1987). The presence of
such reversals suggests that there may be contexts
in which dominant finches allow subordinates to
142
win encounters, possibly to reduce the risk of subordinates leaving the flock. The aim of this paper
is to examine the factors that determine the likelihood of subordinate siskins' (Carduelis spinus)
winning encounters with dominants. We focus on
the relationship between social rank of the opponents and outcome of aggressive encounters, taking into account the form of aggression (whether
threat or attack) and the possession asymmetry
of contestants (whether possessors or intruders).
Material and methods
The study was carried out with two captive groups of siskins.
Each group consisted of five adult birds, color-ringed for individual identification. Each group was housed in an outdoor
cage (100 x 50 x 60 cm) with three perches, two food containers,
and one water source, and was observed through a one way
glass panel. Only one bird at a time was able to feed or drink.
The feeders were replenished once a day, and observations
on interactions were made immediately afterwards. The experiment was conducted during June and July 1986 for one group
and during October and November 1986 for the other; the
mean duration of each daily recording session was half an hour.
A total of 46 (19+27) h of observations were made. In all
interactions, the first bird to display or attack was considered
the Actor, and the bird receiving the display the Reactor (Senar
1985a). The first bird to arrive at a resource (food container,
perch, or water) was considered the Possessor, and birds that
came later and challenged the Possessor were called Intruders.
A n individual was considered to have won an encounter if its
opponent demonstrated a submissive posture or withdrew
(Senar 1982, 1985a). Winners of encounters gained or retained
access to the feeder, water, or perch. Since all birds were highly
familiar with the resources and would be equally hungry, we
assumed that there would be no payoff assymmetries (Maynard
Smith and Parquer 1976) for the contestants.
A total of 3149 agonistic interactions were studied, and
for each one the following data were recorded: a) idemity of
the Actor and Reactor, b) identity of the Possessor and In-
truder, c) type of aggressive behavior used during the agonistic
interaction (i.e., Display or Attack; see Senar 1982), and d)
winner of the encounter.
A bird was considered to be dominant over another if it
won significantly more than 50% of the encounters between
them (Senar 1985a), as indicated by the binomial test (Siegel
1956). Since social hierarchies in siskins are nor linear (Senar
1985a), we use the terms " d o m i n a n t " and " s u b o r d i n a t e " to
refer only to a bird's relationship with the other bird in the
encounter ( t h e " relative value of dominance," Huntingford and
Turner 1987). Dominance relationships were statistically significant between all but two pairs of birds; agonistic interactions
within these pairs were excluded from the analyses. The two
groups had three birds in common; therefore, the duplicated
dyads were combined to produce one data set per pair of birds.
This produced a total of 15 separate dominant-subordinate
dyads, which formed the basis of the analyses.
The influence of possession, type of aggression used, and
dominance status on the success rate of agonistic encounters
were analyzed using a trifactorial ANOVA. Success rates were
calculated as arcsine transformations of the percentage of initiated encounters won by each individual in each dyad; these
were calculated only when there were more than 10 interactions
in each situation.
The relative use of displays and attacks was assessed in
a similar way, by calculating the (arcsine transformed) percentage of interactions that were displays for each bird in each
dyad, when both possessor and intruder.
Results
Dominants won 88% (SD=9.27, n = 15 dyads) of
their agonistic encounters and subordinates 12%
(SD = 9.20, n = 15). Dominants initiating the interaction (Actors) won on 98% of the occasions, and
subordinate actors on 61% (Table 1); this difference was highly significant (t=8.74, dr=24, P <
0.001).
While siskins won the majority of interactions that
they initiated, they were more likely to win if they
Table 1. Trifactorial table showing the success rate of dominant or subordinant siskins when initiating encounters, according
to their Possession status and the Type of Aggression used. The percentage of acts that were displays (as opposed to attacks)
is shown in the right hand column, in relation to dominance and possession status. The standard deviation and sample size
(no. of dyads) is given in parentheses. Significance levels of student's t-tests; * P < 0.05; * * P < 0.01 ;* * * P < 0.001 ; n s : not significant
Dominance
Status
Possession
Status
%
won
% won according to Type of Aggression
Attack
100%
(1,9)
%
Displays
Display
Dominant
98%
Possessor
99%
(1,4 15)
ns
99%
(1,5 15)
**
95%
(2,1 11)
90%
(8,6 15)
(1,7 15)
Intruder
96%
(4,9 15)
99%
(2,5 15)
Subordinate
61%
Possessor
89%
(0,2 9)
87%
(23,1 3)
ns
91%
(11,0 6)
95%
(7,2 6)
(11,7 15)
Intruder
51%
(16,2 9)
75%
(12,5 5)
**
33 ~
(16,9 6)
59%
(16,3 9)
31%
(15,8 15)
143
Table 2. Three-way A N O V A of the effect of dominance, possession status, and type of aggression used on success rate (arcsinetransformed percentage of encounters won) in agonistic encounter
Source of variation
F
df
Dominance
Possession
Type Aggression
Dom. x Possession
Dora. x Type Aggression
Possession x Type Agg.
Dora. x Poss. x Type Agg.
92.39
42.13
17.02
20.00
1.72
7.53
1.78
1,62
1,62
1,62
1,62
1,62
1,62
1,62
P
< 0.001
< 0.001
<0.001
<0.001
0.195
0.008
0.187
Table 3. Two-way A N O V A on the effect of dominance and
possession on the (arcsine-transformed) percentage of acts that
were displays (rather than attacks)
Source of variation
F
df
Dominance
Possession
Dominance x Possession
12.55
155.14
1.43
1,43
1,43
1,43
P
0.001
<0.001
0.239
were possessors than if they were intruders, and
if they used attacks rather than displays (Tables 1
and 2). This effect of possession is even more important for subordinates than for dominants (Tables 1 and 2), subordinates experiencing a greatly
increased success rate when possessors. In spite of
this higher success rate, subordinates were less likely to initiate encounters when they were possessors than when dominants were possessors (percentage of encounters initiated when possessors,
subordinates: 43 %, sd = 16.62, n = 11 ; dominants :
58%, sd=18.9, n = 1 5 ; t student=2.129, df=24,
P < 0.05). The significant interaction between possession and type of aggression indicates that the
behavior that leads to success depends on possession: to be successful, possessors should use displays whereas intruders should attack (Tables 1
and 2), this relationship being true for dominants
as well as subordinates (Table 2). Analysis of the
type of aggression used (i.e., the percentage of encounters that were displays) shows that this prediction holds (Tables i and 3).
Dominance also had a significant effect on the type
of aggression used, subordinates using more displays than dominants (Tables 1 and 3). Possessors
that initiated interactions exhibited similar types
of aggression, regardless of dominance status (t=
1.15, df= 19, ns). However, dominance status had
a significant effect on the behavior of Actors when
they were intruders; dominants often attacked,
whereas subordinates mainly used displays (Table 1; t=4.18, df=22, P<0.001).
Discussion
Success rate oJ" dominants and subordinates." Does
it vary with possession ?
Dominants can displace another individual without difficulty due to their bigger Resource Holding
Power (RHP; Parker 1974). Although their success
rate is significantly higher when possessors than
when intruders, they are very successful in either
situation (99% success vs 96%, respectively). For
subordinates the situation is quite different: they
have a much greater success rate when initiating
encounters against dominants if they are possessors than if they are intruders (success rate of possessors = 89% ; of intruders = 51%). This could be
due to the Prior Ownership effect, whereby the
first of two individuals arriving at a resource is
predicted to have an increased probability of success in encounters, irrespective of the individual's
relative Resource Holding Power (RHP) or social
rank (Maynard Smith 1982). Prior ownership has
been shown to be a significant factor in territorial
defense (Brown 1963; Davies 1978; Krebs 1982;
Nelson 1984) and in the integration of bird flocks
where resident individuals may be dominant over
immigrants (Balph 1979; Yasukawa and Bick
1983). It is clear that possession is far more important for subordinate siskins than for dominants;
dominants may alter their behavior according to
whether they are possessors or intruders (see below), but they tend to win the encounters that they
initiate regardless.
Since subordinates are far more successful
when they are possessors, one might predict that
they would mainly initiate encounters when in that
situation. However, this prediction is not supported by the data, which suggests that other factors are also important (see below).
To attack or to display?
In general, dominants and subordinates mainly use
displays. If physical attacks are unnecessary, displays are to be expected since they have a lower
energetic cost and carry less risk of injury than
attacks (Maynard-Smith 1982; Popp 1987 a).
The success rate of dominants is quite high in
all situations, whether they attack or display, and
whether they are the possessor or the intruder.
However, when intruding, they are slightly (but
significantly) more successful when performing attacks than when displaying. This may explain why
they are more likely to attack than display in this
context, the greater probability of success possibly
144
outweighing the increased cost of attack. In contrast, when dominants were possessors, they won
nearly all the encounters that they initiated whether they displayed or attacked. Given the greater
cost of attacks and the lack of substantial benefits
associated with them, it is, therefore, not surprising
that they performed many more displays than attacks when in this situation.
The behavior used in initiating an aggressive
interaction is even more important for subordinates. When the subordinate bird was the possessor, displays were as successful as attacks. However, when the subordinate was the intruder, it was
far more effective if it attacked. These success rates
did not, however, correctly predict the relative frequencies of attacks and displays. While subordinates favored displays when possessors (as predicted), they continued to use predominantly displays when intruding, despite the lower success of
this behavior. This may possibly be due to the risk
of retaliation: attacks on dominant birds have a
higher success rate but are also associated with
a significantly higher risk of a retaliatory attack,
which can lead to an escalated fight (Hinde 1981 ;
Senar 1982). This risk is possibly taken only when
the value of the resource is higher for the subordinate than for the dominant, so that the low-ranking bird is willing to incur a greater cost in order
to obtain this value (Popp 1987b).
The concept of respect
Maynard Smith and Parker (1976) suggested that
the outcome of encounters should largely be dependent on asymmetries between the protagonists.
These asymmetries are of three types. Differences
in R H P (i.e., fighting ability or dominance) and
in the pay-off (i.e., the value of the resource) are
thought to be most important, and uncorrelated
asymmetries (such as prior ownership (Stevens
1988)) are used only if R H P and pay-off asymmetries are not easily recognizable. In this study there
are clear RHP asymmetries, but nevertheless dominants give way to subordinates in some circumstances. Although dominants win nearly all encounters, they can lose when faced with a displaying subordinate that is a possessor. This has also
been found in baboons contesting for females or
food (Kummer etal. 1973; Sigg and Falet 1985)
and could be interpreted as the showing of respect
by dominants, which allows low-ranking group
members to feed unaffected by, and in the vicinity
of, those of higher rank (Senar 1985b; Sigg and
Falett 1985). The benefit to dominants of showing
such "respect" and reducing despotism might be
that they thereby retain "useful" subordinates
within the flock (Rohwer and Ewald 1981; Senar
1988). Nevertheless, they are capable of driving
off subordinates should the need arise (e.g., in
times of food shortage; Rohwer and Ewald 1981).
In this way, this respectful behavior would reduce
the level of conflict in what would otherwise be
competitive situations. It has an adaptive value for
subordinates as well as dominants since it maintains group cohesion with consequent advantages
such as reducing predation, finding food, etc.
(Senar and Metcalfe 1988).
"Respectful" behavior results in the appearance of reversals between the members of a group
in some agonistic encounters (in the sense given
by Sabine 1949). The fact that reversals appear
mainly in species that are socially organized in
flocks (B. Ens pers.comm.) supports the hypothesis
of birds showing "respect. "Instances of such flocking species that show reversals in a relatively high
proportion of agonistic encounters are Junco hyemalis (Sabine 1949), Quelea quelea (Shawcross
1982), Parus atricapillus (Hartzler 1970), Carduelis
tristis (Coutlee 1967), Carpodacus mexicanus
(Thompson 1960), Sturnus vulgaris (Ellis 1966), Arenaria interpres (Metcalfe 1986), and Tadorna tadorna (Patterson 1977). Species whose aggregations do not have a stable membership, or which
are aggregated merely as a result of overlapping
home ranges, do not appear to show "respectful"
behavior, e.g., Gallus domesticus (Schelderup-Ebbe
1922; Fischel 1927), Phasianus colchicus (Collias
and Taber 1951), and Haematopus ostralegus (Ens
and Goss-Custard 1984; B. Ens pets.comm.). In
these species dominants are recorded as winning
all their encounters with subordinate individuals.
Respectful behavior may, therefore, have evolved
in social species in order to retain subordinates
in the flock.
Acknowledgements. We are most grateful to P. Monaghan, L.
Arias de Reina, F. Uribe, A. Omedes, T. Borras, B. Ens and
P.W. Ewald for critical comments on the manuscript or parts
of it. We also would like to thank J. Nadal for his help, and
R. Nos for her continuous encouragement and advice. This
research was supported by a grant of the Obra Social de la
Caixa de Barcelona to JCS.
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