Environmental Uncertainty and the Global Biogeography of Cooperative Breeding in Birds

Article (PDF Available)inCurrent biology: CB 21(1):72-8 · March 2011with129 Reads
DOI: 10.1016/j.cub.2010.11.075 · Source: PubMed
Abstract
Understanding why organisms as different as amoebas, ants, and birds cooperate remains an important question in evolutionary biology. Although ecology can influence cooperation and conflict within animal societies and has been implicated in species differences in sociality, the environmental predictors of sociality across broad geographic and taxonomic scales remain poorly understood. In particular, the importance of temporal variation in selection pressure has been underestimated in most evolutionary studies. Environmental uncertainty resulting from climatic variation is likely to be an important driver of temporal variation in selection pressure and therefore is expected to impact the evolution of behavioral, morphological, and physiological traits, including cooperation. Using a data set of over 95% of the world's birds, we examine the global geography and environmental, biotic, and historical biogeographic predictors of avian social behavior. We find dramatic spatial variation in social behavior for which environmental and biotic factors--namely, among-year environmental variability in precipitation--are important predictors. Although the clear global biogeographic structure in avian social behavior carries a strong signal of evolutionary history, environmental uncertainty plays an additional key role in explaining the incidence and distribution of avian cooperative breeding behavior.
Current Biology 21, 1–7, January 11, 2011 ª2011 Elsevier Ltd All rights reserved DOI 10.1016/j.cub.2010.11.075
Report
Environmental Uncertainty
and the Global Biogeography
of Cooperative Breeding in Birds
Walter Jetz
1,3,
*
and Dustin R. Rubenstein
2,3
1
Department of Ecology and Evolutionary Biology, Yale
University, 165 Prospect Street, New Haven, CT 06520, USA
2
Department of Ecology, Evolution and Environmental
Biology, Columbia University, 10th Floor, Schermerhorn
Extension, 1200 Amsterdam Avenue, New York, NY 10027,
USA
Summary
Understanding why organisms as different as amoebas,
ants, and birds cooperate remains an important question
in evolutionary biology. Although ecology can influence
cooperation and conflict within animal societies and has
been implicated in species differences in sociality [1], the
environmental predictors of sociality across broad
geographic and taxonomic scales remain poorly understood
[2]. In particular, the importance of temporal variation in
selection pressure has been underestimated in most evolu-
tionary studies [3, 4]. Environmental uncertainty resulting
from climatic variation is likely to be an important driver of
temporal variation in selection pressure and therefore is ex-
pected to impact the evolution of behavioral, morphological,
and physiological traits, including cooperatio n [5]. Using
a data set of over 95% of the world’s birds, we examine the
global geography and environmental, biotic, and historical
biogeographic predictors of avian social behavior. We find
dramatic spatial variation in social behavior for which envi-
ronmental and biotic factors—namely, among-year environ-
mental variability in precipitation—are important predictors.
Although the clear global biogeographic structure in avian
social behavior carries a strong signal of evolutionary
history, environmental uncertainty plays an additional key
role in explaining the incidence and distribution of avian
cooperative breeding behavior.
Results and Discussion
Cooperative breeding systems, in which more than two indi-
viduals in a group care for young, are more common in birds
than once thought [6], with at least 9% of all passerines [7]
and nearly 20% of those species with biparental care [8] exhib-
iting this complex social behavior. Although the inclusive
fitness benefits of helping relatives ultimately set the stage
for the evolution of cooperative breeding in most cases [6],
environmental factors have long been thought to influence
the reproductive costs and benefits of this behavior, as well
as its incidence across species and regions [9]. Numerous
studies have demonstrated the importance of territory quality,
access to breeding sites, resource availability, and other
ecological factors in influencing reproductive and dispersal
decisions in cooperatively breeding species (e.g., [10, 11]).
Despite this long-standing emphasis on the role of ecology
in the evolution of cooperative breeding behavior in birds,
few early studies showed a strong relationship between the
interspecific incidence of cooperative breeding and environ-
mental conditions [12–16]. Although more recent comparative
analyses have suggested that climatic variables like tempera-
ture [17] and rainfall [5] may be related to patterns of sociality in
some groups of birds, evidence for interspecific differences in
the ecologies of cooperative and noncooperative vertebrates
has been equivocal at best [2]. Other studies argue that evolu-
tionary history explains the patterns of avian sociality better
than environmental factors do [18–21]. Whereas some suggest
that life history traits and other biotic factors predispose
certain avian lineages to cooperative breeding, with ecological
conditions only acting to further facilitate the behavior [20, 22],
others argue that the two factors work in concert to promote
avian sociality [23].
Strong environmental effects on avian sociality should leave
a visible geographic signature. Although the prevalence of
cooperative breeding in places like Australia [16, 24] and
Sub-Saharan Africa [13, 14
] has long been recognized, to
date, patterns have not been assessed on a global scale. If
environmental determinants of avian sociality in birds exist,
how much of its geographic distribution can they explain?
And for which clades and regions are other drivers, such as
evolutionary history, clade biogeography, and other determi-
nants unconnected with contemporary environment [18, 19,
21], important? Here, we integrate these different environ-
mental, biotic (life history), and historical biogeographic
(phylogenetic) factors and evaluate their relative contributions
to avian sociality. We begin by addressing, for the first time,
the explicit biogeographic distribution of cooperatively
breeding species using a data set of nearly all birds (see
Supplemental Experimental Procedures available online) [7].
We then evaluate the relative importance of environmental
(mean annual, among-year, and within-year variation in rainfall
and temperature) and biotic (body mass, diet breadth, and diet
type) factors on the global patterns of sociality in birds in
a historical biogeographic framework [25].
We find that the world’s 831 cooperatively breeding bird
species (8.9% of all nonmarine birds) exhibit a distinctly
nonuniform geographic distribution, with the highest species
richness in many parts of Sub-Saharan Africa, southwestern
Australia, parts of the Amazon basin, the Himalayas, and
New Guinea (Figure 1A). Across the world’s primary biogeo-
graphic realms [26, 27], the Afrotropics (268 species, 15%)
and Australasia (169 species, 12%) harbor proportionally
more cooperative breeders than the Nearctic (25 species,
7%), Palearctic (45 species, 6%), Indomalaya (98 species,
7%), and Neotropics (218 species, 6%). These broad-scale
differences are exacerbated at finer scales (Figure 1B), with
cooperative breeders representing over 20% of all bird
species in some African, and over 30% of all bird species in
select Australian, bird assemblages (Figure 1B). Additionally,
cooperative breeding behavior is slightly more common in
passerine (583 species, 10%) than nonpasserine (248 species,
7%) species, the two major evolutionary groups in birds
(generalized linear model, Akaike information criterion
[AIC] of null model = 5604; AIC of model fitting group
*Correspondence: walter.jetz@yale.edu
3
These authors contributed equally to this work
Please cite this article in press as: Jetz and Rubenstein, Environmental Uncertainty and the Global Biogeography of Cooperative
Breeding in Birds, Current Biology (2011), doi:10.1016/j.cub.2010.11.075
membership = 5578; z = 5.16, p < 0.001). Although both groups
contribute to the exceptional prevalence of cooperative
breeders in Africa, the high levels in Australasia are almost
exclusively driven by passerine species (Figures 1C and 1D).
In contrast, nonpasserine cooperative breeders are more
prevalent in the Neotropics and account for much of the rich-
ness of cooperative breeders seen in the Amazon basin
(Figures 1C and 1D). In general, cooperative breeders are
much less common at higher latitudes, as has been pointed
out previously for some groups of birds [19]. Overall, the
dramatic spatial variation reported here is remarkable because
its apparent geographic idiosyncrasy strongly surpasses other
avian traits analyzed at global scale thus far (e.g., clutch size)
[25], suggesting an important role for clade biogeographic
(evolutionary) history in addition to combined effects of envi-
ronmental (e.g., habitat structure and availability, climate)
and/or biotic (life history) predictors of avian social behavior.
Comparative studies of avian cooperative breeding behavior
have generally emphasized evolutionary history [18, 19, 21]
and life history predictors [17, 20] as being more important
than environmental predictors (but see [5]). Much of the diffi-
culty in testing for general environmental correlates of cooper-
ative breeding behavior in birds stems from inconsistencies in
the types of environmental variables used to describe the
ecological settings where most cooperatively breeding birds
occur. Cooperative breeders occur in both stable [17, 28]
and unstable [5, 16, 29], as well as in both seasonal [5, 13]
and aseasonal [15], environments. However, most compara-
tive studies have not actually quantified climatic seasonality
or stability, measures of environmental predictability that
quantify among- and within-year variability in climat e [5, 30].
Instead, those studies that have examined more quantitative
environmental correlates of avian cooperative breeding have
generally emphasized climatic means or extremes [12, 13,
17]. To evaluate the relative importance of different environ-
mental predictors of avian social behavior, we set prevalence
of cooperative breeding in relation to broad-scale environ-
mental niches of species, measured as both mean and
variation in environmental conditions found throughout
species’ global geographic ranges (see Supplemental Experi-
mental Procedures). Specifically, we characterized species in
terms of mean annual conditions (EnvMean), within-year varia-
tion (EnvVar within), and among-year variation (EnvVar among)
across their range using a 30-year climatic database of precip-
itation and temperature. In general, we found that among- and
within-year variation in temperature show latitudinal trends of
increasing variation with increasing latitude (Figures 2A and
2B), whereas among- and within-year variation in precipitation
exhibit more complicated patterns with generally greater vari-
ation in the tropics (Figures 2C and 2D). Analyses were per-
formed on the entire data set of all 9310 nonmarine avian
species (All across), but we also ran analyses separately on
passerines and nonpasserines because of their different
evolutionary histories and biogeographies [21, 31]. To control
for shared evolutionary history within lineages and to look for
evidence of phylogenetic signal in our data set, we also per-
formed a nested phylogenetic, or within-clades, analysis on
all species (All within) [25] using the 121 major avian clades
(see Supplemental Experimental Procedures).
In the across-al l-species analysis (All across), we found that
although cooperative breeders tend to occupy regions with
relatively low annual rainfall and high mean temperatures, vari-
ables capturing environmental variability (EnvVar) emerge as
much stronger predictors than variables capturing environ-
mental mean values (EnvMean) (Table 1). Specifically, both
high among- and within-year variation in precipitation posi-
tively affect cooperative breeding (Table 1; Figure 3A). In
contrast, cooperative breeders are slightly negatively associ-
ated with among-year temperature variation and are not
affected by within-year temperature variation (Table 1; Fig-
ure 3B). When considered in a framework that accounts for
evolutionary history, patterns in the nested phylogenetic anal-
ysis (All within) largely track those of the all-species analysis
(All across); the within-clades analysis confirms the impor-
tance of both high among- and within-year variation in precip-
itation (Table 1). Despite generally small magnitudes in the
Figure 1. Biogeographic Distribution of Cooperative Breeding Behavior in Birds
Total richness of cooperative breeders (all 9310 nonmarine species) (A) and proportional richness of all (B), passerine (C), and nonpasserine (D) cooperative
breeders. Maps are calculated across 110 3 110 km grid cells and displayed in quantile classification (i.e., equal number of cells in each class). The legend
depicts lower and upper values for each color class. See also Table S2.
Current Biology Vol 21 No 1
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Breeding in Birds, Current Biology (2011), doi:10.1016/j.cub.2010.11.075
differences between cooperative and noncooperative species
(Figure 3; see below for further analysis), our results demon-
strate that environmental variation in precipitation among
and within years is a key predictor of cooperative breeding
behavior in birds. Overall, our analysis of the environmental
predictors of avian sociality suggests that in general, (1) envi-
ronmental variability (uncertainty) is a relatively stronger
predictor than mean annual conditions, (2) variation in precip-
itation is a relatively stronger predictor than variation in
temperature, and (3) among-year variation is a relatively
stronger predictor than within-year variation (Table 1).
Although our analysis demonstrates that environmental vari-
ation—particularly in precipitation among years—is an impor-
tant predictor of avian social behavior, the patterns are
different for passerine and nonpasserine species. Whereas
environmental variability is a better predictor than mean annual
conditions for both passerines (Pass across) and nonpasser-
ines (Nonpass across), the relative importance of among-
versus within-year variation, and variation in precipitation
versus temperature, differs (Table 1). Cooperative passerines
are more likely to be found in areas of low mean annual and
high variation in rainfall, whereas cooperative nonpasserines
are found in areas of high mean annual and low variation in
temperature. We note that environmental associations are
much weaker in nonpasserines, where, unlike in passerines,
they become nonsignificant in a nested phylogenetic analysis
(Nonpass within, Table S1). Although this likely indicates key
behavioral ecological differences between the two groups, it
could also be due to the different evolutionary histories of the
groups and the fact that the passerine clades diverged more
recently than nonpasserine clades [31]. Nonetheless, this
distinction between the environmental predictors of coopera-
tive breeding behavior in passerine and nonpasserine species
likely explains much of the long-standing disagreement over
the role of environmental factors in the evolution of avian coop-
erative breeding [5, 12, 13, 15–17, 28, 29].
Because life history traits are also thought to be important
for explaining the incidence of cooperative breeding behavior
in birds [20, 22, 23, 28], we examined a suite of potential biotic
predictors (body mass, diet breadth, and diet type) of cooper-
ative breeding. We found no effect of body mass in the across-
all-species analysis (All across), but the within-clades analysis
(All within) suggests that cooperative breeders tend to be
larger than noncooperative breeders; this pattern is driven
by trends in passerines and not in nonpasserines (Table 1).
Additionally, there is a weak but significant trend for coopera-
tive breeders to have a wider diet breadth than noncooperative
breeders in the across-all-species analysis, but not in the
within-clades analysis; in general, cooperative breeders
(both passerines and nonpasserines) are less likely to be
carnivorous or herbivorous than noncooperative breeders
(Table 1). Overall, whereas in the across-species analysis,
biotic effects (AIC
d
= 102) are slightly stronger than those of
environmental conditions (AIC
d
= 94), this reverses in the
within-clades analysis (AIC
d
= 40 biotic versus 44 environ-
mental). This switch in relative importance confirms the
stronger clade-level phylogenetic signal in biotic predictors
compared to environmental predictors, something that was
also recently documented for other life history traits [25]. More-
over, the increase in overall model fit (to AIC
d
212 and 74 for All
across and All within, respectively) when including biotic
predictors to a model that just includes environmental predic-
tors supports the strong complementary role of life history
traits in addition to broad-scale environmental conditions in ex-
plaining the incidence of avian cooperative breeding (Table 1).
How well can the assessed environmental and biotic factors
together explain the incidence of cooperative breeding among
species and across geographic regions? Models combining all
environmental and biotic effects in the across-all-species (All
across) analysis are the best supported (Table 1) and differen-
tiate reasonably well among cooperative and noncooperative
species in both passerines and nonpasserines (Figures 4A
Figure 2. Global Patterns of Climatic Variability
Temperature (A and B) and precipitation variation (C and D) among (A and C) and within (B and D) years (the basis for log10-transformed species variables
TempVar within, TempVar among, PrecVar within, and PrecVar among in Table 1), calculate d as standard deviation s of log-transformed original values.
Colors range from most variable (dark red) to least variable (dark blue) (TempVar among: min 0.06, median 2.59, max 12.71; TempVar within: min 0.25,
median 18.83, max 79.21; PrecVar among: min 0.01, median 2.36, max 10.16; PrecVar within: min 0.23, median 2.98, max 19.94). Visualized across
55 km equal grid cells, natural breaks classification.
Environmental Uncertainty and Avian Sociality
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Breeding in Birds, Current Biology (2011), doi:10.1016/j.cub.2010.11.075
and 4B insets). Area under the curve (AUC) values of the
receiving operator characteristic (ROC) curve suggest sound
discrimination of the two groups by the across-species
models (AUC
Pass
= 0.67, AUC
Nonpass
= 0.70; Figure S1).
Applying the predictions of these best-supported models
(i.e., combining all variables) to the geographic occurrences
allows us to calculate the mean predicted probability that
bird species in a given assemblage are cooperative breeders.
We find that this prediction provides a strong fit (i.e., goodness
of fit) for the observed geographic variation in proportional
occurrence of cooperative breeders (Figures 1C and 1D;
Figures 4A and 4B; passerines r
s
= 0.66, nonpasserines r
s
=
0.68; n = 11,098 110 km cells). However, significant variation
remains, particularly in the case of passerines, where the vari-
ation is strongly geographically structured with the model
unable to predict the high and low prevalence of cooperative
breeders in Australian and Asian assemblages, respectively
(Figure 4A). Additionally fitting clade membership of species,
as done in the within-clades analyses, strongly improves the
interspecific predictions of cooperative breeding (Figures 4C
and 4D insets; Table S1), resulting in very good discrimination,
particularly for nonpasserines, where environmental and biotic
factors are weaker predictors than in passerines (AUC
Pass
=
0.87; AUC
Nonpass
= 0.92). Evaluation plots indicate higher levels
of specificity and sensitivity compared to the across-species
models (Figure 4; Figure S1). Accordingly, geographic fits for
all birds are strongly improved when controlling for clade-level
variation (Figures 4C and 4D; passerines r
s
= 0.71, nonpasser-
ines r
s
= 0.80; n = 11,098 110 km cells), particularly for Austra-
lian passerine assemblages (Figure 4C). Thus, our best-sup-
ported model—particularly when including phylogenetic
signal—does well in explaining interspecific differences in
avian social behavior. Lacking a fully resolved avian phylogeny
that would allow for the calculation of, e.g., Pagel’s lambda
and an analysis in a generalized least-squares setting [32],
the mixed-effects model used for the within-clades analysis
offers a powerful alternative [25]. However, we note that this
approach implies that some of the signal in the predictor vari-
ables is subsumed in that of phylogeny, as a result of conser-
vation of traits and environmental niches of clades. Because
several highly cooperative clades are restricted to regions
that also have high environmental variability (e.g., select
passerine groups in Australia), the attribution of relative phylo-
genetic versus environmental signal is not straightforward. For
interpretation, we therefore emphasize the consistent emer-
gence of key environmental variables associated with cooper-
ative breeding above and beyond the signal of evolutionary
history.
Overall, we show that birds exhibit dramatic spatial and
biogeographic structure in cooperative breeding behavior
Table 1. Environmental and Biotic Predictors of Cooperative Breeding Behavior in Birds
All across All within Pass across Nonpass across Pass versus Nonpass
z AIC
d
z AIC
d
z AIC
d
z AIC
d
EnvMean 70 27 55 27
Annual Temp 4.76 *** 22 2.02 * 2 2.55 * 5 4.52 *** 26 **
Annual Prec 23.18 ** 6 25.14 *** 24 24.97 *** 22 1.78 1 ***
EnvVar 82 35 82 33
TempVar within 21.72 21 20.80 22 1.22 21 24.12 *** 20 ***
PrecVar within 6.98 *** 42 6.73 *** 41 6.12 *** 33 3.25 ** 8
TempVar among 24.01 *** 14 20.67 22 21.73 1 23.99 *** 18 *
PrecVar among 7.32 *** 47 6.68 *** 41 7.89 *** 56 1.08 21**
Both among 78 43 66 20
Both within 46 42 32 31
TempVar both 26 2323 19
PrecVar both 44 43 55 19
Biotic 102 40 99 40
Body mass 20.06 24 6.46 ** 39 8.61 *** 69 21.85 1 ***
Diet breadth 3.69 *** 10 1.59 0 2.73 ** 5 3.21 8
Diet
Vert 24.02 *** 23.18 ** 20.04 22.97 **
Invert 6.11 *** 1.43 3.55 *** 3.22 **
Mixed 5.16 *** 93 3.80 *** 19 4.92 *** 41 2.42 * 35
Plants, seeds 26.24 *** 20.69 24.20 ** 24.27 ***
Fruits, nectar 21.62 22.02 * 22.52 * 1.70 **
EnvMean + EnvVar 94 44 94 38
EnvMean + EnvVar + Biotic 212 74 201 74
Results are based on generalized linear models across all bird species (All across, n = 9310 nonmarine species), all passerines (Pass across, n = 5756
species), all nonpasserines (Nonpass across, n = 3555 species), and a nested phylogenetic or within-clades model for all birds that controls for evolutionary
nonindependence of clades (All within, distinguishing 121 clades, generalized linear mixed effects model). The column ‘Pass versus Nonpass’ indicates
whether the slopes of single predictors for Nonpass across and Pass across are significantly different (based on the interaction between a predictor and
a categorical Nonpass/Pass variable in the All across data set). Positive z values indicate increased probability of cooperative breeding and for the cate-
gorical predictor Diet are based on linear contrasts. AIC
d
values are the difference between the Akaike information criterion (AIC) of the predictor model and
the null model with only intercept fitted (null AIC values: All across 5602; All within 4458; Pass across 3777; Nonpass across 1801); the AIC
d
values refer to
single/two-predictor models within the three variable groups and to multipredictor models across them (count of predictor variables: EnvMean: 2, EnvVar: 4,
Biotic: 3). Values are comparable within and across each of the three variable groups, with larger values indicating stronger fit and allowing comparisons of
relative importance of single predictors or predictor categories. Within each category, *p < 0.05, **p < 0.01, ***p < 0.001. For evaluation plots of these models,
see Figure 4 and Figure S1. See also Table S1.
Current Biology Vol 21 No 1
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Please cite this article in press as: Jetz and Rubenstein, Environmental Uncertainty and the Global Biogeography of Cooperative
Breeding in Birds, Current Biology (2011), doi:10.1016/j.cub.2010.11.075
and highlight the global ‘hot spots’ (e.g., Australia and Afro-
tropics) and ‘cold spots’ (e.g., Neotropics and Indomalaya)
of avian social diversity. We further demonstrate that geog-
raphy and clade history, together with environmental and
biotic (life history) factors, explain the worldwide distribution
of avian social behavior. Importantly, although exerting
a strong signal, phylogenetic history alone cannot fully explain
the observed overrepresentation of cooperatively breeding
species in places like Australia and Africa. Biotic factors like
body mass and diet are among the strongest overall predic-
tors; however, they exhibit strong phylogenetic signal, consis-
tent with the hypothesis that life history traits may predispose
certain lineages toward cooperative breeding behavior [17, 22]
but then work in concert with environmental factors within
those lineages [23]. Notably, environmental factors are strong
predictors of avian sociality above and beyond the effects of
clade membership. In particular, environmental variability is
a stronger predictor of avian sociality than mean annual condi-
tions, and variation in precipitation—particularly among
years—is a stronger predictor than variation in temperature,
as has been suggested previously [5].
Our results demonstrate that even on a global scale, the inci-
dence of complex avian social behavior may be greatly influ-
enced by the fitness consequences of living in unpredictable
environments. Variable environments encompass a broad
range of climatic conditions that likely have important conse-
quences for behavioral, morphological, and physiological
adaptation because they pose a greater range of challenges
to survival and reproduction than predictable environments.
Individuals may be forced to adopt more generalist reproduc-
tive strategies, and cooperative breeding may therefore be
a conservative, ‘best of a bad job’ strategy to maximize
fitness when breeding conditions vary unpredictably from
year to year. Although this hypothesis emphasizes the role of
environmental variability and variation in offspring mortality
and production, adult mortality and longevity are also likely
to be important and similarly influenced by environmental
uncertainty [17, 20]. This idea is supported by recent empirical
studies of avian cooperative breeding behavior showing that
the fitness benefits of helping are most apparent in harsh
conditions [33] and that flexible reproductive strategies allow
for more individuals to maximize their fitness during benign
conditions [34]. Thus, cooperative breeding as a flexible but
conservative reproductive strategy may allow individuals to
maximize fitness in both good and bad times.
Supplemental Information
Supplemental Information includes one figure, two tables, and Supple-
mental Experimental Procedures and can be found with this article online
at doi:10.10 16/j.cub.2010.11.075.
Acknowledgments
We thank S. Alonzo, A. Cockburn, N. Cooper, C. Botero, I. Lovette,
R. Ricklefs, and M. Uriarte for constructive feedback on previous versions
of this manuscript. We are grateful to C. Sekercioglu for sharing diet data
compiled from the literature. W.J. was supported by National Science Foun-
dation awards BCS 0648733 and DBI 0960550, and D.R.R. was supported
by Columbia University and a Miller Research Fellowship from the University
of California, Berkeley.
Received: August 31, 2010
Revised: October 26, 2010
Accepted: November 29, 2010
Published online: December 23, 2010
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Figure 4. Model Fits across Species and Grid Assemblages
Ability of the best-supported model (EnvVar + EnvAvg + Biotic) to predict species (box plots, insets) and geographic (scatter plots) variation in cooperative
breeding for passerines (A and C) and nonpasserines (B and D). Box plots show the predicted probability of a species to be cooperative according to the
EnvVar + EnvAvg + Biotic model (see Table 1) for known cooperative and noncooperative breeders (n = 9310 species; see Figure S1 for additional ev aluation
plots); for details on box plots, see Figure 3. Scatter plots illustrate how these species predictions, averaged across all members of a 110 km grid cell assem-
blage (‘‘mean predicted probability cooperative’’), are able to predict the observed proportional richness of cooperative breeders, i.e., the geographic
patterns shown in Figure 1B (n = 11,098 cells). The across-species (All across) model does not account for phylogeny, whereas the within-clades (All within)
model addresses phylogeny by fitting clade membership (see Table 1). In the scatter plots, dotted lines illustrate a 1:1 fit, and symbol colors indicate which of
the six primary biogeographic realms (see inset map) a grid cell belongs to. See also Figure S1.
Current Biology Vol 21 No 1
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Breeding in Birds, Current Biology (2011), doi:10.1016/j.cub.2010.11.075
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Environmental Uncertainty and Avian Sociality
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Please cite this article in press as: Jetz and Rubenstein, Environmental Uncertainty and the Global Biogeography of Cooperative
Breeding in Birds, Current Biology (2011), doi:10.1016/j.cub.2010.11.075
    • "Earlier comparative studies have investigated the association between eco-climatic and life-history factors and the occurrence of cooperative breeding, yielding sometimes contradictory findings10111213. While some studies have suggested that cooperative breeding is associated with stable climatic conditions and saturated habitats [10, 12, 15], other studies have indicated that it is associated with unpredictable climatic conditions [11, 16, 17]. These contradictory findings may in part reflect heterogeneity in the quality of data on the parental care mode of birds. "
    [Show abstract] [Hide abstract] ABSTRACT: Cooperative breeding is a widespread and intense form of cooperation, in which individuals help raise offspring that are not their own. This behaviour is particularly well studied in birds, using both long-term and comparative studies that have provided insights into the evolution of reproductive altruism. In most cooperatively breeding species, helpers are offspring that remain with their parents beyond independency and help in the raising of younger siblings. However, many cooperatively breeding species are poorly studied, and in 152 species, this behaviour only has been observed infrequently (i.e., occasional cooperative breeding). Here we argue that the parental care mode of these 152 species needs to be treated with caution, as factors associated with occasional cooperative breeding may differ from those associated with "regular" cooperative breeding. In most cooperatively breeding species, helpers provide alloparental care at the nests of their parents or close relatives; however, only in one occasionally cooperatively breeding species do offspring remain into the next breeding season with their parents. Accordingly, different factors are likely to be associated with regular and occasional cooperative breeding. The latter behaviour resembles interspecific feeding (i.e., individuals feed offspring of another species), which occurs when birds lose their brood and begin feeding at a nearby nest, or when birds mistakenly feed at another nest. Thus, we advise researchers to exclude occasional cooperative breeders in comparative analyses until their status is clarified, or to categorize them separately or according to the typically observed parental care mode. This approach will increase the robustness of comparative analyses and thereby improve our understanding of factors that drive the evolution of cooperative breeding.
    Full-text · Article · Dec 2016
    • "Several environmental factors have been evoked to explain the evolutionary causes of intraand interspecific clutch size variation, including predation pressure, seasonal variation in resource availability, abiotic factors that limit adult populations during the nonreproductive period, and heat exchange between eggs and the environment (Martin et al. 2000; Ricklefs 2000; Cooper et al. 2005 and Jetz et al. 2008). Cooperative breeding, in turn, is usually explained by low annual adult mortality, which is associated with increasing sedentariness and decreased environmental fluctuation (Arnold and Owens 1998; Jetz and Rubenstein 2011). Although empirical studies of clutch size and cooperative breeding in birds provide clear evidence of broadscale variation in reproductive traits, most seek to understand the underlying processes using the simplistic dichotomy between temperate and tropical regions or, at best, using latitude as an explanatory variable. "
    [Show abstract] [Hide abstract] ABSTRACT: Abiotic factors exert direct and indirect influences on behavioral, morphological, and life-history traits. Because some of these traits are related to reproduction, there is a causal link between climatic conditions and the expression of reproductive traits. This link allows us to generate predictions on how reproductive traits vary in large geographic scales. Here we formalize this macroecological framework, present some general predictions, and explore empirical examples using harvestmen as study organisms. Our results show that the length of breeding season in harvestmen is primarily influenced by the number of warm months and that precipitation plays a secondary role in modulating the period devoted to reproduction. Moreover, we show that the probability of resource defense polygyny increases with longer breeding seasons and that the presence of this type of mating system positively affects the magnitude of sexual dimorphism in harvestmen. Finally, the presence of postovipositional parental care is also influenced by the length of breeding season but not by actual evapotranspiration, which is our proxy for the intensity of biotic interactions. We argue that the macroecological framework proposed here may be a fruitful field of investigation, with important implications for our understanding of sexual selection and the evolution of reproductive traits in both animals and plants.
    Article · Sep 2016
    • "Despite the drawbacks and early criticisms of this approach, it has been shown to be largely accurate, and has proved to be an extremely valuable tool for a prolific field of research (Rodrigues et al., 2006). While further revisions and corrections are inevitable, we hope the classifications presented here provide a similar template for further study, both to refine the dataset and to underpin broad-scale tests of evolutionary theory, in line with previously published datasets of similar scope (Cockburn, 2006; Jetz and Rubenstein, 2011; Wilman et al., 2014). "
    [Show abstract] [Hide abstract] ABSTRACT: Communal signalling—wherein males and females collaborate to produce joint visual or acoustic displays—is perhaps the most complex and least understood form of communication in social animals. Although many communal signals appear to mediate competitive interactions within and between coalitions of individuals, previous studies have highlighted a confusing array of social and environmental factors that may explain the evolution of these displays, and we still lack the global synthesis needed to understand why communal signals are distributed so unevenly across large taxonomic and geographic scales. Here we use Bayesian phylogenetic models to test whether acoustic communal signals (duets and choruses) are explained by a range of life-history and environmental variables across 10328 bird species worldwide. We estimate that duets and choruses occur in 1830 (18%) species in our sample, and are thus considerably more widespread than previously thought. We then show that global patterns in duetting and chorusing, including evolutionary transitions between communal signalling and solo signalling, are not explained by latitude, migration, climate or habitat, and only weakly correlated with cooperative breeding. Instead, they are most strongly associated with year-round territoriality, typically in conjunction with stable social bonds. Our results suggest that the evolution of communal signals is associated with the coordinated defence of ecological resources by stable coalitions of males and females, and that other widely reported associations are largely by-products of this underlying trend.
    Full-text · Article · Jun 2016
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