Living with strangers: direct benefits favour non-kin cooperation in a communally nesting bird

The greater ani (Crotophaga major), a Neotropical cuckoo, exhibits an unusual breeding system in which several socially monogamous pairs lay eggs in a single nest and contribute care to the communal clutch. Cooperative nesting is costly-females compete for reproduction by ejecting each other's eggs-but the potential direct or indirect fitness benefits that might accrue to group members have not been identified. In this study, I used molecular genotyping to quantify patterns of genetic relatedness and individual reproductive success within social groups in a single colour-banded population. Microsatellite analysis of 122 individuals in 49 groups revealed that group members are not genetic relatives. Group size was strongly correlated with individual reproductive success: solitary pairs were extremely rare and never successful, and nests attended by two pairs were significantly more likely to be depredated than were nests attended by three pairs. Egg loss, a consequence of reproductive competition, was greater in large groups and disproportionately affected females that initiated laying. However, early-laying females compensated for egg losses by laying larger clutches, and female group members switched positions in the laying order across nesting attempts. The greater ani, therefore, appears to be one of the few species in which cooperative breeding among unrelated individuals is favoured by direct, shared benefits that outweigh the substantial costs of reproductive competition.     

Solitary and group nesting in the orchid bee <Emphasis Type="Italic">Euglossa hyacinthina</Emphasis> (Hymenoptera,Apidae)

Summary Orchid bees (Euglossini) provide a potentially informative contrast for examining origins of advanced social behaviour in bees because they are the only tribe in the apine clade that do not form large colonies or have queens and workers. We investigated natural nests of Euglossa hyacinthina Dressler, an orchid bee that nests singly or in groups. By comparing the two types of nests, we examined if individuals in a group merely share the nest (are communal) or exhibit a level of social organization where there is reproductive division of labour among the females. Observations are consistent with communal nesting, indicating that all females in group nests are reproductively similar to the solitary nesting females because the provisioning of young, as well as the ovary development and mating status of females sharing nests were not different than that of solitary-nesting females. Also, multiple female nests did not produce a female-biased brood as predicted for nests with reproductive division of labour. We also investigated potential advantages of group nesting vs. individual nesting. We demonstrate that per capita offspring production is lower in nests with more than one female. However, we found that nests with single females were left unattended for longer periods of time during foraging, and that there was a high incidence of natural enemy attack in nests when females were absent. Group and solitary nesting may be advantageous under different conditions.Received 3 December 2002; revised 7 March 2003; accepted 2 April 2003.     

Effects of nest size and dispersion on brood production in a North American population of wood ant Formica fusca (Hymenoptera: Formicidae)

We examined several key parameters of the population ecology of a North American population of Formica fusca (L.), including nest dispersion, colony size and brood production. Physical nest size was significantly correlated with colony size, and colony size, in turn, was significantly correlated with brood production. Sex allocation was male biased, although larger nests were more likely to produce reproductive female brood (gynes). Neither nest temperature nor moisture level was significantly correlated with brood production. Formica fusca nests in this population had a comparatively low average nearest-neighbor distance with a significantly even pattern of dispersion, which suggests relatively high intraspecific competition. However, nearest-neighbor distance was not significantly associated with either colony size or relative brood production.     

Fitness consequences of sociality in prairie voles, Microtus ochrogaster: influence of group size and composition

We used 7 years of live-trapping data from an Illinois population of prairie voles to examine how survival and reproductive success varied with size and composition of social groups. Specifically, we examined measures of fitness for residents of single female units, male-female pairs and communal groups. Reproductive success, measured as either the number of young that survived to 12 or 30 days of age per adult female in a group, was higher for groups of three adults than for smaller or larger groups. The ideal group composition with respect to reproductive success appeared to be one adult female and two adult males. Individuals born into groups with three adults survived longer than did those born into smaller or larger groups. There was no clear relationship, however, between group size and survival of adult members of groups. Sudden disappearance of a group, presumably to predation by weasels, was associated with relative group size such that the largest groups were more likely to disappear. Overall, our data indicate that prairie voles living in communal groups with three adult members have higher fitness than those living in smaller groups or in larger communal groups. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Colony size, reproductive success, and colony choice in Cave Swallows Petrochelidon fulva

We investigated how annual reproductive success, as measured by the number of nestlings surviving to day 10 and the percentage of nests that were successful, varied with colony size of the Cave Swallow Petrochelidon fulva in south central Texas. We also studied whether Cave Swallows chose colonies, in part, on the basis of reproductive success at a site the previous year. Neither measure of reproductive success varied significantly with colony size for either first-wave or second-wave nestings. Mean clutch size per colony did not vary significantly with colony size. Mean nestling body mass, an index of parental foraging efficiency, was unrelated to colony size, except for broods of five, in which nestling mass declined significantly with colony size. Colony size was not significantly affected by reproductive success at the site the previous year, although sites with more successful nests during the first wave declined less in size during the second wave within the same season than did sites that had fewer successful first-wave nests. Unlike the closely related Cliff Swallow Petrochelidon pyrrhonota , Cave Swallows did not use breeding performance of conspecifics in choosing nest-sites, because they did not preferentially aggregate at sites that were the most successful the previous year. Coloniality in Cave Swallows did not appear to have a net negative effect on annual fitness, suggesting that colonial nesting was not solely a response to nest-site limitation, but the benefits of breeding colonially (if any) were unclear.     

Variation in a sparrow's reproductive success with rainfall: food and predator-mediated processes

From 1997 to 1999, we monitored the reproductive success of individual rufous-crowned sparrows (Aimophila ruficeps) in coastal sage scrub habitat of southern California, USA. Annual reproductive output of this ground-nesting species varied strongly with annual variation in rainfall, attributed to the El Niño-Southern Oscillation. Birds fledged 3.0 young per breeding pair in 1997, when rainfall was near the long-term mean, 5.1 offspring per pair in 1998, a wet El Niño year, and 0.8 fledglings per pair in 1999, a dry La Niña year. Variation in many components of reproductive output was consistent with the hypothesis that food availability was positively correlated with rainfall. However, the factor most responsible for the high reproductive output in 1998 was low early season nest predation which, combined with favorable nesting conditions, enabled more pairs to multiple-brood. Cool, rainy El Niño conditions may have altered the activity of snakes, the main predator of these nests, in the early season of 1998. Overall, more of the annual variation in fecundity was attributable to variation in within-season components of reproductive output (mean number of nests fledged per pair) than to within-nest components (mean brood size). Annual variation in rufous-crowned sparrow fecundity appears to be driven primarily by food resource-mediated processes in La Niña years and by predator-mediated processes in El Niño years.     

The thermoregulatory benefits of the communal nest of sociable weavers Philetairus socius are spatially structured within nests

Structures built by animals, such as nests, mounds and burrows, are often the product of cooperative investment by more than one individual. Such structures may be viewed as a public good, since all individuals that occupy them share the benefits they provide. However, access to the benefits generated by the structure may vary among individuals and is likely to be an important determinant of social organisation. Here we use the massive, communal nests of sociable weavers Philetairus socius, to investigate whether their thermoregulatory function varies in relation to the size of communal nests, and the position of individual nest chambers within the communal structure. We then examine whether this spatial variation in thermoregulatory function predicts the social organisation of colonies. First, we show that the sociable weavers’ communal nests buffer variation in ambient temperature, and reduce temperature variability within nest chambers. The extent of this buffering effect depends significantly on the position of nest chambers within the communal structure, and on the depth to which chambers are embedded within the nest mass. We detected no effect of nest volume on thermoregulatory benefits, suggesting that there are likely to be additional, non‐thermoregulatory benefits leading to communal nests. Finally, our results indicate that there may be competition for access to the benefits of the public good, since older birds occupied the chambers with the highest thermoregulatory benefits, where breeding activity was also more common. We discuss how the spatial structure of the benefits of the public good might influence social organisation in the unique communal lifestyle of sociable weavers.     

Variation in cuticular hydrocarbon signatures, hormonal correlates and establishment of reproductive dominance in a polistine wasp

In many social insects the relationship between reproductive dominance and physiological correlates is poorly understood. Recent evidence now strongly suggests that cuticular hydrocarbons are important in reproductive differentiation in these societies where they are used as signals of ovarian activity in reproductive females. In this study we investigated the relationship between reproductive dominance, size of the corpora allata (CA, producer of Juvenile Hormone, JH) and the proportions of cuticular hydrocarbons present on the cuticle in overwintering foundresses and both associative (polygynous) and solitary (monogynous) pre-emergence colonies of the social wasp Polistes dominulus. Size of the CA was positively correlated with ovarian development in polygynous colonies. In contrast, solitary foundresses possessed significantly smaller CAs than dominant foundresses from polygynous nests, yet ovarian activity was similar for both female types. CA size variation was associated with variation in cuticular hydrocarbon proportions. Overwintering, solitary, dominant and subordinate (from associative nests) females all possessed distinctive cuticular chemical profiles revealed by multivariate discriminant analyses. Our data indicate that the social environment strongly affects reproductive physiology in this wasp, and we discuss the role of cuticular hydrocarbons in reproductive signaling in P. dominulus and other social insects.     

Reproductive indicators in natural populations of Douglas-fir in Mexico

The reproductive capacity in nine Mexican Douglas-fir populations was determined by analyzing seed production traits from 144 trees collected in 2001. Significant variation was found for all traits among populations; they contributed between 21% and 43% of total phenotypic variation found in these traits, indicating broad differences in reproductive capacity for that particular year. Seed efficiency (filled seed/seed potential) varied from 14% to 42% among populations; all populations from Central Mexico had a seed efficiency below 25%. The proportion of developed that were empty seeds varied from 0.40 to 0.81 among populations, whereas seed size varied also from 0.88 to 1.21 g per 100 seeds among them. Average ratio of filled seed weight to cone weight (reproductive efficiency) was 29.6 mg g⁻¹, but it varied three-fold between populations with extreme values. Populations with larger cones had greater seed potential and heavier seed but not necessarily higher reproductive or seed efficiency. Most reproductive indicators were significantly correlated with latitude, with lower values in the marginal populations from Central Mexico, in the southern extreme of the species range. Given these results, the need for conservation of Douglas-fir populations in Central Mexico is discussed.     

Spatial distribution of communal nests in a colonial breeding bird: benefits without costs?

Abstract The spatial organization of individuals, or groups of individuals, within a population can provide valuable information about social organization and population dynamics. We analysed the spatial distribution of nests of the sociable weaver (Philetairus socius) on two farms in the Kalahari. Sociable weavers build large communal nests on big savannah trees, forming a pattern of trees with and without nests. We used two spatial statistics, Ripley's K and the pair correlation function, to describe characteristics of the point patterns over a range of distances. (i) At distances of 200 and 300 m, communal nests were clustered. (ii) At distances greater than 1000 m, communal nests were regularly distributed. These findings are independent of the spatial distribution of trees. Furthermore, we used Moran's I to analyse spatial autocorrelation of nest sizes. We expected negative autocorrelation because of competition between nests. But on both farms there was no significant autocorrelation of nest sizes for any distance class. The regular distribution observed at larger distances may indicate competition and/or territoriality among different nests, but the lack of spatial autocorrelation between nest sizes suggests that these interactions may happen between nest clusters rather than between single nests. This was confirmed by significant clustering of nests on small scales. We thus suggest, that colonies of P. socius consist of several nests on adjacent trees forming a cluster of subcolonies. The question why sociable weavers establish subcolonies instead of adding more chambers to the natal nest, could not simply be answered by limitation of nesting space. We suggest a strategy to avoid costs due to increasing colony size.     

A description of nesting behaviors,including factors impacting nest site selection,in black‐and‐white ruffed lemurs (Varecia variegata)

Nest site selection is at once fundamental to reproduction and a poorly understood component of many organisms’ reproductive investment. This study investigates the nesting behaviors of black‐and‐white ruffed lemurs, Varecia variegata, a litter‐bearing primate from the southeastern rainforests of Madagascar. Using a combination of behavioral, geospatial, and demographic data, I test the hypotheses that environmental and social cues influence nest site selection and that these decisions ultimately impact maternal reproductive success. Gestating females built multiple large nests throughout their territories. Of these, females used only a fraction of the originally constructed nests, as well as several parking locations as infants aged. Nest construction was best predicted by environmental cues, including the size of the nesting tree and density of feeding trees within a 75 m radius of the nest, whereas nest use depended largely on the size and average distance to feeding trees within that same area. Microhabitat characteristics were unrelated to whether females built or used nests. Although unrelated to nest site selection, social cues, specifically the average distance to conspecifics’ nest and park sites, were related to maternal reproductive success; mothers whose litters were parked in closer proximity to others’ nests experienced higher infant survival than those whose nests were more isolated. This is likely because nesting proximity facilitated communal crèche use by neighboring females. Together, these results suggest a complex pattern of nesting behaviors that involves females strategically building nests in areas with high potential resource abundance, using nests in areas according to their realized productivity, and communally rearing infants within a network of nests distributed throughout the larger communal territory.     

Antifungal skin bacteria,embryonic survival,and communal nesting in four-toed salamanders, <Emphasis Type="Italic">Hemidactylium scutatum</Emphasis>

We examined a novel hypothesis for the maintenance of communal nesting in the salamander, Hemidactylium scutatum, namely that communal nests are more likely than solitary nests to be associated with cutaneous antifungal bacteria, which can inhibit fungal infections of embryos. A communal nest contains eggs of two or more females of the same species. The nesting behavior of H. scutatum females and survival of embryos were determined by frequent nest surveys at three ponds. For communal nests, embryonic survival tended to be higher and catastrophic nest failure was lower. Pure bacterial cultures of resident species were obtained from the salamanders’ skins by swabbing and tested against a fungal pathogen of embryos (Mariannaea sp.) in laboratory assays. We found that 27% of females had skin bacteria inhibitory to Mariannaea sp. Communal nests were more likely to have at least one female with antifungal bacteria than were solitary nests. Using a culture-independent assay (denaturing gradient gel electrophoresis of 16S rRNA gene fragments), we found that bacterial species on females and embryos were more similar to each other than they were to bacterial species found in soil within the nest, suggesting that females transmitted skin bacteria to embryos. The presence of anti-Mariannaea skin bacteria identified from the laboratory assays did not prevent fungal presence in field nests. However, once a nest was visibly infected with fungi, presence of anti-Mariannaea bacteria was positively correlated with survival of embryos. Microbe transmission is usually thought to be a cost of group living, but communal nesting in H. scutatum may facilitate the transmission of antifungal bacteria to embryos.     

Mate choice, egg cannibalism and reproductive success in the river bullhead, Cottus gobio L.

The seasonal pattern and individual variation in reproductive success was studied in a population of the river bullhead, Cottus gobio L. Female fecundity and male reproductive success were correlated with body size. Large males were found to breed early in the season when most of the large females spawned. The diameter of eggs found in male nests indicates that females tend to mate with males larger than themselves. The analysis of stomach contents suggests that guarding males cannibalize some of their own eggs. During parental care, the rate of filial cannibalism increases as guarding male body condition deteriorates.     

Colonial Nesting and the Importance of the Brood Size in Male Parasitic Reproduction of the Mediterranean Damselfish Chromis chromis (Pisces: Pomacentridae)

We investigated male parasitic spawning in a protected natural population of Mediterranean damselfish. Chromis chromis nested in colonies, inside which males showed a high variability in mating success. Our field observations indicate that the egg batches obtained by the most successful fish were five times bigger than the ones obtained by the less successful fish and many males never received ovipositions. On the other hand, reproductive parasitism was a common tactic within the colony. Successful nesting males sneaked into their neighbours' nests depending on the amount of eggs in their nest, with small clutch size inducing the males to parasitic reproduction. Males failing to receive egg depositions on their nests showed a significantly higher parasitism rate than successful males. Non-territorial males occupied stations in the water column above the breeding grounds and whenever the opportunity arose, they disrupted spawning in progress, stealing copulation with females. We observed that the likelihood of males being parasitized by sneakers was not correlated with the size of their own clutch; on the contrary, it depended both on the number of neighbouring nests and on the number of neighbouring males with barren nests (i.e. unsuccessful males). No correlation was found between parasitic behaviour and male size, suggesting males may switch between spawning in their own and in their neighbour's nests depending on mating opportunity. The hypothesis that colonial nesting facilitates parasitic reproduction is here discussed.     

Communally breeding skuas: breeding success of pairs, trios and groups of Catharacta lonnbergi on the Chatham Islands, New Zealand

One-third of territories on South East Island are occupied by trios or groups of communally breeding skuas. Comparisons were made of the reproductive performance of pairs, trios and groups over three (and ten) seasons to assess whether any 'advantage'–in terms of offspring production–could be demonstrated for communal breeding. Egg size was not significantly different between pairs and trios, or between years. There were no significant differences between the chicks of pairs, trios or groups in growth characteristics and chick condition appeared similarly good. Overall breeding success was generally lower for trios than for pairs. Although chick production per territory was not significantly different between pairs and trios, it was substantially lower on a per-adult basis. The breeding success of this population is generally high compared to others studied, however, there is no evidence to suggest that skua communal breeding has advantages in terms of offspring production.     

Annual and seasonal reproductive trends in the Lesser Spotted Woodpecker Dendrocopos minor

Annual and seasonal variation in reproductive timing and performance were studied in a population of the Lesser Spotted Woodpecker Dendrocopos minor over 10 years in southern Sweden. The median laying date of the first egg varied by up to 17 days between years, being generally larger than the variation of laying dates within years. Neither clutch size, brood size in successful nests, fledging success in successful nests nor mean nestling weight differed significantly between years. There was no trend for mean clutch size to vary between early and late years. In spite of a more than threefold variation in population size, no reproductive variable demonstrated an apparent density-dependence. Within the season, clutch size declined steeply with increasing clutch initiation date, whereas fledging success and nesting success did not, leading to a trend in brood size almost identical to the trend in clutch size. The survival prospects of fledged young declined with increasing clutch initiation date, and it is argued that the clutch size laid is a strategic adjustment to laying date. Out of 124 breeding attempts, 34% did not produce fledged young. In 9% of the breeding attempts, pairs laid no eggs. At least 20% of the breeding attempts failed after egg-laying. The most common cause of breeding failure was loss of the breeding partner followed by nest abandonment (40% of the failures). Only 16–28% of the failures were due to predation on the nest. Most complete failures, and also partial losses from nests, occurred at the early breeding stages. It is argued that the early nestling phase may be a critical stage, which the woodpeckers adjust to coincide with the seasonal food peak, explaining the strikingly late breeding season compared with other non-migrant species.     

Variation in the quality of Vespula vulgaris (L.) queens (Hymenoptera: Vespidae) and its significance in wasp population dynamics

Abstract

Density‐related variation in queen quality has been proposed as a possible mechanism regulating population fluctuations in Vespula species. We investigated annual variation in the quality (size, weight, and fat content) of adult V. vulgaris queens representing four stages of their life cycle (spring, founding, developing, and emerged) taken from six sites in beech forest, South Island, New Zealand. For each queen the dry weight, head width, and thorax length was measured. For a subsample of queens, the fat content was determined by ether extraction. The size of queen cells was measured from a subsample of nests. Size, weight, and fat content of queens varied between wasp colonies and sites. The smallest juvenile queens were under‐represented in the reproductive population. There was no direct link between body size and food supply. Size and weight of developing queens increased as the number of cells in the nest increased. The size of the queen cells varied significantly among layers in a nest and among nests. The under‐representation of small queens in the reproductive population suggests that queen quality may affect survival and/or competitive ability by increasing winter fat storage, nest building activity, and/or success in usurpation disputes.     

Group living, breeding behaviour and territoriality in the Subdesert Mesite Monias benschi

In 1997–2000 we studied a population of Subdesert Mesites Monias benschi consisting of 35–68 adults comprising 32 groups of two to nine birds (modal group size of four). The study population was significantly male-biased in 1999 but not in 1997 or 1998. Overall, both sexes were philopatric, but when dispersal (or eviction) occurred, it appeared to be female-biased. Over 40% of groups contained more than two adult males, whilst < 15% contained more than two adult females. Whilst there was no evidence of behavioural dominance by females, intrasexual aggression within groups was observed only amongst females. In contrast to other birds occupying the same habitat, breeding in mesites was not tied to rainfall, and occurred throughout the year. Each breeding unit constructed several nests every year, only one of which was used. All adult males and at least one adult female co-operated to raise one or two clutches of one or two eggs per year. Males and females contributed equally to incubation. Chick production and chick survival were not related to group size or territory size. Groups defended large, permanent, and multipurpose territories and all group members contributed to territory defence. Territory size was positively correlated with the number of males in groups, but not with overall group size. Territories were tightly packed with very few areas unoccupied. Transect surveys conducted throughout the narrow geographical range of this species revealed its presence in a range of semi-arid habitat types. Small groups were more likely to be detected in intact, high-stature forest, whilst large groups were more likely to be detected in low-stature forest containing numerous spiny, xerophytic trees Didierea madagascariensis .     

The effects of dominance rank and group size on female lifetime reproductive success in wild long-tailed macaques,Macaca fascicularis

Demographic changes were recorded throughout a 12-year period for three social groups ofMacaca fascicularis in a natural population at Ketambe (Sumatra, Indonesia). We examined the prediction that females' lifetime reproductive success depended on dominance rank and group size. Average birth rate was 0.53 (184 infants born during 349 female years). For mature females (aged 8–20 yr) birth rate reflected physical condition, being higher in years with high food availability and lower in the year following the production of a surviving infant. High-ranking females were significantly more likely than low-ranking ones to give birth again when they did have a surviving offspring born the year before (0.50 vs 0.26), especially in years with relatively low food availability (0.37 vs 0.10). Controlled comparisons of groups at different sizes indicate a decline in birth rate with rroup size only once a group has exceeded a certain size. The dominance effect on birth rate tended to be strongest in large groups. Survival of infants was rank-dependent, but the survival of juveniles was not. There was a trend for offspring survival to be lower in large groups than in mid-sized or small groups. However, rank and group size interacted, in that rank effects on offspring survival were strongest in large groups. High-ranking females were less likely to die themselves during their top-reproductive years, and thus on average had longer reproductive careers. We estimated female lifetime reproductive success based on calculated age-specific birth rates and survival rates. The effects of rank and group size (contest and scramble) on birth rate, offspring survival, age of first reproduction for daughters, and length of reproductive career, while not each consistently statistically significant, added up to substantial effects on estimated lifetime reproductive success. The group size effects explain why large groups tend to split permanently. Since females are philopatric in this species, and daughters achieve dominance rank positions similar to their mother, a close correlation is suggested between the lifetime reproductive success of mothers and daughters. For sons, too, maternal dominance affected their reproductive success: high-born males were more likely to become top-dominant (in another group). These data support the idea that natural selection has favored the evolution of a nepotistic rank system in this species, even if the annual benefits of dominance are small.     

Group structure,within-group conflict and reproductive tactics in cooperatively breeding Galápagos mockingbirds,Nesomimus parvulus

Reproductive conflict within groups can be an important feature of cooperative breeding systems, especially when more than one individual of a sex breeds within a social group. Relationships between group structure, dominance, within-group conflict and reproductive tactics of cooperatively breeding Galápagos mockingbirds were examined on Isla Genovesa. Territorial groups of 2–24 adults included up to three breeding females, with 42% of the groups containing more than one (plural groups); females in most plural groups nested separately. Territory size increased with group size, but the area available per pair in plural groups was smaller than in singular groups (groups with only one breeding pair). Most pairings were monogamous, and males usually outnumbered females; high-ranking males obtained mates more frequently than subordinate males. In 3 relatively dry years, but not in a wet El Niño year, subordinate pairs in plural groups fledged fewer young than dominant pairs or pairs breeding in singular groups. Interference by dominant breeders, often leading to abandonment of nests by subordinate pairs, appears to account for these differences: through nest disruption in drier years, dominant individuals may reduce the cost of sharing their territories and increase the chances of recruiting helpers. Dominant males in plural groups may also father young through extra-pair copulations with subordinate females. Despite costs imposed by within-group conflict, subordinate breeders have higher long-term reproductive success than birds that defer breeding. Plural group structure is maintained because unpredictable climatic variation favours opportunistic breeding by subordinates.