Vibrational communication between hitchhikers and foragers in leaf-cutting ants (Atta cephalotes)

Abstract In a foraging column of the leaf-cutting ant Atta cephalotes, minim workers (the smallest worker subcaste) hitchhike on leaf fragments carried by larger workers. It has been demonstrated that they defend leaf carriers against parasitic phorid flies. The present study examines the cues used by the potential hitchhikers to locate leaf carriers. As recently reported, foraging workers stridulate while cutting a leaf fragment, and the stridulatory vibrations serve as closerange recruitment signals. We tested the hypothesis that these plant-borne stridulatory vibrations are used by the potential hitchhikers to locate workers engaged in cutting. Three different lines of evidence support this view. Firstly, the repetition rate of the stridulations produced by foraging workers increases significantly as foragers maneuver the leaf fragment into the carrying position and walk loaded to the nest. This is the moment when hitchhikers usually climb on the leaf. Although the leaf-borne stridulatory vibrations are considerably attenuated when transmitted through the workers' legs, they can nevertheless be detected at short distances by minims. This subcaste is several times more sensitive to substrate-borne vibrations than larger workers. Secondly, when a stridulating and a silent leaf were simultaneously presented at the foraging site, minim workers spent significantly more time on the stridulating than on the silent leaf. Thirdly, hitchhiking was more frequent in leaf carriers which cut fragments out of the stridulating leaf than in those cutting the silent leaf.Abstract In a foraging column of the leaf-cutting ant Atta cephalotes, minim workers (the smallest worker subcaste) hitchhike on leaf fragments carried by larger workers. It has been demonstrated that they defend leaf carriers against parasitic phorid flies. The present study examines the cues used by the potential hitchhikers to locate leaf carriers. As recently reported, foraging workers stridulate while cutting a leaf fragment, and the stridulatory vibrations serve as closerange recruitment signals. We tested the hypothesis that these plant-borne stridulatory vibrations are used by the potential hitchhikers to locate workers engaged in cutting. Three different lines of evidence support this view. Firstly, the repetition rate of the stridulations produced by foraging workers increases significantly as foragers maneuver the leaf fragment into the carrying position and walk loaded to the nest. This is the moment when hitchhikers usually climb on the leaf. Although the leaf-borne stridulatory vibrations are considerably attenuated when transmitted through the workers' legs, they can nevertheless be detected at short distances by minims. This subcaste is several times more sensitive to substrate-borne vibrations than larger workers. Secondly, when a stridulating and a silent leaf were simultaneously presented at the foraging site, minim workers spent significantly more time on the stridulating than on the silent leaf. Thirdly, hitchhiking was more frequent in leaf carriers which cut fragments out of the stridulating leaf than in those cutting the silent leaf.Communicated by P. Pamilo     

Global optimization from suboptimal parts: foraging sensu lato by leaf-cutting ants

Central-place foraging theory has been unable to explain the load selection behavior of leaf-cutting ants (Atta spp., Attini: Formicidae). We suggest that this is due to incomplete consideration of the sequence of behaviors involved in resource acquisition by these ants. Unlike most central-place foragers, leaf-cutting ants do not return to their nests with food. Instead, the leaf fragments they gather must be processed within the nest to convert them to substrate for fungal gardens. We have shown previously that leaf fragment size affects the rate of distribution and processing of leaf tissue inside laboratory nests of Atta colombica. Including these tasks in the calculation of foraging rate may help explain load selection and other features of central-place foraging by Atta colonies. Here we develop a mathematical model of the complete sequence of external and internal tasks that lead to addition of substrate to fungal gardens. Using realistic parameter values, the leaf fragment sizes predicted to maximize a colony's rate of foraging in this broad sense correspond well with the mean fragment sizes actually collected by Atta colonies in the field. The optimal fragment size for global performance in the model is below the size that would maximize the delivery rate by above-ground foragers. The globally optimal size also fails to maximize the rate of either fragment distribution or fragment processing within the nest. Our results show how maximum collective performance of an ensemble of linked tasks may require behavior that would appear suboptimal in a piecemeal analysis of tasks.     

Allometry and the geometry of leaf-cutting in Atta cephalotes

Summary This study considers the relationship of both leg length and the geometry of leaf-cutting to load-size determination by the highly polymorphic leaf-cutting ant Atta cephalotes. A. cephalotes workers anchor on the leaf edge by their hind legs and pivot around them while cutting arcs from leaves. I tested the hypothesis that, for an ant cutting a semicircular leaf fragment, fragment area is determined by a fixed reach while cutting. This reach hypothesis predicts that ants should cut the same fragment-area for at all leaf types. Also, if the radius of the semicircular fragment is proportional to hind leg length, this hypothesis predicts that leaf area should be proportional to hind-leg length squared. The field work was carried out in March–April 1990 and June 1991 in Heredia Province, Costa Rica. I measured hind-leg length for workers of different masses. I then measured leaf-fragment area and mass for workers cutting semicircular fragments from leaves of different densities (mass/area). The logarithmic relationship between ant mass (M _a) and hind-leg length L accelerated negatively (Fig. 1). As a result of this complex allometry, relative leg length (L/M _a ^0.33) increased with ant mass up to a mass of 7.4 mg. Above 7.4 mg, relative leg length decreased. For foragers cutting semicircular fragments, the area cut by an ant of a given size showed no significant difference among leaves of different densities (Fig. 2). Leaf area (A) increased as a function of leg length to the 1.9 power (Fig. 4), an exponent not significantly different from the square function expected if the radius of a fragment is determined by the ant's reach. As a result of this consistent mode of fragment-area determination, the mass of fragments cut by an ant of a given size was significantly greater when cutting denser leaves (Fig. 3) and relative area (A/M _a) cut decreased with increasing ant mass. However, because larger ants generally cut denser leaves (Table 1), the increased density of thick leaves was offset by the reduced relative area cut by the larger ants. Overall, 93% of the foragers cut fragments weighing between 1.5 and 6 times their own body mass (Table 1). Earlier studies found that this broad load-mass range maximized the biomass-transport rate (mass/distance/time) and transport efficiency (mass/distance/energy cost). Thus, A. cephalotes does not solve the problem of matching ant mass and load mass at leaves of different densities with flexibility in the leaf-cutting behavior of individual ants. Instead, individual ants employ a single simple behavioral rule, but workers of different sizes and body proportions tend to cut leaves of different densities.     

Effects of parasites on foraging and defense behavior of a termitophagous ant,Pheidole titanis Wheeler (Hymenoptera: Formicidae)

Summary Pheidole titanis Wheeler, an ant that occurs in desert and deciduous thorn forest in the southwestern United States and western Mexico, is a predator on termites. In the dry season well-coordinated raids against termite foraging parties occur early in the morning or late in the afternoon, whereas in the wet season most raids occur at night. This seasonal shift in the timing of raids is due to the increased activity of a fly (Diptera: Phoridae) that is a specialist parasitoid on P. titanis workers and soldiers. When parasitic flies discover P. titanis nest entrances or raiding columns, workers stop foraging and defend themselves against oviposition attacks. Flies are only active during the day and never interfere with foraging at night. However, P. titanis does not increase the frequency of raids at night and, as a result, colonies collect less food in the wet season compared to the dry season. Presence of parasitic flies also interferes with normal defense behavior of P. titanis against conspecific and heterospecific enemy ants. Dissections of P. titanis workers and soldiers suggest that the parasitism rate by flies is less than 2% and observations indicate that parasitic flies are much rarer than their host workers and soldiers. Nonetheless, these parasites exert a strong ecological impact on their host.     

Use of stridulation in foraging leaf-cutting ants: mechanical support during cutting or short-range recruitment signal?

Foraging leaf-cutting ant workers stridulate while cutting a leaf fragment. Two effects of stridulation have recently been identified: (i) attraction of nestmates to the cutting site, employing substrate-borne stridulatory vibrations as short-range recruitment signals, and (ii) mechanical facilitation of the cut via a vibratome-effect. We asked whether foragers actually stridulate to support their cutting behavior, or whether the mechanical facilitation is an epiphenomenon correlated with the use of stridulation as recruitment signal. To differentiate between the two alternatives, workers of two different Atta species were presented with tender leaves of invariant physical traits, and their motivation to initiate recruitment was manipulated by varying the palatability of the leaves and the starvation of the colony. The lower the palatability of the harvested leaves, the lower the percentage of workers that stridulated while cutting, irrespective of the leaf’s physical features. After intense feeding, no workers were observed to stridulate while cutting tender leaves, and the percentage of stridulating workers increased with deprivation time. The results support the hypothesis that leaf-cutting ant workers stridulate during cutting in order to recruit nestmates, and that the observed mechanical facilitation of stridulation is an epiphenomenon of recruitment communication. Received: 25 January 1996/Accepted after revision: 13 July 1996     

Sanitizing the fortress: protection of ant brood and nest material by worker antibiotics

Social groups are at particular risk for parasite infection, which is heightened in eusocial insects by the low genetic diversity of individuals within a colony. To combat this, adult ants have evolved a suite of defenses to protect each other, including the production of antimicrobial secretions. However, it is the brood in a colony that are most vulnerable to parasites because their individual defenses are limited, and the nest material in which ants live is also likely to be prone to colonization by potential parasites. Here, we investigate in two ant species whether adult workers use their antimicrobial secretions not only to protect each other but also to sanitize the vulnerable brood and nest material. We find that, in both leaf-cutting ants and weaver ants, the survival of the brood was reduced and the sporulation of parasitic fungi from them increased, when the workers nursing them lacked functional antimicrobial-producing glands. This was the case for both larvae that were experimentally treated with a fungal parasite (Metarhizium) and control larvae which developed infections of an opportunistic fungal parasite (Aspergillus). Similarly, fungi were more likely to grow on the nest material of both ant species if the glands of attending workers were blocked. The results show that the defense of brood and sanitization of nest material are important functions of the antimicrobial secretions of adult ants and that ubiquitous, opportunistic fungi may be a more important driver of the evolution of these defenses than rarer, specialist parasites.     

Parasitic flies (Diptera: Phoridae) influence foraging rhythms and caste division of labor in the leaf-cutter ant,Atta cephalotes (Hymenoptera: Formicidae)

Summary Three lines of evidence, including interspecific comparisons, temporal division of foraging between size castes, and experimental manipulations, suggest that the diurnal parasitoid Neodohrniphora curvinervis (Diptera: Phoridae) influences both the caste sizes and numbers of leaf-cutter ants (Atta cephalotes) that leave their underground nests to collect leaves. At Parque Nacional Corcovado in Costa Rica, A. cephalotes was attacked by Neodohrniphora during the daytime, and foraged less during the day than at night; a closely related ant at the same site, A. colombica, had no phorid parasites and foraged exclusively during the day. Most daytime foragers of A. cephalotes were smaller than the lower size threshold for attack by Neodohrniphora, while nocturnal foragers, active when parasitoids were absent, were both larger than this threshold and within the energetically optimal size range for foraging. When I supplied artificial lighting to allow phorids to hunt at A. cephalotes colonies past dusk, ants foraged less than when light was provided but flies were removed. The influence of Neodohrniphora on the foraging activity of A. cephalotes may explain why investigations focusing on abiotic factors have largely failed to discover what drives this ant's daily foraging cycles, and suggests that forager sizes are influenced not only by energetic efficiency, but also by the threat of parasitism.     

Food influence on colonial recognition and chemical signature between nestmates in the fungus-growing ant <Emphasis Type="Italic">Acromyrmex subterraneus subterraneus</Emphasis>

Summary. Neotropical Fungus-growing leaf-cutting ants (tribe Attini) live in obligatory symbiosis with a fungus, which they grow on fresh leaves harvested by workers. Colonial recognition is likely based on chemical cues provided by cuticular hydrocarbons that have been found to be partly influenced by environmental odor sources. The diet breadth of Acromyrmex subterraneus subterraneus enabled us to test the impact of different plant diets on colonial recognition. The intermediary of the fungus in the ants feeding habit adds a special angle to the question. From a queenright (QR) mother colony of A. s. subterraneus we formed several groups of queenless (QL) workers with fungus (approx. 700 ants). The QR colony and two of the QL-groups were fed with the same diet of fresh bramble leaves. Two other QL-groups were fed with privet leaves and two with rose flowers. After 4 months, QR workers were significantly more aggressive towards the QL-group fed with rose flowers or privet leaves than towards workers of the QL-groups fed with fresh bramble leaves. Rose-fed QL workers were aggressive towards privet-fed QL workers and vice versa, but never towards workers of their counterpart group that fed on the same diet. These results suggest that the absence of the queen or the separation time between groups played a minor role in shaping nestmate recognition cues as compared to the diet. The behavioral studies were supplemented by chemical analyses of cuticles, postpharyngeal glands (PPG) and plant-food extracts revealing profiles variations that were correlated with the dietary changes. However, although the plant extract contained several hydrocarbons there was no congruency between the plant profile and the respective diet-group ants. These results support the hypothesis that the diet influences indirectly the chemical profiles and consequently the recognition cues in A. s. subterraneus.     

Multiple mating increases the sperm stores of Atta colombica leafcutter ant queens

Multiple mating is likely to be costly for ant queens and yet it is common. Whether multiple mating brings benefits to queens that outweigh the costs has, therefore, received considerable theoretical attention. Empirical tests of hypotheses have been scarce and no clear evidence has been reported. We tested the “multiple-mating-for-more-sperm” hypothesis on individual young queens in a natural population of the leafcutter ant Atta colombica, a monogynous ant characterised by very large colonies and high colony longevity. We found that the number of sperm stored by queens was positively correlated with the number of mates per queen estimated through mother-offspring analysis with microsatellite DNA markers. Queen sperm stores increased on average by 30 million sperm for each additional mate. Life-history information for Atta indicate that the number of stored sperm observed is likely to constrain the reproductive lifespan of queens in nature. Multiple mating, despite costs, may therefore enhance the fitness of Atta queens because it enables them to store more sperm. Received: 19 September 1997 / Accepted after revision: 7 December 1997     

Disease resistance in a weaver ant, <Emphasis Type="BoldItalic">Polyrhachis dives</Emphasis>, and the role of antibiotic-producing glands

Parasites represent one of the main threats to all organisms and are likely to be particularly significant for social animals because of the increased potential for intragroup transmission. Social animals must therefore have effective resistance mechanisms against parasites and one of the most important components of disease resistance in ants is thought to be the antibiotic-producing metapleural gland. This gland is ancestral in ants, but has been lost secondarily in a small number of species. It is unknown whether these evolutionary losses are due to a reduction in parasite pressure or the replacement of the gland’s function with other resistance mechanisms. Here we used the generalist entomopathogenic fungus Metarhizium to compare the disease resistance of a species of a weaver ant, Polyrhachis dives, which has lost the metapleural gland, with that of the well-studied leaf-cutting ant Acromyrmex echinatior and two other ant species, Myrmica ruginodis and Formica fusca, all of which have metapleural glands. The P. dives weaver ants had intermediate resistance when kept individually, and similar resistance to A. echinatior leaf-cutting ants when kept in groups, suggesting that the loss of the metapleural gland has not resulted in weaver ants having reduced disease resistance. P. dives weaver ants self-groomed at a significantly higher rate than the other ants examined and apparently use their venom for resistance, as they had reduced resistance when their venom gland was blocked and the venom was shown in vitro to prevent the germination of fungal spores. Unexpectedly, the leaf-cutting ant A. echinatior also had reduced resistance to Metarhizium when its venom gland was blocked. It therefore appears that the evolutionary loss of the metapleural gland does not result in reduced disease resistance in P. dives weaver ants, and that this at least in part may be due to the ants having antimicrobial venom and high self-grooming rates. The results therefore emphasise the importance of multiple, complementary mechanisms in the disease resistance of ant societies.     

Do leaf-cutter ants Atta colombica obtain their magnetic sensors from soil?

How animals sense, process, and use magnetic information remains elusive. In insects, magnetic particles are candidates for a magnetic sensor. Recent studies suggest that the ant Pachycondyla marginata incorporates iron-containing particles from soil. We used leaf-cutter ants Atta colombica to test whether soil contact is necessary for developing a functional magnetic compass. A. colombica is the only invertebrate known to calculate a path-integrated home vector using a magnetic compass. Here, we show that A. colombica requires contact with soil to incorporate magnetic particles that can be used as a magnetic compass; yet, we also show that ants can biosynthesize magnetic particles. Workers from a soil-free colony ignored a 90° shift in the horizontal component of the geomagnetic field, yet oriented homeward despite the occlusion of any geocentric cues. In contrast, workers from a soil-exposed colony oriented to an intermediate direction between their true and subjective home in the shifted field. Homeward orientations under shifted fields suggest that ants calculated a path-integrated vector using proprioceptive information. Strikingly, ants from the soil-free colony also had magnetic particles; yet, as observed by ferromagnetic resonance, these particles differed from those in soil-exposed ants and were not associated with a magnetic compass sensitive to this experimental manipulation.     

Evidence for differential selection and potential adaptive evolution in the worker caste of an inquiline social parasite

Social parasites exploit the socially managed resources of social insect colonies in order to maximise their own fitness. The inquilines are among the most specialised social parasites, because they are dependent on being fully integrated into their host's colony throughout their lives. They are usually relatives of their host and so share ancestral characteristics (Emery's rule). Closely related inquiline-host combinations offer a rare opportunity to study trade-offs in natural selection. This is because ancestral adaptations to a free-living state (e.g. the production of a worker caste) become redundant and may be replaced by novel, parasitic traits as the inquiline becomes more specialised. The dynamics of such processes are, however, unknown as virtually all extant inquiline social parasites have completely lost their worker caste. An exception is Acromyrmex insinuator, an incipient permanent social parasite of the leaf-cutting ant Acromyrmex echinatior. In the present study, we document the size distribution of parasite and host workers and infer how selection has acted on A. insinuator to reduce, but not eliminate, its investment in a worker caste. We show that the antibiotic producing metapleural glands of these parasite workers are significantly smaller than in their host counterparts and we deduce that the metapleural gland size in the host represents the ancestral state. We further show experimentally that social parasite workers are more vulnerable to the general insect pathogenic fungus Metarhizium than are host workers. Our findings suggest that costly disease resistance mechanisms are likely to have been lost early in inquiline evolution, possibly because active selection for maintaining these traits became less when parasite workers had evolved the ability to exploit the collective immune system of their host societies.     

Short independent lives and selection for maximal sperm survival make investment in immune defences unprofitable for leaf-cutting ant males

The short-lived males of ants and other highly eusocial Hymenoptera are essentially ejaculates with compound eyes, brains and wings to vector sperm to its destination. Males compete for lifetime ejaculate storage by females to produce the equivalent of somatic cells (sterile workers) and new seed-propagules (gynes; males are haploid and have no father) after the colony has become sexually mature. Hymenopteran queens never re-mate later in life, which makes partner commitment between queen and male-ejaculate analogous to a sperm and egg committing when forming a zygote that subsequently sequesters a germ line and produces somatic tissues. This semelparous commitment remains unchanged when queens store ejaculates from multiple males, and colonies become chimeras of patrilines. The soma of eusocial hymenopteran males may thus not be under selection for more than minimal independent life, but eusocial male ejaculates are unusually long-lived, and sperm cells may not be used until years after storage. Somatic repair and immune defence in males should thus be minimal, particularly in response to challenges late in adult life. We tested this idea using males of Atta and Acromyrmex leaf-cutting ants and show that lethal infections with the fungal pathogen Metarhizium brunneum affect male sperm quality, but fail to induce an encapsulation immune response. This result is consistent with expectation because fungal infections are highly unlikely to ever reach immature ant males while they are nursed by their sister workers and because males will die natural deaths after leaving their colonies to mate before new infections can kill them.     

Fight or flight? A geographic mosaic in host reaction and potency of a chemical weapon in the social parasite <Emphasis Type="Italic">Harpagoxenus sublaevis</Emphasis>

Ant social parasites use chemical warfare to facilitate host colony takeover, which is a critical but recurring step in their life cycle. Many slave-making ants use the secretion of the Dufour gland to manipulate host behaviour during parasitic nest foundation and slave raids. Harpagoxenus sublaevis applies this chemical weapon onto defending Leptothorax host workers, which elicits deadly fights amongst them. Host species are expected to evolve counter-adaptations against this behavioural manipulation and in this study we investigated the geographic structure of this co-evolving trait. We compared the effectiveness of the parasitic gland secretion from different H. sublaevis populations in host colonies from various sites and analysed the occurrence of local adaptation. The two host species L. muscorum and L. acervorum generally showed different responses to the parasites’ chemical weapon: L. acervorum attacked nestmates treated with Dufour gland secretion, while L. muscorum workers fled. Flight, instead of intraspecific fights, is an adaptive host reaction as it results in fewer host fatalities during raids. Beside interspecific host differences, we found a geographic mosaic of host resistance: parasites from a German population strongly manipulated the behaviour of both sympatric Leptothorax populations. Russian or Italian hosts instead did not react with intracolonial aggression, but fled when confronted with the gland secretion of their sympatric parasite. Not only variation in host resistance explains differences in the effectiveness of the parasitic gland secretion but also interpopulational differences in its chemical composition, which were revealed by gas chromatography and mass spectrometry.     

Chemical mimicry in an incipient leaf-cutting ant social parasite

Some social parasites of insect societies are known to use brute force when usurping a host colony, but most use more subtle forms of chemical cheating either by expressing as few recognition cues as possible to avoid being recognized or by producing similar recognition cues to the host to achieve positive discrimination. The former “chemical insignificance” strategy represents a more general adaptive syndrome than the latter “chemical mimicry” strategy and is expected to be characteristic of early evolutionary stages of social parasitism. We tested this hypothesis by experimentally analyzing the efficiency by which Acromyrmex echinatior leaf-cutting ants recognize intruding workers of the incipient social parasite Acromyrmex insinuator. The results were consistent with the parasite being “chemically insignificant” and not with the “chemical mimicry” hypothesis. Gas chromatography–mass spectrometry analysis of cuticular hydrocarbon profiles showed that social parasite workers produce significantly fewer hydrocarbons overall and that their typical profiles have very low amounts of hydrocarbons in the “normal” C29–C35 range but large quantities of unusually heavy C43–C45 hydrocarbons. This suggests that the C29–C35 hydrocarbons are instrumental in normal host nestmate recognition and that the C43–C45 compounds, all of which are dienes and thus more fluid than the corresponding saturated compounds, may reinforce “chemical insignificance” by blurring any remaining variation in recognition cues.     

Experimental evidence for the costs and hygienic significance of the antibiotic metapleural gland secretion in leaf-cutting ants

The paired exocrine metapleural glands present in the large majority of ant species produce compounds with antibiotic properties. In the leaf-cutting ant, Acromyrmex octospinosus, the secretion consists of more than 20 different compounds and it has generally been assumed that the glands serve as a general defence against various infectious microbes of fungal and bacterial origin. We present results illuminating the direct costs and benefits of these metapleural gland defences in A. octospinosus. We show that major workers of this leaf-cutting ant experience a significant reduction in their respiration rate when the metapleural glands are experimentally closed, indicating that metapleural gland secretion incurs a substantial cost and that the production of compounds from these glands is terminated when the ants are incapable of secreting them. In another set of experiments, we show that the ability to secrete antibiotic compounds from the metapleural glands is of significant importance when ants are exposed to a general but potentially virulent insect pathogen, Metarhizium anisopliae. Infection with this fungus is lethal within a few days when ants have their metapleural glands experimentally closed, but relatively harmless when the metapleural glands are functional. These findings support experimentally the view that the metapleural glands play an important hygienic role in leaf-cutting ants.     

Waste management in the leaf-cutting ant Acromyrmex echinatior: the role of worker size, age and plasticity

Division of labour is the hallmark of the success of many social animals. It may be especially important with regard to waste management because waste often contains pathogens or hazardous toxins and worker specialisation can reduce the number of group members exposed to it. Here we examine waste management in a fungus-farming, leaf-cutting ant, Acromyrmex echinatior, in which waste management is necessary to protect their vulnerable fungal crop. By marking ants with task-specific paint colours, we found clear division of labour between workers that engage in waste management and those that forage, at least during the fine timescale of the 3-day marking period. This division of labour was influenced by both age and size, with waste management workers tending to be smaller and younger than foragers. The role of preventing contaminated ants from entering the colony was fulfilled mainly by medium-sized workers. When the level of waste was experimentally increased, most of the ants that responded to remove the waste were workers previously engaged in tasks inside the nest rather than external waste workers or foragers. These responding workers tended to be young and medium-sized. Surprisingly, the responding ants were subsequently able to revert back to working within the fungus garden, but the probability of them doing so depended on their age and the length of time they were exposed to waste. The results demonstrate the importance of division of labour with regard to waste management in A. echinatior and show that this is adaptable to changing needs.     

Seminal fluid enhances sperm viability in the leafcutter ant Atta colombica

The seminal fluid that accompanies sperm in ejaculates has been shown or suggested to affect sperm competition and paternity success of insects by preventing female remating, inducing oviposition, and forming mating plugs. In Atta leafcutter ants, queens have multiple mates but never remate later in life, although they may live and produce fertilized eggs for several decades. The mating biology and life history of these ants therefore suggests that the major function of seminal fluid is to maximize sperm viability during copulation, sperm transfer, and initial sperm storage. We tested this hypothesis by comparing the viability of testis sperm and ejaculated sperm (mixed with seminal fluid) and found a significant positive effect of seminal fluid on sperm viability. We further quantified this positive effect by adding accessory gland secretion (a major component of seminal fluid) in a dilution series, to show that minute quantities of accessory gland secretion achieve significant increases in sperm viability. Sperm stored by queens for 1 year benefited in a similar way from being exposed to accessory gland compounds after dissection in control saline solution. Our results provide the first empirical evidence that seminal fluid is important for the production of viable ejaculates and that the accessory glands of Atta males—despite their small size—are functional and produce a very potent secretion.     

Both evaluation of resource quality and speed of recruited leaf-cutting ants (Acromyrmex lundi) depend on their motivational state

Summary During recruitment, running velocity of both outbound and laden workers of the leaf-cutting ant Acromyrmex lundi depended on the information about resource quality they received from the first successful recruiter. In independent assays, single scout ants were allowed to collect sugar solutions of different concentrations and to recruit nestmates. Recruited workers were presented with standardized paper discs rather than the sugar solution given to the original recruiting ant. Outbound recruited workers were observed to run faster the more concentrated the solution found by the recruiter. Speed of disc-laden workers also depended on the concentration of the solution found by the recruiter, i.e. on the information about food quality they received, since they had no actual contact with the sugar solution. Disc-laden workers ran, as intuitively expected, slower than outbound workers. The reduction in speed, however, could not be attributed to the effects of the load itself, because workers collecting discs of the same weight, but with added sugar, ran as rapidly as outbound, unladen workers. Workers collecting standardized sugared discs reinforced the chemical trail on their way to the nest. The percentage of trail-layers was higher when workers were recruited to 10% than to 1% sugar solution, even though they collected the same kind of discs at the source. Their evaluation of resource quality, therefore, depended on their motivational state, which was modulated by the information they received during recruitment. Using previously published data on energetics of locomotion in leaf-cutting ants, travel costs of A. lundi workers recruited to sugar solutions of different concentration could be estimated. For workers recruited to the more concentrated solution, both speed and oxygen consumption rate increased by a roughly similar factor. Therefore, although workers ran faster to the high-quality resource, their actual energy investment per trip remained similar to that made by workers recruited to the low-quality resource. It is suggested that the more motivated workers reduced travel time without increasing energy costs during the trip. The adaptive value of these responses seems to be related to a rapid transmission of information about a newly discovered food source.