Ecological and physiological differences between two colour morphs of the coral Pocillopora damicornis

Pocillopora damicornis (Linnaeus) is a ubiquitous branching coral found throughout the Indo-Pacific region. Like many other species of coral, P. damicornis displays a large range of morphologies. At One Tree Island, it occurs as two distinct morphs that are easily distinguished by the presence or absence of pink pigmentation. The two colour morphs of P. damicornis were found to differ in their distribution and abundance in the One Tree Island Lagoon. The brown morph was more abudant than the pink morph in the shallows (<1 m),whereas the pink morph was more abundant at deeper sites (>3 m). The two morphs also differed physiologically. The brown morph tended to have a greater calcification rate than the pink morph, regardless of environmental conditions. However, the difference in the calcification rate between the two morphs became non-significant under shaded conditions (5% full sunlight), indicating some degree of physiological plasticity of the morphs. The pink colour in P. damicornis was due to a hydrophilic pigment with a major peak absorbance at 560 nm. The expression of pink pigment had both genetic and phenotypic components. The brown morph has a reduced genetic capacity to express the pigment relative to the pink morph. On the other hand, pigment expression could be induced by light in the pink morph. Although genetic differences ultimately determine the differences between the two morphs of P. damicornis, the extent of pigment expression is under some degree of environmental influence.     

Dynamics of external brooding in the sea anemone Epictis Epiactis prolifera

The sexually produced young of the externally brooding actinian Epiactis prolifera Verrill, 1869 are attached to the parent's column just above the base. A transitory brood groove may be formed around the limbus when the parent contracts. In the population studied on the coast of Sonoma County, California, USA, from 27 to 49% of the adult anemones were brooding at any time, the proportion being inversely related to seawater temperature and directly related to size of the anemones. A brood usually consisted of young of various sizes. Overall, number of young being brooded was directly related to parent size and inversely related to size of the brooded juveniles. The growth rate of juvenile anemones was inferred to average about a millimeter in pedal disc diameter per month. Juveniles dislodged from the parent when smaller than 4 mm in basal diameter apparently cannot survive in the intertidal zone. Juveniles which successfully make the transition to adulthood have, therefore, been brooded for at least 3 months. Fewer than 50% of the smallest juveniles survived to a size at which they could live independently. The transition to adulthood was accompanied by a mortality of 80%. The survival rate, from egg to adult, was calculated to be about 1% per season.This work was done in partial fulfillment of the requirements for the Ph.D. degree in the Department of Zoology, University of California, Berkeley, and was supported by a U.S. National Science Foundation Traineeship.     

Foster,but not genetic,father plumage coloration has a temperature-dependent effect on offspring quality

Secondary sexual characters have most likely evolved through sexual selection because such traits indicate the genetic or phenotypic quality of the bearer. While genetic variation in such fitness-related traits should be depleted by directional selection, there are many cases in which variation is higher than expected. One hypothesis explaining this variation is that different phenotypes within a population are adapted to different environmental conditions. We tested this hypothesis by comparing the offspring quality of male pied flycatchers, Ficedula hypoleuca, with different degrees of melanin-based dorsal plumage coloration under different environmental conditions. To create different environmental growth conditions and to be able to separate offspring genotype effects from paternal effects on offspring body mass, we used a partial cross-foster design where the brood size was reduced or enlarged by one chick. We also examined the interactive effects of temperature and male phenotype because previous correlative studies suggest such temperature-dependent effects in this species. We show that, while manipulated brood size did not interact with male phenotype to affect offspring quality, temperature during the nestling period influenced the offspring quality of dark and brown foster (but not genetic) fathers. When the temperature was relatively low during the nestling period, foster offspring of black males were lighter than those raised by brown males; the opposite was true if temperature was relatively high. These results add a new aspect to our understanding of how variation in the degree of melanin-based coloration is maintained in wild populations and how phenotypic variation may be maintained in general.     

Resource-associated divergence in the arctic marine amphipod<Emphasis Type="Italic"> Paramphithoe hystrix</Emphasis>

Host specialization has played an important role in the speciation of groups such as herbivorous insects and parasitic invertebrates. In this study, we provide evidence for its role in the diversification of the amphipod Paramphithoe hystrix Ross 1835 in Arctic marine waters. This species lives and feeds on the tissues of varied invertebrate hosts that are much larger than itself, including poriferans, hydrozoans and echinoderms, acting as a 'micropredator'. We examined the genetic structure of P. hystrix at sites in the Canadian Arctic where it is represented by a white form found on the light-coloured sea star Solaster endeca, a red form on the pink soft coral Gersemia rubriformis and by a spotted form with an unidentified host. These phenotypes occur in microallopatry, sometimes occupying alternate hosts just a few metres apart. Although their variation in host associates might have arisen from ingested host tissue, our results indicate that these morphs are genetically distinct. Mitochondrial (cytochrome c oxidase I) and nuclear (28S ribosomal RNA) gene sequences, as well as allozyme data all show strong genetic divergence between the three colour morphs, demonstrating their reproductive isolation. These results suggest that P. hystrix is a complex of at least three species, which may have arisen as a result of disruptive selection following host switches. Moreover, the depths of genetic divergence indicate that diversification of this complex was complete prior to the Pleistocene.Communicated by R.J. Thompson, St. John's     

Anthopleura stellula (Actiniaria,Actiniidae) and its reproduction by transverse fission

Anthopleura stellula Ehrenberg 1834, a sea anemone from Eilat, Red Sea, is redescribed in detail and shown to be synonymous with Anthopleura elatensis England 1969. Asexual reproduction by autonomous transverse fission has been observed over a period of 8 months in A. stellula maintained in Mediterranean seawater (37.0 to 37.5 S). Fission always occurs at the same level, below the pharynx, and is followed by an asynchronous regenerative process beginning at the initial point of cleavage. The distal portion of a bisected anemone regenerates a new pedal disc, while pharyngeal formation is induced by the mesenteries of the first-budding directive tentacle in the proximal half; the original oral disc pattern is restored. After regeneration has been completed, the newly formed anemone can similarly engage in transverse fission. The fission process spreads infectionsly. If Bunodactis verrucosa, a Mediterranean anemone, is maintained in the same aquarium with A. stellula then the beginnings of fission followed by dedifferentiation are observed in the Mediterranean species, which does not ordinarily reproduce asexually. A. stellula still exhibits its induced ability to divide transversly if transferred into fresh Mediterranean seawater. When these specimens are exposed to low temperatures (12° to 13°C) for 2 months, fission activity still remains, but for only one division; there is no further fission, and no regeneration of a pedal disc. Abrupt increases in salinity to the salinity level of the Red Sea (40.0 to 41.0) interrupt the fission activity and pedal disc regeneration. A. stellula incised at the level where fission usually occurs promptly undergo transverse fission if cultured with other dividing anemones. However, the other anemones merely exhibit wound healing if the salinity is high. It is thought that fission and regeneration are induced by stimulating substances produced by the anemones during low salinity conditions and that the stimulating substances accumulate in the aquarium water.     

Genetic and color morph differentiation in the Caribbean sea anemone<Emphasis Type="Italic"> Condylactis gigantea</Emphasis>

The distribution of phenotypic and genetic variation across environments can provide insights into local adaptation. The tropical sea anemone Condylactis gigantea inhabits a broad spectrum of coral-reef habitats and displays a variety of phenotypes, particularly with respect to color. At the coast of Discovery Bay, Jamaica, individuals with either pink or green tentacle tips show distinct distributions. Pink morphs are more abundant in the lagoon and in deeper areas, while green morphs are more abundant in the forereef and in shallower areas. We use DNA sequence data (ITS1-5.8S) to investigate if variation in color is associated with genetic differentiation in lagoon and forereef habitats about 5 km apart. Population genetic analyses reveal two distinct ITS1-5.8S variants, which differ in relative frequency. The two variants are present in both habitats, but a dearth of intermediates suggests reduced gene flow. In the lagoon, but not the forereef, ITS variants show an association with color. In order to address the potential ecological significance of color, we study UV absorbance and UV acclimatization capacities of pink and green color morphs in the lagoon. Color morphs differed significantly in UV-B absorbance. These results suggest genetic and ecological differentiation in the face of gene flow over short distances.Communicated by O. Kinne, Oldendorf/Luhe     

Tale of two colors: <Emphasis Type="Italic">Cladopsammia gracilis</Emphasis> (Dendrophylliidae) color morphs distinguished also by their genetics and ecology

Cladopsammia gracilis (Dendrophylliidae), an ahermatypic coral inhabits the northern Red Sea. Two color morphs (pink and orange) are found aggregated in caves devoid of hermatypic corals, associated with crustose coralline algae (CCA). Sequencing the rDNA ITS region revealed a separate clustering of members of each color morph. Both morphs grow in shallow waters, with orange corals limited to the upper 4 m, while some pink coral aggregates thrive deeper than 30 m. Planulae were released between June and December. Pink planulae treated with antibiotics and exposed at different intervals to CCA, were competent and metamorphosed even 110 days after release. Maximal competency period for orange planulae was 70 days. All planulae were enhanced to metamorphose in presence of CCA. The mean age at metamorphosis of pink and orange planulae treated with CCA differed significantly. Most orange planulae settled directly on the CCA while most pink planulae settled on the wall of the experiment vial. The morphs differed significantly in the calyx cross-section area of primary polyps. Despite being considered a single species according to skeletal based taxonomy, the significant ecological and molecular differences between pink and orange C. gracilis specimens suggest that they may belong to separate species.     

Frequency-dependent male mate harassment and intra-specific variation in its avoidance by females of the damselfly Ischnura elegans

We focused on male harassment on different female color morphs of the damselfly Ischnura elegans and on variation in morph-specific mating avoidance tactics by females. In I. elegans, one of the female morphs is colored like the conspecific male (andromorphs) while the other morphs are not (gynomorphs). Our first goal was to quantify morph-specific male mating attempts, hence male harassment, in populations with manipulated population parameters (densities, sex ratios, and proportion of andromorphs). Second, we examined the female's perspective by looking for potential differences in morph-specific mating avoidance tactics and success of those tactics in a natural population. Differences in population conditions did influence the number of male mating attempts per morph. The less frequent female morph was always subject to fewer mating attempts, which contradicts earlier hypotheses on mimicry, but supports those that assume that males learn to recognize female morphs. Gynomorphs occupy less open habitat and often fly away when a male approaches, while andromorphs use more open habitat, do not fly large distances and directly face approaching males. Female morphs did not differ in the proportion of successful mating-avoidance attempts. Our results suggest that the maintenance of the color polymorphism is most probably the result of interactive selective forces depending on variation in all population conditions, instead of solely density- or frequency-dependent selection within populations.     

Chemical polymorphism in male femoral gland secretions matches polymorphic coloration in common wall lizards (Podarcis muralis)

Previous studies showed that common wall lizards (Podarcis muralis) are polymorphic in colour, both sexes showing three main ventral morphs (white, yellow and red) within the same population and that the three morphs correlate with many life-history traits, including a positive assortative mating according to colour. Chemical communication plays a key role in intra-specific recognition and in social organization of lizards; thus chemical cues might be involved in morph recognition and mate choice. We used gas chromatography–mass spectrometry (GC–MS) to investigate possible differences in the lipophilic fraction of femoral gland secretions between size/age classes and to explore whether chemical secretions match male colour morphs. As expected, most males shared the same compounds, but smaller males showed significantly higher proportions of aldehydes, alcohols and ketones and significantly lower proportions of tocopherols than larger males. Interestingly, inter-morph differences in the proportion of some compounds (especially tocopherols and furanones) matched ventral colour polymorphism. Pairwise comparisons showed that white lizards had significantly different chemical profiles than both the yellow and red ones, whereas differences between yellow and red males were only marginal. A further canonical analysis of principal coordinates correctly classified 67.2 % on average of the chemical profiles according to colour morph (white 85.0 %, red 60.9 %, yellow 57.1 %). We hypothesized that chemical differences associated with colour polymorphism may play a central role in intra-specific communication and even in sexual selection, allowing individuals to choose their partners according to their age, and more interestingly according to their colour morph, in a non-random mating population system.     

Patterns of brood division and an absence of behavioral plasticity in a neotropical passerine

Studies of parental behavior in various habitats provide an opportunity to gain insight into how different environments may mold strategies of parental care. Brood division by parents has been hypothesized to occur facultatively within and among species. Brood division occurs when each parent cares for specific offspring within a brood. We studied brood division in a neotropical passerine, the western slaty antshrike (Thamnophilus atrinucha). Our results present a unique picture of a highly specialized example of avian brood division. Division was a fixed behavioral pattern in the population studied: all broods divided by fledging and remained divided during the entire post-fledging period. Brood division before fledging, a previously unreported phenomenon, occurred in 40% of nests observed. Parents that preferentially fed a certain offspring (defined as their focal offspring) in the nest fed the same individual after fledging. Each parent fed only its focal offspring in broods of one and two. The male parent cared for the heavier offspring and the first offspring to leave the nest. Siblings were segregated spatially during the time of highest predation risk. These observations suggest that a consistently high risk of predation on offspring has favored initial spatial segregation and inflexibility of brood division behavior in this species. Factors other than predation risk alone may explain the observed patterns of long-term, perfect brood division. Because high predation is common and relatively predictable in the tropics, selection for fixed brood division may be stronger in tropical birds than in the temperate zone.     

Extra-pair paternity, offspring mortality and offspring sex ratio in the socially monogamous coal tit (Parus ater)

Females of many socially monogamous bird species commonly engage in extra-pair copulations. Assuming that extra-pair males are more attractive than the females’ social partners and that attractiveness has a heritable component, sex allocation theory predicts facultative overproduction of sons among extra-pair offspring (EPO) as sons benefit more than daughters from inheriting their father’s attractiveness traits. Here, we present a large-scale, three-year study on sex ratio variation in a passerine bird, the coal tit (Parus ater). Molecular sexing in combination with paternity analysis revealed no evidence for a male-bias in EPO sex ratios compared to their within-pair maternal half-siblings. Our main conclusion, therefore, is that facultative sex allocation to EPO is absent in the coal tit, in accordance with findings in several other species. Either there is no net selection for a deviation from random sex ratio variation (e.g. because extra-pair mating may serve goals different from striving for ‘attractiveness genes’) or evolutionary constraints preclude the evolution of precise maternal sex ratio adjustment. It is interesting to note that, however, we found broods without EPO as well as broods without mortality to be relatively female-biased compared to broods with EPO and mortality, respectively. We were unable to identify any environmental or parental variable to co-vary with brood sex ratios. There was no significant repeatability of sex ratios in consecutive broods of individual females that would hint at some idiosyncratic maternal sex ratio adjustment. Further research is needed to resolve the biological significance of the correlation between brood sex ratios and extra-pair paternity and mortality incidence, respectively.     

Differential breeding investment in bridled and non-bridled common guillemots (Uria aalge): morph of the partner matters

Polymorphism is the coexistence of two or more phenotypically distinct and genetically determinate forms in a population and implies a selective balance between the alternative morphs to permanently exist. Common guillemots Uria aalge occur in two genetically distinct morphs—a non-bridled and a bridled—the latter with white eye rings and a well-defined stripe behind the eyes. In this study, we investigated differences between the morphs with regard to reproductive parameters. We used a detailed family-based sample providing data on mother, father, and chick over three breeding seasons. The mating between morphs was random but pure non-bridled and pure bridled pairs produced smaller chicks at age 15 days than the two mixed pair compositions. Body mass of the adults showed much the same pattern; pure pairs having lower body mass than mixed pairs. There were no differences between the two morphs in reproductive parameters without considering the morph of the partner. This suggests that reproductive decisions in some way not only depend on the morph but also on the tactic of the partner. Different reproductive strategies between morph family groups as found in this study may contribute to the understanding of the existence of a balanced polymorphism in common guillemots. The overall breeding conditions during the years of this study were good. However, over time in a variable environment, we suggest that tactics of different family groups may have different success stabilizing the frequency of non-bridled and bridled birds over time.     

Allozyme electrophoresis demonstrates that the scleractinian coral Montipora digitata is two species

Allozyme electrophoresis at five loci was used to clarify the species status of two morphs of the scleractinian coral Montipora digitata (Dana, 1846), using specimens collected from three locations off the Queensland coast between October 1991 and April 1993. The two morphs, occurring sympatrically, were distinguished by one fixed gene difference at Locus LT-2 and frequency differences at three other loci, indicating reproductive isolation between them. Populations of both morphs were generally in Hardy-Weinberg equilibrium, suggesting that sexual reproduction does occur. Despite sampling designed to reduce the chance of collecting clonemates, the genetic diversity ratio (G _O:G_E) indicated that asexual reproduction also occurs. This is in agreement with the known modes of reproduction for this species. There was significant genetic differentiation between populations of one of the morphs of M. digitata. This could have been influenced by greater degrees of asexual reproduction in this morph, selection within sites, or variation in larval survival and recruitment patterns. Relatively low F _st (differentiation among populations) values found for a broadcast spawning species during this study are in accordance with the hypothesis that brooded planulae are adapted for rapid settlement, whereas planulae produced by broadcast spawners are adapted for widespread dispersal.     

Physiological energetics of the intertidal sea anemone Anthopleura elegantissima

High-intertidal (H) individuals of the sea anemone Anthopleura elegantissima (Brandt) are exposed aerially up to 18 h each day, unlike low-intertidal (L) individuals which may be continuously immersed over many days. Thus, H anemones experience shorter feeding periods compared to L anemones. From 1980 to 1982, H and L anemones were observed and collected at the mouth of Bodega Harbor in North Central California (USA) to determine whether any physiological adaptations mitigate the energetic effects of reduced feeding time in H anemones. Weight of prey in coelenterons of H anemones was three times more than that of L anemones following a single immersion period. This difference is not due to slower digestion rates in H anemones. Prey residence time in coelenterons (4h) was equivalent in both groups. Different prey weights imply that ingestion rates were greater in H individuals. However, all anemones had similar weight-specific feeding-surface areas. Different prey-capture rates result from increased receptivity to prey in H anemones, rather than from increases in feeding surface. Absorption efficiency was inversely related to ration size in anemones from both shore positions. H individuals absorbed food more efficiently than L individuals fed equivalent rations. Ration, not exposure conditions, affected absorption efficiency. Daily growth rates were 1.5 to 1.8% and 1.2 to 1.4% of dry body weight in H and L anemones fed large rations (4.0 to 5.6% of dry body weight), respectively. H anemones fed smaller daily rations, approximating amounts of zooplankton captured naturally (1% of anemone dry weight), had higher growth rates and growth efficiencies than L anemones, which lost mass. Higher growth rates in H anemones, which are supported by higher prey-capture rates, result in attainment of minimum body size for reproduction in a relatively short period of time despite reduction in time available for feeding, thus improving relative fitness of these anemones in the upper intertidal zone.     

Genetic relationships of the colour morphs of Acanthochromis polyacanthus (Pomacentridae) on the northern Great Barrier Reef

Acanthochromis polyacanthus is an unusual tropical damselfish because it lacks pelagic larvae and has lost the capacity for broad-scale dispersal among coral reefs. On the modern Great Barrier Reef, this fish has multiple colour morphs and we have previously described steep genetic clines in the central region between three colour morphs. A similar transition of colour morphs has been found in the far northern region (Latitude 11°S), where at least five colour morphs and intergrading hybrids were found during a survey of 14 locations on nine adjacent reefs. On two of these reefs, monochromatic white and bicoloured morphs coexist, partitioning the reef slope vertically when sympatric, but occupying the full depth range elsewhere. In areas of overlap, they interbreed, producing hybrids of intermediate colour. Although the discriminatory power of our genetic analysis was weakened by the low level of variability in these populations (only four polymorphic loci), allozyme electrophoresis revealed significant genetic differentiation within and among reefs. There was neither simple linkage between colour and protein variation, nor correlation between genetic similarity and proximity of sampled populations. We suggest that the observed chaotic structure is the result of differences among the founding populations, although the two colour-based stocks have been homogenized through a long history of introgression during multiple episodes of isolation and secondary contact. Despite genetic mixing, phenotypic diversity (i.e. colour morphs) seems to have been preserved in these populations by sexual selection, perhaps reinforced by natural selection. The long-term stability of this outcome, however, is suggested by the presence of unique morphs on two reefs that are detached from the continental shelf and surrounded by very deep water. The strong identity of these populations can be attributed to long occupation of these isolated sites compared with those on the continental shelf which have only been established in the last 15 000 yr and still retain differences arising from the initial founder events. Received: 15 January 1997 / Accepted: 15 August 1997     

Feeding and predator-avoidance by the rose anemone Urticina piscivora

We investigated the influence of feeding rate on the behavior of the rose anemone Urticina piscivora. We predicted that anemones would travel less and be less willing to detach from the substrate in response to contact with a predator (Dermasterias imbricata) as feeding rate was experimentally increased. As predicted, anemones receiving 0 mussels (Mytilustrossulus) daily moved significantly more than those receiving either 1 or 2 mussels daily. Anemones receiving 0 mussels daily also detached significantly quicker in response to predator contact compared to the 1 mussel per day group, which in turn detached significantly quicker than did the 2 mussels per day group. A field test of the same prediction gave mixed results. We also examined whether anemones could detect feeding rates of neighbors. Movement rates of anemones receiving 1 mussel daily when neighbors received 0 mussels daily were compared with those when neighbors received 2 mussels daily. We predicted that if the subjects could detect feeding rates of neighbors, they should move less when neighbors received less food than themselves (since their position is relatively good) than when neighbors received more food. The results support this interpretation. Received: 21 August 1996 / Accepted: 22 October 1996     

Genetic connectivity among color morphs and Pacific archipelagos for the flame angelfish, <Emphasis Type="Italic">Centropyge loriculus</Emphasis>

Color variation is used in taxonomic classification of reef fishes, but it may not reliably indicate evolutionary divergence. In the central Pacific, there are three color morphs of the flame angelfish, Centropyge loriculus: a red morph that occurs primarily in the Hawaiian archipelago, the endemic Marquesan color morph with reduced black markings, and an orange morph that occurs throughout the rest of Oceania. The red and orange morphs co-occur at Johnston Atoll (1,300 km south of Hawai’i), but intermediate forms have not been reported. To determine whether the three color morphs represent distinct evolutionary lineages, we compared 641 base pairs of mitochondrial cytochrome b. Forty-one closely related haplotypes were observed in 116 individuals. Analysis of molecular variance (AMOVA) indicated no significant genetic structure among color morphs (Φ_ST = 0.011, P = 0.147). Likewise, there was no significant pairwise structure between sampling locations, separated by up to 5,700 km, after a Bonferroni correction (Φ_ST = 0.000–0.080, P = 0.0130–0.999). Genetic studies in conjunction with larval distribution data indicate that Centropyge species are highly dispersive. While there is a strong geographic component to the distribution of color morphs in C. loriculus, we find no evidence for corresponding genetic partitioning. We do not rule out an adaptive role for color differentiation, but our data do not support emerging species.     

Chemische tarnung. Die stoffliche grundlage der anpassung von anemonenfischen an riffanemonen

Dissolved amino acids (³H-phenylalanine, ³H-proline) were accumulated and incorporated into proteins and glycoproteins by sea anemones. On objects (glass rods, pipe cleaners or filter paper) which had touched labelled anemones, tritiated substances could be detected. The mucus of labelled anemones was analysed by disc-electrophoresis. Tritium activity was found in the bands (stained by amino black). Anemone fishes adapted to labelled anemones had 4 times more tritium activity on their surface than control fishes. Most radioactivity was found on those regions which made closest contact with the anemones. The mucus of anemone fishes adapted to labelled anemones was also analysed by disc-electrophoresis. The analysis revealed protein-containing fractions in which ³H-amino acids were incorporated. The pattern of tritium activity in disc-electropherograms from labelled anemones corresponds to that of fishes adapted to those anemones. Anemones produce specific substances which influence the discharge of their nematocytes. These substances have special functions in the normal behaviour of the anemones. The substances provide protection against self-nettling and prevent the discharge of nematocytes into nearby objects which the tentacles continuously contact. The production of these substances by anemones is completely separate from the association with anemone fishes. The fishes thus simply exploit a mechanism existing independently in the anemones. Therefore, it is possible to form, in an aquarium, unnatural associations between anemone fishes and anemone species which never live in association with fishes in their natural biotope. On the basis of previous information, as well as on these new data, it is possible to develop a model which explains the control of nematocyte discharge: Substances with inhibitory qualities (protecting substances) are produced by the anemones themselves, and de-sensitize the sensory inputs of the anemones (nematocytes and sensory cells). Sensitization takes place as soon as the anemones come into contact with “stimulating substances”. This happens if anemones are touched by food objects or by anemone fishes which have been previously isolated from anemones. The surfaces of these fishes are not impregnated with protecting substances. Adapted anemone fishes, neighbouring anemones of the same species and other “adapted” objects are coated with the inhibitory substances and thus do not induce nematocyte discharge.     

Family structure and variation in reproductive success in blackbirds

In avian families, some offspring are rendered unequal by parental fiat. By imposing phenotypic handicaps (e.g., via asynchronous hatching) upon certain of their offspring and not others, parents structure the sibship into castes of advantaged “core” offspring and disadvantaged “marginal” offspring that results in an asymmetric sibling rivalry. Here, I show how this family structure scales up to population level reproductive consequences. In a 17-year study of red-winged blackbirds (Agelaius phoeniceus), I show that year-to-year variation in the number of surviving offspring is driven primarily by variation in the number of marginal offspring at hatching and their posthatching survival. Clutch size, core brood at hatching, and fledging varied little from year to year and had little direct effect on year-to-year variation in total brood size at fledging; conversely, variation in the size of the marginal brood at hatching and at fledging was much greater. Marginal but not core brood size at hatching rose with mean clutch size; in years where parents laid larger average clutches they did so by adding marginal progeny. The mean posthatching survival of marginal offspring was always lower than that of core offspring in a given year, and there was no overlap in the distributions. The highest mean survival of marginal offspring across years fell below the lowest mean survival of core offspring; broods were deeply structured. There was an overall female bias among fledglings, and the sex ratio varied across years, with a higher proportion of the smaller female nestlings in years of below average reproductive success. Such variation was especially pronounced in the marginal brood where a higher incidence of brood reduction allowed greater potential for sex-biased nestling mortality. In years of the highest average reproductive success, the sex ratio in the marginal brood approached equality, whereas in years of the lowest average reproductive success, more than two thirds of 8-day-old nestlings were female. Structuring the brood into core and marginal elements allowed parents to modulate both offspring number and sex under ecological uncertainty with direct consequences for population-level reproductive success. They produced fewer and less expensive fledglings in below average years and more and more expensive fledglings in above average years.