SEARCH

SEARCH BY CITATION

Keywords:

  • bet-hedging;
  • egg size;
  • growth;
  • investment;
  • metamorphosis;
  • tadpoles;
  • unpredictable;
  • unpredictability

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Variable maternal provisioning may evolve when there is variation in the quality of offspring environments. The frog Crinia georgiana has high variability in egg size both within and between clutches, independent of female phenotype. It breeds in ponds with high spatial and temporal variation in habitat quality. Egg size strongly affected offspring fitness in good and poor quality offspring environments, whether the egg size difference was from between or within clutches. Since there is a trade-off in egg size and number, these fitness consequences translate to strong effects on maternal fitness. In the variable and unpredictable offspring environment of C. georgiana, the maintenance of variable maternal provisioning both within and between clutches is likely to be an evolved response to the offspring environment.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Models that assume fitness increases monotonically with offspring size predict that a single optimal offspring size exists that maximizes parental fitness (Smith & Fretwell, 1974; Lloyd, 1987; Winkler & Wallin, 1987). This is true for a constant offspring environment, but if the offspring environment varies in quality then multiple offspring size optima may exist. Patterns of maternal provisioning are likely to reflect adaptive processes especially if selection occurs on post-hatching offspring (Mousseau & Fox, 1998; Einum & Fleming, 2002). Therefore variation in maternal provisioning is thought to evolve in response to variability and unpredictability in the quality of the offspring environment (Crump, 1981; Kaplan & Cooper, 1984). In attempting to understand the relationship between variability in maternal provisioning and variability in the offspring environment, the fitness of different sized offspring must be measured. More critically, the parental fitness of the various offspring provisioning strategies must also be measured since maximizing offspring fitness does not always maximize parental fitness (Smith & Fretwell, 1974).

Both within and between clutch variation in maternal provisioning occurs in many taxa (see Capinera, 1979; Kaplan & Cooper, 1984), and is well documented in amphibians (Kaplan, 1980; Crump, 1981, 1984; Berven & Chadra, 1988; Williamson & Bull, 1989; Tejedo & Reques, 1992; Beachy, 1993; Kaplan & King, 1997). Females of the myobatrachid frog, Crinia georgiana, lay egg clutches in which egg size can be variable both within and between clutches. The amount of yolk in an egg may be critical to the survival and fitness of offspring. In the aquatic larval stage, offspring are vulnerable to a range of unpredictable biotic (for example predation, competition, food availability), and physical risks, especially desiccation, that are associated with temporary freshwater aquatic habitats (Wassersug, 1975; Alford, 1999). Variation in initial egg size may modulate those unpredictable environmental effects.

Crinia georgiana breed in small, shallow temporary ponds around granite outcrops in the southwest of Australia where ponds fill during winter rainfall. Females enter choruses of males at these breeding sites with eggs already provisioned and seek a male calling from one of these ponds to amplex with and deposit eggs. The rain-filled ponds that C. georgiana use for reproduction can vary spatially in size and depth (Seymour et al., 2000; Dziminski, unpublished data) and this variation coupled with density dependent effects may lead to offspring experiencing extremely limited food resources especially in the smallest ponds. These ponds can dry out and fill several times within the breeding season and the length of time a pond contains water can vary unpredictably (Seymour et al., 2000; Doughty & Roberts, 2003; Dziminski, unpublished data). Larvae are at high risk of death by desiccation. If there is a trade-off in egg size and number then spatial variation in the quality of the offspring environment can lead to selection for high variability in offspring provisioning between clutches (McGinley et al., 1987; Poulin & Hamilton, 2000). Temporal within-season variability and unpredictability in the quality of the offspring environment may be acting as a selective force favouring high variability in offspring provisioning within clutches as a bet-hedging strategy (Crump, 1981; Philippi & Seger, 1989).

For egg size to be selected upon, there must be differences in fitness between offspring derived from eggs of different sizes. Several studies have found egg size to affect hatching size or early development (Crump, 1984; Williamson & Bull, 1989; Tejedo & Reques, 1992; Laugen et al., 2002; Loman, 2002) but these differences disappeared when offspring were reared to metamorphosis. Other studies have shown that offspring from larger eggs reached metamorphosis faster and at larger body sizes than those from smaller eggs (Kaplan, 1992; Parichy & Kaplan, 1992). In contrast, Berven & Chadra (1988) showed that offspring from small eggs had a longer larval period than offspring from large eggs, but only when in low density metamorphosed at a larger size than offspring from large eggs.

The responses of tadpoles to risk of pond drying were modelled by Wilbur & Collins (1973) who predicted rapid drying would lead to increases in development rate at the expense of increases in body size. Yolk provisioning and food availability, even in drying ponds, may modify these outcomes; for example, more yolk may still allow rapid development but also an increase in body size, giving offspring from larger eggs a double advantage.

Our study focused on the relationship between variation in initial maternal provisioning and food available to developing tadpoles. Overall we aimed to determine the fitness consequences of variable maternal provisioning in C. georgiana. Because of the extreme difference in size between large and small eggs we expected a strong positive relationship between egg size and offspring fitness, and therefore strong maternal fitness consequences arising from differential maternal provisioning strategies. We also expected that high food levels might compensate for small egg size in determining size at and time to metamorphosis. Specifically our objectives were:

  • 1
    to determine the patterns of maternal provisioning and if a trade-off between egg size and number exists;
  • 2
    to determine offspring fitness consequences of large and small eggs from different clutches;
  • 3
    to determine offspring fitness consequences of large and small eggs from within clutches, and lastly, if an egg size-number trade-off exists;
  • 4
    using (2) and (3) above, to determine maternal fitness consequences of producing clutches of fewer large eggs, more numerous smaller eggs or clutches that contain large and small eggs.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Patterns of maternal provisioning

Thirty-three nonamplexed, gravid female C. georgiana were collected together with an equal number of calling males from a breeding site in the Darling Ranges, approximately 50 km southeast of Perth, Western Australia over five nights in June 2002. Each night frogs were transported to the laboratory in Perth. Each female together with a randomly chosen male was placed in a clear round plastic container (8.5 cm diameter × 11 cm) with a lid and 1 cm depth of tap water purified by a reverse osmosis filter. Frogs were left in a dark room at 15 °C until eggs were deposited. Every female amplexed with her assigned male and egg deposition was usually complete within 10 min.

Upon completion of egg deposition, the yolk volume of every single egg in every clutch was measured using the method of Dziminski & Alford (2005). Females were candled using a bright light to ensure all ova were deposited and were then weighed to the nearest mg using an electronic balance. No females retained any ova.

Possible relationships between female size, total yolk volume output and clutch parameters: egg number, mean egg size and the coefficient of variation of egg size (CV = SD/mean), were explored using Pearson correlations. The presence or absence of relationships between clutch parameters might be caused by their mutual correlations with parental body size or total yolk volume output; therefore we also calculated partial correlations among the clutch parameters controlling for the effect of female size and total yolk volume output. To illustrate such relationships the residuals from multiple regressions of the clutch parameter with female size and total yolk volume output were plotted.

Fitness of offspring from small and large eggs from different clutches

Over three wet nights in 2003 (25, 27 and 29 June), nonamplexed, gravid female C. georgiana were collected together with an equal number of calling males from the same breeding site described above. Females were set up for oviposition in the laboratory as detailed above. A total of 84 individual eggs, each egg chosen from a separate clutch with an independent set of parents, were randomly assigned to 1 of 12 experimental treatments. The treatments were elements of a 2 × 2 × 3 mixed model factorial design. The first fixed factor, egg size, consisted of two size categories: small and large. On each night, clutches were assigned as either containing large or small eggs. A single egg from each assigned clutch was placed in a round plastic container (8.5 cm diameter × 11 cm) without a lid in 2 cm of purified tap water. To confirm our assignment of egg size categories, the yolk volume of all the eggs in 11 clutches of each size category were measured on the first night and compared using a one-way anova. Clutches assigned as large had significantly larger eggs (Table 1; F1,20 = 32.756, P < 0.001), confirming that our categorization of egg sizes accurately reflects egg size.

Table 1.  Means and standard deviations of egg sizes (mm3) used in experiments.
Source of variationEgg size category
LargeSmall
Interclutch6.63 (1.12)4.49 (0.54)
Intraclutch8.46 (3.56)4.70 (1.66)

The second fixed factor was food level, consisting of two categories: fed and unfed. Tadpoles in fed treatments were fed 5 mg of a ground and sieved (250 μm) 3 : 1 mixture by weight of lucerne (alfalfa) pellets and TetraMin®tropical fish food (TetraWerke, Melle, Germany) beginning 3 days after hatching and then every 3 days until Gosner (1960) stage 42. This feeding regime produces metamorphs of the same size range as found in the field (Doughty & Roberts, 2003). Tadpoles in unfed treatments were not fed at all from hatching through to metamorphosis. Crinia georgiana is known to be able to complete the larval stage without feeding (Doughty, 2002). The final factor, night of collection, was a random factor with three categories: 32 eggs (16 small and 16 large) from each of the first two nights were used and 20 eggs (10 small and 10 large) were used from the night of the 29th. Tadpoles were transferred to a fresh container every three days just prior to feeding. Each experimental unit (one tadpole in one container) was assigned a random position on a bench in a controlled temperature room at 15 °C with a 12/12 h light/dark photoperiod provided by fluorescent overhead lighting. Grow lights (Gro-lux® Sylvania, Danvers, MA, USA) provided additional UV light for 3 h centred on mid-day.

The dependant variables were size at metamorphosis, length of larval period and survival to metamorphosis. The larval period was defined as commencing the day of fertilization and ending at Gosner (1960) stage 46, as this is the stage most larvae emerge from ponds in the field (Dziminski, personal observation). When both forelimbs had emerged, water in the container was replaced with a layer of circular wet sponge material (5 mm thick) until Gosner (1960) stage 46 (complete tail resorbtion), at which point the metamorph was blotted dry with a cotton towel and weighed to 0.1 mg precision on an electronic balance.

Data were normally distributed and the assumption of homogeneity of variance among treatment means was satisfied. anova was used to compare the effects of egg size, food level and the night of collection, and their interactions, on both size at metamorphosis and the length of larval period. The proportion of larvae surviving in each treatment was arcsine transformed (Zar, 1999) into degrees and an anova without replication was used to compare the effects of egg size, food level and the night of collection, and the two-way interactions of these factors. The mean square of the egg size*food level*night interaction was used as the error mean square in the calculation of F for the two-way interactions. anova was performed using SPSS statistical software (Version 12.0.1) that considers random variables and corrects for unequal sample size in treatments due to mortality.

Fitness of offspring from small and large eggs from within clutches

A small egg and a large egg were selected from within each of five clutches that contained small and large eggs produced by females collected on the night of the 29th June 2003 from the same breeding site described above. Females were set up for oviposition in the laboratory as detailed above. The yolk volume of all selected eggs was measured using the method of Dziminski & Alford (2005). The large eggs chosen from within clutches were significantly larger than small eggs chosen from within clutches (Table 1; paired-samples t-test: t4 = 4.419, P < 0.05). Each egg was placed in a container and raised as detailed above for the fed treatment. The dependant variables were again size at metamorphosis and length of larval period as detailed above. A paired-samples t-test was used to compare these variables in the two egg size treatments.

Comparison of fitness of offspring from small and large eggs from within clutches to offspring from equivalent egg sizes from uniform clutches

A one-way anova with planned comparisons revealed that there were significant differences between all large and small eggs of both clutch types (F3,28 = 7.468, P < 0.05), but that there was no difference in the size of large eggs from clutches that contained large and small eggs and from clutches that contained just large eggs (t28 = 0.980, n.s.), and that there was no difference in the size of small eggs from clutches that contained large and small eggs and from clutches that contained just small eggs (t28 = 0.077, n.s.). These planned comparisons were applied to compare the size at and time to metamorphosis of offspring from small and large eggs from within clutches to offspring from equivalent egg sizes from uniform clutches.

Maternal fitness

To determine the effects of egg size on maternal fitness we calculated the maternal fitness values for two egg provisioning strategies (large eggs and small eggs), under the two environmental conditions (food available and no food available) that larvae may experience. In this scenario we assumed ponds persist and therefore the length of the larval period is not critical to fitness. A female has a certain amount of energy available for reproduction, which we measured as the total yolk volume output per clutch. We used the mean of all clutches in our survey for this value (864.68 mm3). If a trade-off between egg size and number exists, this total yolk volume output can be divided up into fewer larger eggs or more numerous smaller eggs. This value was then divided by the mean yolk volume of the two egg size categories used in both experiments above (Table 1; small = 4.49 mm3, large = 6.63 mm3) to calculate the number of eggs of each size category a female can produce as a result of the trade-off. Many studies have used size at metamorphosis of amphibians as a measure of offspring fitness (review in Alford, 1999), and this measure has been proven to reflect the relative probability to survival and reproduction of a metamorph (Altwegg & Reyer, 2003). Therefore, as the measure of offspring fitness, from each treatment of the food*egg size experiment we used the product of size at metamorphosis, survival to metamorphosis and the logarithmic regression of offspring size and egg size in the equivalent food treatment, this includes the assumption of a curvilinear relationship of offspring size and fitness (e.g. Smith & Fretwell, 1974). In the case of the treatments where survival did not differ significantly (offspring from large eggs that were fed and unfed, and offspring from small eggs that were fed) we used the mean proportion of survival in these three treatments, pooled for the calculation. Since maternal fitness is the product of offspring fitness and the number of offspring produced (Smith & Fretwell, 1974), we used the following formula to calculate the maternal fitness value of producing fewer large eggs or more numerous smaller eggs under fed and unfed conditions that offspring experience:

  • image

where Wp is the maternal fitness index, Wy is the offspring fitness, which is the product of offspring size (mg), proportion of survival to metamorphosis and the logarithmic regression of offspring size and egg size in the equivalent food treatment, Ip is the total amount of yolk the parent has to provision offspring in mm3, and Iy is the amount of yolk per offspring (effort per offspring) in mm3.

In the next scenario, we include the fact that some ponds dry earlier and therefore the length of the larval period becomes critical to fitness. To include the effects of time to metamorphosis on maternal fitness, we used the above formula to calculate maternal fitness values in two environmental conditions: (1) ponds that dried early in which only offspring from large eggs have enough time to metamorphose and (2) persistent ponds in which offspring from any egg size will survive. Thus the differential survival due to the length of the larval period of each egg size is included in the calculation of maternal fitness. This time maternal fitness was calculated for three egg provisioning strategies: (1) large eggs only, 2) small eggs only and (3) half of the energy available used for large eggs, and the other half for small eggs resulting in a mixed egg size strategy.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Patterns of maternal provisioning

Egg size varied both between and within clutches (Fig. 1a). There was a significant positive relationship between female size and egg number (Fig. 1b; Pearson correlation: r = 0.799, P < 0.001), and total yolk volume output (Fig. 1c; Pearson correlation: r = 0.799, P < 0.001). Total yolk volume output and egg number were also significantly positively related (Fig. 1d; Pearson correlation: r = 0.849, P < 0.001). Egg number was not related to female size when controlling for total yolk volume output, but was related to total yolk volume output when controlling for female size (Fig. 1e; partial correlation: r = 0.662, P < 0.001). This means that females that produced more yolk used this extra yolk to produce more eggs. Egg size was not related to female size with or without controlling for total yolk volume output (Fig. 1a). Egg size was not related to total yolk volume output with or without controlling for female size. CV was not related to female size or egg number or mean egg size whether controlling for female size and total yolk volume separately, together, or not controlling. When controlling for female size and total yolk volume, together or separately, there was always a significant negative relationship between mean egg size and egg number (Fig. 1f; partial correlation: r = −0.961, P < 0.001).

image

Figure 1. (a) Variation in yolk volume between and within clutches. Bars represent the interquartile range divided by the mean. (b) The relationship between female size and egg number. (c) The relationship between female size and total yolk volume output. (d) The relationship between total yolk volume output and egg number. (e) The relationship between egg number and total yolk volume output controlled for female size. Residuals from multiple regressions of egg number with female size and total yolk volume output, with female size are plotted. (f) The trade-off between egg size and number. Residuals from multiple regressions of egg number with female size and total yolk volume output, and mean yolk volume with female size and total yolk volume output are plotted.

Download figure to PowerPoint

Fitness of offspring from small and large eggs from different clutches

Night of collection was not a significant main effect nor did it have any significant interactions with other terms in any analyses (Tables 2 and 3) so this effect is not discussed further below or included in figures. Both egg size and food level had a significant effect on the size at metamorphosis of offspring (Fig. 2a, Table 2a). The egg size*food level interaction was significant (Table 2a) indicating that offspring from different egg sizes responded differently to food level. The difference in size between fed and unfed metamorphs from small eggs was much greater than the difference in size between fed and unfed metamorphs from large eggs (Fig. 2a).

Table 2. anova of (a) size at metamorphosis and (b) length of larval period.
Sourced.f.MSFP
  1. *Total d.f. are different because some offspring died between metamorphosis and weighing later that day, these five were excluded from the size at metamorphosis analysis.

(a) Size at metamorphosis
 Egg size (E) 1507.938125.844<0.01
 Food level (F) 11499.395306.901<0.01
 Night (N) 212.1251.532n.s.
 E × F 178.78478.232<0.05
 E × N 24.0254.080n.s.
 F × N 24.8774.943n.s.
 E × F × N 20.9870.126n.s.
 Error507.815  
 Total61*   
(b) Length of larval period
 Egg size (E) 1454.83169.886<0.05
 Food level (F) 170.19618.661<0.05
 Night (N) 21.7200.688n.s.
 E × F 121.3232.750n.s.
 E × N 26.5470.838n.s.
 F × N 23.7640.482n.s.
 E × F × N 27.8102.204n.s.
 Error553.543  
 Total66*   
Table 3. anova of arcsineinline image in degrees.
Sourced.f.MSFP
Egg size (E)11093.9436.449n.s.
Food level (F)12004.93457.080<0.05
Night (N)253.1680.345n.s.
E × F13189.78462.869<0.05
E × N2169.6243.343n.s.
F × N235.1250.692n.s.
E × F × N (error)250.737  
Total11   
image

Figure 2. (a) Least squares mean size at metamorphosis of all treatments. (b) Least squares mean length of larval period of all treatments. (c) Least squares mean survival to metamorphosis of all treatments. (d) Mean size at metamorphosis of offspring from small and large eggs from within clutches. (e) Mean length of larval period of offspring from small and large eggs from within clutches. All bars represent ±1 SE. See Table 2a, b, and 3 and text for statistical analyses.

Download figure to PowerPoint

Both egg size and food level had a significant effect on the length of the larval period but this time there was no significant interaction (Fig. 2b, Table 2b). Offspring from large eggs metamorphosed in less time than offspring from small eggs and fed offspring metamorphosed in less time than unfed offspring (Fig. 2b).

The main effect of food level had a significant effect on the survival of offspring to metamorphosis (Table 3), however the significant effect of the egg size × food level interaction (Table 3) indicates that only offspring from small eggs that were unfed experienced much lower survival to metamorphosis (Fig. 2c). Only 28% of offspring in this treatment survived to metamorphosis.

Fitness of offspring from small and large eggs from within clutches

All offspring survived. Egg size had a significant effect on both size at metamorphosis (Fig. 2d; t4 = 3.034, P < 0.05) and the length of the larval period (Fig. 2e; t4 = 5.439, P < 0.01). Within clutches, offspring from large eggs metamorphosed in less time and at a larger size than offspring from small eggs (Fig. 2d,e).

Comparison of fitness of offspring from small and large eggs from within clutches to offspring from equivalent egg sizes from uniform clutches

There were significant differences in size at metamorphosis and the length of the larval period between offspring from large and small eggs originating from both clutch types (size at metamorphosis: F3,45 = 9.622, P < 0.001; larval period: F3,46 = 18.269, P < 0.001). There were no significant differences in the size at metamorphosis (t45 = 1.386, n.s.) or the length of the larval period (t46 = 0.296, n.s.) of offspring from large eggs from clutches that contained large and small eggs and from clutches that contained just large eggs (Fig. 2a,b,d,e). There were no significant differences in the size at metamorphosis (t45 = 0.944, n.s.) or the length of the larval period (t46 = 1.318, n.s.) of offspring from small eggs from clutches that contained large and small eggs and from clutches that contained just small eggs (Fig. 2a,b,d,e). Therefore offspring originating from the same egg size performed the same whether they originated from a clutch with variable egg size or a uniform egg size within a clutch.

Maternal fitness

In a persistent good quality offspring environment, producing clutches of more numerous small eggs results in the highest maternal fitness, but in a poor quality offspring environment producing clutches of fewer larger eggs results in higher maternal fitness (Fig. 3a). When time to metamorphosis becomes a critical factor in offspring fitness, then production of large eggs results in the highest maternal fitness in the shorter duration ponds regardless of whether food is available (Fig. 3b). When food is available, production of large eggs results in the highest maternal fitness in shorter duration ponds. In the longer duration ponds the production of small eggs results in the highest maternal fitness (Fig. 3b). The mixed strategy has intermediate maternal fitness between the small and large strategy.

image

Figure 3. (a) Maternal fitness of two possible offspring provisioning strategies for C. georgiana in two offspring environments. Lines adjoin equivalent offspring provisioning strategies: Dashed line, large eggs; solid line, small eggs. (b) Maternal fitness of three possible offspring provisioning strategies for C. georgiana in two offspring environments. Lines adjoin equivalent offspring provisioning strategies: Dashed line, large eggs; solid line, small eggs; dotted line, mixed egg size strategy (half of the energy available used for large eggs, and the other half for small eggs). All bars represent ±1 SE. See text for calculation of the maternal fitness index.

Download figure to PowerPoint

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Patterns of maternal provisioning

Larger females had a larger total yolk volume output and used this larger amount to produce more eggs. This is not surprising since larger females, as a result of increased size, have an increased amount of energy available for reproduction (Castellano et al., 2004). Larger females, however, did not produce larger eggs. Egg size is independent of female size and is therefore likely to have evolved in response to selection imposed by the offspring environment. There was a trade-off in egg size and number. This means that, regardless of female size and total yolk output, females that produced larger eggs produced fewer eggs. Variability in egg size within clutches was not related to female size, total yolk output, egg number or mean egg size. This supports the idea that variation in egg size within clutches being independent of these other parameters is an adaptive strategy in response to the offspring environment (Crump, 1981; Kaplan & Cooper, 1984; Koops et al., 2003). The egg size-number trade-off also applies to females producing clutches with variable egg sizes. A female producing more larger-sized eggs within a clutch produces fewer eggs in total and a female producing more smaller-sized eggs within a clutch produces a higher total egg number. Egg size therefore is independent of maternal phenotype. A single offspring size has not evolved. If the maintenance of small clutches of large eggs, larger clutches of small eggs and clutches of variable sized eggs in the population is an evolved response then egg size needs to affect offspring fitness and thus these strategies must result in maternal fitness consequences.

Since there was no relationship between egg size and variation in egg size within clutches our results do not support the imperfect information hypothesis (see Koops et al., 2003 for review) in this species. This hypothesis predicts that females should produce variable clutches to offset wrong decisions when trying to produce eggs that are optimal for predicted future conditions. Under this scenario, unequal allocation within clutches decreases with increasing mean egg size. This relationship is not present in C. georgiana.

Fitness of offspring from small and large eggs from different clutches

Several studies examining the effects of egg size have found that effects on offspring size (Crump, 1984; Tejedo & Reques, 1992; Loman, 2002) and the length of the larval period (Crump, 1984; Tejedo & Reques, 1992; Laugen et al., 2002) are evident at hatching or early in the larval period but disappear towards metamorphosis. Lack of fitness consequences of offspring size also occurs sometimes in other taxa (e.g. fish: Sehgal & Toor, 1991; birds: Smith et al., 1995; mammals: Oksanen et al., 2001). In C. georgiana, egg size has an important influence on size at metamorphosis and thus offspring fitness (Altwegg & Reyer, 2003). Furthermore egg size also influences the length of the larval period, which again has important fitness consequences in the temporary freshwater environments that many amphibians use for reproduction (Wassersug, 1975; Alford, 1999). Berven & Chadra (1988) found that at lower densities, offspring from small eggs in higher food treatments metamorphosed at a larger size than offspring from large eggs. This is similar to our result where we detected a significant interaction of egg size with food level, indicating that offspring from small eggs in the fed treatment were growing at a proportionally faster rate and closing on the size of the offspring from large eggs. However in C. georgiana the offspring from small eggs never surpassed the size of offspring from large eggs in the same food level treatment. Parichy & Kaplan (1992) also found this relationship although it disappeared in higher quality (more food) treatments, unlike our results where the size difference persisted in our fed treatments. Our study of C. georgiana is unique in that strong influences of egg size on size at metamorphosis and the length of the larval period persist in both a high quality (fed) and low quality (unfed) larval environment.

In some studies, egg size did not affect larval survival (Crump, 1984; Brooks et al., 1991; Tejedo & Reques, 1992; Smith et al., 1995). Parichy & Kaplan (1992) found that the probability of surviving to metamorphosis was lower for offspring from small eggs across both high quality (high food) and low quality (lower food) treatments. In our study egg size had an important effect on survival to metamorphosis, where greater maternal investment per offspring is needed to ensure survival in a poor quality offspring environment when food resources are not available.

Egg size had a strong influence on offspring fitness, therefore coupled with the egg size-number trade-off, should have strong influences on maternal fitness. Offspring from different egg sizes performed differently under different conditions, therefore the strategies of producing clutches of fewer large eggs or clutches of more numerous smaller eggs should result in different maternal fitness consequences under different quality offspring environments.

Fitness of offspring from small and large eggs from within clutches

Some studies have found no difference in the fitness of offspring from small and large eggs from within clutches (e.g. reptiles: Brooks et al., 1991; amphibians: Tejedo, 1992; birds: Krist et al., 2004). In our study, just as egg size had an important influence on offspring from different clutches, this egg size effect also had important consequences for fitness of offspring from different sized eggs from within a clutch. Therefore producing a clutch with variable egg sizes would lead to different maternal fitness consequences when compared with producing a clutch with a uniform egg size.

Comparison of fitness of offspring from small and large eggs from within clutches to offspring from equivalent egg sizes from uniform clutches

We found that egg size is a driving force for offspring fitness independent of female phenotype and that its effect is equivalent whether the different egg sizes arise from different clutches or from within the same clutch. Therefore the same offspring fitness values, as a result of egg size, can be applied to offspring from equivalent egg sizes from clutches of variable sized eggs or from clutches of uniform sized eggs.

Maternal fitness

In effect this experiment simulated two offspring size categories on the Smith & Fretwell (1974) model of offspring provisioning in two different environments (fitness curves) that offspring may experience: poor quality (unfed treatments) and good quality (fed treatments).

At the breeding site visited in this study, C. georgiana tadpoles are found in ponds ranging from only a few centimetres in diameter to up to 2 m across. Depths of these ponds also vary from 0.5 cm up to 15 cm (Seymour et al., 2000; Dziminski, unpublished data). Therefore there is spatial heterogeneity in pond size. Food resources are likely to vary between these ponds and can be very low in some (Doughty & Roberts, 2003), but even if food resources are equally distributed among ponds per unit of water or per area of pond substrate [since C. georgiana tadpoles are benthic (Main, 1957)], then density effects, if the female was to oviposit in one of the smaller ponds, would reduce the amount of food available per offspring in these crowded ponds. Therefore offspring may encounter high or low food conditions, and in the smallest ponds tadpoles may even experience a total lack of food. If a female produces small eggs and happens to oviposit in a large (food available) pond, this would result in a higher maternal fitness value. However, if she happened to oviposit these eggs in a small (low or no food available) pond, this would then result in a lower maternal fitness value (Fig. 3a). The reciprocal maternal fitness values would result if the female produced large eggs (Fig. 3a). It is unlikely that C. georgiana females choose ponds; rather, they choose males (Smith & Roberts, 2003), and in many cases females are intercepted by sneak or satellite males (Byrne, 2004; Byrne & Roberts, 2004) eliminating the opportunity of mating with a particular male and thus ovipositing in a particular pond. The opportunity for male choice of calling site based on pond quality (Howard, 1978) may exist but this interaction would be complex involving the male attracting a female that is carrying the right sized eggs for the pond he is calling from. Data on oxygen tension at oviposition sites suggest egg deposition is not driven by site quality (Byrne & Roberts, 1999; Seymour et al., 2000). More likely is the possibility that larvae may disperse into nearby ponds that are appropriate for their size. This is possible during periods of high rainfall when ponds become connected by a large amount of surface water (Dziminski, personal observation). This spatial heterogeneity in the quality of the offspring environment coupled with density effects and the opportunity for offspring to disperse into appropriate habitats creates the requirements for the selection of variable maternal investment between clutches (McGinley et al., 1987).

As well as being spatially variable in quality, the ponds C. georgiana use for reproduction can vary temporally in quality within the breeding season. These ponds can dry and fill several times within the breeding season and the length of time a pond contains water can vary unpredictably from persisting only a few days to persisting the entire breeding season (Seymour et al., 2000; Doughty & Roberts, 2003; Dziminski, unpublished data). To escape desiccation larvae must metamorphose. Production of large eggs results in the highest maternal fitness in the shorter duration ponds regardless of whether food is available (Fig. 3b). When food is available, production of large eggs results in the highest maternal fitness in shorter duration ponds (poor offspring environment) because offspring from small eggs cannot metamorphose in time and perish (Fig. 3b). In the longer duration ponds (good offspring environment) the production of small eggs results in the highest maternal fitness (Fig. 3b). The mixed strategy has intermediate maternal fitness to the small and large strategy, but has the benefit of ensuring some offspring surviving in the shorter duration ponds (Fig. 3b): a form of bet hedging (Crump, 1981; Philippi & Seger, 1989). Within-season temporal variation in the quality of the offspring environment is thought to lead to selection for variation in offspring size if parents can assess prevailing conditions and produce offspring of the appropriate size (McGinley et al., 1987). However, it is unlikely that C. georgiana females would assess the quality of the offspring environment taking into account factors noted above plus: (1) the high spatial variation in the quality of ponds in terms of food resources, (2) the temporal within-season variation in pond persistence and the unpredictability of this variation, for example the pond may persist, or dry early depending on the future weather conditions and (3) the interaction of (2) above and spatial variation in the temporal within-season variation of ponds. This complex set of interactions, that can be additive or cumulative, coupled with unpredictability may create the condition where selection favours a bet-hedging strategy such as producing mixed egg sizes within a clutch (Fig. 3b). This relationship though, would depend intrinsically on the proportion of energy invested into small and large offspring, the frequency of good offspring conditions, and the magnitude of the difference in the quality of the offspring environments, spatially and temporally (McGinley et al., 1987; Einum & Fleming, 2004). As the data are for eggs collected from individuals in the wild, variation within and among females may also be induced or partly induced by the environment (Reznick & Yang, 1993; Einum & Fleming, 2004) rather than present adaptive strategies. Long-term laboratory studies, although difficult with amphibians, would test this. Regardless of the actual trigger causing variation in egg size, the fact that there are such strong potential fitness consequences means that variation both within and between clutches would be selected for or against, depending upon the environment. In insects, selection on offspring by the environment has been shown to lead to differences in maternal provisioning strategies (Fox & Mousseau, 1996; Fox et al., 2001; Kudo, 2001).

Our example of the variable strategy involved using half the available energy for small eggs, half for large eggs. Although such extreme discrete bimodal distributions of egg sizes did not occur in natural clutches (Fig. 1a), rather more platykurtic distributions, our theoretical example is still a representative estimate of an alternate provisioning strategy. These more platykurtic allocation strategies would result in different maternal fitness values. These fitness values, however, would still lie in between the upper and lower bounds of producing just large or just small eggs (Fig. 3b). The resulting curves adjoining these strategies in the two different environments would vary in slope or position between the upper and lower bounds as a result of different values of maternal fitness. To measure the exact maternal fitness of these strategies would require measuring fitness of offspring from a range of egg sizes between these upper and lower bounds. Then, using this relationship, the calculation of the fitness of every offspring in the clutch and thus exact maternal fitness would be possible.

In this study we have shown that offspring provisioning is independent of female size or other clutch parameters and variation in offspring size within clutches is also independent. A trade-off occurred between offspring size and number. Maternal provisioning has strong influences on size at metamorphosis and the length of the larval period as well as survival to metamorphosis. This translates to strong influences on offspring fitness and thus maternal fitness. Under the spatially and temporally variable and unpredictable offspring environment that this species uses, the conditions that select for the evolution of variable maternal provisioning both between and within clutches can exist. Since there are positive consequences of alternative offspring provisioning strategies under combinations of these conditions, the high variation in maternal provisioning both within and between clutches in this species is very likely to be an evolved response to the offspring environment.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

This research was funded by grants to M. Dziminski from the Society of Wetland Scientists, The Australian Geographic Society, the Linnean Society of New South Wales and the School of Animal Biology, University of Western Australia. R. Black provided very helpful advice on experimental design and data analysis. A. Hettyey helped in the field. All animals were collected and maintained according to the standards of the Animal Ethics Committee of the University of Western Australia (approval number UWA 01/100/150) and the Department of Conservation and Land Management, Western Australia (permit numbers SF004187 and CE000319).

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
  • Alford, R.A. 1999. Ecology: Resource use, competition and predation. In Tadpoles: The biology of anuran larvae (R. W.McDiarmid & R.Altig, eds). University of Chicago Press, Chicago.
  • Altwegg, R. & Reyer, H.U. 2003. Patterns of natural selection on size at metamorphosis in water frogs. Evolution 57: 872882.
  • Beachy, C.K. 1993. Differences in variation in egg size for several species of salamanders (Amphibia: Caudata) that use different larval environments. Brimleyana 0: 7182.
  • Berven, K.A. & Chadra, B.G. 1988. The relationship among egg size, density and food level on larval development in the wood frog (Rana sylvatica). Oecologia 75: 6772.
  • Brooks, R.J., Bobyn, M.L., Galbraith, D.A., Layfield, J.A. & Nancekivell, E.G. 1991. Maternal and Environmental-Influences on Growth and Survival of Embryonic and Hatchling Snapping Turtles (Chelydra-Serpentina). Can. J. Zool. 69: 26672676.
  • Byrne, P.G. 2004. Male sperm expenditure under sperm competition risk and intensity in quacking frogs. Behav. Ecol. 15: 857863.
  • Byrne, P.G. & Roberts, J.D. 1999. Simultaneous mating with multiple males reduces fertilization success in the myobatrachid frog Crinia georgiana. Proc. Roy. Soc. Lond. B 266: 717721.
  • Byrne, P.G. & Roberts, J.D. 2004. Intrasexual selection and group spawning in quacking frogs (Crinia georgiana). Behav. Ecol. 15: 872882.
  • Capinera, J.L. 1979. Quantitative variation in plants and insects: effect of propagule size on ecological plasticity. Am. Nat. 114: 350361.
  • Castellano, S., Cucco, M. & Giacoma, C. 2004.Reproductive investment of female Green Toads (Bufo viridis). Copeia 2004, 659664.
  • Crump, M.L. 1981. Variation in propagule size as a function of environmental uncertainty for three frogs. Am. Nat. 117: 724737.
  • Crump, M.L. 1984.Intraclutch egg size variability in Hyla crucifer (Anura: Hylidae). Copeia 1984: 302308.
  • Doughty, P. 2002.Coevolution of developmental plasticity and large egg size in Crinia georgiana tadpoles. Copeia 2002: 928937.
  • Doughty, P. & Roberts, J.D. 2003. Plasticity in age and size at metamorphosis of Crinia georgiana tadpoles: Responses to variation in food levels and deteriorating conditions during development. Aust. J. Zool. 51: 271284.
  • Dziminski, M.A. & Alford, R.A. 2005.Patterns and fitness consequences of intraclutch variation in egg provisioning in tropical Australian frogs. Oecologia http://dx.doi.org/10.1007/s00442-005-0177-2 .
  • Einum, S. & Fleming, I.A. 2002. Does within-population variation in fish egg size reflect maternal influences on optimal values? Am. Nat. 160: 756765.
  • Einum, S. & Fleming, I.A. 2004. Environmental unpredictability and offspring size: conservative versus diversified bet-hedging. Evol. Ecol. Res. 6: 443455.
  • Fox, C.W., Czesak, M.E. & Fox, R.W. 2001. Consequences of plant resistance for herbivore survivorship, growth, and selection on egg size. Ecology 82: 27902804.
  • Fox, C.W. & Mousseau, T.A. 1996. Larval host plant affects fitness consequences of egg size variation in the seed beetle Stator limbatus. Oecologia 107: 541548.
  • Gosner, K.L. 1960. A simplified table for staging anuran embryos and larvae with notes on identification. Herpetologica 16: 183190.
  • Howard, R.D. 1978. The evolution of mating strategies in bullfrogs, Rana catesbeiana. Evolution 32: 850871.
  • Kaplan, R.H. 1980. The implications of ovum size variability for offspring fitness and clutch size within several populations of salamanders (Ambystoma). Evolution 34: 5164.
  • Kaplan, R.H. 1992. Greater maternal investment can decrease offspring survival in the frog Bombina orientalis. Ecology 73: 280288.
  • Kaplan, R.H. & Cooper, W.S. 1984. The evolution of developmental plasticity in reproductive characteristics: an application of the ‘‘adaptive coin-flipping’’ principle. Am. Nat. 123: 393410.
  • Kaplan, R.H. & King, E.G. 1997. Egg size is a developmentally plastic trait: Evidence from long term studies in the frog Bombina orientalis. Herpetologica 53: 149165.
  • Koops, M.A., Hutchings, J.A. & Adams, B.K. 2003. Environmental predictability and the cost of imperfect information: Influences on offspring size variability. Evol. Ecol. Res. 5: 2942.
  • Krist, M., Remes, V., Uvirova, L., Nadvornik, P. & Bures, S. 2004. Egg size and offspring performance in the collared flycatcher (Ficedula albicollis): a within-clutch approach. Oecologia 140: 5260.
  • Kudo, S. 2001. Intraclutch egg-size variation in acanthosomatid bugs: adaptive allocation of maternal investment? Oikos 92: 208214.
  • Laugen, A.T., Laurila, A. & Merila, J. 2002. Maternal and genetic contributions to geographical variation in Rana temporaria larval life-history traits. Biol. J. Linn. Soc. 76: 6170.
  • Lloyd, D.G. 1987. Selection of offspring size at independence and other size-versus-number strategies. Am. Nat. 129: 800817.
  • Loman, J. 2002. Microevolution and maternal effects on tadpole Rana temporaria growth and development rate. J. Zool. 257: 9399.
  • Main, A.R. 1957. Studies in Australian Amphibia I. The genus Crinia Tschudi in south-western Australia and some species from south-eastern Australia. Aust. J. Zool. 5: 3055.
  • McGinley, M.A., Temme, D.H. & Geber, M.A. 1987. Parental investment in offspring in variable environments: theoretical and empirical considerations. Am. Nat. 130: 370398.
  • Mousseau, T.A. & Fox, C.W. 1998. The adaptive significance of maternal effects. Trends Ecol. Evol. 13: 403407.
  • Oksanen, T.A., Jonsson, P., Koskela, E. & Mappes, T. 2001. Optimal allocation of reproductive effort: manipulation of offspring number and size in the bank vole. Proc. Roy. Soc. Lond. B 268: 661666.
  • Parichy, D.M. & Kaplan, R.H. 1992. Maternal effects on offspring growth and development depend on environmental quality in the frog Bombina orientalis. Oecologia 91: 579586.
  • Philippi, T. & Seger, J. 1989. Hedging Ones Evolutionary Bets, Revisited. Trends Ecol. Evol. 4: 4144.
  • Poulin, R. & Hamilton, W.J. 2000. Egg size variation as a function of environmental variability in parasitic trematodes. Can. J. Zool. 78: 564569.
  • Reznick, D. & Yang, A.P. 1993. The influence of fluctuating resources on life-history – patterns of allocation and plasticity in female guppies. Ecology 74: 20112019.
  • Sehgal, H.S. & Toor, H.S. 1991. Offspring fitness and fecundity of an indian major carp, labeo-rohita (Ham), in relation to egg size. Aquaculture 97: 269279.
  • Seymour, R.S., Roberts, J.D., Mitchell, N.J. & Blaylock, A.J. 2000. Influence of environmental oxygen on development and hatching of aquatic eggs of the Australian frog, Crinia georgiana. Physiol. Biochem. Zool. 73: 501507.
  • Smith, C.C. & Fretwell, S.D. 1974. The optimal balance between size and number of offspring. Am. Nat. 108: 499506.
  • Smith, H.G., Ohlsson, T. & Wettermark, K.J. 1995. Adaptive significance of egg size in the European starling – experimental tests. Ecology 76: 17.
  • Smith, M.J. & Roberts, J.D. 2003. Call structure may affect male mating success in the quacking frog Crinia georgiana (Anura: Myobatrachidae). Behav. Ecol. Sociobiol. 53: 221226.
  • Tejedo, M. 1992. Absence of the trade-off between the size and number of offspring in the natterjack toad (Bufo calamita). Oecologia 90: 294296.
  • Tejedo, M. & Reques, R. 1992. Effects of egg size and density on metamorphic traits in tadpoles of the natterjack toad (Bufo calamita). J. Herp. 26: 146152.
  • Wassersug, R.J. 1975. The adaptive significance of the tadpole stage with comments on the maintenance of complex life cycles in anurans. Am. Zool. 15: 405417.
  • Wilbur, H.M. & Collins, J.P. 1973. Ecological aspects of amphibian metamorphosis. Science 182: 13051314.
  • Williamson, I. & Bull, C.M. 1989. Life history variation in a population of the Australian frog Ranidella signifera: egg size and early development. Copeia 349356.
  • Winkler, D.W. & Wallin, K. 1987. Offspring size and number: A life history model linking effort per offspring and total effort. Am. Nat. 129: 708720.
  • Zar, J.H. 1999. Biostatistical Analysis. Prentice Hall, Upper Saddle River, NJ.