Marisa Puerta

NUTRITIONAL CONSTRAINTS DETERMINE THE EXPRESSION OF A SEXUAL TRAIT IN THE HOUSE SPARROW, PASSER DOMESTICUS

Many birds have distinctive patches of feathers that in some cases have been shown to function as reliable signals of phenotypic quality. It has generally been assumed that these called ‘badges’ are inexpensive to produce, so other kind of costs have been invoked to explain how they are maintained as honest signals. In the present study we addressed the hypothesis that the black throat badge of male house sparrows functions as an honest signal of condition because it is nutritionally costly to produce, a possibility not yet evaluated for badges showing status. While under field conditions adult birds exhibited larger badges than juveniles, individuals of both age classes developed badges of similar size when moulting in an aviary with ad libitum food. Fat reserves, blood proteins and body mass records of free ranging birds at the start of moult suggest that juveniles were in poorer condition than adults. The reduction of blood proteins during moult negatively correlated with badge size after moult in juveniles but not in adults, and only the variation of blood proteins, but not fat reserves, correlated with the size of the badge when using multiple regression analysis. The results suggest that: (i) a limited access to food sources during moult may account for the smaller badge size of juveniles as previously reported in the same study population; and (ii) the use of more blood proteins, that seems to depend on a worse nutritional condition, constrains the production of a larger badge. The results give support for the hypothesis that badges are costly to produce, though the magnitude of the costs may be age dependent.

Negative effects of elevated testosterone on female fecundity in zebra finches

Although factors influencing androgen deposition in the avian egg and its effects on nestling fitness are recently receiving considerable attention, little is known about the potential costs of high testosterone levels in the females. Our study aimed at determining the effect of injections of testosterone (T) in female zebra finches (Taeniopygia guttata), on clutch size, egg mass, yolk mass, and yolk androgen content. Females were given a single bolus injection of T in a range of doses after laying the first egg. Results show that administration of T negatively affected clutch size; the strength of this effect increased with increasing doses of T. Females injected with the highest testosterone dose showed suppressed oviposition of the third and the fourth eggs. Interestingly, testosterone administration made females produce eggs with relatively large yolks, suggesting that T may mediate the trade-off between number and size of eggs. Testosterone injection resulted in elevated levels of androgen in the eggs, in contrast to control clutches, which showed a decreasing pattern of androgen concentration along the laying sequence. We conclude that high androgen investment in eggs may be limited by physiological requirements of the ovulatory process.

Negative effects of early developmental stress on yolk testosterone levels in a passerine bird.

SUMMARY Female birds incorporate in the yolks of their eggs significant concentrations of a number of different androgens. Yolk androgen has been shown to positively affect several fitness components at the embryo, nestling and juvenile stages. Previous experiments have shown that females lay eggs with higher androgen concentrations when they are paired with highly ornamented males. This pattern suggests that yolk androgens are costly to females. In this study, we experimentally manipulated adult female condition in zebra finches Taeniopygia guttata by modifying the level of developmental stress they suffered as nestlings. This was achieved by cross-fostering nestlings to broods of varying brood size. Subsequently, we measured the yolk testosterone contents of the female offspring that resulted from the experimental manipulation. As predicted, females deposited decreasing concentrations of testosterone with increasing brood sizes experienced as nestlings: testosterone concentration (mean ± s.e.m.) of eggs laid by females from small broods, 20.66±2.08 pg mg–1; medium broods, 15.32±1.94 pg mg–1; and large broods, 14.51±1.66 pg mg–1. Additionally, testosterone concentration decreased with laying order, and varied with clutch size in a complex way. Differences in egg testosterone between females exposed to different brood sizes are in line with previous findings in showing that early developmental stress can affect adult reproductive performance, although our study did not detect an effect in other breeding parameters, such as latency to breed or clutch size. Furthermore, the results are consistent with the hypothesis that there is a cost associated with yolk testosterone. However, it is still unclear what the nature of this cost may be, and whether it is paid by females, offspring, or both.

Reproductive effort affects immune response and parasite infection in a lizard : a phenotypic manipulation using testosterone

Life-history theory predicts that there is a trade-off between reproductive effort and several traits that determine fitness. Infectious disease has gained acceptance as a crucial factor linking both variables. In most instances phenotypic manipulation is necessary to convincingly demonstrate a causal relationship of reproductive effort on parasitism. However, experimental studies that manipulate reproductive effort or parasite load have been rarely conducted in reptiles. In this study. we manipulated reproductive effort of male lizards (Psammodromus algirus) through testosterone implants, and measured the associated response in some haematological variables and parasite load. Testosterone-supplemented males had lower scores than control males in factor I of a PCA for different blood parameters. This factor is correlated with the number of white blood cells, especially lymphocytes, and with plasma glucose levels. Experimental males also had higher scores in factor 3 that is mainly related to protein catabolism. Scores of males in component I tended to be correlated negatively with tick load, while scores in component 3 were correlated positively with the number of haemogregarines in the blood. These results suggest that higher investment in reproduction decreases the immune defences. and conduces to the use of structural resources, which may render individuals more susceptible to some haemoparasites. This is consistent with the idea that an increase in reproductive effort mediated by testosterone has a negative effect on the ability to counteract parasite infections.

Pregnancy-induced hyperphagia is associated with increased gene expression of hypothalamic agouti-related peptide in rats

Pregnancy is characterized by an increase in food intake that, in turn, produce a positive energy balance in order to face the considerable metabolic demands associated with the challenge of reproduction. Since hypothalamus is a key brain region involved in many peripheral signals and neuronal pathways that control energy homeostasis and food intake, we investigated if during pregnancy the increase in food intake is mediated by stimulating orexigenic and/or inhibiting anorexigenic neural pathways. We examined hypothalamic gene expressions of Ob-Rb, NPY, AgRP, POMC, MC4-R, and preproorexins in pregnant Wistar rats at day 19 of gestation. Food intake and body weight were increased progressively during the pregnancy. Visceral fat mass depots and serum leptin levels were also increased when compared with virgin animals. No differences were found in mRNA expression of Ob-Rb, POMC, MC4-R, NPY or preproorexin between virgin and pregnant animals. However, pregnancy produced a selective increase in AgRP mRNA levels. These results indicate that the positive energy balance that occurred during pregnancy can hardly be explained by changes in Ob-Rb despite hyperleptinemia associated with pregnancy. The enhanced expression of AgRP suggests the involvement of this neuropeptide in mediating pregnancy-associated hyperphagia.

Female house martins (Delichon urbica) reduce egg androgen deposition in response to a challenge of their immune system

Female birds deposit in the yolks of eggs substantial amounts of androgens, such as testosterone and androstenedione. These androgens have been shown to speed up nestling development, induce a fast development of ornaments and increase dominance in adults. Experiments in several species have reported that females invest greater amounts of androgens in the eggs fathered by attractive males, suggesting that yolk androgen is a costly investment for either the offspring or the mother. There is some evidence that nestling immunocompetence may be partially suppressed by high levels of yolk androgens, but it is not known whether this is also the case for females. We tested this hypothesis in the house martin by inducing an immune challenge through an injection of sheep red blood cells, a standard challenge of the humoral immune system. Experimental birds laid eggs with lower amounts of yolk androstenedione than controls, and there was a similar non-significant trend for testosterone. Furthermore, the probability of laying a replacement clutch was higher for birds that had laid a first clutch with relatively high levels of yolk testosterone. These results suggest that yolk androgen deposition is limited by immune costs in the female, and that only females in good condition may afford to invest high levels of androgen in eggs in this species.

Increased sibling competition does not increase testosterone or corticosterone levels in nestlings of the spotless starling (Sturnus unicolor)

Nestling begging in passerine birds is a complex behaviour that is shaped by a multitude of ecological factors and could be physiologically mediated by varying levels of steroid hormones. Previous research has shown links between sibling competition and testosterone and corticosterone in several bird species. The spotless starling (Sturnus unicolor) is a medium sized passerine in which nestlings compete intensively for resources, often resulting in marked size hierarchies that can have profound effects on their fitness. We tested the hypothesis that an increase in sibling competition levels would result in increases in testosterone and corticosterone in this species. To this end we conducted a brood size manipulation, creating small, medium and large broods. This manipulation had the expected effect on morphology: nestling size and mass decreased with increasing brood size. Androgen levels varied in response to brood size manipulation but, contrary to expectations, the largest concentrations were found in reduced brood sizes. Corticosterone levels increased with increasing brood size, but this effect disappeared when we corrected for the time taken to process nestlings. Cell-mediated immune response was found to decrease with increasing brood size and testosterone levels. The results suggest that the proposed link between testosterone and corticosterone and sibling competition does not hold in this species, and underlines the diversity of species-specific responsiveness to steroids.

Hematology and plasma chemistry of house sparrows (Passer domesticus) along the summer months and after testosterone treatment

Abstract Young and adult male house sparrows were captured once reproduction had finished (early summer) and before molting had taken place. They remained in outdoor aviaries until molting was completed (late summer). Half of them received a chronic treatment of testosterone. A blood sample was collected both at the beginning and at the end of the captivity period. Both young and adult birds showed a hemoconcentration from the beginning to the end of the summer, which is suggested to be related to the natural cycle of water shortage. White blood cell number was higher in young than in adults but no change was observed during the study period. Plasma proteins and their nitrogenous wastes decreased over the summer months which raised the hypothesis of plasma proteins as a store for amino acid needs during molting. Testosterone increased erythrocyte number and blood hemoglobin content. It also altered leucocyte number, but in an age dependent manner. It also increased plasma proteins, but only in adult birds. Plasma triglycerides were unaltered by age, month of sampling or testosterone treatment.

The PHA Test as an Indicator of Phagocytic Activity in a Passerine Bird

Several techniques in ecological immunology have been used to assess bird immunocompetence thus providing useful information to understand the contribution of the immunological system in life-history decisions. The phytohaemagglutinin (PHA)-skin test has been the most widely employed technique being interpreted as the sole result of T lymphocytes proliferation and hence used to evaluate acquired immunological capacity. However, the presence of high numbers of phagocytic cells in the swelling point has cast some doubt about such an assumption. To address this issue, we collected blood from 14 days-old nestlings of spotless starling (Sturnus unicolor), administered subcutaneous PHA immediately after and then measured the swelling response 24 hours later. Differential counts of white blood cells suggested that an intense development of acquired immunological defences was taking place. The phagocytic activity of both heterophiles and monocytes was also very intense as it was the swelling response. Moreover, our results show, for the first time in birds, a positive relationship between the phagocytic activity of both kinds of cells and the swelling response. This broadens the significance of the PHA test from reflecting T lymphocytes proliferation -as previously proposed but still undetermined in vivo- to evaluate phagocytosis as well. In other words, our data suggest that the PHA swelling response may not be considered as the only consequence of processes of specific and induced immunity –T lymphocytes proliferation- but also of constitutive and nonspecific immunity –heterophiles and monocytes phagocytosis. We propose the extensive use of PHA-skin test as an optimal technique to assess immunocompetence.

Differential effects of yolk testosterone and androstenedione in embryo development and nestling growth in the spotless starling (Sturnus unicolor).

Yolk androgens in avian eggs play a significant role in embryo and nestling development. However, few studies have examined the differential effect of two of the main yolk androgens, testosterone (T) and androstenedione (A4). Here, we injected eggs of spotless starlings with physiological levels of either T, A4, the combination T+A4 or vehicle substance (control), to examine the differential ability of these steroids to influence nestling development. We found that the duration of the embryonic period was increased by T, and less so by A4, but not by the combination T+A4. Body condition was reduced in all experimental treatments where A4 was present, particularly so in the combination T+A4. Tarsus length was increased in males by A4, and in a lower degree by T, whereas the combination T+A4 inhibited growth. However, these differences in tarsus length between groups disappeared at the end of the nestling period. Cell-mediated immune responsiveness was marginally affected by the interaction between treatment and sex. These patterns suggest that in this species, T has a stronger influence during embryo development than A4, whereas during nestling development the capacities of both androgens to influence growth are similar. The combination T+A4 showed non-additive effects, suggesting either some kind of inhibition between the two androgens, or else an excessive effect due to a bell-shaped pattern of response. Our results suggest a complex picture of sex and age-dependent effects of T and A4, and underline the necessity of further research in the metabolism and action of egg androgens.