Mean Offspring Size Research Articles

Geographic variation in offspring size: Long- and short-term climate affect mean seed mass of Streptanthus populations.

Offspring size is a key functional trait that can affect subsequent life history stages; in many species, it exhibits both local adaptation and phenotypic plasticity. Variation among populations in offspring size may be explained by various factors, including local climatic conditions. However, geographic variation in climate may be partitioned into long‐term and interannual sources of variation, which may differ in their effects on population mean offspring size. To assess environmental correlates of offspring size, we evaluated geographic variation in seed mass among 88 populations representing 6 species of Streptanthus (Brassicaceae) distributed across a broad climatic gradient in California. We examined the effects of temperature‐mediated growing season length and precipitation on population mean seed mass to determine whether it is best explained by (1) long‐term mean climatic conditions; (2) interannual climate anomalies (i.e., deviations in climate from long‐term means) during the year of seed development, or (3) interactions between climate variables. Both long‐term mean climate and climate anomalies in the year of collection were associated with population mean seed mass, but their effects differed in direction and magnitude. Relatively large seeds were produced at chronically wet sites but also during drier‐than‐average years. This contrast indicates that these associations may be generated by different mechanisms (i.e., adaptive evolution vs. phenotypic plasticity) and may be evidence of countergradient plasticity in seed mass. In addition, populations occurring in locations characterized by relatively long growing seasons produced comparatively large seeds, particularly among chronically dry sites. This study highlights the need to consider that the responses of seed mass to long‐term versus recent climatic conditions may differ and that climate variables may interact to predict seed mass. Such considerations are especially important when using these patterns to forecast the long‐ and short‐term responses of seed mass to climate change. The results presented here also contribute to our broader understanding of how climate drives long‐term (e.g., local adaptation) and short‐term (e.g., phenotypic plasticity) variation in functional traits, such as offspring size across landscapes.

Viviparity does not affect the numbers and sizes of reptile offspring.

Viviparity (live-bearing) has independently evolved from oviparity (egg-laying) in more than 100 lineages of squamates (lizards and snakes). We might expect consequent shifts in selective forces to affect per-brood reproductive investment (RI=total mass of offspring relative to maternal mass) and in the way in which that output is partitioned (number vs. size of offspring per brood). Based on the assumption that newly born offspring are heavier than eggs, we predicted that live-bearing must entail either increased RI or a reduction in offspring size and/or fecundity. However, our phylogenetically controlled analysis of data on 1,259 squamate species revealed no significant differences in mean offspring size, clutch size or RI between oviparous and viviparous squamates. We attribute this paradoxical result to (1) strong selection on offspring sizes, unaffected by parity mode, (2) the lack of a larval stage in amniotes, favouring large eggs even in the ancestral oviparous mode and (3) the ability of viviparous females to decrease the mass of uterine embryos by reducing extra-embryonic water stores. Our analysis shows that squamate eggs (when laid) weigh about the same as the hatchlings that emerge from them (despite a many-fold increase in embryo mass during incubation). Most of the egg mass is due to components (such as water stores and the eggshell) not required for oviductal incubation. That repackaging enables live-born offspring to be accommodated within the mother's body without increasing total litter mass. The consequent stasis in reproductive burden during the evolutionary transition from oviparity to viviparity may have facilitated frequent shifts in parity modes.

Environmental cues influence parental brood structure decisions in the burying beetle Nicrophorus marginatus

Parents evaluate multiple extrinsic and intrinsic cues when making decisions associated with reproduction. These decisions often reflect classic trade-offs between the cost of a strategy and its perceived fitness payoff. Life history theory predicts that when parents experience austere conditions, reproductive success is increased by producing fewer but larger offspring with a competitive advantage in this environment. Conversely, parents experiencing favorable conditions are expected to increase current reproductive success by favoring quantity over quality of offspring. We tested the predictions of life history theory using Nicrophorus marginatus (Coleoptera: Silphidae), a burying beetle species that exhibits infanticide during biparental care and hypervariable adult size across populations, by employing a factorial design that manipulated density and nutritional quality of food. We measured (1) the average number of offspring produced, (2) the average individual size of offspring, and (3) the sex ratio of the offspring. We found no effect of density or food quality on offspring sex ratio, but mean offspring size and number differed between low and high-density treatments. Nutritional environment interacted with density effects such that parents with access to high quality diets were able to modulate offspring size and number to match the perceived competitive environment, whereas those in poor nutritional condition appeared to exhibit physiological constraints to producing optimal brood structures.

Bet hedging in a warming ocean: predictability of maternal environment shapes offspring size variation in marine sticklebacks.

Bet hedging at reproduction is expected to evolve when mothers are exposed to unpredictable cues for future environmental conditions, whereas transgenerational plasticity (TGP) should be favoured when cues reliably predict the environment offspring will experience. Since climate predictions forecast an increase in both temperature and climate variability, both TGP and bet hedging are likely to become important strategies to mediate climate change effects. Here, the potential to produce variably sized offspring in both warming and unpredictable environments was tested by investigating whether stickleback (Gasterosteus aculeatus) mothers adjusted mean offspring size and within-clutch variation in offspring size in response to experimental manipulation of maternal thermal environment and predictability (alternating between ambient and elevated water temperatures). Reproductive output traits of F1 females were influenced by both temperature and environmental predictability. Mothers that developed at ambient temperature (17°C) produced larger, but fewer eggs than mothers that developed at elevated temperature (21°C), implying selection for different-sized offspring in different environments. Mothers in unpredictable environments had smaller mean egg sizes and tended to have greater within-female egg size variability, especially at 21°C, suggesting that mothers may have dynamically modified the variance in offspring size to spread the risk of incorrectly predicting future environmental conditions. Both TGP and diversification influenced F2 offspring body size. F2 offspring reared at 21°C had larger mean body sizes if their mother developed at 21°C, but this TGP benefit was not present for offspring of 17°C mothers reared at 17°C, indicating that maternal TGP will be highly relevant for ocean warming scenarios in this system. Offspring of variable environment mothers were smaller but more variable in size than offspring from constant environment mothers, particularly at 21°C. In summary, stickleback mothers may have used both TGP and diversified bet-hedging strategies to cope with the dual stress of ocean warming and environmental uncertainty.

Coping with environmental uncertainty: dynamic bet hedging as a maternal effect.

Mothers in a range of taxa manipulate the phenotype of their offspring in response to environmental change in order to maximize their own fitness. Most studies have focused on changes in the mean phenotype of offspring. Focusing on mean offspring phenotypes is appropriate for species in which mothers are likely to successfully predict the environment their offspring will experience, but what happens when the offspring's environment is unpredictable? Theory suggests that when mothers face uncertainty regarding their offspring's environment, they should increase within-clutch variation in the offspring phenotype (i.e. they should bet hedge). While comparative analyses support the idea that mothers do bet hedge in response to environmental unpredictability, empirical tests are very rare and it remains unclear whether mothers adaptively adjust variance in offspring traits (a phenomenon we call dynamic bet hedging). As a first step towards examining dynamic bet hedging, we reanalysed data from five previously published studies. These studies were across a range of taxa, but all manipulated the maternal environment/phenotype and then examined changes in mean offspring size. We found some support for the theoretical predictions that mothers should increase within-clutch offspring size variation when faced with unpredictable environments. We predict that dynamic bet hedging is more common than previously anticipated and suggest that it has some interesting implications for the studies that focus on shifts in mean offspring traits alone. Hence, future studies should examine maternal effects on both the mean and the variance of offspring traits.

Characterization of Small‐size Egg‐bearing Thai ss‐strain Rotifers Brachionus rotundiformis and Their First Offspring

Abstract This study selected small (100–120 μm) egg‐bearing females from a local Thai ss population and characterized their first offspring‘s life history in an attempt to produce small‐size rotifers. Mean lorica length of selected small adult females was 120 ± 6 μm at the time of collection, and this mean length did not increase over their lifetime. While their first offspring life span was only 5.11 ± 0.35 d, the reference population longevity was higher, 9.65 ± 0.4 d. The length of reproductive period of offspring from the small‐size egg‐bearing female was 2.58 ± 0.38 d, shorter than that of reference population 6.83 ± 0.58 d. Although the rate of egg production was not different between small egg‐bearing female and reference population, the total number of eggs produced by the small egg‐bearing female’s first offspring was lower (4.78 ± 0.60) compared to the reference population (11.83 ± 1.26). The small egg‐bearing female’s first offspring had a higher egg to lorica length ratio (76.1 ± 1.64%) than the reference population (56.6 ± 1.3%), indicating a relatively high investment in reproduction. The small egg‐bearing female’s first offspring reached only 125 ± 6 μm in 36 h, and further culture of their offspring over 35 d resulted in a mean offspring size, 163 ± 11 μm, similar to the reference population (159 ± 16 μm). This shows that population mean size reduction is not likely to result from selecting small egg‐bearing females within a single generation. The unique reproductive characteristics of small egg‐bearing female place it in its own category.

Allocation of offspring size and sex by female black ratsnakes

How females allocate resources to each offspring and how they allocate the sex of their offspring are two powerful potential avenues by which mothers can affect offspring fitness. Previous research has focussed extensively on mean offspring size, with much less attention given to variance in offspring size. Here we focussed on variation in offspring size in black ratsnakes, Elaphe obsoleta. We collected and hatched 105 clutches (1283 eggs) over 9 years. We predicted that females should lay larger eggs, or more variable eggs, when the environment is less predictable. We also predicted that females laying early or laying larger eggs should produce mostly sons because adult males are larger than adult female ratsnakes. The largest hatchling was more than twice the length and almost four times the mass of the smallest hatchling. Variation in offspring size was itself highly variable, with CVs in offspring mass among clutches ranging from 1% to 25%. With one exception, the variables we expected should influence variation in offspring size had little effect. We found that clutch size increased with maternal size and that egg size decreased with clutch size, but we found no evidence that variance in egg size among clutches increased as the season progressed or that females increased the mean size of their offspring the later in the season they laid their eggs. Females in better condition after they finish laying their eggs did produce larger eggs. There was no relationship between within‐clutch variation in egg size and laying date or mean egg size. Finally, sex ratio did not vary with mean egg size or hatching date. Given evidence that offspring size in snakes affects survival, selection should reduce variation in offspring size unless that variance enhances maternal fitness and yet we found little support for hypothesized advantages of varying offspring size.

CROSS-GENERATIONAL ENVIRONMENTAL EFFECTS AND THE EVOLUTION OF OFFSPRING SIZE IN THE TRINIDADIAN GUPPY POECILIA RETICULATA

The existence of adaptive phenotypic plasticity demands that we study the evolution of reaction norms, rather than just the evolution of fixed traits. This approach requires the examination of functional relationships among traits not only in a single environment but across environments and between traits and plasticity itself. In this study, I examined the interplay of plasticity and local adaptation of offspring size in the Trinidadian guppy, Poecilia reticulata. Guppies respond to food restriction by growing and reproducing less but also by producing larger offspring. This plastic difference in offspring size is of the same order of magnitude as evolved genetic differences among populations. Larger offspring sizes are thought to have evolved as an adaptation to the competitive environment faced by newborn guppies in some environments. If plastic responses to maternal food limitation can achieve the same fitness benefit, then why has guppy offspring size evolved at all? To explore this question, I examined the plastic response to food level of females from two natural populations that experience different selective environments. My goals were to examine whether the plastic responses to food level varied between populations, test the consequences of maternal manipulation of offspring size for offspring fitness, and assess whether costs of plasticity exist that could account for the evolution of mean offspring size across populations. In each population, full-sib sisters were exposed to either a low- or high-food treatment. Females from both populations produced larger, leaner offspring in response to food limitation. However, the population that was thought to have a history of selection for larger offspring was less plastic in its investment per offspring in response to maternal mass, maternal food level, and fecundity than the population under selection for small offspring size. To test the consequences of maternal manipulation of offspring size for offspring fitness, I raised the offspring of low- and high-food mothers in either low- or high-food environments. No maternal effects were detected at high food levels, supporting the prediction that mothers should increase fecundity rather than offspring size in noncompetitive environments. For offspring raised under low food levels, maternal effects on juvenile size and male size at maturity varied significantly between populations, reflecting their initial differences in maternal manipulation of offspring size; nevertheless, in both populations, increased investment per offspring increased offspring fitness. Several correlates of plasticity in investment per offspring that could affect the evolution of offspring size in guppies were identified. Under low-food conditions, mothers from more plastic families invested more in future reproduction and less in their own soma. Similarly, offspring from more plastic families were smaller as juveniles and female offspring reproduced earlier. These correlations suggest that a fixed, high level of investment per offspring might be favored over a plastic response in a chronically low-resource environment or in an environment that selects for lower reproductive effort.

CROSS-GENERATIONAL ENVIRONMENTAL EFFECTS AND THE EVOLUTION OF OFFSPRING SIZE IN THE TRINIDADIAN GUPPY POECILIA RETICULATA

The existence of adaptive phenotypic plasticity demands that we study the evolution of reaction norms, rather than just the evolution of fixed traits. This approach requires the examination of functional relationships among traits not only in a single environment but across environments and between traits and plasticity itself. In this study, I examined the interplay of plasticity and local adaptation of offspring size in the Trinidadian guppy, Poecilia reticulata. Guppies respond to food restriction by growing and reproducing less but also by producing larger offspring. This plastic difference in offspring size is of the same order of magnitude as evolved genetic differences among populations. Larger offspring sizes are thought to have evolved as an adaptation to the competitive environment faced by newborn guppies in some environments. If plastic responses to maternal food limitation can achieve the same fitness benefit, then why has guppy offspring size evolved at all? To explore this question, I examined the plastic response to food level of females from two natural populations that experience different selective environments. My goals were to examine whether the plastic responses to food level varied between populations, test the consequences of maternal manipulation of offspring size for offspring fitness, and assess whether costs of plasticity exist that could account for the evolution of mean offspring size across populations. In each population, full-sib sisters were exposed to either a low- or high-food treatment. Females from both populations produced larger, leaner offspring in response to food limitation. However, the population that was thought to have a history of selection for larger offspring was less plastic in its investment per offspring in response to maternal mass, maternal food level, and fecundity than the population under selection for small offspring size. To test the consequences of maternal manipulation of offspring size for offspring fitness, I raised the offspring of low- and high-food mothers in either low- or high-food environments. No maternal effects were detected at high food levels, supporting the prediction that mothers should increase fecundity rather than offspring size in noncompetitive environments. For offspring raised under low food levels, maternal effects on juvenile size and male size at maturity varied significantly between populations, reflecting their initial differences in maternal manipulation of offspring size; nevertheless, in both populations, increased investment per offspring increased offspring fitness. Several correlates of plasticity in investment per offspring that could affect the evolution of offspring size in guppies were identified. Under low-food conditions, mothers from more plastic families invested more in future reproduction and less in their own soma. Similarly, offspring from more plastic families were smaller as juveniles and female offspring reproduced earlier. These correlations suggest that a fixed, high level of investment per offspring might be favored over a plastic response in a chronically low-resource environment or in an environment that selects for lower reproductive effort.

Optimal offspring size in a small mammal: an exception to the tradeoff invariant life‐history rule

Offspring size and number were examined in a captive population of wild guinea pigs (Cavia aperea), and findings were compared with models of optimal offspring size for small litters. Median and modal litter size was two, regardless of maternal size or parity. Females producing their second litter tended to have litters that were larger than average. In contrast, young females that were still growing never had litters that were larger than average. Mean offspring size decreased and variation in offspring size tended to decrease with increasing litter size. Optimal offspring size models, in which offspring survival depended on the amount of resources invested, as well as litter size, predict such a trend. Little support was found for Charnov and Downhower's (1995) tradeoff invariant life‐history rule that the range in offspring sizes between litters is inversely proportional to the size of the litter. Cavia aperea may be an exception to this rule because pup mass at birth did not reflect total reproductive investment, because conversion of resources into litter mass may not be linearly related to litter size and because resources were not equally partitioned among offspring within large litters. Experimental data are needed to determine the relevance of these results among mammals in general.

Sex allocation in the solitary bee Osmia cornuta: do females behave in agreement with Fisher's theory?

Fisher's theory of sex allocation predicts that, in a panmictic population, parental investment will be equally distributed between male and female progeny. Most studies on parental investment in nesting solitary bees and wasps use offspring or provision weight as estimators of parental investment and do not corroborate Fisher's theory. The measurement of parental investment may be confounded by several factors. First, the use of offspring or provision size does not account for seasonal variation in foraging costs associated with aging of nesting females. Second, provision or offspring size do not reflect parental investment associated with nest construction. In this two-year study we measured parental investment in a solitary bee. We calculated sex allocation using both provision weight and foraging time as parental investment estimators. Investment in pollen-nectar provisions decreased, while investment in mud structures (nest construction) increased, as the nesting period progressed. Overall investment in provisions per nest was ∼25 times higher than investment in mud. Pollen-nectar foraging trips became longer as the season progressed, but mud trip duration did not vary. Due to weather differences between years, more offspring per female were produced in the first year, but progeny sex ratio and mean offspring size of both sexes were similar between years. Mortality did not differ between sexes. As predicted by Fisher's theory, production cost ratios did not differ from 1 in either year, irrespective of the currency used to estimate parental investment (provision weight or foraging time). Our results strongly support Fisher's theory.

CLUTCH SIZE AND OFFSPRING SIZE IN THE WOLF SPIDER PIRATA SEDENTARIUS (ARANEAE, LYCOSIDAE)

Wolf spiders in the genus Pirata are common, often locally abundant, inhabitants of many moist or mesic habitats. However, relatively little is known about the ecology or life history of these spiders. Here we present data collected during 2000-2001 on female size, offspring size and clutch size for two populations (Ball Gulf, Hardscrabble Creek) of Pirata sedentarius from western New York. In both populations, mean offspring size was less variable than was female size, clutch size or total clutch mass. At Ball Gulf, 67% of females produced two egg sacs and 48% produced three sacs. Clutch size declined across the egg sac sequence for all females. Female size, measured as cephalothorax length, was uncorrelated with mean offspring size in all cases. However, larger females produced larger and heavier clutches during 2001 for both populations; female size was uncorrelated with these variables during 2000 at Hardscrabble Creek. Larger clutches from both populations contained more offspring, and larger clutches contained smaller offspring at Hardscrabble Creek in 2001. We found a significant offspring size-number trade-off at Ball Gulf, while at Hardscrabble Creek this trade-off was marginally significant in 2001 and non-significant in 2000.

GENETIC INFLUENCES ON EXPERIMENTAL POPULATION DYNAMICS OF THE LEAST KILLIFISH

When natural populations differ in density or in the dynamic fluctuations of population size, some of those differences may result from their different ecological conditions, and some may originate from genetically based differences in life history expression. Natural populations of the live-bearing poeciliid fish, Heterandria formosa, vary considerably in their population dynamics, with densities that differ between populations by as much as sevenfold. This system offers an excellent opportunity to explore the potential role of genetically based differences in life history expression in creating different dynamic patterns in a common environment. We created five different genetic stocks of H. formosa by carrying out a series of crosses using fish from two North Florida populations (the Wacissa River and Trout Pond) and used them to initiate replicate experimental populations in artificial ponds. The five stocks consisted of two “controls,” which were pure Wacissa River and Trout Pond stocks, and three types of hybrid stocks. The hybrid stocks differed in a regular way in the proportion of genes from one population or the other. The crossing scheme was designed so that each hybrid stock would have the same proportion of heterozygous (or “heterodemic”) loci but would differ in the proportion and/or identity of homozygous (or “homodemic”) loci from the Wacissa River and Trout Pond populations. These populations were chosen because a previous study had found that population densities in the Wacissa River greatly exceeded those of Trout Pond and exhibited a higher range of population fluctuation during the breeding season. We addressed four questions in this experiment: (1) Are there genetically based differences in life history traits of fish from the two populations? (2) If so, do differences in life history expression produce differences in population dynamics in a common environment? (3) Which traits have the greatest influence on population dynamics? (4) How do changes in density affect the phenotypes of individual traits that govern the rates of birth and death in a population? We followed experimental populations of the five genetic stocks from their initiation at low density through 4–6 generations of population growth and decline. The mean offspring size differed among stocks by as much as 50%. At low densities, offspring size exhibited a trade-off with brood size: Trout Pond alleles were associated with more, smaller offspring. At higher densities, offspring sizes were similar among stocks, and the trade-off with offspring number was not evident. Stocks differed in realized population growth rate by as much as 70%; the rank order differences among stocks with respect to population growth rate appeared to match the genetic relatedness among stocks based on the expected percentage of Trout Pond alleles. Differences in population growth rate appeared to be due to differences in brood size among stocks at low density. Stocks did not differ in the equilibrium population size, which indicated the absence of a trade-off between population growth rate and carrying capacity in this environment. Adult survival and recruitment of juveniles into the adult population both declined linearly with increasing density; and stocks did not generally differ in those rates after the effects of density had been taken into account. The stocks differed in their response to the depressant effects of density on life history trait expression. The offspring size of the pure Wacissa River stock was much more sensitive to density than was the offspring size of the pure Trout Pond stock. However, the brood sizes of the Wacissa River stock were reduced much less than those of the Trout Pond stock when exposed to the same high density. These results suggest that life history distinctions among populations, both in the mean values and plasticity of traits, play a role in creating different dynamics. We discuss the ways in which phenotypic plasticity in reproductive traits potentially acts as a mechanism to stabilize population dynamics in this species.

Reproductive allocation in Daphnia exposed to toxic cyanobacteria

We investigated experimentally how resources were allocated to reproduction in Daphnia pulex and Daphnia longispina when varying levels of toxic Microcystis were added to higher quality food. We used multiple regression models to estimate mean offspring size and clutch size in relation to maternal size and clutch number, and analysed effects of treatments on residuals from the models. We also measured variation in per offspring investment. At a high cyanobacterial level, D.pulex was virtually unable to reproduce. At a lower level, D.pulex produced small clutches. However, the regres- sion model residuals indicated that the presence of cyanobacteria increased the portion of available resources allocated to reproduction. The observed allocation may be a means to maximize repro- duction under diminished longevity. Effects on mean offspring size were marginal in D.pulex, but variation in per offspring investment sometimes decreased in cyanobacterial exposures. Daphnia longispina was affected by a higher cyanobacterial level only, where offspring size was reduced. Devi- ations from the regression model indicated that effects on maternal size alone do not explain this effect. Clutch size residuals and per offspring investment were unaffected by treatments in D.longispina. The observed responses differ from theoretical models on reproductive allocation under food limitation.

Quantity or quality? Determinants of maternal reproductive success in tropical pythons (Liasis fuscus)

A female9s reproductive output (size and number of offspring) may say little about her reproductive success; the ‘quality’ of her progeny and the time that they are produced may be more important in this respect. We marked and released 1224 hatchling water pythons ( Liasis fuscus ) from 116 clutches of laboratory–incubated eggs, in a study site in tropical Australia. Clutches varied widely in the number of eggs, mean offspring sizes, the proportion of eggs that hatched, and recapture rates of offspring. Clutch size was not significantly correlated with the number of recaptured hatchlings in either year, nor was mean offspring size related to recapture probability. Instead, offspring recapture rates varied as a function of the time of hatching. Recapture rates were higher for hatchlings released in a year with high prey availability than in a year with few prey. In addition, recapture rates were higher for earlier–hatching clutches in the year with few prey. The other significant effect on recapture rates involved offspring viability: some clutches had a much higher proportion of surviving offspring than did others, and recapture rates were higher from clutches with higher hatching success (i.e. proportion of eggs hatching). Hence, the usual measures of reproductive output (clutch size and offspring size) offer only a poor index of a female9s reproductive success in this system.

Demographic and Life-History Variation in Two Proximate Populations of a Viviparous Skink Separated by a Steep Altitudinal Gradient

1. Life-history parameters (age and size at first reproduction, female age/size distribution, reproductive output) were compared between two populations of the viviparous water skink Eulamprus tympanum in temperate south-eastern Australia. The two populations were in close proximity to each other (6 km) but separated by 1000 m in altitude. 2. Age and size at first reproduction differed significantly between the two populations. Low-altitude females appear to start reproduction at 2 years of age and at a relatively small size (70 mm snout-vent length) whereas high-altitude females start to reproduce at 5 years of age and at a larger size (90 mm snout-vent length). The data suggest that size at first reproduction at high altitude is larger, not only because thermal conditions slow annual growth rates and therefore delay first reproduction, but because small size is a disadvantage for reproducing females under such conditions. 3. Reproductive output also differed significantly between the two populations, with low-altitude females producing relatively large litters with small young and high-altitude females producing relatively small litters with large young. The total offspring mass a female produced in relation to her body size was significantly smaller in high-altitude E. tympanum, suggesting strong environmental constraints on reproduction compared with low-altitude E. tympanum. 4. To test whether reproductive output can be influenced by local environmental conditions, or whether it requires natural selection to produce a change in this parameter, food intake was manipulated in high-altitude females during their pregnancy. Low-diet females produced a significantly smaller total offspring mass and smaller litters than high-diet females. There was no significant difference in mean offspring size between the two treatments. 5. Effects of age on reproductive parameters were examined to determine whether reproductive output increases with maternal age, for example as a consequence of more experience gained by older females, or decreases, possibly as a result of senescence. In neither population was there evidence for an increase or decline in reproductive output with the age of reproducing females independent of the effects of maternal size.

Reproductive biology of the mosquitofish in a permanent natural lagoon in south-west Spain: two tactics for one species

Introduced Gambusia holbrooki in a natural lagoon of southern Spain consisted of two age groups: 1992 cohort, 7-11 months old and 1993 cohort, <4 months old. In the 1992 cohort, females grew even during the gestation period at about 0.30 mm day-1. In the 1993 cohort, females displayed a high growth rate (0.55 mm day-1) and reached reproductive size in a few weeks, but stopped growing when they matured. All the 1992 cohort reproduced from mid-May to mid-June, but only 50% of the 1993 cohort reproduced, from mid-August to mid-September. Reproducing females were significantly larger in the 1992 cohort (39.8 mm) than in the 1993 one (34.8 mm). The largest 1992 females cohort had reproduced previously; the 1993 cohort had not. The mean dry weight of intra-ovarian embryos decreased to a minimum immediately before birth. These metabolic costs represented 29.8 and 31.4% of the initial weights of the 1992 and 1993 embryos, respectively. Mean dry weight of full-term embryo was significantly higher in the 1992 cohort (0.80+/-0.129 mg; 95% CL) than in the 1993 one (0.70+/-0.086 mg; 95% CL). With more females in 1992, cohort fecundity was considerably higher (number of embryos=7151; 63%) than in the 1993 (4193; 37%) cohort. The population completed two generations each year (spring and summer). The spring stock grew slower than the summer one but continued to grow during the gestation period, reaching larger final lengths, with more synchronous reproduction and clearer evidence of a second reproductive event. Each reproductive stock displayed its own life-history characteristics, with significant differences between mean length of reproducing females, growth rate, mean brood size, offspring size, standard fecundity and minimum length at reproduction.

Opportunity of Selection in the Khamti of Lakhimpur District, Assam, India

In the present communication an attempt has been made to measure the Index of Opportunity for selection in the Khamti, a migrant, endogamous, community of Narayanpur, Bihpuria areas in Lakhimpur district, Assam, based on data on pregnancy outcome of 29 Khamti ever-pregnant women aged 45 + years. The results of the present work were compared with those of the available data on the tribal populations of North Eastern India. It is observed from the present analysis that childhood death and embryonic death are very low but mean offspring size is high. The value of I and its mortality and fertility components are also low. I: is evident that selection is operating with low intensity in the Khamti of Lakhimpur district, Assam and the average general fitness of the population is high.

Offspring Size Variation in Eastern Mosquitofish (Gambusia holbrooki: Poeciliidae) from Contrasting Thermal Environments

Seasonal patterns of offspring size variation in eastern mosquitofish (Gambusia holbrooki, Poeciliidae) are described from a thermally ambient environment and an unpredictable, thermally elevated system in South Carolina. Newborn mosquitofish (n = 1371) were individually lengthed and weighed shortly after birth; mean offspring size (both length and dry weight) varies among females, seasons, and thermal environments. Size variance was high in all cases but less than half was explainable by environment, season, maternal size, or clutch size; environment and season explain the most variation. Although offspring size is certainly subject to selection (i.e., optimality arguments) and even size variation may be selected for (i.e., plasticity arguments) it should not be overlooked that some portion of variation may be due to uncontrollable noise (i.e., the female cannot precisely control offspring size). All three aspects of size variation in mosquitofish remain to be tested. ONE focus of life history studies is the evolution of offspring size. Possible fitness differences among different sized offspring, and relationships between offspring size, clutch size and other life history parameters have resulted in divergent views of the action of selection on

Breeding phenology, variation in reproductive effort and offspring size in a tropical population of the woodlouse Porcellionides pruinosus.

Most species of woodlice in temperate habitats have discrete breeding seasons. It is hypothesised that breeding synchronises with favourable environmental conditions to maximise offspring growth and survivorship (Willows 1984). We measured the breeding phenology of a species introduced to a tropical environment, primarily to consider the assumption that life histories in the tropics will differ fundamentally from those in temperate habitats. In addition to breeding phenology we considered variation in reproductive effort between individual females and the division of this effort between the size and number of young.A continuous breeding phenology was observed in a synanthropic population of Porcellionides pruinosus within the tropics. Reproductive effort varied between months, showed a weak relationship with female size and was independent of female fecundity. Female sizefecundity relationships varied between samples and when the proportion of reproductive females was high size-fecundity slopes were steeper than at other times. Mean offspring size varied between months and there was a wide range in offspring size within broods. Offspring size was not related to female body mass, reproductive effort or fecundity; consequently brood mass increased linearly with an increase in fecundity. Increased reproductive effort goes into more rather than larger offspring.We propose that the continuous breeding in this population was the result of the constant presence of an environmental cue to reproduction evolved in temperate habitats. Continuous breeding is not necessarily equivocal to high individual reproductive success even though overall population growth may be rapid. However, variation in reproductive effort suggests that individuals respond to current environmental conditions on short time scales.