Behavioral Ecology Vol. 10 No. 5: 465-471
© 1999 International Society for Behavioral Ecology
Cooperative breeding in birds: the role of ecology
Department of Zoology and Entomology, University of Queensland, Brisbane 4072, Australia
Address correspondence to K. E. Arnold. E-mail: karnold{at}zoology.uq.edu.au .
Received 6 June 1998; revised 6 December 1998; accepted 18 January 1999.
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ABSTRACT |
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Theory predicts that cooperative breeding should only occur in species in which certain individuals are constrained from breeding independently by some peculiarity of the species' ecology. Here, we use comparative methods to examine the role of variation in ecology in explaining differences between taxa in the frequency of cooperative breeding. We address three questions. First, does the frequency of cooperative breeding vary at just one phylogenetic level, or across several levels? Second, are differences in the frequency of cooperative breeding among closely-related species correlated with ecology? Last, are ecological differences between ancient lineages important in predisposing certain lineages to cooperative breeding? We find that variation in the frequency of cooperative breeding occurs across all phylogenetic levels, with 40% among families and 60% within families. Also, variation in the frequency of cooperative breeding between closely related species is associated with ecological differences. However, differences in the frequency of cooperative breeding among more ancient lineages are not correlated with differences in ecology. Together, our results suggest that cooperative breeding is not due to any single factor, but is a two step-process: life-history predisposition and ecological facilitation. Low annual mortality predisposes certain lineages to cooperative breeding. Subsequently, changes in ecology facilitate the evolution of cooperative breeding within these predisposed lineages. The key ecological changes appear to be sedentariness and living in a relatively invariable and warm climate. Thus, although ecological variation is not the most important factor in predisposing lineages to cooperative breeding, it is important in determining exactly which species or populations in a predisposed lineage will adopt cooperative breeding.
Key words: birds, comparative methods, cooperative breeding, ecological constraints, mating system.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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An important challenge for comparative biology is to identify the key ecological differences among species that have, according to adaptationist explanations, led to behavioral variation. To this end, phylogenetic comparative methods map behavioral traits, such as social systems, onto a phylogeny and then test for concordant changes among ecological traits (e.g., Brooks and McLennan, 1991
;
Harvey and Pagel, 1991). Any
such concordance is taken as evidence of the link between behavior and
ecology, although cause and effect are notoriously difficult to tease apart
(see Harvey and Pagel, 1991).
Here, we use this comparative approach to test the adaptive hypothesis that
differences in the frequency of cooperative breeding between bird species are
ultimately due to differences between species in ecology
(Emlen, 1982,
1984,
1995,
1996,
1997;
Emlen and Vehrencamp, 1985;
Koenig and Pitelka, 1981;
Koenig et al., 1992;
Stacey, 1979).
Cooperative breeding, where more than two individuals provide care at a
single nest, is a rare behavior known to occur in only approximately 3.2%
(308/9672) of extant bird species (Arnold
and Owens, 1998; see also
Brown, 1987;
Cockburn, 1996;
Dow, 1980;
Edwards and Naeem, 1993).
Nevertheless, it is an intriguing behavior because of the apparent paradox of
individuals helping to rear the offspring of conspecifics. Why do some
individuals share or forgo their reproductive opportunities, rather than breed
independently? Theory has focused on the role of ecology in the evolution of
cooperative breeding (e.g., Davies and
Hartley, 1996; Davies et al.,
1995; Faaborg and Bednarz,
1990; Koenig and Mumme,
1987; Rabenold,
1990; Walters et al.,
1992). Most influentially, the ecological constraints hypothesis
suggests that peculiarities in the ecology of many cooperatively breeding
birds appear to limit the opportunities for independent breeding, thereby
promoting cooperative breeding (Brown,
1974; Emlen, 1982,
1984;
Emlen and Vehrencamp, 1985;
Gaston, 1978;
Koenig and Pitelka, 1981;
Koenig et al., 1992;
Selander, 1964;
Stacey, 1979). According to
the ecological constraints hypothesis, there should be consistent ecological
differences between cooperatively and noncooperatively breeding species. In
agreement with this prediction, empirical studies have demonstrated that the
availability of nest sites and food are the most common ecological constraints
on independent breeding for a wide range of cooperatively breeding species
(see Smith, 1990). For
example, manipulative experiments have shown that breeding opportunities for
individual acorn woodpeckers (Melanerpes formicivorus) are controlled
by the distribution of acorn storage trees
(Koenig and Mumme, 1987).
However, in other species the role of ecology is less clear. Pruett-Jones and
Lewis (1990) found that mates,
as well as territory availability, constrain the breeding opportunities of
male superb fairy wrens (Malurus cyaneus).
Problems remain with the ecological constraints hypothesis. Although
certain cooperatively breeding species, such as the acorn woodpecker, have
obvious peculiarities to their ecology that may predispose them to cooperative
breeding, the case is not nearly so clear in other species
(Fry, 1972;
Rowley, 1976;
Smith, 1990). Indeed, it has
proven difficult to identify any common ecological correlates of cooperative
breeding among birds, despite some thorough cross-species studies (e.g.,
Cockburn, 1996;
Dow, 1980;
Du Plessis et al., 1995;
Edwards and Naeem, 1993;
Ford et al., 1988;
Poiani and Pagel, 1997). One
reason for this is that many studies have examined aspects of ecology that do
not translate easily into ecological limitations to breeding opportunities.
For instance, Ford et al.
(1988) found that cooperative
breeding is common in Eucalyptus woodland, but it is unclear why this
should be taken as supporting the idea of ecological constraints. Second,
authors have made sometimes conflicting general conclusions about the ecology
of cooperatively breeding birds based on the idiosyncrasies of their own
particular study species. For example, Emlen
(1982), working on bee-eaters
(Meropidae), states that cooperative species tend to be diet specialists,
whereas Brown (1987), working
on Aphelocoma jays, concludes that most cooperative breeders tend to
be omnivores. Confusing terminology also abounds in the literature.
Cooperative breeders have been described as living in habitats where the
climate is "unpredictable"
(Emlen, 1982;
Ford et al., 1988),
"harsh but stable" (Faaborg
and Patterson, 1981), "seasonal"
(Du Plessis et al, 1995;
Gaston, 1978), or
"aseasonal" (Ford et al.,
1988). Finally, it has been pointed out that noncooperatively
breeding species can also live under ecological conditions in which young
birds have a difficult time obtaining reproductive vacancies
(Koenig et al., 1992;
Smith, 1990;
Stacey and Ligon, 1991).
The overall aim of this study was to use phylogenetic comparative methods and a taxonomically diverse database to test the broad hypothesis that differences between species in cooperative breeding are associated with consistent ecological differences. We did not set out to identify every conceivable ecological correlate of cooperative breeding, but simply to ascertain the relative importance of certain ecological factors suggested in the literature. We aimed to address three specific questions. First, what is the phylogenetic pattern of variation in cooperative breeding? Does the frequency of cooperative breeding vary at just one phylogenetic level, or across several levels? Second, do closely related cooperatively breeding bird species share any common ecological features that distinguish them from noncooperatively breeding species? Last, based on family-level analyses, is there evidence that ecology is an important factor in predisposing certain lineages to cooperative breeding?
Before tackling these questions, we outline what we perceive to be the
weaknesses of our approaches. First, as our main database covers a wide
taxonomic range of birds, and includes data from a diverse range of sources,
the quality of the data may vary. Second, due to a paucity of
genetic-parentage studies, we use the behavioral definition of cooperative
breeding as any situation in which more than two individuals help rear a
single brood (Emlen and Vehrencamp,
1985). We have not split cooperative breeding according to whether
breeding is monopolized by a single pair or shared between several individuals
(Hartley and Davies, 1994;
Vehrencamp, 1983). Third, our
main focus is to understand the incidence of cooperative breeding in birds,
not why individuals show helping behavior. Nor do we distinguish between the
different origins of helpers or the type of help that they provide. Finally, a
general weakness of comparative analyses is that they can only test for
associated changes in traits, not their benefits. Hence, we do not address the
benefits of philopatry hypothesis, which highlights the advantages to helpers
of staying within the natal group, such as the increased probability of
inheriting a high-quality territory
(Koenig et al., 1992;
Stacey and Ligon, 1991).
Nevertheless, despite the limitations of our analyses, we hope that our
results will be useful because no previous study has examined the association
between cooperative breeding and ecology at different taxonomic levels to
determine the probable evolutionary route to cooperative breeding.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Taxonomic level of variation
To address the question of whether the frequency of cooperative breeding varies at just one phylogenetic level or across several levels, we used a modified form of the nested ANOVA method to determine how the total variation in cooperative breeding is distributed among taxonomic levels (Clutton-Brock and Harvey, 1977
; Pagel and Harvey,
1988). The nested ANOVA method was used to quantify the amount of
variation among species within the same family, versus the amount among
families within an order, versus that variation among avian orders. We
examined the variance in phylogenetic distribution of all 308 known
cooperatively breeding bird species (Arnold
and Owens, 1998) relative to the total 9672 extant bird species
(Sibley and Monroe, 1990).
(This estimate of the frequency of cooperative breeding is likely to be
conservative due to the lack of data on tropical species; see
Cockburn, 1996). First, we
scored each species as to whether it was cooperative (1) or noncooperative
(0). We then calculated the total variance among extant lineages and
subsequently calculated the proportion of this total variation that can be
explained by grouping species within families and grouping families within
orders. The remaining variation that cannot be explained by higher taxonomic
grouping is the remaining variation in cooperative breeding among species
within families (Pagel and Harvey,
1988).
Additionally, the incidence of cooperative breeding in key taxa (including
noncooperative outgroups) was mapped onto a phylogenetic tree using the
MacClade computer program (Maddison and
Maddison, 1992) to illustrate the phylogenetic distribution of
cooperative breeding. The topology of the tree used was based on the Sibley
and Ahlquist (1990) tapestry
phylogeny. Groups in which more than 50% of the species are cooperative were
marked as black squares, those in which less than 50% of species are
cooperative were marked as gray squares, and those containing no cooperatively
breeding species were marked as white squares. MacClade then indicated in
which lineages cooperative breeding is probably the ancestral state in black,
those in which it has relatively recently evolved in gray, those in which it
has never evolved in white, and those in which the evolutionary pathway is
unresolved in hatching.
Lower-level analyses
The second question we addressed was, do cooperatively breeding species
share any characteristic ecological features? We explored correlations between
changes in the frequency of cooperative breeding and our indices of breeding
ecology, feeding ecology, climate, and geographic range. Data were collated
from the literature on 79 definitely cooperatively breeding and 103
noncooperatively breeding species (Arnold
and Owens, 1998). We attempted to match each cooperatively
breeding species with one closely related and one more distantly related
noncooperatively breeding, sympatric species from the same family. The
database was balanced by including species from well-studied families in which
cooperative breeding has not been recorded. A total of 139 families
(Sibley and Monroe, 1990) were
represented in the database. We sought data for each species on the extent of
cooperative breeding, nine indices of feeding ecology, seven indices of
breeding ecology, and nine indices of climate.
For each species, frequency of cooperative breeding was scored on a four-point scale based on the proportion of nests at which more than two individuals contributed to the rearing of a single brood: 0 = <5% of nests cooperative (i.e., noncooperative; 103 species in our database), 1 = 6-35% (32 species), 2 = 36-75% (24 species), 3 = 76-100% (23 species). A rank scale was used rather than raw values to minimize the effects of annual and geographical variation in levels of and our knowledge of cooperative breeding.
We then identified indices of species' ecology that may limit breeding opportunities, and when a clear prediction could be made, we scored them from low to high constraint on independent breeding (see Table 1 for the indices and scoring system). First we measured food distribution. We also measured diet breadth for each species on a seven-point scale, which was defined as the total number of food types (carrion, vertebrates, invertebrates, fruit, nectar, seeds, foliage) in the diet (1 = specialist, 7 = generalist).
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We used the following indices to measure differences in breeding ecology between species: nest site availability, territoriality in the breeding season, territoriality in nonbreeding season, grouping in the breeding season, grouping in the nonbreeding season, and adult dispersal (see Table 1 for definitions and scoring system of these traits). Breeding density (nests per hectare) was also recorded for each species. When breeding density data from more than one study were available, we calculated a mean value.
To examine the role of climate, we used the lowest mean daily temperature in the coldest month across the range of a species (centigrade); highest mean daily temperature in coldest month across the range of a species; lowest mean daily temperature in hottest month across the range of a species; highest mean daily temperature in hottest month across the range of a species; maximum temperature range (= highest mean daily temperature in the hottest month—lowest mean daily temperature in the coldest month across the range of a species); minimum mean annual rainfall across the range of a species (millimeters); maximum mean annual rainfall across the range of a species; annual rainfall range (= maximum mean annual rainfall—minimum mean annual rainfall across the range of a species); and the absolute mean degrees latitude from the equator.
Finally, because a species' geographic range size could confound climate variables, multiple regressions were performed, correlating evolutionarily independent changes in cooperative breeding with the climatic variables, while controlling for changes in maximum latitude and longitude range of a species. We were unable to control for altitudinal effects due to the paucity of climatic data from mountainous regions.
Closely related species cannot be used as independent data points because
of the confounding effect of common ancestry (see
Harvey and Pagel, 1991). Those
problems of non-independence were controlled for by using the Comparative
Analysis by Independent Comparisons (CAIC) software program
(Purvis and Rambaut, 1995) to
identify and calculate evolutionarily independent contrasts at the lower nodes
only (Felsenstein, 1985;
Pagel, 1992). This method is
based on comparing evolutionarily independent changes in traits, rather than
species- or family-typical values. To emphasize that we are not referring to
individual species or families, we therefore only refer to changes in traits.
Our four-point scale of cooperative breeding was used as the dependent
variable and our indices of ecology, geography and climate as the independent
variables. A BRUNCH analysis was performed because our cooperative breeding
scores were discrete character states: BRUNCH uses parsimony to identify the
minimum number of changes necessary to account for the observed variation in
the dependent variable and then calculates changes in the independent
variables at those nodes only (Purvis and
Rambaut, 1995). We used the full Sibley and Ahlquist
(1990) phylogeny as our
phylogenetic topology for this and all subsequent analyses. Branch lengths at
family level and above were set from Sibley and Ahlquist's
(1990) tapestry phylogeny.
Branch lengths between genera in the same family were all set at the arbitrary
length of 2, and those between species in the same genus were set at 1.
We used two-tailed Wilcoxon signed-rank tests based on the independent
contrasts to test for associations between changes in the level of cooperative
breeding and changes in each independent variable among genera and species.
The Wilcoxon signed-rank test tests the hypothesis that, at nodes where the
dependent variable (i.e., cooperative breeding) increases, the independent
variable (e.g., diet breadth) is equally likely to increase or decrease
(Arnold and Owens, 1998;
Owens and Hartley, 1998).
However, by performing Wilcoxon signed-rank tests on changes in many
variables, it is possible that we could achieve a few significant results
simply by chance, and this was corrected for by using the Dunn-Sidak method
(Sokal and Rohlf, 1995). The
adjusted critical value when performing 18 tests was calculated to be 0.0027
(equivalent to the 5% confidence level).
Family-level analyses
Our third question was whether ecological differences are an important
factor in predisposing lineages to cooperative breeding. We tackled this
question by comparing the ecology of noncooperative species in families in
which cooperative breeding has been expressed with the ecology of species in
well-studied families in which cooperative breeding has never been recorded.
In this way, we can infer whether variability in ecology observed in species
today is a result of recent evolutionary events or more ancient
diversification (Arnold and Owens,
1998; Owens and Bennett,
1995, 1997;
Sillen-Tullberg and Møller,
1993). For example, if our initial, lower-level analyses reveal
that cooperatively breeding species tend to be omnivores, we can look at
whether the noncooperative species in those families are also omnivores. If
they are, then we can infer that omnivory probably preceded the evolution of
cooperative breeding in those taxa. Furthermore, if no such pattern is found
in noncooperative families, then it is probable that omnivory predisposes taxa
to cooperative breeding.
The incidence of cooperative breeding in each family was scored as either present or absent: 0 = family contains no cooperatively breeding species, or 1 = family contains cooperative species. Families that contain cooperatively breeding species only were excluded from the database, as cooperative breeding was likely to have been the ancestral state in that lineage. In all other analyses we are assuming that cooperative breeding is the derived state, as it is such a rare behavior in general.
Family-typical values of ecology were then estimated for each of 27 cooperative and 20 noncooperative families. Ecological data were collected on at least five noncooperatively breeding species per family (or the entire family if it contains fewer than five species). Each of those five species in each represented families was scored, as in the previous analysis, for feeding and breeding ecology indices (see Table 1), including diet breadth (total number of listed food types in diet) and breeding density (nests per hectare). The modal species-value of each ecological variable was taken as the family-typical value for that index. In a few of cases there was too much variability within a family to attribute a modal score, even when more species from that family were considered. In these cases we did not use that family in analyses with that variable. Cooperative species were excluded from the evaluation of family patterns.
We performed a new CAIC analysis to discover whether changes in the incidence of cooperative breeding in families was correlated with changes in the family-typical ecological indices, while controlling for phylogeny. CAIC's BRUNCH algorithm was again used to generate the evolutionarily independent contrasts at all higher phylogenetic nodes. We used two-tailed, one-sample Wilcoxon signed-rank tests to determine whether the incidence of cooperative breeding in families was associated with our ecological variables. Again, as multiple tests were performed, the Dunn-Sidak method was used to adjust the critical value. The critical value equivalent to the 5% confidence interval when performing nine tests was calculated as approximately 0.0055.
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RESULTS |
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Taxonomic level of variation in cooperative breeding
The frequency of cooperative breeding varied across all taxonomic levels. Of the total variation in cooperative breeding among bird species, 21.14% was among orders and 19.94% among families. The remaining 58.92% of variation was among genera and species. Furthermore, the phylogenetic reconstruction (Figure 1) suggests that, although cooperative breeding is probably the ancestral state in certain lineages, in other lineages it is a more recently derived state.
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Lower-level analyses
When the critical value was adjusted for multiple tests, increases in the
frequency of cooperative breeding were associated with significant increases
in sedentariness, group living in the breeding season, and the lowest
temperature in the coldest month (Table
2). Increases in cooperative breeding were also correlated with
significant decreases in the maximum temperature range
(Table 2). Other associations
were not significant.
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When the correction for multiple tests was not performed, increases in cooperative breeding were associated with increases in diet specialization, territoriality in the nonbreeding season, as well as group living in the nonbreeding season and increases the highest temperature in the coldest month and the lowest temperature in the hottest month (Table 2). Increases in the frequency of cooperative breeding also corresponded with decreases in the distance from the equator (Table 2). When variations in the size of species' maximum longitude and latitude range were controlled for, using a multiple regression model, all of the above climatic variables remained significant (Table 3). Changes in our other index of feeding ecology, food distribution, was not associated with changes in cooperative breeding. Furthermore, no correlations were found with changes in our other indices of breeding ecology, such as nest availability, breeding density, or territoriality in the breeding season, or climate, such as rainfall (Table 2).
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Family-level analyses
When the critical value was adjusted for multiple tests, changes in none of
our indices of ecology and climate were significantly correlated with changes
in the frequency of cooperative breeding
(Table 4). When the correction
for multiple tests was not made, changes in the incidence of cooperative
breeding in families were associated with increases in the incidence of
nesting in restricted sites and, in contrast to the species-level analysis,
decreases in sedentariness (Table
4).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Variation between species in the frequency of cooperative breeding does appear to be associated with ecological variation. However, ecological variation is not the sole determinant of differences between taxa in the frequency of cooperative breeding. Our results suggest that variation in the frequency of cooperative breeding is evenly spread across taxonomic levels, with approximately 40% of variation among families and orders and 60% below that level. These analyses support our earlier suggestion that certain lineages are predisposed to cooperative breeding (Arnold and Owens, 1998
; see
also Edwards and Naeem, 1993),
but emphasize the extent of plasticity within some lineages. This conclusion
is also supported by Figure 1,
which shows that, while cooperative breeding seems to be ancestral in some
lineages (e.g., corvids; Cockburn,
1996; Edwards and Naeem,
1993; Russell,
1989), it has also evolved more recently in other lineages (e.g.,
rails). This suggests that a single factor is not likely responsible for all
the observed variation in cooperative breeding. Hence, we performed analyses
at different phylogenetic levels. Furthermore, many lineages appear to be
predisposed to cooperative breeding, despite the fact that it is only
expressed in a limited number of species. Our species-level analyses of the ecological differences between cooperative and noncooperative species support the theoretical prediction that ecology is important in determining differences in closely related species. Compared to noncooperative sister species, cooperatively breeding species tend to live in regions where the winters are warm and the variation in temperature is low, thus enabling species to become sedentary and maintain territories all year round. Consequently, independent breeding opportunities will be limited as individuals will be forced to compete for breeding spots against year-round cooperatively breeding residents, who are familiar with the habitat.
Interestingly, several of the traditional ecological variables used to
measure variation in the availability of breeding opportunities (see
Emlen, 1991;
Ford et al., 1988;
Koeing and Pitelka, 1981;
Ligon et al., 1991;
Noske, 1991) were not
correlated with differences in cooperative breeding at the species level.
Neither changes in food distribution nor diet breadth seem to be correlated
with changes in cooperative breeding. Although not significant when we
controlled for the use of multiple tests, our analyses indicate that increases
in cooperative breeding corresponded with increases in diet specialization.
This supports the prediction that cooperative breeders have sufficiently
specialized requirements to limit not only optimal habitat for breeding, but
also marginal habitat in which to float
(Emlen and Vehrencamp, 1985;
Koenig and Pitelka, 1981).
Similarly, independent breeding opportunities do not seem to be consistently
constrained by absolute breeding density, although cooperative breeders may be
living at higher densities because groups maintain territories. Although
certain species, such as the red-cockaded woodpecker, may be limited by the
availability of nest sites, this pattern does not seem to be true for the
majority of cooperative breeders.
Conversely, the results of our family-level analyses, which examined the
role of ecology in predisposing lineages to cooperative breeding, found little
evidence for a link between ecology and cooperative breeding. Surprisingly,
changes in none of our ecological variables seemed to explain variation in the
presence of cooperative breeding in families when we controlled for the use of
multiple tests. This suggests that, overall, ecological variation is
relatively unimportant in predisposing lineages to cooperative breeding. For
example, there is a weak trend for the incidence of cooperative breeding among
families to covary with nest site availability. Although this result was not
significant when we controlled for the use of multiple tests, it may indicate
that cooperative breeding is more likely to occur in families that use rare
nest sites. The conventional explanation for this is that the limited
availability of nest sites has been a force selecting for cooperative
breeding. Individuals are thought to be prevented from breeding independently
due to this shortage of nest sites. However, we suggest another scenario, as
the majority of cooperatively breeding species build open nests in trees. The
key feature of nest sites such as holes and cliff ledges is not that they are
rare, but that they are safe. The move to hole and ledge nesting occurred
early in the evolutionary history of birds (140-40 million years ago) and is
associated with lowered adult mortality
(Martin, 1995;
Owens and Bennett, 1995). A
previous paper of ours examined the role of life history in the evolution of
cooperative breeding and showed that low adult mortality is a key factor
predisposing lineages to cooperative breeding
(Arnold and Owens, 1998). Thus,
we suggest that the low mortality associated with hole nesting, rather than
the low availability of such nest sites, is the key factor predisposing these
lineages to cooperative breeding.
Conclusions
Comparative studies alone cannot prove the direction of causality
unambiguously. However, by performing analyses at different taxonomic levels,
we attempted to determine the probable evolutionary sequence of events leading
to cooperative breeding in birds. Our analyses have demonstrated that,
although changes in ecology are associated with variation in cooperative
breeding among closely related species within a lineage, variation in ecology
is not the most important factor predisposing a lineage to cooperative
breeding. We suggest that the evolution of cooperative breeding, like that of
other mating systems (Owens and Bennett,
1997), is due to a combination of life-history predisposition and
ecological facilitation. In a previous paper
(Arnold and Owens, 1998) we
suggested that the key factor in predisposing lineages to cooperative breeding
is a slow life history with high adult survivorship. We argued that such high
survivorship would lead to slow territory turnover and thereby limit the
opportunities for independent breeding
(Arnold and Owens, 1998;
Cockburn, 1996;
Russell, 1989). Here, we
extend our previous argument to suggest that ecological variation is the key
factor that determines exactly which species or populations in predisposed
lineages will adopt cooperative breeding. In such predisposed lineages,
increases in factors such as sedentariness and year-round territoriality are
likely to lead to further reductions in territory turnover and an increased
likelihood of cooperative breeding. Thus, differences in ecology do play an
important role in the evolution of cooperative breeding, but ecology's role is
to facilitate such behavior in specific populations rather than predisposing
entire lineages to cooperative breeding. Predisposition to cooperative
breeding occurs primarily via changes in life history.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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We thank A. Purvis and M. Rambaut for providing a copy of the CAIC program; W. Koenig, R. Heinsohn, P. Weeks, and D. Putland for providing unpublished data; and P. Bennett, S. Blomberg, A. Cockburn, P. Dwyer, A. Goldizen, I. Hartley, B. Hatchwell, and I. Jamieson for discussing ideas and/or commenting on the manuscript. K.E.A. was supported by a Northcote Trust postgraduate scholarship. The database is available from K.E.A. on request.
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