Patterns of reproductive skew in the polygynandrous acorn woodpecker

We compared observed levels of reproductive skew in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus) with those predicted by two alternative transactional models. "Concession" models predict the degree to which parentage is shared assuming that a single dominant is in complete control of reproduction. Alternatively, "restraint" models predict reproductive sharing assuming that the dominant controls only whether subordinates remain in the group but does not control its share of reproduction. Reproductive skew is high among males: on average, the most successful male sires more than three times as many offspring as the next most successful male. Females share parentage equally and have lower constraints on dispersal and lower survival rates compared with males, which is consistent with predictions from the concessions model. Also as predicted by the concessions model, yearly variation in opportunities for dispersal before the breeding season correlates positively with skew. However, in contrast to concessions but consistent with the restraint model, skew decreases with relatedness. Thus, neither model consistently predicts patterns of reproductive skew in this species. We suggest that models of reproductive skew will need to include competitive interactions among potential breeders and mate choice before they will adequately predict patterns of reproductive partitioning in most vertebrate societies.     

Habitat saturation and communal breeding in the acorn woodpecker

The acorn woodpecker (Melanerpes formicivorus) typically lives in social groups that breed communally: all group members help to feed the young of a single nest. The behaviour of acorn woodpeckers was compared in two locations (New Mexico and California) to test the hypothesis that saturation of the habitat by established groups may lead to more than a single male-female pair living in groups during reproduction. Several lines of evidence indicated that the habitat was less saturated in New Mexico. This difference was reflected in a lower tendency for juveniles and adults to remain in groups, higher reproductive rates and, most important, smaller group sizes. These results suggest that habitat saturation plays an important role in communal breeding in this species.     

Shared parentage and incest avoidance in the cooperatively breeding acorn woodpecker

Social groups of acorn woodpeckers (Melanerpes formicivorus) range in size from unaided pairs to 15 adults. Behavioural indicators of mate guarding, assumed incest avoidance and observations of egg-laying indicate that social organization ranges from monogamous pairs to groups with up to seven male and three female putative cobreeders plus up to 10 nonbreeding helpers. In addition, groups occasionally lack a putative breeder throughout the breeding season. Here we report results from multilocus DNA fingerprinting of 372 nestlings from 123 nests in groups with putative cobreeders of one or both sexes. No extra-group fertilizations were found. Putative cobreeding males within social groups shared paternity. However, the most reproductively successful male was, on average, almost three times as successful as the next most successful and additional males only occasionally sired offspring. In contrast, cobreeding females shared parentage equally. Helpers never bred incestuously when their opposite-sex parent (or another relative, such as their uncle) held breeding status in the group. However, during breeding male vacancies, 14 nestlings were produced when helper males bred incestuously with their mother. Both male and female helpers usually became successful cobreeders with their same-sex parent following replacement of the opposite-sex breeder(s) by unrelated individuals.     

Space competition in the acorn woodpecker: Power struggles in a cooperative breeder

During a 5-year study of the cooperatively living acorn woodpecker Melanerpes formicivorus, I observed 26 irruptions of agonistic behaviour involving large numbers of intruders invading a territory. Evidence from several sources suggests that these invasions are usually contests among potential immigrants for the opportunity to replace a missing group member (hence, ‘power struggles’). Thus, power struggles indicate intense competition among individuals to transfer out of their natal group. Such dispersal is not necessarily excluded by the hypothesis that individuals gain directly by living in cooperative groups through either increased foraging efficiency or better predator defence. However, birds involved in power struggles expend considerable energy and incur considerable risks. These costs are likely to be an acceptable part of dispersal only if cooperative living is a result of birds being forced to remain in groups as a result of resource localization.     

Reproductive roles in the cooperatively breeding acorn woodpecker: incest avoidance versus reproductive competition

Incest is rare in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus) despite a polygynandrous mating system in which nearly all group members are close relatives. Here we test the relative importance of avoiding matings between close relatives (incest avoidance) and within-sex competition for breeding opportunities (reproductive competition) in determining the mating system of acorn woodpeckers by examining how reproductive roles change following breeding vacancies. In 83% of cases in which helpers of the same sex were present in the group, reproductive vacancies were resolved when new unrelated immigrants filled the vacancy to the exclusion of resident same-sex helpers, who generally emigrated or did not breed while they remained in the group. Helpers of the opposite sex, especially when male, were significantly more likely to remain in their natal group and in about half the cases inherited and bred following reproductive vacancies. This result was not explainable by reproductive competition, since the number of immigrants was often less than or equal to the number of same-sex helpers in the group. Apparent incest resulted in 5% of cases. The time required to resolve reproductive vacancies was significantly longer for groups with helpers of the same sex as the vacancy. These results confirm that both incest avoidance and reproductive competition are important factors determining reproductive roles within groups of this highly social species.     

Variable helper effects, ecological conditions, and the evolution of cooperative breeding in the acorn woodpecker

The ecological conditions leading to delayed dispersal and helping behavior are generally thought to follow one of two contrasting scenarios: that conditions are stable and predictable, resulting in young being ecologically forced to remain as helpers (extrinsic constraints and the habitat saturation hypothesis), or that conditions are highly variable and unpredictable, leading to the need for helpers to raise young, at least when conditions are poor (intrinsic constraints and the hard life hypothesis). We investigated how variability in ecological conditions influences the degree to which helpers augment breeder fitness in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus), a species in which the acorn crop, territory quality, and prior breeding experience all vary in ways that have important effects on fitness. We found that the relationship between ecological conditions and the probability that birds would remain as helpers was variable but that helpers generally yielded greater fitness benefits when ecological conditions were favorable, rather than unfavorable, for breeding. These results affirm the importance of extrinsic constraints to delayed dispersal and cooperative breeding in this species, despite its dependence on a highly variable and unpredictable acorn crop. Our findings also confirm that helpers can have very different fitness effects, depending on conditions, but that those effects are not necessarily greater when breeding conditions are unfavorable.     

Group sex ratio and sex reversal

In hermaphroditic fishes, the initiation of sex reversal by male removal explains the replacement of lost males but does not explain how the number of males in a group may increase. Since numerous species apparently cannot produce primary males, a second means of initiating sex reversal must exist. In the present study we formulate a model which suggests the existence of an additional mechanism governing sex change: as soon as the ratio of adult females to males within a group exceeds a certain threshold value, a female changes sex even though no male has been removed. This process is inferred from comparison of data collected in the Red Sea and the western Indian Ocean with the model's predictions concerning size at sex reversal and the sex ratio of groups. The results suggest how several ecological factors may influence the occurrence rate of sex reversal and the development and growth of social groups.     

Experimental tests of sex allocation theory with two species of simultaneously hermaphroditic acorn barnacles

Sex allocation theory for simultaneous hermaphrodites predicts increases in relative allocation to male-specific function as competition for fertilizations increases. Theoretical models developed specifically for competing acorn barnacles predict that the proportional allocation to male function increases toward an asymptote of 50% as the number of competitors for fertilizations increases. Experimental manipulations were used to investigate how mate competition affected both relative and absolute allocation to the sex functions for two species of acorn barnacle: Semibalanus balanoides and Balanus glandula. The ratio of male to female allocation did not increase with the number of competitors for either species. However, both species showed increased allocation to male function (estimated as total mass of sex-specific tissues) with increased crowding. Allocation to female function seemed to be limited by other factors and did not vary with mating group size as predicted. Allocation to male and female function were both positively related to body size, but a trade-off between male and female function, a key assumption of prior models, was not observed.     

Division of labour during incubation in a woodpecker Colaptes auratus with reversed sex roles and facultative polyandry

The contribution of males to incubation has rarely been studied in altricial birds because the pursuit of extra mating opportunities is believed to conflict with incubation. Woodpeckers show reversed sex roles in parental care with males doing most of the nest construction, incubating and brooding of the young while females may be polyandrous. I investigated incubation by each sex at 71 monogamous and four polyandrous nests of the Northern Flicker Colaptes auratus , predicting that males would contribute to incubation according to their energy reserves (body condition) whereas females would contribute based on alternate reproductive opportunities. Nest attendance was 99% with males contributing a mean of 66% of the total incubation including nocturnal incubation. The length of daytime bouts averaged about 2 h and did not differ between the sexes. Consistent with predictions of investment strategies, structurally larger males and those in poorer body condition incubated less than smaller males, perhaps because they required more recess time to forage or to conserve energy. Older females contributed less incubation than young females and polyandrous females contributed less incubation at their secondary nests than monogamous females. Incubation period, nest depredation rate and hatching success were not influenced by bout length, number of bouts or relative contribution of the sexes. Hatching success was 86% at nests of both monogamous and polyandrous females because males compensated for reduced female participation. Because incubation of the sexes is compensatory and not additive, incubation pattern did not influence short-term reproductive success. I conclude that males invest in incubation according to their energy needs, and females may adjust their contributions based on alternate reproductive tactics.     

Mitochondrial DNA based phylogeny of the woodpecker genera Colaptes and Piculus,and implications for the history of woodpecker diversification in South America

The woodpecker genus Colaptes (flickers) has its highest diversity in South America and the closely related genus Piculus is restricted to South and Central America. Two species of flickers occur in North America, and one species is endemic to Cuba. We conducted a Bayesian phylogenetic analysis of three mitochondrial encoded genes (cyt b, COI, 12S rRNA) and confirmed that the two genera are paraphyletic. Three species historically classified as Piculus are actually flickers. We found that the Cuban endemic C. fernandinae is the most basal species within the flickers and that the Northern Flicker is the next most basal species within the Colaptes lineage. The South American clade is most derived. The age of the South American diversification is estimated to be 3.6 MY, which is synchronous with the emergence of the Isthmus of Panama. The pattern of diversification of South American flickers is common among South American woodpeckers. Although woodpeckers have their greatest diversity in South America, we hypothesize that woodpeckers (Family Picidae) originated in Eurasia, dispersed to North America via the Bering land bridge, and multiple lineages entered South America as the Isthmus approached its final closing.     

Classical polyandry found in the three-toed woodpecker Picoides tridactylus

We report two cases of simultaneous, social polyandry in the three-toed woodpecker Picoides tridactylus found in Berchtesgaden National Park, Germany during an 11-year study. The first case was between two neighbouring pairs in 2003. A female incubated both of her clutches during a period of at least 8 days. The second case was in 2004 between a widowed male and a female of a neighbouring pair. This time, a female divided parental care between her two clutches during a period of at least 27 days. We hypothesise that social polyandry in the three-toed woodpecker may be linked to the parental quality of the first male.     

Foraging behaviour of the middle spotted woodpecker Dendrocopos medius in Sweden

During winter and spring the oak Quercus robur , was the tree species most utilized by middle spoiled woodpeckers. In March, saptrees, mainly maples Acer platanoides , were preferred for a short period. In winter the birds foraged in the crowns of oaks but in spring they preferred lower stem parts. Dead branches were utilized more in winter than in spring. The birds mostly used a gleaning and probing technique, which may render the species more senstitive to severe weather conditions than other woodpecker species. Arthropods collected from trees were mostly spiders, beetles, thrips or springtails. The nestlings were mainly fed on scale insects and larvae of moths and sawflies. The oak habitat specialization is believed to be caused by competition for food with other gleaning forest birds. Food shortage might possibly occur during periods of bad weather, causing increased mortality.     

Phylogeographic patterns in the great spotted woodpecker Dendrocopos major across Eurasia

We sequenced parts of three mitochondrial genes in 67 great spotted woodpeckers Dendrocopos major taken from 17 sites across Eurasia and Japan. Two groups, differing by ca. 3% sequence divergence, were determined to have had independent evolutionary histories: samples from the southeast (Sakhalin, Primor'e, Hokkaido), and the rest of Eurasia. The southeastern group corresponds to the subspecies D. m. japonicus , which will likely merit species status upon study of all subspecies; at present it should be considered an evolutionarily significant unit. From the United Kingdom to the Russian North Pacific, no phylogeographic divisions were found, which is likely a result of recent range expansion following retreat of permafrost after the last ice age.     

Organohalogen constituents of the acorn worm Ptychodera bahamensis

• 1.1. Twenty organohalogen compounds, primarily phenols, were detected in the volatile extracts of the acorn worm Ptychodera bahamensis.
• 2.2. Five chlorinated compounds, previously undescribed from acorn worms, were identified.
• 3.3. Enteropneusts can be significant contributors of halogenated organics to the marine environment.

     

昆虫寄生对辽东栎种子命运的影响

报道了昆虫寄生对辽东栎 Quercus liaotungensis 种子命运的影响。结果表明：（1）被昆虫寄生的种子发芽率明显低于正常成熟的种子,被昆虫寄生的种子或者不能发芽,或者能发芽但随后死亡;（2）从虫卵到老龄幼虫的发育在橡实内完成,当橡实落地后幼虫破壳而出,整个种子库的幼虫破壳时间延续2～50天,并非高度同步化;种子库的虫寄生率高达45.4%,以柞栎象Curculio dentipes为主,还有少量其它种类,如另一种象虫Curculio sp.、栗白小卷蛾 Cydia kurokoi 和螟蛾科（Pyralidae）的一未定名种;（3）昆虫寄生影响橡实成熟,测量种子大小分布表明,被昆虫寄生的种子的大小分布介于未成熟和成熟种子之间;（4）象虫寄生取食引起16.05% 的种子能量损失,显著低于对照组。结果证明昆虫寄生对辽东栎种子的命运有重要影响,是影响辽东栎种群更新的关键因素之一。     

Social structure and winter survival in acorn ants

Fluctuating selection is a major theme in the evolutionary and ecological literature, yet attempts to measure how differential selection across time or space affects long-term change in life history traits or behaviors are still rare. Social evolution among the insects has been broadly studied with respect to how key parameters such as queen number and relatedness vary to influence colony fitness. However, a primary focus on fertility selection in the warm months must be complemented by parallel investigations on survivorship selection during the cold months. Here we provide the first assessment of social structure and overwintering survivorship in the field. We studied the acorn ant, which stays aboveground throughout the cold winters in North America, by varying queen number and colony size over two consecutive winters. We found that winter survival was quite low but unconnected to variable colony structure. Therefore previous studies on how social structure affects fertility selection in acorn ants have not been confounded by countervailing selection during the cold months. Our data support the assumption of the larger literature that selective forces molding social behavior in ants act primarily on fertility selection during the reproductive season.