Causes and consequences of living in closed societies: lessons from a long-term socio-genetic study on Bechstein's bats

Understanding the ecological, behavioural and genetic factors influencing animal social systems is crucial to investigating the evolution of sociality. Despite the recent advances in population genetic methods and the analysis of social interactions, long-term studies exploring the causes and consequences of social systems in wild mammals are rare. Here, we provide a synthesis of 15?years of data on the Bechstein's bat (Myotis bechsteinii), a species that raises its young in closed societies of 10-45 females living together for their entire lives and where immigration is virtually absent. We discuss the potential causes and consequences of living in closed societies, based on the available data on Bechstein's bat and other species with similar social systems. Using a combination of observational and genetic data on the bats together with genetic data on an ecto-parasite, we suggest that closed societies in Bechstein's bats are likely caused by a combination of benefits from cooperation with familiar colony members and parasite pressure. Consequences of this peculiar social system include increased sensitivity to demographic fluctuations and limits to dispersal during colony foundation, which have broad implications for conservation. We also hope to illustrate by synthesizing the results of this long-term study the diversity of tools that can be applied to hypothesize about the factors influencing a species' social system. We are convinced that with the expansion of the number of social mammals for which comparably detailed socio-genetic long-term data are available, future comparative studies will provide deeper insights into the evolution of closed societies.     

High gene diversity at swarming sites suggest hot spots for gene flow in the endangered Bechstein's bat

Our study shows that endangered Bechstein'sbats utilise distinct habitats at differentstages of their reproductive cycle, a findingthat has implications how habitat should beselected for preservation. Using nuclear andmitochondrial microsatellite DNA markers wecompared gene diversity of Bechstein's batswithin breeding colonies and at potentialmating sites. Bechstein's bat is one of themost threatened European bat species. Duringsummer it depends largely on mature deciduousforests. Females exhibit strict natalphilopatry. They form demographicallyindependent breeding colonies comprisingmaternally closely related bats. Males aresolitary. Like other temperate bats,Bechstein's bats swarm at the end of summer infront of caves. Because the sexes meet there,such swarming sites are potentially importantfor gene flow. Our genetic analyses reveal thatswarming sites have greater mitochondrial DNAgene diversity than colonies. Furthermore,field data on the phenology and reproductivestatus of several hundred individuals suggestthat Bechstein's bats mate during swarming. Incombination our field and genetic data showthat swarming sites provide the opportunity forgene flow among bats originating from differentcolonies. Therefore, we suggest that swarmingsites should be strictly protected to maintainthe observed high levels of gene flow amongcolonies.     

Group decision making in fission-fusion societies: evidence from two-field experiments in Bechstein's bats

Group decisions are required when group coordination is beneficial, but individuals can choose between alternatives. Despite the increased interest in animal group decision making, there is a lack of experimental field studies that investigate how animals with conflicting information make group decisions. In particular, no field studies have considered the influence of fission-fusion behaviour (temporary splitting into subgroups) on group decisions. We studied group decision making in two wild Bechstein's bat colonies, which are fission-fusion societies of stable individual composition. Since they frequently switch communal roosts, colony members must regularly make group decisions over where to roost. In the two-field experiments, we provided marked individuals with conflicting information about the suitability of potential roosts. We investigated whether conflicting information led to group decisions that followed a 'unanimous' or a 'majority' rule, or increased colony fission. Individual behaviour suggests that bats considered both their own information and the behaviour of others when deciding where to roost. Group decisions about communal roosts reflected the information available to a majority of the bats roosting together, but conflicting information led to an increased fission in one colony. Our results suggest that fission-fusion societies allow individuals to avoid majority decisions that are not in their favour.     

Communally breeding Bechstein's bats have a stable social system that is independent from the postglacial history and location of the populations

Investigating macro-geographical genetic structures of animal populations is crucial to reconstruct population histories and to identify significant units for conservation. This approach may also provide information about the intraspecific flexibility of social systems. We investigated the history and current structure of a large number of populations in the communally breeding Bechstein's bat ( Myotis bechsteinii ). Our aim was to understand which factors shape the species' social system over a large ecological and geographical range. Using sequence data from one coding and one noncoding mitochondrial DNA region, we identified the Balkan Peninsula as the main and probably only glacial refugium of the species in Europe. Sequence data also suggest the presence of a cryptic taxon in the Caucasus and Anatolia. In a second step, we used seven autosomal and two mitochondrial microsatellite loci to compare population structures inside and outside of the Balkan glacial refugium. Central European and Balkan populations both were more strongly differentiated for mitochondrial DNA than for nuclear DNA, had higher genetic diversities and lower levels of relatedness at swarming (mating) sites than in maternity (breeding) colonies, and showed more differentiation between colonies than between swarming sites. All these suggest that populations are shaped by strong female philopatry, male dispersal, and outbreeding throughout their European range. We conclude that Bechstein's bats have a stable social system that is independent from the postglacial history and location of the populations. Our findings have implications for the understanding of the benefits of sociality in female Bechstein's bats and for the conservation of this endangered species.     

Does the mode of transmission between hosts affect the host choice strategies of parasites? Implications from a field study on bat fly and wing mite infestation of Bechstein's bats

In a two-year field study, we analyzed the distribution of two hematophagous ectoparasites, the bat fly Basilia nana and the wing mite Spinturnix bechsteini , within and among 14 female colonies and among 26 solitary male Bechstein's bats Myotis bechsteinii . Our goal was to investigate whether differences in the transmission mode of the parasites, which result from differences in their life cycle, affect their distribution between host colonies and among host individuals within colonies. Bat flies deposit puparia in bat roosts, allowing for the transmission of hatched flies via successively shared roosts, independent of body contact between hosts or of hosts occupying a roost at the same time. In contrast, wing mites stay on the bat's body and are transmitted exclusively by contact of bats that roost together. As expected in cases of higher inter-colony transmissibility, bat flies were more prevalent among the demographically isolated Bechstein's bat colonies and among solitary male bats, as compared to wing mites. Moreover, the prevalence and density of wing mites, but not of bat flies, was positively correlated with colony size, as expected in cases of low inter-colony transmissibility. Within colonies, bat flies showed higher abundance on host individuals in good body condition, which are likely to have high nutritional status and strong immunity. Wing mites showed higher abundance on hosts in medium body condition and on reproductive females and juveniles, which are likely to have relatively weak immunity. We suggest that the observed infestation patterns within host colonies reflect different host choice strategies of bat flies and wing mites, which may result from differences in their inter-colony transmissibility. Our data also indicate that infestation with wing mites, but not with bat flies, might be a cost of sociality in Bechstein's bats.     

Spatio-temporal population genetic structure of the parasitic mite Spinturnix bechsteini is shaped by its own demography and the social system of its bat host

Information about the population genetic structures of parasites is important for an understanding of parasite transmission pathways and ultimately the co-evolution with their hosts. If parasites cannot disperse independently of their hosts, a parasite's population structure will depend upon the host's spatial distribution. Geographical barriers affecting host dispersal can therefore lead to structured parasite populations. However, how the host's social system affects the genetic structure of parasite populations is largely unknown. We used mitochondrial DNA (mtDNA) to describe the spatio-temporal population structure of a contact-transmitted parasitic wing mite ( Spinturnix bechsteini ) and compared it to that of its social host, the Bechstein's bat ( Myotis bechsteinii ). We observed no genetic differentiation between mites living on different bats within a colony. This suggests that mites can move freely among bats of the same colony. As expected in case of restricted inter-colony dispersal, we observed a strong genetic differentiation of mites among demographically isolated bat colonies. In contrast, we found a strong genetic turnover between years when we investigated the temporal variation of mite haplotypes within colonies. This can be explained with mite dispersal occuring between colonies and bottlenecks of mite populations within colonies. The observed absence of isolation by distance could be the result from genetic drift and/or from mites dispersing even between remote bat colonies, whose members may meet at mating sites in autumn or in hibernacula in winter. Our data show that the population structure of this parasitic wing mite is influenced by its own demography and the peculiar social system of its bat host.     

Secretions of the interaural gland contain information about individuality and colony membership in the Bechstein's bat

Mammals use chemical signals for individual and kin recognition, to establish social hierarchies, mark territories and choose mates. The nocturnal and social lifestyle of bats suggests that, besides acoustic signals, they also use scent to communicate. We investigated in the communally breeding Bechstein's bat, Myotis bechsteinii, whether secretions of the facial interaural gland contain information that can be used for individual and colony recognition. Since female Bechstein's bats live in closed societies and show cooperative behaviour, we predicted they would recognize colony members. We analysed interaural gland secretions, which we repeatedly sampled from 85 females belonging to four free-ranging colonies. Gas chromatography/mass spectrometry profiles were individually specific and differed between colonies. Comparing odour profiles between colonies we found a relation between chemical similarity and the mitochondrial haplotype of colony members. Within colonies there was no correlation between mass spectrometer profile similarity and genetic relatedness. Our results suggest that female Bechstein's bats may use interaural gland secretions for individual and colony recognition but not to infer kinship directly.Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

Mitochondrial DNA (mtDNA) reveals that female Bechstein's bats live in closed societies

We present a microgeographic analysis of mitochondrial DNA (mtDNA) in Bechstein's bats using three sources of control region sequence variability, including a novel mtDNA microsatellite, to assess individual relatedness both within and among 10 maternity colonies. Comparison of marker variability among 268 adult females revealed little genetic variability within each colony. However, most colonies were clearly distinguished by colony-specific mitochondrial haplotypes (total n = 28). Low intracolony variability and strong haplotype segregation among colonies, was reflected by an extraordinary high FST of 0.68, indicating a very low intercolony dispersal rate of approximately one female in five generations. Haplotype distribution among 18 solitary males showed that males frequently disperse between colony locations, indicating the absence of dispersal barriers. Bechstein's bat maternity colonies are thus closed groups that comprise 20-40 females probably belonging to only one or, at most, two matrilines. The genetic population structure of Bechstein's bats is in agreement with the hypothesis that females seek familiar and, at least, partially related cooperation partners for raising their young. Alternatively strong philopatry might reflect the importance of profound roost or habitat knowledge for successful reproduction in female Bechstein's bats.     

Spatial Proximity between Newborns Influences the Development of Social Relationships in Bats

Although bats are highly social mammals, the mechanisms influencing the establishment of social structures are far from being fully understood. So far, no study has addressed the effects of spatial proximity between newborns such as that occurring in nursery clusters on the development of preferential associations among individuals. We tested such effects on captive pups of Kuhl’s pipistrelle Pipistrellus kuhlii. During the first 6 wks, we kept them in separate rearing groups. Once able to fly, bats were allowed to freely interact in a common flight room, where those reared in the same groups showed higher rates of amicable interactions (proximity during roosting, allogrooming, huddling) but no effect on aggressive behaviour. Sex also influenced such frequencies, females being more likely to interact amicably. The phenomenon we describe may have significant implications for the development of bat social structures, including colony aggregation, within‐colony cryptic subunits or preferred association of individuals in fission–fusion dynamics. Our study adds a further dimension to bat sociality, highlighting the implications of spatial proximity in an early age phase.     

The structure of a bottlenose dolphin society is coupled to a unique foraging cooperation with artisanal fishermen

Diverse and localized foraging behaviours have been reported in isolated populations of many animal species around the world. In Laguna, southern Brazil, a subset of resident bottlenose dolphins (Tursiops truncatus) uses a foraging tactic involving cooperative interactions with local, beach-casting fishermen. We used individual photo-identification data to assess whether cooperative and non-cooperative dolphins were socially segregated. The social structure of the population was found to be a fission-fusion system with few non-random associations, typical for this species. However, association values were greater among cooperative dolphins than among non-cooperative dolphins or between dolphins from different foraging classes. Furthermore, the dolphin social network was divided into three modules, clustering individuals that shared or lacked the cooperative foraging tactic. Space-use patterns were not sufficient to explain this partitioning, indicating a behavioural factor. The segregation of dolphins using different foraging tactics could result from foraging behaviour driving social structure, while the closer association between dolphins engaged in the cooperation could facilitate the transmission and learning of this behavioural trait from conspecifics. This unique case of a dolphin-human interaction represents a valuable opportunity to explore hypotheses on the role of social learning in wild cetaceans.     

Roost switching, roost sharing and social cohesion: forest-dwelling big brown bats, Eptesicus fuscus, conform to the fission-fusion model

We used radiotelemetry to quantify roost switching and assess associations between members of maternity colonies of forest-dwelling big brown bats. Bats remained loyal to small roosting areas of forest within and between years and switched trees often (). For radiotagged bats from the colony in one of these areas, roost-switching frequency was positively correlated with the number of different individuals with which tagged bats shared roosts. We quantified associations between pairs of bats using a pairwise sharing index and found that bats associated more often than predicted when roost and roostmate selection were random but that all tagged bats spent at least some days roosting in different trees, apart from preferred roostmates. Our results suggest that forest-dwelling big brown bats conform to a fission-fusion roosting pattern. Roost switching in forests may reflect the maintenance of long-term social relationships between individuals from a colony that is spread among a number of different trees on a given night. In this fission-fusion scenario, switching between trees, within a local area, could serve to increase the numbers of individuals with which bats maintain associations. We contend that roosting areas in forests are analogous to spatially large roosts in caves, mines and buildings.     

Social interactions among wild female Bechstein's bats (<Emphasis Type="Italic">Myotis bechsteinii</Emphasis>) living in a maternity colony

Although sociality is common in bats, few studies have investigated individual social behaviour in free-ranging colonies. This study quantifies social interactions among wild female Bechstein's bats (Myotis bechsteinii) belonging to one maternity colony. Our main goal was to analyse allogrooming and nose rubbing, which are both regularly displayed by adult females. Based on data of individually marked bats with known degrees of pairwise relatedness, we suggest that allogrooming has both a social and a hygienic function. Females groomed colony mates mainly on parts of the body that are difficult to reach by a bat itself. Thus, allogrooming may function to remove ectoparasites from inaccessible body parts. Allogrooming was rare compared to self-grooming (on average 0.7% vs 37.7% of a female's total observation time), and there was no significant correlation between the rate at which a bat groomed itself and the frequency with which it was groomed by conspecifics. Therefore, we assume that allogrooming also has a social purpose in addition to its assumed hygienic function. We suggest that allogrooming could strengthen social bonds among colony members that live together for many years. Mothers and adult daughters groomed each other preferentially. Thus, allogrooming may reflect special mother–daughter bonds. Nose rubbing occurred mainly within minutes (median: 80 s) after the arrival of a female in a night roost, and there was no correlation with relatedness. Therefore, it probably allows recognition of colony mates and may also be a greeting behaviour. Communicated by M.E. dos Santos     

Spinner dolphins in a remote Hawaiian atoll: social grouping and population structure

Spinner dolphins (Stenella longirostris) commonly use inshoreisland and atoll habitats for daytime rest and social interactionsand forage over deep waters at night. In Hawaii, they occurthroughout the archipelago. We applied photoidentification mark-recapturetechniques to study the population structure of spinner dolphinsassociated with remote Midway Atoll, far-western Hawaii. AtMidway, spinner dolphins live in stable bisexually bonded societiesof long-term associates, with strong geographic fidelity, noobvious fission-fusion, and limited contacts with other populations.Their large cohesive groups change little over time and arebehaviorally/socially discrete from other spinner dolphin groups.This social pattern differs considerably from the fluid fission-fusionmodel proposed previously for spinner dolphins associated witha large island habitat in the main Hawaiian Archipelago. Thesedifferences correspond to geographic separation and habitatvariation. While in the main islands there are several daytimeresting places available at each island habitat; in far-westernHawaii, areas of suitable habitat are limited and separatedby large stretches of open pelagic waters with potentially highrisk of shark predation. We hypothesize that with deepwaterfood resources in close proximity and other atolls relativelyfar away for easy (day-to-day) access, it is energetically morebeneficial in the remote Hawaiian atolls to remain "at home"than to travel to other atolls, so there is stability insteadof variability; there is no fission-fusion effect. Thus, thegeographic isolation and small size of remote atolls triggera process in which the fluidity of the fission-fusion spinnerdolphin society is replaced with long-term group fidelity andsocial stability.     

A novel mammalian social structure in Indo-Pacific bottlenose dolphins (Tursiops sp.): complex male alliances in an open social network

Terrestrial mammals with differentiated social relationships live in 'semi-closed groups' that occasionally accept new members emigrating from other groups. Bottlenose dolphins (Tursiops sp.) in Shark Bay, Western Australia, exhibit a fission-fusion grouping pattern with strongly differentiated relationships, including nested male alliances. Previous studies failed to detect a group membership 'boundary', suggesting that the dolphins live in an open social network. However, two alternative hypotheses have not been excluded. The community defence model posits that the dolphins live in a large semi-closed 'chimpanzee-like' community defended by males and predicts that a dominant alliance(s) will range over the entire community range. The mating season defence model predicts that alliances will defend mating-season territories or sets of females. Here, both models are tested and rejected: no alliances ranged over the entire community range and alliances showed extensive overlap in mating season ranges and consorted females. The Shark Bay dolphins, therefore, present a combination of traits that is unique among mammals: complex male alliances in an open social network. The open social network of dolphins is linked to their relatively low costs of locomotion. This reveals a surprising and previously unrecognized convergence between adaptations reducing travel costs and complex intergroup-alliance relationships in dolphins, elephants and humans.     

Information transfer about roosts in female Bechstein's bats: an experimental field study

Information transfer among group members is believed to play an important part in the evolution of coloniality in both birds and bats. Although information transfer has received much scientific interest, field studies using experiments to test the underlying hypotheses are rare. We used a field experiment to test if communally breeding female Bechstein's bats (Myotis bechsteinii) exchange information regarding novel roosts. We supplied a wild colony, comprising 17 adult females of known relatedness, with pairs of suitable and unsuitable roosts and monitored the arrival of individuals marked with transponders (PIT-tags) over 2 years. As expected with information transfer, significantly more naive females were recruited towards suitable than towards unsuitable roosts. We conclude that information transfer about roosts has two functions: (i) it generates communal knowledge of a large set of roosts; and (ii) it aids avoidance of colony fission during roost switching. Both functions seem important in Bechstein's bats, in which colonies depend on many day roosts and where colony members live together for many years.     

Day roost selection in female Bechstein's bats (Myotis bechsteinii): a field experiment to determine the influence of roost temperature

The decision where to live has far-reaching fitness consequences for animals. In contrast to most other mammals or birds that use sheltered nest sites, female Bechstein's bats frequently switch day roosts during one breeding season, and therefore must often decide where to spend the day. Selecting the right roost is important, because roost quality, e.g. microclimatic condition, influences survival and reproduction in bats. Although thermal factors are very important for the quality of roosts occupied by bats, whether bats base their day roost selection directly on roost temperature has not been tested in the field. Over one summer, we examined and tested the roost choice of 21 individually marked female Myotis bechsteinii living in one maternity colony. In a field experiment, we allowed the bats to choose between relatively warm versus cold bat boxes, while controlling for site preferences. We expected females to exhibit a preference for warm roosts during pregnancy and lactation to accelerate gestation and shorten the period of growth of their young. Roost occupancy over 160 census days reflected significant temperature differences among 89 surveyed roosts (14 tree holes and 75 bat boxes), and preferences changed with the season. Females significantly preferred cold roosts before parturition, whereas post-partum, they significantly favoured warm roosts. Temperature preferences were independent of the roost site, and thus roost selection was based directly on temperature. Boxes with significantly different daytime temperatures did not differ significantly at night. Consequently, bats would have to spend at least 1 day in a new roost to test it. Information transfer among colony members might facilitate knowledge of roost availability. Access to many roosts providing different microclimates is likely to be important for successful reproduction in the endangered Bechstein's bat.     

Social Grooming in Bats: Are Vampire Bats Exceptional?

Evidence for long-term cooperative relationships comes from several social birds and mammals. Vampire bats demonstrate cooperative social bonds, and like primates, they maintain these bonds through social grooming. It is unclear, however, to what extent vampires are special among bats in this regard. We compared social grooming rates of common vampire bats Desmodus rotundus and four other group-living bats, Artibeus jamaicensis, Carollia perspicillata, Eidolon helvum and Rousettus aegyptiacus, under the same captive conditions of fixed association and no ectoparasites. We conducted 13 focal sampling sessions for each combination of sex and species, for a total of 1560 presence/absence observations per species. We observed evidence for social grooming in all species, but social grooming rates were on average 14 times higher in vampire bats than in other species. Self-grooming rates did not differ. Vampire bats spent 3.7% of their awake time social grooming (95% CI = 1.5–6.3%), whereas bats of the other species spent 0.1–0.5% of their awake time social grooming. Together with past data, this result supports the hypothesis that the elevated social grooming rate in the vampire bat is an adaptive trait, linked to their social bonding and unique regurgitated food sharing behavior.     

Temporal dynamics of European bat Lyssavirus type 1 and survival of Myotis myotis bats in natural colonies

Many emerging RNA viruses of public health concern have recently been detected in bats. However, the dynamics of these viruses in natural bat colonies is presently unknown. Consequently, prediction of the spread of these viruses and the establishment of appropriate control measures are hindered by a lack of information. To this aim, we collected epidemiological, virological and ecological data during a twelve-year longitudinal study in two colonies of insectivorous bats (Myotis myotis) located in Spain and infected by the most common bat lyssavirus found in Europe, the European bat lyssavirus subtype 1 (EBLV-1). This active survey demonstrates that cyclic lyssavirus infections occurred with periodic oscillations in the number of susceptible, immune and infected bats. Persistence of immunity for more than one year was detected in some individuals. These data were further used to feed models to analyze the temporal dynamics of EBLV-1 and the survival rate of bats. According to these models, the infection is characterized by a predicted low basic reproductive rate (R(0) = 1.706) and a short infectious period (D = 5.1 days). In contrast to observations in most non-flying animals infected with rabies, the survival model shows no variation in mortality after EBLV-1 infection of M. myotis. These findings have considerable public health implications in terms of management of colonies where lyssavirus-positive bats have been recorded and confirm the potential risk of rabies transmission to humans. A greater understanding of the dynamics of lyssavirus in bat colonies also provides a model to study how bats contribute to the maintenance and transmission of other viruses of public health concern.     

Studying the microbiota of bats: Accuracy of direct and indirect samplings

Given the recurrent bat‐associated disease outbreaks in humans and recent advances in metagenomics sequencing, the microbiota of bats is increasingly being studied. However, obtaining biological samples directly from wild individuals may represent a challenge, and thus, indirect passive sampling (without capturing bats) is sometimes used as an alternative. Currently, it is not known whether the bacterial community assessed using this approach provides an accurate representation of the bat microbiota. This study was designed to compare the use of direct sampling (based on bat capture and handling) and indirect sampling (collection of bat's excretions under bat colonies) in assessing bacterial communities in bats. Using high‐throughput 16S rRNA sequencing of urine and feces samples from Rousettus aegyptiacus, a cave‐dwelling fruit bat species, we found evidence of niche specialization among different excreta samples, independent of the sampling approach. However, sampling approach influenced both the alpha‐ and beta‐diversity of urinary and fecal microbiotas. In particular, increased alpha‐diversity and more overlapping composition between urine and feces samples was seen when direct sampling was used, suggesting that cross‐contamination may occur when collecting samples directly from bats in hand. In contrast, results from indirect sampling in the cave may be biased by environmental contamination. Our methodological comparison suggested some influence of the sampling approach on the bat‐associated microbiota, but both approaches were able to capture differences among excreta samples. Assessment of these techniques opens an avenue to use more indirect sampling, in order to explore microbial community dynamics in bats.     

Mating patterns, relatedness and the basis of natal philopatry in the brown long-eared bat, Plecotus auritus

The brown long-eared bat, Plecotus auritus, is unusual among temperate zone bats in that summer maternity colonies are composed of adult males and females, with both sexes displaying natal philopatry and long-term association with a colony. Here, we describe the use of microsatellite analysis to investigate colony relatedness and mating patterns, with the aim of identifying the evolutionary determinants of social organization in P. auritus. Mean colony relatedness was found to be low (R=0.033 +/- 0.002), with pairwise estimates of R within colonies ranging from -0.4 to 0.9. The proportion of young fathered by males in their own colony was investigated using a Bayesian approach, incorporating parameters detailing the number of untyped individuals. This analysis revealed that most offspring were fathered by males originating from a different colony to their own. In addition, we determined that the number of paternal half-sibs among cohorts of young was low, inferring little or no skew in male reproductive success. The results of this study suggest that kin selection cannot account for colony stability and natal philopatry in P. auritus, which may instead be explained by advantages accrued through the use of familiar and successful roost sites, and through long-term associations with conspecifics. Moreover, because the underlying causes of male natal dispersal in mammals, such as risk of inbreeding or competition for mates, appear to be avoided via extra-colony copulation and low male reproductive skew, both P. auritus males and females are able to benefit from long-term association with the natal colony.