Bigger is not always better: conflicting selective pressures on seed size in Quercus ilex

Most theoretical treatments of the evolutionary ecology of offspring size assume a simple and direct effect of investment per offspring on offspring fitness. In this paper I experimentally determine the relationship between seed mass and several main fitness components of the oak Quercus ilex, to estimate phenotypic selection acting on seed mass during the early life cycle and to discover any potential selective conflicts occurring between different stages from dispersal to establishment. I found a positive effect of acorn size on most fitness components related to seedling establishment. Large size increased germination rate and seedling survival, accelerated germination timing, and enhanced seedling growth. Nevertheless, there was also a direct negative effect of acorn size on survival to predation, because large acorns were highly preferred by the main postdispersal seed predators at the study site, wild boars and wood mice. Because of the low probability of escape from predation, the fitness of large acorns estimated on this component was significantly lower than the fitness of smaller acorns. Therefore, seed size affected fitness in two different ways, yielding opposing and conflicting selective forces. These findings suggest that the general assumption that offspring fitness is a fixed positive function of seed size needs to be reconsidered for some systems. The existence of conflicting selection might explain the occurrence of an optimal seed size in some plant species without invoking a seed number-size trade-off.     

How optimal life history changes with the community size-spectrum

This paper derives optimal life histories for fishes or other animals in relation to the size spectrum of the ecological community in which they are both predators and prey. Assuming log-linear size-spectra and well known scaling laws for feeding and mortality, we first construct the energetics of the individual. From these we find, using dynamic programming, the optimal allocation of energy between growth and reproduction as well as the trade-off between offspring size and numbers. Optimal strategies were found to be strongly dependent on size spectrum slope. For steep size spectra (numbers declining rapidly with size), determinate growth was optimal and allocation to somatic growth increased rapidly with increasing slope. However, restricting reproduction to a fixed mating season changed optimal allocations to give indeterminate growth approximating a von Bertalanffy trajectory. The optimal offspring size was as small as possible given other restrictions such as newborn starvation mortality. For shallow size spectra, finite optimal maturity size required a decline in fitness for large size or age. All the results are compared with observed size spectra of fish communities to show their consistency and relevance.     

Predation effects on the evolution of life-history traits in a clonal oligochaete

Although size at maturity and size and number of offspring are life-history traits widely studied in sexual and parthenogenetic reproduction, there is no such research on animals reproducing asexually without the involvement of gametes. Here we present an individual-based model in combination with experiments to study the clonal growth of Stylaria lacustris, an oligochaete reproducing through fission. We studied the effect of individual size at fission and fission ratio on clone fitness. Our results show that in benign environments without predators, fitness is higher when small worms produce small offspring. Then we included size-specific sublethal predation and found that the fitness of the clone is maximized when parental worms start fission at a large size and produce large descendants intercalated in the middle of the parental worm's body. These results agree with empirical findings. Furthermore, the results of our own laboratory experiment revealed that when S. lacustris is exposed to chemical alarm signals from injured conspecifics, it alters its life history in the same direction as predicted by the model. Our findings suggest that the effect of size-specific sublethal predation is similar to the effect of size-specific lethal predation because both modes of predation result in size-dependent prey mortality.     

Production of offspring using current income and reserves: size-number trade-off and optimal offspring size

To analyse the effects of current income on the nature of size-number trade-off and optimal offspring size, we developed a model in which offspring grow by absorbing current income and reserves. The offspring continue to grow while the current income is available or the reserves exist, and they cease to grow when the reserves are depleted and the current income ceases. We showed that the size-number trade-off is nonlinear in the region where the number of offspring is smaller than the critical number and linear in the region where the number of offspring is greater than the critical number. In the former region, the reserves are not depleted by the time the current income ceases and the offspring cease to grow when the reserves are depleted, whereas in the latter region, the reserves are depleted before the current income ceases and the offspring production is completed when the current income ceases. The optimal offspring size is the same as that shown in Sakai and Harada (Evolution 55 (2001) 467) if this optimal size is realized in the region of nonlinear trade-off, whereas the optimal offspring size is the same as that shown in Smith and Fretwell (Am. Natur. 108 (1974) 499) if this optimal size is realized in the region of linear trade-off.     

REPRODUCTIVE COST,AGE-SPECIFIC SURVIVAL AND A COMPARISON OF THE REPRODUCTIVE STRATEGY IN TWO EUROPEAN TITS (GENUS PARUS)

Theoretical analyses of optimal reproductive rates usually assume a trade-off between offspring production and parental survival. This study verified a survival cost for willow tit males; nonbreeding males survived better than males attending a brood. Theory also predicts a smaller clutch size in birds that are less successful in transforming reproductive investments into mature offspring. As predicted, we found that crested tits, suffering a higher nest predation rate, laid smaller clutches than willow tits. The generally lower survival rate of willow tit adults may largely be attributed to their higher reproductive commitment (larger willow tit clutch size), because no significant interspecific survival difference remained between nonbreeding males. Finally, in willow tits we found a positive correlation between average clutch size and juvenile survival rate (density-dependent) the ensuing year, suggesting that willow tits may adjust clutch size in response to changing survival prospects for their young by using the breeding density as a cue.     

Cannibalism as a selective force on offspring size in fish

Cannibalism may cause considerable mortality on juvenile fish and it has been hypothesised that it may exercise selection on offspring size in that larger offspring may enjoy a size refuge. For this to be evolutionarily advantageous the survival of individual offspring must compensate for the reduced fecundity implied by larger offspring size. We develop a model which combines standard assumptions of size‐dependent mortality with adult cannibalism to investigate the potential for cannibalism to act as selective force on offspring size. We find that for this potential to be realised, the mortality due to cannibalism must exceed a threshold value that is a decreasing function of non‐cannibalistic predation intensity, cannibalized size range width and the average cannibalized size. If cannibalism exceeds this threshold, the model predicts evolution of offspring size towards refuges above or below cannibalized size range depending on initial offspring size. Cannibalistic mortality cannot be so great that the population is non‐viable, however, the range of parameter values describing cannibalistic intensity allowed within these boundaries is wide. On this basis, we suggest that cannibalism is a potential mechanism for offspring size selection.     

Variable fledging age according to group size: trade-offs in a cooperatively breeding bird

Group living can provide individuals with several benefits, including cooperative vigilance and lower predation rates. Individuals in larger groups may be less vulnerable to predation due to dilution effects, efficient detection or greater ability to repel predators. Individuals in smaller groups may consequently employ alternative behavioural tactics to compensate for their greater vulnerability to predators. Here, we describe how pied babbler (Turdoides bicolor) fledging age varies with group size and the associated risk of nestling predation. Nestling predation is highest in smaller groups, but there is no effect of group size on fledgling predation. Consequently, small groups fledge young earlier, thereby reducing the risk of predation. However, there is a cost to this behaviour as younger fledglings are less mobile than older fledglings: they move shorter distances and are less likely to successfully reach the communal roost tree. The optimal age to fledge young appears to depend on the trade-off between reduced nestling predation and increased fledgling mobility. We suggest that such trade-offs may be common in species where group size critically affects individual survival and reproductive success.     

Egg size plasticity corresponding to maternal food conditions in an annual fish,Ayu <Emphasis Type="Italic">Plecoglossus altivelis</Emphasis>

The classic model of Smith and Fretwell predicts that the optimal egg size will vary according to the shape of the relationship between offspring size and offspring fitness, which may vary among environments. Adaptive significance of intrapopulation egg size variation was examined using Ayu (Plecoglossus altivelis). The species has an annual and migratory life history. Fish under controlled rearing conditions become sexually mature with a trend that smaller females produced larger eggs later in the season. Observed egg size variation was explained by the maternal specific growth rate, which was composed of maternal body size and growing period. Hatchlings from larger eggs had a larger notochord length, larger yolk-sac and grew faster. Such offspring traits provide general advantages of increased larval size, which confer competitive ability for assuring early survivorship. In conclusion, egg size plasticity in Ayu suggests higher offspring fitness through enhancement of their accessibility to food.     

Growth pattern and survival in populations of Poecilia vivipara (Teleostei; Poeciliidae) inhabiting an environmental gradient: a common garden study

We performed a common garden experiment to assess the existence of genetic differences on growth and body size between two populations of Poecilia vivipara inhabiting extremes of an environmental gradient caused by water salinity in lagoons of Northern Rio de Janeiro State, Brazil: the Campelo lagoon (freshwater) and Açu lagoon (brackish/saltwater). The two populations show extreme differences in average phenotypes for body size, shape and life history (freshwater populations with smaller body size, lower fecundity and larger reproductive allotment). Pregnant females were brought to the lab and the offspring from both groups were kept in a common recirculating system with freshwater. Standard length and survival were measured weekly over a period of 200 days and growth models were fitted and selected with information criteria. The offspring originally from the brackish water lagoon presented larger asymptotic length, higher maximum growth rate but lower survival than the offspring originally from the freshwater lagoon. Potential confounding variables such as density differences due to mortality and maternal effects (offspring size) were included as covariates in comparisons of growth rates between groups. The results are consistent with phenotypic differences among populations having some genetic basis, and with the existence of a trade-off between growth and maintenance due to the high growth/low survival observed in the group that changed from salt to freshwater. Comparisons of captive and natural populations suggest that the influence of environmental factors, such as salinity, food availability, fish density and predation should also be considered relevant to explain phenotypic variation in this system.     

Vigilance and fitness in grey partridges Perdix perdix: the effects of group size and foraging-vigilance trade-offs on predation mortality

1. Vigilance increases fitness by improving predator detection but at the expense of increasing starvation risk. We related variation in vigilance among 122 radio-tagged overwintering grey partridges Perdix perdix (L.) across 20 independent farmland sites in England to predation risk (sparrowhawk Accipiter nisus L., kill rate), use of alternative antipredation behaviours (grouping and use of cover) and survival. 2. Vigilance was significantly higher when individuals fed in smaller groups and in taller vegetation. In the covey period (in early winter when partridges are in flocks), vigilance and use of taller vegetation was significantly higher at sites with higher sparrowhawk predation risk, but tall vegetation was used less by larger groups. Individuals were constrained in reducing individual vigilance by group size and habitat choice because maximum group size was determined by overall density in the area during the covey period and by the formation of pairs at the end of the winter (pair period), when there was also a significant twofold increase in the use of tall cover. 3. Over the whole winter individual survival was higher in larger groups and was lower in the pair period. However, when controlling for group size, mean survival decreased as vigilance increased in the covey period. This result, along with vigilance being higher at sites with increasing with raptor risk, suggests individual vigilance increases arose to reduce short-term predation risk from raptors but led to long-term fitness decreases probably because high individual vigilance increased starvation risk or indicated longer exposure to predation. The effect of raptors on survival was less when there were large groups in open habitats, where individual partridges can probably both detect predators and feed efficiently. 4. Our study suggests that increasing partridge density and modifying habitat to remove the need for high individual vigilance may decrease partridge mortality. It demonstrates the general principle that antipredation behaviours may reduce fitness long-term via their effects on the starvation-predation risk trade-off, even though they decrease predation risk short-term, and that it may be ecological constraints, such as poor habitat (that lead to an antipredation behaviour compromising foraging), that cause mortality, rather than the proximate effect of an antipredation behaviour such as vigilance.     

Optimal allocation of reproductive effort: manipulation of offspring number and size in the bank vole

The number of offspring attaining reproductive age is an important measure of an individual's fitness. However, reproductive success is generally constrained by a trade-off between offspring number and quality. We conducted a factorial experiment in order to study the effects of an artificial enlargement of offspring number and size on the reproductive success of female bank voles (Clethrionomys glareolus). We also studied the effects of the manipulations on growth, survival and reproductive success of the offspring. Potentially confounding effects of varying maternal quality were avoided by cross-fostering. Our results showed that the number of offspring alive in the next breeding season was higher in offspring number manipulation groups, despite their smaller body size at weaning. Offspring size manipulation had no effect on offspring growth or survival. Further, the first litter size of female offspring did not differ between treatments. In conclusion, females may be able to increase the number of offspring reaching reproductive age by producing larger litters, whereas increasing offspring size benefits neither the mother nor the offspring.     

Top-down and bottom-up control of life-history strategies in coho salmon (Oncorhynchus kisutch)

Sexual maturation profoundly affects population dynamics, but the degrees to which genetic, top-down, and bottom-up controls affect age at maturity are unclear. Salmonid fishes have plastic age at maturity, and we consider genetic and environmental effects on this trait by developing fitness functions for coho salmon (Oncorhynchus kisutch). The functions are based on size-specific survival and reproductive success, where reproductive success is the product of fecundity and ability to defend nests (females) or the product of sperm volume and ability to mate (males). We model genetic and bottom-up controls (e.g., food availability) with an environmentally explicit growth function and top-down control (predation mortality) with survival functions that consider both size-dependent and size-independent mortality. For females, we predict that early maturation rarely maximizes fitness, but males can maximize fitness by maturing early if they grow well in freshwater. We predict that early maturation is most affected by the bottom-up effects of resource distribution at sea, followed by bottom-up and genotypic effects in freshwater. Top-down processes are predicted to have strong effects on the likelihood of delayed maturation.     

Density-dependent effects of mortality on the optimal body size to shift habitat: Why smaller is better despite increased mortality risk

Many animal species across different taxa change their habitat during their development. An ontogenetic habitat shift enables the development of early vulnerable-to-predation stages in a safe “nursery” habitat with reduced predation mortality, whereas less vulnerable stages can exploit a more risky, rich feeding habitat. Therefore, the timing of the habitat shift is crucial for individual fitness. We investigate the effect that size selectivity in mortality in the rich feeding habitat has on the optimal body size at which to shift between habitats using a population model that incorporates density dependence. We show that when mortality risk is more size dependent, it is optimal to switch to the risky habitat at a smaller rather than larger body size, despite that individuals can avoid mortality by staying longer in the nursery habitat and growing to safety in size. When size selectivity in mortality is high, large reproducing individuals are abundant and produce numerous offspring that strongly compete in the nursery habitat. A smaller body size at habitat shift is therefore favored because strong competition reduces growth potential. Our results reveal the interdependence among population structure, density dependence, and life history traits, and highlight the need for integrating ecological feedbacks in the study of life history evolution.     

The offspring size/fecundity trade-off and female fitness in the Atlantic molly (<Emphasis Type="Italic">Poecilia mexicana</Emphasis>, Poeciliidae)

Across a variety of taxa, large offspring have been demonstrated to have a fitness advantage over smaller offspring of the same species. However, producing large offspring often comes at the cost of having to produce fewer young, and the payoff (and thus, evolutionary outcome) of this trade-off is expected to vary between environments. Atlantic mollies (Poecilia mexicana: Poeciliidae, Teleostei), inhabiting a sulfidic cave and various non-sulfidic surface habitats in Tabasco (Mexico), are reproductively isolated and evolved divergent female life-history traits: females of the cave ecotype produce considerably fewer, but larger offspring. Stressful (sulfidic) environments may favor the production of larger offspring, as they are better able to cope with chemical stressors. It remains to be determined though if increased offspring survival outweighs the fitness cost of producing fewer but larger offspring even under benign laboratory conditions. We tested 30-day newborn survival of offspring from wild-caught P. mexicana females from diverging populations in a low-density, no predation, no cannibalism, and ad-libitum-food, benign laboratory environment. Survival rates were highly skewed towards larger cave molly offspring; however, surface molly females still had a higher fitness than cave molly females in terms of higher total numbers of surviving offspring. Our study provides evidence for an innate fitness advantage of larger cave molly offspring. Furthermore, the observed differences in life-history strategies could promote further divergence and reproductive isolation among these ecotypes of P. mexicana, because cave molly females immigrating into the adjacent surface habitats would most likely be selected against.     

Sink-limitation and the size-number trade-off of organs: production of organs using a fixed amount of reserves

To analyze the nature of size-number trade-off of organs, we develop models in which the effects of sink-limitation in the growth of organs and the loss of resources by maintenance respiration are taken into consideration. In these models, the resource absorption rate of an organ is proportional to either its absolute size or its surface area and either the initial size of an organ or the total initial size of the organs produced is fixed. In all models, organs are produced using a fixed amount of reserved resources and no additional resources become newly available for their growth. We theoretically show that size-number trade-offs are nonlinear if the resource absorption rate of an organ is proportional to the absolute size of the organ and the initial size of the individual organs is fixed or if the resource absorption rate of an organ is proportional to the surface area of the organ. In these nonlinear size-number trade-offs, the size of individual organs increases less rapidly than in linear trade-offs with a decrease in the number of organs and the total size of organs is an increasing function of the number of organs produced. This implies that increasing the number of organs produced is advantageous in terms of resource-use efficiency. In contrast, size-number trade-off is linear if the resource absorption rate of an organ is proportional to the absolute size of the organ and there is a linear trade-off between the initial size of organs and their number. To exemplify the effects of those size-number trade-offs on the life-history evolution, we calculate the optimal offspring sizes that maximize the number of offspring successfully being established. In the case of nonlinear size-number trade-offs, the optimal offspring sizes are smaller than the optimal offspring size in the case of linear size-number trade-offs, namely, that in the model of Smith and Fretwell (1974). Our optimal offspring size depends on the metabolism of organ development; the optimal offspring size decreases with an increase in maintenance respiration rate relative to the growth coefficient of organs.     

Offspring size-number strategy in the bethylid parasitoid Laelius pedatus

I observed clutch size and body size of resulting offspringfor the parasitoid Laelies pedatus (Say) (Hymenoptera: Bethylidae)on hosts of different sizes. Results were compared with thepredictions of offspring size-number models and clutch-sizemodels. Larger clutches were laid on larger hosts. However,even after females had adjusted dutch size to the size of thehost, offspring size was larger in larger broods. The variancein offspring size between broods decreased with increasing dutchsize as expected, but the decrease was smaller than predictedby Charaov and Downhower's trade-off invariant rule. Theorypredicts such trends when the shape of the trade-off betweenper capita investment and per capita offspring fitness dependson dutch size or host size. By observing how this assumptionmight apply to bethylid wasps, I generate a number of testablehypotheses to explain the observed trends.     

Clutch size in frugivorous insects as a function of host firmness: the case of the tephritid fly Anastrepha ludens

Abstract. 1. Optimal clutch size theory predicts that individuals will oviposit the number of eggs that increases their fitness. In Anastrepha ludens Loew (Diptera: Tephritidae), females oviposit larger clutches in unripe (firm) fruits than in ripe (soft) fruits. The following hypotheses were tested: (1) Using fruit firmness as an indicator of fruit quality, A. ludens females vary the number of eggs per clutch every time they reach an oviposition decision. (2) Maximising offspring survival with respect to either unripe or ripe fruit requires placing large clutches in firm fruit and smaller clutches in soft fruit. 2. Agar spheres were used as artificial hosts. Three agar concentrations resulted in three degrees of firmness. Mango fruits Mangifera indica L. served as natural hosts. Ripe and unripe fruits were used to test soft and firm host conditions respectively. Females laid significantly larger clutches in the firmer artificial hosts than in the softer hosts. They also laid significantly more eggs in artificial hosts without sugar than in hosts with sugar. Firm (unripe) mangoes also received significantly larger clutches than soft (ripe) mangoes. 3. When an individual female was first presented with a firm artificial host, it laid a large clutch. If subsequently offered a soft host, the female laid a significantly smaller clutch. Finally, if again offered a firm host, clutch size was increased significantly. 4. Possible trade‐offs in offspring fitness were explored in ripe and unripe mangoes by measuring offspring egg‐to‐adult survival, pupal weight, mean adult longevity, and fecundity. Despite the fact that larval survival was greater in soft fruit than in firm fruit, parameters such as pupal weight, mean longevity, and fecundity of adults stemming from both fruit types did not differ significantly. 5. A probable trade‐off between high offspring mortality caused by host unsuitability and low offspring and adult mortality caused by parasitism and predation is discussed as the reason for the exploitation of sub‐optimal hosts.     

Are there trade-offs between pre- and post-fledging survival in black brent geese?

1.?The growth period is an important determinant of fitness later in life through its effects on first-year survival and future reproduction. Choices by adult females about where to rear their offspring strongly affect growth rates and offspring fitness in geese. 2.?Individual female black brent (Branta bernicla nigricans) tend to raise their broods in the same areas each year, and these areas are consistently ranked with respect to growth rates of goslings. Therefore, some females consistently rear their broods on areas resulting in lower post-fledging fitness. 3.?We explore the potential that growth rates of offspring (and associated fitness consequences) are traded off against other vital rates influencing fitness of either adult females or goslings. Growth of goslings primarily influences fitness after fledging, so one hypothesis is that survival before fledging, which is influenced by predation, is traded off against growth rates and post-fledging survival. 4.?We estimated pre-fledging and post-fledging survival for goslings reared on areas used by broods from the Tutakoke River black brent colony. We examined recaptures, recoveries by hunters and resightings of brent marked as goslings with webtags and standard leg rings. These data were analyzed using capture-mark-recapture models in program mark to derive separate estimates of pre- and post-fledging survival for 18 cohorts (1987-2004) of black brent goslings across seven brood rearing areas (BRAs). 5.?Estimates of pre-fledging survival probability varied from 0·00?±?0·00 (mean?±?95% confidence interval) to 0·92?±?0·1; and estimates of post-fledging survival probability varied from 0·00?±?0·00 to 1·00?±?0·08. Substantial variation existed both among BRAs and years but post-fledging survival declined substantially during the study. 6.?Pre- and post-fledging survival were positively correlated, exhibiting a quadratic relationship (?(post-fledging survival) =?1·00 (±0·47)x-0·83 (±0·480)x(2) , where x?=?pre-fledging survival). Therefore, we did not find a trade-off between pre- and post-fledging survival in black brent goslings across BRAs, suggesting that factors other than foraging conditions and predation on goslings must influence selection of BRAs.     

Invertebrate mesopredators are larger in streams with fish

Species with complex life cycles (e.g., aquatic larvae, terrestrial adults) are expected to shorten the time spent in the larval stage if mortality risks are high, a trade-off that lowers predation risk at the cost of reduced time for growth and thus smaller adult size. We tested these predictions by comparing the timing of and size at emergence for two relatively large and common invertebrate mesopredator species (Isoperla montana and Rhyacophila vibox) that inhabit small coastal streams, with and without predatory fish, in eastern Canada. Contrary to expectations based on predation risk–foraging trade-off theory, individuals of both invertebrate species tended to be larger rather than smaller in streams with fish than in fishless streams. The patterns were consistent, however, with the expected ecological effects of top predators on food webs, where fish lower abundances of invertebrate mesopredators, increasing resource availability and thus growth rates for the remaining individuals. We conclude that variation among streams in size at emergence is better explained by the impact of fish on resource availability than to behavioural or life history trade-offs occurring under risk of predation.     

Resource-Dependent Clutch Size Decisions and Size-Fitness Relationships in a Gregarious Ectoparasitoid Wasp, <Emphasis Type="Italic">Bracon brevicornis</Emphasis>

Gregarious parasitic wasps, which lay more than one egg into or onto a host arthropod’s body, are usually assumed to lay an optimal number of eggs per host. If females would lay too few eggs, some resources may be wasted, but if females lay too many eggs, offspring may develop into substantially smaller-sized adults or may not develop successfully and die. The availability of hosts can further influence a female’s clutch size decision, as more eggs should be laid when hosts are scarce. Here, we analyzed clutch size decisions and the fitness consequences thereof in the ectoparasitic wasp Bracon brevicornis (Hymenoptera: Braconidae), a potential biocontrol agent against pest moth species. For experiments, larvae of the Mediterranean flower moth, Ephestia kuehniella (Lepidoptera: Pyralidae) were used. Using artificially created as well as naturally laid clutches of eggs, the effects of clutch size on fitness of first (F1) and second (F2) generation offspring were investigated. Our results revealed that the fitness consequences of large clutches included both increased mortality and smaller adult sizes of the emerging offspring (F1). Smaller F1 females matured fewer eggs during their lifetime and their offspring (F2) had reduced egg-to-adult survival probability. Naturally laid clutches varied with host size up to a maximum, which probably reflects egg limitation. Clutches remained smaller than the calculated optimal (Lack) clutch size and females responded to high host availability with a decreased number of eggs laid. We thus conclude that large clutches may result in significantly smaller offspring with reduced fitness, and that host size as well as host availability influence the clutch size decision made by B. brevicornis females.