Breeding Sex Ratio Research Articles

SPERM COMPETITION AND SEX CHANGE: A COMPARATIVE ANALYSIS ACROSS FISHES

Current theory to explain the adaptive significance of sex change over gonochorism predicts that female-first sex change could be adaptive when relative reproductive success increases at a faster rate with body size for males than for females. A faster rate of reproductive gain with body size can occur if larger males are more effective in controlling females and excluding competitors from fertilizations. The most simple consequence of this theoretical scenario, based on sexual allocation theory, is that natural breeding sex ratios are expected to be female biased in female-first sex changers, because average male fecundity will exceed that of females. A second prediction is that the intensity of sperm competition is expected to be lower in female-first sex-changing species because larger males should be able to more completely monopolize females and therefore reduce male-male competition during spawning. Relative testis size has been shown to be an indicator of the level of sperm competition, so we use this metric to examine evolutionary responses to selection from postcopulatory male-male competition. We used data from 116 comparable female-first sex-changing and nonhermaphroditic (gonochoristic) fish species to test these two predictions. In addition to cross-species analyses we also controlled for potential phylogenetic nonindependence by analyzing independent contrasts. As expected, breeding sex ratios were significantly more female biased in female-first sex-changing than nonhermaphroditic taxa. In addition, males in female-first sex changers had significantly smaller relative testis sizes that were one-fifth the size of those of nonhermaphroditic species, revealing a new evolutionary correlate of female-first sex change. These results, which are based on data from a wide range of taxa and across the same body-size range for either mode of reproduction, provide direct empirical support for current evolutionary theories regarding the benefits of female-first sex change.

Estrus cycle asynchrony in wild female chimpanzees, Pan troglodytes schweinfurthii

Although estrous synchrony has been reported in a number of mammalian species, most often among primates, methodological and analytical problems make it difficult to interpret these results. We developed a novel estrous synchrony index and employed a randomization procedure to analyze long-term observations of female chimpanzee (Pan troglodytes schweinfurthii) estrous cycles at the Mahale Mountains National Park, Tanzania. Our results revealed that female chimpanzees at Mahale avoid synchronizing their estrous periods with each other. We also found that birthrates decreased as the breeding sex ratio increased. We suggest that estrous asynchrony decreases female–female competition for mates. Asynchrony may also reduce the potential for male sexual coercion by nonpreferred mating partners.

Demographic Characteristics of Extinction in a Small, Insular Population of House Sparrows in Northern Norway

In conservation ecology there is an urgent need for indicators that can be used to predict the risk of extinction of populations. Identifying extinction-prone populations has been difficult because few data sets on the demographic characteristics of the final stage to extinction are available and because of problems in separating out stochastic effects from changes in the expected dynamics. We documented the demographic changes that occurred during the period prior to extinction of a small island population of House Sparrows (Passer domesticus) after the end of permanent human settlement. A mark-recapture analysis revealed that this decline to extinction was mainly due to increased mortality after closure of the last farm that resulted in a negative long-term-specific growth rate. No change occurred in either the structural composition (breeding sex ratio and age distribution) of the population or in female recruitment. No male, however, recruits were produced on the island after the farm closure. Based on a simple, stochastic, density-dependent model we constructed a population prediction interval (PPI) to estimate the time to extinction. The 95% PPI slightly overestimated the time to extinction with large uncertainty in predictions, especially due to the influence of demographic stochasticity and parameter drift. Our results strongly emphasize the importance of access to data on temporal variation that can be used to parameterize simple population models that allow estimation of critical parameters for credible prediction of time to extinction.

Sexual selection: lessons from hermaphrodite mating systems

Over the last 130 years, research has established that (a) sexual selection exists and is widespread in the plant and animal kingdoms; (b) it does not necessarily entail sexual dimorphism; even hermaphrodites have it; (c) it does not require intelligence or a sophisticated sense of esthetics; even tapeworms and plants choose mates; and (d) it does not require brawn or even mobility for competition; plants may compete for pollinators, and broadcast spawning invertebrates may also compete for matings. Although discussions of sexual selection often focus on sexual dimorphism, several phenomena that are commonly associated with sexual selection are widespread and highly developed in hermaphrodites. These phenomena include (a) bizarre and expensive courtship and copulatory behavior, (b) multiple mating and sperm competition, (c) rapid evolution of genitalia, (d) special structures associated with courtship, and (e) sexual polymorphism. The skewed breeding sex ratios associated with sequential hermaphroditism have long been recognized as contributory to sexual selection. In many simultaneous hermaphrodites, although the sex ratio at mating may be one to one, the actual reproductive sex ratio may also be skewed, creating a high potential for sexual selection. Reproductive biology in hermaphroditic taxa also involves a lot of complexity unknown in dioecious taxa, such as sex change, facultative sex allocation and conditional reciprocity that offers opportunities to enrich our understanding of sexual selection and to test the assumptions and predictions of theory.

Reconciling nuclear microsatellite and mitochondrial marker estimates of population structure: breeding population structure of Chesapeake Bay striped bass (Morone saxatilis)

Comparative analyses of nuclear and organelle genetic markers may help delineate evolutionarily significant units or management units, although population differentiation estimates from multiple genomes can also conflict. Striped bass (Morone saxatilis) are long-lived, highly migratory anadromous fish recently recovered from a severe decline in population size. Previous studies with protein, nuclear DNA and mitochondrial DNA (mtDNA) markers produced discordant results, and it remains uncertain if the multiple tributaries within Chesapeake Bay constitute distinct management units. Here, 196 young-of-the-year (YOY) striped bass were sampled from Maryland's Choptank, Potomac and Nanticoke Rivers and the north end of Chesapeake Bay in 1999 and from Virginia's Mataponi and Rappahannock Rivers in 2001. A total of 10 microsatellite loci exhibited between two and 27 alleles per locus with observed heterozygosities between 0.255 and 0.893. The 10-locus estimate of R(ST) among the six tributaries was -0.0065 (95% confidence interval -0.0198 to 0.0018). All R(ST) and all but one theta estimates for pairs of populations were not significantly different from zero. Reanalysis of Chesapeake Bay striped bass mtDNA data from two previous studies estimated population differentiation between theta=-0.002 and 0.160, values generally similar to mtDNA population differentiation predicted from microsatellite R(ST) after adjusting for reduced effective population size and uniparental inheritance in organelle genomes. Based on mtDNA differentiation, breeding sex ratios or gene flow may have been slightly male biased in some years. The results reconcile conflicting past studies based on different types of genetic markers, supporting a single Chesapeake Bay management unit encompassing a panmictic striped bass breeding population.

Effective population size in Hymenoptera with complementary sex determination.

Complementary sex determination in the haplodiploid Hymenoptera leads to the production of inviable or effectively sterile diploid males when diploid progeny are homozygous at the sex-determining locus. The production of diploid males reduces the number of females in a population and biases the effective breeding sex ratio in favor of haploid males. This in turn will reduce the effective population size (Ne) of hymenopteran populations with complementary sex determination relative to the expected reductions due to haplodiploidy alone. The effects of diploid male production on Ne in hymenopterans with complementary sex determination when diploid males are either inviable or effectively sterile are assessed theoretically. In both models, low allelic diversity at the sex locus reduces the Ne of hymenopteran populations, and this effect is largest when diploid males are effectively sterile.

Red‐tail monkey groups in forest patches outside the protected area system in the ‘Kampala area’

AbstractThis study was conducted in the ‘Kampala area’, a forest‐savanna‐agricultural mosaic, and was aimed at investigating the ecology and survival of the red‐tail monkey, Cercopithecus ascanius schmidti in fragmented forest patches outside the protected area system. The relationship between habitat variables and red‐tail monkeys' distribution and other demographic factors were investigated. The average group size for red‐tail monkeys in the forest patches was 11.9, while that in Mpanga Forest Reserve it was 23.2. Most of the red‐tail monkey groups had breeding females but there were small populations with no breeding females. The number of breeding females and young varied in different forest patches with the breeding sex ratio ranging from 0 to 1:3. The residential status of red‐tails in the patches was partly influenced by the presence of the vervet monkey and the intensity of human activities in the vicinity.

Differential seasonal mortality of the sexes in willow ptarmigan Lagopus lagopus in northern British Columbia, Canada

We assessed sex ratio, annual survival rates and patterns of seasonal mortality of female and male willow ptarmigan Lagopus lagopus in northwestern British Columbia. Annual sex ratio of birds on the breeding area did not statistically differ from a 1:1 ratio over eight years, although in six of the eight years males were more numerous. Over eight years, annual survival, based on return rates of banded birds, was 41–63% for males and 30–54% for females. We analysed seasonal patterns of mortality using radio‐tagged birds. During four springs and summers, female survival varied from 74 to 97%. They suffered the highest mortality during late prelaying, incubation and pre‐fledging brood rearing periods, and most were killed by raptors. Broodless females survived better (100%) than females accompanied by broods (77%). No radio‐tagged males died during two breeding seasons. Survival for both sexes was lower in autumn/winter (40–60%) than in spring/summer (75–100%). In autumn, the highest mortality for both sexes occurred during the late brood rearing and winter moult periods, coinciding with the autumn raptor migration. Both sexes had similar mortality rates over winter, despite separating by habitat. Another peak in male mortality occurred in late winter/early spring during the period of transient territoriality, whereas no females were killed during this time. From 1.4 to 3 times as many males as females were killed by hunters. Higher mortality of males during the hunting season and in late winter may balance the higher mortality of females during the breeding season, resulting in a breeding sex ratio that is close to unity.

Harassment during arrival on land and departure to sea in southern elephant seals

During the breeding season female elephant seals spent most of their time on land inside harems. When they arrive on land before joining harems, and when they leave harems to return to sea, they are exposed to secondary males and may suffer intense harassment. Hence, arrival and departure present an ideal opportunity to test hypotheses concerning female tactics of harassment reduction. We studied harassment during arrival and departure in two southern elephant seal populations at Punta Delgada (Valdés Peninsula; DEL hereafter) and Sea Lion Island (Falkland Islands; SLI hereafter). Females were less likely to be intercepted by males during arrival than during departure. They also arrived mostly at high tide, thereby reducing the distance from water to the harems. Interception rate and harassment during departure were higher at DEL, where male density and the breeding sex ratio affected the likelihood of interception; on SLI, the socionomy had a small effect. Harassment was higher at low tide at DEL but not at SLI, because tide level variation was larger at DEL and this resulted in a larger variation in the distances of the harems from the water. Females departed more often than expected at high tide at DEL but not at SLI. In both populations females departed directly to sea, rarely stopping before reaching the water, and they never sought contact with males. Social distraction during departure significantly reduced the likelihood of interception. Departures were more frequent during periods of high social activity, and females departing just after other females were less prone to harassment. Accepting copulations with secondary males may reduce the dangerous effects of harassment: interactions occurring during departure were less frequently protested, but we found no indication that departing females were facilitating copulations in a special manner. Quantity and quality of protest during departures was similar to protest during the last days of residence of the females in the harem.

Do females adjust the sex of their offspring in relation to the breeding sex ratio?

Abstract When the adult sex ratio differs between years in local populations, but still is predictable between adjacent years, it has been proposed that the best strategy would be to bias the offspring sex ratio in favour of the rare sex. We tested this hypothesis using a data set of great reed warbler offspring, sexed by molecular techniques, that were collected over 11 breeding seasons at two adjacent reed marshes. Three important assumptions for this hypothesis are fulfilled in the studied great reed warbler population. First, a substantial proportion of great reed warblers are living in small local populations where sex ratio distortions would be sufficiently large and common. Second, breeding adults and their offspring return to breed in the local population to a high degree. Third, females have a possibility to assess the breeding sex ratio before laying their eggs. At our study site, the breeding sex ratio was positively correlated between successive years. However, contrary to our prediction, female great reed warblers seemed not to adjust their offspring sex ratio in relation to the local breeding sex ratio.

DEMOGRAPHY AND BREEDING BIOLOGY OF A SMALL, LOCALIZED POPULATION OF SOUTHERN ELEPHANT SEALS (MIROUNGA LEONINA)

AbstractSouthern elephant seals have been studied in depth in most of their breeding range. One notable exception is the Falkland Islands population. We present data on demography and breeding biology of elephant seals of Sea Lion Island, the main breeding site of this species in the Falklands. Sea Lion Island shelters a small, localized population of southern elephant seals (516 breeding females in 1995 and 518 in 1996). Comparison with the few available census data collected prior to our study suggests that the population has been stable in the short term (1989‐1996). Females produced pups at maximum rate and pup mortality was low (2.13%). Breeding sex ratio was strongly unbalanced, with about 14 females per breeding male and 47 females per harem‐holding male at peak haul‐out. Survival rate between breeding seasons was 67.4% for females and 50% for males. Timing of the breeding season was very similar to that recorded in other populations and was in accordance with clinal variation with latitude. Sex ratio at birth was balanced, and no significant weight dimorphism at weaning between sexes was detected (males: 135.4 kg; females: 132.0 kg). Weaning weight was correlated with size class of the mother.

A Stochastic Dynamic Programming Model for the Management of the Saiga Antelope

A stochastic dynamic programming model for the optimal management of the saiga antelope is presented. The optimal hunting mortality rate and proportion of adult males in the harvest are found as functions of the size and structure of the saiga population before hunting. The effects of stochastic climatic variation on the population are taken into account in this model. It is shown that key assumptions must be made about the effects of the breeding sex ratio on female fecundity, and about whether poaching is occurring. If incorrect assumptions are made about either of these factors, the calculated optimal strategy can become severely suboptimal. A simple suboptimal decision rule that takes the population size and structure into account is shown to be more able to buffer against these factors than the optimal strategy, which has proved too complicated for analytical solution. The model predictions are robust to parameter changes.

A STOCHASTIC DYNAMIC PROGRAMMING MODEL FOR THE MANAGEMENT OF THE SAIGA ANTELOPE

A stochastic dynamic programming model for the optimal management of the saiga antelope is presented. The optimal hunting mortality rate and proportion of adult males in the harvest are found as functions of the size and structure of the saiga population before hunting. The effects of stochastic climatic variation on the population are taken into account in this model. It is shown that key assumptions must be made about the effects of the breeding sex ratio on female fecundity, and about whether poaching is occurring. If incorrect assumptions are made about either of these factors, the calculated optimal strategy can become severely suboptimal. A simple suboptimal decision rule that takes the population size and structure into account is shown to be more able to buffer against these factors than the optimal strategy, which has proved too complicated for analytical solution. The model predictions are robust to parameter changes.

The Proximate Mechanisms of Natal Dispersal in Female Horses

Abstract The aim of this study was to investigate proximate mechanisms of natal dispersal by female horses, Equus caballus (i.e. proximate causes and the factors influencing the timing), and to test predictions from two functional hypotheses: the intrasexual competition hypothesis, and inbreeding avoidance. The data concerned 40 individuals born between 1974-1985 in a closely monitored herd which developed a natural social structure during this period. All the females dispersed from their natal groups; none became solitary; 80% transferred to existing harems, the others formed new groups with bachelor stallions. Abduction by stallions affected only a quarter of the females whose transfers were observed. The results of this study do not support the hypothesis that a function of natal dispersal is to reduce intrasexual competition. The young females were not expelled by resident females of their natal groups, and did not, as a rule, experience increased aggression from these females before emigration. Their social bonds with members of their natal groups showed no progressive weakening prior to departure, and there was no gradual strengthening of bonds with individuals in the groups to which they transferred. There was no evidence for reproductive competition between the young females and resident mares of their natal groups, since the young females always refused the sexual approaches by males of these groups. Finally, age at dispersal did not decrease with the number of resident females in the groups they left. In contrast, as predicted by the inbreeding avoidance hypothesis, the primary cause of dispersal appeared to be sexual attraction to unfamiliar males. When in oestrus and before dispersing, the young females accepted matings only with males of other groups in spite of frequent sexual approaches by males of their natal groups (normally close relatives). In addition, none dispersed before first oestrus, and most did so during an oestrous period, at or before the conception of their first foal. The mothers of most young females interposed themselves when close kin males of the natal group approached their daughters sexually; this could contribute to the avoidance of close inbreeding. Among the other factors examined, some did not influence dispersal of the young females : they experienced low levels of aggression by adult stallions of their natal groups, particularly at the time of departure; their weight and body condition had no significant effects on leaving age nor did their mother's rank, the number of siblings, or the birth of another. In contrast, as the number of groups and the breeding sex ratio increased, dispersal age declined, occurring at a median age of 23 months (range 12-42 months) in the later years, when the herd had developed a natural social system.

Polygyny in the Willow Warbler Phylloscopus trochilus in Swedish Lapland

The breeding sex ratio of Willow Warblers Phylloscopus trochilus in a subalpine birch forest was strongly female biased. Very few territorial males remained unmated and most became polygynous. Bigyny was the rule, but some males probably had three females. Male assistance at secondary nests varied and was probably dependent on temporal distance between the females' nests. There was no significant reduction in reproductive output for these females. Variation in the sex ratio of breeding birds can be explained, at least partly, by variation in the timing of male settlement.

SEXUAL DIMORPHISM AND SEX RATIO IN THE YELLOWSHOULDERED WIDOWBIRD EUPLECTES MACROURUS SOROR

Summary Savalli, U.M. 1994. Sexual dimorphism and sex ratio in the Yellowshouldered Widowbird Euplectes macrourus soror. Ostrich. 65: 297–301. Yellowshouldered (Yellowbacked) Widowbirds Euplectes macrourus soror are sexually dimorphic in plumage and size. At Kakamega, western Kenya, adult males were all black except for the yellow shoulders, contrary to previous reports that yellow-mantled individuals were predominant in this area. Males were larger than females, with wing length the best single measure to distinguish sex (since immature males are streaky brown, like females). Immature males had shorter wings and tails than did adult males, but did not differ in other measures of size. The sex ratio of netted birds was near 1:1, but nearly half of the males were immature. The breeding sex ratio was nearly even in one year but heavily male biased the second, suggesting that not all females may breed.

Reproductive Biology of Pelobates cultripes (Anura: Pelobatidae) in Central Spain

We studied the mating system of a population of western spadefoot toads Pelobates cultripes for a single season. The breeding season lasted 35 days, during which there were four periods of arrival at the breeding pond. Only males arrived at the breeding site in the first peak; additional males and females arrived in subsequent peaks. Minimum temperature and rainfall influenced activity, and maximum temperature was correlated with recruitment and total number of toads that remained in the pond. The breeding sex ratio was slightly male-biased (1.15:1), whereas average operational sex ratio was more strongly male-biased (average 2.97:1). Males migrating early to the breeding pond were larger than males that migrated late. Males remained in the pond longer than females, and fighting between males was observed

A Polygamous Social System of the Fan‐tailed Warbler Cisticola juncidis

AbstractThe social system of an individually‐marked population of the fan‐tailed warbler Cisticola juncidis was studied in Japan, over six breeding seasons from 1978 to 1983. More than 127 males established territories, some 50–70% were polygynists each year. Territorial males were replaced frequently within seasons. Females were less faithful than males to their first breeding sites. Perennial or seasonal pair bonds were rare, maintained over two successive breeding attempts, by only 13.6% of females. Half the females left the area after one breeding attempt. Frequent divorce, rapid and multiple remating of females, multiple breedings, and female movement over a wide area all combine to skew breeding sex ratio from unity and favor polygyny.

Spatial and temporal components of the mating systems of kudu bulls and red deer stags

Observations were made on greater kudu ( Tragelaphus strepsiceros) in the Kruger National Park in South Africa. Data on movement patterns, courtship, dominance interactions and proportion of time spent in various activities by males during the breeding season are presented. Comparative data on the apportionment of time by red deer ( Cervus elaphus) setages were obtained on the Isle of Rhum, Scotland. While spatial components of male reproductive behaviour were similar in the two species, time-investments differed markedly. Kudu bulls showed little change in time spent feeding and moving during the breeding season, did not herd females and tolerated the presence of younger males. In contrast, red deer stags almost ceased feeding and greatly increased their time spent moving and in sexual or dominance-related activities. Thus the spatial and temporal components of male mating strategies can vary independently and require separate evolutionary explanaions. Comparative data for other ungulates are sparse, but suggest that time-investments are influenced by the duration of the breeding season and the breeding sex ratio. However, kudu bulls incur high mortality costs despite their low time-investments in reproductive activities.

Evolution of social organization and life-history patterns among grouse.

Correlations of social structure, life-history patterns, and ecology among the seventeen species of grouse exemplify some general patterns in the evolution of mating systems among higher vertebrates. The species of grouse differ in the aggregation of displaying males, the permanence of heterosexual affiliations, the contributions of males to parental care, and the breeding sex ratio. Promiscous species (no durable heterosexual affiliation) are probably all polynous (the breeding sex ratio less than unity), but fall into two groups depending on whether the displaying males congregate at leks or disperse relatively evenly. In all of these promiscuous, polygymous species and in three monogramous species, the female cares for the young; dual parental care appears only in one monogamous species. During their first year, the males of polynous species do not mate or mate much less frequently, although females normally breed at one year of age, a situation termed sexual bimaturism. At least among birds and mammals, polygyny is normally associated with sexual bimaturism. Sexual bimaturism and polygyny among grouse correlated well both with large overall size, as indicated by female weight, and with greater sexual dimorphism in weight. These correlations also recur in other vertebrate families. Among the promiscuous grouse, the dispersion patterns of displaying males probably relate to the differences in predation pressures in open and forested habitats. In contrast with some other avian families, differences in social structure among grouse have little relationship to major differences in diet. Single parental care is not a sufficient condition for the evolution of polygyny among grouse. The association of sexual bimaturism with polygyny requires, in addition, an explanation of the adaptive advantages of deferred reproduction among males. Postponed reproduction by males will reduce the spread of their genes in a population unless compensated by a sufficient gain in early survival or increased fecundity later. Calculations of rates of reproductive increase for hypothetical lineages of males indicate that these compensating conditions can plausibly explain the evolution of delayed reproduction among male grouse. Theoretical considerations further suggest that larger size could favor the evolution of deferred reproduction, especially in males, and thus could contribute to the evolution of sexual bimaturism and polygyny in the larger species of grouse. Ecological circumstances, including the details of food dispersion, could thus influence the evolution of mating systems indirectly, through effects on the evolution of body sizes. As polygyny among higher vertebrates is normally associated with sexual bimaturism and is incompatible with full dual parental care, polygyny should evolve under ecological conditions in which the adaptive advantages of sexual bimaturism otuweigh those of dual parental care.