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Behavioral Ecology Vol. 10 No. 1: 1-6
© 1999 International Society for Behavioral Ecology
Do life-history variables of European cuckoo hosts explain their egg-rejection behavior?
Departamento de Biología Animal y Ecología, Facultad de Ciencias, Universidad de Granada, E-18071 Granada, Spain
Address correspondence to J. J. Soler. E-mail: jsolerc{at}goliat.ugr.es
Received 26 August 1997; revised 4 January 1998; accepted 25 March 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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Recently, Brooker and Brooker suggested an equilibrium in the level of host defense and parasite counter-defense despite the passage of sufficient time for the evolution of host defenses through coevolution between brood parasites and their hosts. A long coevolutionary history of brood parasitism and nest predation has favored an adjustment of the host's life-history pattern to the point where total acceptance of a cuckoo egg is now an evolutionarily stable strategy. In a comparative study based on host species as independent observations, some predictions were tested for the European cuckoo (Cuculus canorus) and Horsfield's bronze cuckoo (Chrysococcyx basalis). In this article I reanalyze the predictions made by Brooker and Brooker using information on the European cuckoo and its hosts in the British Isles while controlling for common phylogenetic descent. Only 1 of the 12 predictions of Brooker and Brooker was supported using the new analyses, and none of the life-history variables was related to rejection behavior of the hosts of the European cuckoo, implying weak support for the hypothesis. Therefore, we conclude that when analyzing life-history variables that have a phylogenetic component, the use of modern comparative analyses is essential.
Key words: brood parasitism, coevolution, evolutionary equilibrium.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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Brood parasitism is a reproductive strategy in which the parasite lays its eggs in the nests of other species, which incubate the parasite's eggs and take care of the chicks. The high costs imposed by brood parasites on their hosts have been proposed to select for host defenses against the brood parasite (Rothstein, 1990)
. Host-egg
discrimination and rejection is one of the most important defense tactics,
which simultaneously selects for parasitic counter-defenses such as egg
mimicry (Davies and Brooke, 1988;
Rothstein, 1990). Levels of host defense
and parasite counter-defense are predicted to change in relation to the
duration of sympatry (Davies and Brooke,
1989a; Lotem and Rothstein,
1995; Øien et al.,
1995), an aspect that has received some experimental support
(Briskie et al., 1992;
Soler and Møller, 1990;
Soler et al., 1994). This scenario
indicates reciprocal selective influences between the parasite and its hosts
(Davies and Brooke, 1989b;
Moksnes et al., 1990), possibly resulting
in a coevolutionary arms race (Dawkins and Krebs,
1979).
Although parasites and hosts have been involved in a coevolutionary process
for a long time, not all host species are rejectors, and some host species
almost always accept parasitic eggs and raise parasitic chicks
(Brooke and Davies, 1988;
Brooker and Brooker, 1989;
Mason, 1986;
Rothstein, 1975;
von Haartman, 1981). There are two
general explanations for the lack of rejection behavior in hosts: (1)
the "evolutionary lag" hypothesis states that rejection behavior
has not had time to evolve in the host population
(Davies and Brooke, 1989b;
Dawkins and Krebs, 1979;
Rothstein, 1990), whereas (2) the
"evolutionary equilibrium" hypothesis states that, although there
has been time for the evolution of rejection behavior in hosts, putative costs
of rejection apparently outweigh the benefits, thereby preventing the
evolution of rejection (Davies and Brooke,
1989a; Lotem et al.,
1992, 1995;
Marchetti, 1992;
Rohwer and Spaw, 1988). Alternatively,
variation in host populations, related to individual host quality, its life
history, or environmental characteristics could allow an equilibrium at the
level of host recognition and rejection of cuckoo eggs
(Brooker and Brooker, 1996).
Brooker and Brooker (1996) recently
analyzed different host life-history and environmental variables in relation
to the lack of rejection behavior in the splendid fairy-wren (Malurus
splendens), a common host of Horsfield's bronze cuckoo (Chrysococcyx
basalis), concluding that a long coevolutionary history of brood
parasitism and nest predation has favored an adjustment of the host's
life-history pattern to the point where total acceptance of the cuckoo egg is
now an evolutionarily stable strategy. Moreover, a comparative study of both
hosts of Horsfield's bronze cuckoo and those of the European cuckoo
(Cuculus canorus) were used to analyze the relationship between
rejection rate, as an index of duration of coevolution and life-history traits
of suitable hosts. These researchers found that these two kind of variables
were related and that consequently host rejection behavior can be explained
only by life-history variables. This information was used to suggest a change
in the current paradigm concerning the European cuckoo and its hosts as a
classical example of an escalating coevolutionary arms race.
The comparative analysis fails in several respects: (1) the authors
did not control for common phylogenetic descent and therefore treated species
and different populations of the same species as statistically independent
observations. This point is highly important, given that many examples in the
literature demonstrate that lack of control for common phylogenetic descent
leads to erroneous conclusions (see examples in
Harvey and Pagel, 1991) or masks real
relationships (Soler and Møller,
1996). (2) The authors used data from different host populations
(even from different continents), which may imply differences in the value of
life-history traits (Martin and Clobert,
1996) and in the selection pressures suffered by hosts (e.g.,
parasitism rate; Davies and Brooke,
1989a; Moksnes et al.,
1993). Moreover, (3) the authors mixed hosts of the European
cuckoo and those of Horsfield's bronze cuckoo in the same analysis, but these
are two species that could impose different selection pressures on their
hosts. Because of this, the level of host defense against parasitism (e.g.,
rejection rate) could depend on the particular brood parasite species rather
than the life history of the host. For example, both dependent (rejection
rate) and independent variables (life-history traits, see above) may be
affected by confounding factors leading to an erroneous conclusion.
In summary, variables used in the comparative analysis by Brooker and
Brooker (1996) may be influenced by (1)
common phylogenetic descent of the host species, (2) the geographical area
from which the data came, and (3) the brood parasite species. Therefore,
further comparative analyses are needed to test the predictions by Brooker and
Brooker (1996) for the European cuckoo,
while controlling for all confounding factors by using (1) life-history data
of hosts from the same area, (2) hosts of only one species of brood parasite,
and (3) modern comparative methodology that controls for the effects of common
phylogenetic descent. In this article, I report tests of the predictions by
Brooker and Brooker (1996) for the
European cuckoo and its hosts, while controlling for common phylogenetic
descent, using data from hosts in Britain. The predictions based on the
Brooker and Brooker's (1996)
"evolutionary equilibrium" hypothesis are as follows:
- Frequency of rejection should increase as host mobility increases.
- Frequency of rejection should increase as the breeding season becomes
shorter.
- The type of strategy used by hosts (acceptance, desertion, or ejection)
should be correlated with the duration of the breeding season.
- Frequency of acceptance should increase as clutch size decreases.
- Species with the highest rejection rates should not be those whose eggs
are mimicked by the cuckoo.
- Species with the lowest rejection rates should be those that are most
frequently parasitized.
- The three categories of host strategies (acceptors, deserters, or
ejectors) are distinct when plotted against ability to renest, migration
status, putative costs of raising a cuckoo, and putative costs of
ejection.
- Acceptors should be sedentary species with a high probability of
renesting.
- Rejector species should be migratory species with low probability of
renesting.
- Acceptor species are those for whom the putative cost of raising the
cuckoo is lowest.
- Ejectors species are those for whom the putative cost of ejection is
lowest.
- Deserter species have costs between acceptors and ejectors (higher
putative cost of raising a cuckoo than acceptors and higher putative cost of
ejection than ejectors).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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Potential host species used in the analyses
To analyze features of potential cuckoo hosts, I used all British passerine species that had been recorded with a cuckoo egg in their nests and for which information was available in the literature for all variables used in the analyses (n = 22). However, when testing differences between acceptor, ejector and deserter species, I used only species that had previously been classified in one of these groups by Moksnes et al. (1991)
, or other host species for which
their behavior had been recorded (accepting, ejecting, or deserting) by Davies
and Brooke (1989a).
Variables analyzed in the model
I assembled information on the following variables for each potential
cuckoo host: (1) body mass: the mean value of those reported for
male and female by Perrins (1987). (2)
Clutch size, as the mean of maximum and minimum values reported by Perrins
(1987). (3) Number of broods per season,
from Perrins (1987). (4) Rejection rate,
as the mean value of those reported from various sources
(Davies and Brooke 1989a;
Moksnes et al. 1990), not only from
studies in the British Isles, but also from other European countries. I used
the mean value for the rejection rate because Soler and Møller
(1996) demonstrated a high repeatability
of estimates from different countries (repeatability = 0.73; SE =
0.13; F = 7.12; df = 13,16; p
=.0002; from Soler and Møller,
1996). (5) Degree of mimicry of European cuckoo eggs parasitizing
different host species as the percentage of cuckoo eggs found in their
corresponding host species reported by Moksnes and Røskaft
(1995). (6) Bill length, from the values
of British populations reported by Cramp and Perrins
(1993-1994). (7) Migratory status:
as did Brooker and Brooker (1996), I
classified host species in seven different groups based on their winter
quarters reported by Cramp and Perrins
(1993-1994) for the British
population: (i) year-round residents, (ii) primarily residents (migrate
<50 km), (iii) migrate >50 km but stay in the British Isles, (iv)
migrate outside the British Isles but to surrounding countries, (v) migrate to
southern Europe or North Africa, (vi) migrate south of the Sahara but not
farther than the equator, and (vii) migrate south of the equator. (8) Duration
of the breeding season in months, as the season for the occurrence of eggs
without the margins for early eggs and late broods reported in annual cycle
diagrams by Cramp and Perrins (1993-1994).
(9) Degree of renesting, calculated following Brooker and Brooker
(1996), as the duration of the breeding
season divided by the clutch size. (10) The putative cost of rejection,
calculated following Brooker and Brooker
(1996), as 1/(body mass + bill length).
(11) The putative cost of raising a cuckoo was defined by Brooker and Brooker
(1996) as the relative difference in size
between the host and the cuckoo fledgling; therefore we used only host
body mass. Finally, (12) when testing differences between acceptor, ejector,
and deserter species, using data from Moksnes et al.
(1991) and Davies and Brooke
(1989a), I classified species as (i)
acceptors if they accepted more than 50% of experimental eggs;
(ii) ejectors if they rejected more than 50% of the experimental eggs
and in more than 50% of the rejector nests the experimental eggs were
ejected; and (iii) deserters if they rejected more than 50% of the
experimental eggs and more than 50% of the rejector nests were
abandoned. See the appendix for the data set.
Statistical procedures
Because it is necessary to distinguish between the value of a trait due to
common phylogenetic descent and that due to coevolutionary history of hosts
and brood parasites (see Introduction), I used available comparative
methods.
I have used the passerine classification given in Howard and Moore
(1991) as a phylogeny
(Figure 1). Although the use of
phylogenies based on morphology (traditional cladistic classification) could
involve some problems, it is preferable to use the most available complete
information rather than making no analyses at all, although analyses should be
revised when the phylogeny involved becomes better known
(Garland et al., 1991). Moreover, several
recent studies have suggested that phylogenies based on molecular changes may
also include inaccuracies (Harvey et al.,
1992; Nee et al.,
1993), and there are many examples in the literature where
traditional cladistic classification has been used in comparative studies
(e.g., Hartley and Davies, 1994;
Keller and Genoud, 1997;
Owens and Bennett, 1994).
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In performing the analyses, I assumed polytomies between different species
within the same genus and between different genera from the same family;
i.e., I assumed that all species from the same genus (or all genera from the
same family) evolved simultaneously from a common ancestor (multiway
speciation events; see Purvis and Garland,
1993, for problems with polytomies, their implications, and
possible solutions). Hence, I set branch lengths of all species to the same
value (=1) (Garland et al., 1993;
Purvis and Garland, 1993). I also used
two methods to solve polytomies and assigned branch length, one developed by
Grafen (1989) and another developed by
Pagel and Harvey (1989). These methods
can be applied to imperfectly resolved phylogenies, as might be the case if a
taxonomy is used instead of a phylogeny, as in this study. To control for the
possible effects of common phylogenetic descent, I used Felsenstein's
(1985) independent comparison method as
implemented in a computer program developed by Garland et al.
(1993). This method finds a set of
independent pairwise differences or contrasts, assuming that changes along the
branches of the phylogeny can be modeled by a Brownian motion process
(successive changes are independent of one another), and that the expected
total change added together over many independent changes is zero
(Harvey and Pagel, 1991). Therefore,
pairwise differences in the phylogenetic tree are independent of each other
(Harvey and Pagel, 1991). The advantage
of independent-comparison approaches is that, by partitioning the variation
appropriately, all contrasts can be used to assess a hypothetical comparative
relationship (Harvey and Pagel, 1991).
Moreover, by the use of three different methods of assigning branch length,
the conclusions are stronger when the results are the same, regardless of
assuming polytomies or using methodologies to resolve them.
Some variables in the analyses could be interrelated, and to solve this problem, I previously carried out a principal component analysis (PCA; factor rotation: varimax normalized) using the values of contrasts of all dependent variables. However, the variables shared little variance, the eigenvalue of the second factor being less than 2 (eigenvalue factor 2 = 1.66), and the three first axes explaining only 67.5% of the variance. Therefore, for a better understanding of the results, I used the contrast value for each variable instead of the principal component coordinate for each factor.
Some variables introduced in the analyses were transformed to obtain approximately normal distributions of variables: body mass and clutch size were transformed to log (n); rejection rates and degree of mimicry were transformed to arcsin (n); the other variables already had approximately normal distributions. All variables with the calculated phylogenetic independent contrasts had approximately normal distributions (Kolmogorov-Smirnov tests, ns). All tests were two-tailed.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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With the use of European cuckoo hosts in the British Isles and control for common phylogenetic descent, only prediction number 5 given by Brooker and Brooker (1996)
was supported
(Table 1). Rejection rates were unrelated
to egg mimicry. All other predictions by Brooker and Brooker
(1996) were unsupported
(Table 1). Therefore, it appears that the
use of host species of different brood parasites and the absence of control
for common phylogenetic descent confounded the results presented by Brooker
and Brooker (1996).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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The association between brood parasites and their hosts has traditionally been viewed as a classical example of an escalating coevolutionary arms race (Davies and Brooke, 1989b
;
Dawkins and Krebs, 1979;
Rothstein, 1990). Under the arms race
hypothesis, differences between host species in rejection behavior reflect
different stages of a coevolutionary race (Davies and
Brooke, 1989b), with acceptors at an early stage (experiencing
evolutionary lag) and rejectors at a later stage. However, Brooker and Brooker
(1996) recently reported a correlation
between acceptance rates and host life-history traits and environment and a
negative relationship between rejection rate and the "resultant"
degree of egg mimicry.
Therefore, it seems that acceptance by cuckoo hosts cannot be due to
evolutionary lag, and egg rejection behavior cannot be responsible for egg
mimicry. Rather, host rejection has forced cuckoos to specialize mainly on
those acceptors and partial acceptors whose life-history strategies and
habitat allow them to cope with parasitism. Therefore, at least in the
Cuculus clade of cuckoos, mounting evidence supports a hypothesis of
evolutionary equilibrium in brood parasitism (Brooker
and Brooker, 1996).
In this study I reanalyzed data and predictions from Brooker and Brooker
(1996), while controlling for factors such
as common phylogenetic descent, and using data from only one country and only
one species of brood parasite, the European cuckoo. This new analysis clearly
shows no relationship between any life-history trait of hosts and their level
of rejection. Contrary to this prediction, Soler and Møller
(1996) showed that the rejection rate was
positively related to traits that evolved to facilitate cuckoo-egg
recognition, such as a low degree of intraclutch variation and a high degree
of interclutch variation, evidence that clearly supports the idea that egg
pigmentation is an evolutionary response to brood parasitism and that
coevolution took place (Øien et al.,
1995).
Brooker and Brooker (1996) also found
that acceptors, ejectors, and deserters varied in life-history traits,
although none of the predictions were supported when controlling for
confounding factors. On the contrary, Davies and Brooke
(1989a) found that species with smaller
bills suffered greater rejection costs (own eggs damaged; see also
Moksnes et al., 1991;
Rohwer and Spaw, 1988) and that these
species were more likely to reject model eggs by desertion than species with
larger bills, which tended to reject by ejection. Therefore, the apparent
relationship between host life-history traits and whether they are acceptors,
ejectors, or deserters could be simply because this kind of behavior is
related to bill length, which is a trait with a strong phylogenetic effect
(closely related species have similar bill lengths and, therefore, similar
costs of rejecting cuckoo eggs and life-history traits).
The only one of Brooker and Brooker's predictions supported by the new
analysis is number 5: species with the highest rejection rate should not
be those whose eggs are mimicked by the cuckoo. If the cuckoo egg mimicry
appears as a counter-defense mechanism against the ability of hosts to reject
cuckoo eggs, we should expect a positive relationship between these two
variables. However, because egg coloration and patterns have a phylogenetic
component (closely related species have similar kind of eggs compared with
nonrelated species), it is difficult to know which host species is mainly
responsible for the evolution of cuckoo egg mimicry. In this scenario, it is
possible to detect cuckoo egg mimicry in a group of species not responsible
for the evolution of that parasite counter-defense. Then, to test that
relationship it is first necessary to detect host species potentially
responsible for cuckoo egg mimicry. However, even in this case, a change in
cuckoo host selection to a closely related species will make the
interpretation of the results difficult because (1) although degree of mimicry
will be the same, the former host may gradually lose its capability of such
rejection behavior (see Cruz and Willey,
1989), and (2) the new host, not responsible for the cuckoo egg
mimicry, will have a low rejection rate. Moreover, if cuckoos select similar
nesting sites (Moksnes and Røskaft,
1987, 1995;
Wyllie, 1981), or seek nests completely
at random in the habitat where they were born and reared
(Brooke and Davies, 1991;
Moksnes and Røskaft, 1995), it
will be difficult to interpret the results. Therefore, to make clear
predictions of cuckoo-host coevolution, it is first necessary to clarify the
mechanisms governing cuckoo host selection and subsequently make
predictions.
In conclusion, in a new analysis of the predictions by Brooker and Brooker
(1996), I found weak support for the
evolutionary equilibrium hypothesis described by the authors between the
European cuckoo and its hosts. However, Brooker and Brooker's hypothesis might
still be correct, but they were not allowed to argue this based on current
data. Moreover, it is clear that when analyzing life-history variables, which
have a phylogenetic component, the use of modern comparative analyses is
essential.
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APPENDIX |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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Information on body mass (g), clutch size, rejection rate (%), classification of host species, degree of mimicry (from Moksnes and Roskaft, 1995)
, bill length
(mm), winter quarters (from Cramp and Perrins
1993-1994), migratory status, duration of the breeding season
(months), renesting ability (duration of breeding season divided by clutch
size), and cost of rejection (inverse of body mass multiplied by bill
length)
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXACKNOWLEDGEMENTSREFERENCES |
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I am most grateful to Juan Gabriel Martínez, Anders Pape Møller, Jose Javier Palomino, Manuel Soler, and Carmen Zamora for valuable comments on the manuscript. Funds were provided by a postdoctoral grant from the Ministerio de Educacion y Ciencia included in the project (DGCYT PB94-0785).
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