The evolution of migration in a seasonal environment

Despite the fact that migration occurs in a wide variety of taxa worldwide, little is known about the conditions under which migration is expected to evolve from an ancestral resident population. We develop a model that focuses on ecological factors affecting the evolution of migration in a seasonal environment within a genetically explicit framework. We model the evolution of migration for two common types of migration: ‘shared breeding’ where migrants share a breeding ground with residents and migrate to a separate non-breeding area, versus ‘shared non-breeding’, where migrants share a non-breeding ground with residents and migrate to a separate breeding area. Ecologically, migration is more easily established in the shared-breeding case versus the shared-non-breeding case. Genetically, the additive effect of a migratory allele affects its establishment more in the shared-non-breeding case versus the shared-breeding case, whereas the dominance effect of the allele affects its establishment more in the shared-breeding case versus the shared-non-breeding case. Generally, migratory alleles can invade even when residents are competitively superior to migrants during the shared season. Partial migration occurs when the population is polymorphic for migratory and non-migratory alleles, and is dependent upon which season is shared and the additive and dominance behaviour of the migratory allele.     

Ancestry and evolution of seasonal migration in the Parulidae

Seasonal migration in birds is known to be highly labile and subject to rapid change in response to selection, such that researchers have hypothesized that phylogenetic relationships should neither predict nor constrain the migratory behaviour of a species. Many theories on the evolution of bird migration assume a framework that extant migratory species have evolved repeatedly and relatively recently from sedentary tropical or subtropical ancestors. We performed ancestral state reconstructions of migratory behaviour using a comprehensive, well-supported phylogeny of the Parulidae (the 'wood-warblers'), a large family of Neotropical and Nearctic migratory and sedentary songbirds, and examined the rates of gain and loss of migration throughout the Parulidae. Counter to traditional hypotheses, our results suggest that the ancestral wood-warbler was migratory and that losses of migration have been at least as prevalent as gains throughout the history of Parulidae. Therefore, extant sedentary tropical radiations in the Parulidae represent losses of latitudinal migration and colonization of the tropics from temperate regions. We also tested for phylogenetic signal in migratory behaviour, and our results indicate that although migratory behaviour is variable within some wood-warbler species and clades, phylogeny significantly predicts the migratory distance of species in the Parulidae.     

The impact of seasonal and year-round transmission regimes on the evolution of influenza A virus

Punctuated antigenic change is believed to be a key element in the evolution of influenza A; clusters of antigenically similar strains predominate worldwide for several years until an antigenically distant mutant emerges and instigates a selective sweep. It is thought that a region of East-Southeast Asia with year-round transmission acts as a source of antigenic diversity for influenza A and seasonal epidemics in temperate regions make little contribution to antigenic evolution. We use a mathematical model to examine how different transmission regimes affect the evolutionary dynamics of influenza over the lifespan of an antigenic cluster. Our model indicates that, in non-seasonal regions, mutants that cause significant outbreaks appear before the peak of the wild-type epidemic. A relatively large proportion of these mutants spread globally. In seasonal regions, mutants that cause significant local outbreaks appear each year before the seasonal peak of the wild-type epidemic, but only a small proportion spread globally. The potential for global spread is strongly influenced by the intensity of non-seasonal circulation and coupling between non-seasonal and seasonal regions. Results are similar if mutations are neutral, or confer a weak to moderate antigenic advantage. However, there is a threshold antigenic advantage, depending on the non-seasonal transmission intensity, beyond which mutants can escape herd immunity in the non-seasonal region and there is a global explosion in diversity. We conclude that non-seasonal transmission regions are fundamental to the generation and maintenance of influenza diversity owing to their epidemiology. More extensive sampling of viral diversity in such regions could facilitate earlier identification of antigenically novel strains and extend the critical window for vaccine development.     

The effects of the mating system on the evolution of migration in a spatially heterogeneous population

Summary Verbal explanations for the evolution of migration and dispersal often invoke inbreeding depression as an important force. Experimental work on plant populations indicates that while inbreeding depression may favor increased migration rates, adaptation to local environments may reduce the advantage to migrants. We formalize and test this hypothesis using a two-locus genetic model that incorporates lowered fitness in offspring produced by self-fertilization, and habitat differentiation. We also use the model to address questions about the general theory of genetic modifiers and the modifier reduction principle. We find that even under conditions when migration would increase the mean fitness of a population, migration may not be favored. This result is due to the associations that develop between genotypes at a locus subject to overdominant selection and at a neutral locus controlling the migration rate. Thus, it appears that, in this model, the forces of local adaptation, which favor a reduction in the migration rate, overwhelm those of inbreeding depression, which may favor dispersal.     

Range-wide migration corridors and non-breeding areas of a northward expanding Afro-Palaearctic migrant,the European Bee-eater Merops apiaster

Across their ranges, different populations of migratory species often use separate routes to migrate between breeding and non-breeding grounds. Recent changes in climate and land-use have led to breeding range expansions in many species but it is unclear whether these populations also establish new migratory routes, non-breeding sites and migration phenology. Thus, we compared the migration patterns of European Bee-eaters Merops apiaster from two established western (n = 5) and eastern (n = 6) breeding populations in Europe, with those from a newly founded northern population (n = 19). We aimed to relate the breeding populations to the two known non-breeding clusters in Africa, and to test for similarities of migration routes and timing between the old and new populations. Western Bee-eaters used the western flyway to destinations in West Africa; the eastern birds uniformly headed south to southern African non-breeding sites, confirming a complete separation in time and space between these long-established populations. The recently founded northern population, however, also used a western corridor, but crossed the Mediterranean further east than the western population and overwintered mainly in a new non-breeding area in southern Congo/northern Angola. The migration routes and the new non-breeding range overlapped only slightly with the western, but not with the eastern, population. In contrast, migration phenology appeared to differ between the western and both the northern and the eastern populations, with tracked birds from the western population migrating 2–4 weeks earlier. The northern population thus shares some spatial traits with western Bee-eaters, but similar phenology only with eastern population. This divergence highlights the adjustments in the timing of migration to local environmental conditions in newly founded populations, and a parallel establishment of new breeding and non-breeding sites.     

Incorporating the geometry of dispersal and migration to understand spatial patterns of species distributions

Dispersal and migration can be important drivers of species distributions. Because the paths followed by individuals of many species are curvilinear, spatial statistical models based on rectilinear coordinates systems would fail to predict population connectivity or the ecological consequences of migration or species invasions. I propose that we view migration/dispersal as if organisms were moving along curvilinear geometrical objects called smooth manifolds. In that view, the curvilinear pathways become the ‘shortest realised paths’ arising from the necessity to minimise mortality risks and energy costs. One can then define curvilinear coordinate systems on such manifolds. I describe a procedure to incorporate manifolds and define appropriate coordinate systems, with focus on trajectories (1D manifolds), as parts of mechanistic ecological models. I show how a statistical method, known as ‘manifold learning’, enables one to define the manifold and the appropriate coordinate systems needed to calculate population connectivity or study the effects of migrations (e.g. in aquatic invertebrates, fish, insects and birds). This approach may help in the design of networks of protected areas, in studying the consequences of invasion, range expansions, or transfer of parasites/diseases. Overall, a geometrical view to animal movement gives a novel perspective to the understanding of the ecological role of dispersal and migration.     

The influence of seasonal migration on range size in temperate North American passerines

Seasonal migration has been alternately proposed to promote geographic range size in some contexts and to constrain it in others, but it remains unclear if migratory behavior has a general effect on range size. Because migration involves movement, most hypotheses about the relationship between migration and range size invoke an influence of migration on the process of dispersal-mediated range expansion. Intuitively, a positive relationship between migratory behavior and dispersal ability could bolster range expansion among migratory species, yet some biogeographic patterns suggest that long-distance migration may instead impede range expansion, especially in the temperate zone. We conducted a comparative analysis of the relationship between migratory behavior and range size by testing the effect of migratory status, migration distance and morphological dispersal ability on breeding range size among all temperate North American passerines. Further, we assessed whether these traits affect range expansion into suitable habitat by analyzing their relationship with range filling (the proportion of climatically-suitable area occupied, or ‘filled’ by a species). Contrary to previous studies, we found migration and dispersal ability to be poor predictors of range size and range filling in North America. Rather, most variation in range size is explained by latitude. Our results suggest that migratory behavior does not affect range size within the scale of a continent, and furthermore, that temperate North American passerines’ breeding ranges are not influenced by their dispersal abilities. To better understand why migratory behavior appears to promote range size in some contexts and constrain it in others, future studies should investigate how migratory behavior affects dispersal at the individual level, as well as the relationship between the evolution of migratory behavior and the breadth of species’ climatic niches.     

A polar system of intercontinental bird migration

Studies of bird migration in the Beringia region of Alaska and eastern Siberia are of special interest for revealing the importance of bird migration between Eurasia and North America, for evaluating orientation principles used by the birds at polar latitudes and for understanding the evolutionary implications of intercontinental migratory connectivity among birds as well as their parasites. We used tracking radar placed onboard the ice-breaker Oden to register bird migratory flights from 30 July to 19 August 2005 and we encountered extensive bird migration in the whole Beringia range from latitude 64 degrees N in Bering Strait up to latitude 75 degrees N far north of Wrangel Island, with eastward flights making up 79% of all track directions.The results from Beringia were used in combination with radar studies from the Arctic Ocean north of Siberia and in the Beaufort Sea to make a reconstruction of a major Siberian-American bird migration system in a wide Arctic sector between longitudes 110 degrees E and 130 degrees W, spanning one-third of the entire circumpolar circle. This system was estimated to involve more than 2 million birds, mainly shorebirds, terns and skuas, flying across the Arctic Ocean at mean altitudes exceeding 1 km (maximum altitudes 3-5 km). Great circle orientation provided a significantly better fit with observed flight directions at 20 different sites and areas than constant geographical compass orientation. The long flights over the sea spanned 40-80 degrees of longitude, corresponding to distances and durations of 1400-2600 km and 26-48 hours, respectively. The birds continued from this eastward migration system over the Arctic Ocean into several different flyway systems at the American continents and the Pacific Ocean. Minimization of distances between tundra breeding sectors and northerly stopover sites, in combination with the Beringia glacial refugium and colonization history, seemed to be important for the evolution of this major polar bird migration system.     

Insect migration: Variability and success in a capricious environment

Stochastic effects of climate and weather have a pervasive influence on the induction, performance and evolution of migration. In wing-dimorphic species, their influence on habitat quality, and on rates of development of the migrant itself, maintains variation in responses to environmental cues determining wing-form and migratory behaviour. Migrants flying above their flight boundary layer rely on winds to disperse them across landscapes in which their habitats are distributed. Patterns of distribution of habitat patches, and the influence of changing windspeeds and direction on the displacements of migrants, result in selection for variation in migratory potential at each migration. In subsequent migrations, this variation and stochastic effects of the winds on groundtracks of individual migrants ensure that their destinations ‘sample’ the landscapes they travel over. The extent and resolution of this sampling, by which migrants reach favourable habitats, depend on the components of migratory potential, their mode of inheritance, and genetic correlations between them, as well as on the characteristics of the winds on which they travel.     

The strength of migratory connectivity for birds en route to breeding through the Gulf of Mexico

The strength of migratory connectivity is a measure of the cohesion of populations among phases of the annual cycle, including breeding, migration, and wintering. Many Nearctic‐Neotropical species have strong migratory connectivity between breeding and wintering phases of the annual cycle. It is less clear if this strength persists during migration when multiple endogenous and exogenous factors may decrease the cohesion of populations among routes or through time along the same routes. We sampled three bird species, American redstart Setophaga ruticilla, ovenbird Seiurus aurocapilla, and wood thrush Hylocichla mustelina, during spring migration through the Gulf of Mexico region to test if breeding populations differentiate spatially among migration routes or temporally along the same migration routes and the extent to which within‐population timing is a function of sex, age, and carry‐over from winter habitat, as measured by stable carbon isotope values in claws (δ¹³C). To make quantitative comparisons of migratory connectivity possible, we developed and used new methodology to estimate the strength of migratory connectivity (MC) from probabilistic origin assignments identified using stable hydrogen isotopes in feathers (δ²H). We found support for spatial differentiation among routes by American redstarts and ovenbirds and temporal differentiation along routes by American redstarts. After controlling for breeding origin, the timing of American redstart migration differed among ages and sexes and ovenbird migration timing was influenced by carry‐over from winter habitat. The strength of migratory connectivity did not differ among the three species, with each showing weak breeding‐to‐spring migration MC relative to prior assessments of breeding‐wintering connectivity. Our work begins to fill an essential gap in methodology and understanding of the extent to which populations remain together during migration, information critical for a full annual cycle perspective on the population dynamics and conservation of migratory animals.     

Population genetic analysis of a recent range expansion: mechanisms regulating the poleward range limit in the volcano barnacle Tetraclita rubescens

As range shifts coincident with climate change have become increasingly well documented, efforts to describe the causes of range boundaries have increased. Three mechanisms—genetic impoverishment, migration load, or a physical barrier to dispersal—are well described theoretically, but the data needed to distinguish among them have rarely been collected. We describe the distribution, abundance, genetic variation, and environment of Tetraclita rubescens, an intertidal barnacle that expanded its northern range limit by several hundreds of kilometres from San Francisco, CA, USA, since the 1970s. We compare geographic variation in abundance with abiotic and biotic patterns, including sea surface temperatures and the distributions of 387 co‐occurring species, and describe genetic variation in cytochrome c oxidase subunit I, mitochondrial noncoding region, and nine microsatellite loci from 27 locations between Bahia Magdalena (California Baja Sur, Mexico) and Cape Mendocino (CA, USA). We find very high gene flow, high genetic diversity, and a gradient in physical environmental variation coincident with the range limit. We infer that the primary cause of the northern range boundary in T. rubescens is migration load arising from flow of maladapted alleles into peripheral locations and that environmental change, which could have reduced selection against genotypes immigrating into the newly colonized portion of the range, is the most likely cause of the observed range expansion. Because environmental change could similarly affect all taxa in a region whose distributional limits are established by migration load, these mechanisms may be common causes of range boundaries and largely synchronous multi‐species range expansions.     

The constant philopater hypothesis: a new life history invariant for dispersal evolution

Surprising invariance relationships have emerged from the study of social interaction, whereby a cancelling‐out of multiple partial effects of genetic, ecological or demographic parameters means that they have no net impact upon the evolution of a social behaviour. Such invariants play a pivotal role in the study of social adaptation: on the one hand, they provide theoretical hypotheses that can be empirically tested; and, on the other hand, they provide benchmark frameworks against which new theoretical developments can be understood. Here we derive a novel invariant for dispersal evolution: the ‘constant philopater hypothesis’ (CPH). Specifically, we find that, irrespective of variation in maternal fecundity, all mothers are favoured to produce exactly the same number of philopatric offspring, with high‐fecundity mothers investing proportionally more, and low‐fecundity mothers investing proportionally less, into dispersing offspring. This result holds for female and male dispersal, under haploid, diploid and haplodiploid modes of inheritance, irrespective of the sex ratio, local resource availability and whether mother or offspring controls the latter's dispersal propensity. We explore the implications of this result for evolutionary conflict of interests – and the exchange and withholding of contextual information – both within and between families, and we show that the CPH is the fundamental invariant that underpins and explains a wider family of invariance relationships that emerge from the study of social evolution.     

Phylogeny and biogeography in the evolution of migration: shorebirds of the Charadrius complex

Aim We explore whether molecular phylogeny and biogeography can complement evolutionary ecology in developing a method to address a long-standing issue in the evolution of migration: have migrations between breeding and non-breeding grounds, which may be on different continents, evolved through origins in the breeding grounds with successive shifts of the non-breeding distribution or vice versa? Methods To accommodate the biology of migration, we treated breeding and non-breeding distributions as characters to be mapped onto a phylogeny derived from mitochondrial DNA sequence data and so examined the ancestral home issue as a study in the direction of character evolution. Results Our main findings from applying this approach to a subset of the Charadrius complex of shorebirds (Aves: Charadriinae) are that a case can be made for shifts of breeding distributions having occurred in the ancestries of C. alexandrinus and C. veredus as those species evolved their present migration patterns. Our results also argue for a southern hemisphere origin (specifically South America) for the Charadrius complex as a whole. A South American origin implies other shifts in breeding distributions having occurred in the evolution of the species C. semipalmatus and C. vociferus. On applying the methods we developed for dealing with phylogenetic uncertainty, these results are reinforced and the merit of testing them further is suggested. Conclusions By way of a new approach to the evolution of migration, our study adds to a consensus emerging from the evolutionary ecology of migrant birds, arguing that shifts of breeding distributions are commonly, though not necessarily exclusively, involved in the evolution of migration.     

Does the migration programme constrain dispersal and range sizes of migratory birds?

Aim  It is generally believed that the migration programme constrains the dispersal and hence range sizes of migratory bird species. This conclusion is based on analyses of breeding ranges of migratory versus non-migratory (resident) terrestrial bird species, and rests on the assumption that there are no ecological or evolutionary constraints on extending the non-breeding range. To investigate this assumption, the abilities of migrant and resident terrestrial species to colonize new wintering areas were compared.
Location  Three major wintering regions of long-distance migrants: South America, sub-Saharan Africa and the Indian Subcontinent.
Methods  It was determined whether the relative numbers of residents and short- and long-distance migrants were the same in those species that have dispersed to a novel wintering region as in the source species pools.
Results  At the species level, long-distance migratory species are more likely to have non-breeding ranges that include more than one of the above regions than resident species. This indicates that the dispersal of migratory species is less constrained than that of resident species. The pattern holds irrespective of the inclusion or exclusion of species associated with coastal, freshwater and wetland habitats, and also holds for ecological groups such as aerial feeders. The pattern is most pronounced between the regions separated by the strongest dispersal barriers (South America and sub-Saharan Africa).
Main conclusions  It is unlikely that the migration programme per se constrains dispersal, but rather that difficulties in establishing new non-breeding areas prevent range expansions in migrant species.     

Origin of the Sulawesi macaques (Cercopithecidae: Macaco) as suggested by mitochondrial DNA phylogeny

One of the sharpest biogeographical transitions in the world occurs between the Indonesian islands of Borneo and Sulawesi; this transition is demarcated by Wallace's line. Macaque monkeys represent an interesting anomaly to faunal distributions in this region as they occur on both sides of Wallace's line, with Macacafascicularis, M. nemestrina and other species to the west and seven Sulawesi species to the east. We have investigated macaque evolution and dispersal in the Sunda region and Sulawesi using phylogenetic analysis of mitochondrial DNA sequences. Female philopatry of macaques, which causes sharp geographic clustering of maternally inherited mitochondrial DNA haplotypes, makes mitochondrial phytogenies particularly useful for investigating ancient patterns of dispersal. Results of this study suggest the following: (1) M. fascicularis is not a sister taxon to any species of Sulawesi macaque; (2) haplotypes of some M. nemestrina have a sister relationship to northern and central Sulawesi macaques, while haplotypes of other M. nemestrina have a sister relationship to soudiern Sulawesi macaques; (3) Sulawesi was probably colonized by macaques twice, once to the base of the northern peninsula now occupied by M. hecki and once to the southwestern peninsula now occupied by M, mama; and (4) within north/central and southern Sulawesi, patterns of dispersal are largely consistent with contemporary and past geography of the island, with the exception of a geographically discontinuous relationship between M. nigra and a portion of M. tonkeana from a region in northwest central Sulawesi.     

Estimability of migration survival rates from integrated breeding and winter capture–recapture data

Long‐distance migration is a common phenomenon across the animal kingdom but the scale of annual migratory movements has made it difficult for researchers to estimate survival rates during these periods of the annual cycle. Estimating migration survival is particularly challenging for small‐bodied species that cannot carry satellite tags, a group that includes the vast majority of migratory species. When capture–recapture data are available for linked breeding and non‐breeding populations, estimation of overall migration survival is possible but current methods do not allow separate estimation of spring and autumn survival rates. Recent development of a Bayesian integrated survival model has provided a method to separately estimate the latent spring and autumn survival rates using capture–recapture data, though the accuracy and precision of these estimates has not been formally tested. Here, I used simulated data to explore the estimability of migration survival rates using this model. Under a variety of biologically realistic scenarios, I demonstrate that spring and autumn migration survival can be estimated from the integrated survival model, though estimates are biased toward the overall migration survival probability. The direction and magnitude of this bias are influenced by the relative difference in spring and autumn survival rates as well as the degree of annual variation in these rates. The inclusion of covariates can improve the model's performance, especially when annual variation in migration survival rates is low. Migration survival rates can be estimated from relatively short time series (4–5 years), but bias and precision of estimates are improved when longer time series (10–12 years) are available. The ability to estimate seasonal survival rates of small, migratory organisms opens the door to advancing our understanding of the ecology and conservation of these species. Application of this method will enable researchers to better understand when mortality occurs across the annual cycle and how the migratory periods contribute to population dynamics. Integrating summer and winter capture data requires knowledge of the migratory connectivity of sampled populations and therefore efforts to simultaneously collect both survival and tracking data should be a high priority, especially for species of conservation concern.     

Dispersal evolution in temporally variable environments: implications for plant range dynamics

Dispersal—the movement of an individual from the site of birth to a different site for reproduction—is an ecological and evolutionary driver of species ranges that shapes patterns of colonization, connectivity, gene flow, and adaptation. In plants, the traits that influence dispersal often vary within and among species, are heritable, and evolve in response to the fitness consequences of moving through heterogeneous landscapes. Spatial and temporal variation in the quality and quantity of habitat are important sources of selection on dispersal strategies across species ranges. While recent reviews have evaluated the interactions between spatial variation in habitat and dispersal dynamics, the extent to which geographic variation in temporal variability can also shape range-wide patterns in dispersal traits has not been synthesized. In this paper, we summarize key predictions from metapopulation models that evaluate how dispersal evolves in response to spatial and temporal habitat variability. Next, we compile empirical data that quantify temporal variability in plant demography and patterns of dispersal trait variation across species ranges to evaluate the hypothesis that higher temporal variability favors increased dispersal at plant range limits. We found some suggestive evidence supporting this hypothesis while more generally identifying a major gap in empirical work evaluating plant metapopulation dynamics across species ranges and geographic variation in dispersal traits. To address this gap, we propose several future research directions that would advance our understanding of the interplay between spatiotemporal variability and dispersal trait variation in shaping the dynamics of current and future species ranges.     

Beyond habitat requirements: individual fine-scale site fidelity in a colony of the Galapagos sea lion (<Emphasis Type="Italic">Zalophus wollebaeki</Emphasis>) creates conditions for social structuring

Site fidelity has been widely discussed, but rarely been related explicitly to a species’ social context. This is surprising, as fine-scale site fidelity constitutes an important structural component in animal societies by setting limits to an individual’s social interaction space. The study of fine-scale site fidelity is complicated by the fact that it is inextricably linked to patterns of habitat use. We here document fine-scale site fidelity in the Galapagos sea lion (Zalophus wollebaeki) striving to disentangle these two aspects of spatial behaviour. Regardless of sex and age, all individuals used small, cohesive home ranges, which were stable in size across the reproductive and non-reproductive season. Home ranges showed a large individual component and did not primarily reflect age- or sex-specific habitat requirements. Site specificity could be illustrated up to a resolution of several metres. Long-term site fidelity was indicated by home range persistence over 3 years and the degree of site fidelity was unaffected by habitat, but showed seasonal differences: it was lower between reproductive and non-reproductive periods than between reproductive seasons. We further examined static and social interaction within mother–offspring pairs, which constitute a central social unit in most mammalian societies. Regardless of the occupied habitat type, adult females with offspring had smaller home range sizes than non-breeding females, demonstrating the importance of spatial predictability for mother–offspring pairs that recurrently have to reunite after females’ foraging sojourns. While social interaction with the mother dropped to naught in both sexes after weaning, analysis of static interaction suggested female-biased home range inheritance. Dispersal decisions were apparently not based on habitat quality, but determined by the offspring’s sex. We discuss the implication of observed fine-scale site fidelity patterns on habitat use, dispersal decisions and social structure in colonial breeding pinnipeds.     

Effects of seasonal migration on the latitudinal distribution of west Palaearctic bird species

This paper examines seasonal changes in the latitudinal distribution of birds (excluding seabirds) that breed in the western Palaearctic. Some resident species occupy ranges that span less than 5° of latitude year-round, while some migrant species range over more than 120° of latitude during the course of a year. Among migrant species of land and freshwater habitats, the latitudinal spans of breeding and wintering ranges are correlated. In general, species that breed over a narrow span of latitude also winter over a narrow span of latitude, and vice versa. Among both groups, for any given span of breeding range, species that winter partly in Eurasia and partly in Africa winter over a wider latitudinal span than those that winter entirely within Eurasia or entirely within Africa. Among coastal birds, there is no correlation between the latitudinal spans of breeding and winter areas; most shorebird species breed over a narrow span of (northern) latitude and winter over a wide span distributed linearly along coastlines. Several migration patterns can be distinguished, from complete overlap of breeding and wintering ranges in year-round residents, through partial separation of summer and winter ranges, to complete separation of summer and winter ranges in different geographical regions. In some such species, the gap between breeding and wintering ranges spans up to 55° of latitude (6000 km).     

Genetic structure of African buffalo herds based on variation at the mitochondrial D-loop and autosomal microsatellite loci: Evidence for male-biased gene flow

Sexual differences in herding behaviour ofAfrican buffalo (Syncerus caffer) werestudied by analysing at the herd levelmitochondrial D-loop hypervariable region I andfourteen autosomal microsatellites. Three herdsfrom Arusha National Park in Tanzania wereanalysed with mtDNA and five herds from KrugerNational Park in South Africa with mtDNA andmicrosatellites. Significant mtDNAdifferentiation was observed among herds inArusha NP (F_ST = 0.12, based on haplotypefrequencies). Assignment tests withmicrosatellite data from Kruger NP showed thatmost frequent migration between herds is bymales two years. This was confirmed bytests for herd differentiation and analyseswith Lynch and Ritland's relatedness estimator.Within a herd, males younger than two years andfemales showed a higher relationship through acommon father rather than a common mother,indicating that female herd members mate withonly a few dominant males. This in turnsuggests a female:male sex ratio larger than5:1. The migration rate per generation betweenherds was estimated to be 5–2% for femalesand close to 100% for males. Finally, theimplications for the management of buffalopopulations are being discussed.