Protection against herbivores of the myrmecophyte Leonardoxa africana (Baill.) Aubrèv. T3 by its principal ant inhabitant Aphomomyrmex afer Emery

Leonardoxa africana T3 is a myrmecophyte, a plant with specialized structures (domatia) that shelter ants. Adult trees are essentially all occupied by the ant Aphomomyrmex afer. One tree possesses one ant colony. Ants tend homopterans inside the domatia. The plant provides ants with nest sites and food via production of extrafloral nectar and via honeydew produced by homopterans. Workers patrol the young leaves, although their nectaries are not yet functional. This study was conducted to investigate the nature of the relationship between the plant and its ants. In order to determine whether ants protect the plant against herbivorous insects, we placed microlepidopteran larvae on young leaves of several trees, and measured the time until discovery of the larvae by the workers. We then studied the responses of workers as a function of insect size. We showed that workers patrolled the young leaves of the majority of trees. There was, however, inter-colony variability in intensity of patrolling. Workers attacked every larva they found, killing and eating the smaller ones, and chasing larger ones off the young leaf. Most of the phytophagous insects attacking young leaves of L. africana T3 were inventoried in this study. We showed that the larvae of microlepidopterans, one of the most important herbivores of this species, form part of the diet of A. afer. The function of the stereotyped behaviour of ant patrolling on young leaves may be in part to obtain insect protein to complement carbohydrate-rich nectar and honeydew, and in part to protect the host and thus increase its production of resources for ants. Our study shows that ants protect the tree against herbivores, and that even if this protection is less pronounced and more variable than that demonstrated for their sister species L. africana sensu stricto and Petalomyrmex phylax, the association between L. africana T3 and A. afer is a mutualism.     

Worker size polymorphism and ethological role of sting associated glands in the harvester ant Messor barbarus

Summary: Though harvester ants are closely similar in ecology, species differ in their worker size polymorphism as well as in the glandular source of their trail pheromones and defensive compounds. In the harvester ant Messor barbarus, we find that the recruitment trail pheromone is located in the Dufour gland, while defence-alarm substances are produced in the poison gland. We also investigated how the glandular development and the ethological response to these abdominal glands are related to worker body size. For both glands, M. barbarus workers show monophasic and nonisometric growths with slopes of allometric regression lines lower than 1. The highest trail-following response is elicited by the Dufour gland secretion from media workers, responsible for most foraging activities in M. barbarus. Aggressive behaviour is more frequently observed in the presence of poison gland secretions from medium and large-sized workers. Differences between species and between worker size classes in the ethological role of sting associated glands are discussed in relation to the foraging ecology and defensive characteristics of harvester ants.     

Ant visitation of extrafloral nectaries of Passiflora: the effects of nectary attributes and ant behavior on patterns in facultative ant-plant mutualisms

Extrafloral nectary (EFN) plants are widespread and can be quite species-rich in some communities. Thus, ants that utilize extrafloral nectar may have the opportunity to discriminate among a wide variety of nectar sources, resulting in variation in the ant attention EFN plants receive. In this study, we compare ant visitation rates of three Passiflora species that coexist in an early successional neotropical forest. These three vine species (Passiflora auriculata, P. biflora, and P. oerstedii) differ in their extrafloral nectary structure and placement, and thus may attract different numbers or species of ants. Through censuses of ants tending extrafloral nectaries, we found that P. auriculata received significantly higher numbers of ant visitors than P. oerstedii, but did not differ significantly from P. biflora in its attractiveness to ants. We also found that termite worker baits (simulating herbivores) placed on P. auriculata and P. biflora were discovered by ants significantly more quickly than baits placed on P. oerstedii. In both ant visitation censuses and in termite bait trials, we found no significant associations between Passiflora species and the species of ant visitors. We also performed experimental manipulations of several characteristics of P. auriculata, which resulted in changes in levels of ant visitation. When petiolar nectaries of P. auriculata were experimentally blocked, visitation by the common ant Ectatomma ruidum declined, even though nectaries on the leaf surfaces were still functional. Connections with other vegetation also had an effect on ant visitation. Though experimental creation of connections between growing P. auriculata shoots and other vegetation did not enhance ant visitation, eliminating connections resulted in a significant decline in the number of ant visitors. The results of this study suggest factors that may contribute to variation in ant visitation of extrafloral nectary plants. In addition, this study demonstrates that extrafloral nectary plants co-occurring in a habitat and available to the same ants may differ in patterns of visitation by ants and perhaps in the quality of protection from herbivores that they receive.     

How plants shape the ant community in the Amazonian rainforest canopy: the key role of extrafloral nectaries and homopteran honeydew

Ant-plant interactions in the canopy of a lowland Amazonian rainforest of the upper Orinoco, Venezuela, were studied using a modified commercial crane on rails (Surumoni project). Our observations show a strong correlation between plant sap exudates and both abundance of ants and co-occurrence of ant species in tree canopies. Two types of plant sap sources were compared: extrafloral nectaries (EFNs) and honeydew secretions by homopterans. EFNs were a frequent food source for ants on epiphytes (Philodendron spp., Araceae) and lianas (Dioclea, Fabaceae), but rare on canopy trees in the study area, whereas the majority of trees were host to aggregations of homopterans tended by honeydew-seeking ants (on 62% of the trees examined). These aggregations rarely occurred on epiphytes. Baited ant traps were installed on plants with EFNs and in the crowns of trees from three common genera, including trees with and without ant-tended homopterans: Goupia glabra (Celastraceae), Vochysia spp. (Vochysiaceae), and Xylopia spp. (Annonaceae). The number of ant workers per trap was significantly higher on plants offering one of the two plant sap sources than on trees without such resources. Extrafloral nectaries were used by a much broader spectrum of ant species and genera than honeydew, and co-occurrence of ant species (in traps) was significantly higher on plants bearing EFNs than on trees. Homopteran honeydew (Coccidae and Membracidae), on the other hand, was mostly monopolised by a single ant colony per tree. Homopteran-tending ants were generally among the most dominant ants in the canopy. The most prominent genera were Azteca, Dolichoderus (both Dolichoderinae), Cephalotes, Pheidole, Crematogaster (all Myrmicinae), and Ectatomma (Ponerinae). Potential preferences were recorded between ant and homopteran species, and also between ant-homopteran associations and tree genera. We hypothesize that the high availability of homopteran honeydew provides a key resource for ant mosaics, where dominant ant colonies and species maintain mutually exclusive territories on trees. In turn, we propose that for nourishment of numerous ants of lower competitive capacity, Philodendron and other sources of EFNs might be particularly important.     

Inter‐seasonal persistence and size‐structuring of two minnow species within headwater streams in the Eastern Cape,South Africa

This study examined temporal variation in population dynamics and size structuring of two cyprinid minnows, Pseudobarbus afer and Barbus anoplus, in relation to their proximate physical habitats. Population estimates were determined using three‐pass depletion sampling during both summer and winter. The habitats were characterised by seasonal variation in all physico‐chemical conditions and spatial variation in substrata compositions. Whereas significant differences in population size were noted between seasons for B. anoplus, no differences were found between seasons for density and capture probability for either species. An increase in boulders was associated with increase in population size and density for P. afer; for B. anoplus, increased percentages of bedrock and bank vegetation were associated with an increase in population size and probability of capture, respectively. According to Canonical Correspondence Analysis, size structuring in P. afer was explained predominantly by seasonality, with smaller length classes associated with the seasonal variable of summer, while larger length classes were associated with pH that was higher in winter. By comparison, for B. anoplus, the habitat variables – bank vegetation and bedrock – accounted for much of the explained variance for size structuring. Recruitment appeared to be the major driver of size structuring for the two species; refugia, especially boulders and bank vegetation, also appeared to be important. Overall, the two species were adapted to the headwater streams that were generally variable in environmental conditions. Potential invasions by non‐native invasive fishes that occur within the mainstream habitats threaten these two species. Efforts should continue to protect these minnows from such invasions by constructing barriers to upstream migration of non‐native fishes into these headwater habitats.     

Effects of familiarity and nestmate number on social interactions in two communal bees, Andrena scotica and Panurgus calcaratus (Hymenoptera, Andrenidae)

Summary: Communality is considered a widespread form of social organization in bees and wasps, one in which two or more females of the same generation share a common nest and amongst whom there is no reproductive division of labor. The evolution and maintenance of egalitarian communal societies poses theoretical difficulties where 'cheating' amongst nestmates is practised. Yet there is little knowledge of the intranidal social interactions among communal conspecifics that allow an understanding of the degree of cooperation versus exploitation amongst nestmates in such societies. Using a circle-tube arena, we staged interactions in the laboratory among pairs of female conspecifics using two communal andrenid species, Andrena scotica and Panurgus calcaratus, to score their cooperative and aggressive behaviors. For both species, females did not discriminate between familiar (nestmate) and unfamiliar (non-nestmate) conspecifics in their cooperative and aggressive behaviors, suggesting that there is universal acceptance of conspecifics. Levels of cooperation and aggression did not vary with the number of nestmates in the nest of origin for either species. In addition, frequencies of social interactions did not vary with body size, wing wear or ovarian development in P. calcaratus. Interspecific comparisons were made with published data on social interactions of other bee species at varying levels of social organization (solitary to eusocial) derived from analogous circle-tube experiments. Panurgus calcaratus displayed highly cooperative behavior, similar to that of a previously studied communal bee, Lasioglossum hemichalceum (Kukuk, 1992a). Andrena scotica displayed lower levels of cooperation, and was in this sense more similar to solitary species and to interactions amongst non-nestmate individuals of eusocial species. Both A. scotica and P. calcaratus displayed very low levels of aggressive behavior, again similar to L. hemichalceum. As a working hypothesis, we suggest the existence of a communal behavioral syndrome comprising high levels of intranidal cooperation and low aggression, both directed indiscriminately at conspecifics.     

Evolution of advanced social traits in phylogenetically basal ants: striking worker polymorphism and large queens in Amblyopone australis

The ecological success of ants results notably from their ability to derive several phenotypes from the same genotype. In higher ants, polyphenism can be extreme: high queen–worker size dimorphism and worker polymorphism allow for better success at colony founding by lone queens, better division of tasks, and ultimately access to new ecological niches. However in basal ants, polyphenism is much more limited, restricting them to narrow niches. It is often assumed that basal ants lack the ability to produce a pronounced polyphenism. In Amblyopone, most species nest and hunt centipedes in soil and leaf litter, and polyphenism is weak, e.g. A. pallipes. Here we studied A. australis, which forages and nests in rotten logs. Using analyses of morphometry and allometry, we showed that queen–worker dimorphism and worker polymorphism are much higher in A. australis than in A. pallipes. Workers of A. australis exhibit various sizes and shapes, and large individuals could be better adapted for digging galleries in rotten logs. Moreover, larger queens are probably more efficient during non-claustral colony foundation. We conclude that the evolution of advanced social traits typical of higher ants is also possible in basal ants, but it is not often selected for.     

Brood-size and sex-ratio variation in field populations of three species of solitary aphid parasitoids (Hymenoptera: Braconidae, Aphidiinae)

Solitary parasitoids of colony-forming hosts may produce quasi-gregarious broods, which favours sibmating on the natal patch and local mate competition (LMC). We examined seasonal variations in brood size and sex ratio in three species of solitary parasitoids of aphids associated with trophobiotic ants. Adialytus arvicola, a parasitoid of Sipha agropyrella on grasses, had the smallest broods (mean=4.2, maximum 19), while Lysiphlebus hirticornis, a parasitoid of Metopeurum fuscoviride on tansy, had the largest broods (mean=32.0, maximum 265). In Pauesia pini, a parasitoid of Cinara piceicola on Norway spruce, broods comprised an average of 5.8 (maximum 41) individuals; brood size increased during early summer when hosts became more available but remained the same later in the season. In all three species the sex ratio at eclosion was female-biased, with broods containing approximately two daughters for each son in both A. arvicola and L. hirticornis; the degree of female-bias was least in P. pini. The sex ratio did not vary with brood size. In A. arvicola, the variance of the number of sons declined with an increase in brood size, consistent with "precise" sex allocation. In contrast, in L. hirticornis, the overall sex-ratio variance was greater than its binomial expectation, while it did not differ from binomial in P. pini. A large proportion of broods contained only sons or only daughters, especially in A. arvicola. An excess of male-only broods is expected if constrained females (which can produce only sons) contribute offspring prior to mating. The number of male-only broods, however, did not differ from the number expected if all females are mated and allocate offspring sexes binomially, except in P. pini. In the latter species, broods with two daughters (as opposed to two sons) exceeded binomial expectations. We propose that P. pini is largely outbreeding, while the strongly female-biased sex ratio in A. arvicola and L. hirticornis is consistent with partial sibmating and LMC. Ant-parasitoid interactions could account for a different population mating structure in the three parasitoid species. Both A. arvicola and L. hirticornis mimic the epicuticular hydrocarbon pattern of their aphid hosts. Eclosing wasps are ignored by honeydew-collecting ants and hence can mate and forage on the natal patch. In contrast, P. pini generally depart the mummy area to avoid attacks by trophobiotic ants and mate off patch.     

Non-native Ants Are Smaller than Related Native Ants

I compare the sizes of non-native and native ants to evaluate how worker size may be related to the ability of a species to invade new habitats. I compare the size of 78 non-native ant species belonging to 26 genera with the size of native congeneric species; native ants are larger than non-native ants in 22 of 26 genera. Ants were sorted by genera into fighting and nonfighting groups, based on observations of interspecific interactions with other ant species. In all of the genera with monomorphic worker castes that fight during competition, the non-native species were smaller than the native species. The genera that engage in combat had a higher frequency of significantly smaller size in non-native ants. I selected Wasmannia auropunctata for further studies, to compare native and non-native populations. Specimens of W. auropunctata from non-native populations were smaller than conspecific counterparts from its native habitat. I consider hypotheses to explain why non-native ants are smaller in size than native ants, including the role of colony size in interspecific fights, changes in life history, the release from intraspecific fighting, and climate. The discovery that fighting non-natives are smaller than their closest native relatives may provide insight into the mechanisms for success of non-native species, as well as the role of worker size and colony size during interspecific competition.     

Semi-claustral colony founding in the seed-harvester ant Pogonomyrmex californicus: a comparative analysis of colony founding strategies

The evolution of queens that rear their first brood solely using body reserves, i.e. fully claustral, is viewed as a major advance for higher ants because it eliminated the need for queens to leave the nest to forage. In an apparently unusual secondary modification, the seed-harvester ant Pogonomyrmex californicus displays obligate queen foraging, i.e. queens must forage to garner the resources necessary to survive and successfully rear their first brood. I examined the potential benefits of queen foraging by comparing ecological and physiological traits between P. californicus and several congeners in which the queen can rear brood using only body reserves. The primary advantage of foraging appears to lie in providing the queens of P. californicus with the energy to raise significantly more brood than possible by congeners that use only body reserves; the workers reared in the first brood were also heavier in mass than that predicted by their head width. Other correlates of queen foraging in P. californicus relative to tested congeners included a significantly lower total fat content for alate queens, a small queen body size, and a low queen to worker body mass ratio. Queens also forage in several other well-studied species of Pogonomyrmex, suggesting the possibility that queen foraging may be more common than previously thought in higher ants.     

Polyethism and the adaptiveness of worker size variation in the attine antTrachymyrmex septentrionalis

Division of labor was studied in colonies from Long Island and Florida populations of the fungus-gardening antTrachymyrmex septentrionalis. Workers showed age polyethism and a weak size polyethism, and these patterns of division of labor were not different in colonies from the two populations. Individual workers had repertoires comprised of three to five roles but tended to concentrate their labor within a single role. The data are consistent with the hypothesis that worker polymorphism and the elaboration of size-related behavior in the attine ants evolved along with the use of fresh vegetation as the fungal substrate. The data do not support the hypothesis that size variation in colonies ofT. septentrionalis evolved to promote efficient division of labor. Division of labor within the worker caste is based mainly on age and appears to be an attribute of the species rather than an adaptation to a particular habitat.     

Physical worker castes in colonies of an acacia-ant (Crematogaster nigriceps) correlated with an intra-colonial division of defensive behaviour

Summary: Crematogaster nigriceps (Emery) is one of the acacia-ants inhabiting the swollen thorns of Acacia drepanolobium trees in East Africa. The ants show two distinctive behaviours in response to being disturbed. Some ants ("Defenders") readily leave the swollen thorns to defend the tree, whilst others ("Domestics") remain inside the swollen thorns and will not move unless provoked into doing so. Analysis of head length and head width measurements of the ants shows conclusively that the two behavioural castes are also physical castes. Defenders have significantly larger heads (Head length 0.97 mm - 0.006, Head width 0.98 mm - 0.006) than the Domestics (Head length 0.78 mm - 0.008, Head width 0.81 mm - 0.007). It is concluded that there is an intra-colonial division of defensive behaviour within the worker ants of C. nigriceps, mirrored by a physical sub-division of the worker caste.     

Sea-level changes, river capture and the evolution of populations of the Eastern Cape and fiery redfins (Pseudobarbus afer and Pseudobarbus phlegethon, Cyprinidae) across multiple river systems in South Africa

Aim The phylogeography of the two closely related species Pseudobarbus afer and Pseudobarbus phlegethon was investigated to assess the association of evolutionary processes, inferred from mitochondrial DNA (mtDNA) sequence variation, with hypothetical palaeoriver systems and other climatic and landscape changes. Location One western and several southern river systems in South Africa. Methods We sampled known populations and confirmed known distribution gaps. This was followed by an assessment of mtDNA control region sequence variation for 31 localities across 17 river systems across the range of the species complex. A map of possible offshore drainage patterns during the last major regression event was constructed based on bathymetry and geological studies. Results The genetic distinction of four major lineages of P. afer strongly correspond with proposed palaeoriver systems. However, a western ‘Forest’ lineage, is widespread across two such proposed systems and is closely related to P. phlegethon on the west coast of South Africa. Both the ‘Krom’ and ‘St Francis’ lineages were identified in the single palaeoriver system proposed for St Francis Bay. A fourth ‘Mandela’ lineage is restricted to the one or two palaeoriver systems proposed for Nelson Mandela Bay. Four minor lineages were identified within the Forest lineage and two within the Mandela lineage. Main conclusions The close relationship between P. phlegethon and the Forest lineage of P. afer can only be explained by a series of river captures. We suggest the Gourits River system as a historical link that could account for this relationship. On the south coast, lower sea levels than at present allowed confluence between currently isolated river systems, offering opportunities for dispersal among these populations. At present, isolation between different river systems rather than dispersal appears to have a dominant influence on mtDNA diversity.     

A phylogenetic analysis of the fungus-growing ants (Hymenoptera: Formicidae: Attini) based on morphological characters of the larvae

A phylogenetic hypothesis of the fungus-growing ants (subfamily Myrmicinae, tribe Attini) is proposed, based on a cladistic analysis utilizing forty-four morphological characters (109 states) of the prepupal worker larva. The fifty-one attine species analysed include representatives of eleven of the twelve currently recognized attine genera, excluding only the monotypic workerless parasite Pseudoatta ; the non-attines include two outgroups (species of the basal myrmicine genera Myrmica and Pogonomyrmex ), two myrmicine species presumed to be distantly related to the attines, and twelve species representing taxa that have been proposed by prior workers as possible sister groups of the Attini. There is strong character support for the monophyly of the Attini and for a sister-group relationship of the Attini and the Neotropical Blepharidatta brasiliensis. The Attini are divided into two distinct lineages, an 'apterostigmoid' clade (containing Apterostigma and Mycocepurus) and an 'attoid' clade (containing all other attine genera except Myrmicocrypta). The attine genus Myrmicocrypta appears to be paraphyletic with respect to these two groups; the species M.buenzlii in particular retains many attine plesiomorphies.
These results indicate that the fungus-growing behaviour had a single evolutionary origin in the ants. They also indicate that mycelium cultivation is plesiomorphic and that yeast cultivation is derived within the Attini, overturning the long-standing assumption that the yeast-growing Cyphomyrmex species are the most primitive Attini. Behavioural and ecological investigations into the origin and evolution of the fungus-growing behaviour might more profitably focus on species in the attine genus Myrmicocrypta , as well as those in the closely related non-attine genera Blepharidatta and Wasmannia.     

Influence of forest type and tree species on canopy ants (Hymenoptera: Formicidae) in Budongo Forest, Uganda

The arboreal ant fauna was investigated in Budongo Forest, a seasonal rain forest in Uganda, using the insecticidal fogging technique. Ants were collected from 61 trees, between 7 and 33 m in height, belonging to four tree species. Trees were growing in adjacent plots of forests characterized by different use and structure: an old primary forest, a primary swamp forest along a small river, and a secondary forest where selective logging was carried out for 30 years. A total number of 37,065 ants, belonging to 161 species in 30 genera were collected. Considering the high number of species found only once, the completeness of the canopy ant fauna was relatively high and of relatively similar magnitude as samples from the Neotropics or the Oriental region. Up to 37 ant species on a single tree, with an average of 18.2 species per tree, were found. Forty-four ant species (28.1%) were found only once, less than ten individuals were found for each of 88 species (54.7%), but 64.0% of all individuals belonged to one of five species. Considering the high numerical dominance of a few ant species like a Pheidole sp., Tetramorium aculeatum (Mayr) and a Crematogaster sp., there is some evidence for an ant mosaic in the lower canopy of the Budongo Forest. Individual numbers of ants were strongly correlated with nests in the fogged tree, though the ants were not homogeneously distributed in the tree crowns. Diversity measures that strongly depend on individual numbers such as the Morisita-Horn index or rarefaction methods were calculated, but results were not concordant with those of incidence-based estimates such as jack-knife calculations. Differences in ant species richness and faunal composition between tree species were low, but more significant between forest types. The ant fauna in the secondary forest was less diverse with 12.6% fewer species compared to the primary forest sites. The average number of ant species per tree was significantly lower in the secondary forest (<20% of the species; F=8.03, df=59, P<0.01) than in the undisturbed forest types. Cataulacus, Leptothorax, Tetraponera, and Polyrhachis, which are typical canopy-dwelling ant genera, had a significantly higher diversity and frequency in the two primary forest types (F=4.17, df=53, P<0.05). Secondary forest trees are often younger, lacking dead branches and epiphytes which are important requisites for ant colonization on trees.     

Long-term dynamics of the distribution of the invasive Argentine ant, Linepithema humile, and native ant taxa in northern California

Invasive species, where successful, can devastate native communities. We studied the dynamics of the invasive Argentine ant, Linepithema humile, for 7 years in Jasper Ridge, a biological preserve in northern California. We monitored the distributions at the hectare scale of native ant taxa and L. humile in the spring and fall from 1993 to 1999. We also studied the invasion dynamics at a finer resolution by searching for ants in 1-m² plots. Our results are similar at both scales. The distributions of several native species are not random with regard to L. humile; the distributions of several epigeic species with similar habitat affinities overlap much less frequently than expected with the distribution of L. humile. We found that season had a significant influence on the distributions of L. humile and several native taxa. Over the 7-year period, L. humile has increased its range size in Jasper Ridge largely at the expense of native taxa, but there is seasonal and yearly variation in this rate of increase. Studies of invasions in progress which sample across seasons and years may help to predict the spread and effects of invasive species.     

The relationship between the distribution of worker sizes and new worker production in the ant Formica neorufibarbis

While it is commonly assumed that variation in worker sizes within a single ant colony increases colony efficiency, there is little causal evidence of a link between worker size variation and colony performance. I tested whether the range of worker sizes within colonies of the ant species Formica neorufibarbis affected new worker production. Removing large workers from colonies lowered the rate of new worker production. A study of unmanipulated colonies indicated that colonies did not maintain a full range of worker sizes; mean worker head widths varied from 0.89-1.24 mm. Colonies naturally missing large workers did not have lower rates of worker production, suggesting that the relative size, not the absolute size, of workers within colonies was important. These are the first results to directly link the range of worker sizes to a component of colony fitness in a natural setting.     

PCR primers for polymorphic microsatellite loci in the facultatively polygynous plant‐ant Petalomyrmex phylax (Formicidae)

Fourteen microsatellite loci were isolated and their level of polymorphism characterized in two populations of the facultatively polygynous plant‐ant Petalomyrmex phylax (Formicinae). High levels of within‐population variation were observed at most loci, with number of alleles ranging from two to 15, and heterozygosity from 0.050 to 0.925. Cross‐species amplification of these loci was also tested in four plant‐ant species belonging to three other genera, Aphomomyrmex, Cladomyrma (both Formicinae) and Cataulacus (Myrmicinae).     

Worker thermal tolerance in the thermophilic ant Cataglyphis cursor (Hymenoptera, Formicidae)

In the thermophilic ant genus Cataglyphis, species differing in their physical caste system have developed alternative mechanisms to face extreme heat by physiological and/or behavioural adaptations. In this study, we tested whether thermal tolerance is related to worker size in the ant Cataglyphis cursor that presents intermediate worker size compared with previously studied species (size range 3.5–10 mm). Thermal tolerance at two temperatures was tested in the laboratory on colonies originating from two habitats (seaside versus vineyard), known to differ in average worker size. As expected large workers were more resistant to high temperature than small workers, but the effect of worker size on thermal resistance was less pronounced under the more extreme temperatures. The pattern of thermal tolerance was similar in the two habitat types. After controlling for worker size, worker thermal tolerance significantly varied amongst colonies, but this variation was not related to colony size. Our results suggest that a higher thermal tolerance can confer an advantage to larger workers especially during foraging and are discussed in the context of the evolution of worker size in ants.     

The comparative reproductive styles of two closely related African minnows (Pseadobarbus afer andP. asper) inhabiting two different sections of the Gamtoos River system

Synopsis Two species of a monophyletic lineage of flexible-rayed redfin minnows,Pseudobarbus afer andP. asper, were studied to establish if there were any significant differences in their reproductive styles. They are sister species with few morphological or meristic differences.P. afer andP. asper are open substrate benthic spawners on coarse bottoms (rocks) and non-guarders of non-adhesive eggs. Their young are photophobic as free embryos. Riverine spawning sites indicated a conservative tendency and represented a phylogenetic constraint as compared to the more variable attributes, such as egg size, which were under environmental control within the limits expressed by the genotype. The combination of life-history attributes, gonadosomatic index, fecundity, egg size, investment per clutch, number of clutches per season and reproductive lifespan was found to be different forP. afer andP. asper. P. asper is derived and atypical of otherPseudobarbus species studied to date. Differences betweenP. afer andP. asper are directly related to the two distinct environments inhabited by these species-coastal Cape Fold Belt mountain streams and the inland Karoo streams of the Gamtoos River system.P. asper may have reverted, by juvenilization, to a more altricial form to survive the turbid, intermittent Karoo stream.