Temporal variation in predation risk: stage-dependency, graded

responses and fitness costs in tadpole antipredator defences

Anssi Laurila, Maria Jarvi-Laturi, Susanna Pakkasmaa and Juha Merila

Laurila, A., Jarvi-Laturi, M., Pakkasmaa, S. and Merila, J. 2004. Temporal variation inpredation risk: stage-dependency, graded responses and fitness costs in tadpoleantipredator defences. �/ Oikos 107: 90�/99.

Temporal variation in predation risk may be an important determinant of preyantipredator behaviours. According to the risk allocation hypothesis, the strongestantipredator behaviours are expected when periods of high risk are short andinfrequent. We tested this prediction in a laboratory experiment where common frogRana temporaria tadpoles were raised form early larval stages until metamorphosis. Wemanipulated the time a predatory Aeshna dragonfly larva was present and recordedbehavioural responses (activity) of the tadpoles at three different time points during thetadpoles’ development. We also investigated how tadpole shape, size and age atmetamorphosis were affected by temporal variation at predation risk. We found thatduring the two first time points activity was always lowest in the constant high-risksituation. However, antipredator response in the two treatments with brief high-risksituation increased as tadpoles developed, and by the third time point, when thetadpoles were close to metamorphosis, activity was as low as in the constant high-risksituation. Exposure to chemical cues of a predation event tended to reduce activityduring the first time period, but caused no response later on. Induced morphologicalchanges (deeper tail and shorter relative body length) were graded the response beingstronger as the time spent in the proximity of predator increased. Tadpoles in the briefrisk and chemical cue treatments showed intermediate responses. Modification of lifehistory was only found in the constant high-risk treatment in which tadpoles hadlonger larval period and larger metamorphic size. Our results indicate that bothbehavioural and morphological defences were sensitive to temporal variation inpredation risk, but behaviour did not respond in the manner predicted by the riskallocation model. We discuss the roles of concentration of predator chemical cues andprey stage-dependency in determining these responses.

A. Laurila, S. Pakkasmaa and J. Merila, Dept of Population Biology, EvolutionaryBiology Center, Uppsala Univ., Norbyvagen 18d, SE-75236 Uppsala, Sweden(anssi.laurila@ebc.uu.se). Present address for SP: Natl Board of Fisheries, Inst. ofFreshwater Research, SE-17893 Drottningholm, Sweden. Present address for JM: Deptof Bio- and Environmental Sciences, P.O. Box 65, FIN-00014 University of Helsinki.�/ M. Jarvi-Laturi, Dept of Bio- and Environmental Sciences, P.O. Box 65, FIN-00014University of Helsinki, Finland.

Prey animals often show highly sensitive responses in

behaviour, morphology and life history to variation in

predation risk. Although a large number of studies have

described and analysed these responses in various

systems (Kats and Dill 1998, Lima 1998a, b, reviewed

by Tollrian and Harvell 1999), a potentially important

element of predator�/prey interactions remains obscure.

In most natural systems predation risk varies tempo-

rally: predator activity patterns and densities vary both

diurnally and seasonally, and the theory implies that

such variation may have a profound impact on prey

behaviour (Houston et al. 1993, Clark 1994, Lima and

Accepted 8 March 2004

Copyright # OIKOS 2004ISSN 0030-1299

OIKOS 107: 90�/99, 2004

90 OIKOS 107:1 (2004)

Bednekoff 1999). However, empirical studies on the

effects of temporal variation in predation risk are very

few (Hamilton and Heithaus 2001, Sih and McCarthy

2002, Van Buskirk et al. 2002), and understanding their

effects on prey behaviour and performance remains an

issue important for a proper understanding of predator�/

prey interactions (Lima 1998a, 2002, Sih et al. 2000).

Recently, Lima and Bednekoff (1999) presented a

model, in which animals feeding under temporally

varying predation risk are faced with a problem of the

optimal allocation of antipredator behaviour across

various levels of risk. This risk allocation model predicts

that temporal variation in predation risk has a major

impact on prey antipredator behaviour: animals should

exhibit strongest antipredator behaviour in high-risk

situations that are brief and infrequent, whereas in

situations where the risk is predominantly or constantly

high, antipredator behaviours should be weaker. This is

expected because under prolonged high-risk situations,

animals are forced to feed during high risk in order to

meet their energy demands, whereas during short pulses

of high risk, feeding can be postponed to occur during

periods of low risk (Lima and Bednekoff 1999).

For growing prey animals optimization of antipreda-

tor behaviour is important because diminished foraging

returns are likely to impact their later fitness due to

reduced growth rates (Crowl and Covich 1990, Skelly

and Werner 1990, Ball and Baker 1996, reviewed by

Lima 1998a, b). Although actual tests are lacking, such

effects are likely to be strongest when high-risk situations

are common, as prey may otherwise be able to compen-

sate for foraging losses during long periods of safety

(Lima and Bednekoff 1999, Sih and McCarthy 2002).

Induced plasticity in morphological traits is another

widespread form of antipredator defences (Tollrian and

Harvell 1999). However, morphological defences are

characterized by considerable time lags in their expres-

sion, and plasticity may become disadvantageous when

lags in response are long relative to the time scale of

environmental variation (Padilla and Adolph 1996). This

suggests that morphology may be less plastic in response

to temporal variation in predation risk than behavioural

traits (West-Eberhard 1989, Clark and Harvell 1993,

Padilla and Adolph 1996).

While empirical tests on the effects of temporal

variation in predation risk on life history or morphology

are largely lacking (Van Buskirk 2002, Relyea 2003),

such information is essential for proper understanding of

predator�/prey interactions. Van Buskirk and Arioli

(2002) found that mere predator presence was sufficient

to induce responses in morphological traits of tadpoles,

but behavioural responses were mostly affected by the

number of prey consumed, possibly reflecting the

different time scales of threat the two types of cues

present. Similarly, different types of empirical studies

differ in the main technique how predation risk treat-

ments are implemented; behavioural studies often use a

single pulse of high predation risk (which according to

risk allocation hypothesis may overestimate the magni-

tude of antipredator behaviour), whereas studies on

effects of predators on life-history and morphology

have typically employed prolonged periods of constant

high predation risk (Lima and Bednekoff 1999, Sih et al.

2000). Clearly, studies integrating temporal variation in

predation risk to several measures of antipredator

defence are needed to understand how different techni-

ques of experimentation may affect the outcome of the

experiment.

We investigated the effects of temporal variation in

predation risk on behaviour, morphology and larval life-

history traits of common frog Rana temporaria tadpoles.

R. temporaria tadpoles express behavioural and mor-

phological antipredator defences, as well as modifica-

tions in life-history (body size and larval period; Laurila

et al. 1998, Lardner 2000, Laurila 2000, Van Buskirk

2001). We experimentally tested the predictions of risk

allocation hypothesis on tadpole behaviour by creating

high and low risk situations and by varying the time

predator was present in an experimental container. We

expected to find stronger antipredator behaviour when

high-risk situations were infrequent and of short dura-

tion as compared to the situation where the predator was

continuously present (Fig. 1). An alternative hypothesis,

the risk-spreading theorem by Houston et al. (1993),

predicts that feeding effort minimizing predation risk is

constant and independent of the temporal variation in

predation risk, i.e. it is the lowest effort that still allows

the foraging animals to meet their energetic requirements

(Fig. 1).

Morphological defences are slow to develop, making it

impossible for morphological responses to track varia-

tion in predation risk as tightly as behaviour. Hence, the

Fig. 1. A schematic illustration of predictions from the riskallocation model. Least foraging activity is expected when thetime spent in high risk is short in relation to the time spent inlow risk. In the alternative hypothesis assuming no temporalrisk allocation, foraging activity is expected to be independentof the time spent in high risk, however, also here less foraging isexpected under high risk. Partly modified from Lima andBednekoff (1999).

OIKOS 107:1 (2004) 91

morphological response to temporal variation in risk

could be simple polyphenism (McCollum and Van

Buskirk 1996), in which only defended or undefended

phenotypes are found, and morphology is insensitive to

temporal variation in risk. Alternatively, induced mor-

phology is a continuous trait, but time lags in the

development of induced defence are too long in relation

to the time scale of environmental variation, reducing

the profitability of morphological defences (Padilla and

Adolph 1996). In this case we expect to find the most

defended individuals in the constant high-risk treatment,

whereas individuals in the varying risk treatments should

show intermediate defences. Finally, as chemical cues are

important carriers of information in aquatic systems

(Dodson et al. 1994, Kats and Dill 1998), we had an

additional treatment that allowed us to differentiate the

effects of chemical cues from the situation where both

visual and chemical cues were present.

Methods

Rana temporaria breeds in a variety of freshwater

habitats from lake shore marshes to small ephemeral

ponds that vary widely in predator regimes (Laurila

1998, Van Buskirk 2001). Five amplectant pairs of

R. temporaria were captured in a mating aggregation

50 km NW from Uppsala and brought to the laboratory.

The frogs were stored in cool (48C) and dark for one

week before each female was mated with one male

following Merila et al. (2000). About 200 eggs from each

mating were placed in separate 3-l plastic vials filled with

2 l of water, and the eggs were allowed to develop until

hatching in 168C. Throughout the experiment, we used

reconstituted soft water (RSW; APHA 1985) to assure

homogeneous water quality. After hatching, the tadpoles

from all the families were pooled together and trans-

ferred to two 85-l plastic containers, in which they were

raised for six days in 168C. They were fed slightly boiled

spinach ad libitum.

Late-instar dragonfly larvae of the genus Aeshna are

voracious predators of tadpoles and commonly co-occur

with R. temporaria (Laurila 1998, Van Buskirk 2001).

We captured Aeshna larvae from ponds near Uppsala

and transported them to laboratory where they were

maintained in individual 0.2-l plastic cups in 168C. When

not in the experiment, they were fed R. temporaria and

R. arvalis tadpoles every second day.

Experimental design

We used opaque plastic containers (38�/28�/13 cm)

arranged on four shelves and filled with 10 l of RSW as

experimental units. In each container, we hung a

cylindrical predator cage (diameter 11 cm, height 21

cm) made of transparent plastic film and with a double

net bottom (mesh size 1.5 mm) two centimetres above

the container floor. Hence, the tadpoles were able to get

both visual and chemical cues from the predator.

Temperature during the experiment was maintained at

constant 208C and the light rhythm was 12L:12D.

Fourteen randomly selected tadpoles were placed in

each container. The predator treatments were initiated

four days later (day 1 of the experiment). During the

experiment, the tadpoles were fed slightly boiled spinach,

and food was continuously provided ad libitum. Com-

plete water change in all experimental units was done

once weekly.

The applied predator treatments were:

1) no risk (NR): empty predator cage.

2) infrequent risk (IR): caged Aeshna larva present for

2 h every third day.

3) frequent risk (FR): caged Aeshna larva present for

2 h every day.

4) frequent chemical cues (FCC): the entire water

volume from a vial, in which an Aeshna larva had

captured and eaten one R. temporaria tadpole,

added daily.

5) continuous risk (CR): caged Aeshna larva continu-

ously present.

The predator treatments were always applied at

10 A.M. In IR and FR treatments, one tadpole was

added to the cage simultaneously with the predator. The

predators captured and ate the tadpole within 30 min.

We removed 0.8 l of water from all the experimental

units before the application of the predator treatments.

In FCC treatment, an Aeshna larva was allowed to eat

one tadpole in a 0.9 l plastic vial. After half an hour, the

water (0.8 l) was filtered to remove any solid remains of

the predation event and poured into the experimental

units. In the other treatments, 0.8 l of fresh RSW

was poured into the predator cage. When in the

experiments, the predators were fed one R. temporaria

tadpole (200�/300 mg) daily.

Response variables

To investigate how predator treatments affect tadpole

behaviour, we recorded the behaviour of the tadpoles at

three time points (day 3, 12 and 24 of the experiment).

At each time point, we first twice scored (recordings

separated by ca 30 min) the number of actively moving

tadpoles an hour before the predators or chemical cues

were added to IR, FR and FCC treatments (low risk

situation). One hour after the predators had been

introduced, two further recordings were made (high

risk situation).

92 OIKOS 107:1 (2004)

Six randomly chosen tadpoles from each container

were sampled and stored in 70% alcohol on day 15, and

body length (from the tip of the nose to the end of the

body wall), maximum body width, maximum body

depth, tail length (from the body terminus to the tip of

the tail), maximum tail depth, maximum tail muscle

depth and maximum tail muscle width (both at the base

of the tail) of the preserved individuals were later

measured with a digital calliper.

The containers were checked daily for metamorphos-

ing animals. At the emergence of the first forelimb (stage

42, Gosner 1960), the animals were removed from the

containers and their snout vent length and maximum

body width (between front and hind limbs) were

measured with a digital calliper. The length of larval

period was determined as days elapsed from the start of

the experiment (day 0).

Statistical analyses

To investigate the influence of predator treatment, risk

situation (low or high risk) and observation day (day 3,

12 or 24) on activity levels, the data were first analysed

with repeated-measures ANOVAs by using day and risk

as repeated factors. Upon finding significant interaction

between the repeated factors, or between repeated

factors and predator treatment, we conducted separate

analyses for each time point. Tukey tests performed

within the risk situations were used to test for differences

among the predator treatments. As the behavioural data

were proportional, arcsin-transformed container means

of the two recordings conducted within the risk situa-

tions were used as response variables.

Since morphological traits are not independent of

each other, we performed a principal component analysis

(PCA) with a correlation matrix to produce composite

multivariate measures of size and shape. All traits were

strongly positively loaded on the first axis (Table 1)

revealing this axis to be a size component. The second

axis was interpreted as a shape component describing the

contrast between tail shape and body length (Table 1)

and was used as an indicator of inducible morphology.

To obtain size-corrected measurements on individual

morphological traits, we regressed the traits against the

PC1 scores. Treatment effects on morphological (PC1

and PC2, size-corrected individual traits) and meta-

morphic (body length and width, length of larval period)

traits were analysed with univariate ANOVAs. Container

means were always used as response variables. Tukey

tests were performed to test for differences among the

predator treatments. All statistical analyses were per-

formed with SYSTAT 10.0 (Wilkinson 2000) statistical

package.

Results

Tadpole behaviour

The initial repeated-measures ANOVA on tadpole

activity found strong effects in both the repeated factors,

treatment and all their possible interactions (Table 2).

We therefore continued by analysing each observation

day separately.

There was a strong general treatment effect on activity

at each of the three time points (Table 3). This was

mainly due to the reduced activity level in CR treatment,

which was always more than 80% lower than the activity

in NR treatment (Fig. 2). On the two first time points the

effects in the other predator treatments were much

weaker than in CR treatment (Fig. 2a, b). On day 3

activity in FCC treatment in high-risk situation tended

to be lower than in NR treatment (Fig. 2a; Tukey test,

P�/0.078). Also on day 12 activity was unaffected in IR,

FR and FCC treatments in the low-risk situation

(Fig. 2b). However, both in IR (Tukey test: P�/0.017)

and FR (P�/0.012) treatments activity was roughly 50%

lower in the high-risk situation as compared to NR,

whereas in FCC treatment there was no difference

(Fig. 2b).

The general activity on day 24 was much lower

than during the two previous observation days (Fig. 2,

Table 3). While the general response to predation risk on

day 24 was still very strong in CR treatment (Fig. 2c,

Table 3), some important differences to the two previous

Table 1. Summary of the results of PCA on tadpole morphol-ogy. PC1 and PC2 refer to loadings of the first and secondeigenvectors.

Trait PC1 PC2

Body length 0.769 0.356Body depth 0.848 0.114Body width 0.870 0.069Tail length 0.800 0.162Tail depth 0.781 �/0.456Tail muscle depth 0.751 �/0.531Tail muscle width 0.765 0.255

% variation explained 63.8 10.4Eigenvalue 4.47 0.73

Table 2. Repeated measures ANOVA table for effects ofobservation day, risk and predator treatments on tadpoleactivity.

Source df F P

Between subjectsTreatment 4,35 66.25 B/0.001

Within subjectsDay 2,70 124.60 B/0.001Day�/treatment 8,70 8.57 B/0.001Risk 1,35 27.98 B/0.001Risk�/treatment 4,35 15.14 B/0.001Day�/risk 2,70 5.77 0.005Day�/risk�/treatment 8,70 6.46 B/0.001

OIKOS 107:1 (2004) 93

periods emerged. First, activity in the low-risk situation

was significantly lower in IR (P�/0.04), and almost so in

FR (P�/0.11) than in NR treatment (Fig. 2). Second, in

the high-risk situation activity in IR and FR treatments

was as low as in CR treatment (PB/0.001 as compared

to NR in each case), whereas there was no difference

between NR and FCC treatments (Fig. 2c). Hence, while

IR and FR treatments had only relatively weak effects on

activity of younger tadpoles, individuals near metamor-

phosis had as strong responses to short-term as to

constant predation risk.

Tadpole morphology

All morphological traits were strongly and significantly

influenced by the predator treatments (Table 4). PC1

scores, describing general body size, were significantly

higher in CR than in the other treatments (Tukey test

PB/0.001, Fig. 3), followed by the IR and FR treat-

ments. PC1 was lowest in FCC treatment, which tended

to differ (PB/0.083) from all the other treatments except

NR. PC2, describing inducible morphology, in NR

treatment differed strongly from all the other treatments

(PB/0.001, Fig. 3). The strongest inducible effect was

again found in CR treatment, which also differed from

the other treatments (PB/0.005, Fig. 3). IR, FR and

FCC treatments were intermediate and did not differ

from each other (Fig. 3).

Individual traits differed considerably in their re-

sponses to predator treatments. Residual body length

and tail depth responded in a similar graded manner as

PC2 (in which their factor loadings were high; Table 1);

Table 3. Repeated measures ANOVA tables for tadpole activity in different treatments during the three observation days.

Between subjects Within subjects

Treatment Risk Risk�/treatment

F4,35 P F1,35 P F4,35 P

Day 3 20.46 B/0.001 0.64 0.427 1.90 0.133Day 13 47.66 B/0.001 12.70 B/0.001 7.87 B/0.001Day 23 19.87 B/0.001 29.22 B/0.001 19.46 B/0.001

Fig. 2. Mean activity (9/SE) of R. temporaria tadpoles indifferent predator treatments and risk situations on (A) day 3,(B) day 12 and (C) day 24. Low risk: predator absent, high risk:predator present. In CR treatment predator was presentcontinuously. For abbreviations, see text.

Table 4. One-way ANOVA tables for effects of predatortreatments on tadpole morphology.

F4,35 P

PC1 13.03 B/0.001PC2 32.86 B/0.001Residual body length 9.20 B/0.001Residual body width 9.93 B/0.001Residual body depth 6.16 0.001Residual tail length 5.75 0.001Residual tail depth 21.95 B/0.001Residual tail muscle depth 15.66 B/0.001Residual tail muscle width 5.74 0.001

94 OIKOS 107:1 (2004)

NR and CR treatments represented the opposite ends of

the gradient and the three other treatments lied in

between (Fig. 3). The second type of response, shown

in both tail muscle traits, was the grouping of all

predator cue treatments into one group that differed

from NR treatment (Fig. 3). The third type of response

was somewhat unexpected variation among the predator

treatments so that some of the treatments differed from

NR treatments whereas some did not. In body width,

NR and CR treatments were intermediate between IR

and FCC treatments in which tadpole body was

relatively wide, and FR treatment were they were narrow

(Fig. 3). In body depth, tadpoles tended to have

relatively shallow body on FR and FCC treatments,

whereas CR treatment was intermediate (Fig. 3). Tail

length tended to be relatively shorter in the treatments

with predators. However, in FR treatment tail length was

equal to that in NR treatment (Fig. 3).

Metamorphic traits

There was a significant treatment effect on length of

larval period (F4,35�/7.19, PB/0.001, Fig. 4a), tadpoles

in CR treatment metamorphosing later than those in the

Fig. 4. Mean (9/SE) age (length of larval period), body length,body width and the first principal component of body size atmetamorphosis of R. temporaria in different predation risktreatments. For abbreviations, see text.

Fig. 3. Morphology (PCA scoresand individual trait values9/SE)of R. temporaria tadpoles indifferent predation risktreatments. PC1 describes generalbody size, PC2 describes shape ofthe tadpoles. For abbreviations,see text.

OIKOS 107:1 (2004) 95

other treatments (PB/0.016). While body length at

metamorphosis was not influenced by the treatments

(F4,35�/1.91, P�/0.130, Fig. 4b), there was a strong

effect on body width (F4,35�/8.54, PB/0.001). In gen-

eral, widest bodies were found in CR and narrowest in

FCC treatments while the other three treatments were

situated uniformly in between these (Fig. 4c). To

combine these two measures of body size we conducted

a principal component analysis and analysed the com-

ponent scores on PC1 indicating body size. We found a

strong treatment effect (F4,35�/7.35, PB/0.001) that was

due to CR treatment having larger scores than the other

treatments (PB/0.028). Hence, tadpoles in CR treatment

metamorphosed later but at a larger size than in the

other treatments.

Discussion

We did not find unequivocal support for the prediction

that animals express stronger antipredator behaviour in

brief and infrequent risk situations as compared to more

continuous risk (Lima and Bednekoff 1999). Neither did

we find support for the alternative hypothesis that

antipredator behaviour is independent of temporal

variation in risk. Instead, antipredator behaviour was

generally most pronounced under constant high risk.

The same was true for the morphological and life-history

traits; the strongest responses were found in CR treat-

ment, and in morphological traits the effects in the other

treatments were usually intermediate between CR and

NR treatments. Interestingly, close to metamorphosis

the tadpoles in IR and FR treatments showed increased

antipredator behaviour suggesting that sensitivity to

temporal variation in predation risk may depend on

the developmental stage.

Risk allocation hypothesis and tadpole behaviour

We found the strongest antipredator behaviour in CR

treatment, whereas reduction in activity in IR and FR

treatments was much weaker during the first two

observation periods. The weak response, although failing

to meet the predictions of the risk allocation hypothesis,

is in accordance with previous empirical studies on risk

allocation hypothesis (Sih and McCarthy 2002, Van

Buskirk et al. 2002). Indeed, in a study of Physa snails

Sih and McCarthy (2002) found a very similar beha-

vioural response in activity as we did on day 13. Van

Buskirk et al. (2002) found that R. lessonae tadpoles did

not allocate more feeding to low risk situations when

predation risk varied temporally. In the same study the

tadpoles were able to maintain high growth rates even

when they spent 83% of time in high-risk treatment. Our

results indicate that high growth rates were maintained

even in the continuous presence of predators, and hence

there were no energetic constraints that would have

prevented strong antipredator behaviour during short

periods of high risk. The study by Hamilton and

Heithaus (2001) is the only one so far to find evidence

for increased antipredator behaviour during shorter

pulses of risk. They found that activity of Littorina

snails increased with increasing time spent in the high-

risk environment.

In all time points behavioural responses in IR and FR

treatments were nearly identical indicating that tadpoles

did not differentiate between these treatments. Yet

tadpoles in FR treatment were facing the predation

threat three times as often as those in IR treatment,

suggesting that temporal variation in this time scale

made little difference for their behavioural antipredator

decisions. This weak behavioural response to varying

predation risk is in accordance with a previous study, in

which it took over 20 h for the tadpoles to fully adjust

their behaviour to a changed predation regime (Van

Buskirk 2002).

It seems likely that high activity level in the presence

of a dragonfly larva would be detrimental to tadpoles, so

why do they have such a slow behavioural response to

changes in predation risk? Memory constraints has been

proposed as a possible explanation for time delays in

behavioural responses (Van Buskirk 2002), however, an

alternative explanation seems possible. Chemical cues of

predators and predation events are important determi-

nants of prey behaviour in aquatic systems (Dodson et

al. 1994, Kats and Dill 1998), and in tadpoles there is

direct evidence that the concentration of chemical cues

plays an important role in antipredator behaviour with

higher concentration of cues creating stronger response

(Horat and Semlitsch 1994, Eklov 2000). It seems

possible that the concentrations of chemical cues may

have been different between CR and the other treat-

ments, and we suggest that this may, at least partly,

explain our results. Although the number of tadpoles

eaten was the same, the predators in the CR treatments

were present continuously, which may have increased the

concentration of the cues through, for example, predator

excretion. The present results suggest that when cue

concentration and amount of temporal variation in

predation risk are correlated, the straightforward appli-

cation of the risk allocation model may be challenged in

systems where chemical cues are important. While our

study can be criticized as being conducted in laboratory,

similar variation in chemical cue concentrations is likely

to occur in natural systems (Turner and Montgomery

2003).

Antipredator behaviour of the tadpoles in IR and FR

treatments increased as development proceeded, and

when tadpoles were close to metamorphosis the reduc-

tion in activity was as strong in IR and FR treatments as

in CR treatment. We suggest that the differences among

96 OIKOS 107:1 (2004)

time points could be explained in terms of stage-

dependency. Because of changes in body shape and

drag due to hind limbs, tadpoles close to metamorphosis

may be more vulnerable to predators than less developed

individuals of similar size (Wassersug and Sperry 1977,

Brown and Taylor 1995). Furthermore, close to meta-

morphosis tadpoles stop growing and strongly reduce

foraging (Denver 1997, Laurila and Kujasalo 1999) and

may hence have little to gain with high activity levels. A

plausible mechanism allowing the response could be that

tadpoles close to metamorphosis are more sensitive to

predator cues.

There are good theoretical grounds to expect state-

dependency in tadpole behaviour. Houston et al. (1993)

distinguished between cases where an animal must reach

a fixed state (i.e. maturation, metamorphosis) its fitness

depending on when this state is attained, as opposed to

cases where animals must survive to a fixed time, its

fitness depending on its final state. While the original

risk allocation model deals with the latter possibility

(Lima and Bednekoff 1999), it seems more suitable

scenario for tadpoles close to metamorphosis. Increased

antipredator behaviour close to metamorphosis is also in

accordance with the asset protection principle (Clark

1994): the larger the current asset in terms of fitness, the

more important its protection becomes and the stronger

is the sensitivity for predation risk. Hence, tadpoles close

to the end of the larval stage may be less willing to take

risks.

Temporal variation in predation risk and inducible

morphology

Inducible defences were most evident in PC2, relative

body length, and tail and tail muscle depth in CR

treatment, and trait values in the other three predator

treatments were often, but not always, intermediate

between CR and NR treatments. As such, the results

indicate that morphological responses are continuous

rather than threshold traits (with the possible exception

of tail muscle width, Van Buskirk and Arioli 2002), and

that even relatively infrequent encounters with predators

are enough to induce significant plastic responses in

R. temporaria morphology.

Our results are in accordance with the predictions of

Padilla and Adolph (1996): when the time lag in the

plastic response is longer than the scale of environmental

variability, plasticity becomes disadvantageous and less

or no plasticity is expected. Hence, the time periods of 22

h (FR treatment) or 70 h (IR treatment) in between the

exposures to high risk may have been sufficient to

prevent morphological defences from fully developing.

This reasoning is supported by the observations that

morphological defences in tadpoles are labile and

converge steeply towards the non-induced phenotype

when predation risk is removed (Van Buskirk 2002,

Relyea 2003). Mechanistically, the variation in morpho-

logical responses could be explained with the same

dose-dependency of chemical cues as the behavioural

responses.

We found considerable variation in the pattern of how

individual morphological traits were affected by pre-

dator treatments. Relative body size and tail fin depth

were strongly affected in CR treatment and the responses

in the other predator treatments were intermediate,

reflecting the pattern found in behaviour at day 13.

Both tail muscle traits also showed strong induced

response with little variation among the predator treat-

ments. These traits have been shown to be under strong

selection by dragonfly predation (Van Buskirk and

Relyea 1998). However, tail length and body length

and depth showed a more variable pattern of plasticity

with strongest effects often found in short-term risk

treatments. While the adaptive value of these plastic

changes is unclear, we suggest that they may be of minor

importance and that selection imposed on these traits

may be weaker.

In FCC treatment, chemical cues from a predation

event did not have an effect on behaviour after day 3.

However, the response in induced morphology was in

most traits as strong as in IR and FR treatments. This

indicates that, in repeated exposure to predators, visual

cues are needed for the expression of antipredator

behaviour, whereas chemical cues of a predation event

are sufficient to induce morphological defences. There is

evidence that when exposed to chemical cues the

behavioural response of prey animals may wane over

time (Holomuzki and Hatchett 1994, Turner 1997),

suggesting that habituation may be an issue in long-

lasting behavioural studies using only chemical cues.

Life-history

In amphibians, both size at and time of metamorphosis

have been found to affect later fitness through effects on

juvenile survival and adult fecundity (Smith 1987,

Berven 1990, Scott 1994, Altwegg and Reyer 2003). We

found that larval period was longer and tadpoles were

larger (especially in terms of body width) in CR

treatment indicating that increased investment in de-

fences entailed a delayed metamorphosis at a larger size.

Interestingly, the effects of other predator treatments on

life history traits were weak, possibly because the

tadpoles adjusted their defence levels to minimise the

effects on life history. Paradoxically, tadpoles metamor-

phosing from CR treatment may actually enjoy en-

hanced fitness because of their larger body size. For

example, Altwegg and Reyer (2003) found that meta-

morphic size had generally much stronger effects on later

survival of juvenile frogs than timing of metamorphosis

OIKOS 107:1 (2004) 97

(Van Buskirk and Saxer 2001). As there are strong

theoretical arguments for adaptive plasticity to be costly,

it seems that the costs either are more complex (i.e.

genetic; Tollrian and Harvell 1999, Agrawal 2001) than

simple growth and development effects detectable at

metamorphosis, or that they arise from imperfect match

between the phenotype and the environment (Padilla

and Adolph 1996, Tollrian and Harvell 1999, Agrawal

2001, Van Buskirk and Saxer 2001, Relyea 2002).

In conclusion, we found that R. temporaria tadpoles

exhibit strongest antipredator behaviour when the pre-

dators are continuously present. This result was largely

repeated in morphological defenses and in life history

traits. However, while the results were not as predicted

by the risk allocation model, the effects of temporal

variation in predation risk has to be taken into account

in future investigations. The reasons for the discrepancy

between model predictions and our results are unclear,

but differences in chemical cue concentrations may

provide an explanation, at least in many aquatic systems.

We also found that behavioural response to temporal

variation in predation risk was stage-dependent, and

understanding the causes of such stage-dependency

provides a challenge for future research.

Acknowledgements �/ We thank C. Teplitsky for comments on anearlier draft of the manuscript. Our research was funded by theAcademy of Finland (SP, JM) and the Swedish ResearchCouncil (AL).

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