Control theory predictions of reproductive allocation in female dusky salamanders

A model of the reproductive ecology of female dusky salamanders was used to investigate the allocation scheme that a female might use to maximize her reproductive success. Analysis of the model with techniques of optimal control theory suggests that fecundity is maximized either by allocating food resources to reproduction for the entire time period prior to egg laying, or by growing first, then switching to reproduction later in the year. The analysis also indicates that egg mortality will be minimized if the female provides maternal care at the maximum level throughout egg brooding. These results are not specific to dusky salamanders, but can be extended to other organisms with similar reproductive characteristics.     

Unusual incubation rhythms the Spotted Barbtail, Premnoplex brunnescens

The Spotted Barbtail (Premnoplex brunnescens) inhabits the understory of humid montane forests in Central and South America. Apart from basic information on eggs and nest form, little has been published on its breeding ecology. Using temperature sensors in nest cups, I have collected data on the diurnal patterns of egg-coverage from three nests in eastern Ecuador and reveal a remarkable incubation rhythm. After providing near-constant coverage during the morning, adults leave the eggs unattended for most of the afternoon, returning to the nest only in the late afternoon. The mean duration (±standard deviation) of this period of absence, across the entire incubation period at three nests, was 6.4 ± 1.9 h. These results are discussed in relation to their physiological and ecological significance.     

A critique of methods for measuring life history trade-offs

Constraints have important effects on the evolution of life history strategies, but several difficulties have been encountered in determining constraints empirically. Here we investigate methods for measuring a specific type of constraint known as a trade-off. A trade-off between two traits implies a perfect negative correlation between the traits. Trade-offs may involve more than two traits, however, and pairs of traits involved in such a higher-dimensional trade-off may be positively correlated. If some of the traits involved in a trade-off are omitted from the experimental design, the trade-off may not even be detectable. Direct measures of trade-offs are thus complicated, and indirect means of identifying trade-offs may often provide the only feasible measures.     

Reproductive tactics of four sympatric sticklebacks (Gasterosteidae)

Synopsis The life history traits (longevity, growth, age at maturity, fecundity, egg size and length of breeding season) were compared for four sympatric sticklebacks (Gasterosteidae). Significant differences occurred for growth, longevity, age at maturity and length of breeding season, but not for egg diameters. Differences in life history traits are interpreted in terms of mechanisms permitting the coexistence of these closely-related territorial fish. Egg numbers were lowest inApeltes quadracus and highest inGasterosteus aculeatus withGasterosteus wheatlandi andPungitius pungitius having similar and intermediate fecundities.     

Selective Foraging Behavior in Nest-Provisioning Females of Parastrachia japonensis (Hemiptera: Cydnidae): Cues for Preferred Food

Females of Parastrachia japonensis (Hemiptera: Cydnidae) provision nymph-containing nests with drupes of the single host tree, Schoepfia jasminodora (Rosidae: Santales). P. japonensis feeds on the endosperm of these drupes. Foraging females approached and accepted only a few selected drupes for transport to their nests from among the hundreds of drupes scattered on the ground. The characteristics of the drupes that elicited an acceptance response under field conditions were examined. Females estimated the degree of endosperm development using volatile and nonvolatile chemicals in the outer skin as cues and selected only those drupes which had a well-developed endosperm. Although the cues that prompted approach and inspection of a drupe are in the fleshy outer skin, females presented with various substitute drupes coated with outer skin juices of acceptable drupes selected only those they could recognize as a solid mass, suggesting that they used volatile chemicals in the outer skin as cues for approaching and inspecting a drupe and then taste and tactile stimuli as cues for acceptance. Thus, this bug possesses a complex, ecologically sensitive cognitive system that enables it to recognize valuable food items by using reliable cues. The importance of selective foraging by P. japonensis females and the evolutionary significance of this behavior are discussed.     

Plasticity or fixed adaptive traits? Strategies for predation avoidance in Rana arvalis tadpoles

Tadpoles of Rana arvalis originating from seven island populations were tested for responses to non-lethal predator presence. In general, tadpole growth was reduced and the relative tail depth was increased at predator presence. There was no effect of predator presence on the predicted size at metamorphosis. The differentiation rate, translating as length of the larval period, was lower at predator presence, but this seems to be merely an effect of the reduced growth. Although populations differed with respect to growth, relative tail length, relative tail depth, differentiation rate and predicted size at metamorphosis, no obvious differences were found in their responses to predator presence. Data on predator occurrences in the source ponds show that tadpoles originating from ponds with a high predation pressure have a higher differentiation rate, i.e. they will metamorphose at an earlier date than those from “safe” ponds (if raised under the same conditions). Moreover, they are also predicted to metamorphose at a smaller size, which is in accordance with theoretical models. Despite the fact that populations differed in growth, no correlation was found between growth and predation risk in the source ponds. Received: 16 March 1998 / Accepted: 18 July 1998     

Dynamic energy budget models with size-dependent hazard rates

We formulate and analyze two dynamic energy budget models, a net assimilation model with constant allocation strategy and a net production model with a 2-stage allocation strategy, with the objective of determining strategies that maximize the expected lifetime reproductive energy. The per capita death rate depends on the organisms size, as for example when the main cause of death is predation. In the analysis of the net production model, the size at maturity is calculated along with the probability of reaching that size. We show that a small probability of survival to maturity is incompatible with the simple assumption of an exponential survival probability. We demonstrate that when the hazard rate is significantly greater for small individuals than large ones, it is possible for the optimum strategy to be for an individual to grow to a large size in spite of an arbitrarily small probability of survival to maturity. Numerical simulations indicate how the optimal allocation strategies depend on the parameter values.     

The invasion of the carnivorous carabid beetle Trechisibus antarcticus on South Georgia (sub-Antarctic) and its effect on the endemic herbivorous beetle Hydromedion spasutum

Recently two species of carabid beetle were accidentally introduced onto the sub-Antarctic island of South Georgia. Both species are carnivorous and flightless. One of the species, Trechisibus antarcticus, is locally very abundant and in the process of invading the coastal lowland area, where the endemic herbivorous beetle Hydromedion sparsutum (Perimylopidae) is common. Field samples showed the abundance of the endemic species to be much lower, and its adult body size to be larger, in carabid-infested locations than in carabid-free locations. The sample data allowed us to estimate the growth rate of the H. sparsutum larvae and to reconstruct the most likely life-cycle of both species. A laboratory experiment showed a high mortality for the first three (out of six) larval instars of H. sparsutum in groups which had been subjected to predation by T. antarcticus. The duration of the period during which the larvae are vulnerable to predation was shown in a growth experiment to depend on food type. Quantitative and qualitative aspects of the interaction between the introduced predator and the endemic prey, and conditions which allowed the former to invade are discussed.     

Effects of mating delay and nutritional signals on resource recycling in a cyclically breeding cockroach

Ovarian apoptosis has been found to occur in the female cockroach Nauphoeta cinerea in response to lack of mates. It has been proposed that ovarian apoptosis in continuously breeding insects is an adaptive mechanism for recouping resources in poor conditions (oosorption). However, N. cinerea is a cyclically breeding insect and ovarian apoptosis may represent ageing and clearance of old unused oocytes. To test the hypothesis that oocyte resorption via apoptosis reflects the reclamation of resources, we delayed mating in combination with positive and negative nutritional signals. Females without access to food during sexual maturation invested less in reproduction and had elevated rates of ovarian apoptosis in the terminal oocyte. Starvation also induced apoptosis in non-vitellogenic oocytes of the vitellarium and germinarium, which would be used for future reproductive events. This is paradoxical as theory states that oosorption is an adaptive means of rerouting resources into investment in future reproduction, yet these oocytes do not represent a cache of resources and their loss could limit future reproduction.     

Nest defense and aggressive interactions between a small benthic fish (the johnny darter Etheostoma nigrum) and crayfish

Synopsis Aggression by nest-guarding male johnny darters, Etheostoma nigrum, against intruding crayfish was investigated in laboratoy experiments and field observations. In the laboratory, darter success in chasing crayfish, Orconectes rusticus, from the nest site was inversely related to crayfish size. Small crayfish (less than 15 mm carapace length) were routinely evicted from the nest area by nips directed at the posterior end of the abdomen. Although such aggressive behavior was less successful against larger crayfish, even the largest crayfish tested (carapace length 30–32 mm) were chased from the nest area in 33% of the trials. Those large crayfish that entered nests often remained despite repeated attacks by the male johnny darter and egg predation was observed. In a small Ohio stream, openings to johnny darter nests were generally between 7 and 13 mm. Thus crayfish with a carapace height greater than 13 mm (corresponding to a carapace length greater than 29 mm) would not be able to enter johnny darter nests. In field observations, male Johnny darters successfully defended nests against another crayfish species (Orconectes sanborni, carapace length 12–29 mm). Together, aggressive behavior and small size of nest entrances allow the johnny darter to successfully reproduce in areas with abundant crayfish. The Unit is sponsored jointly by the United States Fish and Wildlife Service, The Ohio Department of NaturalResources, and The Ohio State University.     

Developmental responses to predation risk in morphologically defended mayflies

Densities and species composition of predators could affect morphological defences, larval development and the timing of emergence of their prey. To address this issue we studied the morphology and life history of an ephemerellid mayfly, Ephemerella invaria, from two streams in a deciduous forested drainage basin in central New York. Both streams contained predatory fish, but densities and species composition of fish differed. A field survey provided evidence that Ephemerella inhabiting a stream with 10 fish species and high relative densities of fish emerged several weeks earlier and at smaller sizes than Ephemerella inhabiting a nearby tributary with ~2 fish species and low relative densities of fish. However, the two populations of mayflies showed no differences in defensive morphology or growth rates. In laboratory rearing experiments, we exposed Ephemerella larvae from these two locations to fish chemical cues or control water (no fish) over 2 months to test whether differences in life histories could be attributed to fish. Fish cues induced faster larval development, but also smaller size of mature Ephemerella individuals from both high and low predator locations. Although shorter development times in more dangerous environments could increase larval survival, smaller size of females results in a fecundity cost associated with this life history shift. Consistent with the field studies, laboratory rearing experiments revealed no effects of fish cues on Ephemerella's morphological defences. These data suggest that variation in the density or species composition of predators may favour the evolution of developmental plasticity to reduce mortality in the larval environment.     

Offspring growth, survival and reproductive success in the bank vole: a litter size manipulation experiment

To estimate the optimality of brood size, it is essential to study the effects of brood size manipulation on offspring survival and reproductive success. Moreover, testing the generality of the hypothesis of reproductive costs requires experimental data from a diversity of organisms. Here I present data on the growth, survival and reproductive success of bank vole Clethrionomys glareolus individuals from manipulated litters. Furthermore, the survival of mothers whose litter size was manipulated was studied. At weaning, the mean weight of pups from enlarged litters was lower and from reduced litters higher compared to control litters. After winter, at the start of the breeding season, individuals from enlarged litters, especially males, were still lighter than individuals from the other two treatments. Litter enlargements did not increase the number of reproducing female offspring per mother, nor did the litter sizes of female offspring differ between treatments. There were no differences between treatments in winter survival of offspring after weaning, but among female offspring, weaning weight explained the survival probabilities over winter. A higher weight of females at winter determined the probability of starting to reproduce in spring. The survival of mothers did not seem to be influenced by litter manipulation performed the previous year. According to the results, mothers nursing enlarged or reduced litters do not gain any fitness benefits in terms of number of offspring surviving to breeding. The results are consistent with the majority of experiments conducted in birds, which have found costs of enlarged brood appearing as offspring trade-offs rather than parent trade-offs. Received: 14 December 1997 / Accepted: 1 March 1998     

Effects of body size, age and maturity stage on diet in a large shark: ecological and applied implications

Ontogenetic diet shifts are a widespread phenomenon among vertebrates, although their relationships with life history traits are poorly known. We analyzed the relative importance of body size, age and maturity stage as determinants of the diet of a marine top predator, the copper shark, Carcharhinus brachyurus, by examining stomach contents using a multiple-hypothesis modeling approach. Copper sharks shifted their diet as size and age increased and as they became sexually mature, incorporated larger prey as they grew, and had a discrete shift in diet with body size, with only individuals larger than ≈200 cm total length able to prey on chondrichthyans. Body size was the most important trait explaining the consumption of chondrichthyans, while age determined the consumption of pelagic teleosts. Pelagic teleosts were consumed mostly by medium-aged sharks, a result, probably, of a risk-reducing feeding strategy at young ages coupled with either a senescence-related decline in performance or a change in sensory capabilities as sharks age. Copper sharks of all sizes were able to cut prey in pieces, implying that gape limitation (i.e., the impossibility of eating prey larger than a predator’s mouth) did not play a role in producing the diet shift. Our results suggest that, contrary to the current practice of setting minimum but not maximum size limits in catches, any plan to conserve or restore the ecological function of sharks, through their predatory control of large prey, should aim to maintain the largest individuals.     

Optimizing offspring: the quantity–quality tradeoff in agropastoral Kipsigis

The tradeoff between offspring quantity and offspring quality is at the heart of most evolutionary approaches to the fertility transition, as it is for demographers oriented towards economic explanations for this transition. To date, however, there have been few empirical tests of the key idea that humans trade offspring quantity for quality, and no strictly comparative work designed to identify the specific environmental conditions that favor such a tradeoff. This study suggests that in an East African community where the principal form of intergenerational inheritance is land, intermediate levels of offspring production are favored for women but not men. Women produce approximately the optimal number of surviving children, whereas men produce far fewer than the optimal number. The result highlights the significance of inheritable extrasomatic capital, in conjunction with evolved psychological mechanisms, in shaping fertility strategies that emphasize quality over quantity.     

Seasonal trends in body size of adult male mosquitofish,Gambusia affinis,with evidence for their social control

Synopsis Collections from a natural population of mosquitofish,Gambusia affinis, in southern Indiana showed that males born early in the breeding season mature rapidly at a small body size, while males born later in the breeding season delay maturation and achieve larger body sizes. A field experiment, involving removal of males from pond populations, was conducted to test the hypothesis that delayed maturation by late summer males is, at least in part, under social control. Mean total length (TL) of adult and maturing males in late summer in ponds from which large numbers of males had previously been removed was significantly lower than mean TL of males in control ponds. These data support the hypothesis of social control and support Sohn's competition-predation hypothesis for control of adult male body size inGambusia.     

The relevance of ants as seed rescuers of a primarily bird-dispersed tree in the Neotropical cerrado savanna

Sex ratios can influence mating behaviour, population dynamics and evolutionary trajectories; yet the causes of natural sex ratio variation are often uncertain. Although secondary (birth) sex ratios in guppies (Poecilia reticulata) are typically 1:1, we recorded female-biased tertiary (adult) sex ratios in about half of our 48 samples and male-biased sex ratios in none of them. This pattern implies that some populations experience male-biased mortality, perhaps owing to variation in predation or resource limitation. We assessed the effects of predation and/or inter-specific resource competition (intraguild predation) by measuring the local catch-per-unit-effort (CPUE) of species (Rivulus killifish and Macrobrachium prawns) that may differentially prey on male guppies. We assessed the effects of resource levels by measuring canopy openness and algal biomass (chlorophyll a concentration). We found that guppy sex ratios were increasingly female-biased with increasing CPUE of Macrobrachium, and perhaps also Rivulus, and with decreasing canopy openness. We also found an interaction between predators and resource levels in that the effect of canopy openness was greatest when Macrobrachium CPUE was highest. Our study thus also reveals the value of simultaneously testing multiple environmental factors that may drive tertiary sex ratio variation. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Risk-sensitive decisions during nesting may increase maternal provisioning capacity in the subsocial shield bug Parastrachia japonensis

Abstract 1. Females of the monophagous shield bug Parastrachia japonensis Scott provision their nymph-containing nests with high-quality drupes of the single host tree, Schoepfia jasminodora , a resource that is poor and unstable, under an array of variable environmental constraints.
2. Although the number of drupes provisioned is correlated positively with enhanced nymphal development and survival, there is great variability in the number of drupes that females provide.
3. The ability to adjust behaviour and/or physiology, i.e. make risk-sensitive decisions, should be adaptive for organisms such as P. japonensis that are confronted with extreme variability in food availability and in the conditions under which they must forage. To assess whether females of this species use risk-sensitivity to improve their provisioning success, data on the parameters of provisioning activity obtained during a long-term field study were analysed. The relationships between variation in three female-controlled factors during nesting (nest site, active stage during the provisioning season, duration of provisioning activity) and the conditions of three environmental factors (drupe availability, intraspecific competition, weather) were examined. The relationships between all of these factors and provisioning capacity of females were investigated.
5. Nest site, intraspecific competition, and timing of provisioning activity only affected provisioning capacity when drupe availability was extremely good or bad or weather was particularly bad.
6. The findings suggest that females use risk-sensitive decisions to increase provisioning capacity under extreme conditions of low and high drupe availability and inclement weather.     

Productivity, dispersal and the coexistence of intraguild predators and prey

A great deal is known about the influence of dispersal on species that interact via competition or predation, but very little is known about the influence of dispersal on species that interact via both competition and predation. Here, I investigate the influence of dispersal on the coexistence and abundance-productivity relationships of species that engage in intraguild predation (IGP: competing species that prey on each other). I report two key findings. First, dispersal enhances coexistence when a trade-off between resource competition and IGP is strong and/or when the Intraguild Prey has an overall advantage, and impedes coexistence when the trade-off is weak and/or when the Intraguild Predator has an overall advantage. Second, the Intraguild Prey's abundance-productivity relationship depends crucially on the dispersal rate of the Intraguild Predator, but the Intraguild Predator's abundance-productivity relationship is unaffected by its own dispersal rate or that of the Intraguild Prey. This difference arises because the two species engage in both a competitive interaction as well as an antagonistic (predator-prey) interaction. The Intraguild Prey, being the intermediate consumer, has to balance the conflicting demands of resource acquisition and predator avoidance, while the Intraguild Predator has to contend only with resource acquisition. Thus, the Intraguild Predator's abundance increases monotonically with resource productivity regardless of either species' dispersal rate, while the Intraguild Prey's abundance-productivity relationship can increase, decrease, or become hump-shaped with increasing productivity depending on the Intraguild Predator's dispersal rate. The important implication is that a species' trophic position determines the effectiveness of dispersal in sampling spatial environmental heterogeneity. The dispersal behavior of a top predator is likely to have a stronger effect on coexistence and spatial patterns of abundance than the dispersal behavior of an intermediate consumer.