Females In Good Condition Research Articles

Female body condition and brood sex ratio in Yellow‐legged Gulls Larus cachinnans

In the Yellow‐legged Gull Larus cachinnans, males are the larger sex, and show more reproductive variance than females. We predicted that the proportion of male chicks in a brood should increase with female body condition. We investigated brood sex ratio by using DNA markers taken from samples of hatchlings or dead embryos, and female body condition using plasma cholesterol concentration as a reliable indicator. The brood sex ratio of females in good condition was male biased and the sex ratio of females in poor condition was female biased. This relationship was also significant in those nests where all the eggs laid were sexed. Thus, manipulation of embryo mortality cannot explain the biases reported in this study, suggesting that the sex ratio of the eggs was biased prior to laying. These results confirm that sex‐ratio manipulation in gulls operates under natural conditions, and supports earlier experimental findings.

State-dependent incubation behaviour in the zebra finch

State dependence is thought to be an important factor in resource allocation decisions, particularly in those decisions relating to parental care. Incubation behaviour is a costly part of avian parental care, and therefore likely to depend on the parent's body condition. We manipulated the body condition of zebra finches, Taeniopygia guttata, during incubation by using prelaying diets of different protein content, and induced birds to lay a similar number of eggs. We tested the hypothesis that birds in better body condition would invest more in incubation than birds in poorer condition when incubating on the same diet. Females that had received a high-protein prelaying diet lost more body mass than those that had received a low-protein diet. They also increased the length of their incubation bouts between early and middle incubation, whereas females that had received a low-protein prelaying diet increased bout length only between middle and late incubation. There were no differences between males from the two diet groups, and males were responsible for a lesser proportion of incubation than females. These results indicate that incubation behaviour is state dependent in female zebra finches, and that birds of different body condition adopt different incubation strategies. We found no differences in incubation duration and hatching success between the two incubation strategies in captivity, but the potential risk of nest predation in the wild may differ. We suggest that only females in good condition can afford to adopt a strategy of increasing bout length early in incubation; females in poorer condition first have to recover their body condition after having produced a clutch. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour

Condition-dependent sex allocation in a lek-breeding wader, the ruff (Philomachus pugnax).

Sex allocation theory predicts that females should bias the production of offspring towards the sex that will maximize maternal fitness. Here we demonstrate evidence for nonrandom sex allocation by female ruffs (Philomachus pugnax), at both the individual and population level in relation to female condition. At the population level, female condition varies significantly across 3 years and is mirrored by population sex ratio, such that in years when females are in poor condition the population offspring sex ratio is female-biased, while in years when females are in better condition there was little or no bias. In the year when females were in overall poor condition, females in better condition produced more daughters. The same relationship is also revealed by comparing the sex ratios of individual females breeding in two consecutive years in different condition. As the condition of an individual female improves (across years) she tends to produce more female offspring. Although we have shown that, as in other birds, female condition is an important determinant of sex allocation, our results also suggest that such nonrandom allocation does not occur in every year, being particularly strong in a year when females, on average, are in poorer condition. We suggest that our results are consistent with the idea that skewing the sex ratio is likely to carry a cost to females and that it is adaptive only when the fitness differential between sons and daughters is sufficient to outweigh probable costs.

Experimental manipulation of maternal effort produces differential effects in sons and daughters: implications for adaptive sex ratios in the blue-footed booby

Sex allocation theory predicts that mothers in good condition should bias their brood sex ratio in response to the differential benefits obtained from increased maternal expenditure in sons and daughters. Although there is well-documented variation of offspring sex ratios in several bird species according to maternal condition, the assumption that maternal condition has different fitness consequences for male and for female offspring remains unclear. The blue-footed booby (Sula nebouxii) is a sexually size-dimorphic seabird, with females approximately 31% heavier than males. It has been reported that the sex ratio is male biased in years with poor feeding conditions, which suggests that either females adjust their sex ratio in accordance with their condition or that they suffer differential brood mortality before their sex can be determined. In this study I tested whether the condition of mothers affected their daughters' fitness more than their sons' fitness. I manipulated maternal investment by trimming the flight feathers and thereby handicapping females during the chick-rearing period. Adult females in the handicapped group had a poorer physical condition at end of chick growth, as measured by mass and by the residuals of mass on wing length compared to control birds. Female chicks were affected by the handicapping experiment, showing a lower mass and shorter wing length (reduced approximately 8% in both measures) than controls. However, this effect was not found in male chicks. Hatching sex ratios were also related to female body condition at hatching. The brood sex ratio of females in poor condition was male biased but was female biased for females in good condition. Overall, these results suggest that the variation in the sex ratio in blue-footed boobies is an adaptive response to the disadvantage daughters face from being reared under poor conditions. Copyright 2002.

What did Trivers and Willard really predict?

Correspondence: J. Carranza, Catedra de Biologia y Etologia, Facultad de Veterinaria, Universidad de Extremadura, 10071 Caceres, Spain (email: carranza@unex.es) The issue raised by Trivers & Willard (1973) on the ‘natural selection of parental ability to vary the sex ratio of offspring’ has produced many publications. Probably no other case exists in behavioural ecology where a couple of pages have sired so many studies. With so many descendants, however, mutations are likely to emerge. One main problem is that the Trivers–Willard (TW) hypothesis was a verbal argument and hence semantic interpretations are possible. For instance, TW wrote that ‘females in better condition tend to invest in males’. This may be interpreted in different ways, regarding either the way of allocating the investment or the nature of the comparison, as follows: (1) females in better condition tend to produce bigger (or costlier) male offspring than female offspring or they tend to produce bigger (or costlier) males than females in poor condition; (2) females in better condition tend to produce more male offspring than female offspring or they tend to produce more males than females in poor condition. Furthermore, these statements may lead to complementary ones such as: (1) females in poor condition tend to produce bigger female than male offspring or they tend to produce bigger females than other females; (2) females in poor condition tend to produce more female than male offspring or they tend to produce more females than other females. One needs only to read the original TW paper carefully (or subsequent reviews discussing it: e.g. Frank 1990; Godfray & Werren 1996; Hardy 1997) to see that some of these interpretations are not in concordance with Trivers and Willard’s ideas. I do not intend to review the TW hypothesis comprehensively here, although a simple graphical model may help to put these thoughts into a framework. Figure 1a represents the hypothetical relationship between the fitness return of a female or a male offspring and the amount of care it receives (r). I believe this represents the conditions of the TW model, and I

Effects of supplementary feeding on the offspring sex ratio of kakapo: a dilemma for the conservation of a polygynous parrot

The kakapo ( Strigops habroptilus) is a large, flightless, nocturnal parrot, endemic to New Zealand. It is critically endangered, with a world population of ca. 62 individuals and a male-biased adult sex ratio. The species has a polygynous “lek” mating system and adult males typically weigh 30–40% more than females. The kakapo is subject to intensive conservation efforts, including the provision of supplementary food to wild birds to encourage successful nesting. There is mounting evidence that, in polygynous species with large variance of male reproductive success, females in better condition may maximise their fitness by producing more offspring of the larger, more costly sex to be reared. We used data on the sex ratio of progeny of female kakapo that had or had not received supplementary food, to test the hypothesis that supplementary feeding might cause a male-biased offspring sex ratio. There was a significant excess of males in the clutches of females provided with supplementary food, suggesting that changes need to be made to the feeding regime to increase recruitment of females. This is an example of applying evolutionary theory to a practical conservation problem.

MATERNAL CONDITION AND NESTLING SEX RATIO IN HOUSE WRENS

Abstract Parents are expected to vary the sex ratio of their offspring in relation to the sex-specific fitness benefits. However, benefits of producing sex-biased broods may be dependent on condition of the female. For example, mothers in good condition could achieve greater fitness if they produced high-quality sons, whereas, mothers in poor condition would gain more by producing daughters rather than poor-quality sons. As a consequence, we would expect to see a relationship between female condition and sex ratio of offspring. We examined effect of maternal condition on nestling condition and sex ratio in the House Wren (Troglodytes aedon). Overall sex ratio of nestlings in the population was not biased, but females in better condition produced relatively more sons. Overall positive relationship between female condition and proportion of male offspring was due to second broods, which were significantly male-biased and more likely to be produced by females in good condition. Females in better condition al...

Maternal Condition and Nestling Sex Ratio in House Wrens

AbstractParents are expected to vary the sex ratio of their offspring in relation to the sex-specific fitness benefits. However, benefits of producing sex-biased broods may be dependent on condition of the female. For example, mothers in good condition could achieve greater fitness if they produced high-quality sons, whereas, mothers in poor condition would gain more by producing daughters rather than poor-quality sons. As a consequence, we would expect to see a relationship between female condition and sex ratio of offspring. We examined effect of maternal condition on nestling condition and sex ratio in the House Wren (Troglodytes aedon). Overall sex ratio of nestlings in the population was not biased, but females in better condition produced relatively more sons. Overall positive relationship between female condition and proportion of male offspring was due to second broods, which were significantly male-biased and more likely to be produced by females in good condition. Females in better condition also tended to provision young more often and produced both male and female nestlings in better condition. Polygyny and extrapair mating are common in House Wrens. If males in good condition are more likely to be successful breeders as adults, then it may benefit mothers in good condition to produce more sons.

Breeding strategy and morphological characters in an urban population of blackbirds, Turdus merula

Pairing patterns in monogamous birds are thought to be the consequence of mutual mate choice by males and females for characters linked to individual quality in the opposite sex. Although this may result in assortative pairing for a single character, there may be no simple match between male and female phenotype if the traits linked to individual quality are sex specific. For instance, in several passerine species, females tend to select males on the basis of territory quality or characters that may reflect health or vigour, whereas males benefit from pairing with females in good condition that breed early because early breeding increases reproductive success. We investigated pairing patterns in an urban population of the sexually dimorphic European blackbird in relation to bill colour in males and body condition in females, over a 3-year period. Females breeding early were in significantly better condition than those breeding late. The number of breeding attempts per season was significantly positively correlated with female condition, while the number of fledglings reared by a female in a breeding season was positively correlated with the number of breeding attempts, independently of female condition. Bill colour varied in males and was related to body weight, although not to condition. Female condition was correlated with male bill colour within pairs, with males showing more orange bills being paired to females in better condition. We discuss the observed pairing pattern in relation to individual or territory quality and to the constraints and costs associated with reproduction in each sex.

FEMALE CONDITION: A PREDICTOR OF HATCHING SYNCHRONY IN THE HOUSE WREN?

Abstract The degree of hatching synchrony in clutches of passerine birds frequently varies among species and among individuals of the same species. Many hypotheses have been proposed to explain why some eggs hatch several days after others in a clutch. We tested one of these hypotheses, the energetic-constraints hypothesis, which proposes that females in poor physical condition postpone initiating incubation and hatch their clutches synchronously, whereas females in good condition begin incubation early and hatch their clutches asynchronously. We tested the hypothesis using the House Wren (Troglodytes aedon) because recent studies have found little difference in productivity between synchronously and asynchronously hatching clutches in this species, suggesting that the degree of hatching synchrony varies for reasons unrelated to nestling growth and survival. We used logistic regression to test the dependence of the degree of hatching synchrony on each of two measures of female condition. We found no relat...

Offspring sex ratios in tree swallows: females in better condition produce more sons.

Organisms are expected to adjust the sex ratio of their offspring in relation to the relative fitness benefits of sons and daughters. We used a molecular sexing technique that amplifies an intron of the CHD1 gene in birds to examine the sex ratio at egg-laying in socially monogamous tree swallows (Tachycineta bicolor). We examined all individuals in 40 broods (210 young), including all unhatched eggs and nestlings. Thus, the sex ratio we measured was the same as the sex ratio at laying. Overall, the mean sex ratio per brood (+/- SD) was biased significantly towards males (57 +/- 2% male). Within broods, male-biased sex ratios were associated with females in better body condition, and these females were more likely to produce sons in better condition. Tree swallows have one of the highest known levels of extra-pair paternity in birds (38-76% extra-pair young), and, as a consequence, variance in male reproductive success is greater than that of females. Thus, in tree swallows, investment in sons has the potential for higher fitness returns than investment in daughters, assuming that sons in better condition have greater reproductive success.

Conditions for the trivers-willard hypothesis to be valid: A minimal population-genetic model

The very insightful Trivers-Willard hypothesis, proposed in the early 1970s, states that females in good physiological conditions are more likely to produce male offspring, when the variance of reproductive success amongst males is high. The hypothesis has inspired a number of studies over the last three decades aimed at its experimental verification, and many of them have found adequate supportive evidence in its favour. Theoretical investigations, on the other hand, have been few, perhaps because formulating a population-genetic model for describing the Trivers-Willard hypothesis turns out to be surprisingly complex. The present study is aimed at using a minimal population genetic model to explore one specific scenario, viz. how is the preference for a male offspring by females in good condition altered when 'g', the proportion of such females in the population changes from a low to a high value. As expected, when the proportion of such females in good condition is low in the population, i.e., for low values of 'g', the Trivers-Willard (TW) strategy goes to fixation against the equal investment strategy. This holds true up to gmax, a critical value of 'g', above which the two strategies coexist, but the proportion of the TW strategy steadily decreases as 'g' increases to unity. Similarly, when the effect of well-endowed males attaining disproportionately high number of matings is more pronounced, the TW strategy is more likely to go to fixation. Interestingly, the success of the TW strategy has a complex dependence on the variance in the physiological condition of females. If the difference in the two types of conditions is not large, TW strategy is favoured, and its success is more likely as the difference increases. However, beyond a critical value of the difference, the TW strategy is found to be less and less likely to succeed as the difference becomes larger. Possible reasons for these effects are discussed.

Energetic consequences of reproduction in Atlantic cod (Gadus morhua) in relation to spawning level of somatic energy reserves

The influence of a lower condition on reproductive investment, somatic energy losses, and postspawning condition of Atlantic cod (Gadus morhua) was examined under the hypothesis that females, in response to lower available energy reserves, would reduce reproductive investment in order to limit somatic energy losses. Laboratory experiments revealed that female cod with high prespawning condition factors ended reproduction in better condition than females with low prespawning condition factors. Fecundity and total egg dry weight were significantly lower in poor-condition females. The loss in somatic mass and energy in these poor-condition females was nevertheless higher, in relative terms, than the losses experienced by females in good condition. Consequently, energy reserves invested in reproduction by poor-condition females increase their risk of mortality. In the northern Gulf of St. Lawrence during the early 1990s, reproductive females had lower fecundities and were in worse prespawning and postspawning condition. The condition of spent females suggested a greater impact of changes in environmental conditions on adult than on immature cod. Reproductive potential and possibly recruitment may have suffered from that situation and could have contributed to the failure of that stock to recover despite the moratorium on commercial fishing.

Energetic consequences of reproduction in Atlantic cod (<i>Gadus morhua</i>) in relation to spawning level of somatic energy reserves

The influence of a lower condition on reproductive investment, somatic energy losses, and postspawning condition of Atlantic cod (Gadus morhua) was examined under the hypothesis that females, in response to lower available energy reserves, would reduce reproductive investment in order to limit somatic energy losses. Laboratory experiments revealed that female cod with high prespawning condition factors ended reproduction in better condition than females with low prespawning condition factors. Fecundity and total egg dry weight were significantly lower in poor-condition females. The loss in somatic mass and energy in these poor-condition females was nevertheless higher, in relative terms, than the losses experienced by females in good condition. Consequently, energy reserves invested in reproduction by poor-condition females increase their risk of mortality. In the northern Gulf of St. Lawrence during the early 1990s, reproductive females had lower fecundities and were in worse prespawning and postspawning...

Factors Affecting Neonate Size Variation in Northern Water Snakes, Nerodia sipedon

The theoretical construct of an optimal offspring size remains widely cited in life history literature, despite the regular failure of the central prediction of that theory, that offspring size should not vary within populations. Here we used data from 77 litters collected over four years from captive and free- living northern water snakes (Nerodia sipedon) to test several hypotheses proposed to explain within-pop- ulation variation in offspring size. Neonate mass varied from 1.5 to 6.1 g. Neonate mass did not increase as a simple function of maternal size, contrary to the hypothesis that maternal body size physically constrains offspring size. Holding litter size constant, larger females and females in better condition produced larger offspring. The variance in neonate size greatly exceeded the maximum that could be accounted for by the fractional-offspring hypothesis. We found some evidence that females that acquire food during the pro- tracted period of follicular enlargement can invest that energy into those follicles, and thus increase offspring size. We found no evidence that embryos trade off condition with body size when they are born earlier. The strongest correlate of offspring size was litter size: offspring were smaller in larger litters. Because larger litters were born earlier, we propose that large females (i.e, those producing large litters) may have an energetic advantage that allows them to ovulate earlier. If early birth increases neonate fitness, then a small offspring born early may have the same fitness as a larger offspring born later, thereby allowing larger females to trade off offspring size against offspring number. Collectively, our results suggest that offspring size in northern water snakes varies as a consequence of constraints imposed by reproductive

Brood Reduction in White Storks Mediated through Asymmetries in Plasma Testosterone Concentrations in Chicks

We hypothesized that increasing chick plasma testosterone concentrations, transmitted from the mothers via their eggs, enhances survival of their offspring and that the fitness of the young, depending on the maternal hormones, is influenced by parental quality. To test our hypotheses we distinguished the broods of white storks Ciconia ciconia L. where chicks died and those where all chicks survived. We analysed the plasma testosterone concentrations in the chicks, the ability of the chicks to be first to receive food and the mass of chicks before fledging in relation to their hatching order and recorded the body mass of parents and food mass delivered by them.Female storks used the asymmetries in testosterone concentrations within a brood to control brood size and adjusted the number of young hatched to match the parental ability to rear offspring. Females of poor condition altered the testosterone concentrations to produce large differences between the chicks: The first‐hatched chicks, which had high plasma testosterone levels, responded faster to the feeding parent and received more food than did their younger siblings. One or two later‐hatched chicks, which had lower testosterone levels, died in these broods. Females in good condition produced small differences in testosterone concentrations between the chicks and all chicks survived in their brood. Chicks that were raised by the females of poor condition in reduced broods were heavier than chicks that were raised by females of good condition in broods where all chicks survived.We suggest that the control of brood size by testosterone concentration, transmitted by the mother to the chicks, is a hormonal means of condition‐dependent reproductive strategy in the white stork.

Assortative mating and female clutch investment in black grouse

Variation in female behaviour has only recently received attention in studies of sexual selection. It has been suggested that females may invest differentially in their offspring in relation to the quality of their mate. This may lead to females that mate with high-quality and/or attractive males laying larger clutches. Females may also differ in their ability to choose between males. For example, females in good physical condition may make better choices. If physical condition and clutch size are positively correlated, this hypothesis could also produce a relationship between male attractiveness and female clutch size. We found, in lekking black grouse,Tetrao tetrix, that females mated to the highest ranked males laid the largest clutches. Furthermore we found, regardless of female age, a positive relationship between a measure of female condition and male rank but not between female condition and her clutch size. In addition, females in good condition visited a larger number of different male territories, and old females produced the largest clutches. Our results suggest two mechanisms to explain our findings. First, females in good physical condition tend to mate with the top males, suggesting an assortative mating pattern. Second, females mating with the highest ranked males lay larger clutches as a consequence of their choice. In general, our result calls for caution in evaluating studies that look at the consequences of mate choice. It may be that differences in female quality produce effects that may be wrongly interpreted as male quality effects.

Individual Optimization May Explain Differences in Breeding Time in the European Kestrel Falco tinnunculus

Two hypotheses have been suggested to explain why individual variation in breeding time persists. The constraint hypothesis posits that the optimal laying date will be the same for every pair of a population. However, because egg formation is proximately constrained by the food supply, laying occurs after the ecologically optimal time. Females in poor condition cannot lay as early as females in better condition. By contrast, the individual optimization hypothesis proposes that individual variation in timing of breeding is adaptive since the optimal laying date depends on parental quality. To test these hypotheses I manipulated hatching date in a European Kestrel Falco tinnunculus population by cross-fostering clutches of equal size but differing in laying dates. Experimentally advanced clutches produced fewer fledglings and fledglings of worse quality than control clutches. Moreover, control clutches were also more productive than experimentally delayed clutches despite young being of similar quality. These results are consistent with the individual optimization hypothesis.

Social status and physical condition of mother and sex ratio of offspring in cervids

In cervids, the degree of polygyny and litter size vary between species, and both these variables can potentially influence the relationship between maternal condition and offspring sex ratio. Good foraging conditions have usually been found to favour a trend toward female-biased population birth sex ratios. This appears to apply to even the most polygynous species, red deer ( Cervus elaphus) and reindeer ( Rangifer tarandus). However, analysis of offspring sex ratio within populations of these species indicates that high-ranking females produce more males than low-ranking ones. Owing to monoparity and a high degree of polygyny, these species probably offer the clearest starting points for evaluating Trivers and Willard's hypothesis that, in the case of polygynous mammals, females in good condition produce more males than females in poorer condition. In the genera Odocoileus and Alces litter size is not fixed to one, and parental investment per litter may therefore be affected by both offspring sex and litter size. Available evidence of offspring sex ratio variation within these genera is less conclusive than in Cervus and Rangifer.

Body condition affects implantation date in the European badger, Meles meles

The European badger has been used for many years as a model for the study of delayed implantation. Extensive experimental work has shown that implantation, which occurs around the winter solstice, is triggered by photoperiod. This paper presents data on estimated implantation dates in a wild population of badgers, measured by ultrasound scanning. It shows that females' nutritional state prior to implantation also influences implantation date, with females in good condition implanting relatively early. Photoperiod is known to influence implantation on a timescale of months, but body condition acted on a timescale of days; thus while photoperiod may be used to set the time of breeding to correspond approximately with seasonal variation in good availability, females may use body condition to calibrate their reproduction in response to their own local conditions. Giving birth early in the year provides cubs with the maximum period of growth prior to the summer, when food availability is very low. Since females feed little during gestation and the early phases of lactation, females in good condition will be able to afford to implant earlier than those with smaller fat reserves.