Philopatry and within-colony movements in Columbian ground squirrels

Philopatry and dispersal result in selection of habitat locations that may differ in resources and social environment and thus should influence fitness components like survival and reproduction. We examined short-distance movements of young and adult females from natal or previous nesting sites within a colony of Columbian ground squirrels (Urocitellus columbianus) in the Rocky Mountains of Alberta, Canada, over a 17-year period. Females of all ages were strongly philopatric, yet a few (10-15%) exhibited movements that took them to new home ranges. We tested three hypotheses to explain the pattern of female natal and breeding movements: (1) that movements of philopatric females promote proximity to close kin; (2) that range shifts favour close kin via bequeathal of territory and (3) that dispersers move to lower density areas where competition for resources is lower. Tests of these three hypotheses revealed that: (1) philopatry and movements of young and older philopatric females led to proximity to mothers and local presence of close kin; (2) breeding dispersal did not result in bequeathal of home range to daughters, but movements of philopatric females suggested that they shared space with close kin and (3) adult females moved to new ranges with lower local densities, though dispersing females also left ranges where local density was significantly lower than for philopatric females. Natal and breeding movements among years produced two opportunities for territorial females: close spatial proximity to close kin via short philopatric movements, and habitats with fewer competitors via longer dispersal movements.     

Relatedness among females in a peak phase population of Japanese field voles

Spatial and genetic relationships among females in a peak population ofMicrotus montebelli were studied by live-trapping and DNA fingerprinting. In the early period of the spring breeding season, a close genetic relationship was not observed between neighboring overwintered females, while in the late period, a proximity among highly related females and a dispersed distribution among intermediately related females was observed. These associations among relatives did not last very long, however, as female pairs who remained in the neighborhood for more than 150 days, covering two breeding seasons were not relatives. The possibility of cooperation among relatives in this species may thus be low, except among philopatric females in the late period of a single breeding season.     

Social bonds enhance reproductive success in male macaques

For animals living in mixed-sex social groups, females who form strong social bonds with other females live longer and have higher offspring survival [1-3]. These bonds are highly nepotistic, but sometimes strong bonds may also occur between unrelated females if kin are rare [2, 3] and even among postdispersal unrelated females in chimpanzees and horses [4, 5]. Because of fundamental differences between the resources that limit reproductive success in females (food and safety) and males (fertilizations), it has been predicted that bonding among males should be rare and found only for kin and among philopatric males [6] like chimpanzees [7-9]. We studied social bonds among dispersing male Assamese macaques (Macaca assamensis) to see whether males in multimale groups form differentiated social bonds and whether and how males derive fitness benefits from close bonds. We found that strong bonds were linked to coalition formation, which in turn predicted future social dominance, which influenced paternity success. The strength of males' social bonds was directly linked to the number of offspring they sired. Our results show that differentiated social relationships exert an important influence on the breeding success of both sexes that transcends contrasts in relatedness.     

Effects of female kin groups on reproduction and demography in the gray-tailed vole, Microtus canicaudus

I conducted an experiment with gray-tailed voles, Microtus canicaudus , to test the hypothesis proposed by Charnov and Finerty that populations of voles comprised of female kin groups would grow more rapidly and reach higher densities more quickly than populations in which female kin groups were disrupted. The experiment was conducted in 0.2-ha semi-natural enclosures planted with mixed grasses. In four enclosures, females were unmanipulated (control) and in four enclosures all newly caught females were removed from their natal enclosures and replaced with females of comparable age from another enclosure, such that juvenile females did not settle near their siblings or parents (treatment). I found no significant differences in survival, reproduction, juvenile recruitment, population growth rates, or population size between control and treatment populations. The only difference was the time to sexual maturation for young females, which was 3.1 weeks for control enclosures compared with 4.2 weeks for treatment enclosures. I could not measure reproductive success for individual females, but my results did not support the hypothesis that the presence or absence of kin groups resulted in any biologically meaningful population-level effects. Female voles that have nesting territories adjacent to relatives may accrue some individual benefits, but these benefits are unlikely to contribute to population regulation in gray-tailed voles.     

Lifetime philopatry in the blue-footed booby: a longitudinal study

Philopatry over the lifetime and its relationship with reproductivesuccess were examined using longitudinal records of nest locationand reproduction of individual blue-footed boobies. Males showedshorter natal dispersal than females, and natal dispersal distanceof both sexes were unrelated to either first reproductive successor lifetime reproductive success. Throughout the early lifetime,males and females nested closer to their first breeding sitesthan to their natal sites, and comparison with a simulationof successive breeding dispersals in random directions showedthat male and female blue-footed boobies are philopatric tothe first breeding site. Therefore, throughout the early lifetime,the first breeding site seems to function as a point of referencefor breeding site use together with the previous season's site.Males and females with shorter natal dispersal distances showedstronger lifetime philopatry to their first breeding sites,suggesting stable individual variation in competitive abilityor dispersal phenotype. However, early lifetime philopatry tofirst breeding sites was unrelated to annual breeding success.Compared with simple fidelity to previous breeding sites, lifetimephilopatry to first breeding sites should result in increasedkin interactions and greater selection for kin recognition,altruism and inbreeding avoidance, as well as long-term familiaritywith neighbors.     

Reproductive tactics of the ringed plover Charadrius hiaticula

Reproductive tactics of ringed plovers Charadrius hiaticula were studied at three localities in SW Sweden during five seasons. The usual clutch size is four, but removal experiments showed that females can produce five eggs in succession, with similar intervals between all eggs. High predation made mean breeding success per clutch low, 6.3% of eggs resulting in fledged young. Replacement clutches were common, and many pairs laid again after rearing their first brood to fledging. Egg laying spanned three months, much longer than for other waders in this region. Between years, reproductive success varied unpredictably with time of the season, but averaged over several years, the expected success was low and similar for the different parts of the breeding season. Chicks from late clutches had similar survival and recruitment as others. Because of the long breeding season and high rate of nest failure a female may produce up to five clutches of four eggs per season, containing in total about 3.7 times her own mass. Yearly local survival of adult females and males was estimated to 84.6 and 88.6%, respectively. Ability to produce many clutches with similar expected success throughout the season favours a long reproductive period, sometimes leading to double-brooding. Possible life-history trade-offs are discussed.     

Habitat selection by dispersing yellow-headed blackbirds: evidence of prospecting and the use of public information

In migratory birds individuals may prospect for potential breeding sites months before they attempt to breed and should use the cues most predictive of future reproductive success when selecting a breeding site. However, what cues individuals use when prospecting and which cues are used in selecting a breeding site are unknown for most species. I investigated whether yellow-headed blackbirds (Xanthocephalus xanthocephalus) prospect for future breeding sites and whether they select breeding habitats based on food availability, male or female density, or the average number of young produced per female in the previous year. Although it is often assumed that migratory birds prospect for potential breeding sites at the end of the breeding season, I investigated this by recording all visits to sites early and late in the breeding season. I found that males and females who visited sites other than the site at which they bred were more likely to disperse than individuals only observed at the site where they bred, and that males and females were more likely to prospect late in the breeding season. Both food availability and density in year ^x were not predictive of the number of young per female in year ^x+1; however, the number of young produced per female at a site in year ^x was predictive of the number of young per female in year ^x+1. As expected, dispersers used the most informative cue, the number of young per female and moved to sites with a relatively high number of young per female. This study suggests that individuals prospect for potential breeding sites late in the breeding season when they can use information gathered from the reproductive success of other individuals (i.e., public information) to select a breeding site.     

HIGHER FITNESS FOR PHILOPATRIC THAN FOR IMMIGRANT MALES IN A SEMI-ISOLATED POPULATION OF GREAT REED WARBLERS

To compare the fitness of philopatric and immigrant individuals we examined the lifetime reproductive success of 116 male and 137 female great reed warblers. The study was carried out in a semi-isolated population in Sweden and covered breeding adults hatched between 1985 and 1993. Lifetime fitness, measured as life time number of fledglings and offspring recruits, was lower for immigrant than for philopatric males. We found no such relationships for females. The difference in reproductive success could not be explained by immigrant males having lower phenotypic quality because they had similar life span, spring arrival date, and territory quality as philopatric males. The lower lifetime fitness among immigrant than philopatric males appeared to result from reduced mating success. This suggests that females are reluctant to mate with immigrant males despite their apparently similar phenotypic quality. Though it is not known whether females gain in fitness by avoiding matings with immigrant males, it is notable that immigrant males have smaller song repertoires than philopatric males. Large repertoires, previously shown to sexually arouse great reed warbler females, correlate with the occurrence of extrapair paternity and postfledging survival of offspring in our population.     

Banded mongooses avoid inbreeding when mating with members of the same natal group

Inbreeding and inbreeding avoidance are key factors in the evolution of animal societies, influencing dispersal and reproductive strategies which can affect relatedness structure and helping behaviours. In cooperative breeding systems, individuals typically avoid inbreeding through reproductive restraint and/or dispersing to breed outside their natal group. However, where groups contain multiple potential mates of varying relatedness, strategies of kin recognition and mate choice may be favoured. Here, we investigate male mate choice and female control of paternity in the banded mongoose (Mungos mungo), a cooperatively breeding mammal where both sexes are often philopatric and mating between relatives is known to occur. We find evidence suggestive of inbreeding depression in banded mongooses, indicating a benefit to avoiding breeding with relatives. Successfully breeding pairs were less related than expected under random mating, which appeared to be driven by both male choice and female control of paternity. Male banded mongooses actively guard females to gain access to mating opportunities, and this guarding behaviour is preferentially directed towards less closely related females. Guard–female relatedness did not affect the guard's probability of gaining reproductive success. However, where mate‐guards are unsuccessful, they lose paternity to males that are less related to the females than themselves. Together, our results suggest that both sexes of banded mongoose use kin discrimination to avoid inbreeding. Although this strategy appears to be rare among cooperative breeders, it may be more prominent in species where relatedness to potential mates is variable, and/or where opportunities for dispersal and mating outside of the group are limited.     

Subordinate superb fairy-wrens (Malurus cyaneus) parasitize the reproductive success of attractive dominant males

Explanations of cooperative breeding have largely focused on the indirect benefits philopatric offspring gain from investing in kin. However, recent molecular studies have revealed that in many species subordinates provision unrelated offspring. This has led to the re-evaluation of the direct and indirect benefits of helping behaviour. In this study, we used microsatellite genotyping to assess the extra-group reproductive success of subordinate superb fairy-wrens (Malurus cyaneus), a species with extremely high rates of extra-group paternity. Extra-group subordinate males sired 10.2% (193 out of 1895) of all offspring sampled between 1993 and 2000 and 21.4% (193 out of 901) of all illegitimate offspring sired by known males. The extra-group success of subordinates was greatly influenced by the attractiveness of their dominant male. Subordinates of attractive dominants sired more extra-group young than did average dominants. Evidence suggests that mate choice in superb fairy-wrens is error-prone and subordinates can gain direct reproductive benefits through parasitizing the reproductive success of attractive dominants.     

Is Divorce in Young Common Terns,Sterna hirundo,after Recruitment just a Question of Timing?

The probability of divorce in birds has been linked with age, breeding experience, reproductive output and synchrony in return. Here, we investigate the consequences of first breeding attempts in common terns for mating in the subsequent season. Nearly 20% of all first‐time breeders disappeared or skipped at least one season after recruitment. In 84 pairs, which consisted of at least one recruit and of which both partners returned to the colony, the divorce rate was 45%. We compared reproductive success, arrival dates, and asynchrony in arrival dates of pairs of the first breeding season against the second season, for both reunited and divorced pairs and males and females separately. First, in pairs of which both members came back to the colony, we found an increase of reproductive success most pronounced in males. In the second season reproductive success of divorced compared with reunited pairs was higher, as only divorced pairs significantly improved the number of fledglings, and again this relation was stronger in males. Secondly, females of reunited pairs arrived significantly earlier from the first to the second season and by far more days than their males. However, in divorced pairs former mates did not differ in the number of days they advance their arrivals. Finally, divorced males arrived on average 4 d earlier than their former mates, whereas divorced females arrived 5 d later compared with their former mates of the recruitment season. Contradictory to nearly all other divorce studies in birds so far, we found a clear fitness gain in divorced males. We suggest that the improvement in reproductive success of young males stems from a side‐effect of the birds’ quality and ability to reach the breeding site in appropriate time and earlier as potential competitors. In long‐lived bird species the heterogeneity among young individuals in the timing of arrival at the colony seems to explain why former recruit‐pairs reunite or split. For young males we suggest as best explanation of divorce that they profit from ‘pushing for an empty chair’, while females seem to profit from their choosiness and may actively decide between former and other mates.     

Social organization and space use in California voles: seasonal,sexual, and age-specific strategies

We intensively monitored space use and movement in Microtus californicus over a 2-year period that included 1 year of high density (maximum 618/ha) and one of low (minimum 5/ha); historically this population has exhibited cycles of 2 or 4 years. Adults of both sexes dispersed at the start of the breeding season, culminating in the establishment of intrasexually exclusive territories. In females, these territories persisted throughout life, except that many young females recruiting during the breeding season established contiguous, overlapping, or adjacent home ranges with their mothers. This female philopatry explains the conclusion of previous workers that females of this species are non-territorial. In the dry (non-breeding) season, females had smaller ranges that often overlapped and were clustered. Adult males moved breeding territories at a modal interval of 6 weeks; this is consistent with their avoidance of inbreeding with philopatric daughters. Ranges overlapped 1–4 adult females at any one time, and a cohort of 7 long-lived males overlapped an average of 16.4 females during their tenure on the grid. The period of maximum overlap with adult females varied among individual males, and did not correlate with the time of maximum body weight. Ranges of males in the dry season overlapped extensively, with persistent associations among some individuals. In the lowdensity year, ranges of some adults failed to overlap intersexually. Juvenile males dispersed gradually between 3 and 13 weeks of age (half before 9 weeks), with some leaving after reaching sexual maturity; a few remained philopatric. Of juvenile females, 47% remained philopatric with the rest disappearing before 9 weeks of age. New understanding of vole social behavior, dispersal, and space use is achieved by focusing on the seasonal dynamics of spatial relationships among individuals with respect to age, sex, and relatedness.     

Age-related patterns of reproductive success in house sparrows Passer domesticus

A common life history pattern in many organisms is that reproductive success increases with age. We report a similar pattern in house sparrows Passer domesticus , older individuals performed better than yearlings for most measures of reproductive success. Older males and females began breeding earlier in a given season and fledged more young than their yearling counterparts. Individual males also fledged more young in their second breeding season than they did in their first, but individual females did not show consistent improvement in reproductive success from year one to two. A path analysis indicated that age in both sexes acted primarily through the timing of breeding; earlier nesters laid more eggs and hence fledged more young but did not have more nesting attempts. We tested whether the increased reproductive success with age arose from high quality individuals surviving to be older (selection hypothesis). In contrast to the main prediction of this hypothesis that reproductive success and survival should be positively related, we found that survival from one year of age to two years of age was negatively related to reproductive success in the first year for males and females combined. Additionally, individuals that survived to breed as two-year-olds did not differ in total young fledged in their first year from those that did not survive to their second season of breeding. Our results indicate that fledgling production increases with age due to improvements in timing of breeding, particularly in females, and not because of the loss of poor breeders or increased output. Mechanisms producing age-related differences in timing of breeding warrant further study.     

Selection for synchronous breeding in the European starling

Colonial birds often demonstrate considerable breeding synchrony. In southern Sweden the semi-colonial European starling initiated the vast majority of clutches within one week. Laying dates were positively skewed so that many birds initiated clutches at similar dates early in the season. Breeding was further synchronised by a particularly strong clutch-size reduction equivalent to one third of an egg per day during the first part of the breeding season. The decline in clutch size with season also held true for separate age-classes of females, for individual females laying at different times at different years and for individual females laying at different times the same year. Trends in breeding success during nestling rearing were unlikely to explain the high degree of breeding synchrony or the seasonal decline in clutch size; nestling survival and growth were weakly related or unrelated to reproductive timing. In contrast recruitment success of fledged offspring declined sharply with season. Even within the synchronous laying period, defined as clutches initiated during the first week each year, local recruitment success declined. It is suggested that the early seasonal decline is caused by selection for synchronous fledging permitting the immediate formation of flocks after fledging, whereas the late seasonal trends may be caused by either population differences in female quality or deteriorating conditions for raising young.     

Male ornamentation, timing of breeding, and cost of polygyny in the collared flycatcher

Highly ornamented males are often thought to be better ableto provide females with resources, parental assistance, or goodgenes. Individual variation in such male abilities may overridethe costs of polygyny and therefore largely explain within-populationvariation in mating patterns. We investigated the influenceof variation in male ornamentation and the environment on thecosts of polygyny for female collared flycatchers (Ficedulaalbicollis), using data from a long-term study involving 2733breeding attempts over 19 years. We show that females sufferreduced reproductive success when mated polygynously but thatthe costs of polygyny depend on an interaction between maleornamentation and timing of breeding. Among early breeders,polygynously mated females experience higher reproductive successwhen mated to less ornamented males, but among late breeders,females mated polygynously to highly ornamented males were moresuccessful. We suggest that a high effort spent on obtainingextrapair matings early in the season renders highly ornamentedmales less able to assist two females in caring for the young.Thus, a male's ability to simultaneously gain from extrapairmatings and polygyny may be limited through direct effects onfemale reproductive success. Given such limitation, extrapairmatings may be expected to be less frequent in species withbiparental care and a high level of social polygyny.     

Seasonal variation in male alternative reproductive tactics

Genetic parentage analyses reveal considerable diversity in alternative reproductive behaviours (e.g. sneaking) in many taxa. However, little is known about whether these behaviours vary seasonally and between populations. Here, we investigate seasonal variation in male reproductive behaviours in a population of two‐spotted gobies (Gobiusculus flavescens) in Norway. Male two‐spotted gobies guard nests, attract females and care for fertilized eggs. We collected clutches and nest‐guarding males early and late in the breeding season in artificial nests and used microsatellite markers to reconstruct parentage from a subset of offspring from each nest. We hypothesized that mating, reproductive success and sneaking should be more prevalent early in the breeding season when competition for mates among males is predicted to be higher. However, parentage analyses revealed similar values of mating, reproductive success and high frequencies of successful sneaking early (30% of nests) and late (27% of nests) in the season. We also found that multiple females with eggs in the same nest were fertilized by one or more sneaker males, indicating that some males in this population engage in a satellite strategy. We contrast our results to previous work that demonstrates low levels of cuckoldry in a population in Sweden. Our results demonstrate marked stability in both the genetic mating system and male alternative reproductive tactics over the breeding season. However, sneaking rates may vary geographically within a species, likely due to local selection influencing ecological factors encountered at different locations.     

THE EFFECT OF BIRTH DATE ON FITNESS OF FEMALE DWARF PERCH,MICROMETRUS MINIMUS (PERCIFORMES: EMBIOTOCIDAE)

Lifetime reproductive success may vary considerably with birth date. I measured phenotypic selection on female birth date in a viviparous teleost fish (Embiotocidae: Micrometrus minimus) by sampling birth-date cohorts over time in Tomales Bay, California. Four episodes of selection were measured: survival from birth to first reproduction, reproductive success in the first breeding season, survival to second reproduction, and reproductive success in the second season. Birth date had a significant impact on fitness in the first two episodes. Early born females were more successful in their first breeding season than late born females (directional selection on birth date), but early born females were less likely to survive the period between birth and first reproduction, relative to females born in the middle of the season (stabilizing selection on birth date). The final two episodes of selection had no detectable effect on birth date. Because of the relationship between birth date and survival in the first year, overall selection on female birth date was stabilizing.     

Cooperative enhancement of reproductive success in white-winged choughs

Summary White-winged choughs are a cooperatively breeding species which provide parental care to their young over an entire year. I traced the reproductive success of groups of white-winged choughs from the start of one breeding season to the next over 3 years. I examined the effect of helper number on timing of breeding, the success of each effort, the number of efforts made in a season, and the final reproductive success at the end of each year. Timing of commencement of breeding varied between years but was not related to group size. Early broods were not more successful than late broods. Nest building (July–September) commenced earlier in years which had high rainfall in July; choughs rely on rainfall for supplies of mud for nest construction. Most nest failures occurred gradually and were attributed to starvation of nestlings, although some sudden failures were attributed to predation. Large groups have more young by the beginning of the following season; this is due to higher fledging success and a greater likelihood of having second broods. Disappearance of young after fledging and during the transition to independence was not dependent on group size. Only groups of seven and above produce more than one young on average over the entire year; choughs provide one of the most marked cases for helpers enhancing the reproductive success of breeders. Large groups are virtually guaranteed of reproductive success over the whole year and grow more quickly than small groups. These results highlight the need to consider the effect of helpers over the entire period of reproduction and care of young, rather than just at fledging.     

Differences in prolactin levels between three alternative male reproductive tactics in striped mice (Rhabdomys pumilio)

In male fishes, birds and mammals, increased prolactin secretion is thought to play a role in species showing paternal behaviours. This hypothesis was investigated in the striped mouse (Rhabdomys pumilio). This paper compares serum prolactin levels in 71 free-living male striped mice following three different reproductive tactics: (i) paternal group-living breeders, (ii) alloparental philopatric group-living males, and (iii) roaming non-paternal solitary males. Prolactin levels of breeding males were significantly higher than that of roamers. Alloparental philopatric males had low prolactin levels, which concur with studies of cooperatively breeding mammals, but contrasts with studies of cooperatively breeding birds. Both breeding males and females showed a decrease in prolactin levels after the breeding season, but not alloparental philopatric males. Prolactin levels were correlated with neither corticosterone levels nor age. These results are in agreement with the hypothesis that prolactin is one proximate mechanism of male reproductive tactics, possibly regulating differences in male parental care.     

An increase in parental investment during the breeding season

The intensity of nest-defence aggression by female redwinged black birds (Agelaius phoeniceus) with eggs decreased late in the breeding season, while aggression at this time increased amongst females defending broods. High predation pressure late in the breeding season decreased the probability of survival of the late nesting attempts, so that females with young had a much greater chance of success than females with eggs at this time. The observed changes in aggressive responses are predicted by the theory of parental investment.