Isotopic segregation between sympatric seabird species increases with nutritional stress

Dietary segregation is essential for the coexistence of closely related species of animals. However, little is known about how changes in availability of food resources might affect trophic interactions of wild animals breeding in sympatry. Here, we examined how interannual variations in relative food availability (as reflected in blood levels of stress hormone corticosterone, CORT) affect food partitioning (assessed via a comparison of stable isotope δ(15)N and δ(13)C ratios of blood) between the common murre (Uria aalge) and thick-billed murre (Uria lomvia), breeding on a single colony in the Bering Sea. During a 6-year study, CORT varied among years but not between species, whereas stable isotope ratios varied among years and between species. Isotopic distance between species increased with increasing CORT. These results indicate that, when food was not limiting, both species relied on similar food resources. As foraging conditions deteriorated, murres diverged in their diets. We conclude that the degree of dietary segregation between Uria spp. varies with changes in the availability of food and is greatest during food shortages.     

Regulation of food provisioning and parental body condition in Leach’s storm-petrels, Oceanodroma leucorhoa: Experimental manipulation of offspring food demand

We examined the effects of offspring food demand on parental regulation of food provisioning and body condition in a small long-lived seabird, Leachs storm-petrels (Oceanodroma leucorhoa). In one experimental group, food demand of chicks on their parents was increased by removing one parent (single), and in another group these food demands were decreased by supplementary feeding of chicks (supplement). A further unmanipulated group provided a control. Feeding frequencies by one parent were higher in the single but lower in the supplement than in the control group, in accordance with the food demand of chicks. The size of meals appeared to be not different among the experimental groups. However, as single parents did not compensate perfectly for the increase of chick food demand by food provisioning, single chicks grew at slower rates and fledged at smaller masses than control chicks. Supplement chicks grew at similar rates and fledged at similar masses as control chicks, because parents decreased food provisioning and food processing capacity of the chicks might be limited. The body condition of parents, which was determined by body mass loss and feather regrowth rate, did not differ among the groups. These results indicate that feeding frequency was regulated by parental decision in this storm-petrel species. Parents may adjust their food provisioning to match the food demand of chicks but within a certain range so as not to deteriorate their own body condition.     

Corticosterone facilitates begging and affects resource allocation in the black-legged kittiwake

Parent black-legged kittiwakes (Rissa tridactyla) and their dependent chicks respond to food shortages by increasing circulatinglevels of corticosterone. To examine the behavioral significanceof corticosterone release, we experimentally increased levelsof circulating corticosterone in parents and chicks up to thelevels observed during food shortages. We found that corticosterone-implantedchicks begged more frequently than sham-implanted controls.Corticosterone-implanted chicks in broods of two begged morefrequently than singletons. Parent kittiwakes then respondedto the increase in corticosterone levels in their chicks byincreasing chick-feeding rates. However, feeding rates werenot different among corticosterone-implanted chicks in broodsof two and singletons. We also found that corticosterone-implantedparents spent more time away from the nest—perhaps foraging—andless time brooding/guarding chicks than sham-implanted controls.Untreated mates of the corticosterone-implanted bird did notcompensate for the change in their partner's behavior; consequently, chicks were left unattended about 20% of the time compared to1% at the control nests. However, corticosterone-implantedparents did not decrease their chick-feeding rates. Our findingssuggest two functional implications of the increased corticosteronesecretion during food shortages in the black-legged kittiwake:it facilitates begging in chicks, and it affects time allocatedby parents to guarding young at the nest. Thus, release of corticosterone might provide a mechanistic link between physiological condition and behavioral interactions among adults and theiryoung.     

Colony performance and immunocompetence of a social insect, Bombus terrestris, in poor and variable environments

1. Feeding and defence against parasites are among the major simultaneous requirements for survival and successful reproduction of any animal, including colonies of social insects. Harsh environments may therefore not only negatively affect overall success, but force animals to allocate resources in different ways, for example, to reduce immune defence under adverse conditions while maintaining growth rate.
2. This study examines how colony growth, reproduction and immune defence of the bumble bee, Bombus terrestris , varies with environmental condition. In particular, the manner in which limited or unpredictable food availability affects the number and size of workers and sexuals produced, the timing of reproduction, and the level of encapsulation of a novel antigen was tested. A new experimental paradigm was applied by splitting colonies into two halves while ensuring integrity by regular swaps of the residing queen.
3. The results show that limited food availability, as expected, led to fewer and smaller workers and sexuals. Unpredictable food availability, in contrast, was associated with higher numbers of workers and heavier sexuals. Also, reproductive success was highest under unpredictable conditions, due to an increased rate of collection of resources.
4. The timing of reproduction did not vary among treatments. Contrary to expectation, also immunocompetence did not vary among treatments.     

A general model for ontogenetic growth under food restriction

Food restriction (FR) retards animals' growth. Understanding the underlying mechanisms of this phenomenon is important to conceptual problems in life-history theory, as well as to applied problems in animal husbandry and biomedicine. Despite a considerable amount of empirical data published since the 1930s, there is no relevant general theoretical framework that predicts how animals vary their energy budgets and life-history traits under FR. In this paper, we develop such a general quantitative model based on fundamental principles of metabolic energy allocation during ontogeny. This model predicts growth curves under varying conditions of FR, such as the compensatory growth, different age at which FR begins, its degree and its duration. Our model gives a quantitative explanation for the counterintuitive phenomenon that under FR, lower body temperature and lower metabolism lead to faster growth and larger adult size. This model also predicts that the animals experiencing FR reach the same fraction of their adult mass at the same age as their ad libitum counterparts. All predictions are well supported by empirical data from mammals and birds of varying body size, under different conditions of FR.     

Resource allocation in Wilson's storm-petrels<Emphasis Type="Italic"> Oceanites oceanicus</Emphasis> determined by measurement of glucocorticoid excretion

When resources are limited, life-history theory predicts that long-lived animals should allocate available resources to body maintenance rather than to reproduction in order to maximise their lifetime reproductive success. In the present study, we estimated physiological stress in a small procellariiform seabird, the Wilson's storm-petrel Oceanites oceanicus, as a means of understanding how limited resources are partitioned between provisioning parents and their chicks. We analysed adrenocortical activity of Wilson's storm-petrels during the breeding season by measuring glucocorticoid (GC) excretion, using an enzyme immunoassay measuring tetrahydrocorticosterone concentrations in extracts of faeces and urine of chicks and adults. Faecal GC measures were negatively correlated with chick body condition, suggesting that measures of tetrahydrocorticosterone in faeces and urine can be used to assess adrenal activity characteristic for physiological stress in Wilson's storm-petrels. In the breeding season of 1999, the colony was subject to low food availability, and the faecal and urine GC levels of chicks were elevated during these months of chronic starvation. In contrast, adults did not show elevated GC levels. The data thus suggest that Wilson's storm-petrels respond to unfavourable conditions by maintaining their own body condition and reducing provisioning of food to their chicks. Communicated by R.F. Oliviera     

Potential food resources available to malleefowl Leipoa ocellata in marginal mallee lands during drought

Abstract Prime mallee lands that once supported high densities of malleefowl, Leipoa ocellata in New South Wales have been extensively cleared for agriculture. Malleefowl are now restricted largely to more and habitat. Given the added competition from introduced herbivores, there is concern that these marginal habitats may not contain sufficient food resources to support viable populations of malleefowl, particularly in times of drought. An opportunistic feeding strategy and omnivorous diet enable malleefowl to exploit a diverse array of food items. Vegetative and invertebrate food resources available to malleefowl in marginal mallee lands were monitored over a period of 1 year during drought. Food resources were varied, transient and patchily distributed. Composition and abundance fluctuated considerably, but potential food was never entirely absent. Despite the prevailing drought, food was obtainable throughout the year provided malleefowl adapted to the fluctuating availability of each of the various potential food resources. Reduced food resources in marginal habitat is not implicated as a major factor in the demise of the malleefowl. Food was most abundant between August and November, inclusive, due largely to the abundance of herbs, the diverse supply of buds, flowers and fruit on perennial shrubs, and the sporadic proliferation of cockroaches. Food availability was least, and therefore most critical, during February, March and June. The most favourable time of the year, in terms of available food resources, for the release of captive-bred individuals was during spring, around September.     

Immune response covaries with corticosterone plasma levels under experimentally stressful conditions in nestling barn swallows (Hirundo rustica)

Traits related to fitness are often pleiotropically linked orotherwise constrained in their expression. Organisms thereforetrade between fitness components such as number and viabilityof their offspring. The physiological mechanisms mediating suchtrade-offs, however, have been poorly investigated. We manipulatedbrood size and satiation of nestling barn swallows, Hirundorustica, to simulate the effect of two kinds of natural stresses,i.e., long-term intense competition in a large brood and acutefood deprivation, and we measured their effect on body condition,T cell–mediated immune response, and corticosterone, themain hormone mediating the adrenocortical stress-response. Broodenlargement increased corticosterone levels compared with thosefor brood reduction, and brood enlargement depressed immuneresponse, body mass, and condition. Corticosterone levels markedlyincreased after food deprivation. Immune response negativelycovaried with corticosterone levels measured after long-termstress. Hence, living in a crowded nest and with food deprivationelicited a stress response mediated by corticosterone, and depressedan important component of offspring fitness such as T cell–mediatedimmunity. The negative covariation between circulating corticosteroneand immunity suggests that the trade-off between offspring numberand quality is mediated by variation in plasma levels of corticosterone,which has immunosuppressive effects.     

Rapid reset of the corticosterone rhythm by food presentation in rats under acircadian restricted feeding schedule

Corticosterone levels were determined in the 7-week-old male rat maintained under different feeding and lighting schedules. At 4 weeks of age, the animals were kept either under a natural photoperiod (LD) or were subjected to continuous illumination (LL). Access to food was either ad libitum or restricted to an 8 hr span per 24 hr (circadian) or 32 hr (acircadian).

The food signal seemed able to synchronize the corticosterone rhythm to its own circadian periodicity, irrespective of the lighting regimen. No synchronization was observed in serially sampled LL or LD rats under an acircadian feeding schedule. Instead, the group acrophase appeared 24 hr subsequent to food presentation. Regarding individual patterns, many rats showed an acrophase or a peak also at that time. We speculate that an endogenous circadian mechanism was reset by the food signal, whenever it appeared.     

King penguin demography since the last glaciation inferred from genome-wide data

How natural climate cycles, such as past glacial/interglacial patterns, have shaped species distributions at the high-latitude regions of the Southern Hemisphere is still largely unclear. Here, we show how the post-glacial warming following the Last Glacial Maximum (ca 18 000 years ago), allowed the (re)colonization of the fragmented sub-Antarctic habitat by an upper-level marine predator, the king penguin Aptenodytes patagonicus. Using restriction site-associated DNA sequencing and standard mitochondrial data, we tested the behaviour of subsets of anonymous nuclear loci in inferring past demography through coalescent-based and allele frequency spectrum analyses. Our results show that the king penguin population breeding on Crozet archipelago steeply increased in size, closely following the Holocene warming recorded in the Epica Dome C ice core. The following population growth can be explained by a threshold model in which the ecological requirements of this species (year-round ice-free habitat for breeding and access to a major source of food such as the Antarctic Polar Front) were met on Crozet soon after the Pleistocene/Holocene climatic transition.     

Effects of Group Membership and Size Distribution within a Group on Growth Rates of Juvenile Sablefish Anoplopoma fimbria

Group membership can confer both advantages and disadvantages to growth in juvenile fishes. The balance between costs and benefits of social interactions can shift depending on such factors as the composition of the group (density and size disparity) and the availability of food. We examined the effect of these factors on absolute growth and growth depensation in juvenile sablefish, Anoplopoma fimbria. Increasing density and increasing size disparity had little influence on absolute growth rates of juvenile sablefish and the effects of these social factors were not modified by ration level. In experiments testing density effects, absolute growth did not differ among groups of 1, 3, or 10 fish held at high rations, but at low rations single fish exhibited a different pattern of size-dependent growth compared to fish in groups. In experiments testing disparity effects, absolute growth did not differ between groups with an even size distribution and groups with a mixed size distribution. The relative size of an individual within a group, i.e., small, medium, or large, also did not modify growth, despite evidence of higher chasing behavior in mixed size distributions. Although the growth of small fish was not diminished in the presence of large fish, negative impacts of size disparity were expressed in high levels of cannibalism, which occurred in 42% of groups with a mixed size distribution. Significant growth depensation over time occurred in the density experiment, but not in the size disparity experiment, possibly due to the shorter duration of the latter experiment. We suggest that growth depensation was generated by individual variability in growth capacity rather than social effects on growth rates. Schooling behavior, measured by group cohesion indices, increased with fish size and was higher in groups with an even vs. a mixed size distribution. These results for sablefish are consistent with other schooling species in which growth variability is determined by exploitative competition and/or genetic variability in growth capacity rather than interference competition.