Life History Model Research Articles

Thyroid hormones determine developmental mode in sand dollars (Echinodermata: Echinoidea)

Evolutionary transitions in larval nutritional mode have occurred on numerous occasions independently in many marine invertebrate phyla. Although the evolutionary transition from feeding to nonfeeding development has received considerable attention through both experimental and theoretical studies, mechanisms underlying the change in life history remain poorly understood. Facultative feeding larvae (larvae that can feed but will complete metamorphosis without food) presumably represent an intermediate developmental mode between obligate feeding and nonfeeding. Here we show that an obligatorily feeding larva can be transformed into a facultative feeding larva when exposed to the thyroid hormone thyroxine. We report that larvae of the subtropical sand dollar Leodia sexiesperforata (Echinodermata: Echinoidea) completed metamorphosis without exogenous food when treated with thyroxine, whereas the starved controls (no thyroxine added) did not. Leodia sexiesperforata juveniles from the thyroxine treatment were viable after metamorphosis but were significantly smaller and contained less energy than sibling juveniles reared with exogenous food. In a second starvation experiment, using an L. sexiesperforata female whose eggs were substantially larger than in the first experiment (202+/-5 vs. 187+/-5 microm), a small percentage of starved L. sexiesperforata larvae completed metamorphosis in the absence of food. Still, thyroxine-treated larvae in this experiment completed metamorphosis faster and in much higher numbers than in the starved controls. Furthermore, starved larvae of the sand dollar Mellita tenuis, which developed from much smaller eggs (100+/-2 microm), did not complete metamorphosis either with or without excess thyroxine. Based on these data, and from recent experiments with other echinoids, we hypothesize that thyroxine plays a major role in echinoderm metamorphosis and the evolution of life history transitions in this group. We discuss our results in the context of current life history models for marine invertebrates, emphasizing the role of egg size, juvenile size, and endogenous hormone production for the evolution of nonfeeding larval development.

An individual‐based model of the early life history of mackerel (Scomber scombrus) in the eastern North Atlantic, simulating transport, growth and mortality

AbstractThe main purpose of this paper is to provide the core description of the modelling exercise within the Shelf Edge Advection Mortality And Recruitment (SEAMAR) programme. An individual‐based model (IBM) was developed for the prediction of year‐to‐year survival of the early life‐history stages of mackerel (Scomber scombrus) in the eastern North Atlantic. The IBM is one of two components of the model system. The first component is a circulation model to provide physical input data for the IBM. The circulation model is a geographical variant of the HAMburg Shelf Ocean Model (HAMSOM). The second component is the IBM, which is an i‐space configuration model in which large numbers of individuals are followed as discrete entities to simulate the transport, growth and mortality of mackerel eggs, larvae and post‐larvae. Larval and post‐larval growth is modelled as a function of length, temperature and food distribution; mortality is modelled as a function of length and absolute growth rate. Each particle is considered as a super‐individual representing 106 eggs at the outset of the simulation, and then declining according to the mortality function. Simulations were carried out for the years 1998–2000. Results showed concentrations of particles at Porcupine Bank and the adjacent Irish shelf, along the Celtic Sea shelf‐edge, and in the southern Bay of Biscay. High survival was observed only at Porcupine and the adjacent shelf areas, and, more patchily, around the coastal margin of Biscay. The low survival along the shelf‐edge of the Celtic Sea was due to the consistently low estimates of food availability in that area.

Characterizing the effects of dwarf mistletoe and other diseases for sustainable forest management

Many insects, fungi, and plants in forest ecosystems can damage trees and forests, depending on stand and environmental conditions. Natural disturbances, harvesting, and other forest practices can retard or increase the spread and the effects of dwarf mistletoe and other diseases on tree growth. To monitor the effects of diseases, certification and monitoring programs typically use incidence and severity of infestations as criteria and indicators. However, these are often insufficient to characterize the impact of the disease or to measure the effects of new management practices, such as variable retention silviculture, on sustainability. Long-term observations and models of stand development are advocated as better methods for characterizing disease effects. For dwarf mistletoe (Arceuthobium tsugense), we are designing and monitoring installations in infested stands of western hemlock (Tsuga heterophylla) and constructing a spatial and life history model of stand and disease development. Disease spread and effects are influenced by several factors including site quality, stand density, and the spatial arrangement of infected trees, which are sources of mistletoe spread into new stands. Potentially, these factors could be manipulated to either reduce or encourage the spread and the effects of dwarf mistletoe.

Environment, krill and squid in the Monterey Bay: from fisheries to life histories and back again

The fishery for market squid in California is extremely valuable and only recently subject to regulations (in that sense, it is one of the last great open-access fisheries on the western coast of North America). We briefly review the history of the fishery for Loligo opalescens, on an annual time scale. Within-season analyses suggest that cohorts of squid persist offshore and move inshore for reproduction throughout the season. In order to predict the timing of inshore movement of squid, we develop a life history model using stochastic dynamic programming. This model allows us to link squid growth and reproduction with environmental factors, particularly, upwelling, temperature, and food abundance, in a consistent Darwinian framework. Using the model, we predict the timing of squid movement inshore in cold and warm environments, and the amount of biomass inshore in weekly intervals. We compare the model predictions with unpublished fishery-dependent data subsampled from landings and discuss the implications of our work for management of the squid fishery.

HETEROCHRONIC DEVELOPMENTAL SHIFT CAUSED BY THYROID HORMONE IN LARVAL SAND DOLLARS AND ITS IMPLICATIONS FOR PHENOTYPIC PLASTICITY AND THE EVOLUTION OF NONFEEDING DEVELOPMENT

Recent work on a diverse array of echinoderm species has demonstrated, as is true in amphibians, that thyroid hormone (TH) accelerates development to metamorphosis. Interestingly, the feeding larvae of several species of sea urchins seem to obtain TH through their diet of planktonic algae (exogenous source), whereas nonfeeding larvae of the sand dollar Peronella japonica produce TH themselves (endogenous source). Here we examine the effects of TH (thyroxine) and a TH synthesis inhibitor (thiourea) on the development of Dendraster excentricus, a sand dollar with a feeding larva. We report reduced larval skeleton lengths and more rapid development of the juvenile rudiment in the exogenous TH treatments when compared to controls. Also, larvae treated with exogenous TH reached metamorphic competence faster at a significantly reduced juvenile size, representing the greatest reduction in juvenile size ever reported for an echinoid species with feeding larvae. These effects of TH on D. excentricus larval development are strikingly similar to the phenotypically plastic response of D. excentricus larvae reared under high food conditions. We hypothesize that exogenous (algae-derived) TH is the plasticity cue in echinoid larvae, and that the larvae use ingested TH levels as an indicator for larval nutrition, ultimately signaling the attainment of metamorphic competence. Furthermore, our experiments with the TH synthesis inhibitor thiourea indicate that D. excentricus larvae can produce some TH endogenously. Endogenous TH production might, therefore, be a shared feature among sand dollars, facilitating the evolution of nonfeeding larval development in that group. Mounting evidence on the effects of thyroid hormones in echinoderm development suggests life-history models need to incorporate metamorphic hormone effects and the evolution of metamorphic hormone production.

Factors affecting soldier allocation in clonal aphids: a life-history model and test

Aphid species using a defensive soldier caste offer us the opportunity to study allocation decisions by eusocial groups, without the hindrance of genetic dissimilarity between colony members, which often impair studies involving Hymenopteran or Isopteran systems. When the entire aphid clone is considered the adaptive unit of organization, understanding soldier allocation strategies is tantamount to understanding the outcome of the tradeoff between clonal growth (i.e., asexual reproduction) and clonal defense. Under this framework, we present the results of a dynamic programming effort aimed at determining the optimal ontogeny of defensive allocation strategies by eusocial clonal organisms. We consider the allocation decision for clones with both obligately and facultatively sterile soldiers, under various levels of predation, and favorable and unfavorable ecological conditions. We test predictions of the model with the eusocial aphid, Pemphigus spyrothecae. Our model predicts that defensive investment should be dependent on the time of the season, with clones discounting defense nearer the end of season. Defensive investment should also vary inversely with clonal productivity and be sensitive to the current state (e.g., level of defense) of the clone. Census data collected in Burnaby, British Columbia, Canada, conform to patterns of clonal composition derived from allocation decisions generated in the model. Finally, qualitative predictions about patterns of clonal organization under ‘‘good’’ and ‘‘poor’’ ecological conditions were upheld by comparing clones in preferred and less-preferred galling sites. Key words: colony defense, dynamic programming model, eusocial aphids, Pemphigus spyrothecae, soldier allocation strategies, tradeoffs. [Behav Ecol 15:94–101 (2004)]

HETEROCHRONIC DEVELOPMENTAL SHIFT CAUSED BY THYROID HORMONE IN LARVAL SAND DOLLARS AND ITS IMPLICATIONS FOR PHENOTYPIC PLASTICITY AND THE EVOLUTION OF NONFEEDING DEVELOPMENT

Recent work on a diverse array of echinoderm species has demonstrated, as is true in amphibians, that thyroid hormone (TH) accelerates development to metamorphosis. Interestingly, the feeding larvae of several species of sea urchins seem to obtain TH through their diet of planktonic algae (exogenous source), whereas nonfeeding larvae of the sand dollar Peronella japonica produce TH themselves (endogenous source). Here we examine the effects of TH (thyroxine) and a TH synthesis inhibitor (thiourea) on the development of Dendraster excentricus, a sand dollar with a feeding larva. We report reduced larval skeleton lengths and more rapid development of the juvenile rudiment in the exogenous TH treatments when compared to controls. Also, larvae treated with exogenous TH reached metamorphic competence faster at a significantly reduced juvenile size, representing the greatest reduction in juvenile size ever reported for an echinoid species with feeding larvae. These effects of TH on D. excentricus larval development are strikingly similar to the phenotypically plastic response of D. excentricus larvae reared under high food conditions. We hypothesize that exogenous (algae-derived) TH is the plasticity cue in echinoid larvae, and that the larvae use ingested TH levels as an indicator for larval nutrition, ultimately signaling the attainment of metamorphic competence. Furthermore, our experiments with the TH synthesis inhibitor thiourea indicate that D. excentricus larvae can produce some TH endogenously. Endogenous TH production might, therefore, be a shared feature among sand dollars, facilitating the evolution of nonfeeding larval development in that group. Mounting evidence on the effects of thyroid hormones in echinoderm development suggests life-history models need to incorporate metamorphic hormone effects and the evolution of metamorphic hormone production.

Effects of temperature and circulation on the population dynamics of Calanus finmarchicus in the Gulf of St. Lawrence and Scotian Shelf: Study with a coupled, three‐dimensional hydrodynamic, stage‐based life history model

We developed a physical‐biological model for the Gulf of St. Lawrence (GSL) and Scotian Shelf (SS) by coupling a stage‐based life‐history model of the planktonic copepod Calanus finmarchicus to a three‐dimensional ocean circulation model. The life‐history model consists of 13 morphologically distinct life stages of C. finmarchicus, with stage‐specific and temperature‐dependent molting rates. The model also includes stage‐specific vertical distribution and seasonally varying diapause, egg production, and stage‐specific mortality rates. The model domain covers the eastern Canadian shelf from 55°W to 72°W and from 39°N to 52°N, including the Gulf of St. Lawrence, Scotian Shelf, and Gulf of Maine. A comparison of a 1‐year simulation with observations indicates that the physical‐biological model reasonably describes the observed abundance and distribution of C. finmarchicus in this region. To determine the effects of ocean circulation in the C. finmarchicus population dynamics, we divided the GSL‐SS region into eight sub‐areas and compared the net fluxes of C. finmarchicus across lateral boundaries to the net production in each sub‐area. We found that the annual cross‐boundary exchange rates constitute from <1% to 39% of the local net production, indicating that the horizontal transport of C. finmarchicus by the ocean currents can play a very important role in the dynamics of local C. finmarchicus populations. The results provide insights into the mechanisms of exchange in the GSL‐SS system, as put forward in recent hypotheses.

Life-history models of extinction: a test with island spiders.

This study analyzes extinction patterns for two species of orb spiders monitored annually on 77 islands over a continuous 20-yr period. One species, Argiope argentata, has large populations sometimes crashing quickly to extinction and a much weaker relation of extinction likelihood to population size than does the other species, Metepeira datona. Demographic models were built for both species and matched against observations. Differences between the species in life-history traits-estimated with measurements from the field-together with incorporation of demographic stochasticity, a population ceiling, and environmental stochasticity, were necessary to fit the observed extinction curves. As predicted from life-history patterns, long-term population growth rates (and hence predicted extinction probabilities) are relatively very sensitive to values of juvenile survivorship. Models are also sensitive to variation in the population ceiling and environmental noise, which tend to act in a complementary manner. A simple model with no age structure was able to fit the data on large initial population sizes but not on small initial population sizes, showing that life cycle characteristics interact with the various sources of stochasticity and hence have to be taken into account to produce a precise model of the extinction process.

Gluttony in migratory waders – unprecedented energy assimilation rates in vertebrates

Maximum energy assimilation rate has been implicated as a constraint on maximal sustained energy expenditure, on biomass production, and in various behavioural and life history models. Data on the upper limit to energy assimilation rate are scarce, and the factors that set the limit remain poorly known. We studied migratory waders in captivity, given unlimited food supply around the clock. Many of these waders assimilated energy at rates of seven to ten times basal metabolism, exceeding maximum rates reported for vertebrates during periods of high energy demand, for example during reproduction and in extreme cold. One factor allowing the high energy assimilation rates may be that much of the assimilated energy is stored and not concomitantly expended by muscles or other organs. The remarkable digestive capacity in waders is probably an adaptation to long and rapid migrations, putting a premium on high energy deposition rates. The upper limit to daily energy assimilation in vertebrates is clearly higher than hitherto believed, and food availability, total daily feeding time and, possibly, the fate of assimilated energy may be important factors to take into account when estimating limits to energy budgets in animals.

Host immune status affects maturation time in two nematode species--but not as predicted by a simple life-history model.

In theory, the age at which maturation occurs in parasitic nematodes is inversely related to pre-maturational mortality rate, and cross-species data on mammalian nematodes are consistent with this prediction. Immunity is a major source of parasite mortality and parasites stand to gain sizeable fitness benefits through short-term adjustments of maturation time in response to variation in immune-mediated mortality. The effects of thymus-dependent immune responses on maturation in the nematode parasites Strongyloides ratti and Nippostrongylus brasiliensis were investigated using congenitally thymus-deficient (nude) rats. As compared with worms in normal rats, reproductive maturity of parasites (presence of eggs in utero) in nude rats occurred later in S. ratti but earlier in N. brasiliensis. Immune-mediated differences in maturation time were not associated with differences in worm length. Thymus-dependent immunity had no effect on prematurational mortality. Results are discussed in relation to theoretical expectations and possible explanations for the observed patterns in parasite maturation.

Patterns of age-specific mortality and molding of senescence by natural selection derived via the life-history regression model

An individual based life-history regression setup is introduced not only as an alternative to the ‘reproductive effort model’ in life-history theory, but as a new platform on which the nature of reproductive costs can be explicitly determined and tested under different demographic environmental conditions. Distinctively this regression model is composed of two age-specific features: one is an estimable baseline mortality rate describing the life-history of a population hypothetically undertaking no reproduction, but investing all vital resources into somatic maintenance and growth; the other is a time-dependent covariate encoding dynamic impacts incurred from individual's schedule of reproduction. Regression parameters embedded in the time-dependent covariate explicitly stand for various effects of reproductive costs on future survival relative to the standard described by the baseline mortality rate. Consequently the age-specific mortality is in a compositional structure and gives rise to a wide spectrum of well known mortality curves. Also this compositional structure renders molding forces on senescence by natural selection crucially dependent on patterns of the schedule of reproduction. All numerical evaluations of patterns are performed with two distinct reproductive schedules to illustrate the essential differences from classic results in literature of evolution of life-history.

Small-scale invasion pattern, spread rate, and lag-phase behavior of Rhamnus frangula L.

We tested the ability of the simple reaction–diffusion spread model to quantify the rate of spread of Rhamnus frangula L., a non-indigenous shrub, at the scale of a single forest stand in southeastern New Hampshire. Cut stems of R. frangula were collected in a Cartesian grid pattern within the invaded stand and aged. A contour map of the invasion pattern was produced using kriging. The empirical invasion rate was calculated as the linear distance per year since initial invasion. The intrinsic rate of increase and diffusion coefficient were estimated in the field and the model prediction calculated. Observed mean rate of linear expansion was measured to be 6.7 m per year with a standard deviation of 1.24 m per year, while the predicted linear asymptotic rate of spread was calculated be 6.3 m per year. Thus, the simple reaction–diffusion model yielded a very accurate prediction of actual spread rate. There may have been a lag-phase in the invasion at this site as the spread rate was found to be slower in the early stages of invasion compared to later stages. At the scale of single stands, the simple reaction–diffusion model may be accurate enough to predict the spread of new invaders without recourse to complex life-history spread rate models. Additionally, if lag-phases in spread rate occur with regular frequency, then attempts to retrospectively predict which species may be invasive will be hampered by incorrectly classifying a future invader as currently non-invasive.

Unexpected discontinuities in life-history evolution under size-dependent mortality.

In many organisms survival depends on body size. We investigate the implications of size-selective mortality on life-history evolution by introducing and analysing a new and particularly flexible life-history model with the following key features: the lengths of growth and reproductive periods in successive reproductive cycles can vary evolutionarily, the model does not constrain evolution to patterns of either determinate or indeterminate growth, and lifetime number and sizes of broods are the outcomes of evolutionarily optimal life-history decisions. We find that small changes in environmental conditions can lead to abrupt transitions in optimal life histories when size-dependent mortality is sufficiently strong. Such discontinuous switching results from antagonistic selection pressures and occurs between strategies of early maturation with short reproductive periods and late maturation with long reproductive cycles. When mortality is size-selective and the size-independent component is not too high, selection favours prolonged juvenile growth, thereby allowing individuals to reach a mortality refuge at large body size before the onset of reproduction. When either component of mortality is then increased, the mortality refuge first becomes unattractive and eventually closes up altogether, resulting in short juvenile growth and frequent reproduction. Our results suggest a new mechanism for the evolution of life-history dimorphisms.

Adolescent females' strategic behavior within romantic relationships

Three romantic strategies are inferred from an evolutionary life-history model of adolescence, in which girls must choose between beginning reproduction and acquiring skills. These strategies differ in the level of commitment sought and in the type of information gathered during relationships. I surveyed 158 heterosexual high school girls in New Mexico. The three strategies, as well as age, sexual maturity, parents' education, and father absence, were used as predictors of romantic relationship duration. Three significant results were obtained using the Cox Proportional Hazards Model. First, girls scoring high in the ''committer'' scale were less likely to have relationships end than were girls scoring higher on ''information-gatherer'' scales. Also, Anglo girls and Anglo-Hispanic multi-racial girls were more likely to have a relationship end than were Hispanic girls. Third, girls from a higher socioeconomic background were likely to have longer relationships. The implications of these results are discussed.

Variability of demographic parameters and population dynamics of Atlantic salmon Salmo salar L. in a south-west French river

Abstract The abundance of the salmon population in the Nivelle River was assessed for 11 cohorts during all the stages of their life cycle, from eggs to spawners. A stochastic life history model was used to simulate the changes in numbers at each stage over several years and to evaluate the parameters of a Ricker-type Stock and Recruitment (S–R) relationship. Parameters necessary for managing the exploitation of the species were also estimated. The results indicated that an average deposition of 611 700 eggs (values varying in a proportion of 1 to 3, depending on the year) produced 4870 0+ parr in autumn (variation from 1 to 5.6); 71.8% of which belonged to the group of future 1-year old smolts. The age 1+ parr were eight times less numerous. Survival from egg to 0+ parr was on average 0.97%, but highly variable (varying from 1 to 15). It was density-dependent and followed Ricker S–R model with an optimum of 7800 parr for a survival rate of 3%. During their second year, the survival of 1+ parr reached 53.4% and varied little. The adult runs of complete cohorts amounted to 196 maiden salmon (range, 88 to 382) and previous spawners comprised only 0.9% of adults. Grilse (1 year in the sea) constituted the majority (88.7%). The overall survival rates from 0+ parr to adult returns (6.2% on average) varied three-fold. The majority was females among the grilse (56.2%) and 2-sea-year salmon (88.6%); all 3-sea-year adults were female. Eggs deposited per female averaged 4200, 8500 and 12 750 eggs in each age group, respectively. Simulations of population abundances at various life stages were in good agreement with the observed data. The S–R relationship revealed the low productivity and the vulnerability of this stock, mainly due to the low survival rate of the young during their first year of life. The maximum yield of 12.2% of recruits could be obtained from a deposition of 1 424 000 eggs, which is twice the present average level.

Life history and the competitive environment: trajectories of growth, maturation, and reproductive output among chacma baboons.

The social environment is a key feature influencing primate life histories. Chacma baboons (Papio hamadryas ursinus) are a female-bonded species with a strict linear dominance hierarchy. In this species, the allocation of energy to competing demands of growth and reproduction is hypothesized to vary as a function of competitive ability, which in turn increases with social rank. Since growth rate is a major component of life history models, measures of age-specific growth were used to analyze variation in life history traits across social ranks. Weights of 42 immature baboons were obtained without sedation or baiting from a troop of well-habituated chacma baboons in the Okavango Delta, Botswana. Using demographic and weight data from this wild population, five main findings emerged: 1) Weight for age and growth rate of infant and juvenile females are positively associated with maternal rank. 2) Male growth is not influenced by maternal rank. 3) Female growth shows smaller variation across feeding conditions than male growth. 4) Low-ranking adult females continue investment in offspring through prolonged lactation until they reach a weight comparable to that of high-ranking infants. 5) The benefit of rank to reproductive success shown in this study is 0.83 additional offspring. Reproductive span determined predominantly by age at maturation contributes 27-38% to the difference in expected number of offspring by rank, vs. 62-73% due to reproductive rate. These findings have major implications for understanding the role of social environment in phenotypic plasticity of life history traits, and in the evolution of primate life histories.

Sexual and geographical variation in life history parameters of the shorthorn sculpin

A total of 293 shorthorn sculpins Myoxocephalus scorpius from Tromsø, northern Norway, were sampled between November 1998 and April 1999 to determine sex, total length, age, growth, maturity and mortality. Females grew to larger sizes (L∞=26·9 v. 18·5 cm), matured later (2 v. 1 year of age) at larger size (maturation length=16 v. 14 cm LT), and had lower instantaneous mortality rates (0·93 v. 1·20 year−1) than males. The life history parameters of shorthorn sculpins in northern Norway were more similar to the parameters of short‐lived central European populations than to the parameters of the long‐lived population of Newfoundland. This study confirms that northern Norwegian shorthorn sculpins exhibit sexual dimorphism as in other shorthorn sculpin populations. The relationships between growth pattern, age at maturity and mortality rates observed in the Tromsø population and in other shorthorn sculpin populations, correspond well with the predictions from a published life history model.

Sexual and geographical variation in life history parameters of the shorthorn sculpin

A total of 293 shorthorn sculpins Myoxocephalus scorpius from Tromso, northern Norway, were sampled between November 1998 and April 1999 to determine sex, total length, age, growth, maturity and mortality. Females grew to larger sizes (L-infinity=26.9 v. 18.5 cm), matured later (2 v. 1 year of age) at larger size (maturation length= 16 v. 14 cm L-T), and had lower instantaneous mortality rates (0.93 v. 1.20 year(-1)) than males. The life history parameters of shorthorn sculpins in northern Norway were more similar to the parameters of short-lived central European populations than to the parameters of the long-lived population of Newfoundland. This study confirms that northern Norwegian shorthorn sculpins exhibit sexual dimorphism as in other shorthorn sculpin populations. The relationships between growth pattern, age at maturity and mortality rates observed in the Tromso population and in other shorthorn sculpin populations, correspond well with the predictions from a published life history model. (C) 2002 The Fisheries Society of the British Isles, Published by Elsevier Science Ltd. All rights reserved.

Adaptive reproductive variation along a pollution gradient in a wolf spider.

When populations are exposed to environmental pollutants, growth and reproduction might be strongly reduced due to an increased detoxification effort. Sublethal metal pollution is therefore to be expected to cause the same selection pressure as a low resource habitat and might alter the reproductive strategy. Optimality models of life history theory predict that when resource availability is reduced, growth and reproductive output are reduced and that the release of fewer but larger propagules will be favoured. This was tested by applying a life history model to reproductive trait measurements in six populations of the wolf spider Pirata piraticus in which the assumptions of the model are satisfied. Internal Cd, Cu and Zn body burden were strongly correlated with each other, and differed strongly between the populations, indicating consistently differing metal exposure at the different sites. Pb levels were extremely variable within each population and did not differ between the populations. Females from populations with high concentrations of the first three heavy metals showed a strongly reduced reproductive output and fecundity, indicating a high reduction in resource availability due to detoxification processes. Egg size in contrast was negatively correlated with fecundity and reproductive output and as a consequence positively related with internal metal burden. Our results are thus in strong agreement with the predictions of the optimality models and confirm the benefits of a larger propagule size when resource availability is reduced.