Egg size and clutch size in two Daphnia species grown at different food levels

The effect of three different Scenedesmus food concentrations(0.04, 0.2 and 1 mg C l^–1) on maternal investment wasstudied in two cladoceran species of similar size, Daphnia pulicariaand D.hyalina. It was observed that as food concentration decreased(between 1 and 0.2 mg C l^–1), there was an increase insize, protein content, lipid content, carbon and mass of theegg, while, at the same time, the clutch size of the femalesbecame smaller. Such an increase in ‘per offspring investment’was reflected in an increase in body length, body carbon andbody mass of neonates as the food available for females decreased.However, in D.pulicaria this tendency was not maintained downat 0.04 mg C l^–1 in which there was a decrease of theegg characteristics mentioned above. Although, there are notavailable all the egg and neonate parameters of D.hyalina at0.04 mg C l^–1, the body length of the neonates was largerthan at 0.2 and 1 mg C l^–1. These results show that, asfood diminishes, these two cladoceran species are able to respondby decreasing clutch size, but increasing the size of egg, therebyincreasing the probability of neonate survival. This tendencyis probably maintained until the food concentration is too lowand the females have to reduce the energy allocated for reproduction.     

Recurrent violations of invariant rules for offspring size: evidence from turtles and the implications for small clutch size models

Smith and Fretwell’s classic model predicts that parents can maximize fitness by dividing the energy available for reproduction into offspring of an optimal size. However, this model breaks down when clutch size is small (~1–10 offspring). Invariant rules are an extension of the Smith–Fretwell model, and these rules predict how offspring size will vary among and within individuals that produce small clutch sizes. Here, we provide a narrow test of invariant rules using three turtle species, then we synthesize and re-analyze existing data from 18 different species (comprising five Orders) to evaluate whether invariant rules are followed across broad taxa. We do not find support for most invariant rules in turtles, and our re-analysis demonstrates a general mismatch between observed and expected values across all taxa evaluated, suggesting that invariant rules fail to predict reproductive patterns in nature. Morphological constraints on offspring size and reproductive effort may be important reasons for disparities between theory and observation both in turtles and other taxa. Paradoxically, morphological constraints are most common in small-bodied species and individuals, but these same candidates are also those which produce the small clutch sizes that are necessary to test invariant rules, such that a fair test of invariant rules will often be challenging. Mismatches between theory and observation might also occur because theory assumes that mothers exert control over resource allocation to offspring. In fact, there is evidence of widespread genetic correlations among investment per offspring and reproductive effort, such that these traits are not independent.     

Testing demographic models of effective population size

Due to practical difficulties in obtaining direct genetic estimates of effective sizes, conservation biologists have to rely on so-called 'demographic models' which combine life-history and mating-system parameters with F-statistics in order to produce indirect estimates of effective sizes. However, for the same practical reasons that prevent direct genetic estimates, the accuracy of demographic models is difficult to evaluate. Here we use individual-based, genetically explicit computer simulations in order to investigate the accuracy of two such demographic models aimed at investigating the hierarchical structure of populations. We show that, by and large, these models provide good estimates under a wide range of mating systems and dispersal patterns. However, one of the models should be avoided whenever the focal species' breeding system approaches monogamy with no sex bias in dispersal or when a substructure within social groups is suspected because effective sizes may then be strongly overestimated. The timing during the life cycle at which F-statistics are evaluated is also of crucial importance and attention should be paid to it when designing field sampling since different demographic models assume different timings. Our study shows that individual-based, genetically explicit models provide a promising way of evaluating the accuracy of demographic models of effective size and delineate their field of applicability.     

Testing the geometric clutch hypothesis

The Geometric Clutch hypothesis is based on the premise that transverse forces (t-forces) acting on the outer doublets of the eukaryotic axoneme coordinate the action of the dynein motors to produce flagellar and ciliary beating. T-forces result from tension and compression on the outer doublets when a bend is present on the flagellum or cilium. The t-force acts to pry the doublets apart in an active bend, and push the doublets together when the flagellum is passively bent and thus could engage and disengage the dynein motors. Computed simulations of this working mechanism have reproduced the beating pattern of simple cilia and flagella, and of mammalian sperm. Cilia-like beating, with a clearly defined effective and recovery stroke, can be generated using one uniformly applied switching algorithm. When the mechanical properties and dimensions appropriate to a specific flagellum are incorporated into the model the same algorithm can simulate a sea urchin or bull sperm-like beat. The computed model reproduces many of the observed behaviors of real flagella and cilia. The model can duplicate the results of outer arm extraction experiments in cilia and predicted two types of arrest behavior that were verified experimentally in bull sperm. It also successfully predicted the experimentally determined nexin elasticity. Calculations based on live and reactivated sea urchin and bull sperm yielded a value of 0.5 nN/microm for the t-force at the switch-point. This is a force sufficient to overcome the shearing force generated by all the dyneins on one micron of outer doublet. A t-force of this magnitude should produce substantial distortion of the axoneme at the switch-point, especially in spoke or spoke-head deficient motile flagella. This concrete and verifiable prediction is within the grasp of recent advances in imaging technology, specifically cryoelectron microscopy and atomic force microscopy.     

Parent-offspring conflict over clutch size

Summary We present a model for sexually-reproducing diploids in which a female can produce a variable (generally large) clutch size, where the sibs then compete over some fixed resource, and where certain offspring use siblicide to reduce the primary clutch/brood size created by the mother. Where siblicide involves neither direct energy loss (e.g. fighting cost) nor gain (e.g. cannibalism) to an offspring, the optimal clutch size for an offspring can differ from the optimum for the mother, i.e. there can be parent-offspring conflict over clutch size. The magnitude of this evolutionary conflict (measured in terms of difference between clutch size optima) increases with multipaternity of the brood and with the steepness of the initial decline in offspring survivorship (through sib-competition as further offspring are added to the brood). However, the disparity in clutch size optima may not be great. Where the integer clutch size optima are the same, there will clearly be no conflict. Where this differs, resolution of the evolutionary conflict could involve much apparent behavioral conflict, commonly manifest as siblicidal aggression.The ESS (evolutionarily stable strategy) for such a game will depend upon the direct costs and benefits of siblicide, as well as on the indirect costs to sibs via relatedness. If the only costs of siblicide arise through relatedness, then offspring will win in the sense that the eventual clutch size will match the offspring optimum. Whether or not the mother will produce this clutch size depends on the mechanism controlling siblicide. A siblicidal ESS will occur when offspring are programmed to kill a fixed number/proportion of a brood (victim-based siblicide), but not if programmed to reduce the sibship to the offspring optimum (survivor-based siblicide). With survivor-based siblicide, the mother can do no better than to lay the offsprings' optimal clutch size.     

Testing the relationship between clutch size and brood size in the Coot (Fulica atra)

The time between egg laying and chick fledging is of crucial importance for the survival of young birds. I analyzed breeding output at consecutive phases of growth of young Coots (Fulica atra) relative to the clutch size and laying date. Considering the specific breeding biology of the Coot, I tested whether chick survival reveals clutch size-dependent variability. Clutch size did not affect hatching success; it only affected brood size, and that merely temporarily. During the first 20 days after hatching, i.e. during the time of the highest chick mortality, birds with larger clutches lost chicks at a higher rate. As a result, the number of fledged chicks was independent of the initial number of chicks, and pairs with different clutch sizes had a similar number of fledglings. The laying date had no effect. This pattern of age-related chick survival points to the greater role of the type of chick growth (semi-precocial) and behavior in their survival.     

Is clutch size individually optimized?

Brood size manipulations were carried out to test whether clutchsize variation in individual great tits (Parus major) controlledfor laying date was tuned to their phenotypic quality and/orlocal food abundance (individual optimization hypothesis; IOH).Broods with different original clutch sizes, but equal hatchingdates, were manipulated to a common brood size. A third broodwas kept as a control. Under the IOH, we expected a positiveassociation between reproductive success and original clutchsize. Fledgling production varied in an inconclusive way aftermanipulation, with data from 1 out of 3 years favoring the IOH.The effect of manipulation on the probability of a second clutchwas consistent with the IOH in another 1 out of the 3 years.When fitness accrued to second broods was also taken into considerationin terms of annual fledgling production, results from 2 outof 3 years tended to support the IOH. There was no effect ofthe manipulation on fitness (estimated as the number of recruitsplus parents breeding in the next season). Both the clutch component(local recruitment) and the parental component (survival tillnext breeding season) varied inconclusively with respect tothe IOH. On the basis of fitness measurements, the IOH couldnot be confirmed as an explanation for clutch size variationin this population. In 2 out of 3 years one of the three fitnesscomponents measured varied in accordance with the IOH. Overallthe evidence for the IOH in this data set is therefore weak.     

Parasitoid clutch size and irreversible evolution

Previously, theoretical and empirical studies suggested that parasitoids developing in small multiple-egg broods would evolve siblicidal behaviour, making such brood sizes rare and single-egg broods an evolutionary absorbing state. Recent evidence, however, suggests that small gregarious broods are relatively stable in many parasitoid taxa, and that gregarious development has evolved many times from solitary development. This suggests that new research is needed to assess how nonsiblicidal behaviour can spread and become stable. We discuss some potentially rewarding possibilities.     

Evolution of clutch size in insects. I. A review of static optimality models

Abstract This paper reviews the importance of constraint assumptions to the predictions of static optimality models of insect clutch size. This allows us to identify predictions that distinguish between models embodying different constraints on female oviposition behaviour and hence to determine which resources or other factors limit clutch size evolutionarily. We conclude that while some models may be distinguished using qualitative criteria, others require the testing of quantitative predictions. In a companion paper (Wilson 1994) these models are tested using the bruchid beetle Callosobruchus maculatus.     

Models for clutch size and sex ratio with sibling interaction

General models are constructed to predict sex ratio and clutch size simultaneously for those organisms in which members of a clutch interact to affect each other's fitness. The same set of underlying factors can be shown to predict both optimal sex ratios and clutch sizes. The presence or absence of these factors enables different life histories or models to be classified. Previous sibling interaction sex ratio and clutch size models are special cases of the general model. The circumstances in which it is wrong to consider clutch size and sex ratio in isolation are identified. A distinction is made between those models that maximise sex ratio or clutch size with respect to a single clutch and those that consider either several clutches or lifetime fitness.     

Experiments on clutch size and nest size in passerine birds

Summary Results of experiments on three passerine species suggest that brood size may be constrained by nest size, since the breeding success of pairs provided with large nestcups was greater than that of those provided with small artificial nestcups. These results may have important implications, e.g. to the design of experiments involving manipulation of clutch and brood size. A small nestcup is requisite for successful hatching during the incubation period, but a large one for successful rearing during the nestling period. In nature this difference may select for types of nesting materials that are elastic, such as mosses and lichens. However, experiments showed that such materials rapidly absorb rainwater but only slowly dry out. In addition, because large nests dry out more slowly than small nests, selection will favour small nests among those open-nesting species that have exposed nests. A further possible nest size constraint on open-nesters is nest predation. However, no difference in the predation rate was found in experiments with small and large artificial nests.     

Models of clutch size in insect oviposition

Models for clutch size in species where a female deposits eggs into a larval resource of limited carrying capacity are developed. Previous models of clutch size related mainly to vertebrates (notably birds) where parental care limits clutch size. Our models cover cases where a single female “saturates” a larval food patch with larvae. The main predictions are that (1) extra eggs should be laid to compensate for larval moratility; (2) clutches should generally be smaller than the size that yields the maximum number of surviving larvae/clutch; (3) in species that gain resources for eggs in the adult stage, clutch size will be unaffected by age-independent parental mortality between clutches; (4) clutch size should reduce throughout life in species that gain resources for eggs before the adult stage; (5) similar species, but which are constrained to produce constant-sized clutches, should lay smaller clutches if their total potential egg production is low; (6) clutch size should increase with increasing search costs for oviposition sites. An ESS model of double-oviposition (where two females sometimes lay in the same larval food patch) indicates that the first female should generally lay more eggs than the second female; the difference in clutch size should decrease as the probability of double-oviposition increases, and should decrease as the search costs for larval food plants decreases. Many of the predictions have some support from data on insect oviposition.     

Age-dependent clutch size in a koinobiont parasitoid

Abstract.  1. The Lack clutch size theory predicts how many eggs a female should lay to maximise her fitness gain per clutch. However, for parasitoids that lay multiple clutches it can overestimate optimal clutch size because it does not take into account the future reproductive success of the parasitoid.
2. From egg-limitation and time-limitation models, it is theoretically expected that (i) clutch size decreases with age if host encounter rate is constant, and (ii) clutch size should increase with host deprivation and hence with age in host-deprived individuals.
3. Clutch sizes produced by ageing females of the koinobiont gregarious parasitoid Microplitis tristis Nees (Hymenoptera: Braconidae) that were provided daily with hosts, and of females ageing with different periods of host deprivation were measured.
4. Contrary to expectations, during the first 2 weeks, clutch size did not change with the age of the female parasitoid, neither with nor without increasing host-deprivation time.
5. After the age of 2 weeks, clutch size decreased for parasitoids that parasitised hosts daily. The decrease was accompanied by a strong decrease in available eggs. However, a similar decrease occurred in host-deprived parasitoids that did not experience egg depletion, suggesting that egg limitation was not the only factor causing the decrease in clutch size.
6. For koinobiont parasitoids like M. tristis that have low natural host encounter rates and short oviposition times, the costs of reproduction due to egg limitation, time limitation, or other factors are relatively small, if the natural lifespan is relatively short.
7. Koinobiont parasitoid species that in natural situations experience little variation in host density and host quality might not have strongly evolved the ability to adjust clutch size.     

Geographic variation in calcium and clutch size

For over a half century numerous hypotheses have surfaced aimed at explaining a key life history trait, the evolution of clutch size in birds. A principal goal has been to explain why clutch size generally increases with latitude both within species and among closely related species. Most hypotheses have stressed food limitation, predation, or seasonality. I present a novel hypothesis to explain geographic variation: a limitation of calcium resulting from broad scale variation in this element. Because the storage capacity of avian medullary bone is limited or nonexistent, during egg formation, female birds must intake supplemental calcium. Yet calcium and other exchangeable bases are much rarer in tropical soils. I briefly review the abundant experimental and observational evidence supporting how calcium limitation affects clutch size and other life history traits, and I present a series of predictions (and apparent support for them), stemming from the calcium variation and limitation hypothesis. The balance of evidence suggests that variability in availability of environmental calcium plays a proximate and ultimate role in the evolution of clutch size. Although this hypothesis is not mutually exclusive with others, it highlights another factor that needs to be considered in studies of the geographic variation in clutch size.     

The effect of clutch size on whole clutch cannibalism in the beaugregory damselfish

Parental male beaugregory damselfish Stegastes leucostictus commonly cannibalize entire egg clutches under natural conditions. Clutch size was experimentally reduced, but a relationship between the extent of clutch reduction and the frequency of total cannibalism was not detected.     

Termination of parental care due to small clutch size in the temperate damselfish,Chromis notata

Synopsis Reproductive behavior of the temperate damselfish, Chromis notata, was investigated on the island of Mukaishima, Japan, almost daily during the breeding season in 1982. Both males and females repeated reproductive cycles many times during the breeding season. Females had a strong tendency to spawn a whole clutch on one nest during a few hours. The average number of eggs which a male gained per reproductive cycle was estimated at 38560 (480–131100 eggs). Males ordinarily cared for eggs until just prior to hatching, but abandoned more than half of the nests with the eggs numbering less than 11568. Contribution 207 from the Mukaishima Marine Biological Station.     

Encountering competitors reduces clutch size and increases offspring size in a parasitoid with female-female fighting

Understanding the size of clutches produced by only one parent may require a game-theoretic approach: clutch size may affect offspring fitness in terms of future competitive ability. If larger clutches generate smaller offspring and larger adults are more successful in acquiring and retaining resources, clutch size optima should be reduced when the probability of future competitive encounters is higher. We test this using Goniozus nephantidis, a gregarious parasitoid wasp in which the assumption of size-dependent resource acquisition is met via female-female contests for hosts. As predicted, smaller clutches are produced by mothers experiencing competition, due to fewer eggs being matured and to a reduced proportion of matured eggs being laid. As assumed, smaller clutches generate fewer but larger offspring. We believe this is the first direct evidence for pre-ovipositional and game-theoretic clutch size adjustment in response to an intergenerational fitness effect when clutches are produced by a single individual.