Ecological determinants of divorce: a structural approach to the explanation of Japanese divorce.

This paper examines the ecological determinants of contemporary Japanese divorce rates on the prefectural level. LISREL and computer-generated graphics are the analytic methods used. The aggregate level of analysis demands the use of the ecological model which posits that demographic changes, economic activities, migration patterns, and the level of urbanization are significant predictors of divorce rate. Our analysis demonstrates that sex ratio, female labor force participation, female in-migration patterns, population increase, and net household income all play a significant role in affecting the divorce rate. Our findings also confirm the well-supported hypothesis that both population density and modernization positively influence modern Japan's divorce rates. The residual analysis also points out that in order to account for the large proportion of the unexplained variance of Japanese divorce, behavioral-related variables and island- or prefecture-specific dimensions need to be included in the ecological model of divorce.     

Implications of mate search, mate choice and divorce rate for population dynamics of sexually reproducing species

In this paper we examine how the process of mate search, degree of mate choice and degree of mate fidelity may interact to affect long‐term population dynamics of sexually reproducing species. In particular, we address the following questions: are degree of mate choice and degree of mate fidelity correlated? How does mate search shape this relationship? How does longevity affect mating behaviour? To resolve these questions, we develop a spatially explicit, individual‐based model of a sexually reproducing population with single (i.e. unpaired) males, single females, and pairs as focal individuals. Both this model and its non‐spatial approximation give rise to the Allee effect due to lack of mating possibilities, and sufficiently small/sparse populations always go extinct. We quantify combinations of mate choice and divorce rate under which populations persist or go extinct even from high sizes. We thus show that there exist ecological constraints for possible (co)evolution of mate choice and pair maintenance behaviour. Our models also suggest that colonial species with active mate search strategy survive at higher divorce rates than less colonial animals that search for their mates randomly, and that long‐lived species sustain at higher degrees of mate choice and lower degrees of mate fidelity compared to the short‐lived ones. Connection of these findings to other theoretical results and some empirical observations is discussed.     

Divorce in contemporary Japan

Data from the 1985-86 Japanese census are analysed to explore the determinants of the divorce rates in Japan's forty-seven prefectures, using two theoretical models: (a) the social integration model, which is shown to have a greater utility in predicting Japanese divorce levels than (b), the human capital model. Female emigration patterns play a significant role in affecting the divorce rate. Population increase and net household income are also important predictors of the Japanese divorce rate and urbanization has a great influence in modern Japan. Demographic and aggregate variables such as migration, urbanization, and socioeconomic factors are useful when organized under a social integration model.     

High divorce rates in Corsican blue tits: how to choose a better option in a harsh environment

We investigate which hypothesis, the “better mate hypothesis” or the “better territory hypothesis” best explains the unusually high divorce rate (59%) in a population of blue tits (Parus caeruleus) living in a sclerophyllous habitat characterised by severe environmental constraints (trophic, parasitic, climatic) on the island of Corsica, France. Using data from the breeding seasons 1985–1998 and from a brood size experiment (1990–1993) we examined the causes of divorce and their consequences on breeding performance, mate assortment and territory choice. Breeding performance had no significant effect on whether birds re‐united or divorced in the next breeding season. Re‐uniting pairs did better than divorced females and the latter improved their breeding performance compared to prior to divorce, but this was mainly due to age and territory effects. There were no differences in male performance depending on whether they re‐united or divorced. The age combination of pairs did not differ between re‐uniting and divorcing pairs, but mate assortment changed after divorce with males re‐mating more often with older partners than females. Manipulation of brood size showed a trend for birds with enlarged broods to divorce more. Pairs responded significantly to territory quality by divorcing more often in poor than in good breeding sites. Both faithful pairs and male divorcees had shorter breeding dispersal distances than female divorcees. Divorce rates were determined by the large differences in quality among breeding sites. Males, whatever their status, usually retained their previous territory whereas divorced females moved significantly longer distances and improved their breeding site. Moving to a better territory after divorce benefits only females which appear to be the choosing sex in the decision to divorce. This study strongly supports the “habitat mediated hypothesis” and we suggest that the large observed intraspecific variation in the magnitude of divorce rates in many species of birds is mostly determined by habitat characteristics.     

Trading up: the fitness consequences of divorce in monogamous birds

Social and genetic mating systems play an important role in natural and sexual selection, as well as in the dynamics of populations. In socially monogamous species different genetic mating patterns appear when individuals mate outside the breeding pair within a breeding season (extra‐pair mating) or when they change partners between two breeding seasons (widowing or divorce). Divorce can be defined as having occurred when two previously paired individuals are alive during the next breeding season and at least one of them has re‐mated with a new partner. In socially monogamous birds divorce is widespread, but it is not clear whether it is a behavioural adaptation to improve the quality of a mating decision or whether, alternatively, it results as a non‐selected consequence of other processes: existing studies suggest a heterogeneous set of results with respect to this central question. This heterogeneity could result from a number of factors, ranging from the methodological approaches used, to population‐ or species‐specific characters. In this review we use phylogenetic meta‐analyses to assess the evidence that divorce is adaptive (in terms of breeding success) across 64 species of socially monogamous birds. Second, we explore biological and methodological reasons for the heterogeneity in the results of previous studies. Results of our analyses supported the hypothesis that divorce is, in general, an adaptive behavioural strategy as: (1) divorce is triggered by relatively low breeding success; (2) there is a positive change in breeding success as a result of divorce. More specifically, while controlling for methodological moderators, we show that: (i) earlier stages of breeding are better predictors of divorce than later stages (r = 0.231; 95% CI: 0.061–0.391 for clutch size; similar for laying date); (ii) females benefited from divorce more than males in terms of increasing breeding success between successive breeding attempts, with different stages of the breeding cycle improving at different rates (e.g. r = 0.637; 95% CI: 0.328–0.817 for brood‐level measures). We show that the effect size was dependent on the methodological approach used across studies and argue that research on the adaptive nature of divorce should be cautious when designing the study and interpreting the results. Altogether, by providing strong evidence that divorce is an adaptive strategy across monogamous birds, the results of our analysis provide a firm ground for further exploration of external covariates of divorce (e.g. demographic factors) and the mechanisms underlying the differences in the effect sizes of the proximal fitness causes and consequences of divorce.     

A two-sex demographic model with single-dependent divorce rate

Divorce appears to be one of the least studied demographic processes, both empirically and in two-sex demographic models. In this paper, we study mathematical as well as biological implications of the assumption that the divorce rate is positively affected by the amount of single (i.e., unmarried/unpaired) individuals in the population. We do that by modifying the classical exponential two-sex model accounting for pair formation and separation. We model the divorce rate as an increasing function of the single population size and show that the single population pressure on the established couples alters the exponential behavior of the classical model in which the divorce rate is assumed constant. In particular, the total population size becomes bounded and a unique positive equilibrium exists. In addition, a Hopf bifurcation analysis around the positive equilibrium shows that the modified model may exhibit sustained oscillations.     

Mate and territory availability affect breeding dispersal and divorce in a resident Southern House Wren Troglodytes aedon musculus population

Breeding dispersal is the movement of an individual between breeding attempts and is usually associated with the disruption of the social pair bond, although mates may disperse together as a social unit. In monogamous territorial species, the decision to disperse may be affected by individual attributes such as sex, age and condition of the disperser. However, environmental and social contexts may also play a crucial role in the decision to disperse. We analysed capture‐resighting data collected over 9 years to study breeding dispersal and divorce rates of a Southern House Wren Troglodytes aedon musculus population in South Temperate Argentina. Between‐season dispersal was more frequent than within‐season dispersal, with females dispersing more often than males, both between and within seasons. Both within‐season and between‐season breeding dispersal probability was affected by territory availability, but not by previous breeding success. When the adult sex ratio (ASR) was more skewed towards males, male between‐season dispersal was also affected by mating status, with widowed and single males dispersing more often than paired males. Within‐season divorce increased the reproductive success of females but not males, and was affected by the availability of social partners (with increasingly male‐skewed ASR). Our results suggest that territorial vacancies and mating opportunities affect dispersal and divorce rates in resident Southern House Wrens, highlighting the importance of social and environmental contexts for dispersal behaviour and the stability of social pair bonds.     

Projection of the Mexican National Labor Force, 1980–2005

Abstract

Mexico has a large and rapidly growing labor force. This paper projects the Mexican national labor force from 1980 to 2005, with varying assumptions of vital rates, economic activity, and international migration. Projections are also made for the urban and rural components of the Mexican population, assuming inter‐component migration flows. Results indicate that the Mexican labor force will grow over the projection period at an average annual rate of 907,000 to 1,183,000 workers; will age slightly; and will have a much higher proportion female. Implications are discussed in terms of Mexican‐U.S. migration, possible agreements on free trade, and global trends in workforce.     

Influence of reproductive output on divorce rates in polar seabirds

The high occurrence of social monogamy in birds has led to questions about partner fidelity, or the perennial nature of monogamy from one breeding season to another. Despite the evolutionary advantages of partner fidelity, divorce occurs among 95% of bird species. We aimed to describe patterns of divorce and partner fidelity in five seabird species breeding in Arctic and Antarctic regions and investigated the influence of breeding status on pair bond maintenance. For four out of the five species considered, we observed low divorce rates (respectively 1.9%, 3.3%, 2.5%, and 0.0% for Brünnich''s guillemot, glaucous gull, Antarctic petrel, and south polar skua), while the divorce rate was much higher (19.1%) for the black‐legged kittiwake. For kittiwakes, the divorce rate was lower for pairs that managed to raise their chick to 15 days of age, while the effect of breeding success on divorce in the four other species could not be tested due to the rareness of divorce events. Our results emphasize the potentially large temporal (interannual) variations that should be taken into account in understanding divorce and partner fidelity in seabirds.     

Fitness consequences of divorce in the oystercatcher, Haematopus ostralegus

We investigated the fitness consequences of divorce in oystercatchers. We made a distinction between two types of divorce: in desertions the disruption of the pair bond is initiated by one of the pair members, and in usurpations by a conspecific individual. Survival and reproduction prospects for oystercatchers are largely determined by their social status (nonbreeder or breeding bird at a site of a specific quality). Changes in social status in relation to divorce showed that birds taking the initiative to leave their mate increased in fitness, relative to birds that were forced to leave their partner. Status of individuals that remained in their territory after divorce was unaffected if their mate was expelled, but declined if their mate deserted. Survival after divorce was significantly lower for birds that were expelled than for those deserting. Divorce rate, and especially desertion rate, was higher among occupants of low- than high-quality territories. In general, divorce rate increased following elevated mortality. In high-quality territories usurpations increased with increasing breeder mortality, but at low-quality territories this relation was absent. Desertion rates were similarly related to mortality in both territory types. Divorce participants thus differed strongly in their fitness prospects, depending on the type of divorce, the role played in the divorce and the quality of the territory where divorce took place. Studies that do not observe the birds during divorce cannot determine the type of divorce and the role played by the individuals, and this may lead to misleading conclusions on the costs and benefits of divorce. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Degree of mutual ornamentation in birds is related to divorce rate

Many bird species have ornaments that are expressed equally in both sexes. I use comparative analysis to investigate why some monomorphic birds are highly ornamented, whereas others are drab. The results show a significant positive association between the degree of mutual ornamentation and divorce rate. This result is robust to the removal of the effects of phylogeny, site fidelity, residency, coloniality, nest type, mortality, body size and body-size dimorphism. The level of extra-pair paternity was not related to the degree of mutual ornamentation. I argue that these results are compatible with a process of mutual sexual selection, in which both sexes compete for access to mates. The coupled evolution of ornamentation and divorce rate, from the probable ancestral state of a high degree of ornamentation and a low divorce rate, appears to result mainly from a loss of ornamentation under mate fidelity.     

Modelling irruptions and population dynamics of the great spotted woodpecker – joint effects of density and cone crops

Irruptive migrants are partially migrating species showing pronounced temporal variation in the number of migrants. Occasional irruptions are often explained by increased population density (per area) or lack of food. Similarly to population dynamics, these explanations are not mutually exclusive, but probably act in concert. Here we do a parallel analysis on the migration intensity and population dynamics of an irruptive migratory bird species, the great spotted woodpecker Dendrocopos major, in southern Finland. Both analyses include simultaneous effects of breeding density and cone crops of Scots pine Pinus sylvestris and Norway spruce Picea abies. A novel model for statistical analyses of irruptive migration intensity is developed. Population dynamics are investigated using log‐linear state‐space models. Woodpecker migration intensity is well explained by lack of spruce cones and increases proportionally to population density. Population dynamics is most successfully described with a density dependent model, where the natural logarithm of spruce cone availability previous autumn has a strong positive effect on population growth. This is likely to be due to lowered winter mortality and lowered emigration rates resulting from plentiful food availability. Although the species uses pine cones as a food resource in winter, the estimated impacts of pine cone crops on migration intensity and population dynamics are low, probably due to low annual variation in crop comparing to spruce cones. Large irruptions also tend to occur earlier in season, often before cones are a crucial food resource for the woodpeckers, suggesting that the woodpeckers use the amount of maturing cones as a cue for upcoming conditions. Our results show that the ecological role of different resources can differ considerably from the expected pattern based on the proportion of the resources in a species’ diet. Advantages of examining patterns of bird migration, population density and dynamics in unison are apparent.     

Female mate choice of male signals is unlikely to promote ecological adaptation in Enchenopa treehoppers (Hemiptera: Membracidae)

A key question in speciation research is how ecological and sexual divergence arise and interact. We tested the hypothesis that mate choice causes local adaptation and ecological divergence using the rationale that the performance~signal trait relationship should parallel the attractiveness~signal trait relationship. We used female fecundity as a measure of ecological performance. We used a species in the Enchenopa binotata treehopper complex, wherein speciation involves adaptation to novel environments and divergence in sexual communication. We used a full‐sibling, split‐family rearing design to estimate genetic correlations (r_G) between fecundity and signal traits, and compared those relationships against population‐level mate preferences for the signal traits. Animal model estimates for r_G between female fecundity and male signal traits overlapped zero—rejecting the hypothesis—but could reflect sample size limitations. The magnitude of r_G correlated with the strength of the mate preferences for the corresponding signal traits, especially for signal frequency, which has the strongest mate preference and the most divergence in the complex. However, signal frequencies favored by the population‐level mate preference are not associated with high fecundity. Therefore, mate preferences do not appear to have been selected to favor high‐performance genotypes. Our findings suggest that ecological and sexual divergence may arise separately, but reinforce each other, during speciation.     

When can ecological speciation be detected with neutral loci?

It is not yet clear under what conditions empirical studies can reliably detect progress toward ecological speciation through the analysis of allelic variation at neutral loci. We use a simulation approach to investigate the range of parameter space under which such detection is, and is not, likely. We specifically test for the conditions under which divergent natural selection can cause a ‘generalized barrier to gene flow’ that is present across the genome. Our individual‐based numerical simulations focus on how population divergence at neutral loci varies in relation to recombination rate with a selected locus, divergent selection on that locus, migration rate and population size. We specifically test whether genetic differences at neutral markers are greater between populations in different environments than between populations in similar environments. We find that this expected signature of ecological speciation can be detected under part of the parameter space, most consistently when divergent selection is strong and migration is intermediate. By contrast, the expected signature of ecological speciation is not reliably detected when divergent selection is weak or migration is low or high. These findings provide insights into the strengths and weaknesses of using neutral markers to infer ecological speciation in natural systems.     

The effects of investigator disturbance on procellariiform seabirds: A review

Abstract

Despite long‐held concerns about the effects of researchers on breeding birds, few studies have focused on the impact of investigators on Procellariiformes. In this review, we summarise published investigations concerning the effects of investigators on physiology, behaviour, reproductive success, offspring quality, and population trends of procellariiform seabirds. Many of the smaller procel‐lariid taxa, such as storm‐petrels (Hydrobatidae, and some shearwater Puffinus species) are particularly sensitive to handling during the incubation period, resulting in lowered reproductive success, burrow shifts, and increased divorce between pairs. However, repeated handling of chicks does not seem to have a discernible effect on chick quality. In larger procellariiforms, the presence of researchers within the breeding colony is associated with significant increases in heart rates. Therefore, researchers seeking unbiased estimates of demographic and ecological parameters of birds should be encouraged to measure their own impact and report the findings in the literature, even if they detect no adverse effects of investigator disturbance. Techniques should be developed that ensure the accurate recording of birds’ natural behaviour while minimising the impact of investigator disturbance. If investigators are willing to study disturbance problem s, it should be possible to reduce biases caused by their activities.     

Optimal feeding under stoichiometric constraints: a model of compensatory feeding with functional response

When the nutrient content of food is limited, herbivores often increase their feeding rates. Such an increase in the feeding rate is called ‘compensatory feeding’. Although it has a number of implications for herbivore population and plant–forager dynamics, the compensatory feeding is not yet functionally formulated especially in relation with ecological stoichiometry. Therefore, we constructed a simple mathematical model by incorporating the optimal feeding rate into the type II functional response to maximize a forager's growth rate under constraints of carbon or nutritionally important element like phosphorus (P). We used the planktonic herbivore Daphnia as a model herbivore. The model revealed that the optimal feeding rate increased by using excess carbon when relative P content of food was less than a certain level, which is known as the threshold elemental ratio. This level changed with the change of food abundance. It also showed that whether or not foragers should exhibit compensatory feeding depends on their stoichiometric characteristics and digestive traits, and also on the assimilability of a given food. These findings are helpful to test the feeding conditions under which compensatory feeding is advantageous for a given animal. Our model can be easily incorporated into forager population dynamics and prey‐consumer interaction models because the optimal feeding rate can be analytically given.     

Association between serum uncarboxylated osteocalcin levels and nutritional intake in Japanese female athletes

[Purpose] The current study aimed to determine the association between serum uncarboxylated osteocalcin (ucOC) levels and dietary vitamin K intake in female Japanese athletes.[Methods] The nutritional profile and food group intake of 52 Japanese female athletes were investigated using a digital photographic method with data obtained from 3-day dietary records. The food groups were categorized into 18 groups in line with the standard tables of food composition in Japan. Fasting blood samples were collected for serum ucOC levels, and dual-energy X-ray absorptiometry (DXA) evaluated body composition and bone parameters.[Results] The results showed that dietary vitamin K intake level was 235 ± 148 µg/day, and approximately 70% (n = 36) of participants consumed more than the adequate intake (AI) level, based on the Dietary Reference Intakes for Japanese females aged 18–29. Serum ucOC levels were negatively associated with daily vitamin K intake (r = −0.388, P = 0.004) and calcium (r = −0.596, P = 0.004) after adjusting for energy intake.[Conclusion] Our study revealed that serum ucOC levels were negatively associated with dietary vitamin K intake in female Japanese athletes. Serum ucOC levels reflected dietary vitamin K intake in female athletes. In summary, female athletes consume more vitamin K than the general population to maintain bone health. Furthermore, our results indicated that serum ucOC levels might be linked to dietary calcium intake.     

Migration rates as inferred from the 2002 Russian census

Migration rates in the modern Russian population have been studied on the basis of the results of the 2002 census. The migration rate at the population level corresponding to a city or rural administrative district is m = 0.443. The migration rate for women (m(F) = 0.463) is 11% higher than for men (m(M) = 0.418); the migration rate for urban populations (m(U) = 0.459) is 15% higher than for rural populations (m(R) = 0.398). The geographic variation of migrations has been studied; the geographic distributions of the migration rates for rural, urban, male, and female populations are presented. Comparison of migrations in the rural population of Russia at two temporal cross-sections (1970 and 2002) has shown that the geographic structure of migrations has remained the same in many respects. The ecological factor has remained the main determinant of the migration geography, which is expressed in the relationship of the settled way of life with the forest-steppe and broad-leaved forest landscapes.     

Genetic dissimilarity between mates, but not male heterozygosity, influences divorce in schistosomes

Background

Correlational studies strongly suggest that both genetic similarity and heterozygosity can influence female mate choice. However, the influence of each variable has usually been tested independently, although similarity and heterozygosity might be correlated. We experimentally determined the relative influence of genetic similarity and heterozygosity in divorce and re-mating in the monogamous endoparasite Schistosoma mansoni.

Methodology/Principal Findings

We performed sequential infections of vertebrate hosts with controlled larval populations of parasites, where sex and individual genetic diversity and similarity were predetermined before infection. Divorce rate increased significantly when females were given the opportunity to increase genetic dissimilarity through re-mating with a new partner, independently of the intensity of male-male competition. We found however no evidence for females attempting to maximize the level of heterozygosity of their reproductive partner through divorce.

Conclusions/Significance

Female preference for genetically dissimilar males should result in more heterozygous offspring. Because genetic heterozygosity might partly determine the ability of parasites to counter host resistance, adaptive divorce could be an important factor in the evolutionary arms race between schistosomes and their hosts.     

What shapes local density? The importance of migration rates and local growth for density‐patch size relationships in two Cionus weevils

1. The relative effect of migration and local growth on the spatio‐temporal density‐distribution of two co‐existing herbivorous weevils, Cionus scrophulariae L. and C. tuberculosus Scop., in 32 host plant Scrophularia nodosa L. patches of varying sizes was investigated. 2. Predictions of the temporal development of the slope in the density‐patch size relationships were derived from a basic population model with scale‐dependent migration rates. The model indicated that the slopes in the density‐patch size relationships during the early season should be reflected by the net scaling of immigration and emigration rates, whereas the slopes during the later season should increase as a result of local growth. 3. Emigration rates of the weevils were estimated in a field experiment, were the weevils coexisted in space and time. These results were then combined with a previous estimate of immigration rates in order to determine the net scaling of migration rates. 4. The emigration rate differed between species, caused by different movement rates in small patches, which could explain differences in the general slope of the density‐patch size relationships of the weevils in the natural figwort patches throughout the summer. The slopes in the relationships in the early season were largely predicted by the net scaling of migration rates. The slope also increased in the later season for C. tuberculosus, whereas the slope decreased for C. scrophulariae. 5. It was concluded that the understanding of both inter‐ and intra‐specific variations in density‐patch size relationships of insect herbivores can be improved using population models incorporating scale‐dependent migration and local growth.