A general structured model for a sequential hermaphrodite population

This paper introduces and analyzes a model of sequential hermaphroditism in the framework of continuously structured population models with sexual reproduction. The model is general in the sense that the birth, transition (from one sex to the other) and death processes of the population are given by arbitrary functions according to a biological meaningful hypotheses. The system is reduced to a single equation introducing the intrinsic sex-ratio subspace. The steady states are analyzed and illustrated for several cases. In particular, neglecting the competition for resources we have explicitly found a unique non-trivial equilibrium which is unstable.     

Body size-dependent gender role in a simultaneous hermaphrodite freshwater snail, Physa acuta

We examined whether gender role in the simultaneous hermaphroditefreshwater snail, Physa acuta, is determined by relative bodysize in a manner predicted by the size-advantage model. We observedthe body-size combinations of pairs in the laboratory by usingfield-collected populations. Smaller individuals tended to playthe "male" role (sperm donor), and larger snails the "female"(sperm recipient). Next, we analyzed the mating behaviors involvedin gender-role decision in snail pairs of three different body-sizecombinations, using "large" and "small" snails. Smaller snailswere more likely to approach the partner as a male in different-sizecombination (large/small), whereas frequent initial approachesas a male and rejection behavior as a female were observed inthe large/large combination. Third, we examined the body sizepreference when a snail can freely choose the partner from twoother individuals of different body sizes (large/large/smallor large/small/small). Small individuals had a significant tendencyto act as the male and positively selected large snails as thefemale partner in both triple combinations. However, the largeindividual acted as both the male and the female with nearlyequal frequency. In the size-differing pairings, copulationsoccurred after fewer male approaches and fewer rejections thanin pairings involving two large snails, suggesting that bodysize difference is one of the behavioral solutions in genderconflict. Clear gender-role switching associated with body sizewas not seen. Smaller snails thus have a tendency to play themale role more frequently but adopt both gender roles when theirbody size is sufficiently large.     

Terminal reproductive effort in a marsupial

Life-history theory predicts that as organisms approach the end of their life, they should increase their reproductive effort (RE). However, studies on mammals often find that measures of RE do not vary with maternal age. This might be because offspring have some control over energy transfer which may constrain adaptive variation in RE by mothers, particularly in eutherian mammals where placental function is primarily controlled by offspring. However, in marsupials, energy transfer is primarily by lactation and under maternal control, leaving marsupial mothers free to vary RE. Here, we provide the first analysis, to our knowledge, of age-specific RE in a marsupial, the common brushtail possum. RE, measured as the proportion of maternal mass lost during lactation, was strongly correlated with offspring mass as a yearling. Older females had higher RE, gave birth earlier in the season and were more likely to produce two offspring in a year. Females with high RE in one year were lighter at the beginning of the next breeding season. These results provide the clearest support yet for terminal RE in a mammal.     

Redhead reproductive strategy choices: a dynamic state variable model

Female redhead ducks (Aythya americana) exhibit one of the highest frequenciesof facultative parasitic egg laying, extending reproductive choiceswithin a season beyond nesting only. The occurrence of alternative strategieson a population level within and among years and the factorsthat influence choices are not well documented or understood.We developed a dynamic state variable model to predict reproductivestrategy choice and the influence of female age, body mass,food availability, and host availability on strategy choice.The model predicts a general distribution of strategy choiceby body mass and a strong influence of both age and host availability onstrategy choice. As body mass increases, females choose morecostly reproductive strategies from nonbreeding to parasitizingto nesting to a dual strategy, which is defined as a parasiticallylaid clutch of eggs followed by another clutch laid in the females'own nest. Comparatively, food availability only influenced strategychoice by slightly increasing the use of more costly strategies.Predictions of strategy choice by body mass reflect relationships similarto those proposed by others. Previous studies of the influenceof food availability on observed parasitic frequencies producedmixed and often conflicting results. We propose that femaleredheads are assessing the host environment before making reproductivechoices and food availability functions to fine tune this assessmentby encouraging or discouraging more costly strategies at a lowerbody mass.     

Social queuing in animal societies: a dynamic model of reproductive skew

Previously developed models of reproductive skew have overlooked one of the main reasons why subordinates might remain in a group despite restricted opportunities to breed: the possibility of social queuing, i.e. acquiring dominant status in the future. Here, we present a dynamic ESS model of skew in animal societies that incorporates both immediate and future fitness consequences of the decisions taken by group members, based on their probability of surviving from one season to the next (when post-breeding survival probabilities drop to zero, our analysis reduces to the model produced by Reeve and Ratnieks in 1993, which considered only a single breeding season). This allows us to compare the delayed benefits of philopatry and the immediate opportunities for independent breeding. We show that delayed benefits greatly reduce the need for dominants to offer reproductive concessions to retain subordinates peacefully in the group. Moreover, this effect is strong enough that differences in survival have a much greater impact on the group structure than differences in other parameters, such as relatedness. When the possibility of acceding to dominant status is taken into account, groups where the dominant completely monopolizes reproduction can be stable, even if they consist of unrelated individuals, and even if subordinates have a reasonably high probability of winning a fight for dominance. Finally, we show that stable groups are possible even if association leads to a decrease in current productivity. Subordinates may still stand to gain from group membership under these circumstances, as acquiring breeding positions by queuing may be more efficient than the attempt to establish a new territory. At the same time, the dominant may be unable to exclude unwelcome subordinates, may enjoy increased survival when they are present, or may gain indirect benefits from allowing relatives to stay and queue for dominance. We conclude that reproductive skew in animal groups, ranging from eusocial insect colonies to mating aggregations (leks), will be strongly influenced by the future prospects of group members.     

An analysis of age-and size-dependent flowering: A critical-production model

Three critical phenomena of flowering can be recognized in nature: critical age, critical initial size and critical switchover size. In order to understand the ecological significance of these flowering phenomena from the viewpoint of matter production, a simple model of flowering phenomean (critical-production model) was studied, assuming that plant flowering is controlled by the productive capacity of the plant (critical-production model, under the condition that plants forecast environmental conditions affecting matter production. Thus, we concluded that all three critical phenomena are various manifestation of the same critical-production principle. Then, using the model simulation, the reliability of each critical phenomenon for securing a given critical production was investigated in relation to the photosynthetic productivity of the habitat. The main predictions obtained from the simulation were as follows: 1) Only annual and biennial plants will show critical age phenomena, and most biennials will be facultative. 2) Among perennials, the critical initial size phenomenon will appear in low productive habitats, whereas in high productive habitats the critical switchover size phenomenon will be observed.     

Sex determination: ways to evolve a hermaphrodite

Most species of the nematode genus Caenorhabditis reproduce through males and females; C. elegans and C. briggsae, however, produce self-fertile hermaphrodites instead of females. These transitions to hermaphroditism evolved convergently through distinct modifications of germline sex determination mechanisms.     

Population density and group size effects on reproductive behavior in a simultaneous hermaphrodite

Background  

Despite growing evidence that population dynamic processes can have substantial effects on mating system evolution, little is known about their effect on mating rates in simultaneous hermaphrodites. According to theory, mating rate is expected to increase with mate availability because mating activity is primarily controlled by the male sexual function. A different scenario appears plausible in the hermaphroditic opisthobranch Chelidonura sandrana. Here, field mating rates are close to the female fitness optimum, suggesting that mating activity remains unresponsive to variation in mate availability.     

Pollen limitation of reproductive effort in willows

Summary Pollen limitation of seed set differs from resource limitation in its implications for the evolution of floral traits. Willow flowers attract insects, but also abundantly produce wind-dispersed pollen. I demonstrated pollen limitation in single branches bearing 2–4 inflorescences (catkins) in a field experiment with five species by artificially increasing or decreasing the pollen load. Because the responses by single branches might be explained by diversion of resources to better-pollinated branches within a plant, a second experiment with one species tested both pollen limitation of whole plants and the autonomy of catkins. Seed set of single willow catkins is unaffected by experimental alterations of seed set in other catkins on the same plant. Hand-pollination of single catkins and of whole plants increased seed set to the same degree, suggesting there is little or no competition for resources between catkins only 5–10 cm apart. Thus, seed set in willows appears to be pollen limited, favoring insect pollination and the evolution of entomophilous traits. The data support previous views that willows have a dual pollination system utilizing wind and insects.     

Lifetime reproductive output in a hermaphrodite cestode when reproducing alone or in pairs: a time cost of pairing

The cestode Schistocephalus solidus is a facultatively self-fertilising simultaneous hermaphrodite. Here we test for differences in the starting point, the rate, and the magnitude of egg production between individuals allowed to reproduce alone (only self-fertilisation possible) or in pairs (both self- and cross-fertilisation possible). Specifically, we want to distinguish between alternative processes responsible for the lower egg production in paired individuals observed in an earlier study (Wedekind et al., 1998). We designed an improved in vitro system, replacing the bird final host that allows us to measure, with high temporal resolution, the timing and magnitude of lifetime egg production of worms in these two social situations. We found that the experimental groups did not differ significantly in the starting point of egg production. However, the temporal pattern in egg production differed between them, in that paired individuals had a lower rate of egg production. This, however, did not lead to a significant reduction in lifetime egg production, as pairs compensated for the lower rate by producing eggs longer than single individuals. We argue that the lower rate of egg production may nevertheless lead to a time cost of pairing in the study species, and that this cost is likely to represent a cost of outcrossing due to sexual selection. This revised version was published online in July 2006 with corrections to the Cover Date.     

The timing of male reproductive effort relative to female ovulation in a capital breeder

1. In large herbivores, the timing of breeding is important for females to hit peak plant protein levels. For males, the timing of reproductive effort is important to maximize the number of females they can mate during autumn rut in competition with other males. The latter depends on when most females are ovulating, but also on how other males with a different competitive ability are timing use of their capital (fat); it may pay younger males to invest more heavily later when prime aged males are exhausted. 2. Based on estimates of body mass loss, we quantify how much timing (start, peak and end dates) of male reproductive effort during rutting varies depending on male age, density and climate as well as timing of female ovulation. 3. Ovulation in adult females was delayed by 5 days from low to high density, and ovulation was also more synchronous at high density. The starting date of decline in male body mass was only later in yearlings than among other age groups. However, at low density, peak and end dates of rut became increasingly earlier and close to peak female ovulation with increasing age up to 7 years of age. Prime-aged males matched peak effort closely with peak rate of prime-aged female ovulation, while younger males were delayed. This is consistent with the view that younger males have a better chance when the prime-aged males are becoming exhausted. 4. Apart from yearlings, male age groups were synchronized in both the starting, peak and end dates of mass decline at high density. Thus, this partly follows change in female ovulation patterns, but also suggests that competition among males decreased with increasing density due probably to lower intensity of sexual selection. The lowered sexual selection may be due not only to more synchronous female ovulation, but also increasingly female-biased sex ratios and a younger male age structure with increasing density. 5. The onset of rutting is somewhat independent of male age (apart from the youngest males), but the peak and end of rutting effort is dependent strongly upon age, density and peak female ovulation. Male rutting phenology is thus best interpreted as a compromise between hitting peak female ovulation and intensity of sexual selection.     

Condition-dependent reproductive effort in frogs infected by a widespread pathogen

To minimize the negative effects of an infection on fitness, hosts can respond adaptively by altering their reproductive effort or by adjusting their timing of reproduction. We studied effects of the pathogenic fungus Batrachochytrium dendrobatidis on the probability of calling in a stream-breeding rainforest frog (Litoria rheocola). In uninfected frogs, calling probability was relatively constant across seasons and body conditions, but in infected frogs, calling probability differed among seasons (lowest in winter, highest in summer) and was strongly and positively related to body condition. Infected frogs in poor condition were up to 40% less likely to call than uninfected frogs, whereas infected frogs in good condition were up to 30% more likely to call than uninfected frogs. Our results suggest that frogs employed a pre-existing, plastic, life-history strategy in response to infection, which may have complex evolutionary implications. If infected males in good condition reproduce at rates equal to or greater than those of uninfected males, selection on factors affecting disease susceptibility may be minimal. However, because reproductive effort in infected males is positively related to body condition, there may be selection on mechanisms that limit the negative effects of infections on hosts.     

Fruit production in cranberry (Ericaceae: Vaccinium macrocarpon): a bet-hedging strategy to optimize reproductive effort

In the cultivated cranberry (Vaccinium macrocarpon), reproductive stems produce 1-3 fruit even though they usually have 5-7 flowers in the spring. We undertook experiments to test the hypothesis that this was an adaptive life history strategy associated with reproductive effort rather than simply the result of insufficient pollination. We compared fruit production on naturally pollinated plants with those that were either manually pollinated or that were caged to exclude insects. Clearly, insects are necessary for the effective pollination of cranberry plants, but hand pollination of all flowers did not result in an increase in fruit number. Most of the upper flowers, which had significantly fewer ovules than did the lower flowers, aborted naturally soon after pollination. However, when the lower flower buds were removed, the upper flowers produced fruit. This suggests that the upper flowers may serve as a backup if the earlier blooming lower ones are lost early in the season. Furthermore, the late-blooming flowers may still contribute to the plant's reproductive success as visiting pollinators remove the pollen, which could serve to sire fruit on other plants. These results are discussed in the context of their possible evolutionary and proximate causes.