Egg sex ratio and paternal traits: using within-individual comparisons

Empirical studies of sex ratios in birds have been limited dueto difficulties in determining offspring sex. Since molecularsexing techniques removed this constraint, the last 5 yearshas seen a great increase in studies of clutch sex ratio manipulationby female birds. Typically these studies investigate variationin clutch sex ratios across individuals in relation to environmentalcharacteristics or parental traits, and often they find no relationships. In this study we also found that clutch sex ratiosdid not vary in relation to a number of biological and environmentalfactors for 238 great tit Parus major nests. However, interestingsex ratio biases were revealed when variation in clutch sexratios was analyzed within individual females breeding in successiveyears. There was a significant positive relationship betweenthe change in sex ratio of a female's clutch from one yearto the next and the relative body condition of her partner.Females mating with males of higher body condition in yearx + 1 produced relatively male-biased sex ratios, and the oppositewas true for females mated with lower condition males. Within-individualanalysis also allowed investigations of sex ratio in relationto partner change. There was no change in sex ratios of femalespairing with the same male; however, females pairing with anew male produced clutches significantly more female biased. Comparisons of clutch sex ratios within individuals may be apowerful method for detecting sex ratio variation, and perhapsfemale birds may indeed manipulate egg sex but require personalcontextual experience for such decisions.     

When should a female avoid adding eggs to the clutch of another female? A simultaneous oviposition and sex allocation game

Summary Avoidance of double oviposition (ADO) is the strategy not to oviposit on food patches where another female has oviposited before. If two females oviposit on the same patch, competitive and mating interactions within and between broods may lead to both a clutch size game and a sex allocation game between the two visitors. Though the two games interact, they are usually considered separately. Here, the ESS conditions for ADO are investigated in an analysis that combines the two games into one. The analysis strengthens the notion that it is really ADO that needs to be explained, because role-dependent net pay-off from an additional egg is most likely to favour double oviposition (DO). To a first female, the net payoff includes the effect on the eggs already present, whereas to a second female only the egg's gross pay-off matters. ADO is the evolutionary stable strategy (ESS) if there are enough patches still without eggs and either (1) the fitness of an additional egg is so low that the first female would not lay it even in the absence of detrimental effects on earlier offspring, so neither would a second female, or (2) differences in either the survival probability of the offspring or their reproductive success are sufficient to counterbalance the differential interest in the eggs already present. The first condition requires that eggs are relatively large, because then the decrease in pay-off between two successive eggs can be large. The second condition may be met when there is a time interval between ovipositions of subsequent females. The resulting developmental lag of the second clutch will (1) diminish its ability to compete for food and (2) lower its reproductive success when there is local mate competition and sons are too late to mate with daughters of the first female. If sons of first and second females compete on equal terms, however, ADO is unlikely. Male migration between patches reduces the influence of sex allocation strategies on clutch size decisions; the same holds for small clutch sizes. To illustrate the importance of considering sex allocation and clutch size decisions in an integrated way, oviposition strategies of plant-inhabiting predatory mites (Acari: Phytoseiidae) are discussed.     

Size, operational sex ratio, and mate-guarding success of the carrion beetle, Necrophila americana

When male insects guard females until oviposition, the benefitsfrom last-male sperm precedence must outweigh the costs of relinquishingadditional fertilizations. The profitability of guarding isincreased when males guard large, fecund females and when femalesare scarce because fewer fertilizations are sacrificed. However,the male reproductive success is not only determined by theprofitability of guarding but also by his ability to maintainguarding. In this study, we used male carrion beetles (Necrophilaamericana) to examine the effects of sex ratio, male relativesize, and female quality on the ability to guard. First, wepresent a model of mate guarding that explores factors, suchas sperm precedence, sex ratio, male size, and female quality,that influence the profitability of postcopulatory riding. Ourmodel predicts that large N. americana males should preferentiallyguard the largest female only when the sex ratio is male biasedand sperm precedence is above 80%. In contrast, small malesgain little from guarding because they are not likely to maintainit and be the last male to mate. Then, we tested these predictionsby manipulating sex ratio, relative male size, and female quality.All males in equal sex ratio and large males in male-biasedsex ratio guarded females significantly longer than did malesin female-biased sex ratio. In male-biased sex ratio, largemales guarded significantly longer and achieved more takeoversthan small males. Large females were guarded longer. The successof guarding males in this beetle depends on their size relativeto other males and the operational sex ratio.     

Group composition affects male reproductive partitioning in a cooperatively breeding cichlid

Individuals within groups of cooperatively breeding species may partition reproduction, with the dominant pair often taking the largest share. The dominant's ability to reproductively control subordinates may depend on differences in competitive ability, due to, e.g. body size differences, but may also depend on the number of same‐sex competitors inside the group. We tested experimentally whether subordinates reproduce more when these subordinates are large or when a second subordinate of the same sex need to be controlled by the dominants, using the cooperatively breeding cichlid Neolamprologus pulcher. Dominant pairs were assisted by a large and a small unrelated subordinate; sexes of these fish were varied in a full‐factorial design (giving four treatments). Dominant males lost significantly more parentage to the large subordinate male when a small subordinate male was also present, compared to when a small subordinate female was present. However, subordinate paternity was generally low and did not significantly curb total dominant male reproductive output, which was more affected by the sizes and numbers of reproductive females present inside his group. Dominant female maternity, clutch sizes and total output did not depend on the treatments. Subordinate–subordinate reproduction was virtually absent (one out of 874 offspring). Female subordinates were more likely to provide care for their own broods. In contrast, male subordinates did not adjust their level of care to their parentage. Variability in female subordinate alloparental brood care was particularly high, with females showing more care than males in general. We also detected effects of growth rate and food ration on parentage independent of the treatments, most notably: (i) a trade‐off between dominant male growth rate and paternity; (ii) a decrease in dominant male paternity with increasing food ration; (iii) a positive effect of growth rate on paternity in small males. We conclude that dominant males should be sensitive to the number and sizes of subordinate males present in their group, particularly when these subordinates are not helpful or grow fast, and food is plentiful. Dominant females should be less sensitive, because female subordinates do not appear to impose reproductive costs and can be helpful through alloparental brood care.     

Predation risk affects trade-off between nest guarding and foraging in Seychelles warblers

The fitness costs of egg loss for Seychelles warblers (Acrocephalus sechellensis)on Cousin Island are considerable because warblers have a single-eggclutch and no time to lay a successful replacement clutch. Onthe islands of Cousin and Cousine, with equal densities of Seychellesfodies (Foudia sechellarum), nearly 75% of artificial eggs placedin artificial nests were predated by fodies after 3 days. OnAride Island with no fodies present, loss of artificial eggswas not observed. Female warblers incubate the clutch, and malewarblers guard the clutch when females are absent. Deterrenceof fodies by male warblers is efficient: loss rate of eggs fromunattended warbler nests was seven times as high as from attendednests, and the more nest guarding, the lower the egg loss andthe higher the hatching success. Egg loss is independent ofthe amount of incubation by females. There is no trade-off betweenincubating and foraging by females. Nest guarding competes withforaging by males, and this trade-off has a more pronounced effecton egg loss when food availability is low. The transfer of breeding pairsfrom Cousin to either Cousine with egg-predating fodies or toAride without fodies allowed us to experimentally investigatethe presumed trade-off between nest guarding and foraging. OnCousine, individual males spent the same amount of time nestguarding and foraging as on Cousin, and egg loss was similarand inversely related to time spent nest guarding as on Cousin.Males that guarded their clutch on Cousin did not guard theclutch on Aride but allocated significantly more time to foragingand gained better body condition. Loss of warbler eggs on Aridewas not observed. Time allocation to incubating and foragingby individual females before and after both translocations remainedthe same.     

Brood desertion in Kentish plover: sex differences in remating opportunities

To understand the evolution of parental care, one needs to estimatethe payoffs from providing care for the offspring and the payoffsfrom terminating care and deserting them. These payoffs arerarely known. In this study we experimentally estimated therewards from brood desertion in a species that has a variablepattern of parental care. In particular, either the female or themale parent may desert the brood in Kentish plover Charadrius alexandrinus,so some broods are attended by one parent of either sex, whereasin other broods both parents stay with the brood until the chicks fledge.We created single males and single females by experimentallyremoving the other parent and the clutch. The expected rematingtime of males was significantly higher (median: 25.4 days) thanthat of the females (5.3 days, p <.0001). The expected rematingtime tended to increase over the breeding season in both sexes,although the increase was significant only in females. The newnest of remated males was closer to their previous territory (mean± SE, 46 ± 8 m) than that of the remated females(289 ± 57 m, p <.001). Hatching success of new nestswas not different between remated males and females. Our resultsdemonstrate that the remating opportunities are different formale and female Kentish plovers and these opportunities varyover the season. We propose that the remating opportunitieswere influenced by the male-biased adult sex ratio and the seasonaldecrease in the number of breeders. However, we stress thatmeasuring remating times is a more direct measure of matingopportunities than calculating the operational sex ratio.     

Interval between clutches, fitness, and climate change

Timing of optimal reproduction can be affected by the presenceof multiple broods, with multi-brooded species breeding earlier(and later) than the optimal timing of breeding as comparedwith single-brooded species that only need to optimize the timingof a single brood. Approximately two-thirds of barn swallowsHirundo rustica produce 2 broods per year, and I tested whetherthe constraints on timing of reproduction were affected by climatechange because climatic amelioration would allow both an earlierstart and a later termination of reproduction. The durationof the interval between first and second clutches and the variancein the duration increased during 1971–2005 when temperatureduring spring, but not summer, increased rapidly. Interclutchinterval was shorter when mean date of breeding was late andalso among late-breeding individuals during individual years.When clutch size and brood size of the first clutch were large,interval until the second brood increased. Pairs with a longinterval produced more fledglings than pairs with a short interval.Pairs with first broods with strong mean T-cell–mediatedimmune responses took shorter time to start their second clutch,whereas mean body mass or tarsus length of first broods werenot significantly related to interclutch interval. Interclutchinterval increased with the size of a secondary sexual character,the length of the outermost tail feathers of adult male barnswallows, but not with tail length of females, or with sizeof several other phenotypic characters in either sex. Thesefindings are consistent with the hypothesis that the durationof the interclutch interval is determined by a combination ofenvironmental conditions, reproductive effort, and sexual selection.     

Hungry females show stronger mating preferences

Female mating decisions that are based on condition-dependenttraits, such as male nutritional state, may be associated witha female's own condition. In the swordtail fish, Xiphophorusbirchmanni, females prefer the chemical cues of well-fed malesto cues of food-deprived males. Here we show that this preferenceis significantly stronger in females when they were experimentallyfood deprived than when they were well fed. Our results suggestthat if females have limited access to food resources, and arethemselves food deprived, they will attend to cues indicatingmale nutritional condition more than when environmental conditionsallow for greater access to food. Furthermore, not only is theslope of the preference function condition dependent but also,in all trials, the latency to respond to the presented stimuliwas shorter in food-deprived females, suggesting that responsivenessto environmental cues is condition dependent as well. Undernatural conditions, females of many species likely experiencevariation in resource availability. Thus, we predict that covariancebetween the strength of female preferences and resource availabilitymay be widespread and may represent a common source of femalepreference variation within and between populations.     

On the evolutionary pathway of parental care in mouth-brooding cichlid fishes

Evolutionary theory predicts that differences in parental care patterns among species arose from interspecific differences in the costs and benefits of care for each sex. In Galilee St Peter''s fish, Sarotherodon galilaeus (Cichlidae), male care, female care and biparental care all occur in the same population. We exploit this unusual variability to isolate conditions favouring biparental versus uniparental mouth-brooding by males or females. We first review a game-theoretic model of parental care evolution, predictions of which we test experimentally in this paper. Manipulations of the operational sex ratio show that males and females desert their offspring more frequently when the costs of care are high (in terms of lost mating opportunities). Breeding trials with males of different sizes show that small fathers desert more frequently than large fathers. We attribute this to the associated difference in the fitness benefit of biparental care relative to female-only care. Our experimental results confirm that in St Peter''s fish the probability of caring is determined facultatively according to current conditions at each spawn. The experiments and model together suggest that interspecific variation in remating opportunities and clutch size may be responsible for differences in care patterns within the sub-family Tilapiini. Our results support the hypothesis that biparental mouth-brooding was the ancestral state of both male and female uniparental mouth-brooding in cichlid fishes.     

甜菜夜蛾交配行为和能力

在(27±1)℃,光周期L14∶D10的条件下对甜菜夜蛾I>Spodoptera exigua的交配行为及能力进行了研究。结果表明：成虫在羽化当晚即可进行交配,交配率以羽化后头三个晚上的较高(>82%),但从第4天起则显著下降。成虫一天中的交配时间出现于23:30～05:30之间,交配高峰出现在01:30～02:30和03:00～04:00 之间, 其中以第1高峰的发生频率较高。成虫交配持续时间从22～191 min不等,但以30～60 min的为多(40.8%, n=97), 60～90 min的次之(19.4%),超过180 min的较少(10.2 %)。另外,交配持续时间与蛾龄紧密相关。蛾龄越大,交配持续的时间越长,且差异显著。雄蛾一生的交配能力由1～11次不等,但受性比的影响显著：在性比为1∶1的条件下,雄蛾平均交配次数仅为3.0 次,而在2♀∶1♂至5♀∶1♂时,则增加到5.1～6.0 次。雌蛾交配比例及次数受性比的影响也很大：没有交配的雌蛾比例从1∶1时的8.3%增加到5♀∶1♂时的32%,仅交配一次的比例从16.7%增加到38.7%,而交配≥5 次的比例则从 25%下降到0。最后,对这些结果在甜菜夜蛾防治中应用的可能性进行了讨论。