Male-skewed Sex Ratios Research Articles

Male-Skewed Sex Ratio in Saltmarsh Sparrow Nestlings

Abstract. Sex ratios of adult birds are frequently skewed, but in few species is the sex ratio of offspring also skewed. Adult females, however, should modify their offspring's sex ratio and invest more in the sex with the greatest future reproductive potential. We investigated the relationship between nestlings' morphology, timing of reproduction, and sex ratios in broods of the Saltmarsh Sparrow (Ammodramus caudacutus), a species with extreme levels of multiple paternity: males provide no parental care and sex ratios of adults may be highly skewed. By molecular methods, we sexed 157 nestlings from 52 Saltmarsh Sparrow nests in Connecticut over 2 years. Their sex ratio was significantly skewed toward males (59%) and did not vary with brood size, but this imbalance is insufficient to fully explain the reported skew in adults' sex ratio. Just prior to fledging, male nestlings were larger than female nestlings, which may suggest that they are more costly to produce. Survivorship is greater in nests initiate...

The practicality of Trojan sex chromosomes as a biological control: an agent based model of two highly invasive Gambusia species

Invasive fish species are a primary threat to aquatic ecosystems. Owing to the high fecundity of some fish, conventional control methods (e.g. specific removal) can be ineffective and the use of poisons is not desirable due to their non-specificity. Trojan sex chromosomes (TSC) are a theoretical method of invasive species control, where sex-reversed fish that are only able to produce male offspring are released into the target population. These Trojan individuals subsequently breed, causing a male skewed population sex ratio and eventually population collapse. Previous publications have explored TSC as an invasive species control, but assume that wild-type and Trojan fish have equal fitness, an assumption that may not be valid. What is more, models from closely related fields suggest that differential fitness between Trojans and wild-type fish maybe influential in the efficacy of TSC as a bio-control. Here we use agent based modeling to test how effectively TSC can be used to control two common invasive species of mosquitofish (Gambusia affinis and G. holbrooki) when Trojans have compromised fitness. We manipulated the fecundity, probability of mating and offspring survival of Trojan fish. Overall, our models found that fecundity holds the most influence over how effectively TSC theory can be used to control fish populations. However, a recent meta-analysis demonstrates that the fecundity of sex-reversed fish is compromised. It may be possible to compensate for reduced fecundity by increasing the rate of Trojan introductions. Surprisingly, our models also found that Trojans are a more effective bio-control when consistently introduced into the same place, rather than being randomly distributed at introduction.

Sex determination in southern flounder, Paralichthys lethostigma, from the Texas Gulf Coast

In marine flatfish of the genus Paralichthys, temperature plays a large role in sex determination. The most susceptible areas to temperature fluctuation are shallow water environments, particularly estuaries, which serve as essential nursery habitats for juvenile southern flounder. While in the estuaries, juveniles develop, and sex is determined. Southern flounder exhibit genotypic sex determination; however, the sex of females is highly influenced by temperature, which can result in phenotypic sex reversal. The temperature-sensitive enzyme complex responsible for estrogen biosynthesis in vertebrates is aromatase cytochrome P450 (P450arom), a critical component in ovarian differentiation. In this study, mRNA expression of P450arom was used to identify presumptive males and females exposed to a gradient of temperatures (18, 21, 24, 27, and 30°C) beginning at 35–40 or at 65–70mm total length (TL). This research identifies that sex is influenced by temperature between 35 and 65mm TL and establishes that increases in temperature from 18°C during this size range, produce increasingly male skewed sex ratios in southern flounder from Texas. The findings presented here are essential for understanding population dynamics of southern flounder and the potential effects climate change may have on wild populations.

Elevated mortality among birds in Chernobyl as judged from skewed age and sex ratios.

BackgroundRadiation has negative effects on survival of animals including humans, although the generality of this claim is poorly documented under low-dose field conditions. Because females may suffer disproportionately from the effects of radiation on survival due to differences in sex roles during reproduction, radiation-induced mortality may result in male-skewed adult sex ratios.Methodology/Principal FindingWe estimated the effects of low-dose radiation on adult survival rates in birds by determining age ratios of adults captured in mist nets during the breeding season in relation to background radiation levels around Chernobyl and in nearby uncontaminated control areas. Age ratios were skewed towards yearlings, especially in the most contaminated areas, implying that adult survival rates were reduced in contaminated areas, and that populations in such areas could only be maintained through immigration from nearby uncontaminated areas. Differential mortality in females resulted in a strongly male-skewed sex ratio in the most contaminated areas. In addition, males sang disproportionately commonly in the most contaminated areas where the sex ratio was male skewed presumably because males had difficulty finding and acquiring mates when females were rare. The results were not caused by permanent emigration by females from the most contaminated areas because none of the recaptured birds had changed breeding site, and the proportion of individuals with morphological abnormalities did not differ significantly between the sexes for areas with normal and higher levels of contamination.Conclusions/SignificanceThese findings are consistent with the hypothesis that the adult survival rate of female birds is particularly susceptible to the effects of low-dose radiation, resulting in male skewed sex ratios at high levels of radiation. Such skewed age ratios towards yearlings in contaminated areas are consistent with the hypothesis that an area exceeding 30,000 km2 in Chernobyl’s surroundings constitutes an ecological trap that causes dramatic excess mortality.

Optimizing laboratory production of Strelkovimermis spiculatus (Nematoda: Mermithidae) with a discussion of potential release strategies for mosquito biological control

We assessed Strelkovimermis spiculatus preparasite (J2) infectivity and postparasite (J4) production in rearing pans using a laboratory host, second-instar Aedes aegypti mosquitoes, and evaluated first the production efficiency by testing increasing J2 concentrations (preparasite:host ratios). At 10 000 host larvae/rearing pan, we obtained the maximum yield of J4s (3.50 g), number of J4s/larva (3.1), and parasitism frequency (94.9 ± 9.5%) at a 10:1 J2/larva ratio. This 10:1 ratio, however, resulted in a significantly male-skewed sex ratio, whereas at ratios between 4:1 and 7:1 females predominated and parasite frequencies were still acceptable. Next, postparasite-inoculation methods were compared under culture conditions reflecting two categories of mosquito habitat (permanently flooded or desiccated cultures). The J4s were left to develop into adults, mate, and oviposit in the substrate. The flooded-culture J2s hatched at week 3 and after 30 weeks gave a total production of 6.2 × 104 per pan. Maximum J2 yield in desiccated cultures occurred at the first flooding (5 weeks), but emergence continued through drying flooding cycles for 10 weeks, giving a total J2 production of 4.5 × 105 per pan. The J2s infected mosquito larvae under both conditions. S. spiculatus preparasites or postparasites would serve as mosquito bioregulatory agents in temporary or permanent mosquito habitats. Relevant strategies are discussed.

Ethiopian village dogs: Behavioural responses to a stranger's approach

We studied the behavioural ecological characteristics of free-roaming dogs ( Canis familiaris) in four Ethiopian villages via observational surveys. The Ethiopian village dogs surveyed in this study have similar characteristics to other free-roaming dog populations in the world: (1) they are almost entirely unrestrained, (2) there is a male-skewed sex ratio in the adult population, (3) the majority appear not to be owned (or at least not declared so) and (4) many of them are found around people's dwellings. We also evaluated ‘village’ dogs’ attitudes towards people using a stranger-approach test. Eighty-five dogs were approached by an unfamiliar Caucasian woman in different contexts and the dogs’ vocal behaviour before the approach, reaction to the approach and approaching distance between dogs and observer were systematically recorded. We found variability in village dog behavioural responses to a stranger depending on dogs’ location (inside house, outside house, street), social setting (alone, pair, group) and village. Dogs inside homes and dogs that were alone were more likely to vocalize towards the observer than dogs in the street. Avoiding was the most common reaction exhibited by village dogs, especially when approached in the streets, suggesting that most of them are shy of people. However, 11% responded aggressively towards the observer; all of these dogs vocalized towards the observer before she approached them. One third of the dogs maintained a neutral attitude apparently ignoring the observer. Slightly less than one third of village dogs could be approached closely (0–2 m), one third could be approached at a medium distance (2–5 m), and over one third could not be approached at all (>5 m). Altogether, our findings showed that the dog populations in the villages we studied were not behaviourally homogeneous, within and between villages. Our study suggests that behavioural investigations are useful for characterizing free-roaming/village dogs’ attitudes, especially when assessing any risk dogs pose to the human population. Our observations suggest that people might have greater chances of being bitten by dogs that: vocalize towards people, are alone or in pairs, and are found inside households. Our “approach test” proved to be a simple, effective way of measuring dog attitudes, which could be easily implemented in different parts of the world by training local people. Quantifying behavioural responses of village dogs towards people would be particularly important in areas of the world where potentially fatal dog-transmitted diseases, such as rabies, are still prevalent.

Reproductive effort and costs of reproduction do not explain female‐biased sex ratios in the moss Pseudocalliergon trifarium (Amblystegiaceae)

A fundamental assumption in life-history theory is that reproduction is costly. Higher reproductive investment for fruits than for flowers may result in larger costs of reproduction in females than in males, which is often used to explain male-skewed sex ratios in unisexual seed plants. In contrast, bryophytes have predominantly female-biased sex ratios, suggested to be a product of a higher average cost of sexual reproduction in males. Empirical evidence to support this notion is largely lacking. We investigated sex-specific reproductive effort and costs in the unisexual moss Pseudocalliergon trifarium that has a female-dominated expressed sex ratio and rarely produces sporophytes. Annual vegetative segment mass did not differ among male, female, and non-expressing individuals, indicating that there was no threshold-size for sex expression. Mean and annual mass of sexual branches were higher in females than in males, but branch number per segment did not differ between sexes. Prefertilization reproductive effort for females was significantly greater (11.2%) than for males (8.6%). No cost for sexual branch production in terms of reduced relative vegetative growth or decreased investment in reproductive structures in consecutive years was detected. A higher realized reproductive cost in males cannot explain the unbalanced sex ratio in the study species.

Cloning of the promoter from the gonadal aromatase gene of the European sea bass and identification of single nucleotide polymorphisms

Aromatase contributes to sex differentiation by catalysing the conversion of androgens into estrogens. We have cloned the promoter of the gonadal aromatase gene from the sea bass, Dicentrarchus labrax, a species whose farming is complicated by male-skewed sex ratios. The promoter shows a conserved binding site for SF-1 as well as two cAMP response elements (CREs) and putative binding sites for transcription factors belonging to the Sox and forkhead families. Analysis of promoter sequences from individual fish suggests the presence of three promoter alleles that arise due to three single nucleotide polymorphisms (SNPs) in linkage disequilibrium.

When prey mating increases predation risk: the relationship between the flatworm Mesostoma ehrenbergii and the copepod Boeckella gracilis

The zooplanktivorous flatworm Mesostoma ehrenbergii and the calanoid copepod Boeckella gracilis were observed to coexist in Patagonian fishless ponds. In laboratory experiments, we studied the vulnerability of B. gracilis to M. ehrenbergii predation, testing the attack rates on copulating pairs and single adults in different abundances. We also determined B. gracilis dimorphism, sex ratio and copulating pair ratio on two occasions in a temporary pond, with and without M. ehrenbergii. Our results indicated that B. gracilis exhibited a male-skewed sex ratio irrespective of the presence of the predator. A marked dimorphism characterized this copepod species (females are about 40 % larger than males) and a large proportion of adults were observed participating in copulating pairs that lasted for days. M. ehrenbergii ate sim- ilar quantities of single males and females of B. gracilis but significantly more copu- lating pairs. The use of mucus threads allowed Mesostoma to ingest both members of the pairs instead of only one in most attacks. Larger prey may create more turbulence in the water while swimming, so the hydrodynamic signals produced by pairs should be greater than those produced by single individuals, making them more vulnerable. Besides, the attack rates obtained in the different prey abundances showed that en- counter rate is the factor that determines M. ehrenbergii predation. We suggest that B. gracilis prolonged mating duration (days) is dangerous because it increases predation risk.

Mate Guarding in a Population of White-Fronted Chats, Ephthianura-Albifrons Jardine and Selby (Passeriformes, Ephthianuridae) - a Response to Group Living and a Male-Skewed Sex-Ratio

Mate-guarding behaviour and three attributes considered to affect its intensity (nesting dispersion, territoriality and sex ratio) were studied in a population of white-fronted chats. All-purpose territories were not defended and there was a tendency for nests to be clustered. A 1.7:1 male-skewed sex ratio was evident, with males accounting for 83% of birds in flocks during the breeding season. Paired males followed their mates prior to and during nest building, and attacked other males that approached their mates. Males copulated with dummy females which they defended from dummy and live males. Mate guarding protects males from cuckoldry and perhaps prevents loss of females to other males during a breeding season characterised by multiple broods.

POPULATION DENSITY AND SEX RATIO DYNAMICS OF OVERWINTERING MAIZE WEEVILS (COLEOPTERA: CURCULIONIDAE) INFESTING FIELD CORN

Natural infestations of the maize weevil, Sitophilus zeamais (Motschulsky) were monitored in selected corn fields located in each of five major climatic or edaphic environments of Georgia. The population density and sex ratio of each population was recorded on a monthly basis during the fall and winters of 1982–83–84. Females suffered higher winter mortality than males, resulting in increasingly male-skewed sex ratios (from 0.8/1.0 to 1.6/1.0) as winter progressed. The degree of male-skewedness was positively correlated with the amount of subzero weather experienced by each population. Surviving weevil populations decreased in size as winter progressed, with the greatest reductions at the higher latitudes. No weevils survived through spring north of 33° 57′N where winter weather extremes of −15°C occurred. No true diapausing stage was found; however, cold tolerance tests indicated that winter-collected weevils were significantly more cold hardy than laboratory-reared weevils.

Sex ratio regulation in Brachymeria intermedia, a pupal gypsy moth parasitoid

The solitary pupal gypsy moth endoparasitoid, Brachymeria intermedia (Nees) (Hymenoptera: Chalcididae), regulates the sex ratio of its progeny such that the ratio is generally female skewed. Parents with a male-skewed sex ratio produce progeny that are female biased more than parents with a female-skewed sex ratio. Variables such as host size, host density, and intraspecific competition played no role in determining the sex ratio of the progeny. We conclude that sex ratio regulation of the progeny in this species is a function of male density.

Invasion Patterns and Sex-ratio Dynamics of the Maize Weevil (Coleoptera: Curculionidae) Infesting Field Corn

Field populations of maize weevils, Sitophilus zeamais Motschulsky, were monitored during the 1981, 1982, and 1983 growing seasons in 19 cornfields located in the Georgia Piedmont and Coastal Plain. Colonizing weevils were predominately males (1.6 males:1.0 females) with the weevils per corn ear and weevil-infested corn ears clumped. Superior colonizing capabilities of males in conjunction with poor overwintering abilities of females are proposed to cause the male-skewed sex ratio. F1 and laboratory weevil populations were composed primarily of females (0.6 males:1.0 females), indicating female skewedness may be the common condition under periods of continuous reproduction.

Female infanticide, European diseases, and population levels among the Mackenzie Dene

Selective female infanticide by the Dene hunting tribes of Subarctic Canada is noted in the historical and ethnographic record. Unusually extensive and complete census data on two generations of Mackenzie Dene in the infanticide era manifest male-skewed sex ratios that suggest that the number of females in those generations was reduced through infanticide by 20 percent or more. These and later census data falsify the assumption of depopulation of the Mackenzie Dene by exogenous diseases in the historic era, rendering invalid the depopulation theory on which arguments for the “loss” of putative aboriginal unilocal-unilineal organization have been based. Diagrammatic population models are employed to explore the capacity of the population to sustain mortality from introduced diseases when counterbalanced by the elimination of selective female infanticide as a mortality factor. [infanticide, population, hunter-gatherers, subarctic Indians, Northern Athapaskans]

Survival and Longevity of the Little Brown Bat (Myotis lucifugus) in Southeastern Ontario

Survival and longevity in a hibernating population of Myotis lucifugus were studied for 35 years with mark-recapture techniques. Two males were recaptured 29 and 30 years after being banded; these two bats showed no outward signs of senescence. Survival rates were calculated with Cormack's stochastic technique. Average annual survival rates were 0.816 for males and 0.708 for females. Climatic and geographic factors may account for differences in survival rates between populations in Ontario and in southern Indiana, where female survival rate exceeds male. The differences in survival rate between the sexes are not alone sufficient to account for the male-skewed sex ratios in hibernating populations.