Direct fitness of group living mammals varies with breeding strategy, climate and fitness estimates

1. Understanding how variation in fitness relates to variation in group living remains critical to determine whether this major aspect of social behaviour is currently adaptive. 2. Available evidence in social mammals aimed to examine this issue remains controversial. Studies show positive (i.e. potentially adaptive), neutral or even negative fitness effects of group living. 3. Attempts to explain this variation rely on intrinsic and extrinsic factors to social groups. Thus, relatively more positive fitness effects are predicted in singularly breeding as opposed to plural breeding species. Fitness effects of sociality in turn may depend on ecological conditions (i.e. extrinsic factors) that influence associated benefits and costs. 4. We used meta-analytic tools to review how breeding strategy or ecological conditions influence the effect size associated with direct fitness-sociality relationships reported in the mammalian literature. Additionally, we determined how taxonomic affiliation of species studied, different fitness and sociality measures used, and major climatic conditions of study sites explained any variation in direct fitness effect size. 5. We found group living had modest, yet positive effects on direct fitness. This generally adaptive scenario was contingent not only upon breeding strategy and climate of study sites, but also on fitness measures examined. Thus, positive and significant effects characterized singular as opposed to plural breeding strategies. 6. We found more positive fitness effects on studies conducted in tropical as opposed to temperate or arid climates. More positive and significant effects were noted on studies that relied on group fecundity, male fecundity and offspring survival as measures of fitness. 7. To conclude, direct fitness consequences of mammalian group living are driven by interspecific differences in breeding strategy and climate conditions. Other factors not examined in this study, namely individual variation in direct and indirect fitness benefits and potential interactions between social and ecological conditions, may be important and require further studies.     

Cultural transmission of vocalizations in ravens, Corvus corax

As ravens have pronounced learning capabilities, especially in the vocal domain, cultural transmission might be a relevant factor affecting the composition of their vocal repertoires. We investigated how important cultural transmission is in establishing a raven's vocal repertoire and whether there are different pathways of transmission. We studied individually known free-ranging breeding pairs whose vocalizations were recorded during interactions with a caged pair. The mean repertoire size of these vocalizations was 12 call types per individual (from 79 types recorded from 74 individuals), with no difference between the sexes. In 81% of individuals the repertoire was composed entirely of calls also used by other individuals. Analysis of the distribution of call types among individuals regarding sex, partnership and neighbourhood and aspects of the geographical distribution of calls showed that the main pathway of cultural transmission was within the sexes. In addition, transmission occurred between pair partners, as well as between neighbouring and more distant pairs. About 40% of the calls in a raven's repertoire were transmitted within a sex and 10% between pair partners. Any two given birds in the population shared on average 20% (0-77%) of their calls. Cultural transmission within the sexes led to a pronounced sexual dimorphism in vocal behaviour. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

Growth and morphology of nestling Hooded crows Corvus corone cornix, a sexually dimorphic bird species

The growth and morphology of nestling Hooded crows, Corvus corone cornix , were studied at Trondheim, Central Norway, during 1980 and 1981. The sexual size dimorphism of the nestlings developed while they were in the nest, but was less pronounced than that of the juvenile birds. Female young grew relatively faster than the males. The size variation of the morphological characters studied was greater in the males than in the females, during the entire nestling period. The order of development of the different morphological characters seemed to be adapted to the needs of the nestlings during the nestling period. Growth of the bone and bill characters was the highest priority, body weight and feather characters the lowest. In general, the sizes of the different morphological characters were highly intercorrelated, particularly those that developed at the same time during the growth period. In the favourable breeding season of 1981, the nestlings were larger than in the unfavourable season of 1980. The high priority characters of the males increased more in size than did those of the females, and the low priority characters of the females increased more in size than did those of the males. About 75% of the nestlings could be correctly sexed from their external morphological characteristics. The findings support the view that the parental cost of rearing a male offspring is greater than that of a female.     

Common ravens, Corvus corax, preferentially associate with grey wolves, Canis lupus, as a foraging strategy in winter

One foraging strategy that scavengers can employ to discover unpredictable food sources is to associate directly with predators who inadvertently provide food. The common raven, a well known feeding generalist, is also a prominent scavenger of wolves' kills and is found to be in close association with this predator. We tested the hypothesis that ravens preferentially associate with wolves in winter as a kleptoparasitic foraging strategy. The presence, absence and behaviour of ravens was documented during winter observations of wolves, coyotes, Canis latrans, and elk, Cervus elaphus, as well as the landscape in the absence of these three species. Ravens were found to be in close association with wolves when they were travelling, resting and hunting prey. In comparison, ravens showed no significant association with coyotes, elk or areas on the landscape in the absence of wolves. We also compared ravens' discovery success of wolf-killed and nonwolf-killed carcasses and their behavioural response upon discovery. Ravens found all wolf kills almost immediately and remained at the carcass to feed alongside wolves after the death of the prey. In contrast, ravens were less successful discovering experimentally placed carcasses in the same study region, and did not land or feed despite the availability of fresh, exposed meat. Our results show that ravens' association with wolves is not just an incidental and proximate by-product of the presence of fresh meat. Instead, we show that ravens preferentially associate with wolves in both the presence and absence of food, resulting in the discovery of carcasses and suppression of ravens' innate fear of novel food sources. Through this mode of social foraging, ravens may experience increased foraging efficiency in the use of an otherwise spatially and temporally unpredictable food source.Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved .     

Disease-mediated inbreeding depression in a large,open population of cooperative crows

Disease-mediated inbreeding depression is a potential cost of living in groups with kin, but its general magnitude in wild populations is unclear. We examined the relationships between inbreeding, survival and disease for 312 offspring, produced by 35 parental pairs, in a large, open population of cooperatively breeding American crows (Corvus brachyrhynchos). Genetic analyses of parentage, parental relatedness coefficients and pedigree information suggested that 23 per cent of parental dyads were first- or second-order kin. Heterozygosity–heterozygosity correlations suggested that a microsatellite-based index of individual heterozygosity predicted individual genome-wide heterozygosity in this population. After excluding birds that died traumatically, survival probability was lower for relatively inbred birds during the 2–50 months after banding: the hazard rate for the most inbred birds was 170 per cent higher than that for the least inbred birds across the range of inbreeding index values. Birds that died with disease symptoms had higher inbreeding indices than birds with other fates. Our results suggest that avoidance of close inbreeding and the absence of inbreeding depression in large, open populations should not be assumed in taxa with kin-based social systems, and that microsatellite-based indices of individual heterozygosity can be an appropriate tool for examining the inbreeding depression in populations where incest and close inbreeding occur.     

Limited indirect fitness benefits of male group membership in a lekking species

In group living species, individuals may gain the indirect fitness benefits characterizing kin selection when groups contain close relatives. However, tests of kin selection have primarily focused on cooperatively breeding and eusocial species, whereas its importance in other forms of group living remains to be fully understood. Lekking is a form of grouping where males display on small aggregated territories, which females then visit to mate. As females prefer larger aggregations, territorial males might gain indirect fitness benefits if their presence increases the fitness of close relatives. Previous studies have tested specific predictions of kin selection models using measures such as group‐level relatedness. However, a full understanding of the contribution of kin selection in the evolution of group living requires estimating individuals' indirect fitness benefits across multiple sites and years. Using behavioural and genetic data from the black grouse (Tetrao tetrix), we show that the indirect fitness benefits of group membership were very small because newcomers joined leks containing few close relatives who had limited mating success. Males' indirect fitness benefits were higher in yearlings during increasing population density but marginally changed the variation in male mating success. Kin selection acting through increasing group size is therefore unlikely to contribute substantially to the evolution and maintenance of lekking in this black grouse population.     

Mechanisms and fitness effects of antibacterial defences in a carrion beetle

Parents of many species care for their offspring by protecting them from a wide range of environmental hazards, including desiccation, food shortages, predators, competitors, and parasites and pathogens. Currently, little is known about the mechanisms and fitness consequences of parental defences against bacterial pathogens and competitors. Here, we combine approaches from microbiology and behavioural ecology to investigate the role and mechanistic basis of antibacterial secretions applied to carcasses by parents of the burying beetle Nicrophorus vespilloides. This species rears its larvae on vertebrate carcasses, where larvae suffer significant fitness costs due to competition with bacterial decomposers. We first confirm that anal secretions produced by parents are potently bactericidal and that their effects are specific to gram-positive bacteria. Next, we identify the source of bacterial killing as a secreted lysozyme and show that its concentration changes throughout the breeding cycle. Finally, we show that secreted lysozyme is crucial for larval development, increasing survival by nearly two-fold compared to offspring reared in its absence. These results demonstrate for the first time that anal secretions applied to carrion is a form of parental care and expand the mechanistic repertoire of defences used by parent insects to protect dependent offspring from microbial threats.     

Duplicated growth hormone genes in a passerine bird, the jungle crow (Corvus macrorhynchos)

Molecular cloning, molecular phylogeny, gene structure and expression analyses of growth hormone (GH) were performed in a passerine bird, the jungle crow (Corvus macrorhynchos). Unexpectedly, duplicated GH cDNA and genes were identified and designated as GH1A and GH1B. In silico analyses identified the zebra finch orthologs. Both GH genes encode 217 amino acid residues and consist of five exons and four introns, spanning 5.2 kbp in GH1A and 4.2 kbp in GH1B. Predicted GH proteins of the jungle crow and zebra finch contain four conserved cysteine residues, suggesting duplicated GH genes are functional. Molecular phylogenetic analysis revealed that duplication of GH genes occur after divergence of the passerine lineage from the other avian orders as has been suggested from partial genomic DNA sequences of passerine GH genes. RT-PCR analyses confirmed expression of GH1A and GH1B in the pituitary gland. In addition, GH1A gene is expressed in all the tissues examined. However, expression of GH1B is confined to several brain areas and blood cells. These results indicate that the regulatory mechanisms of duplicated GH genes are different and that duplicated GH genes exert both endocrine and autocrine/paracrine functions.     

Direct fitness benefits explain mate preference,but not choice,for similarity in heterozygosity levels

Under sexual selection, mate preferences can evolve for traits advertising fitness benefits. Observed mating patterns (mate choice) are often assumed to represent preference, even though they result from the interaction between preference, sampling strategy and environmental factors. Correlating fitness with mate choice instead of preference will therefore lead to confounded conclusions about the role of preference in sexual selection. Here we show that direct fitness benefits underlie mate preferences for genetic characteristics in a unique experiment on wild great tits. In repeated mate preference tests, both sexes preferred mates that had similar heterozygosity levels to themselves, and not those with which they would optimise offspring heterozygosity. In a subsequent field experiment where we cross fostered offspring, foster parents with more similar heterozygosity levels had higher reproductive success, despite the absence of assortative mating patterns. These results support the idea that selection for preference persists despite constraints on mate choice.     

Direct benefits and the evolution of female-biased cooperative breeding in Seychelles warblers

Abstract.— Inclusive fitness benefits have been suggested to be a major selective force behind the evolution of cooperative breeding. We investigated the fitness benefits selecting for cooperative breeding in the Seychelles warbler, Acrocephalus sechellensis . A microsatellite-based genotyping method was used to determine the relatedness of subordinates to group offspring in an isolated population of Seychelles warblers. The indirect and direct breeding benefits accruing to individual subordinates were then calculated for every successful breeding event over a three-year period. We show that female subordinates frequently gained parentage and that this, combined with high levels of extra group paternity, resulted in low levels of relatedness between subordinates and non descendent offspring within a territory. Direct breeding benefits were found to be significantly higher than indirect kin benefits for both female and male subordinates. As predicted, female subordinates gained significantly more direct breeding opportunities and therefore higher inclusive fitness benefits by being a subordinate within a group than did males. This may explain why most subordinates in the Seychelles warbler are female.