Clocks for sex: loss of circadian rhythms in ants after mating?

This paper describes experiments on the locomotor activity rhythm of queens of the ant species Camponotus compressus, which were performed to investigate the consequences of mating on circadian clocks. Locomotor activity rhythm of virgin and mated queens was monitored individually under constant conditions of the laboratory. The locomotor activity rhythm of virgin queens entrained to a 24 h (12:12 h) laboratory light/dark (LD) cycle and free-ran under constant dim red light (RR) with a free-running period () of approximately 24 h. The locomotor activity of the mated queens on the other hand was arrhythmic during the period when they were laying eggs, and robust rhythmicity appeared soon after the egg-laying phase was over. The of the locomotor activity rhythm of mated queens was significantly greater than that of virgin queens. These results are contrary to the commonly held belief that the role of circadian clocks in ant queens ceases after mating flights, thus suggesting that circadian clocks of ant queens are adaptively plastic and display activity patterns, perhaps depending on their physiological state and tasks in the colony.     

Precision of a Mammalian Circadian Clock

 This paper reports study of day-to-day instability in the locomotor activity rhythm of the nocturnal field mouse Mus booduga. The free-running period (τ) of this rhythm was estimated in constant darkness in n=347 adult male mice. The "onset" and "offset" of locomotor activity rhythm were used as phase markers of the circadian clock. The precision of the onset of locomotor activity was observed to be a non-linear function of τ, with maximal precision at τ close to 24 h. The precision of the offset of locomotor activity was found to increase with increasing τ. These results suggest that the homeostasis of τ is tighter when τ is close to 24 h. Received: 5 May 1998 / Accepted: 11 February 1999     

Small queens and big-headed workers in a monomorphic ponerine ant

Evolution of caste is a central issue in the biology of social insects. Comparative studies on their morphology so far suggest the following three patterns: (1) a positive correlation between queen–worker size dimorphism and the divergence in reproductive ability between castes, (2) a negative correlation among workers between morphological diversity and reproductive ability, and (3) a positive correlation between queen–worker body shape difference and the diversity in worker morphology. We conducted morphological comparisons between castes in Pachycondyla luteipes, workers of which are monomorphic and lack their reproductive ability. Although the size distribution broadly overlapped, mean head width, head length, and scape length were significantly different between queens and workers. Conversely, in eye length, petiole width, and Weber’s length, the size differences were reversed. The allometries (head length/head width, scape length/head width, and Weber’s length/head width) were also significantly different between queens and workers. Morphological examinations showed that the body shape was different between queens and workers, and the head part of workers was disproportionately larger than that of queens. This pattern of queen–worker dimorphism is novel in ants with monomorphic workers and a clear exception to the last pattern. This study suggests that it is possible that the loss of individual-level selection, the lack of reproductive ability, influences morphological modification in ants.     

Diploid males, diploid sperm production, and triploid females in the ant Tapinoma erraticum

Under complementary sex determination (CSD), females of Hymenoptera arise from diploid, fertilized eggs and males from haploid, unfertilized eggs. Incidentally, fertilized eggs that inherit two identical alleles at the CSD locus will develop into diploid males. Diploid males are usually unviable or sterile. In a few species, however, they produce diploid sperm and father a triploid female progeny. Diploid males have been reported in a number of social Hymenoptera, but the occurrence of triploid females has hardly ever been documented. Here, we report the presence of triploid females, diploid males, and diploid sperm (produced by diploid males and stored in queen spermathecae) in the ant Tapinoma erraticum. Moreover, we show variations in the frequency of triploids among female castes: Triploid females are more frequent among workers than virgin queens; they are absent among mated, reproductive queens. The frequency of triploid workers also varies between populations and between nests within populations.     

Factors inducing the intense burrowing activity of the red-swamp crayfish,<Emphasis Type="Italic"> Procambarus clarkii</Emphasis>, an invasive species

The burrowing activity of the invasive red-swamp crayfish, Procambarus clarkii, was studied along a 25-m-long transect in an irrigation ditch system in Italy. Our objective was to understand the factors inducing this species intense digging, which can result in bank collapse and consequently in severe damage to both agricultural fields and natural ecosystems. Burrow morphology and position, together with their occupancy by crayfish and digging, were recorded once every 6 h for 10 consecutive days. The majority of burrows were simple, although a few had a chimney and were constructed at a farther distance from the water surface than simple burrows. Burrow occupancy and digging, together with their plugged/unplugged status, were constant throughout a 24-h cycle and were not related to any abiotic parameter of the habitat. Crayfish occupied and dug a burrow for a relatively short time (6 h on average). Once abandoned, old burrows were rarely reoccupied and often collapsed, while crayfish excavated new ones. As a result, the overall number of burrows increased. This massive use of banks by P. clarkii seems to be related to soil composition and humidity, which favour crayfish digging but also cause the easy collapse of burrows.     

Degeneration of sperm reservoir and the loss of mating ability in worker ants

Workers never mate in the large majority of ants, and they have usually lost the spermatheca, an organ specialized for long-term storage of sperm. Such ‘non-sexual’ workers are restricted to laying unfertilized eggs that give rise to males, and they cannot compete with the queens for the production of female offspring. In sharp contrast, workers in 200–300 species from phylogenetically basal subfamilies can reproduce sexually (‘gamergates’) because they retain a functional spermatheca like the queens. Importantly, ‘non-sexual’ workers in closely related species have a vestigial spermatheca. In this study, we compared the reservoir epithelium of ‘sexual’ workers to that of congeneric queens and ‘non-sexual’ workers using 21 species of Amblyoponinae, Ponerinae and Ectatomminae. We show that a pronounced thickening of the epithelium near the opening of the sperm duct is strictly associated with sexual reproduction in both castes. This is unlike ‘non-sexual’ workers in which this epithelium is always very thin, with few organelles; but all other structures remain intact. We discuss this evolutionary degeneration of the spermatheca and how it relates to behavioural or physiological modifications linked to mating. Our results help understand the loss of sexual reproduction by ant workers, a critical step in the extreme specialization of their phenotype.     

<Emphasis Type="Italic">Gnamptogenys hartmani</Emphasis> Wheeler (Ponerinae: Ectatommini): an agro-predator of<Emphasis Type="Italic"> Trachymyrmex</Emphasis> and<Emphasis Type="Italic"> Sericomyrmex</Emphasis> fungus-growing ants

The fungus gardens of fungus-growing ants are a potentially valuable resource for exploitation by natural enemies, but few of these antagonistic interactions have been studied. Here we describe key aspects of the behavioral ecology of Gnamptogenys hartmani (Ponerinae: Ectatommini), a specialized agro-predator of Trachymyrmex and Sericomyrmex fungus-growing ants in Panama. Raiding columns of G. hartmani attack and usurp nests with remarkably little effort: a few intruding workers are sufficient to cause panic among the attine ants and make them abscond from the nest. Both G. hartmani larvae and adults consume the fungus and the host brood, after which the colony migrates to a new fungus-growing ant nest discovered by scouting workers. The morphology of the G. hartmani larval mouthparts is similar to that of Gnamptogenys species with a non-fungal diet. However, we suggest that the presence of long spinules on the larval mandibles in the genus Gnamptogenys, comparable to those found in attine larvae, may have pre-adapted G. hartmani to fungus eating. G. hartmani workers do not actively maintain or modify fungus gardens, which makes them less efficient exploiters than Megalomyrmex, the only other agro-predatory ant species known so far.     

Intrinsic worker mortality depends on behavioral caste and the queens’ presence in a social insect

According to the classic life history theory, selection for longevity depends on age-dependant extrinsic mortality and fecundity. In social insects, the common life history trade-off between fecundity and longevity appears to be reversed, as the most fecund individual, the queen, often exceeds workers in lifespan several fold. But does fecundity directly affect intrinsic mortality also in social insect workers? And what is the effect of task on worker mortality? Here, we studied how social environment and behavioral caste affect intrinsic mortality of ant workers. We compared worker survival between queenless and queenright Temnothorax longispinosus nests and demonstrate that workers survive longer under the queens’ absence. Temnothorax ant workers fight over reproduction when the queen is absent and dominant workers lay eggs. Worker fertility might therefore increase lifespan, possibly due to a positive physiological link between fecundity and longevity, or better care for fertile workers. In social insects, division of labor among workers is age-dependant with young workers caring for the brood and old ones going out to forage. We therefore expected nurses to survive longer than foragers, which is what we found. Surprisingly, inactive inside workers showed a lower survival than nurses but comparable to that of foragers. The reduced longevity of inactive workers could be due to them being older than the nurses, or due to a positive effect of activity on lifespan. Overall, our study points to behavioral caste-dependent intrinsic mortality rates and a positive association between fertility and longevity not only in queens but also in ant workers.     

Reclaiming the crown: queen to worker conflict over reproduction in <Emphasis Type="Italic">Aphaenogaster cockerelli</Emphasis>

In many social taxa, reproductively dominant individuals sometimes use aggression to secure and maintain reproductive status. In the social insects, queen aggression towards subordinate individuals or workers has been documented and is predicted to occur only in species with a small colony size and a low level of queen–worker dimorphism. We report queen aggression towards reproductive workers in the ant species Aphaenogaster cockerelli, a species with a relatively large colony size and a high level of reproductive dimorphism. Through analysis of cuticular hydrocarbon profiles, we show that queens are aggressive only to reproductively active workers. Non-reproductive workers treated with a hydrocarbon typical for reproductives are attacked by workers but not by queens, which suggests different ways of recognition. We provide possible explanations of why queen aggression is observed in this species.     

Costs and benefits of subordinate queens in colonies of the ant,<Emphasis Type="Italic"> Leptothorax gredleri</Emphasis>

Queens of some species of social insects form dominance hierarchies in which only the top-ranking individual reproduces. Such unequal partitioning of reproduction can be stable when subordinate queens increase their inclusive fitness indirectly by helping to rear a related dominant queen's offspring. We investigated whether subordinate queens of the ant, Leptothorax gredleri, affect the reproductive success of the dominant queen. In laboratory colonies with two queens, first eggs were laid approximately 20 days later than in colonies with a single queen, presumably due to aggressive competition among nestmate queens. Nevertheless, two-queen colonies produced on average more brood than one-queen colonies, although egg laying was completely or almost completely monopolized by one single queen. The presence of a subordinate queen therefore appears to increase the productivity of the dominant.     

Cold resistance depends on acclimation and behavioral caste in a temperate ant

Adjusting to low temperatures is important for animals living in cold environments. We studied the chill–coma recovery time in temperate ant workers (Temnothorax nylanderi) from colonies collected in autumn and spring in Germany. We experimentally acclimated these ant colonies to cold temperatures followed by warm temperatures. As expected, cold-acclimated workers recovered faster from freezing temperatures, but subsequent heat acclimation did not change the short recovery times observed after cold acclimation. Hence, either heat acclimation improves cold tolerance, possibly as a general response to stress, or at least it does not negate enhanced cold tolerance following cold acclimation. Colonies collected in spring showed similar cold tolerance levels to cold-acclimated colonies in the laboratory. Next, we compared the chill–coma recovery time of different worker castes and found that exterior workers recovered faster than interior workers. This difference may be related to their more frequent exposure to cold, higher activity level, or distinct physiology. Interior workers were also heavier and showed a higher gaster-to-head ratio and thorax ratio compared to exterior workers. An obvious difference between exterior and interior workers is activity level, but we found no link between activity and cold tolerance. This suggests that physiology rather than behavioral differences could cause the increased cold tolerance of exterior workers. Our study reveals the importance of acclimation for cold tolerance under natural and standardized conditions and demonstrates differences in cold tolerance and body dimensions in monomorphic behavioral castes of an ant.     

A unique strategy of host colony exploitation in a parasitic ant: workers of<Emphasis Type="Italic"> Polyrhachis lama</Emphasis> rear their brood in neighbouring host nests

The formicine ant Polyrhachis lama is a social parasite, exploiting its ponerine host ant species Diacamma sp. In most social parasitic associations, the parasitic species are closely related to their host species group, evolving directly from independent ancestors of the host species. However, in the Polyrhachis lama–Diacamma sp. association, the associated species belong to different ant subfamilies. Based on preliminary field surveys, we had presumed that P. lama might have given up its reproductive division of labour, i.e. workers would be able to produce males as well as workers and females parthenogenetically. In this study, this hypothesis was disproved: Polyrhachis lama workers cannot be fertilized and are only able to produce males. In the host–parasite association originating from nests possessing a P. lama queen, workers penetrate surrounding Diacamma sp. nests, carrying brood for rearing within these satellite nests. In this peculiar way, a single P. lama colony is able to exploit several Diacamma sp. colonies simultaneously.     

Opposable spines facilitate fine and gross object manipulation in fire ants

Ants inhabit diverse terrestrial biomes from the Sahara Desert to the Arctic tundra. One factor contributing to the ants’ successful colonization of diverse geographical regions is their ability to manipulate objects when excavating nests, capturing, transporting and rendering prey or grooming, feeding and transporting helpless brood. This paper is the first to report the form and function of opposable spines on the foretarsi of queens and workers used during fine motor and gross motor object manipulation in the fire ant, Solenopsis invicta. In conjunction with their mandibles, queens and workers used their foretarsi to grasp and rotate eggs, push or pull thread-like objects out of their way or push excavated soil pellets behind them for disposal by other workers. Opposable spines were found on the foretarsi of workers from seven of eight other ant species suggesting that they might be a common feature in the Formicidae.     

Winged queens replaced by reproductives smaller than workers in <Emphasis Type="Italic">Mystrium</Emphasis> ants

In ants, winged queens that are specialized for independent colony foundation can be replaced by wingless reproductives better adapted for colony fission. We studied this shift in reproductive strategy by comparing two Mystrium species from Madagascar using morphometry, allometry and dissections. Mystrium rogeri has a single dealate queen in each colony with a larger thorax than workers and similar mandibles that allow these queens to hunt during non-claustral foundation. In contrast, Mystrium ‘red’ lacks winged queens and half of the female adults belong to a wingless ‘intermorph’ caste smaller and allometrically distinct from the workers. Intermorphs have functional ovaries and spermatheca while those of workers are degenerate. Intermorphs care for brood and a few mate and reproduce making them an all-purpose caste that takes charge of both work and reproduction. However, their mandibles are reduced and inappropriate for hunting centipedes, unlike the workers’ mandibles. This together with their small thorax disallow them to perform independent colony foundation, and colonies reproduce by fission. M. rogeri workers have mandibles polymorphic in size and shape, which allow for all tasks from brood care to hunting. In M. ‘red’, colonial investment in reproduction has shifted from producing expensive winged queens to more numerous helpers. M. ‘red’ intermorphs are the first case of reproductives smaller than workers in ants and illustrate their potential to diversify their caste system for better colonial economy.     

Alternative mating behaviors of the queen polymorphic ant <Emphasis Type="Italic">Temnothorax longispinosus</Emphasis>

Mating behaviors of ants fall into two categories: female calling, in which a female alate releases pheromones that attract males, and male swarming, in which large male aggregations attract females. Female calling is common in species with queens that return to their natal nest to found colonies dependently after mating, while male swarming is common in species with queens that disperse to found independently. In some species that display both founding strategies, a queen-size polymorphism has evolved in which dependent-founding queens are smaller than independent-founding queens. Dependent founding is likely difficult if gynes (virgin queens) are mating in distant swarms. Therefore, a queen may adopt one or the other mating strategy based on its size and founding behavior. We investigated mating behaviors in the queen-polymorphic ant, Temnothorax longispinosus. Observations in laboratory mating arenas indicated that small gynes exhibited significantly lower flight activity than large gynes. Both forms mated in male swarms, and neither form exhibited female calling. The reduced flight activity of the small morph may facilitate returning to the natal nest after mating, provided the mating swarm is located nearby. Therefore, alternative colony-founding behaviors may be possible without the evolution of female-calling behavior; however, the reduced flight activity of small morphs may require that mating swarms are not distant from the natal nest.     

Sumo wrestling in ants: major workers fight over male production in<Emphasis Type="Italic"> Acanthomyrmex ferox</Emphasis>

In the myrmicine ant Acanthomyrmex ferox, major workers have the same number of ovarioles as queens, thrice that of minor workers, making them well suited for egg-laying. In the queen's presence, infrequent aggression allows ranking of majors but they lay only unviable trophic eggs. Major workers engage each other, but not the minors, in antennal boxing and spectacular shaking contests, a novel interaction in ants. The absence of reversals allows a clear ranking of major workers. After queen removal, aggression becomes very frequent, but previous ranking is maintained. All majors start laying reproductive eggs although they show a skew in ovary development according to ranking. The dominant major, however, actively patrols the egg-pile and cannibalises eggs laid by subordinates.     

Rapid modification in the olfactory signal of ants following a change in reproductive status

In insect societies, the presence and condition of egg-layers can be assessed with pheromones. Exocrine secretions are expected to vary in time in order to give up-to-date information on an individuals reproductive physiology. In the queenless monogynous ant Streblognathus peetersi, we allowed a previously infertile high-ranking worker to accede to the alpha rank, thus triggering the onset of her oogenesis (15 replicates). We then studied her interactions with an established egg-layer from the same colony after different durations, ranging from 20 h to several days. Even though her eggs are only ready to be laid after 30 days, the new alpha was recognised within 1–2 days. Detection occurred at a distance of a few millimetres, suggesting the involvement of a pheromone with low volatility, such as cuticular hydrocarbons. When the new alpha had differentiated for >48 h, she was attacked by the established egg-layer. In all cases, low-ranking workers eventually immobilised one of the two alphas: the new alpha was the target if she had differentiated only recently, suggesting that police workers select the dominant worker with the less fertile odour. Using the behaviour of ants as our measure, we demonstrate that a dominants olfactory signal changes rapidly with a modification in her social status, and it occurs well before the onset of egg-laying.     

Thelytokous parthenogenesis by queens in the dacetine ant Pyramica membranifera (Hymenoptera: Formicidae)

Thelytokous parthenogenesis in which diploid females are produced from unfertilized eggs, was recently reported for some ant species. Here, we document thelytokous reproduction by queens in the polygynous species Pyramica membranifera. Queens that emerged in the laboratory were kept with or without workers under laboratory conditions. Independent colony founding was successful for a few queens if prey was provided. All artificial colonies, which started with a newly emerged queen and workers produced new workers and some of the colonies also produced female sexuals. Some of the female sexuals shed their wings in the laboratory and started formation of new polygynous colonies. Workers had no ovaries and thus, were obligatorily sterile.     

Lethal fighting between honeybee queens and parasitic workers (<Emphasis Type="Italic">Apis mellifera</Emphasis>)

Pheromonal signals associated with queen and worker policing prevent worker reproduction and have been identified as important factors for establishing harmony in the honeybee (Apis mellifera) colony. However, "anarchic workers", which can evade both mechanisms, have been detected at low frequency in several honeybee populations. Worker bees of the Cape honeybee, Apis mellifera capensis, also show this anarchistic trait but to an extreme degree. They can develop into so called "pseudoqueens", which release a pheromonal bouquet very similar to that of queens. They prime and release very similar reactions in sterile workers to those of true queens (e.g. suppress ovary activation; release retinue behavior). Here we show in an experimental bioassay that lethal fights between these parasitic workers and the queen (similar to queen–queen fights) occur, resulting in the death of either queen or worker. Although it is usually the queen that attacks the parasitic workers and kills many of them, in a few cases the workers succeeded in killing the queen. If this also occurs in a parasitized colony where the queen encounters many parasitic workers, she may eventually be killed in one of the repeated fights she engages in.