Parental investment and pair-bond behavior among canvasback ducks (Aythya valisineria,Anatidae)

Summary Hypothetical patterns of male and female cumulative reproductive effort in canvasback ducks were used to predict the efforts of males and females in pair-bond reinforcing behavior through a breeding sequence. The predictions were that, at any point in time in a monogamous breeding system, given certain assumptions, (a) the sex investing less should be more active in reinforcing the pair bond, and (b) this relationship should shift within and between breeding attempts with the pattern of cumulative reproductive effort per brood by the male and female. Data on the behavior of newly-formed pairs were obtained by observing apparently paired but unmarked canvasbacks during spring migration. Later, 11 individuallymarked pairs were followed through breeding sequences, including repeated nesting attempts. Observed affiliative behavior fit the predictions reasonably well. That sex whose cumulative reproductive effort was less tended to be more active in maintaining proximity to its mate, initiating bouts of mutual display, and coordinating pair activities. Predictions were met in 14 of 24 cases, results were contrary in 3 cases, and in 7 cases trends were not statistically different (P>0.05). Shifts in relative male and female reinforcement efforts that coincided with major changes in investment were apparent. These results support the proposition that factors other than simple parental investment-related desertion costs might be important to an individual's behavioral decisions regarding pairbond reinforcement and ultimately mate abandonment. Discrepancies in a few predicted outcomes further suggest that males and females may use and balance various pair-reinforcement tactics differently through a breeding season.     

The optimal harvest and management in the models of animal populations

We extended zero-cost optimization model for population of domestic animals. Also the model of hiring of labor in cattle-breeding farm is constructed and investigated.     

Sex ratio, sexual dimorphism and mating structure in bethylid wasps

Sexual dimorphism has been linked to parasitoid mating structure by several authors. In turn mating structure has an important influence on predicted sex ratio optima. Here we test the relationship between sexual dimorphism and sex ratio using data from 19 species of bethylid wasps. Using phylogenetically based comparative methods we confirm the findings of a previous cross-species analysis that sex ratio (proportion of males) is strongly and negatively correlated with clutch size. Using cross-species comparisons we show an additional positive correlation of sex ratio and relative male size, as predicted. The relationship however is not significant when using phylogenetically based methods. The cross-species result is largely due to differences between two bethylid sub-families: the Epyrinae have relatively large males and relatively high sex ratios, whereas the Bethylinae have relatively small males and lower sex ratios. Our study illustrates the benefits and drawbacks of using cross-species versus phylogenetically based comparisons. Received: 13 May 1997 / Accepted after revision: 12 January 1998     

Sex ratio,local mate competition and mating behaviour in the aphid Pemphigus spyrothecae

Summary The primary sex ratio and the selective factors associated with it were investigated in the aphid Pemphigus spyrothecae. The sexuparae gave birth to an average of 1.99 ± 0.01 (1–2) males and 5.06 ± 0.09 (2–6) females [± SE (range), n = 147]. The average weight of individual males was 16.2±0.86 (13–18) g (n = 5) and of females 40.7 ± 0.71 (38–46) g (n = 15). The overall investment ratio by the mother sexuparae was therefore 1:6.4, equivalent to a proportionate investment in sons of 0.135 (interquartiel range: 0.117–0.165). There was a highly significant correlation between the size of the sexupara and the number of her daughters: the number of sons was independent of sexupara size. Field observations and laboratory experiments showed that the sexuparae aggregate on poplar bark before giving birth to the sexuals: they appear to behave in such a way that there is always at least one other sexupara in the crevice with them. The average number of sexuparae forming a foundress group, within which offspring could potentially mate with each other, was 2.48±0.19 (1–9) (n = 80). The life-time activity patterns of the sexuals (which moult four times but do not feed) was described. Both sexes are active shortly after birth; they are then immobile for a long period but become very active again after emerging as adults (after about 42 h in males, 51 h in females). Each male can mate with up to 14 females. The females are receptive to a second male for only a relatively short period (ca. 15–20 min) after their first mating. There is considerable competition between males during mating (just as intense between clonal sibs as non-sibs) and evidence for post-copulatory mate-guarding by the males. There is no evidence that females prefer to mate with non-sibs rather than sibs. The observed sex ratios and the mating behaviour of the aphids are discussed in relation to models of local mate competition assuming variable female fecundity, as developed by Frank (1987a, b) and Yamaguchi (1985) (the constant male hypothesis) and by Stubblefield and Seger (1990).     

Sex ratio adjustment and maternal condition in two aphid species

We analyzed how offspring sex ratio varies with maternal condition in order to obtain evidence on the population structure in two aphid species with different life cycles. When fitness returns per unit investment differ for the production of daughters and sons, selection will favor an increasing investment into the sex with the higher returns. Therefore, the offspring sex ratios of individual mothers should become more biased towards the sex with the higher fitness returns as their condition or fecundity improves. The pattern of sex ratio adjustment we found in Uroleucon cirsii indicates local mate competition among males, while the pattern we found in Rhopalosiphum padi suggests local resource competition among sexual females. This might be the first evidence for local resource competition among females in an invertebrate species. Local mate competition means that fitness returns are limited by the availability of females as mates within local breeding groups, whereas local resource competition means that fitness returns are limited by the availability of resources for females competing within local groups. We discuss how the life cycles of both species fit to these hypotheses.

Sex ratio,monogamy and breeding success in the Midas cichlid (Cichlasoma citrinellum)

Summary The Midas cichlid (Cichlasoma citrinellum) is a large, aggressive monogamous fish from the Great Lakes of Nicaragua. It lives in an ecosystem where breeding sites are at a premium and where breeding success can be low. I tested the strength of the pair bond by manipulating the sex ratios in captive groups, using ratios of 1:1, 2:1, 4:1, 1:2 and 1:4 (:). No polygamy was seen at any of the ratios. Departure from a ratio of 1:1 decreased breeding success for those pairs that formed. Both sexes appeared to enforce the pair bond but did so in different ways: females attacked males and females equally but males attacked other males significantly more than they did females. These results, coupled with field observations, indicate the Midas cichlid is obligated to monogamy by the demands of competition for breeding sites, the need for two parents to protect the fry and by energetic constraints.     

Sex ratio determination and worker reproduction in the slave-making ant Harpagoxenus sublaevis

Summary In a population of the monogynous slave-making ant Harpagoxenus sublaevis in S.E. Sweden, the mean proportion of dry weight investment in queens was 0.54. This result differed significantly from 0.75 but not from 0.5, matching the prediction from the genetic relatedness hypothesis of sex ratio applied to slave-makers, given (as confirmed by this study) single mating of queens, population-wide mate competition, and relatively low levels of worker male production. Sex investment appeared unaffected by resource availability. In the same 47 colony population sample, fertile slave-maker workers were found in every queenless colony (ca. 30% of all colonies), and in 58% of queen-right colonies. Fertile workers occurred at a significantly higher frequency in the queenless colonies (19.2%) than in the queenright ones (9.8%), confirming that queenless conditions promote worker fertility. Fertile and sterile workers were similar in size. Electrophoretic allozyme analysis of ants from 49 colonies showed that: 1) queens mated singly; 2) female nestmates were full sisters (their regression coefficient of relatedness (±SE) was 0.735±0.044); 3) inbreeding did not occur; 4) queen and worker siblings were not genetically differentiated. Worker male production in queenright colonies was neither confirmed nor ruled out by the genetic data. However, production data indicated that queenless workers produced between 4.4 and 21.6% of all males. Overall colony productivity was largely determined by slave number, itself positively correlated with the number of slave-maker workers. There was an abrupt switch from all worker to all sexual production as colony size rose, as predicted by life history models. In queenright colonies, fertile slave-makers did not discernibly reduce colony productivity. Such workers occurred in queenright colonies with most slaves, suggesting they exploited energetic surpluses. Worker reproduction in H. sublaevis therefore appears to have greater influence at the level of individual behaviour than at colony or population level.     

Sex ratio and hermaphroditism in Oregon intertidal populations of the echinoid Strongylocentrotus purpuratus

Examination of large numbers of the echinoid Strongylocentrotus purpuratus (Stimpson) in the years 1969, 1970 and 1971 indicated that the sex ratio at Yaquina Head, Oregon (USA) was significantly different from 1:1, with males predominating. During 1968, 1970 and 1971 at Boiler Bay and Whale Cove, Oregon, the sex ratio did not differ significantly from 1:1 but, in 1969 at Boiler Bay, the proportion of males was 0.61. Two functionally hermaphroditic individuals were found among the 1,354 Yaquina Head echinoids examined, but none were found at Boiler Bay in an equal number of individuals. The hypothesis is proposed that S. purpuratus is a labile gonochorist, with a multiple, autosomal, sex-determining mechanism whose expression may be influenced by the environment.     

A parsimonious optimal foraging model explaining mortality patterns in Serengeti wildebeest

Based on data collected over 24 years in the Serengeti in Tanzania, Sinclair and Arcese (1995) indicated that the sensitivity of blue wildebeest Connochaetes taurinus to predation risk by lions Panthera leo may cause them to change habitats between open (low risk) and wooded (risky) habitats. They found that, in poor rainfall years, predators kill wildebeest that are in better condition than those that die of natural causes. In good rainfall years, predators kill wildebeest that are in worse condition than those that die of natural causes. Sinclair and Arcese (1995) proposed the “predation-sensitive food” hypothesis. This hypothesis suggests that, as food becomes limiting, animals take greater risks to obtain more food, and some of these animals are killed. I propose a more parsimonious hypothesis based on the marginal value theorem that is consistent with the observations made by Sinclair and Arcese (1995). Wildebeest follow a single decision rule in good and poor rainfall years, viz. move when foraging elsewhere increases your rate of intake of nutritious food. Similarly, predators follow a single decision rule in good and poor rainfall years, viz. take the prey item that maximizes the intake of energy per unit effort expended. This parsimonious model does not require differences in predator sensitivity as required by Sinclair and Arcese's (1995) model. I indicate ways in which my model can be falsified.     

A state-dependent model for the optimal management of an invasive metapopulation.

Management of invasive species involves choosing between different management strategy options, but often the best strategy for a particular scenario is not obvious. We illustrate the use of optimization methods to determine the most efficient management strategy using one of the most devastating invasive forest pests in North America, the gypsy moth (Lymantria dispar), as a case study. The optimization approach involves the application of stochastic dynamic programming (SDP) to a metapopulation framework with different infestation patch sizes, with the goal of minimizing infestation spread. We use a novel "moving window" approach as a way to address a spatially explicit problem without being explicitly spatial. We examine results for two cases in order to develop general rules of thumb for management. We explore a model with limited parameter information and then assess how strategies change with specific parameterization for the gypsy moth. The model results in a complex but stable, state-dependent management strategy for a multiyear management program that is robust even under situations of uncertainty. The general rule of thumb for the basic model consists of three strategies: eradicating medium-density infestations, reducing large-density infestations, and reducing the colonization rate from the main infestation, depending on the state of the system. With specific gypsy moth parameterization, reducing colonization decreases in importance relative to the other two strategies. The application of this model to gypsy moth management emphasizes the importance of managing based on the state of the system, and if applied to a specific geographic area, has the potential to substantially improve the efficiency and cost-effectiveness of current gypsy moth eradication programs, helping to slow the spread of this pest. Additionally, the approach used for this particular invasive species can be extended to the optimization of management programs for the spread of other invasive and problem species exhibiting metapopulation dynamics.     

Sex ratio and maternal rank in wild spider monkeys: when daughters disperse

Summary Data from a long-term field study of the spider monkey, Ateles paniscus, in Peru indicate that a strongly female-biased sex ratio exists from birth in this population. Of 46 infants born between July 1981 and June 1986, 12 were male, 32 were female and 2 were of undetermined sex. This effect is consistent between years as well, with more females than males born in each year of the study (Table 1). This bias is driven by the fact that low-ranking females produce daughters almost exclusively, while high-ranking females bias their investment somewhat less strongly towards sons (Table 2). The unusual pattern of female-biased maternal investment observed in this population of Ateles probably occurs for a combination of the following reasons: (1) maternal investment in individual male offspring is somewhat greater than in individual female offspring; (2) males remain with their natal groups, and the sons of high-ranking females are likely to be competitively superior to the sons of low-ranking females; (3) males compete for mates, and only the one or two most dominant males within a community are likely to achieve significant reproductive success. Two possible mechanisms of sex-ratio adjustment and the evidence for each are discussed.     

Sex ratio and the sexual conflict about brood care in a biparental mouthbrooder

The pay-off of deserting and leaving a mate to care for the offspring alone is generally assumed to depend mainly on the availability of alternative mating partners and on the potential spawning rate of males and females. Eretmodus cyanostictus is a monogamous mouthbrooding cichlid in which the clutch is successively incubated first by the female and then by the male. It has been suggested that parents are constrained to monogamy due to low remating probabilities for both sexes. We tested this hypothesis by varying the sex ratio experimentally. Mate desertion by either sex was not significantly higher when additional potential mates were present (males: 8.3%, females: 0%) than when there were no other same-sex conspecifics present (males: 0%, females: 0%). Males lost their mate to a male intruder during their incubation in 26.7% of cases. Pair members were more active and showed more aggression when same-sex conspecifics were present. Behavioural differences between treatments were strongest during the incubation period of a given sex. If no desertion takes place, sexual conflict may be expressed also on a second level, the amount of parental care each parent provides. Indeed, males took the offspring later when additional females were present, although male incubation time did not differ between treatments. A hitherto undescribed display behaviour of females was clear evidence of a conflict about the timing of shift of young. In conclusion, offering alternative mating opportunities did not strongly favour mate desertion in E. cyanostictus. It rather revealed a conflict between mates about when to shift the young.Communicated by M. Abrahams     

Sex ratio shifts within litters of meadow voles (Microtus pennsylvanicus)

Summary Changes in the sex ratio of juvenile recruits into a population of meadow voles (Microtus pennsylvanicus) were correlated with shifts in the weight and mortality of pups within the population. The biased recruitment of female juveniles in the spring was reflected in differential allocation of energy within the litters, as measured by female pups being heavier than male pups (n=245). In the fall, the shift in recruitment to male juveniles was reflected within litters by male pups being heavier than female pups (n=139). Nestling mortality showed a similar gender bias. Skewed sex ratios were most evident within the litters of larger mothers, indicating the gender bias was not trigered by energy limitations. We postulate that gender differences in social spacing and behavior result in spring/fall fluctuations in the reproductive success of offspring, based on their gender.     

A simulation model to explore the relative value of stock enhancement versus harvest regulations for fishery sustainability

Harvest restrictions and stock enhancement are commonly proposed management responses for sustaining degraded fisheries, but comparisons of their relative effectiveness have seldom been considered prior to making policy choices. We built a population model that incorporated both size-dependent harvest restrictions and stock enhancement contributions to explore trade-offs between minimum length limits and stock enhancement for improving population sustainability and fishery metrics (e.g., catch). We used a Murray cod Maccullochella peelii peelii population as a test case, and the model incorporated density-dependent recruitment processes for both hatchery and wild fish. We estimated the spawning potential ratio (SPR) and fishery metrics (e.g., angler catch) across a range of minimum length limits and stocking rates. Model estimates showed that increased minimum length limits were much more effective than stock enhancement for increasing SPR and angler catches in exploited populations, but length limits resulted in reduced harvest. Stocking was predicted to significantly increase total recruitment, population sustainability, and fishery metrics only in systems where natural reproduction had been greatly reduced via habitat loss, fishing mortality was high, or both. If angler fishing effort increased with increased fish abundance from stocking efforts, fishing mortality was predicted to increase and reduce the benefits realized from stocking. The model also indicated that benefits from stock enhancement would be reduced if reproductive efficiency of hatchery-origin fish was compromised. The simulations indicated that stock enhancement was a less effective method to improve fishery sustainability than measures designed to reduce fishing mortality (e.g., length limits).     

Sex,cannibalism and sticklebacks

Summary Two closely related species of sticklebacks (Pisces:Gasterosteidae), breeding in the same site at the same time, showed different patterns of egg cannibalism. In Gasterosteus aculeatus the intensity of cannibalism was density-dependent and females were the most important cannibals. In G. wheatlandi cannibalism was not density-dependent and males ate more eggs than females. This study indicates that sex-specific patterns of cannibalism can differ widely between species despite their similar ecology, and it provides evidence inconsistent with a currently popular explanation for egg cannibalism.     

Sex ratio and sex change in wild and laboratory populations ofTyposyllis prolifera (Polychaeta)

Sex determination and sex differentiation in three local Istrian (Yugoslavia) populations of the polychaeteTyposyllis prolifera were studied both in the field and the laboratory. Significant differences in sex habits were revealed among the populations, the biological significance of which is unknown. (1) Sex ratios (% ) in natural populations from the Adriatic Sea amounted to 50% (Rovinj), 62% (Pore) and 77% (Pula). In the Pula-(but not in the Rovinj-)population, a correlation was found between population density on individualHalopteris scoparia thalli (the favorite habitat ofT. prolifera) and the respective sex ratio: The mean male protion was only a little more than 50% in densely populated thalli, and increased to more than 80% in sparsely populated thalli. Laboratory studies provided the information necessary to explain these field findings. (2) Laboratoryraised progenies showed an overall 1:1 primary (first sexual phase) sex ratio, independent of the local origin of the material and of whether worms were raised singly or under conditions of social contact. Nevertheless, the studied populations may differ with respect to the genetic mechanisms of sex determination: In the Pore- and Pulapopulation individual progenies often departed significantly from the 1:1 sex ratio, whereas individual progenies in the Rovinj- population never did so. (3) The populations proved to be partially protogynous. Sex differentiation in that half of the individuals which differentiated into males (primary males) was absolutely stable during sequential sexual phases. Worms which differentiated into females, however, often changed sex at an earlier or later stage of their life cycle (secondary males). Sex change was irreversible. As to the degree of lability of female differentiation under conditions of isolation, the findings suggest a genetic divergence among both populations and individuals of the same population. Female differentiation in the Rovinj-population was virtually stable, but Pula-females underwent rapid and complete masculinization. Pore-females were between these extremes. (4) In addition to genetic factors, exogenous conditions affected the incidence and time of sex change (Pula-population): Under conditions of social contact, sex change was delayed or suppressed as compared with isolated individuals. The degree of delay or inhibition was independent of the sex of the social partners, yet increased (to the point of saturation) with population density.     

Uncertainty, learning, and the optimal management of wildlife

Wildlife management is limited by uncontrolled and often unrecognized environmental variation, by limited capabilities to observe and control animal populations, and by a lack of understanding about the biological processes driving population dynamics. In this paper I describe a comprehensive framework for management that includes multiple models and likelihood values to account for structural uncertainty, along with stochastic factors to account for environmental variation, random sampling, and partial controllability. Adaptive optimization is developed in terms of the optimal control of incompletely understood populations, with the expected value of perfect information measuring the potential for improving control through learning. The framework for optimal adaptive control is generalized by including partial observability and non-adaptive, sample-based updating of model likelihoods. Passive adaptive management is derived as a special case of constrained adaptive optimization, representing a potentially efficient suboptimal alternative that nonetheless accounts for structural uncertainty.     

Risk spreading, connectivity, and optimal reserve spacing

Two important processes determining the dynamics of spatially structured populations are dispersal and the spatial covariance of demographic fluctuations. Spatially explicit approaches to conservation, such as reserve networks, must consider the tension between these two processes and reach a balance between distances near enough to maintain connectivity, but far enough to benefit from risk spreading. Here, we model this trade-off. We show how two measures of metapopulation persistence depend on the shape of the dispersal kernel and the shape of the distance decay in demographic covariance, and we consider the implications of this trade-off for reserve spacing. The relative rates of distance decay in dispersal and demographic covariance determine whether the long-run metapopulation growth rate, and quasi-extinction risk, peak for adjacent patches or intermediately spaced patches; two local maxima in metapopulation persistence are also possible. When dispersal itself fluctuates over time, the trade-off changes. Temporal variation in mean distance that propagules are dispersed (i.e., propagule advection) decreases metapopulation persistence and decreases the likelihood that persistence will peak for adjacent patches. Conversely, variation in diffusion (the extent of random spread around mean dispersal) increases metapopulation persistence overall and causes it to peak at shorter inter-patch distances. Thus, failure to consider temporal variation in dispersal processes increases the risk that reserve spacings will fail to meet the objective of ensuring metapopulation persistence. This study identifies two phenomena that receive relatively little attention in empirical work on reserve spacing, but that can qualitatively change the effectiveness of reserve spacing strategies: (1) the functional form of the distance decay in covariance among patch-specific demographic rates and (2) temporal variation in the shape of the dispersal kernel. The sensitivity of metapopulation recovery and persistence to how covariance of vital rates decreases with distance suggests that estimating the shape of this function is likely to be as important for effective reserve design as estimating connectivity. Similarly, because temporal variation in dispersal dynamics influences the effect of reserve spacing, approaches to reserve design that ignore such variation, and rely instead on long-term average dispersal patterns, are likely to lead to lower metapopulation viability than is actually achievable.     

Causasilty among wildfire,ENSO, timber harvest,and urban sprawl: The vector autoregression approach

This article demonstrates the applicability of vector autoregression (VAR) modeling in probing the causality relationships among wildfire, El Niño/Southern Oscillation (ENSO), timber harvest, and urban sprawl in the U.S. The VAR approach allows for the multi-directional, multi-faceted interactions among the variables concerned and enables us to portray the temporal impacts of ENSO, the volume of timber harvested, and urban sprawl on wildfire. The empirical analysis, though intended mainly for illustration, reveals that an individual factor may not affect wildfire activity (number of fires and area burned) when acting alone, but can significantly influence fire activity when coupled with other factors, and that wildfire activity has feedback effects on other variables. The impact of a change in ENSO, the volume of timber harvested, and urban population density on wildfire activity could last two decades with the most noticeable impact occurring in the initial 5–10 years. Though ENSO, timber harvest, and urban sprawl all Granger-cause wildfire activity, the impulse response functions show that wildfire activity is more responsive to urban population density than to the volume of timber harvested or ENSO. Thus, controlling urban sprawl represents another option for wildfire mitigation; and integrative wildfire management is essential.     

Sex ratio and oviposition responses to host age and the fitness consequences to mother and offspring in the parasitoid wasp Spalangia endius

In the parasitoid wasp Spalangia endius more offspring and a greater proportion of daughters were oviposited in, and emerged from 0-day-old versus 3-day-old hosts. Offspring that developed on the younger hosts (1) were larger at adulthood, (2) developed more quickly, (3) had higher survivorship to adulthood, and (4) were more often able to chew their way out of the host. Sons and daughters did not differ in how host age affected their size, development rate, or survivorship. The greater proportion of daughters from the younger hosts may be adaptive, as described by the host quality model (a variant of the Trivers and Willard hypothesis). It is adaptive if greater size or more rapid development has a more positive effect on a daughter’s than a son’s fitness and the positive effect is large enough to compensate for sons being trapped disproportionately to daughters in the older hosts. Despite greater success at drilling the younger hosts, mothers did not try to drill them sooner or more often. Having previously oviposited on the older rather than the younger hosts had no detrimental effect on the mother’s subsequent longevity or offspring production. Received: 8 March 2000 / Revised: 9 June 2000 / Accepted: 24 June 2000