Sex ratios

Sex ratio theory attempts to explain variation at all levels (species, population, individual, brood) in the proportion of offspring that are male (the sex ratio). In many cases this work has been extremely successful, providing qualitative and even quantitative explanations of sex ratio variation. However, this is not always the situation, and one of the greatest remaining problems is explaining broad taxonomic patterns. Specifically, why do different organisms show so much variation in the amount and precision with which they adjust their offspring sex ratios?     

Sex ratios in bumble bees

The median proportion of investment in females among 11 populations of seven bumble bee (Bombus) species was 0.32 (range 0.07 to 0.64). By contrast, two species of workerless social parasites in the related genus Psithyrus had female-biased sex allocation, the reasons for which remain unclear. Male-biased sex allocation in Bombus contradicts the predictions of Trivers and Hare''s sex ratio model for the social Hymenoptera, which are that the population sex investment ratio should be 0.5 (1:1) under queen control and 0.75 (3:1 females:males) under worker control (assuming single, once-mated, outbred queens and non-reproductive workers). Male bias in Bombus does not appear to be either an artefact, or purely the result of symbiotic sex ratio distorters. According to modifications of the Trivers–Hare model, the level of worker male-production in Bombus is insufficient to account for observed levels of male bias. There is also no evidence that male bias arises from either local resource competition (related females compete for resources) or local mate enhancement (related males cooperate in securing mates). Bulmer presented models predicting sexual selection for protandry (males are produced before females) in annual social Hymenoptera and, as a consequence (given some parameter values), male-biased sex allocation. Bumble bees fit the assumptions of Bulmer''s models and are protandrous. These models therefore represent the best current explanation for the bees'' male-biased sex investment ratios. This conclusion suggests that the relative timing of the production of the sexes strongly influences sex allocation in the social Hymenoptera.     

Sexuality and hermaphroditism in fishes. I. Synchronous functional hermaphroditism in the serranid fish Serranus scriba L

Anatomical and histological examination of Serranus scriba L. showed the existence of primary females (67%), hermaphrodites (31%) and primary males (2%). Synchronous functional hermaphroditism is described on the basis of an anatomical and histological study of the gonads. Although they function simultaneously, the testicular and ovarian parts of hermaphrodite gonads have completely separate ducts. Females and hermaphrodites have the same annual reproduction cycle. In hermaphrodites, the testicular part matures one month sooner than the ovarian part. Cross fertilization between primary females and hermaphrodite individuals and between two different hermaphrodites probably occurs, while self-fertilization is less likely. The testicular tissues of primary males are of the acinar type and those of hermaphrodites are of the radial type. It is possible that primary males do not take part in reproduction. Serranus scriba in Egyptian Mediterranean waters is a longperiod spawner, which spawns from June to the end of October, i.e. it is a summer-autumn spawner.     

Sex ratios in geographically structured populations

In his book on sexual selection (1), Darwin documented evidence that the primary sex ratio (the proportion of males at conception) is about 1/2 in a wide variety of species. Otherwise, he explained, a newly conceived member of the rare sex will, on average, have more offspring than one of the common sex, since each offspring has one mother and one father; thus there is frequency-dependent selection in favour of parents producing the rare sex. Darwin formulated this explanation in the first edition (1871) for monogamous species, but he failed to extend it to polygamous species, and in the second edition (1874) he retracted it completely. It was left to Fisher (2) to develop the theory in the more general form that there should be equal parental expenditure on the two sexes, allowing for the possibility that one sex may cost more to produce than the other. Despite the wide applicability of Fisher's principle, recent work on sex ratio evolution has focused on situations where it breaks down (3). Hamilton (4) first pointed out that Fisher's argument assumes population-wide competition for mates, whereas most natural populations have a geographical population structure in which limited dispersal imposes constraints on mating patterns. What are the consequences for the sex ratio?     

Borrowing contextual inflection: evidence from northern Australia

Gurindji Kriol is a north Australian mixed language which combines lexical and structural elements from Gurindji (Pama-Nyungan), and Kriol (English-lexifier). One of the more striking features of the grammar of Gurindji Kriol is the presence of the Gurindji case paradigm including ergative and dative case-markers within a Kriol verbal frame. Given the fragility of inflectional morphology in other language contact situations, particularly contextual inflections such as structural case markers, this situation bears closer scrunity. This paper argues that the presence of Gurindji case morphology is the result of pervasive code-switching practices which immediately preceded the genesis of the mixed language. As the code-switching stabilised into a mixed language, case-marking was integrated into predicate argument structure of Gurindji Kriol via nominal adjunct structures. Yet, these case markers were not absorbed unscathed. Although the Gurindji Kriol case paradigm bears a close resemblance to its Gurindji source in form, these case markers have not been perfectly replicated in function and distribution. Contact with Kriol functional equivalents such as prepositions and word order have altered the function and distribution of these case markers. The last part of this paper examines the shift that has occurred in Gurindji-derived case morphology in Gurindji Kriol.     

Sex change and sequential hermaphroditism in Tegillarca granosa (Bivalvia: Arcidae)

This study aimed to confirm sex change in Tegillarca granosa to determine whether this species is a sequential hermaphrodite. Samples of bivalves were divided into two groups, namely, a 1+ year class (14 months) and a 2+ year class (26 months), for analysis. At the beginning of the study, 44.3% of T. granosa were female, and after one year, this increased to 53.9% in the same population. The increase of females in the population was greater in the 1+ year class (12.3%), when compared with the 2+ year class (6.5%). Overall, a sex change ratio of 15.1% (n = 104/688) was recorded for T. granosa. The sex change ratio of the 1+ year class (17.8%) was higher than that of the 2+ year class (12.1%), and displayed the tendency of being higher in the males (21.2%), than the females (6.2%). The results of this study indicate that T. granosa is a sequential hermaphrodite.     

Sex ratios among Canadian liveborn infants of mothers from different countries

Background:

There has been much discussion about whether female feticide occurs in certain immigrant groups in Canada. We examined data on live births in Ontario and compared sex ratios in different groups according to the mother’s country or region of birth and parity.

Methods:

We completed a population-based study of 766 688 singleton live births between 2002 and 2007. We used birth records provided by Ontario Vital Statistics for live births in the province between 23 and 41 weeks’ gestation. We categorized each newborn according to the mother’s country or region of birth, namely Canada (n = 486 599), Europe (n = 58 505), South Korea (n = 3663), China (n = 23 818), Philippines (n = 15 367), rest of East Asia (n = 18 971), Pakistan (n = 18 018), India (n = 31 978), rest of South Asia (n = 20 695) and other countries (n = 89 074). We calculated male:female ratios and 95% confidence intervals (CIs) for all live births by these regions and stratified them by maternal parity at the time of delivery (0, 1, 2 or ≥ 3).

Results:

Among infants of nulliparous women, the male:female ratio was about 1.05 overall. As parity increased, the ratio remained unchanged among infants of Canadian-born women. In contrast, the male:female ratio was significantly higher among infants of primiparous women born in South Korea (1.20, 95% CI 1.09–1.34) and India (1.11, 95% CI 1.07–1.15) than among infants of Canadian-born primiparous women. Among multiparous women, those born in India were significantly more likely than Canadian-born women to have a male infant (parity 2, ratio 1.36, 95% CI 1.27–1.46; parity ≥ 3, ratio 1.25, 95% CI 1.09–1.43).

Interpretation:

Our study of male:female ratios in Ontario showed that multiparous women born in India were significantly more likely than multiparous women born in Canada to have a male infant.Although there are some myths about correctly guessing the sex of a fetus,¹ modern-day prenatal ultrasound enables the identification of whether a fetus is a boy or girl with 99% accuracy.² There has been much discussion about whether female fetuses are at higher risk of pregnancy termination than male fetuses in certain ethnic groups. In India, a study of data from the National Family Health Survey for 265 516 births showed a sharp increase in the male:female ratio among second-order births when the firstborn was a girl, and no significant increase when the firstborn was a boy.³ The authors attributed this trend to the practice of selective abortion of female fetuses. A recent editorial⁴ and news item⁵ in CMAJ suggested that female feticide may also be occurring in Canada.⁶ Rather than using live-birth statistics, the Canadian study cited in CMAJ used data from the 2001 and 2006 Canada Census long-form questionnaires, which were completed by 20% of the population and relied on self-reporting of additional information, including the number of family members in a given household.We used contemporary data on live births in Ontario, Canada’s most populous and ethnically diverse province, and compared sex ratios among infants of Canadian-born women with sex ratios in different immigrant groups. We focused on immigrant groups from countries purported to have the highest rates of preference for a son following the birth of one or more daughters.^3–6 We determined whether the male:female ratio increased with increasing parity in certain immigrant groups, as has been previously suggested.^3,6     

Nuclear RFLP variation in Eucalyptus camaldulensis Dehnh. from northern Australia

Eucalyptus camaldulensis Dehnh. is the most widely planted eucalypt in the tropics. Natural populations are riparian and sampling strategies for breeding programmes have assumed that gene flow among drainage basins is limited. RFLP variation, within and among 31 populations from river systems across northern Australia, was analysed to test this hypothesis. To allow comparisons within and between river systems, trees were sampled from up to three populations per river system. Allele frequencies were correlated with longitude for more than half the 33 RFLP loci surveyed. Genetic identity was greatest between populations in closest geographic proximity, irrespective of river system, suggesting that sampling strategies for breeding programmes should be based on geographic distance rather than river system. The level of genetic variation was similar throughout the geographic range examined (mean H(E) = 0.49). However, there was evidence of a barrier to gene flow between populations in the east and west of the species range. The RFLP data support morphological evidence of hybridisation between E. camaldulensis and E. tereticornis Sm. in several populations in northeast Queensland and the genetic divergence of E. camaldulensis subsp. simulata Brooker and Kleinig.     

Sex ratios observed in 80 species of parrots

A number of potential evolutionary and physiological factors may be involved in avian sex ratio bias so that under certain conditions a sex ratio bias may favour males or females within a population. In addition different factors may be important in manipulating sex ratio bias through the different life stages. In this study sex ratio bias was examined in a total of 16 570 captive parrots, representing 80 species, many of which are endangered in the wild, using database records originating form commercial laboratories that offer genetic sexing. Within the species examined 72% showed a male bias this was significant in three species, when adjusted for multiple comparisons. This preliminary study is limited due to lack of data on the age of the individuals sampled. However, the large dataset do suggest that this phenomenon should be further considered by investigators working at a species level where such data can be collected.     

Sex ratios: human twins and fraternal effects

Historical data from Finnish populations reveals that, for females, exposure to a male twin in the womb may have significant, life-long, effects on subsequent fitness, with profound implications for the evolution of sex ratios and brood size.     

Sex ratios in dairy cattle under various conditions

Current calving information was obtained on 35,102 single births in 2254 dairy herds. The overall proportion of males to females was 50.8%. The 5 dairy breeds did not differ. Only 6 of 111 sires studied produced calves with a sex ratio different from breed average at P≤0.5. This is the number expected by chance alone. A slight bias seems to occur when reporting the sires of the cows according to the sex of the cow's calf. The sex ratio deviated from expected in a small sample of repeat breeder cows, but when a new and larger sample of 2,084 such cows which calved was obtained, there was no change associated with service number. The time of insemination was recorded for 12,764 heifers and cows first seen in estrus in the morning and 4,799 animals first seen in estrus in the evening. There was no effect of time of insemination on sex ratio. Likewise there was no effect of age of cows or season of breeding on sex ratio at birth. Because the sex ratio for cows requiring one insemination per pregnancy was not different from repeat breeders it is suggested that the sex ratio at fertilization and birth may be similar.     

A new species of psychrolutid fish from western Australia

Psychrolutes occidentalis, a new species of psychrolutid fish, is described from mud bottoms at depths of 350–740 m on the continental slopes around Rowley Shoals, northwestern Australia. The new species is illustrated and compared with other species of the genusPsychrolutes. It differs from other species in the following characters: 14–15 dorsal soft rays, 21–23 pectoral rays, 30–31 vertebrae, no cirri on head and body, lateral line not emerging through tubes, and reddish brown head and body color with darker brown marblings. Aspects of the zoogeography of this new species from the tropical region and of the other species ofPsychrolutes are discussed.     

Two new species ofSilhouetted (Gobiidae) from northern Australia

Two new species of the tropical Indo-Pacific gobiid genusSilhouetted, S. evanida andS. hoesei, are described from Darwin and Queensland, and the Cobourg Peninsula, N.T., respectively.S. evanida occurs in pools on intertidal sand flats along beaches and sandy creek mouths, andS. hoesei more sublittorally at 5–6 m on silty sand. A key to western Pacific and Australian species is included.     

Sex ratios and the evolution of eusociality in the hymenoptera

The potential role of sex ratio biassing in the evolution of worker behaviour in male-haploid hymenopteran insects is examined using a deterministic genetic model. The model is based on a bivoltine life cycle with annual colonies and it assumes five gene loci, each of them controlling a specific feature of the life cycle (particularly brood sex ratios). The hypothetical gene controlling worker behaviour is assumed to be expressed either in the mothers (parental manipulation models) or in the female offspring (offspring altruism models). The threshold of the worker efficiency required for the worker behaviour to evolve is 0.5 under parental manipulation and 1.0 under offspring altruism when the sex ratios are not skewed. Worker evolution by offspring altruism can evolve more easily if the first workers initially raise mainly female brood. With such a sex ratio bias, the threshold of worker efficiency allowing eusociality to evolve drops below 1.0, even close to 0.8. Worker evolution is also favoured by the elimination of males from the first of the two annually occurring offspring generations. It is concluded that the male-haploid sex determination can, through the control of sex ratios, play a significant role in the evolution of eusociality in hymenopteran insects.     

Sex ratios and sexual selection in socially monogamous zebra finches

An experiment was performed in which adult sex ratios of zebrafinches, Taeniopygyia guttata castanotis, were varied to testpossible effects of adult population sex ratios on sexual selectionintensity and mating system dynamics in species with biparentalcare. The possibility that sex ratio influences the successof social mating patterns (leading to polygyny when males arerare and polyandry when females are rare) was not supported.Results did support the prediction of the differential allocationhypothesis that individuals of the abundant sex would increasetheir relative parental expenditure (PE). Although total (male+ female) PE did not vary between treatments, relative malePE was significantly higher in the male-biased treatment (MBT;sex ratio 64% male) than in the female-biased treatment (FBT; sexratio 36% male). In both treatments, male PE contributions contributedto female reproductive rate. Results also supported the predictionof the differential access hypothesis that individuals of theabundant sex would experience greater intensity of selectionon sexually selected attributes. Male beak color, a sexuallyselected trait, influenced male social parentage in the MBTbut not in the FBT. Finally, broods in the FBT displayed higher hatchingasynchrony and lower hatching success; we believe this was causedby early onset of incubation, a tactic used as a defense againstintraspecific brood parasitism, which was much higher in theFBT. Population sex ratios may be an important factor affectingfemale ability to influence male parental investment patterns.