School fidelity and homing synchronicity of yellowfin tuna, Thunnus albacares

Thirty-eight yellowfin tuna (Thunnus albacares) were tagged with coded ultrasonic beacons between 6 March and 4 December 1996 near two buoys off the western coast of Oahu, Hawaii. Two to four tuna were captured, tagged, and released on the same day in as rapid succession as possible in an effort to tag members of the same school. Automated “listening” monitors attached to the buoys recorded when these marked individuals entered within a radius of ≤1.1 km of the buoys during a 13 mo period. Twenty-seven of the tuna returned to the site of tagging. The mean number of returns was 4.2 per tuna (max. = 17), and visits ranged from 1 to 910 min (median = 2.7 min, mean = 40.1 min). The intervals between successive returns varied from 1 to 257 d (median = 3.0 d, mean = 17.4 d). Seventy-three percent of the tuna returned together with tunas tagged on the same day, exceeding the frequency of returns of tuna tagged on another day or arriving alone. This social cohesion is supported by the pattern of return visits by five tuna tagged on 6 March at Monitoring Station R. Two or more of these tuna arrived together on 24 of 35 d when tagged tuna were detected. All five individuals visited R on 11 April, a month after tagging, three arrived together 5 mo later on 4 August, and three returned 6 mo later on 1 December 1996. Tuna often arrived at the same time of day, e.g. Individuals 1 and 3 visited R at 09:15 hrs on 12 April and at 09:00 hrs 8 mo later. The returns were also site-specific. The 22 tuna tagged at R made 182 return visits to R (92.4%) and only 15 visits to Monitoring Station K (7.6%), 10 km away. An allegiance of tuna to one school, a predilection for returning to the site of tagging, and precise timing when visiting sites, are consistent with tuna having migratory pathways consisting of “way-points” that are visited with temporal regularity. Received: 30 April 1998 / Accepted: 27 October 1998     

Is Syllis gracilis (Polychaeta: Syllidae) a species complex? An allozyme perspective

The genetic relationships between morphologically indistinguishable marine and brackish populations of Syllis gracilis Grube, 1840 (Polychaeta: Syllidae) were studied by means of allozyme electrophoresis. Samples of S. gracilis from marine coastal and brackish-water habitats were examined for variation at 13 presumptive loci. In addition, a sample of the closely related species S. prolifera (Krohn, 1852) was analysed. Five loci were multiallelic in at least one population of S. gracilis and eight loci in S. prolifera. Low to moderate levels of within-population genetic variability were found, with average expected heterozygosity values ranging from H = 0.068 (±0.043 SE) to 0.187 (±0.069 SE) in the populations of S. gracilis; higher values were found in S. prolifera (H = 0.325 ± 0.076). The presence of various private alleles indicated a marked genetic divergence among populations of S. gracilis, with Nei's genetic distances ranging from D = 0.000 to 0.833 and a highly significant F _ST value. Furthermore, evidence for strong genetic heterogeneity between two sympatric marine populations was found. UPGMA cluster analysis and multidimensional scaling pointed out a clear genetic divergence between brackish and marine populations. At least two genetically divergent entities occurred in marine and brackish habitats. This could be due to local adaptation of individuals coming from marine populations to brackish habitats, but more presumably to the occurrence of a species complex within S. gracilis. Received: 6 June 1999 / Accepted: 7 February 2000     

Do newborn domestic rabbits Oryctolagus cuniculus compete for thermally advantageous positions in the litter huddle?

Competition among mammalian siblings for scarce resources can be severe. Whereas research to date has focused on competition for the mother’s milk, the young of many (particularly altricial) species might also be expected to compete for thermally favorable positions within the nest, den, or litter huddle. We investigated this in newborn pups of the European rabbit Oryctolagus cuniculus, a species in which the altricial young are not brooded by the mother, and in which competition for milk is severe. In eight unculled litters (N = 86 pups) of a domestic chinchilla strain, we calculated huddling indexes for individual pups on postnatal days 2–5 as a measure of the degree of insulation they received from littermates. Pups maintained almost constant physical contact with the litter huddle. They performed brief but frequent rooting and climbing behaviors, which usually improved their huddling index, interspersed with longer periods of quiescence during which their huddling index declined. As expected, we found a significant positive relation between pups’ mean huddling index and body temperature. Unexpectedly, however, we did not find a relation between huddling index and pups’ birth weight, survival, milk intake, or efficiency of converting milk to body mass. We conclude that rather than competing for thermally advantageous positions within the huddle newborn rabbits share out thermally advantageous positions as they move in a continual dynamic flow through it. Thus, in newborn rabbits, competition for the mother’s milk exists alongside mutual “cooperative” benefits of littermate presence. This contribution is part of the special issue “Sibling competition and cooperation in mammals” (guest editors: Robyn Hudson and Fritz Trillmich).     

Males as infant protectors in Hanuman langurs (Presbytis entellus) living in multimale groups – defence pattern, paternity and sexual behaviour

Hypotheses about the evolution of gregariousness and social organisation in primates are based on ecological explanations as well as on social factors such as conspecific threat (especially infanticide by males). The social explanation fits well with the conditions found in strepsirrhine primates and furthermore explains why infanticide in anthropoid primates living in one-male groups mainly occurs when the resident male (protector) is replaced. However, whether it likewise fits to the conditions in multimale groups will depend on the role of resident males as infant protectors, which has rarely been examined. We investigated long-term data of wild Hanuman langurs (Presbytis entellus) inhabiting a forest near the village of Ramnagar (southern Nepal). Twenty-two eye-witnessed attacks on infants by males were analysed in connection with male residency, paternity (DNA analyses) and sexual behaviour. Adult males played a major role in infant defence (65%). Only the genetic father or males who had been residents when the infant was conceived were observed to protect infants. Males who immigrated after a female had conceived may later attack her infant and were never observed to defend it. lt seems that the males took only copulations with potentially fertile females but not with pregnant females as clues for paternity. In the light of these results it seems likely that the risk of infanticide is an important determinant in female-male associations even in anthropoid primate multimale groups. Received: 22 December 1998 / Received in revised form: 30 April 1999 / Accepted: 1 May 1999     

Disease resistance: a benefit of sociality in the dampwood termite Zootermopsis angusticollis (Isoptera: Termopsidae)

The benefit of sociality in relation to disease susceptibility was studied in the dampwood termite Zootermopsis angusticollis. Although contact with high concentrations of fungal conidia is lethal, the survivorship of nymphs exposed to spore suspensions ranging from 6 × 10⁶ to 2 × 10⁸ spores/ml of the fungus Metarhizium anisopliae increased with group size. The survivorship (measured as LT₅₀) of isolated individuals ranged from 3.0 to 4.8 days, but infected nymphs living in groups of 10 and 25 individuals survived significantly longer (5.6–8.3 and 5.6–9.1 days, respectively). In most cases, there were no significant differences in the survival distributions of the 10- and 25-termite groups. When nymphs were infected with concentrations of 7 × 10¹–7 × 10⁴ spores/ml and allowed to interact with healthy nestmates, fungal infections were not contracted by the unexposed termites. Moreover, infected termites benefitted from social contact with unexposed nestmates: their survival rates were significantly higher than those of infected termites living with similarly infected nestmates. Allogrooming, which increased in frequency during and after exposure to conidia, appeared to remove potentially infectious spores from the cuticle, thus increasing termite survivorship. These results suggest that allogrooming plays a crucial role in the control of disease and its death hazard in termites. The infection-reducing advantage of group living may have been significant in the evolution of social behavior in the Isoptera. Received: 18 March 1998 / Accepted after revision: 31 May 1998     

Marine aquaculture and bottlenose dolphins’ (<Emphasis Type="Italic">Tursiops truncatus</Emphasis>) social structure

In this study, we investigate association patterns of 249 bottlenose dolphin feeding groups off Sardinia Island (Italy) from January 2000–May 2007 and describe how their association behaviour is related to their response to food patches created by a marine fin fish farm. We also tested the hypothesis that dolphins have different social structures with different feeding activities: Associations should decrease during opportunistic feeding behaviours as it is easier to capture prey, and cooperation is not as necessary. Sixteen individually identified bottlenose dolphins were observed participating in both opportunistic and not opportunistic feeding activities, with a mean of 30 ± 8 times and 9.6 ± 1 times, respectively. Bottlenose dolphins show non-random social behaviour during feeding and this behaviour differs depending on their specific foraging activity. Dolphin associations during feeding can be divided into three categories: acquaintances, affiliates, and feeding associates. Association behaviour during fish farm feeding is consistent with our hypothesis that during opportunistic behaviours, benefits from cooperation decrease, as it is easier to capture prey. Group size homogeneity in both feeding activities demonstrates that the number of dolphins engaging in foraging is not necessarily related with cooperation levels. Moreover, an adult dolphin may prefer to associate with a specific individual, independent of the sex, who shares the same foraging priorities. This study is the first to show how aquaculture is not only directly affecting marine predators but could also indirectly affect their social structure and behaviour.     

Genetic differentiation among populations of a broadcast spawning soft coral, Sinularia flexibilis, on the Great Barrier Reef

The genetic structure of 12 reef populations of the soft coral Sinularia flexibilis (Octocorallia, Alcyoniidae) was studied along the Great Barrier Reef (GBR) at a maximum separation of 1,300 km to investigate the relative importance of sexual and asexual reproduction, genetic differentiation and gene flow among these populations. S. flexibilis is a widely distributed Indo-Pacific species and a gamete broadcaster that can form large aggregations of colonies on near-shore reefs of the GBR. Up to 60 individuals per reef were collected at a minimum sampling scale of 5 m at two sites per reef, from December 1998 to February 2000. Electrophoretic analyses of nine polymorphic allozymes indicated that genotypic frequencies in most populations and loci did not differ significantly from those expected from Hardy–Weinberg predictions. Analysis of multi-locus genotypes indicated a high number of unique genotypes (N _go) relative to the number of individuals sampled (N) in each reef population (range of 0.69–0.95). The maximum number of individuals likely to have been produced sexually (N*) was similar to the number of individuals sampled (i.e. N*:N ˜ 1), suggesting that even repeated genotypes may have been produced sexually. These results demonstrated a dominant role of sexual reproduction in these populations at the scale sampled. Significant genetic differentiation between some populations indicated that gene flow is restricted between some reefs (F _ST=0.026, 95% CI= 0.011 − 0.045) and even between sites within reefs (F _ST=0.041, 95% CI=0.027 − 0.055). Nevertheless, there was no relationship between geographic separation and genetic differentiation. Analyses comparing groups of populations showed no significant differentiation on a north-south gradient in the GBR. The pattern in the number of significant differences in gene frequencies in pairwise population comparisons, however, suggested that gene flow may be more restricted among inner-shelf reef populations near to the coast than among mid/outer-shelf populations further from the coast. Received: 10 July 2000 / Accepted: 5 October 2000     

Relationship between growth and biochemical indices in laboratory-reared juvenile Japanese flounder (Paralichthys olivaceus), and its application to wild fish

In order to estimate growth rates based on biochemical indices of the liver of wild Japanese flounder (Paralichthys olivaceus), juveniles were reared at six ration levels (0, 0.5, 2, 4, 6 and 8% body weight day⁻¹) in the laboratory for 14 days, and the relationship between their growth rates and biochemical indices (RNA/DNA, protein/DNA, triglyceride/DNA, phospholipid/DNA and cathepsin D activities) were determined. Positive and approximately linear relationships were seen between growth rates and the indices of RNA/DNA, protein/DNA and phospholipid/DNA. The triglyceride/DNA ratio decreased with increasing growth rates up to approximately 1% body weight day⁻¹, then increased linearly with increasing growth rates. There was no significant correlation between growth rates and cathepsin D activity, and the highest values were obtained in the starved fish. Compared with laboratory-reared specimens, wild specimens of similar sizes were found to have significantly larger livers. The RNA/DNA, protein/DNA and phospholipid/DNA ratios of wild specimens fell in a broad range between ration groups of reared juveniles. The protein/DNA ratios of wild specimens were low and outside the range of the reared juveniles at six ration levels. In contrast, the levels of cathepsin D activity of wild fish were highest compared to the reared fish. Estimated growth rates of wild fish from the RNA/DNA, protein/DNA and phospholipid/DNA regressions obtained from the rearing experiment were 1.66, −1.74 and 0.10% day⁻¹, respectively. Based on our results, the RNA/DNA index may be regarded as the most valid and reliable growth estimator. It is noted that the larger liver size, the lower liver protein/DNA ratio and the unexpectedly high level of cathepsin D activities of wild specimens found in this study may reflect the different metabolic conditions of fish reared in the laboratory compared to those collected in the field. Received: 29 February 2000 / Accepted: 26 August 2000     

Estimating paternity with spatial behaviour and DNA fingerprinting in the striped plateau lizard, Sceloporus virgatus (Phrynosomatidae)

The association between spatial proximity and paternity was studied in a population of the striped plateau lizard, Sceloporus virgatus. The relationship between estimated mating success and male phenotypic traits was examined for a sample of 55 males. DNA samples were obtained from 13 female-offspring families. The males with the closest spatial proximity to each female were tested as possible sires within each family. Fingerprinting with two multilocus hypervariable minisatellite probes revealed a strong correspondence between male-female spatial proximity and actual paternity. Paternity could be assigned for 72 of the 100 hatchlings. Most hatchlings with identifiable sires were attributed to a male with the highest category of spatial proximity to the mother. However, there was a low to moderate level of multiple paternity within clutches, and for some clutches probable sires could not be identified even though the most likely behavioural candidates were tested. Thus, nonterritorial males or other males lacking strong social and spatial relationships with females may achieve some degree of reproductive success. Analysis of mating success revealed that male success increased with body size, up to a point beyond which larger size conferred no advantage. Received: 7 January 1997 / Accepted after revision: 16 June 1997     

Ecological and social determinants of spacing behaviour in desert baboon groups

This paper investigates the determinants of individual spacing behaviour in a desert baboon population (Papio cynocephalus ursinus). Patterns of neighbour proximity and neighbour density were examined among adults in four groups under different ecological and social conditions (through instantaneous sampling during focal follows). Initial analysis of these data shows that (1) the use of vertical substrates (refuges such as tall trees and cliff faces) can confound patterns of spacing, and (2) individual differences in spacing can depend on the spatial scale over which it is measured. To minimise these substrate and scale effects, this analysis focuses on animals which are off refuges and examines spacing behaviour through its underlying statistical `dimensions' (identified through factor analysis). Analysis of these dimensions indicates that sex, group size, activity-habitat and female reproductive state can all have independent effects on spacing: (1) males are more dispersed than females in smaller groups, (2) male and female dispersion increases with time spent in foraging habitats, and (3) female dispersion is reduced during lactation. According to the hypotheses tested, these results indicate that feeding competition only affects spacing behaviour during foraging while predation risk plays little or no role in spacing. Most aspects of spacing behaviour are best explained by male reproductive strategies and their social repercussions. Received: 25 May 1998 / Accepted after revision: 18 July 1998     

Escape of the ctenophore Mnemiopsis leidyi from the scyphomedusa predator Chrysaora quinquecirrha

The ctenophore Mnemiopsis leidyi A. Agassiz, 1865 is known to be eaten by the scyphomedusan Chrysaora quinquecirrha (Desor, 1948), which can control populations of ctenophores in the tributaries of Chesapeake Bay. In the summer of 1995, we videotaped interactions in large aquaria in order to determine whether M. leidyi was always captured after contact with medusae. Surprisingly, M. leidyi escaped in 97.2% of 143 contacts. The ctenophores increased swimming speed by an average of 300% immediately after contact with tentacles and 600% by mid-escape. When caught in the tentacles of C. quinquecirrha, the ctenophores frequently lost a portion of their body, which allowed them to escape. Lost parts regenerated within a few days. The striking ability of M. leidyi to escape from C. quinquecirrha may be critically important in maintaining ctenophore populations in situ. Received: 14 November 1996 / Accepted: 4 December 1996     

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     

Greater spear-nosed bats give group-distinctive calls

Individually distinctive vocalizations are ubiquitous; however, group distinctive calls have rarely been demonstrated. Under some conditions, selection should favor calls indicating social group membership in animals that forage in groups. Greater spear-nosed bats (Phyllostomus hastatus) give calls that appear to facilitate recognition of social group mates who are unrelated. Females give loud broadband (4–18 kHz) vocalizations termed screech calls when departing on foraging trips and at foraging sites. Screech calls help to establish foraging groups among social group members, and to maintain contact over the long distances they travel while foraging. I test two hypotheses about how screech calls may be structured to convey caller identity. Individual calls may be distinct and group members may learn to recognize each individual's calls and to associate the individual with the social group. Alternatively, groups may give distinct calls and individuals within groups may share call characteristics. To test these hypotheses I conducted multivariate acoustic analysis of multiple calls from 28 bats from three social groups. Although the ubiquity of individually distinctive calls in other taxa makes this result more likely, the results reveal that group calls are highly distinctive. Individual bats within groups are statistically indistinguishable. Calls appear to decrease slightly in frequency as bats age. Call convergence among unrelated group mates implies vocal learning in this species. Received: 28 March 1996 / Accepted after revision: 6 October 1996     

Spacing and group coordination in a nocturnal primate, the golden brown mouse lemur (Microcebus ravelobensis): the role of olfactory and acoustic signals

In order to remain stable, dispersed social groups have to solve two fundamental problems: the coordination of movement and cohesiveness within a group and the spacing between the groups. Here, we investigate mechanisms involved in intra-group coordination and inter-group spacing using the golden brown mouse lemur, Microcebus ravelobensis, as a model for a nocturnal, solitary foraging mammal with a dispersed social system. By means of radiotelemetry and bioacoustics we studied the olfactory and vocal behaviour during nocturnal dispersal and reunion of five sleeping groups.All groups used 3–17 sleeping sites exclusively, suggesting a sleeping site-related territoriality and competition for them. The occurrence of olfactory and vocal behaviour showed an asymmetrical temporal distribution. Whereas marking behaviour was observed exclusively during dispersal, a particular call type, the trill, was used by all groups during reunions. Interestingly, these trills carried group-specific signatures.Our findings provide the first empirical evidence for nocturnal primates in a natural environment that olfactory signals represent an important mechanism to regulate the distribution of different groups in space, whereas acoustic signals control intra-group cohesion and coordination.     

Discrimination of different social companions in spectacled parrotlets (Forpus conspicillatus): evidence for individual vocal recognition

Individual recognition is generally assumed to be a prerequisite for establishing and maintaining a complex social system. Indeed, there is good evidence that highly social species have complex systems of vocal communication with individual recognition by acoustic cues. In this study, we provide experimental evidence that vocal class and individual recognition is present in a non-passerine bird, the spectacled parrotlet (Forpus conspicillatus). Spectacled parrotlets live in a complex system of social relationships. Soon after fledging, the young establish close sibling relationships which are important for successful socialization, pairing and reproduction. In a series of playback experiments we tested if spectacled parrotlets use contact calls for vocal recognition. The results showed that spectacled parrotlets discriminate between the contact calls of different social categories. Adult birds preferred to respond to the contact calls of their mates. Subadult individuals recognized the contact calls of their siblings. During the period of pair bond formation, the affiliative contacts to the siblings decrease, but the parrotlets continue to respond to the calls of their siblings. This is the first evidence that vocal sibling recognition might outlast the period of strong sibling interaction and extends into the period of pair bond formation. In cases of mate loss or divorce, the acoustic contact to their siblings might facilitate the re-establishment of close sibling relationships. Received: 29 October 1997 / Accepted after revision: 5 April 1998     

Genetic structure and allozyme variation of sea bass (Dicentrarchus labrax and D. punctatus) in the Mediterranean Sea

This paper reports data on 28 allozyme loci in wild and artificially reared sea bass (Dicentrarchus labrax) samples, originating from either coastal lagoon or marine sites in the Mediterranean Sea. F _ST analysis (θ estimator) indicated strong genetic structuring among populations; around 34% of the overall genetic variation is due to interpopulation variation. Pairwise θ estimates showed that, on average, the degree of genetic structuring was much higher between marine populations than between samples from lagoons. Six polymorphic loci showed differences in allele frequencies between marine and lagoon samples. Multivariate analyses of individual allozymic profiles and of allele frequencies suggested that different arrays of genotypes prevail in lagoons compared to marine samples, particularly at those loci that, on the basis of previous acclimation experiments, had been implicated in adaptation to freshwater. On the other hand, variation at “neutral” allozyme loci reflects to a greater extent the geographic location of populations. Allozyme differentiation was also studied in a D. labrax population from the Portuguese coast. Average genetic distance between this population and the Mediterranean populations was quite high (Nei's D = 0.236) and calls into question the taxonomic status of the Portuguese population. Finally, genetic relationships between D. labrax and D. punctatus were evaluated. Average Nei's D was 0.648, revealing high genetic differentiation between the two species, even for two sympatric populations of these species in Egypt; thus gene flow was not indicated between species. Received: 24 October 1996 / Accepted: 27 November 1996     

Amphi-Atlantic phylogeography of direct-developing lineages of Lasaea, a genus of brooding bivalves

Direct-developing intertidal Lasaea spp. occur in the North Atlantic as both continental margin and oceanic island populations. We conducted a molecular phylogenetic analysis of representative populations in order to test colonization hypotheses for North Atlantic oceanic islands. Thirty individuals each were collected in 1995 and 1996 from two continental putative source populations (Florida, Iberia) and two oceanic island populations (Bermuda, Azores). They were sequenced for a 462 nucleotide portion of the mitochondrial large ribosomal subunit (16S) gene. No amphi-Atlantic genotypes were detected: Bermudan lineages co-clustered exclusively with Floridian congeners, and Azorean samples formed an exclusive clade with Iberian haplotypes. Our data indicate that geographical proximity to continental source populations is a better predictor of phylogenetic relationships in North Atlantic Lasaea spp. than present-day oceanic surface circulation patterns. The phylogenetic trees generated are not consistent with colonization of oceanic islands by indirect-developing ancestral lineages or by truly trans-oceanic rafting events. However, they are consistent with predicted topologies resulting from limited (≤ 2000 km), long-distance colonization by rafting (against present-day circulation patterns in the case of the Azores) and from anthropogenic introductions. Received: 17 December 1998 / Accepted: 7 June 1999     

Genetic relationships between marine and marginal-marine populations of Cerithium species from the Mediterranean Sea

Results of isozyme electrophoresis were used to explore the genetic relationships between several Mediterranean morphs of Cerithium (Gastropoda: Prosobranchia), for which taxonomy is currently uncertain because of high intraspecific variability and low interspecific differentiation. The large species, classically known as C. vulgatum Bruguière, 1789 was identified at four sites (two in the French Mediterranean and two in southern Spain). Two different larval types were found in the French sites, but poecilogony could not be demonstrated. Individuals collected from harbours were not genetically distinct from open-sea populations of classic C. vulgatum. However, a population in the Embiez lagoon (French Mediterranean) which morphologically resembles C. vulgatum did display distinct genetic traits, supporting its status as a separate species. Of the small Cerithium species usually known as C. rupestre, two sympatric species (C.“rupestre” Risso, 1826 and C. lividulum Risso, 1826) were distinguished. Genotype frequencies within the analysed populations revealed much heterozygote deficiency. F _ST values (fixation index measuring the effects of population subdivision) suggest a higher genetic differentiation for C. lividulum populations than for C. vulgatum populations. We assume that a high larval dispersal capability (via planktotrophy) allows a high gene flow between populations of C. vulgatum. Received: 24 November 1998 / Accepted: 24 September 1999     

Social monogamy and the threat of infanticide in larger mammals

The evolution of social monogamy in larger mammals is difficult to explain because males usually do not invest much in direct offspring care and might achieve greater fitness by deserting a pregnant female to reproduce with additional females elsewhere. It has been hypothesized that socially monogamous males remain with the female year-round to protect their offspring from infanticide by new immigrant males. We investigated this idea by analyzing all cases of infant loss in a wild population of white-handed gibbons (Hylobates lar; Primates), in which most groups were socially monogamous and some polyandrous (137.5 group years). We examined the influence of (a) male intruder pressure on male immigration rates and (b) the presence of a new male in the group on infant loss. We found no relation between intruder pressure and male immigration rates. Infant loss was lowest (4.5%) for stable monogamy (probable father stayed from conception through infancy) and intermediate (25.0%; p = 0.166) for stable polyandry. If a new male immigrated after conception, however, the infant was lost in all cases (p < 0.01) independent of the presumed father’s presence. Overall, 83.3% of infant losses were associated with the presence of a presumably unrelated male. Although the sample size is small, our results provide the first true support for the idea that the risk of infanticide is an important factor in the evolution of social monogamy in larger mammals.     

Macroplankton/micronekton dynamics in the vicinity of the Prince Edward Islands (Southern Ocean)

Combined trawling (rectangular midwater trawl, RMT-8 net) and acoustic (120 kHz) surveys in the upstream and downstream region of the Prince Edward Islands were undertaken during April and May 1996 and 1997. A total of 49 species of macroplankton and micronekton were encountered within the region investigated. Mesopelagic fishes, euphausiids, chaetognaths and tunicates dominated numerically and by biomass. Average abundance and biomass of macroplankton/micronekton in the top 300 m layer were low, 18 individuals 1000 m⁻³ and 288 mg dry wt 1000 m⁻³, respectively. Numerical analysis revealed the presence of two major groups of stations broadly corresponding to the offshore (upstream and downstream) and inshore (inter-island) realms. Planktonic samples and acoustic measurements revealed that elevated densities of macroplankton/micronekton were associated with the subantarctic front region and in close proximity to the island plateau. Based on trawling and acoustic survey results, it is suggested that large plankton and micronekton are mostly washed around rather than across the inter-island shelf region. This is in contrast with the “replenishing hypothesis” previously proposed for this region by other investigators. Received: 11 May 1998 / Accepted: 25 March 1999