Patterns Of Reproductive Effort Research Articles

Observations on the Reproductive Biology of Two Notropis Shiner Species

Important details of the reproductive ecology of many freshwater fishes of the species-rich southeastern USA are still poorly known. Two such species are Notropis asperifrons (Burrhead Shiner), and Notropis stilbius (Silverstripe Shiner). Both are endemic to the Mobile Basin, AL. To determine timing and patterns of reproductive effort, collections were made of as many as 20 individuals of each species at roughly four-week intervals from March through September. Female gonadal somatic index (GSI) data averages for each collection indicate that both species peak in reproductive activity in April. Both species still had reproductively competent female GSI values as late as July 31. Examination of ovarian tissue in- dicates that Burrhead Shiner oocytes are usually larger at equivalent developmental stages. Burrhead Shiners exhibit sexual size dimorphism with larger females, while Silverstripe Shiners do not.

Ageing and reproductive effort in male moose under variable levels of intrasexual competition

Summary Capital breeding is a resource use tactic common among polygynous mammals, such as many ungulates. Even though large, prime‐aged males stop eating during the rutting season, younger individuals often do not and may adopt alternative mating tactics that are less strenuous. The pattern of reproductive effort is therefore probably very variable among age classes, but has rarely been quantified in male mammals. Based on data of body weight of 9949 male moose (Alces alces L.) aged up to 21 years from seven populations in Norway, we tested hypotheses regarding the pattern of reproductive effort (weight loss during rut) in this capital breeder, and how this may be affected by factors such as age, population sex ratio and density. Reproductive effort increased with age, even after prime‐age was reached around the age of 6 years. This provides the first evidence consistent with the terminal investment hypothesis in male mammals. For the very oldest males (> 12 years) data were limited, but the tendency was that effort stabilized or even decreased slightly. Effort did not depend on the population sex ratio or density for adult males. Yearling males also lost some weight during the rutting season, but this was not related to population sex ratio. Decreasing trends in yearling body weight as seen in many strongly harvested moose populations, therefore are due probably to other causes than earlier onset of rutting. Further, effort in yearlings decreased with increasing density. Despite the lack of a correlation between effort and population sex ratio, a significantly faster ageing after prime‐age was observed with an increasingly female‐biased population sex ratio. This is consistent with the hypothesis that strongly skewed sex ratios may affect the ageing process. Most ungulate populations in Europe and North America are heavily harvested or otherwise managed extensively, harvest being typically male‐biased to varying degrees. The resulting skews in sex ratio provide manipulations that give unique opportunities to study life‐history variation, in particular for large mammals that are otherwise difficult to manipulate. These opportunities are currently not fully utilized.

Patterns of reproductive effort in male ungulates

AbstractIn ungulates, males and females have contrasting life histories, as usually only the females raise the young. How reproductive effort in males varies with individual level and population level characteristics has received little attention in the literature. Using published information on direct (weight loss during the rut) and indirect (rut‐related changes in activity budgets, fighting frequency, etc.) measures of reproductive effort, we tested whether effort in males increased with (H1) increasing age, (H2) increasing body size, (H3) decreasing population density, (H4) increasingly female‐biased sex ratio and with a younger male age structure. Consistent with H1, reproductive effort was consistently higher in prime‐aged than in younger males in a large number of studies. Among younger males, sub‐adult males had an equal or higher effort than yearling males. Prime‐aged males had more typical rutting behaviour (e.g. roaring, tending, fighting and chasing frequencies) and they often lowered their intake of forage during the rut. However, reproductive effort usually declined for very old age classes. Reproductive effort increased with size (H2) also after accounting for age. Data on effort vs (H3) density and (H4) sex ratio/male age structure were inconclusive. Clearly, far more studies relating effort to population characteristics are needed before the rutting ecology of male ungulates can be understood.

Age– and density–dependent reproductive effort in male red deer

Reproductive effort in female ungulates originates from gestation and lactation and has been studied extensively; however, no comparable studies of reproductive effort in males (due to fighting for access to mates) have, to our knowledge, previously been reported. Here, we report on weight loss of male red deer during the annual mating season--a direct measure of male reproductive effort (or somatic reproductive costs). The 'terminal investment' hypothesis predicts that reproductive effort should increase with age, given that costs remain stable. We also propose the 'mating strategy-effort' hypothesis, which predicts that reproductive effort peaks in prime-aged males, since they are most often the harem holders. Consistent with the mating strategy-effort hypothesis, relative weight loss during the rutting season peaked at prime age and was lower in younger and senescent males. Weight loss during the rut was relatively smaller as density increased and more so for older males. This is probably primarily due to males (particularly senescent males) starting their rut in poorer condition at high density. The pattern of reproductive effort in males with regard to age and density therefore differs markedly from the pattern reported for females.

Patterns of reproductive effort and success in birds: path analyses of long‐term data from European ducks

Summary We tested ecological hypotheses about timing of breeding and reproductive effort in birds, by analysing > 15‐year data sets for individually marked females in three species of Latvian ducks (northern shoveler, tufted duck, common pochard). Duckling survival and recruitment declined with advancing hatch date in pochard and tufted duck, after controlling for effects of female age and other factors with path analysis, a novel finding which indicates that fitness advantages associated with early hatching extended beyond the prefledging period. Logistic regression analysis suggested further that individual duckling prefledging survival was moderate in the earliest phase of the breeding season, greatest in mid‐season and lowest later on. However, selection acting against early hatched ducklings was surpassed by strong directional selection favouring recruitment of the earliest hatching females. The absolute and relative numbers of female recruits produced by a breeding female declined sharply with advancing hatch date in all species. Unlike previous studies, an hypothesized intraspecific trade‐off between duckling mass and brood size was detected, being very robust in two of three species. Unexpectedly, female age effects on recruitment were manifested only indirectly by several pathways, the most important being the earlier hatching dates of older females. Size‐adjusted body mass (i.e. condition index) was positively related to reproductive success, and was 2–8‐fold more influential than female size (indexed by wing length). Overall, fecundity‐independent variables (e.g. hatching date, weather, indices of duckling production and habitat quality) generally had 2–10 times greater influence on recruitment rates than did fecundity‐dependent variables such as female size or condition, duckling mass and brood size, suggesting a critical role for external environmental factors vs. individual female‐specific traits in the recruitment process.

Potential ‘costs of reproduction’ in a skink: Inter‐ and intrapopulational variation

AbstractReproductive costs are important determinants of reproductive effort in squamate reptiles. Consequently, differences in costs of reproduction between populations of geographically or climatically widespread species are likely to result in different patterns of reproductive effort. In the present study, the effect of pregnancy on sprint speed was examined in a small viviparous skink,Niveoscincus ocellatus(Gray 1845), from two populations at the climatic extremes of its distribution. Decreased sprint speed has the potential to be an important cost of reproduction in this species, through a reduced ability to avoid predation and/or decreased foraging efficiency. Lizards inhabiting the colder site were larger than those from the warmer site and, contrary to predictions from life history theory, had a higher reproductive effort. In both populations, sprint speed was lower in pregnant lizards than in either the same individuals after birth or non‐pregnant control lizards. Within each population, sprint speed was unrelated to the level of reproductive effort of the female in terms of either absolute mass of the reproductive burden or the burden relative to her post‐partum body mass. However, within each population, the mass of the clutch that an individual female was carrying relative to snout–vent length was an important determinant of her sprint speed while pregnant. Thus, within each population, a relatively high reproductive burden may potentially increase costs of reproduction in this species. Despite this relationship and predictions from life history theory suggesting that annual reproductive effort will be lower in populations with a large body size and delayed maturity, it is suggested that a higher reproductive effort at the cold site is possible because they have a higher absolute sprint speed because of their larger size and a relatively higher abundance of cover at the cold site, and differences in predation pressure may alter selective pressures on reproductive investment.

Patterns of reproductive effort in male and female shrubs of Oemleria cerasiformis: a 6‐year study

1 We monitored flowering and fruiting of individual male and female plants of Oemleria cerasiformis over a 6‐year period in a population in western Canada, and calculated fruit set (percentage of pistils maturing) and reproductive effort (RE) (gram of reproductive tissue per gram of leaf). 2 Over 6 years, male O. cerasiformis had on average much lower total RE, but much higher RE at flowering, than females. 3 In males, strong correlations between RE and light suggested that investment in reproduction was largely determined by light levels. There were strong positive correlations of RE between years, with no evidence of periodic fluctuations. 4 In females, in contrast to males, RE at flowering was not related to light. However, fruit set was strongly correlated with light. Flowering RE and fruit set were uncorrelated in females, indicating that these are affected by different factors. 5 Correlations of RE between years in females, although often significant, were lower than in males, indicating that RE fluctuates more between years in females than in males and may respond to past levels of RE. Flowering may reflect adjustments in response to past reproduction, or may be controlled by resources other than light. Fruit set was not significantly related to previous RE. 6 The greater total RE of females and their limited ability to adjust fruit set are probably major factors contributing to the greater mortality rates of females and the male‐biased sex ratios in O. cerasiformis.

Comparison of size-dependent reproductive effort in two dandelion (<i>Tara×acum officinale</i>) populations

Reproductive effort in dandelions (Taraxacum officinale Weber) from two different habitats was compared. One dandelion population occupied a 5-year-old alfalfa (Medicago spp.) field, an environment subject to regular disturbance but with a relatively low density of neighbours. Individuals from the second population were derived from a number of undisturbed sites where the density of neighbouring grasses was high. Three hypotheses were evaluated with respect to the observed patterns of reproductive effort. One hypothesis, that reproductive effort was a function of differences between habitats in resource availability, did not provide an explanation for our results. A second hypothesis considered patterns of reproductive effort when mortality rates varied with degree of disturbance and neighbour density, which was a consequence of each habitat representing a different successional environment. A third hypothesis used a life-history approach to predict reproductive effort when mortality schedules were size d...

Patterns of reproductive effort with time since last fire in Florida scrub plants

Abstract. In periodically burned ecosystems, fire frequency may be an important selective pressure for the evolution of plant reproductive allocation patterns. We evaluated this hypothesis for Florida (USA) scrub plants by developing three models of reproductive effort with time since last fire given assumptions concerning seed dormancy and seedling establishment. We then examined reproductive effort of five woody, resprouting shrubs at sites representing nine times since last fire (ranging from 0–64 yr). All species showed significant patterns with time since fire in percentage of stems reproductive and fruit production. Stems of all species needed to attain a minimum size before flowering. Four species had the greatest level of reproductive effort (fruit biomass/above‐ground biomass) within 5 yr post‐fire and best fit the Early Peak Model of reproductive effort (i.e. between‐fire seedling recruitment or seed dormancy). A fifth species best fit the Broad Peak Model (i.e. immediate post‐fire seedling establishment), peaking in reproductive effort at 7 yr post‐fire. Both of these models are based on somewhat variable fire‐return intervals, suggesting that the frequency of scrub fires may have been too unpredictable to select for reproductive allocation patterns precisely reflecting particular fire‐return intervals. Early peaks in post‐fire reproductive effort may be a bet‐hedging strategy to allow for greater chances of seedling establishment and survival.

Patterns of reproductive effort inArabidopsis thaliana: Confounding effects of size and developmental stage

AbstractWe demonstrate for the annual Arabidopsis thaliana, that reproductive effort (RE) varies with plant size but is affected more directly by developmental stage; i.e. RE is “stage-dependent”. We evaluated plant size and RE under a constant environment with two harvest criteria, and under a variable environment (a range of densities) with one harvest criterion. Plants of 10 genotypes were grown singly and harvested either at maturity of the first fruit (early) or at complete senescence (late). Plants from a subset of 3 genotypes were grown at 6 additional densities and harvested at maturity of the first fruit. For plants grown singly, later harvest resulted in significantly higher RE (seed biomass / total remaining above ground biomass) and larger plant size than early harvest. Conversely, RE decreased with increasing size for plants grown under a range of densities and harvested at maturity of the first fruit. We argue that these apparently contradictory relationships between RE and size are a conseq...

Size-dependent allocation to sexual and vegetative reproduction in four clonal composites.

Populations of Silphium speciosum, Vernonia baldwinii, Solidago canadensis and Pityopsis graminifolia were studied to determine whether biomass allocation to sexual and vegetative reproduction and the balance between them were size-dependent and whether interpopulation differences in allocation patterns could be predicted from differences in population size distributions. All four species showed strong linear relationships between inflorescence mass and vegetative mass with negative y-intercepts. As a result, sexual reproductive effort (SRE) was a monotonically increasing function of ramet size. Genet size was a poor predictor of SRE. In each species, the regression parameters of these relationships differed significantly between burned and unburned habitats indicating size-independent interpopulation differences in patterns of reproductive effort as well as sizedependent effects. Interpopulation variation in vegetative reproductive effort (VRE) was greater than variation in SRE, but neither VRE nor the pattern of partitioning of VRE among daughter rhizomes showed significant relationships to plant size.

Biomass-allocation patterns in populations of Trillium erectum and T. grandiflorum in southern Ontario

Biomass-allocation patterns to aerial tissues were examined among six populations for each of Trillium erectum L. and T. grandiflorum (Michx.) Salisb. in southern Ontario. Total biomass did not differ among populations of T. erectum, but androecial proportions and reproductive effort generally decreased from southwest to northeast. Total biomass of T. grandiflorum varied significantly among populations and its pattern of reproductive effort was the reverse of that of T. erectum. Androecium–gynoecium ratios decreased across southern Ontario from southwest to northeast for both species. These results suggest that there may be a shift from allogamy to autogamy in a northeasterly direction across southern Ontario.

Regulating Reproductive Effort in a Changing Environment

Using the ratio of effective reproduction to survival rate, with time scaled in units of time till independence, as an index to age specific reproductive effort, we ask how optimal reproductive effort should vary when environmental conditions change. With an idealized portrayal of the effects of environmental change introduced to a fitness set analysis of the trade-off between reproduction and survival, we find that the optimal reproductive effort is indifferent to environmental change. Deviations from this null case are obtained in a more general model, where we find that a pattern of increasing reproductive effort with environmental amelioration and decreasing reproductive effort with environmental deterioration is predicted when older breeding age classes are less affected by environmental change that are younger age classes, or when the potential adult survival rates are less affected by environmental change than are the potential survival rates of dependent young or the potential natality rates. Where the converse of these conditions obtains, the reverse pattern of change of reproductive effort will be favored. The assumptions of the null-case model provide precise definitions of the threshold conditions at which the pattern of reproductive effort will switch from one type to the other. The circumstances under which regulation of reproductive effort is predicted to be expressed as an increase in reproductive effort in more favorable times and a decrease in less favorable times are commonly met, especially in the life histories of many long-lived vertebrates. Thus the analysis of optimal life histories offers a competing explanation for some patterns of reproductive behavior that are otherwise described as social adaptations that evolved in response to the selective advantage of population regulation.