Experience versus effort: what explains dynamic heterogeneity with respect to age?

Age‐related patterns of survival and reproduction have been explained by accumulated experience (‘experience hypothesis’), increased effort (‘effort hypothesis’), and intrinsic differences in phenotypes (‘selection hypothesis’). We examined the experience and effort hypotheses using a 40‐year data set in a population of Leach's storm‐petrels Oceanodroma leucorhoa, long‐lived seabirds for which the effect of phenotypic variation has been previously demonstrated. Age was quantified by time since recruitment (‘breeding age’). The best model of adult survival included a positive effect of breeding age (1, 2, 3+ years), sex (male > female), and year. Among‐individuals variation (fixed heterogeneity) accounted for 31.6% of the variance in annual reproductive success. We further examined within‐individual patterns in reproductive success (dynamic heterogeneity) in the subset of individuals with at least five breeding attempts. Three distinct phases characterized reproductive success – early increase, long asymptotic peak, late decline. No effect of early reproductive output on longevity was found, however, early success was positively correlated with lifetime reproductive success. Reproductive success was lower earlier than later in life. Among the few natally philopatric individuals in the population, age of first breeding had no effect on longevity, lifetime reproductive success, or early reproductive success. No support for the effort hypothesis was found in this population. Instead, age‐specific patterns of survival and reproduction in these birds are best explained by the experience hypothesis over and above the effect of intrinsic differences among individuals.     

Evaluating the contributions of change in investment and change in efficiency to age‐related declines in male and female reproduction

It is commonly observed that reproduction decreases with age, often at a different rate in males and females. This phenomenon is generally interpreted as senescence. Such reproductive declines may stem from at least two sources: a change in resource allocation and a decline in the ability to convert resources into offspring. This distinction is important because a shift in resource allocation may be favoured by selection, while reduced efficiency is purely deleterious. We propose a way to distinguish whether a decline in reproduction is purely deleterious based on estimating reproductive investment, output, and their ratio, efficiency. We apply this approach to the hermaphroditic snail Physa acuta and demonstrate that both male and female functions decline with age. The male decline largely stems from reduced investment into male activity while female decline is due to increased reproductive inefficiency. This shows that age‐related declines in reproduction can occur for a number of different reasons, a distinction that is usually masked by the general term ‘senescence’. This approach could be applied to any species to evaluate age‐related reproductive decline. We advocate that future studies measure age trajectories of reproductive investment and output to explore the potential processes hidden behind the observation that reproduction declines with age.     

Looking for a needle in a haystack: inference about individual fitness components in a heterogeneous population

Studies of wild vertebrates have provided evidence of substantial differences in lifetime reproduction among individuals and the sequences of life history ‘states’ during life (breeding, nonbreeding, etc.). Such differences may reflect ‘fixed’ differences in fitness components among individuals determined before, or at the onset of reproductive life. Many retrospective life history studies have translated this idea by assuming a ‘latent’ unobserved heterogeneity resulting in a fixed hierarchy among individuals in fitness components. Alternatively, fixed differences among individuals are not necessarily needed to account for observed levels of individual heterogeneity in life histories. Individuals with identical fitness traits may stochastically experience different outcomes for breeding and survival through life that lead to a diversity of ‘state’ sequences with some individuals living longer and being more productive than others, by chance alone. The question is whether individuals differ in their underlying fitness components in ways that cannot be explained by observable ‘states’ such as age, previous breeding success, etc. Here, we compare statistical models that represent these opposing hypotheses, and mixtures of them, using data from kittiwakes. We constructed models that accounted for observed covariates, individual random effects (unobserved heterogeneity), first‐order Markovian transitions between observed states, or combinations of these features. We show that individual sequences of states are better accounted for by models incorporating unobserved heterogeneity than by models including first‐order Markov processes alone, or a combination of both. If we had not considered individual heterogeneity, models including Markovian transitions would have been the best performing ones. We also show that inference about age‐related changes in fitness components is sensitive to incorporation of underlying individual heterogeneity in models. Our approach provides insight into the sources of individual heterogeneity in life histories, and can be applied to other data sets to examine the ubiquity of our results across the tree of life.     

Relative contributions of fixed and dynamic heterogeneity to variation in lifetime reproductive success in kestrels (Falco tinnunculus)

Many species show large variation in lifetime reproductive success (LRS), with a few individuals producing the majority of offspring. This variation can be explained by factors related to individuals (fixed heterogeneity) and stochastic differences in survival and reproduction (dynamic heterogeneity). In this study, we study the relative effects of these processes on the LRS of a Dutch Kestrel population, using three different methods. First, we extended neutral simulations by simulating LRS distributions of populations consisting of groups with increasingly different population parameters. Decomposition of total LRS variance into contributions from fixed and dynamic heterogeneity revealed that the proportion of fixed heterogeneity is probably lower than 10% of the total variance. Secondly, we used sensitivities of the mean and variance in LRS to each parameter to analytically show that it is impossible to get equal contributions of fixed and dynamic heterogeneity when only one parameter differs between groups. Finally, we computed the LRS probability distribution to show that even when all individuals have identical survival and reproduction rates, the variance in LRS is large (females: 27.52, males: 12.99). Although each method has its limitations, they all lead to the conclusion that the majority of the variation in kestrel LRS is caused by dynamic heterogeneity. This large effect of dynamic heterogeneity on LRS is similar to results for other species and contributes to the evidence that in most species the majority of individual variation in LRS is due to dynamic heterogeneity.     

Diversity of age‐specific reproductive rates may result from ageing and optimal resource allocation

This paper reports the results of a dynamic programming model which optimizes resource allocation to growth, reproduction and repair of somatic damage, based on the disposable soma theory of ageing. Here it is shown that different age‐dependent patterns of reproductive rates are products of optimal lifetime strategies of resource partitioning. The array of different reproductive patterns generated by the model includes those in which reproduction begins at the maximum rate at maturity and then declines to the end of life, or increases up to a certain age and then drops. The observed patterns reflect optimal resource allocation shaped by the level of extrinsic mortality. A continuous decline in the reproductive rate from the start of reproduction is associated with high extrinsic mortality, and an early increase in the reproductive rate occurs under low extrinsic mortality. A long‐lived organism shows a low reproductive rate early in life, and short‐lived organisms start reproduction at the maximum rate.     

Drivers of age-specific survival in a long-lived seabird: contributions of observed and hidden sources of heterogeneity

1.?We assessed the relative influence of variability in recruitment age, dynamic reproductive investment (time-specific reproductive states) and frailty (unobserved differences in survival abilities across individuals) on survival in the black-legged kittiwake. Furthermore, we examined whether observed variability in survival trajectories was best explained by immediate reproductive investment, cumulative investment, or both. 2.?Individuals that delayed recruitment (≥ age 7) suffered a higher mortality risk than early recruits (age 3), especially later in life, suggesting that recruitment age may be an indicator of individual quality. Although recruitment age helped explain variation in survival, time-varying reproductive investment had a more substantial influence. 3.?The dichotomy of attempting to breed or not explained variability in survival across life better than other parameterizations of reproductive states such as clutch size, brood size or breeding success. In the kittiwake, the sinequanon condition to initiate reproduction is to hold a nest site, which is considered a very competitive activity. This might explain why attempting to breed is the key level of investment that affects survival, independent of the outcome (failure or success). 4.?Interestingly, the more individuals cumulate reproductive attempts over life, the lower their mortality risk, indicating that breeding experience may be a good indicator of parental quality as well. In contrast, attempting to breed at time t increased the risk of mortality between t and t + 1. We thus detected an immediate trade-off between attempting to breed and survival in this population; however, the earlier individuals recruited, and the more breeding experience they accumulated, the smaller the cost. 5.?Lastly, unobserved heterogeneity across individuals improved model fit more (1·3 times) than fixed and dynamic sources of observed heterogeneity in reproductive investment, demonstrating that it is critical to account for both sources of individual heterogeneity when studying survival trajectories. Only after simultaneously accounting for both sources of heterogeneity were we able to detect the 'cost' of immediate reproductive investment on survival and the 'benefit' of cumulative breeding attempts (experience), a proxy to individual quality.     

Identifying population thresholds for flowering plant reproductive success: the marsh gentian (<Emphasis Type="Italic">Gentiana pneumonanthe</Emphasis>) as a flagship species of humid meadows and heathland

The threshold below which population declines impact the effectiveness of plant reproduction is essential for the identification of populations that can no longer spontaneously recover following habitat management or restoration, below the minimum viable population (MVP) size. We hypothesized that risk of reproductive limitation can be evaluated from combined analysis of pollen activity, ovule fertilization and germination in the context of population demographics and fragmentation. The marsh gentian (Gentiana pneumonanthe), a rare emblematic species of European heathland and fen, was investigated at the southern limit of its range in eighteen populations encompassing one to several hundred thousand individuals, spanning small fragments to extensive well-preserved areas. An index of habitat fragmentation was determined from GIS; field surveys determined the ratio of juvenile to reproductive age states; fluorescence microscopy of pistils determined, for each population, the proportion of flowers exhibiting active pollen tube growth. Analysis of seed lots determined the ovule fertilization rate and seed germination capacity. Some of the small populations occupying restricted habitat fragments showed high rates of pollination (100%) and ‘normal’ age state demographics. However, reproductive characters all exhibited exponential rise to maximum relationships with population size, indicating clear tipping points (for pollination, at a threshold of 7 reproductive adults, and for ovule fertilization rate and germination at 42 reproductive adults). Thus although small populations may set seed, exhibit a ‘normal’ age state structure, and may appear viable, reproductive effectiveness declines when population size falls below 42 generative individuals and < 7 is an indicator of strong limitation. Although many remnant populations of G. pneumonanthe are in the order of 50–150 individuals these should be not be considered as MVPs; they are on the brink of calamity.     

Individual quality and age but not environmental or social conditions modulate costs of reproduction in a capital breeder

Costs associated with reproduction are widely known to play a role in the evolution of reproductive tactics with consequences to population and eco‐evolutionary dynamics. Evaluating these costs as they pertain to species in the wild remains an important goal of evolutionary ecology. Individual heterogeneity, including differences in individual quality (i.e., among‐individual differences in traits associated with survival and reproduction) or state, and variation in environmental and social conditions can modulate the costs of reproduction; however, few studies have considered effects of these factors simultaneously. Taking advantage of a detailed, long‐term dataset for a population of feral horses (Sable Island, Nova Scotia, Canada), we address the question of how intrinsic (quality, age), environmental (winter severity, location), and social conditions (group size, composition, sex ratio, density) influence the costs of reproduction on subsequent reproduction. Individual quality was measured using a multivariate analysis on a combination of four static and dynamic traits expected to depict heterogeneity in individual performance. Female quality and age interacted with reproductive status of the previous year to determine current reproductive effort, while no effect of social or environmental covariates was found. High‐quality females showed higher probabilities of giving birth and weaning their foal regardless of their reproductive status the previous year, while those of lower quality showed lower probabilities of producing foals in successive years. Middle‐aged (prime) females had the highest probability of giving birth when they had not reproduced the year before, but no such relationship with age was found among females that had reproduced the previous year, indicating that prime‐aged females bear higher costs of reproduction. We show that individual quality and age were key factors modulating the costs of reproduction in a capital breeder but that environmental or social conditions were not, highlighting the importance of considering multiple factors when studying costs of reproduction.     

An empirical test of evolutionary theories for reproductive senescence and reproductive effort in the garter snake Thamnophis elegans

Evolutionary theory predicts that differential reproductive effort and rate of reproductive senescence will evolve under different rates of external mortality. We examine the evolutionary divergence of age-specific reproduction in two life-history ecotypes of the western terrestrial garter snake, Thamnophis elegans. We test for the signature of reproductive senescence (decreasing fecundity with age) and increasing reproductive effort with age (increasing reproductive productivity per gram female) in replicate populations of two life-history ecotypes: snakes that grow fast, mature young and have shorter lifespans, and snakes that grow slow, mature late and have long lives. The difference between life-history ecotypes is due to genetic divergence in growth rate. We find (i) reproductive success (live litter mass) increases with age in both ecotypes, but does so more rapidly in the fast-growth ecotype, (ii) reproductive failure increases with age in both ecotypes, but the proportion of reproductive failure to total reproductive output remains invariant, and (iii) reproductive effort remains constant in fast-growth individuals with age, but declines in slow-growth individuals. This illustration of increasing fecundity with age, even at the latest ages, deviates from standard expectations for reproductive senescence, as does the lack of increases in reproductive effort. We discuss our findings in light of recent theories regarding the phenomenon of increased reproduction throughout life in organisms with indeterminate growth and its potential to offset theoretical expectations for the ubiquity of senescence.     

Dynamic heterogeneity in life histories

Longitudinal data on natural populations have been analysed using multistage models in which survival depends on reproductive stage, and individuals change stages according to a Markov chain. These models are special cases of stage-structured population models. We show that stage-structured models generate dynamic heterogeneity: life-history differences produced by stochastic stratum dynamics. We characterize dynamic heterogeneity in a range of species across taxa by properties of the Markov chain: the entropy, which describes the extent of heterogeneity, and the subdominant eigenvalue, which describes the persistence of reproductive success during the life of an individual. Trajectories of reproductive stage determine survivorship, and we analyse the variance in lifespan within and between trajectories of reproductive stage. We show how stage-structured models can be used to predict realized distributions of lifetime reproductive success. Dynamic heterogeneity contrasts with fixed heterogeneity: unobserved differences that generate variation between life histories. We show by an example that observed distributions of lifetime reproductive success are often consistent with the claim that little or no fixed heterogeneity influences this trait. We propose that dynamic heterogeneity provides a 'neutral' model for assessing the possible role of unobserved 'quality' differences between individuals. We discuss fitness for dynamic life histories, and the implications of dynamic heterogeneity for the evolution of life histories and senescence.     

Early-life reproduction is associated with increased mortality risk but enhanced lifetime fitness in pre-industrial humans

The physiology of reproductive senescence in women is well understood, but the drivers of variation in senescence rates are less so. Evolutionary theory predicts that early-life investment in reproduction should be favoured by selection at the cost of reduced survival and faster reproductive senescence. We tested this hypothesis using data collected from preindustrial Finnish church records. Reproductive success increased up to age 25 and was relatively stable until a decline from age 41. Women with higher early-life fecundity (ELF; producing more children before age 25) subsequently had higher mortality risk, but high ELF was not associated with accelerated senescence in annual breeding success. However, women with higher ELF experienced faster senescence in offspring survival. Despite these apparent costs, ELF was under positive selection: individuals with higher ELF had higher lifetime reproductive success. These results are consistent with previous observations in both humans and wild vertebrates that more births and earlier onset of reproduction are associated with reduced survival, and with evolutionary theory predicting trade-offs between early reproduction and later-life survival. The results are particularly significant given recent increases in maternal ages in many societies and the potential consequences for offspring health and fitness.     

Condition‐dependent mortality exacerbates male (but not female) reproductive senescence and the potential for sexual conflict

Disentangling the relationship between age and reproduction is central to understand life‐history evolution, and recent evidence shows that considering condition‐dependent mortality is a crucial piece of this puzzle. For example, nonrandom mortality of ‘low‐condition’ individuals can lead to an increase in average lifespan. However, selective disappearance of such low‐condition individuals may also affect reproductive senescence at the population level due to trade‐offs between physiological functions related to survival/lifespan and the maintenance of reproductive functions. Here, we address the idea that condition‐dependent extrinsic mortality (i.e. simulated predation) may increase the age‐related decline in male reproductive success and with it the potential for sexual conflict, by comparing reproductive ageing in Drosophila melanogaster male/female cohorts exposed (or not) to condition‐dependent simulated predation across time. Although female reproductive senescence was not affected by predation, male reproductive senescence was considerably higher under predation, due mainly to an accelerated decline in offspring viability of ‘surviving’ males with age. This sex‐specific effect suggests that condition‐dependent extrinsic mortality can exacerbate survival‐reproduction trade‐offs in males, which are typically under stronger condition‐dependent selection than females. Interestingly, condition‐dependent extrinsic mortality did not affect mating success, hinting that accelerated reproductive senescence is due to a decrease in male post‐copulatory fitness components. Our results support the recent proposal that male ageing can be an important source of sexual conflict, further suggesting this effect could be exacerbated under more natural conditions.     

Energy versus risk: costs of reproduction in free‐ranging pythons in tropical Australia

Abstract Data from a 12‐year field study have allowed us to quantify ‘costs of reproduction’ in a natural population of water pythons (Liasis fuscus) in tropical Australia. Both sexes of pythons cease feeding during the reproductive season. For males, this involves fasting for a 6 week period. Adult males lose weight rapidly over this period (approximately 17% of their body mass) but regain condition in the following months, and do not experience reduced survival. In contrast, reproductive adult females cease feeding for 3 months, lose an average of 44% of their body mass over this period, and experience increased mortality. A causal link between reproductive output and reduced female survival is supported by (i) a decrease in survival rates at female maturation; (ii) a correlation between survival rates and frequency of reproduction, in a comparison among different size classes of adult pythons; and (iii) a lowered survival rate for females that allocated more energy to reproduction. Hence, both sexes experience substantial energy costs of reproduction, but a relatively higher energy cost translates into a survival cost only in females. Such non‐linearities in the relationship between energy costs and survival costs may be widespread, and challenge the value of simple energy‐based measures of 'reproductive effort’.     

Capture–recapture models with heterogeneity to study survival senescence in the wild

Detecting senescence in wild populations and estimating its strength raise three challenges. First, in the presence of individual heterogeneity in survival probability, the proportion of high‐survival individuals increases with age. This increase can mask a senescence‐related decrease in survival probability when the probability is estimated at the population level. To accommodate individual heterogeneity we use a mixture model structure (discrete classes of individuals). Second, the study individuals can elude the observers in the field, and their detection rate can be heterogeneous. To account for detectability issues we use capture–mark–recapture (CMR) methodology, mixture models and data that provide information on individuals’ detectability. Last, emigration to non‐monitored sites can bias survival estimates, because it can occur at the end of the individuals’ histories and mimic earlier death. To model emigration we use Markovian transitions to and from an unobservable state. These different model structures are merged together using hidden Markov chain CMR models, or multievent models. Simulation studies illustrate that reliable evidence for survival senescence can be obtained using highly heterogeneous data from non site‐faithful individuals. We then design a tailored application for a dataset from a colony of black‐headed gull Chroicocephalus ridibundus. Survival probabilities do not appear individually variable, but evidence for survival senescence becomes significant only when accounting for other sources of heterogeneity. This result suggests that not accounting for heterogeneity leads to flawed inference and/or that emigration heterogeneity mimics survival heterogeneity and biases senescence estimates.     

Female reproduction bears no survival cost in captivity for gray mouse lemurs

The survival cost of reproduction has been revealed in many free‐ranging vertebrates. However, recent studies on captive populations failed to detect this cost. Theoretically, this lack of survival/reproduction trade‐off is expected when resources are not limiting, but these studies may have failed to detect the cost, as they may not have fully accounted for potential confounding effects, in particular interindividual heterogeneity. Here, we investigated the effects of current and past reproductive effort on later survival in captive females of a small primate, the gray mouse lemur. Survival analyses showed no cost of reproduction in females; and the pattern was even in the opposite direction: the higher the reproductive effort, the higher the chances of survival until the next reproductive event. These conclusions hold even while accounting for interindividual heterogeneity. In agreement with aforementioned studies on captive vertebrates, these results remind us that reproduction is expected to be traded against body maintenance and the survival prospect only when resources are so limiting that they induce an allocation trade‐off. Thus, the cost of reproduction has a major extrinsic component driven by environmental conditions.     

Characterizing the cost of oviposition in insects: a dynamic model

The development of a consensus model of insect oviposition has been impeded by an unresolved controversy regarding the importance of time costs versus egg costs in mediating the trade-off between current and future reproduction. Here I develop a dynamic optimization model that places time and egg costs in a common currency (opportunity costs expressed as decreased lifetime reproductive success) so that their relative magnitudes can be compared directly. The model incorporates stochasticity in host encounter and mortality risk as well as behavioral plasticity in response to changes in the age and egg load of the ovipositing female. The dynamic model's predictions are congruent with those of a simpler, static model: both time- and egg-mediated costs make important contributions to the overall cost of oviposition. Modest quantitative differences between the costs predicted by the static versus dynamic models show that plasticity of oviposition behavior modulates the opportunity costs incurred by reproducing females. The relative importance of egg-mediated costs increases substantially for oviposition events occurring later in life. I propose that the long debate over how to represent the cost of oviposition should be resolved not by advocating the pre-eminence of one sort of cost above all others, but rather by building models that represent the complementary roles of different costs. In particular, both time and egg costs must be recognized to produce a general model of insect oviposition that incorporates a realistic representation of the cost of reproduction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Female red squirrels fit Williams' hypothesis of increasing reproductive effort with increasing age

Williams predicted that reproductive effort should increase as individuals age and their reproductive value declines. This simple prediction has proven difficult to test because conventional measures of energy expenditure on reproduction may not be a true reflection of reproductive effort. We investigated age-specific variation in female reproductive effort in a stable population of North American red squirrels where energy expenditure on reproduction is likely to reflect actual reproductive effort. We used seven measures of reproductive effort spanning conception to offspring weaning. We found that females completed growth by age 3 and that reproductive value decreased after this age likely because of reproductive and survival senescence. We therefore, predicted that reproductive effort would increase from age 3 onwards. The probability of breeding, litter mass at weaning, and likelihood of territory bequeathal were all lower for 1- and 2-year-old females than for females older than 3 years, the age at which growth is completed. That growing females are faced with additional energetic requirements might account for their lower allocation to reproduction as compared with older females. The probability of attempting a second reproduction within the same breeding season and the propensity to bequeath the territory to juveniles increased from 3 years of age onwards, indicating an increase in reproductive effort with age. We think this increase in reproductive effort is an adaptive response of females to declining reproductive values when ageing, thereby supporting Williams' prediction.     

A dynamic model of size-dependent reproductive effort in a sequential hermaphrodite: a counterexample to Williams's conjecture

In 1966, G. C. Williams showed that for iteroparous organisms, the level of reproductive effort that maximizes fitness is that which balances the marginal gains through current reproduction against the marginal losses to expected future reproduction. When, over an organism's lifetime, the value of future reproduction declines relative to the value of current reproduction, the level of effort allocated to current reproduction should always increase with increasing age. Conversely, when the value of future reproduction increases relative to the value of current reproduction, the level of effort allocated to current reproduction should decrease or remain at zero. While this latter pattern occurs commonly in species that exhibit a delayed age at first reproduction, it may also occur following an initial period of reproduction in some sex-changing organisms that experience a dramatic increase in reproductive potential as they grow larger. Indeed, this schedule of reproductive effort is predicted by models of "early" sex change; however, these models may arrive at this result incidentally because they consider only two reproductive states: on and off. In order to examine the schedule of reproductive effort in greater detail in a system where the potential reproductive rate increases sharply, we adapt the logic and methods of time-dependent dynamic-programming models to develop a size-dependent model of reproductive effort for an example species that experiences a dramatic increase in reproductive potential at large sizes: the bluehead wrasse, Thalassoma bifasciatum. Our model shows that the optimal level of reproductive effort will decline with increasing size or age when increases to the residual reproductive value outpace the increases to current reproductive potential. This result confirms the logic of Williams's analysis of optimal life histories, while offering a realistic counterexample to his conjecture of ever-increasing allocation to current reproduction.     

Reproductive Competition Among Males in Multimale Groups of Primates: Modeling the Costs and Effectiveness of Conflict

Multimale groups of primates are characterized by strong reproductive competition among males, generally resulting in an uneven division of male reproductive success (reproductive skew). The observed patterns of conflict and reproductive skew have often been attributed to the so-called tug-of-war model. We show, however, that two important assumptions of this model are not met in male primates. First, the tug-of-war model assumes that reproductive conflict reduces overall group productivity, but in male primates (and most other vertebrates) conflict likely involves mortality rather than fecundity costs. Second, the tug-of-war model does not account for the possibility that male primates can achieve some reproductive success without engagement in open conflict, such as when a single male cannot guard several receptive females at the same time. We therefore develop a dynamic version of the tug-of-war model, in which reproductive competition causes mortality costs, and in which individuals can gain uncontested shares of reproduction dependent on the degree of female receptive overlap. This model differs substantially from the original tug-of-war model, and derives a new and rich set of comparative predictions. For instance, it predicts that the level of conflict among males declines as the queuing success of subordinate males increases (as survival increases), and also, as their uncontested share of reproduction increases, e.g., as female receptive overlap increases. Our model shows how male–male conflict and female receptive overlap collectively determine the level of reproductive skew among male primates, and illustrates that this relationship is more complex than previously thought.     

Broadcasting fables: Is external fertilization really primitive? Sex,size, and larvae in sabellid polychaetes

Traditionally, broadcast spawning and planktonic larvae have been considered the plesiomorphic ‘ground plan’ for the Polychaeta and other metazoan groups. To assess whether this reproductive mode is in fact ‘primitive’, the study of monophyletic groups with various reproductive modes should be informative. A large range of body sizes would allow testing the ideas that aspects of reproductive mode may be functionally constrained. The family Sabellidac is one such group, with sexual reproductive modes ranging from broadcast spawning to intratubular brooding to ovovivi-parity, and a body size range over more than five orders of magnitude. Sabellids have previously been the subject of detailed cladistic analyses (Fitzhugh 1989, 1991); here we introduce several new characters based on morphology of reproductive structures. Larval development in four brooding sabellid species is also described with the aim of introducing new characters for future systematic analyses. Our cladistic analysis of sabellid genera suggests that gonochorism and brooding of direct-developing larvae are plesiomorphic in the Sabellidae, with external fertilization and swimming larvae limited to apomorphie clades in the subfamily Sabellinae. The presence of sperm with elongate heads may be correlated with the presence of intratubular brooding, though an adequate causal explanation for this relationship can not yet be presented. The concept that ‘modified’ sperm must be derived from ‘primitive’ sperm is shown to be false, with ‘modified’ sperm being plesiomorphic for the Sabellidae, from which ‘primitive’ sperm is derived in apomorphic Sabellinae. All sabellids have lecithotrophic development and appear to be phylogenetically constrained in this regard. Data gathered on body size and reproductive variables in the Sabellidac suggests the following (when phylogenetic effects are not controlled): (1) egg number and total egg volume are significantly correlated with body size, with small animals having fewer, larger eggs than large animals; (2) individual egg volume is not correlated with body size; (3) reproductive mode is significantly correlated with body size; intratubular brooders tend to be small-bodied, whereas broadcast spawners are large. However when the effect of body size is controlled for, then (4) egg number, egg volume and total egg volume all vary significantly with reproductive mode. Broadcast spawners expel a large number of small eggs for a high total egg volurne. Intratubular brooders have a few relatively large eggs for a small total egg volume. When statistics arc performed using phylogenetically independent contrasts there is a significant correlation between total egg volume and body size but not for egg number and body size. The effect of non-independence (due to phylogeny) of our data needs to be more fully controlled in future analyses but methods of incorporating continuous data into cladistic analyses should also be investigated. We show that some predictions can be made about reproductive mode based on body size but ad hoc patterns of reproductive character-state transformation should not be made independent of empirical hypotheses of phylogenetic relationship. Further studies of this kind throughout the Annelida are needed to determine the plesiomorphic reproductive mode for the phylum.