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.     

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.     

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.     

Ontogeny of worker body size distribution in bumble bee (Bombus impatiens) colonies

1. Bumble bees exhibit worker size polymorphisms; highly related workers within a colony may vary up to 10‐fold in body mass. As size variation is an important life history feature in bumble bees, the distribution of body sizes within the colony and how it fluctuates over the colony cycle were analysed. 2. Ten commercially purchased colonies of Bombus impatiens (Cresson) were reared in ad libitum conditions. The size of all workers present and newly emerging workers (callows) was recorded each week. 3. The average size of bumble bee workers did not change with colony age, but variation in body size tended to decrease over time. The average size of callows did not change with population size, but did tend to decrease with colony age. In all measures, there was considerable variation among colonies. 4. Colonies of B. impatiens usually produced workers with normally distributed body sizes throughout the colony life cycle. Unlike most polymorphic ants, there was no increase in worker body size with colony age or colony size. This provides the first, quantitative data on the ontogeny of bumble bee worker size distribution. The potential adaptive significance of this size variation is discussed.     

Colony size predicts division of labour in attine ants

Division of labour is central to the ecological success of eusocial insects, yet the evolutionary factors driving increases in complexity in division of labour are little known. The size–complexity hypothesis proposes that, as larger colonies evolve, both non-reproductive and reproductive division of labour become more complex as workers and queens act to maximize inclusive fitness. Using a statistically robust phylogenetic comparative analysis of social and environmental traits of species within the ant tribe Attini, we show that colony size is positively related to both non-reproductive (worker size variation) and reproductive (queen–worker dimorphism) division of labour. The results also suggested that colony size acts on non-reproductive and reproductive division of labour in different ways. Environmental factors, including measures of variation in temperature and precipitation, had no significant effects on any division of labour measure or colony size. Overall, these results support the size–complexity hypothesis for the evolution of social complexity and division of labour in eusocial insects. Determining the evolutionary drivers of colony size may help contribute to our understanding of the evolution of social complexity.     

Genetic and morphological variation over space and time in the invasive fire ant <Emphasis Type="Italic">Solenopsis invicta</Emphasis>

Social insects are among the most successful and damaging of invasive taxa. We studied spatial and temporal variation in two traits, colony genetic structure and worker mass, associated with social insect success in the introduced fire ant Solenopsis invicta. Our aim was to determine if changes in social structure occurred over time and if variation in worker size was related to worker genotype. We sampled 1139 workers from five multiple-queen S. invicta nests on six dates over a one-year period. The genotypes of workers were determined at ten microsatellite loci and at the selected locus general protein-9 (Gp-9). We found little evidence for genetic differentiation of workers sampled from distinct nests or from different dates at the microsatellite loci. However, worker Gp-9 genotype frequencies varied among nests and over time. In addition, worker mass was affected by nest-of-origin, sampling date, ploidy level, and Gp-9 genotype. Our results suggest that large numbers of queens contribute to the production of workers in introduced S. invicta nests throughout the year. Colony boundaries are semi-permeable, although the among-nest variation in Gp-9 genotype frequencies and worker mass does suggest that boundaries are present. In addition, selection operating on Gp-9 genotype depends on nest environment. Finally, worker mass is affected by both endogenous and exogenous factors in S. invicta. Overall, our data suggests that the key traits of colony social structure and worker size reflect the effects of variable selection in invasive social insects.     

The influence of intraspecific competition on resource allocation during dependent colony foundation in a social insect

Organisms face a trade-off between investment in fewer, larger offspring, or more, smaller offspring. Most organisms can adjust investment through variation in the size and number of offspring in response to factors such as resource availability and competition. In some social animals, established colonies divide into groups of individuals that become autonomous, a process known as colony fission (also dependent colony foundation in social insects). Resource allocation under fission can be fine-tuned by adjusting the number of new groups (offspring number) and the number of individuals in each new group (offspring size). We assessed the influence of competition on resource allocation during fission in the ant Cataglyphis cursor, by allowing colonies to fission in experimental enclosures of high or low conspecific colony density. The pattern of colony fission was similar to that observed in the field: each fissioning colony produced a few new nests comprising a highly variable number of workers and a single queen, the old queen was often replaced, and new queens were produced in excess. The number of new nests produced depended on the available workforce in the parent colony but was not affected by differences in colony density. Comparison with data from fission under natural field conditions, however, indicates that colonies in enclosures produced fewer, larger new nests, suggesting that resource investment patterns during fission are indeed subject to extrinsic factors. The density of conspecific colonies in the immediate surroundings may be an unreliable estimate of competition intensity and other factors should be considered.     

红火蚁工蚁受侵扰的行为反应强度与蚁巢大小和侵扰强度之间的关系研究

红火蚁Solenopsis invicta Buren的防御行为是其被认定为社会性昆虫的关键特征之一。红火蚁蚁巢受到侵扰时,工蚁会快速涌出搜寻侵扰物并展开攻击。然而,蚁巢受侵扰后工蚁的行为反应与蚁巢大小和侵扰强度之间的关系有待进一步明确。为此,本研究通过野外视频记录并结合室内统计分析的方法来明确不同大小的红火蚁蚁巢受侵扰后工蚁反侵扰行为的动态过程。结果表明,蚁巢大小和受侵扰强度对蚁巢受侵扰后工蚁的反应速度具有显著影响。同等侵扰强度下,与中、大型蚁巢相比,小型蚁巢的工蚁往往表现出更慢的行为反应。尤其在低侵扰强度时,小蚁巢工蚁的反应时间平均0.8 s,而中蚁巢和大蚁巢均仅为0.34 s。当侵扰强度相同时,蚁巢越大,受侵扰后召集工蚁的数量越多;当蚁巢大小相同时,侵扰强度越高,召集工蚁的数量越多。但无论是蚁巢大小还是侵扰强度的不同,红火蚁蚁巢受侵扰后,召集工蚁的数量到达峰值的时间均为30 s左右。研究结果为进一步了解红火蚁的防御行为,科学预防红火蚁的攻击,以及有效开展红火蚁的防治提供了重要的实验依据。     

Body size variation and caste ratios in geographically distinct populations of the invasive big‐headed ant,Pheidole megacephala (Hymenoptera: Formicidae)

Body size is an important life history trait that can evolve rapidly as a result of how species interact with each other and their environment. Invasive species often encounter vastly different ecological conditions throughout their introduced range that can influence relative investment in growth, reproduction and defence among populations. In this study, we quantified variation in worker size, morphology and proportion of majors among five populations of a worldwide invasive species, the big‐headed ant, Pheidole megacephala (Fabricius). The sampled populations differed in ant community composition, allowing us to examine if P. megacephala invests differently in the size and number of majors based on the local ant fauna. We also used genetic data to determine if these populations of P. megacephala represented cryptic species or if morphological differences could be attributed to change following introduction. We found significant variation in worker mass among the populations. Both major and minor workers were largest in Australia, where the ant fauna was most diverse, and minor workers were smallest in Hawaii and Mauritius, where P. megacephala interacted with few to no other ants. We also found differences in major and minor worker morphology among populations. Majors from Mauritius had significantly larger heads (width and length) relative to whole body size than those from Hawaii and Florida. Minors had longer heads and hind tibias in South Africa compared with populations from Australia, Hawaii and Florida. The proportion of majors did not differ among populations, suggesting that these populations may not be subject to trade‐offs in investment in major size versus number. Our molecular data place all samples within the same clade, supporting that these morphologically different populations represent the same species. These results suggest that the variation in shape and morphology of major and minor workers may therefore be the result of rapid adaptation or plastic responses to local conditions. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 423–438.     

Habitat-specific clutch size and cost of incubation in eiders reconsidered

The energetic incubation constraint hypothesis (EICH) for clutch size states that birds breeding in poor habitat may free up resources for future reproduction by laying a smaller clutch. The eider (Somateria mollissima) is considered a candidate for supporting this hypothesis. Clutch size is smaller in exposed nests, presumably because of faster heat loss and higher incubation cost, and, hence, smaller optimal clutch size. However, an alternative explanation is partial predation: the first egg(s) are left unattended and vulnerable to predation, which may disproportionately affect exposed nests, so clutch size may be underestimated. We experimentally investigated whether predation on first-laid eggs in eiders depends on nest cover. We then re-evaluated how nesting habitat affects clutch size and incubation costs based on long-term data, accounting for confounding effects between habitat and individual quality. We also experimentally assessed adult survival costs of nesting in sheltered nests. The risk of egg predation in experimental nests decreased with cover. Confounding between individual and habitat quality is unlikely, as clutch size was also smaller in open nests within individuals, and early and late breeders had similar nest cover characteristics. A trade-off between clutch and female safety may explain nest cover variation, as the risk of female capture by us, mimicking predation on adults, increased with nest cover. Nest habitat had no effect on female hatching weight or weight loss, while lower temperature during incubation had an unanticipated positive relationship with hatching weight. There were no indications of elevated costs of incubating larger clutches, while clutch size and colony size were positively correlated, a pattern not predicted by the ‘energetic incubation constraint’ hypothesis. Differential partial clutch predation thus offers the more parsimonious explanation for clutch size variation among habitats in eiders, highlighting the need for caution when analysing fecundity and associated life-history parameters when habitat-specific rates of clutch predation occur.     

The ant Lasius niger shows no relationship between task efficiency and body size variation among workers

In ants, workers of different sizes may perform various tasks, even in so-called monomorphic species with relatively low body size variation. However, it is unclear if the body size diversity of monomorphic workers correlates with task efficiency, especially in stressful contingencies. Here we tested if the body size variation of workers corresponds with its efficiency in transferring pupae. Transferring brood is a pre-set behavioral response to stress, e.g. suboptimal temperature. Here we applied a laboratory experiment simulating nest damage. The study was performed on the common garden ant (Lasius niger (Linnaeus, 1758)) – a species with no distinct worker subcastes. The efficiency of workers was measured as the latency of transferring pupae from a lit part of the experimental colony to a darkened part, while the body size diversity was expressed as the within-colony coefficient of variation in head width. We did not find any significant correlation between efficiency and body size variation. Summarizing the existing studies and the present results, we propose the hypothesis that the body size diversity of L. niger may have implications for workers’ division of labor but not for their task efficiency in a stressful contingency.     

The significance of latitudinal variation in body size in a holarctic ant, Leptothorax acervorum

The mean body size of workers of the holarctic ant Leptothorax acervorum increases with latitude. Workers from populations near the Polar Circle were 10% larger than workers from central Europe. This gradient does not appear to be associated with variation in colony size. According to controlled rearing experiments with brood from populations in Cape Kartesh, Karelia (67°N) and Erlangen, Germany (49.7°N), larger adult body size in boreal populations is not an epiphenomenon of slow cell growth and larger cell size at lower temperatures. Larger workers survived longer without food both at room temperature and <0°C, suggesting that selection for increased fasting endurance in boreal habitats might lead to this Bergmann's rule-like pattern in an ectothermic ant.     

Testing the importance of the distribution of worker sizes to colony performance in the ant species <Emphasis Type="Italic">Formica obscuripes</Emphasis> Forel

We examined the relationship between colony performance and the distribution of worker sizes within colonies in the ant species, Formica obscuripes. We manipulated the distribution of worker sizes within colonies and found that experimental colonies whose distributions mimicked the natural distributions retained a larger percentage of colony biomass over three weeks when fed on honeydew, relative to colonies composed of only large or only small workers. In natural colonies most of the variation in worker sizes was found within, as opposed, to between colonies, suggesting that homeostatic mechanisms within colonies regulated the distribution of worker sizes. While there were no disjunctions in the distribution of worker sizes within colonies, the distribution tended to be bimodal. This study demonstrates that the distribution of worker sizes within colonies is important even for ant species that lack discrete worker castes. Received 31 October 2006; revised 26 December 2006; accepted 4 January 2007.     

The determinants of queen size in a socially polymorphic ant

In social animals, body size can be shaped by multiple factors, such as direct genetic effects, maternal effects, or the social environment. In ants, the body size of queens correlates with the social structure of the colony: colonies headed by a single queen (monogyne) generally produce larger queens that are able to found colonies independently, whereas colonies headed by multiple queens (polygyne) tend to produce smaller queens that stay in their natal colony or disperse with workers. We performed a cross‐fostering experiment to investigate the proximate causes of queen size variation in the socially polymorphic ant Formica selysi. As expected if genetic or maternal effects influence queen size, eggs originating from monogyne colonies developed into larger queens than eggs collected from polygyne colonies, be they raised by monogyne or polygyne workers. In contrast, eggs sampled in monogyne colonies were smaller than eggs sampled in polygyne colonies. Hence, eggs from monogyne colonies are smaller but develop into larger queens than eggs from polygyne colonies, independently of the social structure of the workers caring for the brood. These results demonstrate that a genetic polymorphism or maternal effect transmitted to the eggs influences queen size, which probably affects the social structure of new colonies.     

Colony size affects collective decision-making in the ant Temnothorax albipennis

Social insects are well-known for their ability to achieve robust collective behaviours even when individuals have limited information. It is often assumed that such behaviours rely on very large group sizes, but many insect colonies start out with only a few workers. Here we investigate the influence of colony size on collective decision-making in the house-hunting of the ant Temnothorax albipennis. In experiments where colony size was manipulated by splitting colonies, we show that worker number has an influence on the speed with which colonies discover new nest sites, but not on the time needed to make a decision (achieve a quorum threshold) or total emigration time. This occurred because split colonies adopted a lower quorum threshold, in fact they adopted the same threshold in proportion to their size as full-size colonies. This indicates that ants may be measuring relative quorum, i.e. population in the new nest relative to that of the old nest, rather than the absolute number. Experimentally reduced colonies also seemed to gain more from experience through repeated emigrations, as they could then reduce nest discovery times to those of larger colonies. In colonies of different sizes collected from the field, total emigration time was also not correlated with colony size. However, quorum threshold was not correlated with colony size, meaning that individuals in larger colonies adopted relatively lower quorum thresholds. Since this is a different result to that from size-manipulated colonies, it strongly suggests that the differences between natural small and large colonies were not caused by worker number alone. Individual ants may have adjusted their behaviour to their colony’s size, or other factors may correlate with colony size in the field. Our study thus shows the importance of experimentally manipulating colony size if the effect of worker number on the emergence of collective behaviour is to be studied. Received 13 December 2005; revised 9 May 2006; accepted 15 May 2006.     

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.     

Colony size explains the lifespan differences between queens and workers in eusocial Hymenoptera

Eusocial Hymenoptera show a unique divergence in lifespan of queens and workers; queens belong to the longest lived insects while workers in most eusocial species have significantly shorter lives. The different phenotypes within a colony emerge through reproductive division of labour, which is a characteristic trait of eusocial animals. Division of labour as a measure of organismal complexity increases with colony size in eusocial species similar to the increase of complexity with size that has been shown for the whole range of living organisms. We show that queen and worker lifespan diverge in closely related species representing the transition from solitary to social life and show that queen and worker lifespan are correlated if colony size is taken into account: with increasing colony size the lifespan differential between queen and worker increases, whereas neither queen nor worker lifespan is associated with colony size. Additionally, the lifespan differential is better explained by colony size than by the weight differences between the castes. The divergence of phenotypes found is in line with the increasing specialization of subunits in larger organisms, which leads to increasing complexity. We argue that division of labour is acting to increase colony efficiency, which in turn shapes the investments made into individuals leading to short‐lived workers and long‐lived queens. Additionally, maintenance investments may be shaped due to the variable extrinsic risk faced by different castes. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 710–724.     

The evolution of worker caste diversity in social insects

Morphological diversification of workers is predicted to improve the division of labor within social insect colonies, yet many species have monomorphic workers. Individual-level selection on the reproductive capacities of workers may counter colony-level selection for diversification, and life-history differences between species (timing of caste determination, colony size, genetic variation available) may mediate the strength of this selection. We tested this through phylogenetically independent contrast analyses on a new data set for 35 ant species. Evidence was found that early divergence of queen-worker developmental pathways may facilitate the evolution of worker diversity because queen-worker dimorphism was strongly positively associated with diversity. By contrast, risks for colonies that invest in specialized workers and colony size effects on costs of worker reproduction seem unlikely to strongly affect the evolution of worker diversity because there was no significant association between colony size and diversity when controlling statistically for queen-worker dimorphism. Finally, worker diversity was greater in species with multiple lineages per colony, and it was negatively associated with relatedness in monogynous species. This could be due to high intracolonial genetic variance favoring the expression and evolution of great worker diversity or to diversity evolving more easily when there is selection for repression of worker reproduction (worker policing).     

Queen–worker caste ratio depends on colony size in the pharaoh ant (<Emphasis Type="Italic">Monomorium pharaonis</Emphasis>)

The success of an ant colony depends on the simultaneous presence of reproducing queens and non-reproducing workers in a ratio that will maximize colony growth and reproduction. Despite its presumably crucial role, queen–worker caste ratios (the ratio of adult queens to workers) and the factors affecting this variable remain scarcely studied. Maintaining polygynous pharaoh ant (Monomorium pharaonis) colonies in the laboratory has provided us with the opportunity to experimentally manipulate colony size, one of the key factors that can be expected to affect colony level queen–worker caste ratios and body size of eclosing workers, gynes and males. We found that smaller colonies produced more new queens relative to workers, and that these queens and workers both tended to be larger. However, colony size had no effect on the size of males or on the sex ratio of the individuals reared. Furthermore, for the first time in a social insect, we confirmed the general life history prediction by Smith and Fretwell (Am Nat 108:499–506, 1974) that offspring number varies more than offspring size. Our findings document a high level of plasticity in energy allocation toward female castes and suggest that polygynous species with budding colonies may adaptively adjust caste ratios to ensure rapid growth.     

Nest architecture and colony size of the fungus-growing ant <Emphasis Type="Italic">Mycetophylax simplex</Emphasis> Emery, 1888 (Formicidae,Attini)

Detailed measurements on nest architecture and colony size of the fungus-growing ant Mycetophylax simplex Emery, 1888 (Formicidae, Attini) are reported for the first time, based on excavations of 55 nests from two sites in southern Brazil. All nests were subterranean, with a single entrance hole. Most nests consisted of two chambers, an upper and a lower chamber, but one and three-chamber nests were also found. The chambers were more cone-shaped than rounded, and located at a depth ranging from 4.0 cm to 32.5 cm below the nest entrance. The chamber dimensions generally increased as the depth of the chambers increased, and the lower chamber was mostly wider than the upper one. The fungus garden was always found resting on the chamber floor. The average colony size was 264.1 workers, ranging from 67 to 610 workers. Colonies produced most sexuals during the summer (from December to March) and a few during the winter (July). Direct observations showed that colonies were mostly monogynous, but more than one queen was recorded in two nests, suggesting that polygyny may also occur in this species. Received 30 November 2006; revised 20 April 2007; accepted 23 April 2007.