1Sex allocation theory predicts that when resources are limited in simultaneous hermaphrodites, the allocation to one sexual function will automatically reduce the resources available to the other function. This study examines the effect of nutritional stress on mating behaviour and male and female reproductive output (dry mass and nitrogen contents of spermatophores, sperm delivered and eggs deposited) in individuals of the simultaneously hermaphroditic land snail Arianta arbustorum kept under three different food regimes: ample (100%), restricted (50%) and extremely restricted (25%) food supply.
2Independent of the extent of nutritional stress, 10–12% of the resources taken up were invested in reproductive output (both gender functions together) and 88–90% in maintenance (including faeces and excretion).
3Courtship and copulation behaviour was affected by nutritional stress. Snails with an extremely restricted food supply did not mate, except one pair. Individuals with restricted food supply tended to court longer, and copulated for a shorter period, than individuals with ample food supply.
4Nutritional stress did not affect the number of sperm delivered. However, snails with a restricted food supply produced fewer eggs. Thus, snails kept under nutritional stress invested relatively more resources in the male function than in the female function. Nevertheless, the absolute reproductive output remained highly female biased (>95% in all experimental groups).
5At the individual level, the existence of a trade-off between resources invested in the male vs the female function could not be confirmed. However, there was a trade-off between nitrogen allocated to reproductive function and maintenance in snails with a restricted food supply.
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Reproductive resource allocation is a fundamental aspect of life history with profound ecological and evolutionary consequences (Stearns 1992). Allocation decisions in hermaphroditic plants and animals are particularly interesting because individuals can potentially maximize reproductive success through a wide variety of different strategies. Thus, a key observation for testing sex allocation theory in simultaneous hermaphrodites is the proportion of resources devoted to male vs female function (Charnov 1982). The specific allocation strategy followed by a hermaphrodite may affect the extent of sexual selection and mating behaviour (Michiels 1998). Most models of sex allocation are based on the concept of male and female gain curves (the relationship between relative investment in either male or female gamete production and resulting reproductive success; Charnov, Smith & Bull 1976; Charnov 1979, 1982). These models have received substantial empirical support (e.g. in coral reef fishes, Fischer 1980, 1981, 1984; Fischer & Petersen 1987; Petersen 1987, 1991; in a polychaete worm, Sella 1985, 1990, 1991; and in a barnacle, Raimondi & Martin 1991). All these hermaphrodites have external fertilization. A variety of hermaphroditic invertebrates, however, have some form of copulation, sperm storage and internal fertilization (Michiels 1998).
More recently, sex allocation models for outbreeding hermaphrodites with internal fertilization and sperm storage have been developed (Charnov 1996; Greeff & Michiels 1999). These models consider how various aspects of sperm competition, such as mating frequency, sperm digestion and different mechanisms of sperm displacement, affect sex allocation in simultaneous hermaphrodites. The models predict that a reduced mating rate leads to a reduction in resources allocated to the male function (Charnov 1996; Greeff & Michiels 1999), while sperm digestion leads to an increase in allocation to the male function (Greeff & Michiels 1999). Locher & Baur (2000) found that the reproductive allocation was highly female-biased in the simultaneous hermaphrodite land snail Arianta arbustorum kept under optimal food supply and – in accordance with the predictions of Charnov's (1996) and Greeff & Michiels's (1999) models – that an increased mating frequency led to an increased allocation to the male function. At the individual level, Locher & Baur (2000) found no trade-off between male and female reproductive allocation. In contrast, there was a significant positive correlation between the resources allocated to the male and female function. This finding contradicts current theory regarding sex allocation in simultaneous hermaphrodites. The theory predicts that when resources are limited, the allocation to one function will automatically reduce the resources available to other functions. Trade-offs in resource allocation may not occur or be less pronounced under favourable conditions. Under stressful conditions, such as limited food supply, high temperature or drought, the energy intake might be carefully shared among different functions, including reproduction (Reznick 1992). So far there is little empirical evidence for gender-specific reproductive allocation in internally fertilizing simultaneous hermaphrodites (but see Doums, Perdieu & Jarne 1998 for sex allocation in a self-fertilizing freshwater gastropod).
The research presented here explores sex allocation under nutritional stress in A. arbustorum, a simultaneous hermaphrodite land snail with internal fertilization, sperm storage and sperm digestion. We designed an experiment to investigate (1) whether the overall resource allocation to reproduction is affected by a restricted food supply, (2) whether snails kept under restricted food supply alter the absolute and relative allocation to the male and female reproductive function, and (3) whether the resources spent on spermatophores and sperm are traded off against resources expended on egg production. For this purpose we examined the number of sperm delivered and eggs produced by individual snails that were kept at different levels of food supply. We examined the resources ingested and assessed the dry mass and nitrogen contents of spermatophores, sperm and eggs produced as sex-specific measures of reproductive allocation.
Materials and Methods
Arianta arbustorum is common in moist habitats of north-western and central Europe (Kerney & Cameron 1979). The snail has determinate growth (shell breadth of adults 17–22 mm). Individuals become sexually mature at 2–4 years, and adults live another 3–4 years (maximum 14 years; Baur & Raboud 1988). In the field, snails deposit one to three egg batches consisting of 20–50 eggs per reproductive season (Baur & Raboud 1988; Baur 1990). In contrast to male fecundity (i.e. sperm expenditure), female fecundity (i.e. clutch size and the number of batches produced per season) is positively correlated with adult shell size (B. Baur 1994a; Baur, Locher & Baur 1998). Breeding experiments showed that 27% of virgin snails prevented from mating produced a few hatchlings by self-fertilization in the second and third year of isolation (Chen 1993). The reproductive success of selfing individuals, however, is less than 2% of that of mated snails, suggesting high costs for selfing (Chen 1994).
Mating in A. arbustorum includes elaborate courtship behaviour with optional dart shooting (i.e. the pushing of a calcareous dart into the mating partner's body), and lasts 2–18 h (Hofmann 1923; Baur 1992a). Copulation is reciprocal; after intromission, each snail transfers simultaneously one spermatophore (Haase & Baur 1995). The spermatophore is formed and filled with sperm during copulation (Hofmann 1923). It has a distinctive form consisting of a head, a body (sperm container with 800 000–4 000 000 spermatozoa) and a tail 2–3 cm long (Baur et al. 1998). Fertile sperm can be stored for more than 1 year (Baur 1988). Mating is random with respect to shell size and different degrees of relatedness (Baur 1992a; Baur & Baur 1997). Individuals need at least 8 days to replenish their sperm reserves after a successful copulation (Locher & Baur 1999; Hänggi, Locher & Baur 2002).
No data on mating frequency in natural A. arbustorum populations are available. In 48 snails kept in groups of six individuals under laboratory conditions for 58 days, 79% copulated once or not at all, whereas 17% copulated twice and 4% three times (N. Minoretti, S. Zschokke & B. Baur, unpublished data). This indicates a low mating frequency during the oviposition period. However, mating may also occur after egg-laying or after hibernation prior to egg production (B. Baur, unpublished data).
Paternity analysis in broods of wild-caught A. arbustorum showed a high frequency of multiple insemination (B. Baur 1994b). A controlled laboratory experiment showed that one successful copulation per reproductive season is sufficient to fertilize all the eggs produced by an individual (Chen & Baur 1993). However, there is a probability of 5–8% that a copulation will not lead to fertilization of eggs (no sperm transfer or transfer of infertile sperm; Chen & Baur 1993).
To obtain virgin snails, subadult individuals that had not yet completed shell growth were collected from the embankment along a track in a subalpine forest near Gurnigelbad, 30 km south of Bern, Switzerland (46°45′ N, 7°27′ E) at an altitude of 1320 m above sea level on 13 May 2000. The snails were kept isolated in transparent beakers (8 cm deep, 6·5 cm in diameter) lined with moist soil (approximately 4 cm) at 19 °C with a light : dark cycle of 16 : 8 h.
To examine the effect of nutritional stress on resource allocation to the male and female reproductive function, snails were fed different amounts of fresh lettuce. Individuals were randomly assigned to one of the following treatment groups: snails were offered four disks of lettuce with a total leaf area of 32·16 cm2 per week (ample: 100%, group A); two disks with a total leaf area of 16·08 cm2 per week (restricted: 50%, group B), or one disk with a leaf area of 8·04 cm2 (extremely restricted: 25%, group C). Fresh lettuce was provided twice per week for individuals of group A and B, for those of group C only once. At the same time the beakers were cleaned and remnants of lettuce disks (if present) were removed and their areas determined to the nearest 1 mm2 using mm-paper. In this way the area of lettuce consumed could be assessed for each snail.
To assess nitrogen content of the lettuce consumed, three disks (each 8·04 cm2) from each lettuce head used in the experiment were collected and dried for 72 h at 60 °C. The dry mass of each lettuce sample was recorded to the nearest 0·01 mg. The nitrogen concentration was determined using a CHN analyser (LECO CHN-900, LECO Instruments GmbH, Munich, Germany).
Within 4 weeks, all individuals reached sexual maturity as indicated by the formation of a flanged lip at the shell aperture. The snails were marked individually with letters and numbers written on their shells with a waterproof felt-tipped pen on a spot of correction fluid (Tipp-Ex). The animals showed no visible reaction to the marking procedure.
Mating trials were performed outdoors to expose snails to natural temperature and light conditions. Two randomly chosen active snails (individuals with an extended soft body and everted tentacles) of the same group were allowed to copulate in a transparent plastic container, measuring 14 × 10 × 7 cm3, lined with moistened paper towelling to maintain activity. Mating trials were initiated in the evening and ran during five nights in June and July 2000. The period between the end of May and the middle of July is the time of maximum mating activity in subalpine populations of A. arbustorum.
The snails’ mating behaviour was observed at intervals of 30 min (at night using a torch) following the method described in Baur (1992a) and Baur et al. (1998). Copulating snails were observed at intervals of 15 min. Records included courtship duration (time interval from courtship initiation to copulation) and copulation duration. Observation sessions were terminated either after successful copulation or after 8 h if no snail initiated courtship behaviour in a test arena. Snails that did not mate were tested again 7 days later with a new partner. Between two trials, snails of each group were kept as in the premating phase.
After copulation, one randomly chosen mating partner (hereafter referred to as sperm donor) was kept under the same food regime as in the premating phase. The other mating partner (hereafter referred to as sperm recipient) was frozen immediately after copulation. For group A and B sample size was 15 sperm donors. In group C with extremely restricted food supply, however, only one pair mated.
The sperm recipient was dissected to obtain the spermatophore. The length (L) and width (W) of the sperm-containing part of each spermatophore was measured to the nearest 0·1 mm using a dissecting microscope. Spermatophore size (in mm3) was approximated, by the formula (πLW2/4), assuming a cylindrical volume. Spermatophores were kept singly in Eppendorf tubes at −30 °C until sperm were counted.
The beakers of sperm donors were checked for eggs once per week. The eggs of each batch were collected, counted, and kept in a plastic dish (6·5 mm in diameter) lined with moist paper towelling at 19 °C to determine hatching success. Newly hatched snails were separated from remaining unhatched eggs to prevent egg cannibalism (Baur 1992b). In all treatment groups eggs were collected over a period of 60 days following copulation. The length of this period corresponds to approximately one reproductive season of A. arbustorum living in the wild (Baur & Raboud 1988; Baur 1990).
To assess any effect of the size of the sperm donor on the number of sperm transferred and number of eggs produced, shell breadth and height of the snails were measured to the nearest 0·1 mm using vernier callipers and the shell volume was calculated using the formula:
(measurements in mm; B. Baur, unpublished data). Shell volume is a more reliable measurement of snail size than mass, because mass depends on the state of hydration and thus is highly variable in terrestrial gastropods. Snails from the three treatment groups did not differ in shell size (anova, P = 0·43).
Sperm Counting Procedure
The number of sperm that an individual delivered was evaluated by counting the number of sperm in the spermatophore transferred. This procedure is described in detail in Locher & Baur (1997).
The spermatophore of A. arbustorum consists of a hardened secretion which encapsulates the spermatozoa (Hofmann 1923). The spermatophore was mechanically disrupted in 200 µl PBS buffer (138·6 mm NaCl, 2·7 mm KCl, 8·1 mm Na2HPO4 × 2H2O and 1·5 mm KH2PO4) using a pair of microscissors. The sperm suspension was homogenized with a set of Gilson pipettes for 5–15 min. To count the sperm, the homogenate was stained for 1–3 h with an equal volume of a gallocyanin–chromium complex, which stains the DNA in the head of the spermatozoa. If spermatozoa still occurred in clusters, the sample was treated overnight with a sonicator (35 kHz). Two subsamples of known volume of the sperm suspension were diluted 1 : 3 with PBS-buffer and transferred to a Bürker–Türk counting chamber. This counting chamber consists of 16 cells each with a volume of 25 nl. A count was made of all sperm heads in randomly chosen cells until the total number of sperm heads exceeded 400, and the average of two subsamples was used to calculate the total number of sperm in a spermatophore.
Estimate of Resource Allocation
The resource allocation of individual snails was determined by measuring food consumption and resources allocated to both male (production of spermatophores and spermatozoa) and female reproductive function (production of eggs). An estimate of the resources invested in maintenance (i.e. for respiration, mucus production, faeces and excretion) was obtained by subtracting the resources allocated to reproduction from the resources taken up. Arianta arbustorum has determinate growth. In the present study only fully grown snails were used. Thus, somatic growth can be neglected in the resource budget.
Estimate of Reproductive Allocation
Two measures of reproductive allocation to male and female function were used: (1) the dry mass of spermatophores filled with spermatozoa and that of the eggs produced, and (2) the nitrogen content of spermatophore and eggs. Two individuals of group B copulated successfully (delivered a spermatophore with sperm) but produced no eggs. These snails were excluded from comparisons between male and female reproductive output reducing the sample size for this group to N = 13.
The relationship between the dry mass of spermatophores filled with spermatozoa (Y in mg) and the size of the spermatophores (X = volume of the sperm container in mm3):
was used to calculate the dry mass of the spermatophores produced by individual snails in these experiments (relationship from Locher & Baur 2000). Data on dry mass and nitrogen concentration of spermatophores were obtained from Locher & Baur (2000). In that study spermatophores were obtained from copulating A. arbustorum provided with lettuce ad libitum. The volume of the sperm container was assessed as described above. No differences were found in spermatophore size and number of sperm transferred between snails fed lettuce ad libitum (data on first copulation in Locher & Baur 2000) and snails kept under the three food supply levels of the present study (unpaired t-tests, P > 0·15). Nitrogen content of spermatophores was obtained by multiplying the spermatophore dry mass with the average nitrogen (12·05%) concentration.
Arianta arbustorum kept either on a lettuce or a Petasites albus diet produced eggs of equal composition and concentrations of nutrients (A. Baur 1994). In the present study we used data on dry mass and nitrogen concentrations of 264 eggs from 33 batches obtained from the same population as used in the study by Baur & Baur (1998). The nitrogen content of all eggs produced by an individual snail were obtained by multiplying egg number with the average nitrogen (4·02%) concentration.
The StatView program package (SAS Institute 1998) was used for statistical analyses. Means ± 1 SE are given unless otherwise stated. Mann–Whitney U-tests were used to compare courtship and copulation duration. Data on total number of eggs were log10-transformed and frequency data (hatching success) were arcsine-transformed. In multiple comparisons sequential Bonferroni correction of the significance level is indicated (Rice 1989). Allometric relationships may confound interpretations of differences in observed reproductive output between treatment groups. To examine possible differences in reproductive traits analysis of covariance with treatment as factor and shell size as covariate was used (ancova, type III model, Superanova program, Abacus Concepts 1989). Relative values expressed in the tables were calculated separately for each individual and then averaged in both treatment groups.
Copulations were observed in 31 (20·7%) out of 150 trials. Snails from the three groups differed in percentage of successful mating trials: group A 31·3% (15 matings out of 48 trials), group B 28·9% (15 out of 52), group C 2·0% (1 out of 50; χ2 = 11·55, df = 2, P = 0·003). Thus, snails kept under conditions of extremely restricted food supply (group C) rarely copulated. No difference in mating propensity was found between individuals from groups A and B (χ2 = 0·04, df = 1, P = 0·85). Individuals that did not copulate produced no eggs. Consequently, snails from group C were not considered in further data analyses.
Overall Resource Allocation
Because different measures of resource allocation (dry mass and nitrogen content) were intercorrelated, the resource budget of A. arbustorum is only expressed as nitrogen allocation to reproduction and maintenance. Snails with ample food supply consumed 51·2 ± 2·2 mg nitrogen within 60 days and individuals with restricted food supply 31·9 ± 0·4 mg. The only individual that reproduced under conditions of extremely restricted food supply consumed 16·3 mg nitrogen. Table 1 presents the relative and absolute nitrogen allocation to reproduction and maintenance (including metabolism, faeces and excretion) under different levels of food supply. The relative nitrogen allocation is expressed as percentage of total consumed nitrogen devoted to reproductive function or maintenance. Independent of the extent of nutritional stress, 10–12% of the consumed resources were invested in reproductive function (both gender functions) and 88–90% in maintenance. However, individuals kept under nutritional stress produced fewer eggs (Table 2) and thus allocated relatively more nitrogen to the male reproductive function than snails with ample food.
Table 1. Relative and absolute nitrogen allocation to reproductive output and maintenance (including metabolism, faeces and excretion) in A. arbustorum kept at different levels of food supply. The relative allocation is expressed as percentage of total amount nitrogen taken up. Figures in italics indicate absolute values. Mean values ± 1 SE with sample size in parenthesis are given. P values are from unpaired t-tests
Group (food supply)
A (100%) (N = 15)
B (50%) (N = 13)
Group C (25%, N = 1): reproductive output 10·3%, 1·68 mg; male function 0·8%, 0·13 mg; female function 9·5%, 1·55 mg; maintenance 89·7%, 14·62 mg.
11·77 ± 0·88
11·64 ± 1·65
6·05 ± 0·54
3·70 ± 0·51
0·30 ± 0·03
0·45 ± 0·02
0·14 ± 0·01
0·14 ± 0·01
11·47 ± 0·88
11·19 ± 1·65
5·91 ± 0·54
3·56 ± 0·51
Maintenance (including metabolism, faeces and excretion)
88·23 ± 0·88
88·36 ± 1·65
45·09 ± 1·90
28·19 ± 0·65
Table 2. Effect of different food supply on sex-specific reproductive output in A. arbustorum. The results of analyses of covariance (ANCOVA) with snail size (shell volume) as covariate are presented
Independent variable; covariate
Group (food supply)
Type III SS
A (100%) (N = 15)
B (50%) (N = 13)
Arcsine-transformed. As characters from the same individuals were used in five comparisons (k = 5), sequential Bonferroni correction reveals an adjusted level of significance at P = 0·01.
Individuals with a restricted food supply (group B) tended to court for longer than did snails with ample food (group A; median [range]: 240 [90–390] min vs 180 [60–300] min; Mann–Whitney U-test, P = 0·086). However, copulation duration was shorter in individuals from group B than in those from group A (median [range]: 80 [55–135] min vs 115 [65–170] min; Mann–Whitney U-test, P = 0·017). This suggests that courtship and copulation behaviour is affected by nutritional stress. In both groups neither courtship behaviour nor copulation time was correlated with any male or female reproductive trait (Spearman rank correlation: in all cases rs < 0·6, P > 0·07).
Male and Female Reproductive Output
Table 2 shows the effect of limited food supply on sex-specific reproductive output in A. arbustorum. The ancova revealed no significant interactions between treatment and shell size. Consequently, the non-significant interaction term was dropped from the model (cf. Scheiner & Gurevitch 1993) and the ancova repeated.
Spermatophore size was the only measured trait that was positively influenced by snail size. However, male reproductive output (spermatophore size and number of sperm transferred) was not affected by food restriction (Table 2). In contrast, snails kept under restricted food supply had a reduced female reproductive output: they deposited fewer egg batches and tended to produce fewer eggs than snails kept under ample food supply. Hatching success of eggs was not influenced by different food supply.
Considering snails from both groups, there was no correlation between the number of eggs produced and the number of sperm delivered (group A: r = 0·05, N = 15, P = 0·87; group B: r = 0·12, N = 13, P = 0·71).
Proportion of Resources Devoted to Male vs. Female Function
The amount of resources allocated to the male function was not influenced by nutritional stress (Table 3). Even the sperm donor of the single pair that mated in group C invested a similar amount of resources into the male function as did snails from the other two treatment groups. However, individuals kept under restricted food conditions invested significantly fewer resources into eggs (Table 3).
Table 3. Absolute (in mg) and relative resources allocated to the male and female reproductive function in A. arbustorum kept at different levels of food supply. Figures in square brackets indicate ranges. Values in italics represent relative allocation to the reproductive function (expressed as percentage of the total dry mass or nitrogen taken up). P values are from unpaired t-tests
Group (food supply)
A (100%) (N = 15)
B (50%) (N = 13)
Group C (25%, N = 1): dry mass, male: 1·07 mg, 0·26%; female: 38·52 mg, 9·67%; nitrogen, male: 0·13 mg, 0·80%; female: 1·5 mg, 9·51%.
1·20 ± 0·06
1·18 ± 0·06
0·10 ± 0·01
0·14 ± 0·01
146·95 ± 13·49
88·56 ± 12·66
11·81 ± 0·87
11·45 ± 1·70
0·14 ± 0·01
0·14 ± 0·01
0·30 ± 0·03
0·45 ± 0·02
5·91 ± 0·54
3·56 ± 0·51
11·47 ± 0·88
11·19 ± 1·65
Considering relative resource allocation (expressed as percentage of the total dry mass or nitrogen taken up), snails kept under food restriction allocated relatively more resources to the male function than did snails kept under ambient food supply (Table 3). Comparing both functions, the average resources devoted to the male function were less than 5% of those allocated to the female function (mean values of dry mass were 0·95% in group A and 1·7% in group B, those of nitrogen 2·8% in group A and 4·8% in group B). Considering individual snails, the maximum nitrogen allocation to the male function was 5·9% in group A and 11·1% in group B. Thus, only a minor part of the resources devoted to the reproductive output was allocated to the male function. Furthermore, snail size did not affect the relative reproductive allocation to male vs female function in both groups (in all cases r < 0·14 and P > 0·61).
In both treatment groups, there was no correlation between male reproductive allocation (total dry mass of spermatophores and sperm delivered) and female reproductive allocation (total dry mass of all eggs produced; group A: r = 0·08, N = 15, P = 0·77; group B: r = 0·14, N = 13, P = 0·66).
Considering individual snails with ample food supply, no correlation between the amount of nitrogen invested in reproductive function and the amount invested in maintenance was found (r = 0·35, N = 15, P = 0·21). In contrast, a trade-off between nitrogen allocated to reproductive output and maintenance was found in snails with restricted food supply (r = −0·81, N = 13, P < 0·001; Fig. 1). However, as a consequence of different levels of food supply, snails from group A allocated more nitrogen into reproductive output and maintenance than snails from group B (reproductive function: 6·05 ± 0·54 mg, N = 15 vs 3·70 ± 0·51 mg, N = 13; maintenance: 45·09 ± 1·90 mg, N = 15 vs 28·19 ± 0·65 mg, N = 13).
The present study shows to our knowledge for the first time that nutritional stress affects sex-specific reproductive allocation in a simultaneous hermaphrodite land snail. Considering the resource budget, snails invested only a minor part (10–12%) of the nitrogen consumed into reproduction. In A. arbustorum the assimilation efficiency of consumed lettuce averaged 97·0% (dry mass; Abdel-Rehim 1987) and in Cepaea nemoralis fed on carrot root 94·0% (Williamson & Cameron 1976). Assuming a similar assimilation efficiency in our study, this would indicate that 85–90% of the nitrogen taken up is allocated to maintenance (including metabolism, faeces and excretion).
The present study also confirmed the earlier finding of a strongly female-biased sex allocation in A. arbustorum (Locher & Baur 2000). The female-biased resource allocation is much more pronounced than predicted by the theoretical models of Charnov (1996) and Greeff & Michiels (1999). However, the mating frequency in natural A. arbustorum populations might not be as high as assumed in the models. Individuals of A. arbustorum have been observed to mate with different partners in the course of a reproductive season in a field cage experiment (Baur 1988). In a laboratory experiment, the snails mated one to three times within 58 days, but only 21% copulated twice or three times (N. Minoretti, S. Zschokke & B. Baur, unpublished data).
Nutritional stress reduced the absolute female reproductive output, but did not affect the absolute male reproductive output. Similarly, Simmons (1994) found that food limitation reduced female reproductive output in the Bush Cricket Kawanaphila nartee, whereas male expenditure remained unchanged. In the present study, snails with an extremely restricted food supply did not copulate and could therefore not produce any fertilized eggs with the exception of one pair. The resources supplied to this group of snails were less than those used for maintenance by the other groups (Table 1). However, the sperm donor of this single pair invested a similar amount of resources into the male function as did snails from the other two treatment groups. The energy value of one spermatophore filled with sperm corresponds to less than that of a single egg (Locher & Baur 2000). Thus, the production of an egg batch is extremely costly compared with the cost of a sperm-filled spermatophore. As a consequence of the decreased egg production, snails kept under nutritional stress invested relatively more of the resources available into the male function.
We focused on male and female reproductive output. However, there are other aspects of male and female function in hermaphrodites that are difficult to measure. In simultaneous hermaphroditic land snails such as A. arbustorum, mating in the male role carries costs, which include the costs of courtship behaviour (including the optional dart shooting), spermatophore and sperm production and other possible costs associated with mating. During the long-lasting courtship, terrestrial gastropods produce huge amounts of mucus, an energetically expensive behaviour. The present study does not provide any estimate of the actual costs of courtship behaviour. However, it is not clear whether the costs of courtship can entirely be assigned to the male function. The female function of this outcrossing hermaphrodite needs at least one copulation per reproductive season for the fertilization of the eggs (Chen & Baur 1993; Chen 1994). From the point of view of offspring quality, it has also been suggested that the female function might use (so far unknown) cues from courtship behaviour to assess the quality of the potential mating partner (Baur 1998; Baur et al. 1998). Indeed, there is some evidence for the occurrence of selective sperm digestion (cryptic female choice) in A. arbustorum (Haase & Baur 1995). If this is the case, the costs of courtship behaviour should be distributed to both the male and female function. Finally, in a variety of terrestrial gastropods, including A. arbustorum, copulation per se and/or the transfer of the spermatophore stimulates egg production via hormones in both partners (Saleuddin, Griffond & Ashton 1991; Baur & Baur 1992). Thus, a copulation might be beneficial to both functions even if the sperm delivered in repeated copulations are not used by the female function for egg fertilization. It is difficult to assess the different costs associated with mating behaviour and to assign them to a specific sex function in A. arbustorum as well as in any other pulmonate land snail.
A fundamental assumption of sex allocation theory in hermaphrodites is a trade-off between male and female function, i.e. the animal has a fixed amount of resources to allocate between the sexes (Charnov 1982). A hermaphrodite that suppresses the use of one sex function may automatically free resources for the other sex function. Trade-offs between the male and female functions within a population have been documented in some plant species (e.g. Garnier, Maurice & Olivieri 1993; Ashman 1999), but not in others (e.g. Damgaard & Loeschcke 1994; Mossop, Macnair & Robertson 1994). De Visser, ter Maat & Zonneveld (1994) experimentally suppressed the male function in the simultaneous hermaphrodite freshwater snail Lymnaea stagnalis, which resulted in an increased egg production. This freshwater snail mates frequently, transfers spermatozoa in a sperm suspension and dies after one reproductive period. Copulation is unilateral, but reversal of roles occurs in most matings (Van Duivenboden & ter Maat 1988). Resource allocation considerations generate predictions of a negative quantitative relationship between male and female functions. Trade-offs may occur when resources are in short supply. Arianta arbustorum feeds on vascular plants and dead or senescent herbs, but the snails often supplement their vegetable diet with microorganisms or carrion (Frömming 1954; Grime & Blythe 1969). A short supply of nitrogen is probably critical for the reproduction of this land snail (White 1993). Thus, the nitrogen content of spermatophores, sperm and eggs may be an appropriate measure for examining a possible trade-off in the allocation to the male and female functions.
One of the most fundamental trade-offs in life history occurs between the amount of resources invested per offspring and number of offspring produced (Stearns 1989). Numerous studies showed that diet composition and the amount of energy available to the mother affect offspring size, which in turn influences offspring survival (e.g. Kaplan 1987; Berven 1988; Solbreck et al. 1989). Egg quality is expected to change only under severe nutritional stress (Charnov 1982). In the present study, egg quality did not seem to be influenced by food limitation as the hatching success of the two treatment groups did not differ. Different food plants consumed by A. arbustorum did not influence the egg nutrient composition of A. arbustorum eggs (A. Baur 1994). In the present study, only the number of eggs produced decreased with food limitation. Similar findings were obtained by Rollo & Shibata (1991) in the terrestrial slug Deroceras laeve in which the number of eggs produced decreased with food quality.
At the individual level we found a pronounced trade-off between nitrogen allocated to the reproductive function and to maintenance in snails with a restricted food supply. As predicted by theory (Stearns 1989), this trade-off occurred only under harsh conditions. So far, most of the resource allocation studies focused on the resource allocation trade-off between growth and reproduction in animals with indeterminate growth (for reviews see Glazier 1999; Heino & Kaitala 1999). For instance, in the periwinkle Littorina littorea, 36% of the consumed energy was allocated to reproduction and 64% to shell and somatic growth (Chase & Thomas 1998). Jokela & Mutikainen (1995) studied resource allocation between maintenance, reproduction and growth in the freshwater clam Anodonta piscinalis. Their results support theoretical and empirical studies, implying that organisms with indeterminate growth have priority rules for resource allocation (Glazier & Calow 1992; Perrin 1992). They concluded that A. piscinalis ranked their energy demands so that allocation to maintenance is the top priority, allocation to reproduction the second priority, and allocation to growth is least important. The results of the present study indicate similar priority rules for resource allocation in a species with determinate growth. Snails with an extremely restricted food supply rarely mated and invested no energy in egg production.
Interestingly, courtship and copulation behaviour was affected by nutritional stress. Individuals kept under nutritional stress tended to court for longer and copulated for a shorter period than did individuals with ample food. This finding contradicts the expectation that nutritionally stressed snails would shorten courtship and copulation behaviour to minimize the energetically expensive behaviour of mucus production during the long-lasting courtship (Calow 1977; Davies, Hawkins & Jones 1990). The fact that courtship takes longer under nutritional stress suggests that the assessment of a partner of reduced quality lasts longer, but that the snails eventually reach an agreement.
We thank A. Baur, A. Erhardt, H.-P. Rusterholz, S. Zschokke and two anonymous referees for constructive comments on the manuscript. Financial support was received from the Swiss National Science Foundation (grant no. 31–64855·01).