Body size but not colony size increases with altitude in the holarctic ant,Leptothorax acervorum

1. Bergmann's rule states that organisms inhabiting colder environments show an increase in body size or mass in comparison to their conspecifics living in warmer climates. Although originally proposed for homoeothermic vertebrates, this rule was later extended to ectotherms. In social insects, only a few studies have tested this rule and the results were ambiguous. Here, ‘body size’ can be considered at two different levels (the size of the individual workers or the size of the colony). 2. In this study, data from 53 nests collected along altitudinal gradients in the Alps were used to test the hypotheses that the worker body size and colony size of the ant Leptothorax acervorum increase with increasing altitude and therefore follow Bergmann's rule. 3. The results show that the body size of workers but not the colony size increases with altitude. Whether this pattern is driven by starvation resistance or other mechanisms remains to be investigated.     

Colony structure and reproduction in the ant, Leptothorax acervorum

We analyzed the sociogenetic organization of the ant (Leptothoraxacervorum) from Nrnberger Reichswald in Southern Germany. Accordingto relatedness estimates from allozyme analyses, virgin femalesexuals produced in polygynous colonies were on average fullsisters, whereas workers in a pooled sample of polygynous colonieswere significantly less closely related. Rather than attributingthis to reproductive hierarchies among nest mate queens, weshow how this phenomenon could result from seasonal fluctuationsof colony composition and a decline of the production of femalesexuals in polygynous colonies. We suggest that by queen adoptionand emigration or budding, colonies easily switch from monogynyto polygyny and vice versa. Due to the long developmental timeof sexual larvae, colonies that have become polygynous onlyrecently will still produce the female sexual progeny of a singlequeen. In older polygynous nests, fewer and fewer female sexualsare produced, but colonies may fragment into monogynous budsin which the production of female sexuals may begin again. Relatednessestimates, dissection results, and field observations supportthis suggestion. This pattern of cyclical monogyny and polygynykeeps nest mate relatedness high and probably facilitates colonyfounding in boreal habitats. Preliminary data suggest that thepattern of the production of sexuals in colonies of L. acervorumfits the expectations of sex allocation theory.     

Mating biology and population structure of the ant, <Emphasis Type="Italic">Leptothorax gredleri</Emphasis>

Summary Female sexuals of the ant Leptothorax gredleri attract males by sexual calling. In an experimental set-up allowing for competition among males, both female and male sexuals copulated with up to four partners, with the median being one mate in both sexes. Neither male nor female sexuals invariably mated with the first partner they encountered, but we could not find any morphological difference between sexuals that succeeded in mating multiply and those that copulated only once. Males did not aggressively compete for access to the female sexuals. According to microsatellite genotyping, workers produced by multiply mated queens were all offspring of a single father, i.e. queens appear to use sperm from a single mate to fertilize their eggs. Population genetic studies revealed a strong population subdivision, suggesting that both male and female sexuals mate in the vicinity of their maternal nests and that gene flow is strongly restricted even between forest patches isolated only by a few meters of grassland.     

Ecology, life history and resource allocation in the ant, Leptothorax nylanderi

We aimed at identifying the causal basis of previously shown interrelations between demographic and genetic colony structure, ecological factors and split sex ratios in the ant, Leptothorax nylanderi. Colony-level variation in sex allocation was only partly explained by annual fluctuations during eight study years and by resource availability as indicated by sexual production of colonies. Allocation ratios were highly male-biased in dense populations with ephemeral nest sites and high frequencies of colonies containing several unrelated matrilines. Field observations and experimental manipulations showed that nest site limitation leads to such heterogeneous colonies. Laboratory experiments demonstrated that genetic heterogeneity directly causes male-biased investment, although relatedness asymmetry is not influenced by invasion of unrelated queens. The influence of genetic composition on allocation strategies might either be explained by negative feedback mechanisms connected with habitat saturation or by a lower efficiency of heterogeneous colonies. Our results thus demonstrate which factors other than variation in relatedness asymmetry can explain split sex ratios in ants. An empirical test of a model on reproductive allocation revealed on-going queen-worker conflict over colony growth and sexual reproduction. Workers controlled reproductive allocation, but queen-worker conflict ceased in large colonies with a high survival rate.     

Density‐dependent offspring interactions do not explain macroevolutionary scaling of adult size and offspring size

Most life forms exhibit a correlated evolution of adult size (AS) and size at independence (SI), giving rise to AS–SI scaling relationships. Theory suggests that scaling arises because relatively large adults have relatively high reproductive output, resulting in strong density‐dependent competition in early life, where large size at independence provides a competitive advantage to juveniles. The primary goal of our study is to test this density hypothesis, using large datasets that span the vertebrate tree of life (fishes, amphibians, reptiles, birds, and mammals). Our secondary goal is to motivate new hypotheses for AS–SI scaling by exploring how subtle variation in life‐histories among closely related species is associated with variation in scaling. Our phylogenetically informed comparisons do not support the density hypothesis. Instead, exploration of AS–SI scaling among life‐history variants suggests that steeper AS–SI scaling slopes are associated with evolutionary increases in size at independence. We suggest that a positive association between size at independence and juvenile growth rate may represent an important mechanism underlying AS–SI scaling, a mechanism that has been underappreciated by theorists. If faster juvenile growth is a consequence of evolutionary increases in size at independence, this may help offset the cost of delayed maturation, leading to steeper AS–SI scaling slopes.     

Effect of temperature and social environment on worker size in the ant Temnothorax nylanderi

Warm temperatures decrease insect developmental time and body size. Social life could buffer external environmental variations, especially in large social groups, either through behavioral regulation and compensation or through specific nest architecture. Mean worker size and distribution of worker sizes within colonies are important parameters affecting colony productivity as worker size is linked to division of labor in insect societies. In this paper, we investigate the effect of stressful warm temperatures and the role of social environment (colony size and size of nestmate workers) on the mean size and size variation of laboratory-born workers in the small European ant Temnothorax nylanderi. To do so, we reared field-collected colonies under medium or warm temperature treatments after having marked the field-born workers and removed the brood except for 30 first instar larvae. Warm temperature resulted in the production of fewer workers and a higher adult mortality, confirming that this regime was stressful for the ants. T. nylanderi ants followed the temperature size rule observed in insects, with a decreased developmental time and mean size under warm condition. Social environment appeared to play an important role as we observed that (i) larger colonies buffered the effect of temperature better than smaller ones (ii) colonies with larger workers produced larger workers whatever the rearing temperature and (iii) the coefficient of variation of worker size was similar in the field and under medium laboratory temperature. This suggests that worker size variation is not primarily due to seasonal environmental fluctuations in the field. Finally, we observed a higher coefficient of variation of worker size under warm temperature. We propose that this results from a disruption of social regulation, i.e. the control of nestmate workers over developing larvae and adult worker size, under stressful conditions.     

Evolution of the optimal reproductive schedule in the ant Camponotus (Colobopsis) nipponicus (wheeler): a demographic approach

1. Traits are hypothesised to optimise via natural selection. The schedule of reproduction is an important adaptive trait, but its evolution is difficult to study, as measuring parameters is usually difficult. However, the sufficient amounts of demographic data enable us to estimate these parameters. 2. Here, it is shown that the reproductive schedule of the ant Camponotus (Colobopsis) nipponicus is tuned to maximise the lifetime production of alates. 3. A colony started its reproduction 4 years after the colony founding, at which time they were far smaller than well‐developed colonies. This contradicts the prediction of the bang‐bang strategy theory. The size distribution of colonies in the study area showed that the mortality of small colonies is much higher than that of large colonies. 4. A simulation analysis suggests that the colonies that are smaller than the threshold can still achieve significant improvement in colny survival to the following year by investing all resources in colony growth instead of reproduction. A sensitivity analysis for the starting year of reproduction showed that the observed schedule maximises lifetime alate production. The demographic data suggest a stable population, which is required for optimisation through this maximisation. 5. The observed reproductive schedule must be optimised, and the breakdown of the bang‐bang theory is due to higher mortalities during the incipient stage of colonies. This study demonstrates that having enough demographic data creates a useful tool for studying the evolution of life‐history characteristics.     

The evolution of large brain size in birds is related to social,not genetic,monogamy

There are several hypotheses suggesting that social complexity, including pair bonding, is important in the evolution of increased brain size. I examined whether genetic or social monogamy was related to large brain size in birds. Recent work has indicated that the length and strength of pair bonds are associated with large brain size. I tested several hypotheses for the evolution of large brain size in 42 species of bird by including life history variables in a regression model. A test on 100 phylogenetic trees revealed no phylogenetic signal in brain size. Controlling for body size, a principal components analysis was run on the life history variables and degrees of extra‐pair paternity. The main principal component (PC1) was regressed on brain size revealing a strong, positive association. Social, but not genetic, monogamy was positively related to brain size. Large brain size is related to the selective pressures of procuring extra‐pair copulations whilst maintaining a social partnership. However, other life history variables also loaded positively and significantly on brain size. These results indicate that the evolution of large brain size in birds was driven by several important selective pressures. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 668–678.     

The evolution of menopause in cetaceans and humans: the role of demography

Human females stop reproducing long before they die. Among other mammals, only pilot and killer whales exhibit a comparable period of post-reproductive life. The grandmother hypothesis suggests that kin selection can favour post-reproductive survival when older females help their relatives to reproduce. But although there is an evidence that grandmothers can provide such assistance, it is puzzling why menopause should have evolved only among the great apes and toothed whales. We have previously suggested (Cant & Johnstone 2008 Proc. Natl Acad. Sci. USA 105, 5332–5336 (doi:10.1073/pnas.0711911105)) that relatedness asymmetries owing to female-biased dispersal in ancestral humans would have favoured younger females in reproductive competition with older females, predisposing our species to the evolution of menopause. But this argument appears inapplicable to menopausal cetaceans, which exhibit philopatry of both sexes combined with extra-group mating. Here, we derive general formulae for ‘kinship dynamics’, the age-related changes in local relatedness that occur in long-lived social organisms as a consequence of dispersal and mortality. We show that the very different social structures of great apes and menopausal whales both give rise to an increase in local relatedness with female age, favouring late-life helping. Our analysis can therefore help to explain why, of all long-lived, social mammals, it is specifically among the great apes and toothed whales that menopause and post-reproductive helping have evolved.     

Social and genetic structure of a supercolonial weaver ant, <Emphasis Type="Italic">Polyrhachis robsoni</Emphasis>, with dimorphic queens

We studied a population of the Australian weaver ant Polyrachis robsoni with regard to variation in the morphology of its winged queens using six newlydeveloped microsatellite markers. Morphometrically the queens fell clearly into two groups, macrogynes and microgynes, with the latter an isometric reduction of the former. Aggression tests showed that hostility between ants from different nests was minimal. Nests frequently contained numbers of both queen types, with microgynes about twice as numerous as macrogynes. Nestmate workers, microgynes, and macrogynes, were significantly related to others within their caste, with macrogynes more highly related than the other castes. Relatedness values between these groups of nestmates were also significant. Pairwise relatedness values were consistent with both queen morphs producing workers. At the population level, microgynes from different nests were also significantly related and there was a weak inverse relationship between pairwise relatedness value between individuals and distance between nests.We conclude that this species is supercolonial and that the two queen morphs are part of the same population. Received 19 July 2006; revised 16 October 2006; accepted 25 October 2006.     

Effects of social organization and elevation on spatial genetic structure in a montane ant

Studying patterns of population structure across the landscape sheds light on dispersal and demographic processes, which helps to inform conservation decisions. Here, we study how social organization and landscape factors affect spatial patterns of genetic differentiation in an ant species living in mountainous regions. Using genome‐wide SNP markers, we assess population structure in the Alpine silver ant, Formica selysi. This species has two social forms controlled by a supergene. The monogyne form has one queen per colony, while the polygyne form has multiple queens per colony. The two social forms co‐occur in the same populations. For both social forms, we found a strong pattern of isolation‐by‐distance across the Alps. Within regions, genetic differentiation between populations was weaker for the monogyne form than for the polygyne form. We suggest that this pattern is due to higher dispersal and effective population sizes in the monogyne form. In addition, we found stronger isolation‐by‐distance and lower genetic diversity in high elevation populations, compared to lowland populations, suggesting that gene flow between F. selysi populations in the Alps occurs mostly through riparian corridors along lowland valleys. Overall, this survey highlights the need to consider intraspecific polymorphisms when assessing population connectivity and calls for special attention to the conservation of lowland habitats in mountain regions.     

Flight dimorphism is related to survival,reproduction and mating success in the leaf beetle Oreina cacaliae

1. Alternative life histories may be maintained in populations due to variation in the costs and benefits of the underlying strategies. In this study, potential costs of dispersal by flight were investigated as an alternative life‐history strategy in the mountain‐living chrysomelid beetle Oreina cacaliae. 2. In this species, previous mark–recapture studies showed a dispersal dimorphism in both males and females. While a fraction of the population engages in flight in autumn and spring (in the following referred to as ‘flyers’), the other part does not fly (non‐flyers). Flyers emerge earlier than non‐flyers and feed on a spring host plant before the emergence of the main host plant. 3. In this study, the overwintering and dispersal locations were recorded over 7 years in the field, flyers from the spring host plant were collected, and morphology and lifetime reproductive output and survival of collected flyers and non‐flyers were compared. 4. A potential trade‐off between flight and life‐history traits was observed: flyers were smaller in size, lighter in body mass, had a lower lifetime fecundity and a higher mortality. 5. Mating experiments of field‐caught beetles in the laboratory showed that larger beetles had a higher (multiple) mating success, but there was no evidence for size‐assortative mating. It is hypothesized that one reason for small beetles to disperse by flight might be to escape competition for mates with larger non‐flyers. 6. The overwhelming quantity of beetles found on the spring host every year reveals that the flying strategy is successful, despite the costs and risks.     

Genome size in arthropods; different roles of phylogeny,habitat and life history in insects and crustaceans

Despite the major role of genome size for physiology, ecology, and evolution, there is still mixed evidence with regard to proximate and ultimate drivers. The main causes of large genome size are proliferation of noncoding elements and/or duplication events. The relative role and interplay between these proximate causes and the evolutionary patterns shaped by phylogeny, life history traits or environment are largely unknown for the arthropods. Genome size shows a tremendous variability in this group, and it has a major impact on a range of fitness‐related parameters such as growth, metabolism, life history traits, and for many species also body size. In this study, we compared genome size in two major arthropod groups, insects and crustaceans, and related this to phylogenetic patterns and parameters affecting ambient temperature (latitude, depth, or altitude), insect developmental mode, as well as crustacean body size and habitat, for species where data were available. For the insects, the genome size is clearly phylogeny‐dependent, reflecting primarily their life history and mode of development, while for crustaceans there was a weaker association between genome size and phylogeny, suggesting life cycle strategies and habitat as more important determinants. Maximum observed latitude and depth, and their combined effect, showed positive, and possibly phylogenetic independent, correlations with genome size for crustaceans. This study illustrate the striking difference in genome sizes both between and within these two major groups of arthropods, and that while living in the cold with low developmental rates may promote large genomes in marine crustaceans, there is a multitude of proximate and ultimate drivers of genome size.     

Life history evolution in cichlids 2: directional evolution of the trade-off between egg number and egg size

The negative relationship between offspring number and offspring size provides a classic example of the role of trade-offs in life history theory. However, the evolutionary transitions in egg size and clutch size that have produced this negative relationship are still largely unknown. Since body size may affect both of these traits, it would be helpful to understand how evolutionary changes in body size may have facilitated or constrained shifts in clutch and egg size. By using comparative methods with a database of life histories and a phylogeny of 222 genera of cichlid fishes, we investigated the order of evolutionary transitions in these traits in relation to each other. We found that the ancestral large-bodied cichlids first increased egg size, followed by a decrease in both body size and clutch size resulting in the common current combination of a small-bodied cichlid with a small clutch of large eggs. Furthermore, lineages that deviated from the negative relationship between clutch and egg size underwent different transitions in these traits according to their body size (large bodied genera have moved towards the large clutch/small egg end of the continuum and small bodied genera towards the small clutch/large egg end of the continuum) to reach the negative relationship between clutch size and egg size. Our results show that body size is highly important in shaping the negative relationship between clutch size and egg size.     

Body size changes in ground beetle assemblages – a reanalysis of Braun et al. (2004)'s data

Abstract.  1. Data in Braun et al .'s (2004) recent paper on size trends in ground beetles (Carabidae) are re-analysed, after re-classifying the species according to feeding categories.
2. When ground beetles are classified as predatory species, mixed feeders or herbivores, and evaluated separately, the size trends indicate group-specific differences. Most of these do not support the efficiency-specialisation hypothesis.
3. Taxonomic groups rarely fit into one ecological category and ecological analyses should explicitly consider this hurdle.     

Egg shape and size allometry in geckos (Squamata: Gekkota), lizards with contrasting eggshell structure: why lay spherical eggs?

Hard, highly calcified eggshells evolved several independent times during the history of amniotes. Because of phylogenetic conservatism of this trait, lineages in which closely related taxa differ in eggshell structure are rare. Four gekkotan families (Carphodactylidae, Diplodactylidae, Eublepharidae and Pygopodidae) have eggs with soft shells, while their close relatives (Gekkonidae) lay eggs with hard shells. Geckos thus offer a rare opportunity to compare the impact of the emergence of a hard eggshell on the economy of egg architecture. Because a sphere has the smallest surface area of all three‐dimensional solids of a given volume, spherical eggs in geckos with hard eggshells reduce calcium investment and should therefore be advantageous. Here, we document that hard‐shelled gekkonid eggs are indeed more spherical than those of the other gecko lineages. However, within gekkonids, small species lay more elongated eggs than larger species. We speculate that miniature gekkonid females, which lay larger eggs relative to body size compared with large gekkonids, produce elongate eggs in order to pass the egg through a limited pelvic opening.     

Of chickens and eggs: diverging propagule size of iteroparous and semelparous organisms

Interactive effects of two or more life-history traits on fitness have the potential to create suites of coadapted traits. Propagule (egg or seed) size is one such trait that is believed to have undergone coadaptation with other traits. Phylogenetic analyses of salmonid fishes have revealed an association between large eggs and semelparity, leading to the question of which came first. It has been hypothesized that an increased egg size would have increased juvenile relative to adult survival, favoring a subsequent increase in reproductive effort and eventually semelparity. Others have suggested that this is insufficient to cause a shift in parity, implying to the contrary that semelparity gave rise to larger eggs. In a previous study we showed that environmental unpredictability might select for production of larger propagules. Here we use simulations to directly model how propagule size evolves in response to environmental unpredictability with varying degrees of iteroparity. Our results demonstrate that environmental unpredictability causes pronounced propagule size divergence between iteroparous and purely semelparous species in taxa with a fixed age at maturity (e.g., pure annual species). However, even rare incidents of repeat breeding are sufficient to reduce selection for larger propagules substantially and thus divergence. Furthermore, introducing variation in age at maturity within propagule size genotypes has evolutionary effects similar to that of repeat breeding. Environmental unpredictability is thus unlikely to provide a general alternative explanation for the observed egg size divergence between iteroparous and semelparous salmonids.     

The evolution of dispersal polymorphisms in insects: The influence of habitats,host plants and mates

Wing-dimorphic, delphacid planthoppers were used to test hypotheses concerning the effects of habitat persistence and architectural complexity on the occurrence of dispersal. For reasons concerning both the durational stability of the habitat and the reduced availability of mates, selection has favored high levels of dispersal in species occupying temporary habitats. Flightlessness predominates in species occupying persistent habitats, and is promoted by a phenotypic trade-off between reproductive success and flight capability. Wings are retained in tree-inhabiting species, probably for reasons concerning the more effective negotiation of three-dimensional habitats. In contrast, flightlessness is characteristic of those species inhabiting low profile host plants. For several delphacid genera, migratory species are larger than their sedentary congeners. Because body size and fecundity are positively related in planthoppers, the large body size observed in migratory taxa may result from selection for increased fecundity in colonizing species.     

Genetic consequences of polygyny and social structure in an Indian fruit bat, Cynopterus sphinx. II. Variance in male mating success and effective population size

Variance in reproductive success is a primary determinant of genetically effective population size (Ne), and thus has important implications for the role of genetic drift in the evolutionary dynamics of animal taxa characterized by polygynous mating systems. Here we report the results of a study designed to test the hypothesis that polygynous mating results in significantly reduced Ne in an age-structured population. This hypothesis was tested in a natural population of a harem-forming fruit bat, Cynopterus sphinx (Chiroptera: Pteropodidae), in western India. The influence of the mating system on the ratio of variance Ne to adult census number (N) was assessed using a mathematical model designed for age-structured populations that incorporated demographic and genetic data. Male mating success was assessed by means of direct and indirect paternity analysis using 10-locus microsatellite genotypes of adults and progeny from two consecutive breeding periods (n = 431 individually marked bats). Combined results from both analyses were used to infer the effective number of male parents in each breeding period. The relative proportion of successfully reproducing males and the size distribution of paternal sibships comprising each offspring cohort revealed an extremely high within-season variance in male mating success (up to 9.2 times higher than Poisson expectation). The resultant estimate of Ne/N for the C. sphinx study population was 0.42. As a result of polygynous mating, the predicted rate of drift (1/2Ne per generation) was 17.6% higher than expected from a Poisson distribution of male mating success. However, the estimated Ne/N was well within the 0.25-0.75 range expected for age-structured populations under normal demographic conditions. The life-history schedule of C. sphinx is characterized by a disproportionately short sexual maturation period scaled to adult life span. Consequently, the influence of polygynous mating on Ne/N is mitigated by the extensive overlap of generations. In C. sphinx, turnover of breeding males between seasons ensures a broader sampling of the adult male gamete pool than expected from the variance in mating success within a single breeding period.     

Sex‐specific phenotypic plasticity in response to the trade‐off between developmental time and body size supports the dimorphic niche hypothesis

Female‐biased sexual size dimorphism (SSD) is widespread in many invertebrate taxa. One hypothesis for the evolution of SSD is the dimorphic niche hypothesis, which states that SSD evolved in response to the different sexual reproductive roles. While females benefit from a larger body size by producing more or larger eggs, males benefit from a faster development, which allows them to fertilize virgin females (protandry). To test this hypothesis, we studied the influence of temperature and intraspecific density on the development of Chorthippus montanus. We reared them in monosexual groups under different conditions and measured adult body size, wing length, nymphal mortality, and development time. The present study revealed an inverse temperature–size relationship: body size increased with increasing temperature in both sexes. Furthermore, we found intersexual differences in the phenotypic response to population density, supporting the dimorphic niches hypothesis. At a lower temperature, female development time increased and male body size decreased with increasing density. Because there was no food limitation, we conclude that interference competition hampered development. By contrast to expectations, mortality decreased with increasing density, suggesting that interference did not negatively affect survival. The present study shows that sex‐specific niche optima may be a major trigger of sexual dimorphisms. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115 , 48–57.