Foraging by Atta sexdens (Formicidae: Attini): seasonal patterns, caste and efficiency

Abstract. 1. Atta sexdens changes diel periods of foraging, the size of its foraging territory, the numbers and lengths of foraging trails, and its rate of foraging with respect to seasonality in subtropical Paraguay. Leaf loads are significantly larger in warmer months than loads carried in cooler months.
2. Foragers segregate into three labour groups in the field: a small subset climb trees, cut large quantities of vegetation, and drop them to the ground; the second subset of foragers searches out these leaf caches, cut diem into smaller pieces, and carry and deposit the leaf fragments on the foraging trail; the third subset of workers retrieves leaf fragments on the trail and carry them to the nest.
3. Pitfall trapping shows a large degree of patchiness in activity, with media workers dominating the foraging population, more so closer to vegetation which is being harvested.
4. The recovery efficiency of the multi-staged foraging behaviour is estimated to be only 49%, with the recovery of leaf caches near 50%. The impact of A. sexdens may, thus, be twice as great as previous estimates on their herbivory.     

Grass production and decomposition in Southern Guinea savanna,Nigeria

Summary Annual grass production in ungrazed plots was 2,731 kg ha^-1, litter production was 1,619 kg ha^-1 and decomposition was 1,789 kg ha^-1. In grazed plots the corresponding figures were 3,157 kg ha^-1, 1,440 kg ha^-1, and 1,475 kg ha^-1 respectively; cattle consumed 1,405 kg ha^-1. Litter disappearance was greatest in the dry season: 1,226 kg ha^-1 (69% of the annual total) disappearing in the 4 months of December to March in the ungrazed plots, largely due to consumption (790 kg ha^-1 in December to March) by fungus-growing termites (Macrotermitinae). A positive linear relationship was found between maximum grass biomass and annual rainfall in West Africa.     

Vegetation and litter production in Southern Guinea savanna,Nigeria

Summary This study examines the woody vegetation, annual leaf-fall and annual wood-fall in Southern Guinea savanna near Mokwa, Nigeria. There were 1425±402 (95% limits) trees ha^-1 of which Caesalpiniaceous legumes made up 53%. Annual leaf-fall was 2.387 t ha^-1 a^-1 equivalent to 11,238,932 kcal ha^-1 a^-1 and was highly seasonal, peaking from November to February, with a maximum in January after the annual bush fires. Annual wood-fall was 1.391 t ha^-1 a^-1 equivalent to 7,598,256 kcal ha^-1 a^-1 and was less seasonal but with two peaks, one from January to March after the fires and another from May to July in the rainy season.The litter-fall data currently available from West Africa are reviewed and indicate a negative correlation between litterfall and latitude (P<0.001). Litter production is found to be positively correlated with rainfall (P<0.001) and it is suggested that seasonal distribution of rainfall may also be a factor contributing to the limits of litter production.     

The role of termites in the decomposition of wood and leaf litter in the Southern Guinea savanna of Nigeria

Summary The number of flowers produced by inflorescences of Yucca whipplei (Agavaceae) consistently exceeds the number of fruits produced by about one order of magnitude. To determine the factors responsible for low fruit set, the relation between pollinator availability, the amount of resources spent on reproduction (as indicated by inflorescence size), and the number of fruits matured was studied during 1978 and 1979 at 18 locations in chaparral, coastal sage scrub, and desert scrub communities of southern California.The following results support the conclusion that pollinators do not usually limit fruit production in Yucca whipplei. Rather, fruit production is limited by the amount of resources available to support developing fruits. (1) Fruit production is positively correlated with inflorescence size both within and between populations. The average size of inflorescence for a population is an excellent predictor of mean fruit production. Furthermore, 54% of the total variance in fruit production of individual plants can be explained by inflorescence size. (2) In contrast, although fruit production within most populations is positively correlated with an index of the number of pollinator visits to an inflorescence, the relative abundance of pollinators for a population is a poor predictor of mean fruit production, and only 9% of the total variance in fruit production can be explained by the visitation index. Furthermore, at four sites studied for two years, there was little change in average inflorescence size or fruit production from 1978 to 1979, despite large differences in relative abundance of pollinators at each of the sites. (3) Based on geographic proximity, and physiographic and vegetational similarities, study sites were grouped into regional clusters. Both inflorescence size and fruit production varied considerably between regions. Of the total variation in fruit production, 27% can be attributed to differences between regions. Most of this variation is the result of regional differences in inflorescence size, which in turn influence fruit production.Why does Yucca whipplei produce such large inflorescences if so few fruits can be supported? Two relevant hypotheses are discussed: (1) the floral display is the result of selection for pollen dissemination at the expense of fruit set; and (2) the floral display is the result of selection for a bet-hedging strategy either to increase the probability of adequate pollination when pollinators are unusually rare, or to allow individuals to support more fruits when resources are unusually abundant.     

Raiding behavior of the dulotic antChalepoxenus muellerianus (Finzi) in the field (Hymenoptera: Formicidae,Myrmicinae)

Summary Little information is available regarding the raiding behavior in nature of dulotic ants belonging to the tribeLeptothoracini. Between July 17 and 24, 1991, several raids ofChalepoxenus muellerianus (Finzi) were observed in nature near Tignale/Lago di Garda, Italy. Apparently, this species raids frequently during its summer raiding season (5 raids were observed during 7 days spent observing 8 colonies). A singleChalepoxenus colony sometimes raids more than one host colony more or less simultaneously. Observations during which oneChalepoxenus colony raided another and captured slavemaker brood indicate that intraspecific raids can occur either accidentally, or as a result of competition or territoriality when there is a sufficient dense slavemaker population.     

Effect of mound size and fertilizer on white Guinea yam (Dioscorea rotundata) in Southern Nigeria

Summary Field trials were conducted in the forest zone of southern Nigeria on three soil series, gravelly loamy sand Ibadan soil (Oxic paleustalf), gravelly sandy loam Egbeda soil (Oxic paleustalf) and sandy loam Alagba soil (Oxic paleustalf). The trials were carried out to study the effects of planting on flatversus various mound sizes and NPK fertilizer on performance of white Guinea yam (Dioscorea rotundata) cultivar Laoko.Mound size appeared to have a more pronounced effect on tuber yield than fertilizer even on land which was in the second and third year of cropping after bush fallow. The average tuber yield for the three locations without fertilizers was 7.83 tons/ha on the flat compared with 9.44 tons/ha on large mound (about 30 cm height). With fertilizer application, tuber yields were 7.43 tons/ha on the flat and 11.30 tons/ha on large mound respectively. Total yield reduction on flat may in part be related to physical soil impedence. Planting on large mounds also resulted in longer tubers and shorter harvesting time.     

Spatial distribution, size and density of nests of Myrmicaria opaciventris Emery (Formicidae, Myrmicinae)

Summary: We recorded over the course of one year the location of nests of Myrmicaria opaciventris on five sites in the region surrounding Yaoundé (Cameroon). On these sites, the colonies are polydomous and polygynous. The size of the largest colony, which was made up of 27 nests, was evaluated at 216,575 - 24,750 individuals of which 147,230 - 15,336 were workers. The nests are interconnected by trails that the workers dig during the rainy season. They consist of trenches that are slowly closed to form tunnels. Thanks to these tunnels, the workers are sheltered from their predators when going from one nest to another. They also have easy access to food sources far from the nests. We noted a seasonal variation in the location of the nests. It consists of a kind of budding. Depending on the case, the initial nest might survive or end up by being abandoned during the main rainy season or early in the dry season. The spatial distribution of the nests in the field is, in general, random and the density is lower during the dry season (from 6.2 to 26.4 nests/ha) than during the rainy season (from 12.1 to 50 nests/ha).     

Spatio-temporal Foraging Dynamics in Two Coexisting Harvester Ants (Hymenoptera: Formicidae)

Different aspects of the foraging strategies of two harvester ant species, Messor wasmanni and M. minor, were investigated in a Mediterranean dry grassland area. Baits were used to evaluate the existence of a trade-off between resource discovery and domination as well as the effect of three variables (air temperature, relative humidity and distance) on the trade-off. Baits were also utilized to explore random vs non random use of time by colonies. Random vs non random utilization of space was instead evaluated by mapping the daily foraging area of colonies in a grid of 900 plots of 1 m² each. Results revealed that species coexistence is not preferentially supported by a trade-off in resource utilization with no overall effect of the examined variables. The foraging activity of the two species widely overlapped whilst a clear competition for space occurred. The observed space partitioning could represent an advantageous strategy for the coexistence of the two ant species.     

Biology and myriapod egg predation by the Neotropical myrmicine antStegomyrmex vizottoi (Hymenoptera: Formicidae)

Summary For the first time for a Neotropical ant and for Myrmicinae, the searching behavior and specialized predation of spirobolid millipede eggs byStegomyrmex vizottoi Diniz will be described. The relationship between morphology and habits is studied, as are nest architecture and distribution of the ant population in the nest chambers. We also report on some observations of behavior in the field and laboratory.We dedicate this paper to William L. Brown Junior, on the occasion of his 70^th birthday.     

Lenomyrmex, an enigmatic new ant genus from the Neotropical Region (Hymenoptera: Formicidae: Myrmicinae)

Abstract. We describe a new myrmicine ant genus, Lenomyrmex , with four new species: L. mandibularis (type species) and L. foveolatus from Colombia, L. wardi from Ecuador and Colombia, and L. costatus from Panama. The new taxon is clearly distinguished from other myrmicine ants by the following combination of characters: (1) mandibles elongate-triangular, with crenulate masticatory margin three times longer than basal margin, and bearing a series of ten to twenty minute peg-like denticles that arise along and immediately behind the masticatory margin; (2) frontal lobes poorly expanded laterally, partly covering the antennal fossae; (3) large and deep antennal fossae; (4) antennal scrobes absent; (5) eyes protuberant; (6) petiole pedunculate, with poorly defined node and with an anteroventral subpetiolar process pointed anteriorly. The affinities of this new taxon within Myrmicinae remain unclear and the observed characters do not permit it to be included satisfactorily in any of the known tribes or genus groups of the subfamily.