Mixed sex allocation strategies in a parasitoid wasp

The sex allocation strategy of the parasitoid Laelius pedatus (Hymenoptera: Bethylidae) on different-sized hosts was investigated. The wasp lays from one to five eggs, and clutch size increases with host size. On the smallest hosts, single male eggs are laid, while on slightly larger hosts single female eggs are laid. On still larger hosts, gregarious clutches are laid which nearly always consist of a single male and one or more female eggs. The sex ratio strategy of the wasp appears to be influenced by a combination of local mate competition and conditional sex expression based on host quality. Received: 6 June 1996 / Accepted: 13 October 1996     

Incubation capacity and clutch size determination in two calidrine sandpipers: a test of the four-egg threshold

Several groups of vertebrate taxa, including shorebirds, are unusual in that they produce a fixed number of offspring. The aim of this study was to examine whether the incubation capacity of western sandpipers (Calidris mauri) and semipalmated sandpipers (C. pusilla) limits their maximum clutch size to four eggs. Experimental enlargement of clutch size had no effect on rates of nest abandonment, nest attendance or loss of body mass by incubating sandpipers. The duration of incubation was significantly longer for enlarged five-egg nests, and there were trends towards increased partial clutch loss and asynchrony at hatch, but overall hatching success was unaffected by experimental egg number. I conclude that small, calidrine sandpipers with biparental care are able to compensate for an additional egg in an enlarged nestbowl, despite the constraints of conically shaped eggs and two brood patches. Possibly, shorebirds do not lay more than a fixed clutch size of four eggs because selection on factors acting during egg production or brood-rearing is more important in regulating offspring number. Received: 20 June 1996 / Accepted: 30 September 1996     

The coadaptation of female morphology and offspring size: a comparative analysis in crickets

Few studies of invertebrates have considered combinations of morphological and life history traits in the context of the evolution of reproductive strategies. Cricket species that exploit habitats harsh with respect to egg survival have evolved a long ovipositor, presumably because laying deep in the soil reduces egg mortality. Yet hatchling mortality increases with laying depth, and the ability of hatchlings to climb through the soil increases with egg size. Thus a conflict may exist between survival of the egg and that of the hatchling, inducing a positive covariation between egg size and ovipositor length across species evolving under contrasting selective habitats. We used the phylogenetic autocorrelation method and a path analysis to assess whether egg size coevolved with ovipositor length across 40 species of crickets, and whether egg size was affected by body size or ecological factors that influence egg mortality. Body size and ovipositor length were affected by taxonomic association, whereas common ancestry had no significant effect on egg size, diapausing strategy, and oviposition preference for soil types. The path model indicated that 29.11% of the variance in egg size was explained by independent evolution. As expected, ovipositor length was positively correlated with egg size, and species diapausing in the egg stage produced larger eggs than crickets diapausing in the nymphal stage or with no diapause. Ovipositor length and diapausing strategy were the first and second most important traits, respectively, in term of the proportion of variance in egg size explained by specific values. These results support the hypothesis that the ability of hatchlings to climb through the soil, and variation in diapause strategies, are general selective factors affecting the evolution of egg size in crickets. Phylogeny explained 51.01% of the variance in egg size. Egg size in a current cricket species, however, was not directly determined by egg size in its ancestor. Instead, it was strongly related to the phylogenetic values of body size and ovipositor length. Such indirect phylogenetic effects of body size and ovipositor length may have arisen because clades originating from ancestors with different ovipositor lengths experienced different selective pressures on egg size. Recelived: 13 October 1995 / Accepted: 30 September 1996     

PARENTAL QUALITY AND SELECTION ON EGG SIZE IN THE MAGELLANIC PENGUIN

We examined the relative contributions of egg size and parental quality to hatching success, fledging success, and chick growth in the Magellanic penguin (Spheniscus magellanicus) be exchanging clutches between nests to reduce the covariation between egg and parental factors. Among control nests, fledging success increased slightly with egg size. However, the effect of egg size independently of parental quality was limited to an influence on chick mass and size for the first 10 days post-hatching. In contrast, attributes of the parents influenced nesting success and chick size at fledging, independently of the egg size actually raised. We suggest that the common occurrence of a positive phenotypic correlation between egg size and fledging success is due to two factors: (1) adults laying large eggs tend to be of higher quality; and (2) to the extent that egg size does influence early survival independently of parental quality, the effect on survival is due to a maternal effect on egg composition rather than an inherent effect of egg size.     

CLUTCH SIZE, EGG SIZE, HATCH WEIGHT AND LAYING DATE IN RELATION TO EARLY MORTALITY IN RED GROUSE LAGOPUS LAGOPUS SCOTICUS CHICKS

Clutches of Red Grouse eggs were collected from the wild and subsequent hatching and rearing done in standard conditions in captivity. Variations in chick survival from one clutch to another in the same year were related to differences in hatch weight. Hatch weight was determined only partly by egg size. Weight loss between laying and hatching was related to survival independently of egg size. Variation in this weight loss obscured any simple relationship between egg size and survival, except in eggs laid by captive hens. Intrinsic differences amongst hens caused some variations in laying date, egg size, hatch weight and chick survival. Variations in egg size and hatch weight accounted for less than half the variation in survival; other unmeasured intrinsic factors were also important. Big clutches hatched earlier than small ones. The commonest clutches were of seven and eight eggs, with six and nine frequent. Very big clutches of ten or more eggs were infrequent and chicks from them sometimes survived worse than from smaller clutches. As in other species, the commonest clutch sizes were not the most productive. There was no simple relationship between egg size and clutch size.     

The effect of mermithid parasitism on predation of nymphal Baetis bicaudatus (Ephemeroptera) by invertebrates

We investigated how infection by the mermithid nematode Gasteromermis sp. affected predation on its nymphal mayfly host, Baetisbicaudatus, by two invertebrate predators – the stonefly nymphs of Kogotusmodestus and the caddisfly larvae of Rhyacophilahyalinata. Predation trials and behavioral observations were conducted in stream-side, flow-through experimental chambers. When parasitized and unparasitized prey were offered in equal numbers, K. modestus consumed significantly more parasitized than unparasitized nymphs. R. hyalinata consumed equal numbers of both prey types. Behavioral observations of foraging K.␣modestus on parasitized and unparasitized prey suggested that the increased consumption of parasitized nymphs was due to differences in the behavior of infected mayflies in response to the predator. Specifically, parasitized nymphs drifted less often to escape an approaching predator (non-contact encounters) compared to unparasitized nymphs, which increased the number of contact encounters and attacks that occurred between K.␣modestus and parasitized prey. Because all hosts are castrated, these behavioral alterations affect only the fitness of the parasite, which is killed along with its host by invertebrate predation. We present a number of hypotheses to explain why the parasite causes increased predation on its host. These include the large size of the parasite affecting the sensory abilities of the host, the larger energetic costs of escape behavior for parasitized individuals, and natural selection from fish predation against drifting behavior by parasitized individuals. Received: 27 May 1996 / Accepted: 30 September 1996     

Neighborhood density and reproductive potential in harvester ants

When neighbors compete for resources, the characteristics of a neighborhood may affect fitness. We examined the relationship between reproductive success and the density and size/age characteristics of neighbors in a population of the seed-eating ant, Pogonomyrmex barbatus, in which the ages of all colonies were known. Reproductive success was estimated by trapping and counting the number of alate, reproductive ants emerging from the nest for the annual mating flight. Alate production was negatively related to neighborhood density. Decreased production of alates by more crowded colonies may be due to competition for food with surrounding colonies. Neighbor size/age was unrelated to alate production. If alate production is correlated with lifetime reproductive success, these results suggest that selection favors colonies that monopolize more space, whatever the size of neighboring colonies. Received: 12 February 1996 / Accepted: 6 September 1996     

The separate effects of egg size and parental quality on the development of ornamental plumage coloration

The fitness‐related consequences of egg size, independent of the influences of parental quality, are poorly understood in altricial birds. Not only can egg size and parental quality influence growth and survival, but each could influence the development of condition‐dependent plumage coloration in offspring. The Eastern Bluebird Sialia sialis is an altricial, multi‐brooded, cavity‐nesting passerine in which juveniles display dichromatic UV‐blue plumage. Previous research suggests that plumage coloration acts as a signal of individual quality among juvenile and adult Eastern Bluebirds. Here, we separate the effects of egg size and parental quality (defined by egg size laid) on nestling growth and plumage ornamentation by exchanging clutches of large eggs with clutches of small eggs. Nestlings were significantly larger immediately post‐hatching when hatched from a large egg, but to maintain a larger size, nestlings needed to have hatched from a large egg and to have been reared by high‐quality parents. Nestlings were brighter when reared by high‐quality parents and this relationship was strongest later in the breeding season. Nestlings exhibited greater UV chroma if hatched early in the season, but UV chroma was not significantly affected by egg size or parental quality. These findings demonstrate varying influences of both egg size and parental quality on offspring growth and plumage ornamentation but suggest that quality of post‐hatching investment is more influential than pre‐hatching investment.     

Effects of emergence date and maternal size on egg development and sizes of eggs and first-instar nymphs of a semelparous aquatic insect

We examined whether or not sizes of eggs and offspring were related to emergence date or maternal size in a semelparous aquatic insect (the burrowing mayfly, Hexagenia) in which parental care is lacking and oviposited eggs are passively dispersed. We quantified the size of males and female imagos over the emergence span at a site on the Detroit River, Canada, and investigated relationships between emergence date and female size and (1) egg size and (2) size of first-instar nymphs. Although size of female imagos (H. limbata and H. rigida combined) declined significantly (P<0.025) over the emergence season, there was no significant relationship between body length and emergence date for males of either species. Males were significantly (P<0.001) smaller than females. H. limbata eggs, subsampled from three individuals from each of three size classes of female imagos collected on seven sampling dates, were measured using video image analysis. Eggs (n=100) oviposited by each of 63 H. limbata imagos were inspected daily for hatching. Newly hatched nymphs were removed, counted and measured. Egg size (P<0.001) and size of first-instar nymphs (P<0.001) varied significantly with emergence date, but not maternal size. The largest eggs and newly hatched nymphs occurred at peak emergence of adults. The synchronous release of larger (faster-sinking) eggs may result in reduced predation. Plasticity in egg development time and egg and nymph size may account for the ability of this taxon to recover from episodes of massive population reduction. Received: 12 March 1996 / Accepted: 24 February 1997     

Coding of blend ratios of binary mixtures by olfactory neurons in the Florida spiny lobster, Panulirus argus

The aim of this study was to investigate quality coding of blend ratios of binary mixtures by olfactory receptor cells in the spiny lobster. Three odorants (adenosine-5′-monophosphate, l-glutamate, and taurine) at 0.1–100 μmol · l⁻¹ and seven blend ratios of each of their binary mixtures at a total concentration of 100 μmol · l⁻¹ were used. The olfactory cells recorded (n = 48) evoked across-neuron patterns for single odorants that were well separated from each other. Across-neuron patterns varied with stimulus concentration but less than with stimulus type. Blend ratios of the three mixtures evoked across-neuron patterns that were orderly placed within a continuum between those elicited by the components. Mixture interactions, defined as a lack of independent effects by a mixture's components, occurred in 25, 24 and 37% of responses to blend ratios of glutamate/taurine, adenosine-5′-monophosphate/taurine, and glutamate/adenosine-5′-monophosphate, respectively. These mixture interactions did not have a large enough effect on the across-neuron patterns for the mixtures such they would be novel relative to those of the single components. These results suggest that despite mixture interactions the quality of individual compounds is not lost when mixed. This corroborates behavioral studies showing that spiny lobsters have the ability to elementally process odor mixtures. Accepted: 23 August 1996     

Effects of egg size and composition on the size, quality and survival of lapwing Vanellus vanellus chicks

The influence of egg size and composition on the size, quality and survival of lapwing chicks was examined on two farmland study sites in the Midland Valley of Scotland. Eggs comprised 33.1% yolk, 61.3% albumen and 5.6% shell. Whereas the yolk and shell proportions decreased with increasing egg size, the albumen proportion increased. Most variation in egg size was attributable to differences between females but was also influenced by clutch number (eggs in replacement clutches on the rough grazing, but not the arable, site were smaller), clutch size (eggs were smaller in smaller clutches), maternal body condition (females in good condition produced larger eggs) and habitat (since females on the arable site fed more successfully, they were in better condition and laid larger eggs). Chick size, weight and survival were all influenced by egg size. The incubation period varied between 21 and 28 days (mean = 25.2) and was shorter in clutches laid later in the season.     

Effects of variation in egg size and hatching date on survival of Lesser Scaup Aythya affinis ducklings

The consequences of avian egg-size variation on offspring quality and survival remain unclear. We evaluated the effects of egg-size and hatch-date variation on survival of Lesser Scaup Aythya affinis ducklings in the wild. Duckling mass at hatching increased significantly with increasing egg size. Ducklings from larger eggs survived better than those from smaller eggs. We suspect that ducklings from larger eggs survived better because of advantages associated with larger or more efficient utilization of nutrient reserves, or both. We were unable to detect any within-clutch differences in egg size of survivors and non-survivors, nor any consistent direction in the difference in egg size between survivors and nonsurvivors within clutches. This suggests that within-clutch variation may be insufficient to have survival consequences for offspring. In addition, ducklings that hatched later in the breeding season had a higher probability of survival. We suggest a food-dependent hypothesis as an explanation for the seasonally increasing survival and for later nest initiation of Lesser Scaup compared with other North American ducks.     

Different Temperature and Cooling Patterns at the Blunt and Sharp Egg Poles Reflect the Arrangement of Eggs in an Avian Clutch

Incubation is an energetically demanding process during which birds apply heat to their eggs to ensure embryonic development. Parent behaviours such as egg turning and exchanging the outer and central eggs in the nest cup affect the amount of heat lost to the environment from individual eggs. Little is known, however, about whether and how egg surface temperature and cooling rates vary among the different areas of an egg and how the arrangement of eggs within the clutch influences heat loss. We performed laboratory (using Japanese quail eggs) and field (with northern lapwing eggs) experiments using infrared imaging to assess the temperature and cooling patterns of heated eggs and clutches. We found that (i) the sharp poles of individual quail eggs warmed to a higher egg surface temperature than did the blunt poles, resulting in faster cooling at the sharp poles compared to the blunt poles; (ii) both quail and lapwing clutches with the sharp poles oriented towards the clutch centre (arranged clutches) maintained higher temperatures over the central part of the clutch than occurred in those clutches where most of the sharp egg poles were oriented towards the exterior (scattered clutches); and (iii) the arranged clutches of both quail and lapwing showed slower cooling rates at both the inner and outer clutch positions than did the respective parts of scattered clutches. Our results demonstrate that egg surface temperature and cooling rates differ between the sharp and blunt poles of the egg and that the orientation of individual eggs within the nest cup can significantly affect cooling of the clutch as a whole. We suggest that birds can arrange their eggs within the nest cup to optimise thermoregulation of the clutch.     

Comparing the richness of metazoan ectoparasite communities of marine fishes: controlling for host phylogeny

Parasite communities are the product of acquisitions and losses of parasite species during the evolutionary history of their host. When comparing the parasite communities of different host species to assess the role of ecological variables as determinants of parasite species richness, a correction must be made for the possible phylogenetic inheritance of parasites from ancestral hosts independent of host ecology. We performed a comparative analysis of the metazoan ectoparasite communities on the heads and gills of 111 species of marine fish. The influences of host body size, host schooling behaviour and water temperature were tested after controlling for both sampling and phylogenetic effects. Overall, water temperature correlated positively with both parasite species richness and abundance, whereas fish size only correlated with parasite abundance. The correlation across all fish species between water temperature and parasite species richness was dependent on an outlier point. The results, however, generally held when fish from different biogeographical areas (Pacific and Atlantic) were analysed separately. In all analyses, parasite species richness always correlated strongly with parasite abundance. There was no evidence that schooling fish taxa harboured richer or more abundant ectoparasite communities than their non-schooling sister taxa, possibly because of the small number of contrasts available for that test. Overall, whereas both water temperature and host size affect the number of parasite individuals that can be harboured by a fish, only temperature appears important as a determinant of ectoparasite community richness. Received: 30 May 1996 / Accepted: 23 October 1996     

Rearing conditions determine offspring survival independent of egg quality: a cross-foster experiment with Oystercatchers Haematopus ostralegus

Variation in rearing conditions, due either to parental or to environmental quality, can result in offspring of different quality (e.g. body condition, immune function). However, evidence is accumulating that egg size and composition can also affect offspring quality. In Oystercatchers Haematopus ostralegus , high-quality rearing conditions result in a higher quantity as well as quality of offspring. This is thought to be caused by increased parental food provisioning to the chicks in high-quality environments. However, variation in egg quality between rearing conditions could also affect the quantity and quality of offspring. Determining the mechanism and ontogeny of quality differences is important in unravelling the causes of variation in reproductive success. To disentangle the effects of egg quality, and quality of the rearing conditions, on the future survival of offspring, we cross-fostered complete clutches between nests. When reared under conditions of similar environmental quality, chicks originating from eggs laid in low-quality environments survived as well as chicks originating from eggs laid in a high-quality environment. However, chicks reared in high-quality environments survived twice as long as chicks reared in low-quality environments, independent of the environmental quality in which the eggs were laid. This suggests that variation in the future survival of offspring is primarily caused by differences in environmental and/or parental quality, with no clear effect of egg quality (size).     

Costs of large communal clutches for male and female Greater Rheas Rhea americana

The breeding system of the Greater Rhea Rhea americana is almost unique among birds as it combines harem polygyny and sequential polyandry, with communal egg-laying and uniparental male care. In this species, large communal clutches (more than 30 eggs) are rare and have a lower hatching success than smaller clutches. Here we analyse the proximate causes of hatching failures and the costs of large communal clutches (and therefore the costs of extensive polygyny) for males and females. We evaluated if length of the nesting period, egg viability, egg losses during incubation and male parental activity at the nest were affected by clutch size. We also evaluated if chicks hatched from large clutches have a lower survival during the first 2 months after hatching. Large clutches had longer nesting period and lower hatching success, mainly as a result of bacterial contamination of the eggs and increased hatching asynchrony. In addition, large clutches tended to lose more eggs as a result of accidental breakage or predation. Male activity at the nest and chick survival were not related to clutch size. Low hatching success, nest predation risk and energetic costs associated with large clutches penalize females that join large harems and males that accept additional eggs into the nest.     

Effects of plant hybridization on herbivore-parasitoid interactions

We studied the effects of host plant hybridization on the survival and mortality of the leaf-mining moth Phyllonorycter salicifoliella on hybrid and parental willow plants in the field and in a common garden experiment. P. salicifoliella survival differed significantly among three willow taxa in the field in 1994 but not in the field in 1995 or in the common garden. Parasitism by eulophid wasps differed significantly among taxa in 1994 and appeared to account for the variation in their survival. In the field in 1995, host feeding predation varied significant among taxa. The theory of tritrophic interactions predicts that plant genotype can affect natural enemy impact, and this study supports this prediction. Significant variation in survival and eulophid parasitism was also found among genotypes within taxa in the field in both years and in the common garden experiment. The common garden results show that genetic differences in plants affect the herbivore-parasitoid interaction. Variation among years in the patterns of survival and causes of mortality among field plants suggest that genotype by environment interactions may be important. Received: 1 March 1996 / Accepted: 4 November 1996     

Effects of above-ground browsing by mammals on mycorrhizal infection in an early successional taiga ecosystem

Using an exclosure experiment in the willow stage of primary succession on the floodplain of the Tanana River, we tested the hypothesis that browsing can reduce mycorrhizal infection. We measured the effects winter browsing by moose (Alcesalces) and snowshoe hare (Lepusamericanus) had on mycorrhizal infection and fine root biomass of willow (Salix spp.) and balsam poplar (Populusbalsamifera). We found that protection from winter browsing increased ectomycorrhizal infection by 10% in the top 5 cm of the soil profile, by 23% at 5–10 cm, and by 42% at the 10–15 cm depth. Mammal browsing in taiga forests is now recognized as a major cause of the shift from palatable deciduous species such as willow and balsam poplar to less palatable species such as alder and spruce. We suggest that browsing-induced reduction in ectomycorrhizal infection of salicaceous species plays a central role in this shift in plant community composition. Received: 26 March 1996 / Accepted: 26 September 1996     

Sequential functioning of Sym-13 and Sym-31 , two genes affecting symbiosome development in root nodules of pea ( Pisum sativum L.)

Two Fix⁻ mutants of pea (Pisum sativum L.) which are unable to fix molecular nitrogen, E135f (sym-13) and Sprint-2Fix⁻ (sym-31), were crossed to create the doubly homozygous recessive line, named RBT (sym-13, sym-31). The ultrastructural organization of nodules of the RBT line was compared with that of each of the two parental mutant lines and with the original wild-type genotypes of the cultivars Sparkle and Sprint-2. It was shown that the RBT line is similar to the mutant line Sprint-2Fix⁻ in having abnormal symbiosome composition and bacteroids with relatively undifferentiated morphology. Because the phenotypic manifestation of the sym-31 mutant allele suppresses the phenotypic manifestation of the sym-13 mutant allele, it is concluded that the function of the gene Sym-31 (which is mutated in the Sprint-2Fix⁻ line) is necessary at an earlier stage of symbiosome development than the gene Sym-13 (which is mutant in the E135f line). Received: 28 October 1996 / Accepted: 22 January 1997