Supplemental food affects egg size but not hatching asynchrony in rollers

Maternal effects can function as a mechanism of transgenerational plasticity by which the environment experienced by parents is translated into the offspring phenotype and fitness. In birds, parents may affect the competitive ability of their offspring, and hence their fitness, by modifying their hatching pattern and/or egg size. However, little is known about how mothers can modify offspring phenotypes and their fitness in response to a sudden change in environmental conditions during egg-laying. Here, we studied the effect of supplemental food during egg-laying on hatching asynchrony and egg size in the Eurasian roller (Coracias garrulus), a species with marked hatching asynchrony. We also explored the effects of maternal investment on offspring fitness. Food supplementation did not affect hatching asynchrony. However, females in food-supplemented nests laid eggs that increased in size with laying order except for an ultimate small egg. Meanwhile, size of eggs laid by females in control nests did not change with laying order. Supplemental food positively affected hatchability of the egg laid just before the last one and negatively affected hatchability of the last laid egg, which seemed to be a side effect of egg size. Consequently, food-supplemented nests produced fewer fledglings and had higher probabilities of suffering brood reduction than control nests. We conclude that egg size in rollers is a plastic trait, sensitive to short-term changes in food conditions. Furthermore, our results show that maternal investment in egg size may potentially affect offspring fitness.     

Food supplementation affects egg albumen content and body size asymmetry among yellow-legged gull siblings

Parental decisions can determine offspring experience of environmental conditions. Such ‘maternal’ effects act both before and after hatching via, e.g., egg quality or the social milieu predisposed by parents. Resource availability may constrain the expression of adaptive maternal effects, and the specific pattern of allocation of these effects among offspring depending on their sex or birth order can result in different fitness payoffs to parents. Declining egg mass with laying order observed in several bird species may constitute an adaptive strategy of parental favouritism towards early hatching offspring with larger reproductive value but may also result from nutritional constraints on laying effort. A previous study has shown that the small size of the third, last laid (c-)egg in yellow-legged gull (Larus michahellis) clutches depends on food availability and that food-supplemented mothers increase the size of their female but not male c-eggs. Here, we show that increased mass of c-eggs laid by females supplemented with food after clutch initiation depends on increased albumen mass, which, in turn, enhances the size of daughters at hatching. Because asynchronous hatching results in a competitive disadvantage of c-chicks, present results suggest that mothers relieved from nutritional constraints enhance the size of daughters to compensate for their larger susceptibility to hatching last. The study also confirms the role of egg albumen content in determining hatchling size, previously experimentally detected only in one species in the wild. The effect of increased egg mass on offspring size persisted at least until day 8 after hatching, when, however, it did not vary with sex, suggesting intense negative selection on small female c-chicks in control broods. Hence, maternal effects mediated by egg albumen content had persistent effects on offspring size.     

Potential latitudinal variation in egg size and number of a geographically widespread reef fish,revealed by common-environment experiments

Variation in maternal reproductive traits was examined in field and reared populations of a geographically widespread reef fish, Pomacentrus coelestis (Pomacentridae), drawn from three different latitudes in Japan. Size-specific clutch size and clutch weight of wild fish increased with increasing latitude. Conversely, latitudinal variation in egg size of wild fish was obscure in same-season comparisons, probably because of the temperature effect on egg size. Common-environment experiments conducted at three temperatures showed that egg size decreased with increasing temperature in all populations. In the experiments, egg size, clutch size and clutch weight differed among populations at all temperatures, showing clear latitudinal clines. Females from low latitude spawned larger eggs at every experimental temperature. Size-specific clutch size and weight were greater in females from high latitude. Thus, the northern fish had a larger reproductive output per spawning and a larger number of smaller eggs in a spawning. Such interpopualtion variation in this fish is likely to be partially genetically based, although environmental effects on the variation cannot be entirely ruled out. This study provides evidence of potential latitudinal variation in the egg size and number in a coastal fish, by common-environment experiments. The close correspondence between latitudes and these maternal reproductive traits may be a consequence of local adaptation.     

Early maternal investment in male and female African elephant calves

Summary The suckling behaviour of 130 freeranging elephant calves aged between birth and 4.5 years old was examined in Amboseli National Park, Kenya. Analyses of frequencies of suckling and durations of suckling bouts showed that males attempted to suckle more often, were more successful at their attempts, and as a result were estimated to have a higher milk intake than did female calves. Mothers were equally tolerant of their sons' and daughters' demands to suckle at young ages, but were less tolerant of their older sons' demands. The growth rates of males based on hind footprint length were faster than those of females from birth onwards. During drought years with low food availability, male calf survivorship in the first year was lower than that of female calves. During wet years, there was little difference between sexes in survivorship. It appeared that during dry years mothers were unable to sustain milk production at a level that met the metabolic requirements of their sons, and as result male calves were more likely to die. Females with a surviving son tended to have a longer interbirth interval than did females with a surviving daughter. We suggest that greater early maternal investment in male calves occurs because, in the highly-competitive polygynous mating system of elephants, size in adult male elephants is an important factor in mating success.     

Intra-group relatedness affects parental and helper investment rules in offspring care

In any system where multiple individuals jointly contribute to rearing offspring, conflict is expected to arise over the relative contributions of each carer. Existing theoretical work on the conflict over care has: (a) rarely considered the influence of tactical investment during offspring production on later contributions to offspring rearing; (b) concentrated mainly on biparental care, rather than cooperatively caring groups comprising both parents and helpers; and (c) typically ignored relatedness between carers as a potential influence on investment behavior. We use a game-theoretical approach to explore the effects of female production tactics and differing group relatedness structures on the expected rearing investment contributed by breeding females, breeding males, and helpers in cooperative groups. Our results suggest that the breeding female should pay higher costs overall when helpful helpers are present, as she produces additional offspring to take advantage of the available care. We find that helpers related to offspring through the breeding female rather than the breeding male should contribute less to care, and decrease their contribution as group size increases, because the female refrains from producing additional offspring to exploit them. Finally, within-group variation in helper relatedness also affects individual helper investment rules by inflating the differences between the contributions to care of dissimilar helpers. Our findings underline the importance of considering maternal investment decisions during offspring production to understand investment across the entire breeding attempt, and provide empirically testable predictions concerning the interplay between maternal, paternal and helper investment and how these are modified by different relatedness structures.     

Geographic variation in a predator-induced defense and its genetic basis

Predator-induced morphological defenses are a well-known form of phenotypic plasticity, but we continue to have a limited understanding of geographic variation in these responses and its genetic basis. Here we examine genetic variation and geographic differentiation in the inducible defenses of tadpoles (Rana pirica) in response to predatory salamander larvae (Hynobius retardatus). To do so, we crossed male and female frogs from a "mainland" Japanese island having predaceous salamanders and a more isolated island not having predaceous salamanders and raised resulting offspring in the presence and absence of H. retardatus. Mainland tadpoles exhibited a higher capacity to express the inducible morphology (a more bulgy body) than those from the predator-free island, and expression of the bulgy morph in mainland-island hybrids produced phenotypes that were intermediate to those produced by pure crosses. In addition, parental sex had no effect on expression of the bulgy morph. Our results support the hypothesis that geographic variation in inducible defenses is linked to the additive effects of autosomal alleles that are shaped by differences in historical exposure to the inducing predator.     

Geographic range size and extinction risk assessment in nomadic species

Geographic range size is often conceptualized as a fixed attribute of a species and treated as such for the purposes of quantification of extinction risk; species occupying smaller geographic ranges are assumed to have a higher risk of extinction, all else being equal. However many species are mobile, and their movements range from relatively predictable to‐and‐fro migrations to complex irregular movements shown by nomadic species. These movements can lead to substantial temporary expansion and contraction of geographic ranges, potentially to levels which may pose an extinction risk. By linking occurrence data with environmental conditions at the time of observations of nomadic species, we modeled the dynamic distributions of 43 arid‐zone nomadic bird species across the Australian continent for each month over 11 years and calculated minimum range size and extent of fluctuation in geographic range size from these models. There was enormous variability in predicted spatial distribution over time; 10 species varied in estimated geographic range size by more than an order of magnitude, and 2 species varied by >2 orders of magnitude. During times of poor environmental conditions, several species not currently classified as globally threatened contracted their ranges to very small areas, despite their normally large geographic range size. This finding raises questions about the adequacy of conventional assessments of extinction risk based on static geographic range size (e.g., IUCN Red Listing). Climate change is predicted to affect the pattern of resource fluctuations across much of the southern hemisphere, where nomadism is the dominant form of animal movement, so it is critical we begin to understand the consequences of this for accurate threat assessment of nomadic species. Our approach provides a tool for discovering spatial dynamics in highly mobile species and can be used to unlock valuable information for improved extinction risk assessment and conservation planning.     

Testis size variation in frogs: testing the alternatives

While sperm competition may be a major factor affecting relative testis size in vertebrates as a whole, additional hypotheses have not been given much attention in frogs. This is important because sperm competition is relatively uncommon in frogs and relative testis size varies in frogs that do not have multiple-male breeding systems. This paper tests two additional hypotheses for differences in relative testis size among frogs: relative clutch size (number of eggs/snout vent length) and androgen level. Testis size was measured in 90 species of frogs belonging to five families. Relative testis size was found to be positively correlated with relative clutch size in species that lack sperm competition. Mean androgen levels of species also positively covaried with relative testis size. However, there was no correlation between relative testis size and level of male agonistic behavior among species, despite other work indicating that testosterone levels are positively correlated with agonistic behavior in at least some species. These findings suggest that a number of factors in addition to sperm competition are important in the evolution of testis size in male frogs. Received: 17 January 1997 / Accepted after revision: 23 June 1997     

Geographic variation in polyphenolic levels of Northeastern Pacific kelps and rockweeds

Brown algal polyphenolic compounds are secondary metabolites whose functions may include protecting plants from pathogens or damage by UV radiation, and deterring feeding by herbivores. We present here the first analysis of spatial variation (at scales from tens of meters to hundreds of kilometers) in concentration of these compounds in two orders of brown algae from the northeastern Pacific Ocean. In kelps (order Laminariales), variation among sites was significant in only 25% of species examined and was consistent within families (high in the Alariaceae and low in the Laminariaceae and Lessoniaceae). In rockweeds (order Fucales, family Fucaceae), site variation was high in three of four species examined. Both the proportion of high polyphenolic kelp species and the magnitude of spatial variation within species from both kelps and rockweeds were much higher than would have been predicted from previous studies in other regions. In one kelp (Laminaria groenlandica), significant differences between sites occurred at scales of only tens of meters. No latitudinal clines were observed. Differences in phenolic concentrations of kelps spanned nearly an order of magnitude in one species, Hedophyllum sessile. Phenolic levels were significantly higher in members of the Fucales than the Laminariales, but showed no significant differences between intertidal and subtidal species. Received: 22 July 1996 / Accepted: 26 October 1998     

Helpers and egg investment in the cooperatively breeding acorn woodpecker: testing the concealed helper effects hypothesis

In cooperatively breeding acorn woodpeckers (Melanerpes formicivorus), helper males have a large positive effect on fledging success in good acorn crop years but only a small positive effect in poor acorn crop years, while helper females exhibit the opposite pattern. Based on these findings, we tested the “concealed helper effects” hypothesis, which proposes that laying females reduce investment in eggs (with respect to their size, number, or quality) in a way that confounds helper effects and results in an absence of a relationship between helpers and breeding success. Results generally failed to support the hypothesis. Mean egg size was positively related to temperatures during the 10 days prior to egg-laying and negatively related to the food supply as indexed by the prior fall’s acorn crop, but there were no significant differences vis-à-vis helpers except for interactions with the acorn crop that only partly corresponded to those predicted. With respect to clutch size, females laid larger clutches when assisted by female helpers, opposite the pattern predicted. Although our results suggest that egg size is adjusted to particular ecological circumstances, we conclude that neither egg nor clutch size is adjusted in a way that confounds the apparent effects of helpers, as proposed by the concealed helper effects hypothesis.     

Muscle esterase variation and size variation in the sand launce Ammodytes dubius

Esterase variation detected in homogenates of muscle tissue taken from the sand launce Ammodytes dubius in April, 1970 and 1971 is described. Eight alleles were found segregating at a single locus, and the frequencies of these alleles were homogenous in population samples taken on the Emerald Bank, off Nova Scotia, Canada, in successive years. When ordered by their electrophoretic mobility, the profile of the frequencies of these alleles was unimodal and symmetric. In both of the population samples, there were significant excesses of homozygotes and deficencies of heterozygotes. There was an association between genotype and size of individuals among homozygous genotypes; individual homozygous for the most common, intermediately-migrating allele were larger than homozygotes bearing alleles with relatively fast or slow mobilities.     

Genotype-dependent fecundity and temporal variation of spawning in hybrid mussel (Mytilus) populations

Mussel samples were collected at 4 to 6 wk intervals throughout 1987 from two hybridMytilus edulis/M. galloprovincialis populations, at Croyde Bay and Whitsand Bay, in southwest England. These were analyzed at two polymorphic loci which are diagnostic for allozyme differences which typifyM. edulis andM. galloprovincialis. Dry mantle weight as a function of shell length was determined for all individuals of each sample. Size-frequency data for the two populations was obtained in September 1987 and March 1988. For all genotypes at both sites, fecundity was a function of shell length, and in both populations the frequency ofM. galloprovincialis alleles was positively correlated with shell length. At both sites, allozyme genotype explained a significant amount of variation in mantle weight either when assessed as a main effect or when assessed as an interaction with shell length or time of collection. At Croyde,M. galloprovincialis mussels had greater estimated fecundity per unit length than theM. edulis mussels. Differences in the timing of spawning activity between theM. edulis and theM. galloprovincialis mussels were inferred, and these differences might act to reduce the amount of interbreeding at Croyde. At Whitsand, a reduced level of variability in the timing of spawning activity and fecundity between the genotypes was observed and explained by a higher degree of genetic mixing. Because theM. galloprovincialis mussels had (1) a greater estimated fecundity at any length, and (2) a greater mean length than theM. edulis mussels, the mean genotypic annual fecundity perM. galloprovincialis mussel was 2.8 times greater than an individualM. edulis mussel at Croyde, and 2.2 times greater than an individualM. edulis mussel at Whitsand. This evidence thatM. galloprovincialis mussels have an advantage in fecundity, and thus perhaps in fertility, taken together with the evidence thatM. galloprovincialis also has a higher viability, indicates directional selection in favour of theM. galloprovincialis phenotype. Because of the observed temporal stability of the population it seems likely that this selection is counterbalanced by a massive imigration ofM. edulis spat from neighbouring populations.     

Effects of organic additions to sediment,and maternal age and size,on patterns of offspring investment and performance in two opportunistic deposit-feeding polychaetes

The investment made by a reproducing organism in its offspring can be affected by both extrinsic (environmental) and intrinsic (female condition) factors. The purpose of this study, conducted between December 1990 and June 1992, was to examine the influence of organic additions to sediment, and female age and size, on patterns of offspring investment and performance in the opportunistic polychaete Capitella sp. I (cf. Capitella capitata Farbricius). Sediment treatments were composed of marsh mud enriched with sewage or algae, or spiked with #2 fuel oil. All three of these additions are associated with opportunistic responses in Capitella sp. I populations in the laboratory and field. As a means of comparison,effects of sewage additions were also examined on the spionid Streblospio benedicti Webster. Organic additions to mud had no detectable effect on the C or N content of embryos in either species. However, sewage had a positive effect on juvenile performance in Capitella sp. I. Juveniles produced by adult Capitella sp. I in the sewage treatment grew 2 times larger (in 2 wk) than juveniles produced by adults in the mud treatment when these juveniles were raised in the same sediment type (mud). Such effects may facilitate opportunistic responses in Capitella sp. I offspring by reducing time to first reproduction. In addition to treatment sediment effects, maternal age and size had a strong influence on patterns of offspring investment in Capitella sp. I. The material investiment measured in collected embryos declined with maternal age in the oil and sewage treatments. The C:N ratio of embryos increased with maternal age for Capitella sp. I except in the algae treatment, which showed no effect of maternal age. Juvenile growth decreased with maternal age in the sewage treatment for Capitella sp. I. Maternal size appeared to have a positive effect on embryo investment in the mud and oil treatments, where reproducing worms were smallest. Previous studies of Capitella sp. I have documented a broad range of environmentally induced responses in growth and fecundity.The results of this study emphasize the importance of maternal environment, age, and size effects on the relationship between offspring investment and performance.Such effects may have profound consequences for the dynamics of populations and the fitness of individuals. Present address: WES-ES-F, USAE Waterways Experiment Station, 3909 Halls Ferry Rd., Vicksburg, Mississippi 39180-6199, USA     

Biparental investment and reproductive success in a subsocial desert beetle: the role of maternal effort

Parastizopus armaticeps is a nocturnal subsocial detritivorous desert tenebrionid that produces very few offspring per brood. The two environmental factors that constrain reproduction, rapid sand desiccation rate and food scarcity, are countered by biparental effort. Males dig and extend breeding burrows, maintaining their moisture level; females forage on the surface at night for high-quality detritus, the larval food. This was shown to be a scarce and unpredictable resource for which there is high competition. When food was supplemented in a field experiment, offspring number and survivorship doubled and burrow failure due to desiccation dropped from approximately half, the typical failure rate for unsupplemented burrows, to zero. Food supplementation did not, however, increase larval foodstore size and there was no difference in the size of the offspring produced. Supplemented females reallocated their time, foraging less and digging more with the male. This change in maternal behaviour patterns resulted in deeper burrows which remained moist longer, thus extending the larval production period. Female foraging efficiency, particularly food retrieval speed, determined how much time females could allocate to digging, consequently increasing the reproductive success of the pair. Burrow depth and sand moisture level at the burrow base were the major correlates of reproductive success, but the scarcity and unpredictability of high-quality food on the surface and the competition for this resource influenced the number of offspring indirectly through their effect on female behaviour. Received: 29 November 1996 / Accepted after revision: 7 December 1997     

The influence of maternal rank and infant sex on maternal investment trends in rhesus macaques: birth sex ratios,inter-birth intervals and suckling patterns

Summary In the new colony of rhesus macaques at Madingley, no overall difference was found in the length of the inter-birth intervals following the birth of male and female infants. The lack of overall differences in the reproductive costs of raising sons and daughters, was associated with an absence of differences in suckling patterns between male and female infants. It is argued that similar pre-weaning investment in sons and daughters may be related to the presence of similar growth rates for male and female infants during the first year of life. Most high ranking mothers reproduced in consecutive years, while low ranking mothers were more likely to experience two year long inter-birth intervals. Further analyses revealed that this difference was mainly due to the fact that low ranking mothers always failed to reproduce the year after raising a daughter. Thus, no difference was found in the length of inter-birth intervals between high ranking mothers with infants of either sex and low ranking mothers with sons. The daughters of low ranking females suckled more frequently and making more nipple contacts per bout, and such high nipple stimulation was responsible for the reproductive inhibition experienced by their mothers. It is suggested that the high degrees of nipple stimulation may have been a consequence of the high levels of aggression that low ranking females with daughters have been shown to receive, since their mothers could have allowed frequent ventro-ventral contact in order to protect them. Given that low ranking mothers find daughters reproductively costly to raise, it is possible that such high reproductive costs have played a significant role in the evolution of sex ratio biases at birth. As in other primate populations, in this colony low ranking females produced a greater proportion of sons than daughters, and high ranking mothers showed the opposite trend.     

Geographic variation in the growth rates of Littorina littorea and L. saxatilis

The two intertidal grazers Littorina littorea and L. saxatilis occupy extensively overlapping distributions on both sides of the Atlantic. Morphological, electrophoretic and physiological evidence suggests that the direct developer L. saxatilis is more geographically differentiated than the planktonic developer L. littorea. This hypothesis was tested by collecting both species in 1980 and 1981 from three geographically separated beaches in Massachusetts, USA, marking them, and releasing them in two of the sites. Individuals from all three L. littorea populations, grown at a common site, tended to grow at similar rates, while L. saxatilis exhibited consistent population differences in individual growth. This study supports the view that L. saxatilis achieves its broad geographic distribution via locally adapted ecotypes, while L. littorea achieves its distribution by individual physiological plasticity.     

Philopatry,reproductive success of females,and maternal investment in the red-necked wallaby

Summary Female red-necked wallabies settle within their mothers' home ranges, apparently for life, while males disperse at about two years of age. However, sons spend much more time with their mothers before dispersing than do daughters of similar ages. Females who associate regularly with their subadult offspring are less likely to reproduce successfully at their next breeding attempt than are females who spend little time with their subadults, and sons therefore impose greater short-term reproductive costs on their mothers than do daughters. Females who are generally gregarious also suffer reduced reproductive success, even though reproductive success is independent of local density. It is suggested that the reproductive costs to females of associating with their subadult offspring, and other relatives, are incurred through tolerance of ecological competition from those kin, and therefore reflect a form of prolonged maternal investment, which is initially heaviest in sons but is sustained for longer periods in daughters. Females produce equal numbers of male and female offspring, and spend equal amounts of time suckling them in infancy.     

Intraspecific variation in a predator affects community structure and cascading trophic interactions

Intraspecific phenotypic variation in ecologically important traits is widespread and important for evolutionary processes, but its effects on community and ecosystem processes are poorly understood. We use life history differences among populations of alewives, Alosa pseudoharengus, to test the effects of intraspecific phenotypic variation in a predator on pelagic zooplankton community structure and the strength of cascading trophic interactions. We focus on the effects of differences in (1) the duration of residence in fresh water (either seasonal or year-round) and (2) differences in foraging morphology, both of which may strongly influence interactions between alewives and their prey. We measured zooplankton community structure, algal biomass, and spring total phosphorus in lakes that contained landlocked, anadromous, or no alewives. Both the duration of residence and the intraspecific variation in foraging morphology strongly influenced zooplankton community structure. Lakes with landlocked alewives had small-bodied zooplankton year-round, and lakes with no alewives had large-bodied zooplankton year-round. In contrast, zooplankton communities in lakes with anadromous alewives cycled between large-bodied zooplankton in the winter and spring and small-bodied zooplankton in the summer. In summer, differences in feeding morphology of alewives caused zooplankton biomass to be lower and body size to be smaller in lakes with anadromous alewives than in lakes with landlocked alewives. Furthermore, intraspecific variation altered the strength of the trophic cascade caused by alewives. Our results demonstrate that intraspecific phenotypic variation of predators can regulate community structure and ecosystem processes by modifying the form and strength of complex trophic interactions.     

Genetic variation in defense chemistry in wild cabbages affects herbivores and their endoparasitoids

Populations of wild Brassica oleracea L. grow naturally along the Atlantic coastlines of the United Kingdom and France. Over a very small spatial scale (i.e., <15 km) these populations differ in the expression of the defensive compounds, glucosinolates (GS). Thus far, very few studies have examined interactions between genetically distinct populations of a wild plant species and associated consumers in a multitrophic framework. Here, we compared the development of a specialist (Pieris rapae) and a generalist (Mamestra brassicae) insect herbivore and their endoparasitoids (Cotesia rubecula and Microplitis mediator, respectively) on three wild populations and one cultivar of B. oleracea under controlled greenhouse conditions. Herbivore performance was differentially affected by the plant population on which they were reared. Plant population influenced only development time and pupal mass in P. rapae, whereas plant population also had a dramatic effect on survival of M. brassicae. Prolonged development time in P. rapae corresponded with high levels of the indole GS, neoglucobrassicin, whereas reduced survival in M. brassicae coincided with high levels of the aliphatic GS, gluconapin and sinigrin. The difference between the two species can be explained by the fact that the specialist P. rapae is adapted to feed on plants containing GS and has evolved an effective detoxification system against aliphatic GS. The different B. oleracea populations also affected development of the endoparasitoids. Differences in food-plant quality for the hosts were reflected in adult size in C. rubecula and survival in M. mediator, and further showed that parasitoid performance is also affected by herbivore diet.