Within clutch co-variation of egg mass and sex in the black-headed gull

Female birds of several species have control over the production of daughters and sons. However, most studies failed to find a relationship between egg size and sex. This is intriguing as adjustment of egg size would constitute a powerful tool for the female to meet different resource demands of the sexes, particularly in size dimorphic species. Our results show that, within clutches of black-headed gulls (Larus ridibundus) the proportion of males was positively associated with egg mass. This applied for all three laying positions, independently of the absolute egg mass. There was a significant relationship between the distribution of the sexes over the laying sequence and the egg mass change. When egg mass decreased over the sequence, first-laid eggs were male biased and last-laid eggs female biased, and vice versa. The potential adaptive value of this allocation strategy is evaluated with regard to male sensitivity to egg quality and competitive differences between the sexes.

Effects of 17-beta-estradiol treatment of female zebra finches on offspring sex ratio and survival

Treatment of female zebra finches (Taeniopygia guttata) with 17-beta-estradiol leads to a female-biased sex ratio in their offspring at the age of independence [Horm. Behav. 35 (1999) 135]. It is unclear whether this is due to a bias of the primary sex ratio or to sex-specific survival. We replicated this experiment and found again a significantly higher total number of daughters than sons at independence in the estradiol-treated group. This was due to higher embryonic survival of daughters compared with sons in the estradiol-treated group and the reverse in the control group. There was no effect of the hormone treatment on the primary sex ratio. Treatment with 17-beta-estradiol led to a significantly shorter hatching time and to heavier offspring at day 7 after hatching. This weight was correlated with maternal plasma estradiol levels on the day of the first egg, which were significantly higher in the estradiol-treated group than in the control group. The results do not support the idea that maternal estradiol levels influence the primary sex ratio. They indicate that maternal estradiol differentially affects survival of sons and daughters via an influence on the embryonic environment, possibly enhancing offspring growth.

Maternal androgens in egg yolks: relation with sex, incubation time and embryonic growth

Hormones of maternal origin are known to be transferred to the egg yolks of oviparous species. Several studies have shown that within and between clutch variation of maternal androgens may be adaptively tuned. Moreover, it has recently been hypothesized that sex steroids of maternal origin may play a role in adaptive sex ratio manipulation. For sex determination the eggs have to be incubated to allow the germinal disc to grow and thus extract sufficient DNA. This means that yolk hormone levels are determined after a number of days of incubation and this may hamper interpretation of the data. If yolk utilization or embryonic hormone production are influenced by the sex of the embryo, differences in hormone content at a certain stage of incubation do not reflect the mother's initial allocation. In this experiment we show that testosterone levels in chicken eggs do not change with incubation period. A4 levels decrease between 3 and 5 days of incubation, which we cannot explain. Male eggs did not contain higher levels of testosterone or androstenedione than female eggs, in contrast to the data reported for another galliform species, the peacock. We conclude that it is unlikely that maternal androgens are a key factor in the avian sex determination mechanism.

Genetic and environmental components of variation in eumelanin and phaeomelanin sex-traits in the barn owl

Knowledge of the mechanism underlying the expression of melanin-based sex-traits may help us to understand their signalling function. Potential sources of inter-individual variation are the total amount of melanins produced but also how biochemical precursors are allocated into the eumelanin and phaeomelanin pigments responsible for black and reddish-brown colours, respectively. In the barn owl (Tyto alba), a eumelanin trait (referred to as 'plumage spottiness') signals immunocompetence towards an artificially administrated antigen and parasite resistance in females, whereas a phaeomelanin trait ('plumage coloration') signals investment in reproduction in males. This raises the question whether plumage coloration and spottiness are expressed independent of each other. To investigate this question, we have studied the genetics of these two plumage traits. Crossfostering experiments showed that, for each trait, phenotypic variation has a strong genetic component, whereas no environmental component could be detected. Plumage coloration is autosomally inherited, as suggested by the similar paternal-to-maternal contribution to offspring coloration. In contrast, plumage spottiness may be sex-linked inherited (in birds, females are heterogametic). That proposition arises from the observation that sons resembled their mother more than their father and that daughters resembled only their father. Despite plumage coloration and spottiness signalling different qualities, these two traits are not inherited independent of each other, darker birds being spottier. This suggests that the extent to which coloration and spottiness are expressed depends on the total amount of melanin produced (with more melanin leading to a both darker and spottier plumage) rather than on differential allocation of melanin into plumage coloration and spottiness (in such a case, darker birds should have been less spotted). A gene controlling the production of melanin pigments may be located on sex-chromosomes, since the phenotypic correlation between coloration and spottiness was stronger in males than in females.

A female melanin ornament signals offspring fluctuating asymmetry in the barn owl

Sexual selection theory predicts that males advertise quality by displaying extravagant ornaments. By contrast, whether phenotypic variation in females has a signalling function remains an open question. Here, to our knowledge, we provide the first evidence that a female plumage trait can signal fluctuating asymmetry in the offspring. We experimentally demonstrate in wild barn owls (Tyto alba) that the extent to which females display black spots on their plumage does not only signal offspring parasite resistance as shown in a previous study but also developmental homeostasis in the offspring. A greater number of spotted females produced offspring that had more symmetrical feathers during the period of growth. Males, that pair non-randomly with respect to female plumage spottiness therefore appear to gain substantial benefits by mating with heavily spotted females. Genetic variation in plumage spottiness is nevertheless maintained as the covariation between offspring body mass and mother plumage spottiness varies annually depending on environmental conditions.

Sex differences in yolk hormones depend on maternal social status in Leghorn chickens (Gallus gallus domesticus)

Maternal hormones are known to be present in avian eggs and can have beneficial effects on chick development. Recently, differences in avian yolk steroid concentrations between the sexes have been demonstrated, and in this context steroids have been proposed to be part of the avian sex-determining mechanism. In our study, we show that it is very unlikely that androgen concentrations alone are the decisive part of the sex-determining mechanism. We found that sex-specific differences in the yolk hormones strongly depend on the social rank of the mother. First, dominant females, but not subdominant females, allocated significantly more testosterone to male eggs than to female eggs. Second, subordinate females increased the testosterone concentrations of female eggs. This pattern of yolk hormone deposition can be functionally explained. In polygynous species such as the chicken, reproductive success is more variable in males than in females. Parental investment in sons or daughters is therefore expected to occur in direct relation to parental rearing capacities. We found that the social status of a hen was indeed negatively correlated with her maternal capacities (for example, body mass, egg mass). Differential androgen deposition might thus provide a mechanism for adaptive maternal investment depending on both the sex of the egg and the social status of the mother.

Female barn owls (Tyto alba) advertise good genes

The good genes hypothesis of sexual selection postulates that ornamentation signals superior genetic quality to potential mates. Support for this hypothesis comes from studies on male ornamentation only, while it remains to be shown that female ornamentation may signal genetic quality as well. Female barn owls (Tyto alba) display more black spots on their plumage than males. The expression of this plumage trait has a genetic basis and it has been suggested that males prefer to mate with females displaying more black spots. Given the role of parasites in the evolution of sexually selected traits and of the immune system in parasite resistance, we hypothesize that the extent of female plumage 'spottiness' reflects immunological defence. We assessed the genetic variation in specific antibody production against a non-pathogenic antigen among cross-fostered nestlings and studied its covariation with the plumage spottiness of genetic parents. The magnitude of the antibody response was positively correlated with the plumage spottiness of the genetic mother but not of the genetic father. Our study thereby provides the first experimental support, to our knowledge, for the hypothesis that female ornamentation signals genetic quality.

Energy requirements for growth in relation to sexual size dimorphism in marsh harrier Circus aeruginosus nestlings

Food consumption was measured in six female and seven male hand-raised marsh harrier (Circus aeruginosus) nestlings. Females consumed on average 4,321 g and males consumed 3,571 g of food during the nestling stage from 0 to 36 d. Total consumption until 56 d was 6,960 g and 5,822 g for females and males, respectively. On the basis of Fisher's sex ratio theory, this food intake ratio of 0.46 (intake male/[intake male + female]) would explain the observed male-biased fledging sex ratio of 55% males in marsh harrier broods. Growth, gross energy intake, and metabolizable energy intake were measured, along with metabolism of the nestlings, enabling us to determine energy allocation. The assimilation quotient (Q = 0.72) did not differ systematically between the sexes. Differences in metabolic rates between males and females at 15 and 30 d of age were fully attributable to the difference in body mass. Sexual size dimorphism in marsh harriers (female body mass around 60 d of age is 1.28 times greater than male mass) did not fully explain the difference in food intake between male and female nestlings: an analysis of energy requirements for growth and body mass in 16 avian species shows that energy intake was less than proportional to the average body mass at release. The data presented in this study are in agreement with Fisher's theory of inverse proportionality between the sex-specific ratios of energy requirements for growth and of offspring numbers in the marsh harrier population.

Sex identification in birds using two CHD genes

In theory, birds should control the sex ratio of the offspring they produce. In practice, we have very limited evidence to support this idea because of our difficulty in sexing nestling birds. In addition, extinction is facing an increasing number of birds. Our ability to help includes captive breeding which, again, is difficult if male and female adults cannot be recognized. Here we describe the discovery of a W-linked gene in the Great tit (Parus major). It is named CHD-W (chromodomain-helicase-DNA-binding protein W-linked), it is highly conserved and it is W-chromosome linked in a range of bird species. These birds also possess a second, non-W-linked CHD gene (CHD-NW). A single, simple polymerase chain reaction technique based on both genes can be used to identify the sex in a wide variety of birds.

Mononuclear cells in glomeruli and cytokines in urine reflect the severity of experimental proliferative immune complex glomerulonephritis

Immunohistochemical methods were used to investigate the role of macrophages in the progression of proliferative immune complex glomerulonephritis. The mononuclear cell component of glomerular inflammation was analysed in three different stages of chronic serum sickness, each of which was clearly distinguished by criteria of kidney function. Urinary excretion of the macrophage secretory products interleukin-1 and tumour necrosis factor was also evaluated in relation to the functional severity of kidney disease. T lymphocytes and macrophages began to accumulate in glomeruli at the onset of proteinuria, but not before. Urinary excretion of interleukin-1 also began with proteinuria. Proteinuria increased in direct correlation with increases in the number of glomerular macrophages. Development of the most severe stage of glomerulonephritis, characterized by cachexia, declining kidney function, and necrotizing glomerular pathology, was accompanied by the disappearance of T cells from glomeruli and the expression of highly abnormal phenotypes by most macrophages. In addition, there was a switch from urinary excretion of interleukin-1 to excretion of tumour necrosis factor. The progression of proliferative immune complex glomerulonephritis was associated with qualitative as well as quantitative changes in glomerular macrophage populations. Differentiation and/or activation of those glomerular macrophages may have resulted from local T cell-mediated immunoregulation. Measurements of urinary cytokine excretion provided a reliable means of monitoring disease progression. The local action of tumour necrosis factor probably contributed to declining kidney function in the most severe stage of disease.

Family Planning in the Kestrel (Falco Tinnunculus): the Ultimate Control of Covariation of Laying Date and Clutch Size

Abstract

The theory that individual birds maximize their fitness by the two major decisions in reproduction concerning date (when to start laying eggs) and clutch size (when to stop laying eggs) is empirically approached in the Kestrel by quantifying Fisher's Reproductive Value for both the clutch (Vc = c. Vo/2) and the parents (Vp). The reproductive value of an egg (Vo) was found to decrease monotonically with laying date (d) due to significant associations with d of the components So (probability for an egg to survive till fledging), S1 (probability to survive from fledging till age 1), S2 (survival age 1 till age 2), and P1 (probability of breeding at age 1). Vp declined negligibly with laying date, although there were significant associations between d (laying date) and N (probability of the nest to produce at least one fledging), Pr (probability of a repeat clutch following nest failure), and La (probability of local survival of the parents following breeding). In experiments where brood size at day 10 after hatching was increased or reduced, Vc increasing experimental brood size, while Vp simultaneously decreased. Total reproductive value (V = Vc + Vp) remained unaffected by the experiments. This result suggests that a rather broad range of clutch sizes maximizes total reproductive value, as far as detectable by the data. While the yield of kestrel hunting, and hence the number of young raisable with constant parental effort (and constant Vp), increased with the spring increase in vole population density, reproductive value of the clutches decreased. For any particular food situation (hunting yield) this leads to a unique combination of clutch size and laying date maximizing V. This could be worked out by calculating fitness contours for all combinations and for different yields (Fig. 12). The optimal solutions are on a declining slope, with smaller clutches associated with later dates. 59.4% of all clutches observed obeyed the maximization criteria. Furthermore, there was a reasonable, unbiased association between predicted laying dates and clutch sizes based on individual male hunting yields and observed dates and clutches as laid by the females. Qualitatively, any method predicts a seasonal decrease in the optimal clutch size when the environment improves while reproductive value declines with progressive date. Preliminary results from an experimental approach to test the assumption of a causal effect of date on Vo, using the release of juvenile kestrels reared in captivity under artificial light schedules, are presented.