Downloaded from http://rspb.royalsocietypublishing.org/ on June 14, 2017
rspb.royalsocietypublishing.org
Research
Cite this article: Roznik EA, Sapsford SJ, Pike
DA, Schwarzkopf L, Alford RA. 2015 Conditiondependent reproductive effort in frogs infected
by a widespread pathogen. Proc. R. Soc. B 282:
20150694.
http://dx.doi.org/10.1098/rspb.2015.0694
Received: 25 March 2015
Accepted: 13 May 2015
Subject Areas:
behaviour, ecology, health and disease and
epidemiology
Keywords:
body condition, calling effort, chytridiomycosis,
fitness, life-history trade-offs
Author for correspondence:
Elizabeth A. Roznik
e-mail: [email protected]
†
Present address: Department of Integrative
Biology, University of South Florida, Tampa,
FL 33620, USA.
‡
Present address: School of Veterinary and Life
Sciences, Murdoch University, Murdoch,
Western Australia 6150, Australia.
Electronic supplementary material is available
at http://dx.doi.org/10.1098/rspb.2015.0694 or
via http://rspb.royalsocietypublishing.org.
Condition-dependent reproductive effort
in frogs infected by a widespread
pathogen
Elizabeth A. Roznik†, Sarah J. Sapsford‡, David A. Pike, Lin Schwarzkopf
and Ross A. Alford
School of Marine and Tropical Biology, James Cook University, Townsville, Queensland 4811, Australia
To minimize the negative effects of an infection on fitness, hosts can respond
adaptively by altering their reproductive effort or by adjusting their timing of
reproduction. We studied effects of the pathogenic fungus Batrachochytrium
dendrobatidis on the probability of calling in a stream-breeding rainforest frog
(Litoria rheocola). In uninfected frogs, calling probability was relatively constant
across seasons and body conditions, but in infected frogs, calling probability
differed among seasons (lowest in winter, highest in summer) and was strongly
and positively related to body condition. Infected frogs in poor condition were
up to 40% less likely to call than uninfected frogs, whereas infected frogs in
good condition were up to 30% more likely to call than uninfected frogs. Our
results suggest that frogs employed a pre-existing, plastic, life-history strategy
in response to infection, which may have complex evolutionary implications.
If infected males in good condition reproduce at rates equal to or greater
than those of uninfected males, selection on factors affecting disease susceptibility may be minimal. However, because reproductive effort in infected
males is positively related to body condition, there may be selection on
mechanisms that limit the negative effects of infections on hosts.
1. Introduction
Life-history traits of organisms, including growth, reproduction and longevity,
interact to influence fitness. Because these traits are constrained by resource availability, many organisms can adaptively modify their allocation of energy as their
circumstances change [1,2]. Resource allocation strategies can vary among individuals, with natural selection favouring individuals with resource allocation
patterns that enhance lifetime reproductive success [1,2]. Pathogens can influence
host fitness by reducing survival, and also by affecting reproductive success.
A pathogen can influence reproductive success not only by reducing the total
amount of resources available to a host, but also by altering the optimal pattern
of resource allocation for an individual once it becomes infected [3].
To maximize fitness, individuals typically maintain moderate levels of current reproductive effort; this results in a longer lifespan and production of more
offspring during their lifetime [1,2]. Individuals that are infected by a pathogen
may allocate resources differently, however, depending on their immune
defences and longevity. For example, it might be beneficial for infected individuals to preferentially allocate resources to immune responses to fight their
infections, and to invest fewer resources in gamete production or reproductive
behaviour. In many taxa that rely on energetically expensive vocalizations for
mate attraction (e.g. frogs, birds, insects), males that are infected may vocalize
less, and they may also alter their vocalizations in terms of rate, length, complexity and frequency [4 –6]. Males that vocalize less should attract fewer
mates, mate less often and produce fewer offspring. Other pathogen-induced
changes in sound production could also reduce fitness, because vocalizations
are subject to sexual selection by females. Infected males that reduce the rate,
length or complexity of vocalizations may be less attractive, because these
characteristics are often honest signals of overall genotypic fitness [7,8].
& 2015 The Author(s) Published by the Royal Society. All rights reserved.
Downloaded from http://rspb.royalsocietypublishing.org/ on June 14, 2017
calling behaviour and infection status both spatially (across
six sites differing in elevation) and temporally (this species
calls year-round), allowing robust tests of the hypothesis that
infection by B. dendrobatidis has sublethal effects that interact
with body condition to influence calling probability.
2. Material and methods
Proc. R. Soc. B 282: 20150694
The common mistfrog (L. rheocola) is a treefrog that occurs near tropical rainforest streams in northeastern Queensland, Australia [32].
By the mid-1990s, chytridiomycosis had extirpated this IUCNEndangered species [33] at higher elevations (more than 400 m)
throughout its geographical range [31]. However, many populations have subsequently recovered or recolonized these areas
[34] and now coexist with the pathogen [25]. Several aspects of
the ecology of L. rheocola make it ideal for examining how disease
influences calling effort across seasons and with body condition.
First, this tropical species calls and breeds year-round, although
reproductive behaviour decreases during the coolest weather
[32]. Second, individuals of this species that are calling on a given
night typically call throughout the night, allowing us to easily and
accurately assess calling behaviour. Third, active frogs are located
relatively easily, even when they are not calling. Males remain
near fast-flowing streams, where they are typically found on rocks
and streamside vegetation [32,35]. During a tracking study, male
frogs were located an average of 0.10 m (maximum: 1.80 m) from
the stream during winter, and 0.74 m (maximum: 3.75 m) away
during summer [35]. Finally, infection by B. dendrobatidis does not
alter the detection probability of males of this species, so infected
males are just as likely to be located as uninfected males [25]. As
with many stream-associated rainforest frogs, the behaviour of
females is poorly understood because they are observed much less
frequently than males [25,35,36].
We surveyed L. rheocola at six rainforest streams in northeastern
Queensland, Australia (table 1). All streams were surrounded by
tropical rainforest, characterized by dense vegetation composed
of large trees (10 m in height), vines, epiphytes, shrubs and herbaceous plants. Although our sites were in relatively undisturbed
rainforest, several sites were damaged by a tropical cyclone in
February 2011 [27]. Stream width varied from 5 to 10 m, and
streambeds were composed of rocks ranging in size from small
pebbles to large boulders (10 m in diameter). All streams contained
pools, runs and riffles, and most had several waterfalls.
We located adult L. rheocola by visually surveying frogs along a
400 m transect at each stream. Surveys were conducted over five consecutive nights at each site in each season, from June 2010 through to
October 2011 (except for spring 2011, when we sampled for one
night per site). We conducted two winter (June–July) and two
spring (October–November) surveys (2010 and 2011), one survey
in summer (January–February 2011) and one survey in autumn
(March–April 2011). We could not conduct a summer survey
during 2011 at Bobbin Bobbin Creek, because this site was inaccessible due to cyclone damage. We captured each frog as soon as it was
located visually. Prior to capture, we recorded whether each male
frog was calling. Individuals can be heard distinctly from a distance
of about 50 m, and our period of observation for detecting calling
behaviour was approximately 3–5 min.
We measured the body size of each captured frog (snout–urostyle length to 0.5 mm, and mass to 0.1 g), and we determined its
sex by the presence/absence of distinct nuptial pads. We estimated
a body condition index for each male frog using the residuals from
a linear regression of log10 transformed body mass on squareroot transformed snout–urostyle length for all male frogs sampled
[37]. The resulting positive relationship was strong and highly
significant (r 2 ¼ 0.45, F1,2486 ¼ 1197.98, p , 0.001). To determine
whether frogs were infected by B. dendrobatidis, we swabbed the
ventral surface and all four feet of each frog with a sterile rayon
2
rspb.royalsocietypublishing.org
Life-history theory predicts that iteroparous organisms
should increase their current reproductive effort as life expectancy decreases [1,2,9]. Therefore, the optimal life-history
strategy for infected individuals may be to increase investment
in current reproductive effort [10,11]. Both male and female
hosts can compensate for an increased risk of mortality
imposed by an infection by increasing their investment in
earlier reproduction. For example, among female hosts,
Tasmanian devils (Sarcophilus harrisii) infected by a transmissible cancer mature and breed earlier [12], and crickets (Acheta
domesticus) infected by a bacterium, and water fleas (Daphnia
magna) infected by a microsporidian lay more eggs [13,14].
Among male hosts, frogs (Lithobates pipiens) infected by
a fungus increase sperm production [15], flies (Drosophila
nigrospiracula) infected by a parasitic mite, and amphipods
(Corophium volutator) infected by trematodes increase reproductive effort [16,17], and beetles (Tenebrio molitor) infected
by tapeworms provide higher quality nuptial gifts to their
mates, thus increasing egg production [18]. Whether hosts
increase their reproductive effort in response to an infection
depends on many factors, including resource availability
[19]. Therefore, infected hosts in very poor condition may not
be able to increase reproductive effort [20–22].
Understanding the effects of pathogens on host reproduction, and thus fitness, has direct implications for population
demography and evolution. The primary mechanism of
attracting and locating mates in most frogs is through male
advertisement calls, and frog populations in many regions of
the world are undergoing declines due to chytridiomycosis,
the disease caused by the widespread pathogenic fungus
Batrachochytrium dendrobatidis [23]. This fungus attacks the skin
cells of amphibians and disrupts their osmoregulatory and
transport functions, ultimately altering electrolyte concentrations in the blood, and causing cardiac arrest when the
fungal population on the host reaches a high density [24].
Currently, we have very little understanding of whether
B. dendrobatidis infections influence amphibian reproduction.
Although B. dendrobatidis can influence host survival directly,
many individuals carry sublethal infections, sometimes for
extended periods of time, and can ultimately recover [25].
Many aspects of frog behaviour can differ between frogs infected
by B. dendrobatidis and uninfected individuals (e.g. movements,
microhabitat use, microenvironment use [26,27]), potentially
reflecting changes in the behaviour of infected frogs. Infection
by B. dendrobatidis could also affect direct fitness traits, such
as energetic investment in mate attraction or gamete production [15]. Because calling requires substantial energy, reduced
body condition can also lead to reduction of calling effort or temporal shifts in calling effort [20]. Frogs infected by B. dendrobatidis
often have lower body condition than uninfected frogs [28–30];
when this occurs, they might reduce calling effort. On the other
hand, infected hosts with relatively good body condition may
have the plasticity to respond to the infection by investing
more in present reproductive effort than uninfected individuals
when their expectation of survival to the next reproductive bout
is lower [13–18]. Such a response could at least partially counteract the effects of natural selection on disease resistance, because it
may reduce differences in reproductive success between more
and less vulnerable males.
We studied the effects of B. dendrobatidis infection and body
condition on the probability of calling by the common mistfrog,
Litoria rheocola, a stream-breeding rainforest frog with a history
of declines caused by chytridiomycosis [31]. We sampled frog
Downloaded from http://rspb.royalsocietypublishing.org/ on June 14, 2017
sample size
calling
coordinates
elevation
(m ASL)
infected
Bobbin Bobbin Creek
Frenchman Creek
17.3788 S, 145.7758 E
17.3078 S, 145.9228 E
700
40
28
10
Mena Creek
Stoney Creek
17.6498 S, 145.9878 E
17.9208 S, 146.0698 E
60
20
Tully Creek
17.7738 S, 145.6458 E
Windin Creek
17.3658 S, 145.7178 E
infected
uninfected
total
81
47
37
49
76
270
222
376
33
31
181
93
38
36
100
99
352
259
150
53
241
24
172
490
750
14
46
19
65
144
swab, covering these areas twice. These samples were analysed
using real-time quantitative PCR assays [38]. We also gave each
frog a unique identifying mark using visible implant elastomer
[39]. For analysis, we used the initial capture of each male frog
and excluded all recaptures, resulting in an independent sample
of male frogs.
We used generalized linear mixed-effects models to examine
potential effects of infection status, body condition and season on
the calling probability of individual male frogs. Calling status
was coded as a binomial response variable, so we used models
with a binomial family and a logit link function. We developed
a set of candidate models that included models with all combinations of one, two or three fixed effects, and all two- and
three-way interactions. For all models, we included site identity
as a random effect to control for any overall effects of particular
sites. We ranked models according to Akaike’s information
criterion with adjustment for finite sample size (AICc) to determine the strength of evidence for each model relative to the set
of candidate models, using the criteria of Burnham & Anderson
[40]. Statistical analyses were performed in program R, v. 2.15.2
[41], using the lme4 [42] and MuMIn [43] packages. Because
infection loads were low during our study (98% of infected
frogs had less than 50 zoospores), we could not examine possible
effects of infection load on calling probability.
3. Results
We captured a total of 1843 unique male frogs at six study
sites during six seasonal samples (table 1). Of these frogs, 372
individuals were infected, with fungal loads of up to 912.7
zoospore equivalents per frog. Fungal loads were lowest
during autumn (mean: 0.7 zoospore equivalents per frog,
maximum: 4.1) and summer (mean: 2.2, maximum: 15.2),
and highest during spring (mean: 14.7, maximum: 912.7) and
winter (mean: 3.6, maximum: 308.1). We found that infection
status, body condition and season all influenced the calling
probability of individual frogs (table 2 and figure 1). Four
models with similar DAICc values that were less than 3 were
strongly supported by our data. Because the selected threshold
for model selection should be based on all models in the set,
rather than an arbitrary cutoff [44], we averaged the top four
models that were most strongly supported by our data and
had a total Akaike weight of 65% (table 2). This resulted in a
final, averaged model that included the random effects of site
uninfected
and the main effects of body condition, season and infection
status, plus interactions between infection status and body condition, season and infection status, and season and body
condition (table 2).
Our results clearly demonstrate that the relationship
between frog body condition and calling probability was
strongly influenced by infection status (figure 1). For uninfected frogs, calling probability was relatively constant across
seasons; our models suggest slight decreases with increasing
body condition in all seasons except winter, where there is a
slight increase, but the slopes of the lines are near zero in all
seasons (figure 1). By contrast, calling probability for infected
frogs differed among seasons; calling probability was lowest
in winter, highest in summer and intermediate in spring and
autumn. Calling probability also depended strongly on body
condition in infected frogs; across all seasons, the probability
of calling increased strongly as body condition increased
(figure 1). The predicted body condition at which both infected
and uninfected frogs were equally likely to call changed seasonally: in winter, infected frogs called more than uninfected
frogs only if they were in very good condition (i.e. their body
condition index was well above zero); in both spring and
autumn, infected frogs called more than uninfected frogs if
they were in good condition (i.e. their body condition index
was zero or above) and in summer, infected frogs called
more than uninfected frogs, even if they were in poor condition
(i.e. their body condition index was well below zero).
4. Discussion
We found that the calling probability of male frogs (L. rheocola)
was influenced by interactions among B. dendrobatidis infection
status, body condition and season, strongly suggesting that
males were employing a pre-existing, adaptive, conditiondependent response to infection. For uninfected frogs, calling
probability was relatively constant across seasons (near 50%;
figure 1), consistent with reports that L. rheocola call and
breed year-round [32]. This is probably the maximum sustainable calling rate for healthy males over the long term. Calling
probability was affected very little by body condition in uninfected frogs, but it did decrease slightly with increasing body
condition during all seasons except winter (figure 1), consistent
Proc. R. Soc. B 282: 20150694
site
not calling
3
rspb.royalsocietypublishing.org
Table 1. Study site details and sample sizes of unique male Litoria rheocola frogs (n ¼ 1843) captured at six rainforest streams in northeastern Queensland,
Australia. (Frogs were ether infected or uninfected by the chytrid fungus Batrachochytrium dendrobatidis, and either calling or not calling when located during
stream surveys.)
Downloaded from http://rspb.royalsocietypublishing.org/ on June 14, 2017
candidate models
AICc
DAICc
weight
cumulative
weight
infection, condition, season, infection condition, infection season
condition, season, condition season
2327.371
2329.675
0.000
2.304
0.352
0.111
0.352
0.463
infection, condition, season, infection condition
infection, condition, season, infection condition, infection season,
2329.869
2330.288
2.498
2.917
0.101
0.082
0.564
0.646
infection, condition, season, infection condition, infection season,
condition season, infection condition season
2330.719
3.348
0.066
0.712
infection, condition, season, infection season, condition season
infection, condition, season, condition season
2330.765
2331.490
3.394
4.119
0.064
0.045
0.776
0.821
infection, condition, season, infection condition, condition season
infection, season, infection season
2331.849
2331.987
4.477
4.616
0.038
0.035
0.859
0.894
season
2332.170
4.799
0.032
0.926
infection, condition, season, infection season
condition, season
2332.604
2332.655
5.233
5.284
0.026
0.025
0.962
0.977
infection, season
infection, condition, season
2334.102
2334.547
6.731
7.176
0.012
0.010
0.989
0.999
infection, condition, infection condition
2338.938
11.566
0.001
1.000
condition
intercept only
2341.858
2341.928
14.487
14.557
0.000
0.000
1.000
1.000
infection
infection, condition
2343.176
2343.307
15.805
15.936
0.000
0.000
1.000
1.000
estimate
importance
20.464
—
condition season
final model
model effect
intercept
condition
season (spring)
0.405
0.248
1
1
season (summer)
season (autumn)
0.274
0.463
1
1
20.222
0.83
infection (positive) condition
season (spring) infection (positive)
2.287
0.486
0.83
0.67
season (summer) infection (positive)
season (autumn) infection (positive)
0.729
0.308
0.67
0.67
season (spring) condition
season (summer) condition
20.817
20.779
0.30
0.30
season (autumn) condition
20.718
0.30
infection (positive)
with the expectation that frogs which expend more energy calling should have less energy reserves, and therefore lower body
condition [45]. By contrast, the calling probability of infected
frogs differed among seasons (lowest in winter, highest in
summer) and depended strongly on body condition. In each
season, the calling probability of infected frogs increased
Proc. R. Soc. B 282: 20150694
model effects
4
rspb.royalsocietypublishing.org
Table 2. Generalized linear mixed-effects models (family: binomial, link function: logit) used to examine relationships between calling by individual Litoria
rheocola and their Batrachochytrium dendrobatidis infection status (infected or uninfected) and body condition index. (We included site as a random effect, and
infection status, body condition and season as fixed effects. We developed a set of candidate models combining the random effect of site with all
combinations of one, two or three effects, and all two- and three-way interactions, and we ranked models according to Akaike’s information criterion with
adjustment for finite sample size (AICc). All models that we tested are shown, and four models were strongly supported (DAICc , 3, total weight of 65%) by
our data. We averaged these four models to obtain the final model, which is presented below the candidate models.)
Downloaded from http://rspb.royalsocietypublishing.org/ on June 14, 2017
(b) 80
60
40
40
20
20
0
0
(c) 80
(d) 80
60
60
40
40
20
20
0
–0.8 –0.6 –0.4 –0.2
0
0.2
body condition index
0.4
infected
uninfected
0
–0.8 –0.6 –0.4 –0.2
0
0.2
body condition index
0.4
Figure 1. Predicted calling probability for each individual male Litoria rheocola (n ¼ 1843) sampled in our study during (a) spring, (b) summer, (c) autumn and (d )
winter, based on its body condition and Batrachochytrium dendrobatidis infection status. Predictions were generated from the final, averaged generalized linear
mixed-effects model presented in table 2. Body condition was calculated as the residual for each frog from a regression using data on all male frogs of log10
transformed body mass on square-root transformed snout – urostyle length [37].
with body condition, such that infected frogs in poor body condition were less likely to call than uninfected frogs in similar
condition, but infected frogs in good condition often had a
higher calling probability than uninfected frogs (figure 1).
This pattern of increased calling effort in infected frogs is
consistent with life-history theory, which predicts that reproductive effort should increase as life expectancy decreases
[1,2,9]. Because L. rheocola has not coexisted with B. dendrobatidis
for an extended period, it is likely that the pattern we observed
reflects a generalized, plastic response to a pathogenic infection,
in which infected frogs increase their present reproductive
effort at the expense of possible future reproductive effort.
A functionally similar response to infection by B. dendrobatidis
occurs in northern leopard frogs (Lithobates pipiens); the testes
of infected males are larger and contain more sperm than
those of uninfected males [15]. Studies on other taxa reveal
that present reproductive effort can increase as life expectancy
decreases [10–18]. In our study, however, calling activity did
not increase in all infected frogs; we found that infected frogs
in relatively poor condition were less likely to call than uninfected frogs. This could be because infected frogs were unable
to call owing to physiological changes caused by their infections, or because frogs were adaptively adjusting their energy
expenditure. For example, they may have been allocating less
energy to reproduction and more to other functions required
for immediate survival, such as immune responses to fight
their infections [46].
There are several alternative hypotheses for the pattern we
observed. For example, the pathogen might be manipulating
the host, possibly to increase contact between infected males
and uninfected females, thus increasing rates of pathogen
transmission [47]. However, this seems unlikely. The effects
of rates of physical contact between frogs on rates of transmission of B. dendrobatidis are not known, but frogs can also
become infected by contact with water or substrates, both
of which can harbour infectious B. dendrobatidis zoospores
[48,49]. In addition, zoospores can be carried and released
into the environment by non-amphibian hosts, including
nematodes and crayfish [50,51], and non-amphibian reservoirs,
including reptiles and waterfowl [52,53]. Increased calling
effort could also attract predators [54], which would be detrimental for pathogen transmission. Overall, it seems unlikely
that there would be strong selection for B. dendrobatidis to
increase transmission rates by increasing male calling rates.
Another alternative hypothesis is that changes in the calling
probability of infected frogs were caused by side effects of the
infection. Calling is energetically expensive [45], and reduced
energy availability should lead to decreased calling activity.
This could account for the decreases in activity we observed
in frogs with poor body condition. However, such side effects
cannot explain the pattern that infected frogs in relatively good
body condition had greater calling probabilities than did uninfected frogs in similar condition (figure 1). The most plausible
explanation for this pattern is that infected frogs in relatively
good body condition were exhibiting a generalized, plastic
response to pathogen infection by allocating more energy to
reproductive effort than did uninfected frogs in similar
body condition.
Our results demonstrate that the interaction between
B. dendrobatidis infection status and body condition can strongly
influence the probability of calling. However, increased calling
effort in infected frogs may not lead to increased mating opportunities if females are not attracted to their calls. Female frogs
often prefer calls that are louder, longer and emitted at faster
rates, because they often indicate genetic superiority of males
capable of producing high levels of sound [8,45]. In some frog
species, parasitic infections can reduce male calling rates [6],
and thus possibly reduce mating success. In other frog species,
however, infected males do not change the quality of their calls
and females do not avoid mating with them [55]. In L. rheocola,
Proc. R. Soc. B 282: 20150694
calling probability (%)
60
5
rspb.royalsocietypublishing.org
calling probability (%)
(a) 80
Downloaded from http://rspb.royalsocietypublishing.org/ on June 14, 2017
Ethics. Research was conducted under permits WISP03070208 and
WITK03070508 issued by the Queensland Department of Environment and Resource Management, and protocols A1420 and A1673
approved by the Animal Ethics Committee at James Cook University.
Data accessibility. Data are available as the electronic supplementary
material.
Authors’ contributions. E.A.R., D.A.P. and R.A.A. designed the study;
S.J.S., E.A.R. and D.A.P. conducted the fieldwork; R.A.A., L.S.,
D.A.P. and E.A.R. contributed funding; E.A.R. and R.A.A. analysed
the data; E.A.R. wrote the paper; and all authors revised the paper.
Competing interests. We declare we have no competing interests.
Funding. Funding was provided by a Linkage Grant from the Australian Research Council in partnership with Powerlink Queensland
(LP0776927 to L.S. and R.A.A.), Discovery Grants from the Australian
Research Council (DP0986537 to R.A.A.; DP130101635 to R.A.A., L.S.
and D.A.P.), the Skyrail Rainforest Foundation (to E.A.R.), and the
Graduate Research School at James Cook University (to E.A.R.).
E.A.R. was supported by a Postgraduate Research Scholarship and
a Doctoral Completion Award, both from James Cook University.
Acknowledgements. We thank many volunteers for help with fieldwork,
and Michael Mahony, Dale Roberts and two anonymous reviewers
for helpful comments on the manuscript. Diagnostic quantitative
PCR assays were performed by the Amphibian Disease Diagnostic
Laboratory at Washington State University.
References
1.
2.
3.
4.
Stearns SC. 1992 The evolution of life histories.
New York, NY: Oxford University Press.
Roff DA. 2002 Life history evolution. Sunderland,
MA: Sinauer Associates.
Cressler CE, Nelson WA, Day T, McCauley E. 2014
Disentangling the interaction among host resources,
the immune system and pathogens. Ecol. Lett. 17,
284–293. (doi:10.1111/ele.12229)
Cade WH. 1984 Effects of fly parasitoids on nightly
calling duration in field crickets. Can. J. Zool. 62,
226–228. (doi:10.1139/z84-037)
5.
6.
7.
Garamszegi LZ. 2005 Bird song and parasites.
Behav. Ecol. Sociobiol. 59, 167 –180. (doi:10.1007/
s00265-005-0041-8)
Madelaire CB, da Silva RJ, Gomes FR. 2013 Calling
behavior and parasite intensity in treefrogs,
Hypsiboas prasinus. J. Herpetol. 47, 450–455.
(doi:10.1670/11-315)
Hamilton WD, Zuk M. 1982 Heritable true
fitness and bright birds: a role for parasites?
Science 218, 384 –387. (doi:10.1126/science.
7123238)
8.
Welch AM, Semlitsch RD, Gerhardt HC. 1998 Call
duration as an indicator of genetic quality in male
gray tree frogs. Science 280, 1928 –1930. (doi:10.
1126/science.280.5371.1928)
9. Clutton-Brock TH. 1984 Reproductive effort
and terminal investment in iteroparous
animals. Am. Nat. 123, 212 –229. (doi:10.1086/
284198)
10. Forbes MRL. 1993 Parasitism and host
reproductive effort. Oikos 67, 444–450.
(doi:10.2307/3545356)
6
Proc. R. Soc. B 282: 20150694
male reproductive tactics and female reproduction will aid
in our understanding of how it influences amphibian populations and will provide insight into the potential for
evolutionary responses.
Calling is the primary mechanism of attracting and locating mates in most frogs, and therefore, the changes we
documented in the calling effort of infected males are likely
to affect their mating success. This could lead to a variety
of effects on the evolution of the host species. If infected
males in good body condition reproduce at rates equal to
or greater than those of uninfected males, selection on factors
affecting the probability of acquiring infections may be less
effective than it otherwise would be. However, because
reproductive effort in infected males is strongly and positively related to body condition, males in good condition
may produce more offspring than those in poorer condition.
This could lead to selection favouring mechanisms that limit
the negative effects of infections on body condition. Batrachochytrium dendrobatidis has devastated amphibian populations
in many regions of the world, but many populations are coexisting with the pathogen [25,60]. Elucidating whether
populations that are coexisting with the pathogen are experiencing sublethal effects that influence reproduction and
mating systems is important for understanding potential
changes in population demography and evolution.
rspb.royalsocietypublishing.org
we do not know whether the quality of calls produced by
infected males differs from that of calls produced by uninfected
males. Regardless of the relative attractiveness of calls emitted
by infected males, it is likely that calling more frequently will
attract more mates than not doing so. Determining the influence
of B. dendrobatidis infections on calling behaviour and reproductive success will be necessary to fully understand how this
pathogen influences population dynamics.
Season affected the calling probabilities of infected frogs
more strongly than those of uninfected frogs (figure 1). The
effects of season also interacted with those of body condition;
the average body condition of frogs was lowest in winter,
highest in summer and intermediate in spring and autumn
(figure 1). These changes appeared to be greater in infected
frogs than in uninfected frogs (figure 1). Seasonal changes in
body condition of both uninfected and infected frogs were probably caused by changes in energy acquisition or expenditure.
Reduced energy intake could be associated with low availability
of rainforest arthropods during dry months [56], which can
affect the diets of frogs [57]. The strongly reduced body condition of infected frogs in winter could also be related to
greater infection loads caused by faster growth rates of B. dendrobatidis under cooler temperatures [58]. In infected frogs, calling
probability differed among seasons (lowest in winter, highest
in summer) and was strongly and positively related to body
condition, whereas in uninfected frogs, calling probability was
relatively constant across seasons and body conditions. In
summer, when frogs were in the best condition, infected frogs
were up to 30% more likely to call than were uninfected
frogs (figure 1). In winter, however, when frogs were in the
worst condition, infected frogs were up to 40% less likely to
call than were uninfected frogs (figure 1). During spring and
autumn, infected frogs in poor body condition were less likely
to call than uninfected frogs in similar condition, but when
infected frogs were in good body condition, their calling probability was often higher than that of uninfected frogs (figure 1).
Our findings suggest that infected males in poor condition will have lower fitness than healthier frogs, but that
infected males in good condition may compensate for a
potential loss of future reproductive output by increasing
their current efforts, and thus their fitness. It is possible that
males with reduced calling effort may override these effects
by using alternative mating tactics, such as by attempting
to intercept females attracted to nearby calling males (‘satellite behaviour’) [59]. Likewise, the effects of B. dendrobatidis
infections on female reproductive biology and behaviour
are unknown. Understanding how this pathogen alters
Downloaded from http://rspb.royalsocietypublishing.org/ on June 14, 2017
26.
27.
28.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
not affect short-term movements or survival
rates of the treefrog Litoria rheocola.
Herpetologica 70, 23 –33. (doi:10.1655/
HERPETOLOGICA-D-13-0004)
Burnham KP, Anderson DR. 2002 Model selection
and multi-model inference: a practical informationtheoretic approach, 2nd edn. New York, NY:
Springer.
R Core Team. 2012 R: a language and environment
for statistical computing. Vienna, Austria: R
Foundation for Statistical Computing.
Bates D, Maechler M, Bolker B. 2012 lme4: Linear
mixed-effects models using S4 classes. R package,
version 0.999999-0. See http://CRAN.Rproject.org/
package=lme4.
Barton K. 2013 MuMIn: Multi-model inference. R
package, version 1.9.0. See http://CRAN.R-project.
org/package=MuMIn.
Burnham KP, Anderson DR, Huyvaert KP. 2011 AIC
model selection and multimodel inference in
behavioral ecology: some background, observations,
and comparisons. Behav. Ecol. Sociobiol. 65, 23 –35.
(doi:10.1007/s00265-010-1029-6)
Wells KD. 2001 The energetics of calling in frogs. In
Anuran communication (ed. MJ Ryan), pp. 45 –60.
Washington, DC: Smithsonian Institution Press.
McMahon TA et al. 2014 Amphibians acquire
resistance to live and dead fungus overcoming
fungal immunosuppression. Nature 511, 224 –227.
(doi:10.1038/nature13491)
Poulin R. 2007 Evolutionary ecology of parasites,
2nd edn. Princeton, NJ: Princeton University Press.
Johnson ML, Speare R. 2003 Survival of
Batrachochytrium dendrobatidis in water: quarantine
and disease control implications. Emerg. Infect. Dis.
9, 922 –925. (doi:10.3201/eid0908.030145)
Johnson ML, Speare R. 2005 Possible modes of
dissemination of the amphibian chytrid
Batrachochytrium dendrobatidis in the environment.
Dis. Aqu. Organ. 65, 181–186. (doi:10.3354/
dao065181)
Shapard E, Moss A, San Francisco M. 2012
Batrachochytrium dendrobatidis can infect and cause
mortality in the nematode Caenorhabditis elegans.
Mycopathologia 173, 121–126. (doi:10.1007/
s11046-011-9470-2)
McMahon TA, Brannelly LA, Chatfield MWH,
Johnson PTJ, Joseph MB, McKenzie VJ, RichardsZawacki CL, Venesky MD, Rohr JR. 2013 Chytrid
fungus Batrachochytrium dendrobatidis has
nonamphibian hosts and releases chemicals that
cause pathology in the absence of infection. Proc.
Natl Acad. Sci. USA 110, 210–215. (doi:10.1073/
pnas.1200592110)
Kilburn VL, Ibáñez R, Green DM. 2011 Reptiles as
potential vectors and hosts of the amphibian
pathogen Batrachochytrium dendrobatidis in
Panama. Dis. Aqu. Organ. 97, 127–134.
(doi:10.3354/dao02409)
Garmyn A, Van Rooij P, Pasmans F, Hellebuyck T,
Van Den Broeck W, Haesebrouck F, Martel A.
2012 Waterfowl: potential environmental reservoirs
of the chytrid fungus Batrachochytrium
7
Proc. R. Soc. B 282: 20150694
29.
frog Litoria rheocola. MSc thesis, James Cook University,
Townsville, Australia.
Rowley JJL, Alford RA. 2013 Hot bodies protect
amphibians against chytrid infection in nature. Sci.
Rep. 3, 1515. (doi:10.1038/srep01515)
Roznik EA. 2013 Effects of individual behaviour on
host– pathogen interactions: Australian rainforest
frogs and the chytrid fungus Batrachochytrium
dendrobatidis. PhD thesis, James Cook University,
Townsville, Australia.
Retallick RWR, Miera V. 2007 Strain differences in
the amphibian chytrid Batrachochytrium
dendrobatidis and non-permanent, sub-lethal
effects of infection. Dis. Aqu. Organ. 75, 201– 207.
(doi:10.3354/dao075201)
Harris RN, Lauer A, Simon MA, Banning JL, Alford
RA. 2009 Addition of antifungal skin bacteria to
salamanders ameliorates the effects of
chytridiomycosis. Dis. Aqu. Organ. 83, 11 –16.
(doi:10.3354/dao02004)
Murphy PJ, St-Hilaire S, Corn PS. 2011 Temperature,
hydric environment, and prior pathogen exposure
alter the experimental severity of chytridiomycosis
in boreal toads. Dis. Aqu. Organ. 95, 31 –42.
(doi:10.3354/dao02336)
McDonald K, Alford R. 1999 A review of declining
frogs in northern Queensland. In Declines and
disappearances of Australian frogs (ed. A Campbell),
pp. 14– 22. Canberra, Australia: Environment
Australia.
Dennis AJ. 2012 Common mistfrog, Litoria rheocola.
In Queensland‘s threatened animals (eds LK Curtis,
AJ Dennis, KR McDonald, PM Kyne, SJS Debus), pp.
166 –167. Collingwood, Australia: CSIRO Publishing.
IUCN. 2014 IUCN Red List of Threatened Species.
Version 2014.3. See www.iucnredlist.org.
McDonald KR, Mendez D, Muller R, Freeman AB,
Speare R. 2005 Decline in the prevalence of
chytridiomycosis in frog populations in North
Queensland, Australia. Pacific Conserv. Biol. 11,
114 –120.
Roznik EA, Alford RA. 2015 Seasonal ecology and
behavior of an endangered rainforest frog (Litoria
rheocola) threatened by disease. PLoS ONE 10,
e0127851. (doi:10.1371/journal.pone.0127851)
Sapsford SJ, Alford RA, Schwarzkopf L. 2013 Elevation,
temperature, and aquatic connectivity all influence
the infection dynamics of the amphibian chytrid
fungus in adult frogs. PLoS ONE 8, e82425. (doi:10.
1371/journal.pone.0082425)
Peig J, Green AJ. 2009 New perspectives for
estimating body condition from mass/length data:
the scaled mass index as an alternative method.
Oikos 118, 1883– 1891. (doi:10.1111/j.1600-0706.
2009.17643.x)
Boyle DG, Boyle DB, Olsen V, Morgan JAT, Hyatt AD.
2004 Rapid quantitative detection of
chytridiomycosis (Batrachochytrium dendrobatidis) in
amphibian samples using real-time Taqman PCR
assay. Dis. Aqu. Organ. 60, 141–148. (doi:10.3354/
dao060141)
Sapsford SJ, Roznik EA, Alford RA, Schwarzkopf L.
2014 Visible implant elastomer marking does
rspb.royalsocietypublishing.org
11. Agnew P, Koella JC, Michalakis Y. 2000 Host life
history responses to parasitism. Microb. Infect. 2,
891–896. (doi:10.1016/S1286-4579(00)00389-0)
12. Jones ME, Cockburn A, Hamede R, Hawkins C,
Hesterman H, Lachish S, Mann D, McCallum H,
Pemberton D. 2008 Life-history change in diseaseravaged Tasmanian devil populations. Proc. Natl
Acad. Sci. USA 105, 10 023 –10 027. (doi:10.1073/
pnas.0711236105)
13. Adamo SA. 1999 Evidence for adaptive changes in
egg laying in crickets exposed to bacteria and
parasites. Anim. Behav. 57, 117–124. (doi:10.1006/
anbe.1998.0999)
14. Chadwick W, Little TJ. 2005 A parasite-mediated
life-history shift in Daphnia magna. Proc. R. Soc. B
272, 505–509. (doi:10.1098/rspb.2004.2959)
15. Chatfield MWH, Brannelly LA, Robak MJ, Freeborn L,
Lailvaux SP, Richards-Zawacki CL. 2013 Fitness
consequences of infection by Batrachochytrium
dendrobatidis in northern leopard frogs (Lithobates
pipiens). Ecohealth 10, 90 –98. (doi:10.1007/
s10393-013-0833-7)
16. Polak M, Starmer WT. 1998 Parasite-induced risk of
mortality elevates reproductive effort in male
Drosophila. Proc. R. Soc. Lond. B 265, 2197 –2201.
(doi:10.1098/rspb.1998.0559)
17. McCurdy DG, Forbes MR, Boates JS. 2000 Male
amphipods increase their mating effort before
behavioural manipulation by trematodes.
Can. J. Zool. 78, 606 –612. (doi:10.1139/z99-229)
18. Hurd H, Ardin R. 2003 Infection increases the value of
nuptial gifts, and hence male reproductive success, in
the Hymenolepis diminuta-Tenebrio molitor
association. Proc. R. Soc. Lond. B 270(Suppl. 2),
S172–S174. (doi:10.1098/rsbl.2003.0033)
19. van Noordwijk AJ, de Jong G. 1986 Acquisition and
allocation of resources: their influence on variation
in life history tactics. Am. Nat. 128, 137 –142.
(doi:10.1086/284547)
20. Bevier CR. 1997 Breeding activity and chorus tenure
of two Neotropical hylid frogs. Herpetologica 53,
297–311.
21. Wilson K, Bjørnstad ON, Dobson AP, Merler S,
Poglayen G, Randolph SE, Read AF, Skorping A.
2001 Heterogeneities in macroparasite infections:
patterns and processes. In The ecology of wildlife
diseases (eds PJ Hudson, A Rizzoli, BT Grenfell,
H Heesterbeek, AP Dobson), pp. 6–44. New York,
NY: Oxford University Press.
22. Judge KA, Ting JJ, Gwynne DT. 2008 Condition
dependence of male life span and calling effort in a
field cricket. Evolution 62, 868 –878. (doi:10.1111/j.
1558-5646.2008.00318.x)
23. Kilpatrick AM, Briggs CJ, Daszak P. 2010 The ecology
and impact of chytridiomycosis: an emerging
disease of amphibians. Trends Ecol. Evol. 25,
109–118. (doi:10.1016/j.tree.2009.07.011)
24. Voyles J et al. 2009 Pathogenesis of
chytridiomycosis, a cause of catastrophic amphibian
declines. Science 326, 582– 585. (doi:10.1126/
science.1176765)
25. Sapsford SJ. 2012 Population and disease dynamics of
the amphibian chytrid fungus in the stream-associated
Downloaded from http://rspb.royalsocietypublishing.org/ on June 14, 2017
56. Wolda H. 1978 Seasonal fluctuations in rainfall, food
and abundance of tropical insects. J. Anim. Ecol. 47,
369 –381. (doi:10.2307/3789)
57. Toft C. 1980 Feeding ecology of thirteen syntopic
species of anurans in a seasonal tropical
environment. Oecologia 45, 131–141. (doi:10.
1007/BF00346717)
58. Stevenson LA, Bell SC, Roznik EA, Berger L, Alford
RA, Pike DA. 2013 Variation in thermal performance
of a widespread pathogen, the amphibian chytrid
fungus Batrachochytrium dendrobatidis. PLoS ONE 8,
e73830. (doi:10.1371/journal.pone.0073830)
59. Wells KD. 2007 The ecology and behavior of
amphibians. Chicago, IL: University of Chicago Press.
60. Retallick RWR, McCallum H, Speare R. 2004
Endemic infection of the amphibian chytrid fungus
in a frog community post-decline. PLoS Biol. 2,
e351. (doi:10.1371/journal.pbio.0020351)
8
rspb.royalsocietypublishing.org
dendrobatidis. PLoS ONE 7, e35038. (doi:10.1371/
journal.pone.0035038)
54. Tuttle MD, Ryan MJ. 1981 Bat predation and the
evolution of frog vocalizations in the Neotropics. Science
214, 677–678. (doi:10.1126/science.214.4521.677)
55. Hausfater G, Gerhardt HC, Klump GM. 1990
Parasites and mate choice in gray treefrogs, Hyla
versicolor. Am. Zool. 30, 299 –311. (doi:10.1093/
icb/30.2.299)
Proc. R. Soc. B 282: 20150694