Dispersal, pair formation and social structure in gibbons (Hylobates lar)

We report observations on reproduction, natal dispersal, pair formation, and group structure based on longitudinal observations of several white-handed gibbon (Hylobates lar) groups spanning 18 years. Our observations are at odds with the traditional view that gibbons live in nuclear family groups consisting of a pair of adults and their offspring, and that parents exclude young from the family territory when they reach adult size. In the relatively dense Khao Yai study population, dispersing young usually obtain mates by replacing adults in existing territories, which creates non-nuclear families. Six subadults, five males and one female, matured and dispersed at an average age of 10 years, or about 2 years after reaching adult size. Average natal dispersal distance was 710 m, or between one and two territories away. At least two dispersing males replaced adults in neighboring groups. In one case, forcible displacement of the resident male resulted in a group which included a young juvenile presumably fathered by the previous male, two younger juveniles (probably brothers) from the new male's original group, and (later) offspring of the new pair. Social relations within this heterogeneous group remained harmonious: the adults groomed all the young and play occurred between all preadult members. In only two out of a total of seven cases of dispersal seen did two subadults pair and disperse into new territorial space. Nonreproducing subadults which delay dispersal may be tolerated by the adults provided that they contribute benefits to the adults or their offspring. Possible benefits include behaviors such as grooming, social play with juveniles, and support of the adult male in defending the territory. Delayed dispersal is probably advantageous in a saturated environment where there is no room for floaters, but subadults may also gain indirect fitness benefits by aiding siblings and other relatives. Received: 24 January 1997 / Accepted after revision: 12 January 1998     

Extended family structure in the ant Formica paralugubris: the role of the breeding system

In populations of various ant species, many queens reproduce in the same nest (polygyny), and colony boundaries appear to be absent with individuals able to move freely between nests (unicoloniality). Such societies depart strongly from a simple family structure and pose a potential challenge to kin selection theory, because high queen number coupled with unrestricted gene flow among nests should result in levels of relatedness among nestmates close to zero. This study investigated the breeding system and genetic structure of a highly polygynous and largely unicolonial population of the wood ant Formica paralugubris. A microsatellite analysis revealed that nestmate workers, reproductive queens and reproductive males (the queens' mates) are all equally related to each other, with relatedness estimates centring around 0.14. This suggests that most of the queens and males reproducing in the study population had mated within or close to their natal nest, and that the queens did not disperse far after mating. We developed a theoretical model to investigate how the breeding system affects the relatedness structure of polygynous colonies. By combining the model and our empirical data, it was estimated that about 99.8% of the reproducing queens and males originated from within the nest, or from a nearby nest. This high rate of local mating and the rarity of long-distance dispersal maintain significant relatedness among nestmates, and contrast with the common view that unicoloniality is coupled with unrestricted gene flow among nests. Received: 8 February 1999 / Received in revised form: 15 June 1999 / Accepted: 19 June 1999     

Male care and mating effort among Hadza foragers

Paternal care figures prominently in many scenarios of human evolution. Recently, however, such scenarios have been challenged on two scores. First, the level of male contribution may be insignificant. Second, male care may be provided as a form of mating effort, rather than parenting effort. In theory, since men can enhance their Darwinian fitness both by providing care to their own offspring if this raises offspring fitness and by pursuing additional mates if this leads to additional offspring, men should respond to payoffs from both mating and parenting effort. If men respond to payoffs from parenting effort, paternity ought to make a difference. And if men respond to payoffs from mating effort, mating opportunities ought to make a difference. I analyzed the impact of these two factors on variation in male care among the Hadza, a foraging society in Tanzania. Two predictions were tested: (1) biological children will receive more care than stepchildren, and (2) men will provide less care to their biological children as their mating opportunities increase. Both predictions were supported. These results suggest men provide care, in part, as parenting effort, and that they trade off parenting effort for mating effort when they have greater mating opportunities. Received: 21 January 1998 / Received in revised form: 24 January 1999 / Accepted: 1 February 1999     

Costs of delayed dispersal and alloparental care in the fungus-cultivating ambrosia beetle <Emphasis Type="Italic">Xyleborus affinis</Emphasis> Eichhoff (Scolytinae: Curculionidae)

Body reserves may determine the reproductive output of animals, depending on their resource allocation strategy. In insects, an accumulation of reserves for reproduction is often obtained before dispersal by pre-emergence (or maturation) feeding. This has been assumed to be an important cause of delayed dispersal from the natal nest in scolytine beetles. In the cooperatively breeding ambrosia beetles, this is of special interest because in this group delayed dispersal could serve two alternative purposes: “selfish” maturation feeding or “altruistic” alloparental care. To distinguish between these two possibilities, we have experimentally studied the effect of delayed dispersal on future reproductive output in the xyleborine ambrosia beetle Xyleborus affinis. Females experimentally induced to disperse and delayed dispersing females did not differ in their body condition at dispersal and in their founding success afterwards, which indicates that females disperse independently of condition, and staying adult females are fully mature and would be able to breed. However, induced dispersers produced more offspring than delayed dispersers within a test period of 40 days. This suggests that delayed dispersal comes at a cost to females, which may result primarily from alloparental care and leads to a reduced reproductive output. Alternatively, females might have reproduced prior to dispersal. This is unlikely, however, for the majority of dispersing females because of the small numbers of offspring present in the gallery when females dispersed, suggesting that mainly the foundress had reproduced. In addition, “gallery of origin” was a strong predictor of the reproductive success of females, which may reflect variation in the microbial complex transmitted vertically from the natal nest to the daughter colony, or variation of genetic quality. These results have important implications for the understanding of proximate mechanisms selecting for philopatry and alloparental care in highly social ambrosia beetles and other cooperatively breeding arthropods.     

Individual dispersal status influences space use of conspecific residents in the common lizard, Lacerta vivipara

The effects of immigration on the behaviour of residents may have important implications for the local population characteristics. A manipulative laboratory experiment with yearlings of the common lizard (Lacerta vivipara) was performed to test whether the introduction of dispersing or philopatric individuals influences the short-term spacing behaviour of resident individuals. Staged encounters were carried out to induce interactions within dyads. The home cage of each responding individual was connected by a corridor to an unfamiliar “arrival cage” to measure the latency to leave their own home cage after each encounter. Our results showed that the time that pairs spent in close proximity was longer when a dispersing individual was introduced in the home cage. The latency to leave the home cage was longer after the introduction of a dispersing individual. These response variables were not influenced by the relative body sizes of contestants nor by the level of aggression towards each other. In contrast, the aggressive response was significantly influenced by the residency asymmetry established experimentally (“owner” of the home cage vs introduced individual). Our results suggest that the space use by resident individuals is influenced by the dispersal status of conspecifics. The potential ultimate causes driving this effect are discussed.     

Nestling size rank in the little egret (Egretta garzetta) influences subsequent breeding success of offspring

Few studies have investigated the long-term fitness consequences of nestling size hierarchies in altricial birds. In this study, we investigated whether or not the size rank order of siblings influences subsequent breeding success in the little egret, Egretta garzetta. From a marking program allowing individual recognition of wild birds, we obtained data on the breeding success of 56 pairs comprising individuals for which the size rank order was known. The breeding success in these pairs was positively influenced by the age of the marked bird but negatively affected by the laying date of the pair and the size rank order of the marked individual. There was also a significant difference between breeding colonies. We suggest two main hypotheses for a link between size rank order of individuals and their breeding success and we discuss our results in relation to current hypotheses on the adaptive value of hatching asynchrony. Received: 10 August 1998 / Accepted after revision: 13 December 1998     

Group foraging, patch exploitation time and the finder's advantage

 We use a combination of the marginal value theorem (MVT) of Charnov (1976), and a group foraging model featuring information sharing to address patch residence in an environment where food occurs in discrete patches. We shall show that among equal competitors the optimal patch time for the individual that finds the food patch is shorter than that for the non-finder among equal competitors, T _E < T _N. This is the case if the patch-finder commences food harvesting in the patch earlier and manages to monopolise a fraction of the prey items (finder's advantage) before the other individuals come to take their benefit. When individuals differ in their food-searching abilities so that some of them (producers) contribute proportionally more to food-searching than others (scroungers), and differ in ability to compete for the food found, a difference emerges between producer and scrounger individuals in the optimal patch time. Within a patch we always have the finder's advantage (T _E < T _N) regardless of phenotype. Between patches a suite of optimal patch times for encountering individuals emerges depending on the performance of producers and scroungers when changing from solitary feeding to feeding in a group. The optimal patch time for individuals that are affected more severely by competition is shorter than that for individuals of the phenotype with better competitive ability. When both phenotypes are affected similarly no difference in optimal patch times emerges. Received: 13 February 1996 / Accepted after revision: 28 September 1996     

Sex-biased dispersal in a long-lived polygynous reptile (Crocodylus johnstoni )

We compared natal dispersals of freshwater crocodiles (Crocodylus johnstoni) against the prediction of male dispersal bias for a polygynous mating system. The crocodiles inhabited a linear series of pools and we calculated the net distances from natal pools to recapture locations some 12–18 years later, at maturity. Philopatry was assessed in terms of adult social distances. A female social distance was 0.46 pools and a male social distance was 1.0 pool. By these criteria, both sexes showed low levels of philopatry (7–12%). However, individuals of both sexes dispersed from the natal site long before they were sexually mature. Divergence in dispersal patterns by sex occurred after the maturity threshold, as males dispersed two to three times farther than females. Intrasexual competition by males is resolved by a size-based hierarchy. The displacement of small males from local mating access is a probable cause of the longer dispersals undertaken by males. Competition, rather than inbreeding avoidance, is driving dispersal in this population of freshwater crocodiles. Received: 8 May 1998 / Accepted after revision: 26 June 1998     

Genetic structure,lack of sex-biased dispersal and behavioral flexibility in the pair-living fat-tailed dwarf lemur, <Emphasis Type="Italic">Cheirogaleus medius</Emphasis>

Mating system and dispersal patterns influence the spatio-genetic structure within and between populations. Among mammals, monogamy is rare, and its socio-genetic consequences have not been studied in detail before. The goal of our study was to investigate population history, demographic structure, and dispersal patterns in a population of pair-living fat-tailed dwarf lemurs, Cheirogaleus medius, a small, nocturnal primate from western Madagascar, and to infer their underlying behavioral mechanisms. Tissue samples for DNA extraction were obtained from a total of 140 individuals that were captured in two subpopulations about 3 km apart. Analyses of mtDNA variability at the population level revealed very low levels of genetic variability combined with high haplotype diversity, which is indicative of a recent population bottleneck. We found no evidence for spatial clustering of same-sexed individuals with identical haplotypes within each of two subpopulations but significant clustering between them. Thus, a high level of local subpopulation differentiation was observed (F _ST = 0.230). The sexes showed equal variances in the number of individuals representing each haplotype, as well as equal levels of aggregation of identical haplotypes. Hence, both sexes disperse from their natal area, one pattern expected in a pair-living mammal. There is a possibility of behavioral and social flexibility in this species, however, because we documented pronounced differences in density and sex ratio between the two subpopulations, suggesting that single study sites or populations may not be representative of a given local population or even species.     

The influence of ration level on growth and statolith increment width of the tropical squid Sepioteuthis lessoniana (Cephalopoda: Loliginidae): an experimental approach

Juvenile squids were grown in individual 2.6-l floating enclosures and were fed either a high- or a low-ration diet of fish and the crustacean Acetes. Squids were maintained for a maximum of 44 days in two experiments. The high-ration individuals reached a significantly larger size in both experiments (27, 25.5 mm mean mantle length, ML) compared to their low-ration siblings (19 mm mean ML) in both experiments. The statolith increment widths prior to the start of the experiment were significantly wider (between 3 and 4 μm) compared to the increment widths after the start of the experiment (between 2 and 3 μm) both for the low- and the high-ration squids. High-ration squids also had significantly wider increments and larger statoliths than their low-ration siblings. Even though we detected consistent trends in daily statolith increment widths for the different feeding regimes, we could not detect variation in increment widths at a daily level of resolution (i.e. as a result of differences in day-to-day food intake at an individual level). This was probably due to the relatively consistent diet experienced by each individual. These experiments revealed that ration level influences squid growth rate, statolith size and daily statolith increment width. Received: 30 March 2000 / Accepted: 30 October 2000     

Effect of diet on growth rate and reproductive fitness of Turbo sarmaticus (Mollusca: Vetigastropoda: Turbinidae)

Experiments were conducted to determine the effect of four algal diets (Corallina spp., Gelidium pristoides, Ulva rigida and an equal volume mixture of these seaweeds) on growth and reproduction of the South African gastropod Turbo sarmaticus Linnaeus, 1758. The best growth rate of juveniles (up to 13.8 mm shell length and 34.26 g wet body weight increase in 12 months), reproductive fitness of mature specimens (gonad index up to 33%) and energy reserves (up to 4.76 mg glycogen/100 mg foot tissue) were achieved when T. sarmaticus was fed U. rigida or a mixed diet. In addition, the gonad index of individuals fed these diets was almost twice that of similar-sized field specimens. Juveniles fed Corallina spp. only, grew very little (only 2.4 mm shell length and 4.23 g wet body weight increase in 12 months). The reproductive fitness of adults fed on such a diet was also poor (gonad index <4.5%) and energy reserves were low (<3.5 mg glycogen/100 mg foot tissue). Along the southeastern coast of South Africa, T. sarmaticus has a distinct reproductive cycle with gametogenesis occurring from March/April until August/September, whilst maturity (gonad index = 15%) was maintained until December, after which spawning occurred until March. Received: 6 July 1998 / Accepted: 8 March 1999     

Fitness costs of dispersal in red foxes (Vulpes vulpes)

The costs of dispersal are an important factor promoting natal philopatry, thereby encouraging the formation of social groups. The red fox, Vulpes vulpes, exhibits a highly flexible social system and one that is thought to represent a possible stage in the evolution of more complex patterns of group-living. Although the potential benefits accruing to philopatric offspring have previously been studied in this species, the potential costs of dispersal have received less attention. We contrasted survival rates, nutritional status, injuries and reproductive output of dispersing and non-dispersing male and female foxes in an urban population to assess the relative costs of dispersal versus natal philopatry. Mortality rates were not significantly higher for dispersing foxes, either in the short- or long-term. There was no evidence of increased nutritional stress in dispersing individuals. Dispersing individuals did, however, exhibit greater levels of wounding, although this did not appear to affect survival. Dispersing females were more likely to miss a breeding opportunity early in their reproductive lifespan. In contrast, both dispersing and non-dispersing males were unlikely to breed in their first year. We conclude that the major fitness component in females affected by dispersing is age at first reproduction.     

Oceanic diffusion and the pelagic insects Halobates spp. (Gerridae: Hemiptera)

Five pelagic Halobates species occupy a vast area from 40^∘ north to 40^∘ south in the three major oceans. Oceanic diffusion, constantly acting to disperse these insects, must be an important factor in determining their life history and distribution. We investigated the effects of oceanic diffusion on the following aspects of these insects. (1) The estimated radius of a patch of Halobates could be expanded by oceanic diffusion alone from an initial point of origin to 1250 km in 60 d. This distance is about 1/12 of the maximum distributional range of H. micans in the Pacific Ocean. Mutual encounter rates due to oceanic turbulence could be as high as 11 d⁻¹ even at low population densities (100 ind km⁻²). This suggests that individuals from their original habitat could find mates even when they had been carried a long distance. Thus, extensive gene mixing may occur over the whole range of a species' distribution. (2)␣Estimated growth rates are rather low (0.0026 to 0.0079 d⁻¹) compared with those of other insects. However, they are offset by a long life span (over 90 d) and an extended oviposition period (perhaps over 2␣months). Thus, pelagic Halobates spp. appear to have adopted a strategy of slow growth and prolonged longevity to cope with living in an unstable physical environment that is constantly disturbed by storms and winds. Received: 5 February 1995 / Accepted: 30 October 1997     

Optimal timing of comb construction by honeybee (Apis mellifera) colonies: a dynamic programming model and experimental tests

Honeybee colonies, like organisms, should exhibit optimal design in their temporal pattern of resource allocation to somatic structures. A vital colony structure is the comb which stores honey for overwinter survival. However, the timing of comb construction poses a dilemma to a colony attempting to maximize its honey reserves. On the one hand, plenty of empty comb is needed for efficient exploitation of temporally unpredictable flower blooms. On the other hand, because comb is made from energetically expensive wax, its construction too early or in excessive amounts will reduce the amount of honey available for winter thermoregulation and brood-rearing. A dynamic optimization model concludes that colonies should add new comb only when they have filled their old comb with food and brood above a threshold level. The threshold increases with time until, at the end of the season, building is never an optimal behavior. The temporal pattern of construction predicted by the model – pulses of building coincident with periods of nectar intake and comb fullness – matches that seen in an actual colony observed over the course of an entire foraging season. When nectar sources are rich but temporally clumped, the model also predicts that bees should be sensitive to nectar intake, employing much higher thresholds on days when nectar is not available than on days when it is. Even under poorer and more dispersed nectar regimes, little fitness cost is paid by colonies replacing the optimal strategy with a simpler rule of thumb calling for new construction only when two conditions are met: (1) a fullness threshold has been exceeded, and (2) nectar is currently being collected. Experiments demonstrate that colonies do in fact use such a rule of thumb to control the onset of construction. However, once they have begun building, the bees continue as long as nectar collection persists, regardless of changes in comb fullness. Thus the onset and duration of comb-building bouts appear to be under partially independent control. Received: 30 October 1998 / Received in revised form: 14 December 1998 / Accepted: 16 January 1999     

Consequences of communal gall occupation and a test for kin discrimination in the aphid Tamalia coweni (Cockerell) (Homoptera: Aphididae)

This study investigated the consequences of communal gall induction on individual and group fitness in the aphid Tamaliacoweni. The possibilities that kin discrimination and foundress density are factors favoring communal gall occupation were examined. Clonally produced aphid foundresses were collected to create two treatments: clonal groups and groups of less closely related individuals. These were confined on suitable host plant tissue to compare their respective propensities towards communal behavior. There were no significant differences in the frequencies of communal gall occupation; therefore, active kin discrimination by T.coweni foundresses apparently does not play a role in their communal behavior, within the context of this experiment. In a second experiment, aphid foundress density on the host plant was manipulated in three treatments and was correlated with the frequency of communal gall occupation. Individual fitness was inversely related to the mean number of foundresses per gall. These results suggest that communal gall occupation does not necessarily represent mutual cooperation but may instead be the outcome of competition for limited gall sites on the host plant. However, natural selection at the clonal level may favor communal gall occupation under the conditions of resource limitation and high within-group relatedness. Received: 26 September 1997 / Accepted after revision: 28 March 1998     

Scent-marking behavior in wild groups of common marmosets (Callithrix jacchus)

Scent-marking in mammals has been frequently related to within-group social and reproductive dominance and to defense of territory and resources. We studied the scent-marking behavior of five wild groups of common marmosets, Callithrix jacchus, during 5 months of the fruiting season in northeastern Brazil. Circumgenital marking was the most common type of marking. Marks were distributed throughout the home range and were deposited mainly during travel and intergroup encounters. Although marks were commonly deposited at gum trees, there was no evidence that the animals used scent marks to label fruiting trees or sleeping sites. Contrary to expectations, reproductively dominant females did not mark more than reproductively subordinate females. Moreover, during intergroup encounters, reproductively subordinate females displayed higher frequencies of scent-marking than the reproductively dominant females of their group. Our results suggest that scent-marking is not strictly tied to reproductive dominance or territorial (or resource) defense in common marmosets. Because marks provide information about individual identity and reproductive condition, scent marks could serve different functions when used by different individuals. The high frequency of marking by reproductively subordinate females during intergroup encounters suggests that scent-marking might be used to signal to individuals of neighboring groups. Our data highlight the importance of social and ecological variables in scent-marking behavior. Received: 26 October 1998 / Received in revised form: 20 May 1999 / Accepted: 30 May 1999     

Genetic composition of a green turtle (Chelonia mydas) feeding ground population: evidence for multiple origins

Migratory marine turtles are extremely difficult to track between their feeding and nesting areas, and the link between juvenile and adult habitats is generally unknown. To assess the composition of a feeding ground (FG) population of juvenile green turtles (Cheloniamydas Linnaeus), mitochondrial DNA control region sequences were examined in 80 post-pelagic individuals (straight carapace length = 31 to 67 cm) sampled in September 1992 from Great Inagua, Bahamas, and compared to those of 194 individuals from nine Atlantic and Mediterranean nesting colonies. Evidence from genetic markers, haplotype frequencies, and maximum likelihood (ML) analyses are concordant in indicating that multiple colonies contribute to the Bahamian FG population. ML analyses suggested that most Bahamian FG juveniles originated in the western (79.5%) and eastern (12.9%) Caribbean regions, and these proportions are roughly comparable to the size of candidate rookeries. These data support a life-cycle model in which individuals become pooled in post-hatchling (pelagic) and juvenile (benthic) habitats as a consequence of ocean currents and movement among FGs. A substantial harvest of immature turtles on their feeding pastures will influence the reproductive success of contributing nesting populations over a wide geographic scale. Received: 1 April 1997 / Accepted: 14 October 1997     

Experimental analysis of food detection in capuchin monkeys: effects of distance, travel speed, and resource size

Knowing how far away animals can detect food has important consequences for understanding their foraging and social behaviors. As part of a broader set of field experiments on primate foraging behavior, we set out artificial feeding platforms (90 × 90 cm or 50 × 50 cm) throughout the home range of one group of 22 brown capuchin monkeys, at sites where they had not seen such platforms previously. Whenever the group approached such a new platform to within 100 m, we recorded the group's direction and speed of approach, and the identity and distance from the platform of the group member that detected the platform or came closest to it without detecting it. We used logistic regression on these data to examine the effects of group movement speed, platform size and height, and focal individual age and sex on the probability of detecting the platform as a function of distance. Likelihood of detecting a platform decreased significantly at greater distances – the probability of detecting a platform reached 0.5 at 41 m from the group's center and 25.5 m from the nearest group member. These results show that detectability of platforms by the entire group (9 adults, 13 juveniles) was less than twice that for single group members. Detectability at a given distance decreased severely as the group moved faster; at their fastest speed, individuals had to approach a platform to within less than 10 m to find it. The large platforms were significantly more likely to be detected than the small ones, suggesting that increased use of larger food patches by wild primates may not necessarily reflect foraging preferences. Received: 20 May 1996 / Accepted after revision: 5 April 1997     

Microgeographic genetic differentiation in a colonial ascidian (Botryllus schlosseri) population

Limited gene flow via the restricted dispersal of larvae and gametes is expected to result in the genetic differentiation of populations of clonal invertebrates on small spatial scales. However, occasional dispersal events over greater distances may generate sufficient gene flow to maintain genetic homogeneity. We applied a spatial autocorrelation approach that does not require a priori definitions of subdivision boundaries to examine genetic differentiation within a continuous population of the colonial ascidian Botryllus schlosseri (Pallas) at two allozyme and five polychromatism loci. Colonies were sampled in July 1992, on a 12 by 18 m grid superimposed on a shallow subtidal (1 to 3 m) population in the Damariscotta River estuary in Maine, USA. Low but significant levels of positive autocorrelation were detected over very small spatial scales (<5 m), with negative autocorrelation occurring on larger scales (>8 m). This pattern indicates significant genetic differentiation over distances of 8 to 21 m, and is consistent with genetic drift and inbreeding creating small scale genetic structure. Received: 18 October 1999 / Accepted: 11 July 2000     

Biased parental investment and reproductive success in Gabbra pastoralists

Demographic data from 848 Gabbra households are used to examine the relationships between herd size and reproductive success in relation to sex, in a traditional, pastoralist population. The number of camels in the household herd has a significant positive effect on the reproductive success of both men and women, although the effect of wealth is greater for men, as predicted from evolutionary theory. The greater the number of elder brothers a man has, the lower his reproductive success, as a result of a smaller initial herd and a later age at marriage. This is not true for women –number of elder sisters does not have a measurable effect on a woman’s fertility, although it does have a small, negative effect on the size of her dowry. These results are interpreted as competition between same-sex siblings for parental investment, in the form of their father’s herd, which is more intense between sons than daughters as parental investments are greatest in males. Received: 30 June 1995/Accepted after revision: 23 October 1995