Strong maternal effects and extreme heterogeneity of progeny development in the caridean shrimp Sclerocrangon boreas (Crangonidae)

Sclerocrangon boreas is uncommon among marine coastal carideans in having a non-dispersing, abbreviated (2-stage) larval phase. We investigated the implications of this strategy in terms of fecundity, offspring provisioning and brood care in S. boreas from the St. Lawrence Gulf and Estuary in 2009–2010. Fecundity scaled positively to female body size but was low due to the production of large, lipid-rich eggs. Offspring size at all stages of development was positively related to female size. Larval traits and lipid dynamics indicate obligatory lecithotrophic development from hatching to juvenile. The larva becomes a juvenile on the mother and remains associated with her for sometime after. The co-occurrence of early egg stages among many juveniles in some clutches raises the possibility that maternal care of juveniles includes the provisioning of trophic eggs or eggs reclaimed from other females.     

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.     

Otolith microstructure and daily increment validation of marbled sole (<Emphasis Type="Italic">Pseudopleuronectes yokohamae</Emphasis>)

We examined the daily deposition of otolith increments of marbled sole (Pseudopleuronectes yokohamae) larvae and juveniles by rearing experiments, and estimated the growth pattern of wild larvae and juveniles in Hakodate Bay (Hokkaido Island, Japan). At 16°C, prominent checks (inner checks; ca. 19.8 µm in diameter) were observed on the centers of sagittae and lapilli extracted from 5-day-old larvae. On both otoliths, distinctive and regular increments were observed outside of the inner checks, and the slopes of regression lines between age and the number of increments (n_i) (for sagittae: n_i=0.98×Day–5.90; for lapillus: n_i=0.96×Day–5.70) did not significantly differ from 1. Inner check formations were delayed at lower temperature, and the inner checks formed 13 days after hatching at 8°C. Over 80% of larvae, just after their yolk-sac has been absorbed completely (stage C), had inner checks on both their otoliths. On the lapilli, other checks (outer check) formed at the beginning of eye migration (stage G). To validate the daily deposition of increments during the juvenile stage, wild captured P. yokohamae juveniles were immersed in alizarin complexone (ALC)-seawater solutions and reared in cages set in their natural habitat. After 6 days, the mean number of rings deposited after the ALC mark was 5.7. The age–body length relationship of wild P. yokohamae larvae and juveniles caught in Hakodate Bay was divided into three phases. In the larval period, the relationship was represented by a quadratic equation (notochord length=–0.010×Age²+0.682×Age–2.480, r²=0.82, P<0.001), and the estimated instantaneous growth was 0.38 mm day^–1 at 15 days, 0 mm day^–1 at 34 days and –0.12 mm day^–1 at 40 days. The age–body length relationship in the early juvenile stage (<50 days) and the late juvenile stage (>50 days) were represented by linear equations (standard length=0.055×Age+5.722 and standard length=0.345×Age–9.908, respectively). These results showed that the growth rates in the late larval periods and the early juvenile stage were lower than those in the early larval stage and late juvenile stage; during the slow growth period, energy appears to be directed towards metamorphosis rather than body growth. This study provided the information needed to use otolith microstructure analysis for wild marbled sole larvae and juveniles.Communicated by T. Ikeda, Hakodate     

The secondary adjustment of clutch size in red-winged blackbirds (Agelaius phoeniceus)

More than a half century ago, the British ornithologist David Lack suggested that parent birds may use brood reduction to track uncertain food, a process facilitated by the asynchronous hatching of their young. Lack sketched the logic of asymmetric sibling rivalry: the phenotypic handicap imposed upon last-hatched marginal offspring renders their growth and survival conditional upon uncertain ecological conditions while buffering first-hatched core offspring from the inimical effects of overcrowding during periods of stringency. Though subjected to numerous indirect tests in short-term studies, the central prediction of Lack's hypothesis - that parents use marginal offspring to track unpredictable brood-rearing conditions and thus achieve a secondary adjustment of clutch size - has never been tested directly. Here we present the results of a 7-year study of marsh-nesting red-winged blackbirds (Agelaius phoeniceus) showing that (1) brood size tracks interannual variability in growth and survival of nestlings, (2) the growth and mortality of marginal but not core offspring is contingent upon stochastic environmental conditions (mean air temperature) during brood rearing, (3) the mortality of marginal but not core offspring is strongly affected by developmental uncertainty in the form of both experimental and natural alterations of brood size, (4) the phenotypic handicap of hatching asynchrony buffers core offspring from poor growth conditions, but (5) its effects upon marginal nestlings are reversible when growth conditions are favourable and especially when brood size is reduced either experimentally or via hatching failure. The presence of marginal offspring ensures that blackbird parents are not left with a too small brood when brood-rearing conditions are favourable. Parents create two castes of progeny: marginal offspring that are strongly affected by both ecological and developmental stochasticity, and core offspring that are not.     

Asymmetric sibling rivalry and nestling growth in red-winged blackbirds (Agelaius phoeniceus)

Many birds hatch their offspring asynchronously, and the adaptive significance of this trait, if any, is controversial. David Lack suggested long ago that by facilitating brood reduction when resources are scarce, hatching asynchrony provides relief from the effects of overcrowding. Some field workers interpret this to mean that the growth and survival of survivors should rise following partial brood loss. Here we show in a 6-year study of red-winged blackbirds (Agelaius phoeniceus) that the presence or absence of marginal offspring in experimentally manipulated broods had virtually no effect upon the growth of core offspring, whereas alterations of the size of core brood had strong and significant effects. Nestling growth was, not surprisingly, slower in broods with partial brood loss. Intriguingly, marginal offspring showed significantly greater variation in mass. Core offspring are less sensitive to, but not exempt from, the inimical effects of resource shortfall than are marginal offspring. The phenotypic handicap appears to marginal offspring a caste of high-variance progeny whose fitness prospects rest upon levels of parental input (stochastic resources) and the size of the core brood (stochastic development). Received: 21 June 1999 / Revised: 5 June 2000 / Accepted: 25 June 2000     

Egg size and laying order in relation to offspring sex in the extreme sexually size dimorphic brown songlark,<Emphasis Type="Italic"> Cinclorhamphus cruralis</Emphasis>

In some bird species, mothers can advantage the offspring of one sex either by elevating them in the laying order to promote earlier hatching or by allocating greater resources to eggs of the preferred sex. In size dimorphic species, the predictions as to which sex should benefit most from such pre-laying adjustments are ambiguous. The smaller sex would benefit from an initial size advantage to help compensate for the faster growth rate of the larger sex. However, an early advantage to offspring of the larger sex might have a greater effect on their lifetime reproductive success than an equivalent advantage to offspring of the smaller sex. We investigated these hypotheses in the polygynous brown songlark, Cinclorhamphus cruralis, which is one of the most sexually size dimorphic birds known. We conducted within-clutch comparisons and found that females hatched from larger eggs and were initially heavier (but not structurally larger) than their brothers. This may afford females an early competitive advantage, as egg volume remained correlated with chick mass until at least 5 days of age. Similarly, we found that hatch order was still positively associated with nestling mass and size when the brood was 10 days of age, but there was no clear relationship between offspring sex and hatching order. During this study, food was plentiful and there were few obvious cases of nestling starvation. When food is limited, we suggest that the greater nutrient reserves of female hatchlings could not only help compensate for their slower growth, but could also give them a survival advantage over their brothers early in the nestling period. Consequently, egg size dimorphism may be an adaptation that facilitates an early shift in brood sex-ratio towards cheaper daughters in conditions of low food availability.     

Factors influencing egg size in the gammarid amphipod Gammarus insensibilis

Developmental and seasonal changes in egg volume were examined in a population of the amphipod Gammarus insensibilis Stock occurring on the south coast of England, towards its northern limit of distribution. Results showed a marked increase in egg volume during development (2.9 times by Egg Stage V), resulting from water uptake and from the conversion of yolk reserves into structural elements. The maximum rate of increase coincides with the period of organ and limb development. At hatching, after initial rupture of the egg membrane by urosome spines, egg volume increases rapidly over a short period (15 to 20 min) by a further 30% (uptake rate 3.6×10^–5 mm³s^–1), followed by a post-hatching decrease in juvenile volume. Increase in size at hatching is the result of drinking by embryos, although changes in body-wall permeability may contribute. Females carrying eggs in an advanced stage of development exhibit egg-collecting behaviour. This is seen as an adaptation to an increased likelihood of egg loss with increase in volume of the brood as hatching approaches. Seasonal changes in Stage I (early) egg size are marked in this species, with winter eggs as much as 60% greater in volume than summer eggs. Egg size is inversely related to the temperature during oocyte development. A simple model has been derived to account for the observed seasonal pattern in egg size. The consequences of seasonal variation in egg and juvenile size are considered.     

Costs and benefits of surplus offspring in the lesser kestrel (Falco naumanni )

Lesser kestrels (Falco naumanni) lay clutches which appear excessive as only 3% of them yield as many young as eggs laid. Four hypotheses may explain the adaptive value of producing surplus eggs: (1) the bet-hedging hypothesis assumes that the environment varies unpredictably and surplus eggs serve to track uncertain resources; (2) the ice-box hypothesis suggests that surplus offspring serve as a reserve food during a period of shortage; (3) the progeny choice hypothesis says that parents produce surplus offspring in order to choose these with higher fitness; and (4) the insurance-egg hypothesis proposes that extra eggs are an insurance against the failure of any egg. To test the significance of this strategy in the lesser kestrel, an experiment manipu-lating brood size at hatching was carried out over 2 years, with good and bad feeding conditions. The experiment consisted of adding a chick to experimental broods where one egg failed to hatch or removing a randomly selected chick from experimental broods where all eggs had hatched. Independently of annual food availability, pairs with brood sizes reduced by one chick fledged more nestlings than pairs with brood size equalling their clutch sizes. Body condition of young was also better in the former group, but only in 1993 (a high-food year). Independently of year, mean local survival of parents with complete broods at hatching was lower than for parents raising reduced broods. These results supported only the insurance-egg hypothesis which says that surplus eggs may be an insurance against the failure of any egg, but parents may suffer reproductive costs when all eggs hatch. Received: 17 January 1997 / Accepted after revision: 27 April 1997     

Hatching asynchrony and survival of insurance offspring in an obligate brood reducing species,the American white pelican

American white pelicans (Pelecanus erythrorhynchos) lay two eggs but typically rear only one young owing to siblicidal brood reduction affecting the later-hatched, or B-chick. When the A-offspring fails at an early age, the B-chick may survive as a replacement (insurance) offspring. Using a combination of nests with natural and artificially manipulated hatching asynchrony, I examined the hypothesis that hatching asynchrony in this species is adaptively tuned to permit B-chicks to survive during the time they are most likely to be needed as replacements, with brood reduction following when they become redundant. Hatching asynchrony over the natural range of 0–4 days significantly increased within-brood mass differentials and reduced B-chick lifespan. Full synchrony had a marginally negative effect on A-chick mass. Greater asynchrony did not significantly affect the number of days B-chicks survived after hatching of the A-chick, owing to a corresponding extension of time B-offspring were protected from harassment while still within the egg. This resulted in a high probability (> 0.8) of B-chicks surviving through the initial period (5–7 days) of maximum early A-chick loss. Redundant B-chicks were subject to heavy brood reduction, with both chicks likely to have survived at only one each of 94 natural and 84 manipulated (0, 2, and 4 days asynchrony) nests. Hatching asynchrony in American white pelicans, in combination with a rapid development of senior chick siblicidal competence, appears to result in a time course of brood reduction appropriate for an effective insurance reproductive strategy. Received: 2 February 1996 /Accepted after revision: 18 May 1996     

Settlement patterns and post-settlement survival in two Mediterranean littoral fishes: influences of early-life traits and environmental variables

We examined the relationships between daily pattern of settlement and environmental parameters during two consecutive years in two littoral fishes, Lipophrys trigloides (Blenniidae) and Chromis chromis (Pomacentridae), in the NW Mediterranean Sea. We also used individual early-life traits (pelagic larval duration, size at hatching and size at settlement) calculated from otoliths, to study the proximate causes of settlement variability and size-selective mortality after settlement. Several early-life characteristics of L. trigloides (planktonic larval duration and size at hatching), and environmental variables averaged during the whole planktonic period (e.g. water temperature, wave height, solar radiation) were related with the magnitude of settlement. In contrast, C. chromis showed no significant relationships between early-life traits and the magnitude of settlement, and a weak relationship between settlement magnitude and environmental variables. Furthermore, juvenile survivors showed larger size at hatching than settlers, indicating that size at hatching affected the juvenile survival of the two species. These results suggest that survival was linked largely to conditions at hatching for both species.     

Sex-specific effects of albumen removal and nest environment manipulation on Barn Swallow nestlings

In avian species, maternal provisioning to the eggs is predicted to be more valuable for the offspring under adverse environmental conditions and intense sibling competition. However, studies manipulating both the amount of maternal pre-hatching resources and the harshness of post-hatching environment have seldom been performed to date. In this experimental study of Barn Swallow (Hirundo rustica) nestlings, we tested the consequences of a reduction in the albumen content of the eggs for fitness-related offspring traits, while performing an unbalanced partial cross-fostering soon after hatching, either increasing or decreasing brood size by one nestling. By molecular sexing of the chicks, we additionally tested for sex-specific sensitivity of individual nestlings to experimental treatments and to sex ratio variation in nestmates. We predicted that chicks hatching from albumen-deprived eggs should suffer more than control chicks from the harsher rearing conditions of enlarged broods. However, although albumen removal depressed chick body mass, chicks hatching from control eggs did not fare better than those hatching from eggs with reduced albumen content in enlarged vs. reduced broods. Albumen removal had sex-specific effects on immunity, with males, but not females, hatching from eggs with reduced albumen content showing a lower T-cell-mediated immune response than controls, suggesting that the two sexes were differentially susceptible to resource deprivation during early ontogeny. In addition, both immune response and chick body mass at age 7 days, when maximum growth rate is attained, declined with an increasing proportion of male nestmates. The effect of brood size manipulation on chick body mass at age 12 days, when peak body mass is attained, was also found to depend on brood sex composition, in that an increase in the proportion of male nestmates depressed offspring body mass in reduced broods, while the reverse was true in enlarged broods. On the whole, these findings suggest that sex differences may exist in environmental sensitivity and patterns of resource allocation among different body functions, and that brood size variation and sex composition may affect offspring fitness-related traits.     

In vitro reproduction of the turbellarian<Emphasis Type="Italic"> Urastoma cyprinae</Emphasis> isolated from<Emphasis Type="Italic"> Mytilus galloprovincialis</Emphasis>

Urastoma cyprinae is a species infecting the gills of several marine bivalves. Although there is some literature on this turbellarian, its life cycle remains unknown. In our work we have demonstrated that reproduction of U. cyprinae can be completed out of its host. More than 50% of turbellarians isolated from mussels (Mytilus galloprovincialis) secreted and cemented a cocoon to the well bottom during the first 72 h of incubation in seawater (34 salinity) at 14°C. Oviposition started at days 1–18 (average 4.8 days) and occurred in most cases inside the protective cocoon. Each Urastoma laid an average of 2.9 egg capsules (range 1–10) and 3.9 embryos were developed inside each egg capsule (range 1–11). Hatching started at days 20–43 (average 24 days). An average of 12.8 juvenile forms (range 1–64) escaped from the cocoon after hatching. The free-swimming juveniles showed a positive phototactic response and survived about a month after hatching. On the basis of our results, we propose a life cycle for U. cyprinae involving a sexual maturation parasitic period in the bivalve gills and a reproduction period including cocoon secretion, egg laying, and hatching that is entirely completed in the external environment.Communicated by S.A. Poulet, Roscoff     

Phototaxis and cannibalism in gammaridean amphipods

Development changes in phototaxis were investigated in three species of estuarine amphipods: Gammarus lawrencianus. G. tigrinus, and G. mucronatus. In water of salinity 16 S, juveniles of G. tigrinus and G. mucronatus were initially photoneutral and became photonegative within two days after hatching. Juveniles of G. lawrencianus were photopositive on hatching and became photoneutral approximately 11 d later. In all three species, the change in phototactic behaviour coincided with the age at which juveniles became less vulnerable to cannibalism by adults. Evidence is presented showing genetic variation for phototactic behaviour in G. lawrencianus. Our data are consistent with the hypothesis that juvenile phototaxis is an evolutionary adaptation to minimise cannibalism by adults.     

Distribution,growth and life cycle of the mesopelagic amphipod Primno abyssalis (Hyperiidea: Phrosinidae) in the southern Japan sea

The vertical distribution, growth, maturation, brood size and life cycle of the hyperiid amphipod Primno abyssalis (formerly P. macropa) were investigated using seasonal samples collected from Toyama Bay, southern Japan Sea, during the period June 1986 to September 1992. Over four different seasons of the year, P. abyssalis was most abundant in the 200 to 350 m strata at night and the 350 to 400 m strata during the day, indicating 100 to 150 m as the general distance of diel vertical movement. Some differences in vertical migrating behavior were noted among juveniles, adult males and females. Population-structure analysis revealed the occurrence of three cohorts aged 0+, 1+ and 2+ yr. Growth as body length in this species is linear with time. Estimated time to complete one life cycle is 1.8 to 2.5 yr for females, but only 0.8 yr for males. Maximum longevity is 2.8 yr. Instar analysis based on the segment number of pleopod rami indicated that newly hatched juveniles molt ten times to reach adult male, and four more times to reach adult female. Adult instar number was found to be only 1 for males and 5 for females. Ovigerous females occurred throughout the year, but the annual peak of release of juveniles from the female's marsupium is estimated to be arly March. Brood size was not correlated with female size, a maximum brood size of 214 eggs was recorded. The dry and ash-free dry weights of instars suggested that juveniles in the female marsupium, adult males, and older adult females are less active feeding or non-feeding stages. Except for the reduced growth rate and the occurrence of small, short-lived males, most characteristics of P. abyssalis are consistent with the present view of the life modes of mesopelagic animals, including linear growth in length, aseasonal reproduction, and smaller brood size coupled with larger eggs.     

The effects of size-dependent predation risk on the interaction between behavioral and life history traits in a stream-dwelling isopod

In field surveys, laboratory observations and field-based assays of behavior, I examined the effects of size-dependent predation risk on the interaction between size at reproductive maturity and maternal care behavior in the stream-dwelling isopod, Lirceus fontinalis. L. fontinalis exhibit population-specific sizes at reproductive maturity which result in population differences in predation risk during the adult phase. Females from streams containing salamander larvae (that prefer small prey) mature at large sizes and then become relatively safe from predation. Females from streams containing fish (that consume all size classes of prey equally) mature at small sizes and remain at risk. I tested whether these differences in expected survival were reflected in the behavior of females during the maternal phase (i.e., the period during which females exhibit maternal care). Female L. fontinalis carry developing juveniles inside a brood pouch. I simulated predatory attacks on gravid female L. fontinalis from the different population types and found that female behavior correlated with population differences in risk. When “attacked”, females from streams with predatory fish (that experience high risk to adult females) released juveniles from the brood pouch, whereas females from populations with predatory salamander larvae (that pose relatively little risk to adult females) did not release juveniles. I discuss the results with reference to the joint evolution of behavioral and life history traits. Received: 6 March 1996 /Accepted after revision: 12 August 1996     

Paternity alone does not predict long-term investment in juveniles by male baboons

Adult male chacma baboons (Papio hamadryas ursinus) form preferential associations, or friendships, with particular lactating females. Males exhibit high levels of affiliative contact with their friends’ infants and defend them from potentially infanticidal attacks (Palombit et al. 1997). Little is known about males’ associations with juveniles once they have passed the period of infanticidal risk. We conducted an observational, experimental, and genetic study of adult male and juvenile chacma baboons in the Moremi Reserve, Botswana. We identified preferential associations between males and juveniles and used behavioral data and a playback experiment to explore whether those associations have potential fitness benefits for juveniles. We determined whether males preferentially invest in care of their own offspring. We also determined how often males invest in care of their former friends’ offspring. The majority of juveniles exhibited preferential associations with one or two males, who had almost always been their mother’s friend during infancy. However, in only a subset of these relationships was the male the actual father, in part because many fathers died or disappeared before their offspring were weaned. Male caretakers intervened on behalf of their juvenile associates in social conflicts more often than they intervened on behalf of unconnected juveniles, and they did not appear to differentiate between genetic offspring and unrelated associates. Playbacks of juveniles’ distress calls elicited a stronger response from their caretakers than from control males. Chacma males may provide care to unrelated offspring of former friends because the costs associated with such care are low compared with the potentially high fitness costs of refusing aid to a juvenile who is a possible offspring.     

Reproductive biology of a wood-boring isopod, Sphaeroma terebrans, with extended parental care

The isopod Sphaeroma terebrans Bate, 1866 burrows in aerial roots of the red mangrove Rhizophora mangle L. The burrows serve as shelter and as a reproductive habitat, and females are known to host their offspring in their burrows. I examined the reproductive biology of S. terebrans in the Indian River Lagoon, a shallow lagoon stretching for ∼200 km along the Atlantic coast of Florida, USA. Reproductive isopods were found throughout the year, but reproductive activity was highest in the fall and during late spring/early summer. During the latter periods, large numbers of subadults established their own burrows in aerial roots. The average numbers of S. terebrans per root were high during the fall, but decreased during the winter and reached lowest levels at the end of the summer. Females reached maturity at a larger size than males, but also grew to larger sizes than the males. The average size of females varied between 8 and 10 mm, the average size of males between 6.5 and 8.5 mm. The number of embryos female⁻¹ was strongly correlated with female body length. No indication for embryo mortality during development was found. Parental females (i.e. with juveniles in their burrows) hosted on average between 5 and 20 juveniles in their burrows (range 1 to 59). Most juveniles found in female burrows were in the manca stage and 2 to 3 mm in body length. Juveniles did not increase in size while in the maternal burrow, and juveniles of similar sizes could also be found in their own burrows. Males did not participate in extended parental care, since most of them left the females after copulation. Many females that were born in the summer produced one brood in the fall and a second during winter/early spring. Females that were born in the fall produced one brood during spring/early summer, but then probably died. Extended parental care in S. terebrans is short compared to other peracarid crustaceans. It is concluded that this reproductive strategy in S. terebrans serves primarily to shelter small juveniles immediately after they emerge from the female body, when their exoskeleton is still hardening and their physiological capabilities are still developing. Thus, in S. terebrans, extended parental care probably aids to protect small juveniles from adverse physical conditions in their subtropical intertidal habitat. Received: 9 December 1998 / Accepted: 24 June 1999     

Solitary behavior in a high-altitude population of the social sweat bee Halictus rubicundus (Hymenoptera: Halictidae)

In the subalpine region of the Rocky Mountains of Colorado, United States, Halictus rubicundus has a solitary life cycle, but it is social in other parts of its known range. The brood is protandrous, with a nearly equal investment in the sexes. Productivity averages 6.5 offspring per foundress female, similar to the second brood of social nests in New York, but less than the combined productivity of both New York broods. Leucophora sp. (Diptera: Anthomyiidae) is the principal cause of brood mortality in Colorado. Foundress females in about half the nests survive until brood emerge as adults. Retention of these foundresses decreases offspring mortality by 68%. Comparable abilities to express solitary behavior with a single brood may characterize other eusocial halictine lineages that have successfully invaded high altitudes in the Rocky Mountains. The apparent inability to do this may help explain the absence of other eusocial halictine bees and polistine wasps at high altitudes, despite their success at lower elevations in the same mountains. Presence or absence of this ability may help explain latitudinal distributions of these lineages in North America. Holarctic distributions of lineages with eusocial behavior can be explained by migration as solitary populations from Eurasia to North America across Pleistocene Bering land bridges, with re-expression of double-brooded, eusocial behavior when the species then extended their ranges southward in North America. Received: 4 November 1994/Accepted after revision: 23 October 1995     

Apparent sibling rivalry in the freshwater fingernail clam<Emphasis Type="Italic"> Sphaerium striatinum</Emphasis>

In Sphaerium striatinum, a freshwater brooding bivalve, up to 97.5% of offspring that adults initially produce fail to reach independence. Marsupial sacs, specialized extensions of gill filaments that act as nurseries, initially contain multiple offspring in various sizes and stages of development. However, by the time offspring reach later stages of development, marsupial sacs typically contain only one offspring. Brood mortality is hypothesized to be the result of competition among embryos for nutrients and/or space. Sphaeriid eggs do not contain enough yolk for offspring to complete development. Adults supply additional nutrients required to reach independence. Brood capacity is limited by adult size. Adults cannot physically brood all offspring they produce. Here, we examine the validity of the competition hypothesis for brood mortality. We reared offspring, in vitro, through metamorphosis under varying nutrient levels and embryo densities. While hatching success and time to hatching were not influenced by nutrients or density, both factors had significant effects on the percentage of embryos completing metamorphosis and timing of metamorphosis. A higher percentage of offspring completed metamorphosis in higher nutrient levels and lower densities. Offspring reared with higher nutrient levels and lower densities also completed metamorphosis more rapidly. We discuss these results in relation to hypotheses for the overproduction of offspring, sibling rivalries, as well as factors that might explain brood mortality in this species.Communicated by T. Czeschlik     

Rising costs of care make spiny chromis discerning parents

When the costs of parental care do not scale with the number of offspring being cared for, inclusion of non-descendant young into broods can be advantageous, leading to systems of alloparental care. However, if the cost of care scales with the number of offspring, selection may act against misdirected parental care. The spiny chromis, Acanthochromis polyacanthus, is a marine fish with extended biparental care, and broods that increase in size over the care period strongly suggest that alloparental care occurs in this species. However, A. polyacanthus parents directly provision their offspring by producing ectodermal mucus for their fry to feed on. The costs of such provisioning may scale with brood size, potentially increasing the costs of parental care. Using wild A. polyacanthus pairs, we tested whether foreign offspring are accepted into established broods, and measured how brood defence effort and mucal feeding scale with brood size. We found that A. polyacanthus discriminate between their own and foreign young, vigorously expelling experimentally introduced foreign offspring. Although defensive effort did not scale with brood size, mucal feeding was strongly dependent on brood size, and this increasing cost of care likely acts as the primary selective force on parental discrimination and rejection of foreign fry in A. polyacanthus.