Leaf-cutting ants cut fragment sizes in relation to the distance from the nest

Short Communications

Regardless of the processes responsible for the acquisition of the preference, the above data indicate that females have the ability to discriminate between males carrying different t-haplotypes. This discriminatory capacity may have consequences for the population genetics of t-complex genes. If females preferentially mate with males carrying dissimilar t-haplotypes, the frequency of lethal thaptotypes will be higher, owing to the production of compound heterozygote females, than if females preferentially mate with males carrying the same t-haplotype. We thank J. Nadeau from Jackson Laboratories for supplying the LT.MA mice, U. W. Huck for the preference apparatus, L. Abramovitch for statistical advice and R. Weeks for her excellent animal care. CAROL B. COOPERSMITH SARAHLENINGTON Institute of Animal Behavior, 101 Warren Street, Rutgers University-Newark, Newark, NJ07102, U.S.A.

References

Bennett, D. 1975. The T-locus of the mouse. Cell, 6, 441-454. Bennett, D. 1978. Population genetics of Tit complex mutations. In: Origins of Inbred Mice (Ed. by H. Morse), pp. 615-632. Academic Press: New York. Egid, K. & Brown, J. 1989. The major histocompatibility complex and female mating preferences in mice. AnOn. Behav., 38, 548-550. Huck, U. W. & Banks, E. M. 1981. Olfactory discrimination of social status in the brown lemming. Behav. neural Biol., 33, 364-371. Klein, J., Sipos, P. & Figueroa, F. 1984. Polymorphism of t-complex genes in European wild mice. Genet. Res. Cam., 44, 39-46. Lenington, S., Egid, K. & Williams, J. 1988a. Analysis of a genetic recognition system. Behav. Genet., 18, 549-564. Lenington, S., Franks, P. & Williams, J. 1988b. Distribution of t-haplotypes in natural populations of wild house mice. J. Mammal., 69, 489-499. Silver, L. M. 1985. Mouse t-haplotypes. A. Rev. Genet., 19, 179-208. Yamazaki, K., Beauchamp, G. K., Kupniewski, D., Bard, J., Thomas, L. & Boyse, E. A. 1988. Familialimprinting determines H-2 selective mating preferences. Science, 240, 1331-1332. Yamazaki, K., Boyse, E. A., Mike, V., Thaler, H. T., Mathieson, B. J., Abbott, J., Boyse J. & Zayas, Z. A. 1976. Control of mating preferences in mice by genes in the major histocompatibility complex. J. exp. Med., 144, 1324-1325. (Received 19 March 1990; initialacceptance 3 May 1990;finalacceptance 6 July 1990; MS. number: As-708)

1181

Leaf-cutting Ants Cut Fragment Sizes in Relation to the Distance from the Nest

Optimal foraging models have been developed to predict the response of animals to foraging problems (Emlen 1966; Schoener 1971). F o r animals that carry the food they harvest to a fixed place, i.e. central place foragers, optimal foraging arguments predict enhanced selectivity as the distance from the central place to the foraging site increases (Orians & Pearson 1979; Schoener 1979). If animals bring one prey item at a time, Orians & Pearson's (1979) model predicts that they should select .the prey size that maximizes the net rate of resource delivery to the central place, i.e. animals should select large prey at larger distances. If natural selection shaped foragers of eusocial insects to be efficient food harvesters (Oster & Wilson 1978 ), leaf-cutting ants should be well suited to test optimal foraging predictions because they can reach their own decisions about the prey (leaf) size to be retrieved through their cutting behaviour. I present the first test of the prediction of enhanced selectivity with increasing distance in leaf-cutting ants by comparing the food fragment sizes cut by recruited Acromyrmex lundi workers at two distances from the nest. I performed experiments on a laboratory colony of 1500 workers reared from the founding queen. The mean + SE body mass of forager workers was 2"39___0.08 mg (N=78), not significantly different from that found in natural adult colonies (2.42_0.14 mg; t=0.17, N = 110, P>0.25). I performed load-size selectivity assays with workers foraging at two consecutively connected foraging tables, 1 and 5 m from the nest. Field colonies of this species have foraging trails of different distances, with maximal mean trail length of 45 m (Fowler et al. 1986). F o r these assays, I took advantage of the olfactory conditioning phenomenon found in recruited A. lundi workers (Roces 1990): they cut and retrieve the laboratory film Parafilm (Marathon R) if it is impregnated with the same odour of the food source initially found by scout workers. By using the film, I avoided variations in physical or chemical features of leaves. In each assay I presented 15 g of wheat germ flakes impregnated with OH-citronellal as the food source, at only one of the two foraging tables. Immediately after the first scout worker picked up a flake and returned to the nest displaying recruiting behaviour, i.e. trail-laying, I removed the flakes and replaced them with a piece of OH-citronellal impregnated Parafilm, measuring 8 x 8 cm, securely pinned to the floor so that weight of the piece was not a variable during the assays. Recruited workers

1182

Animal Behaviour , 40, 6

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Figure 1. Regressions of ant weight on load carried for two distances from the nest to the food site. Least squares curve fit for 5 m (9 Y=exp(-0.7+0.66X), (R=0.798, N=41, P<0.00001); for 1 m (O): Y=exp(-2.22+0.84X), (R = 0.725, N= 37, P < 0.00001). Sample sizes for each distance correspond to four replicates.

cut the film and carried fragments to the nest. Workers were therefore not disturbed during this procedure, which closely resembled a normal foraging-recruiting process. In each assay I collected the first 10 ants that cut a film fragment and weighed ants and their loads to the nearest 0.01 mg. The assays were performed in a random sequence, one each day. Two results are clear. (1) Ants cut larger fragments further from the nest (Fig. 1). Previous tests, performed with Pogonomyrmex and Veromessor seed-harvester ants, have shown either no correlation between seed size and distance (Taylor 1977; Rissing & Pollock 1984; Holder Bailey & Polis 1987) or an increased specialization at larger dis, tances from the nest, i.e. Pogonomyrmex rugosus colonies collected a narrow range of seed sizes at longer distances (Davidson 1978). (2) For each distance, there is a significant regression (P<0.001) between the weight of the fragment cut and that of the ant (Fig. 1). Correlation between ant size and the size of food items has been found in several studies (e.g. Davidson 1977; Rudolph & Loudon 1986) but was not found in Rissing & Pollock (1984). Weber (1972) suggested that, because ants anchor their hind legs on the leaf edge and cut an

arc, the load size selected by leaf-cutting ants may be directly determined by their body size. My results contrast with this causal relationship: recruited A. lundi workers also cut an arc circumscribed by their hind legs, but for an ant of a given size, the reach while cutting was different at the two different distances from the nest. This finding and the negative correlation between leaf density (mass/ area) and leaf fragment area cut found by Cherrett (1972) and Rudolph & Loudon (1986) suggest a more complex mode of load-size selection in leafcutting ants than the method proposed by Weber (1972). Orians & Pearson's (1979) model predicts a change in load size as a function of distance, and that incomplete loads should only be gathered when animals feed in a patch where intake rate decreases with time. My results, although consistent with this prediction, are somewhat unexpected because the Parafilm piece is, for a given ant, a nondepleting patch, and therefore raises the question about what leaf-cutting ants optimize when selecting a certain load size. I thank J. A. NOfiez for encouragement and fruitful discussions, and an anonymous referee for comments that improved the manuscript. This research was supported by a grant of CONICET

Short Communications (National Research Council) of Argentina to J. A. Nfifiez. FLAVIOROCES Departamento de Biologla, Universidad de Buenos Aires, Pabellbn H Ciudad Universitaria, RA-1428 Buenos Aires, Argentina. References Cherrett, J. M. 1972. Some factors involved in the selection of vegetable substrate by Atta cephalotes (L.) (Hymenoptera: Formicidae) in tropical rain forest. J. Anita. Ecol., 41, 647-660. Davidson, D. W. 1977. Species diversity and community organization in desert seed-eating ants. Ecology, 58, 711 724. Davidson, D. W. 1978. Experimental tests of the optimal diet in two social insects. Behav. Ecol. Sociobiol., 4, 35-41. Emlen, J. M. 1966. The role of time and energy in food preference. Am. Nat., 100, 611-617. Fowler, H. G., Pereira-da-Silva, V., Forti, L. C. &Saes, N. B. 1986.Population dynamics of leaf-cutting ants: a brief review.In: FireAnts and Leaf-cutting Ants. Biology and Management (Ed. by C. S. Lofgren & R. K. Vander Meer), pp. 123-145. Boulder, Colorado: Westview Press. Holder Bailey, K. & Polis, G. A. 1987. Optimal and central-place foraging theory applied to a desert harvester ant, Pogonomyrmex californicus. Oecologia (Berl.), 72, 440-448. Orians, G. H. & Pearson, N. E. 1979. On the theory of central place foraging, In: Analysis of Ecological Systems (Ed. by D. J. Horn, G. R. Stairs & R. D. Mitchell), pp. 155-177. Columbus: Ohio State University Press. Oster, G. F. & Wilson, E. O. 1978. Caste andEcology in the Social Insects. Princeton, New Jersey: Princeton University Press. Rissing, S. W. & Pollock, G. B. 1984. Worker size variability and foraging efficiencyin Veromessorpergandei (Hymenoptera: Formicidae). Behav. Ecol. Sociobiol., 15, 121-126. Roces, F. 1990. Olfactory conditioning during the recruitment process in a leaf-cutting ant. Oecologia (Berl.), 83, 261-262. Rudolph, S. G. & Loudon, C. 1986. Load size selectionby foraging leaf-cutter ants. Ecol. Entomol., 11,401-410. Schoener, T. W. 1971. Theory of feeding strategies. A. Rev. Ecol. Syst., 2, 369-404. Schoener, T. W. 1979. Generality of the size-distance relation in models of optimal feeding. Am. Nat., 114, 90~914. Taylor, F. 1977. Foraging behavior of ants: experiments with two species of myrmecine ants. Behav. Ecol. Sociobiol., 2, 147-168. Weber, N. A. 1972. Gardening Ants, the Attines. Philadelphia: American Philosophical Society. (Received 19 March 1990; initial acceptance 13 April 1990;finalacceptance 13 June 1990; MS. number: AS-644)

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Sparring and Access to Food in Female Caribou in the Winter Female caribou and reindeer, Rangifer tarandus, are the only female cervids that regularly carry antlers (Chapman 1975; Goss 1983). Both male and female caribou grow their antlers each year during the summer. Males cast their antlers from December to March whereas females carry theirs for about 4 months longer, casting them only in early June, typically a few days after giving birth (Lent 1965; Espmark 1971; Bergerud 1976). It has been suggested that females use their antlers against males during the winter in competition for food (Henshaw 1968; Bubenik 1975; Clutton-Brock 1982). A caribou can displace a conspecific over 80% of the time merely by approaching it, i.e. without having to hit or even touch it (Vandal & Barrette 1985). The possession, as well as the size, of antlers seem to be important in this respect, allowing females with antlers to displace males without antlers (Barrette & Vandal 1986). However, antlers are considered to be primarily useful in actual combats (Geist 1966; Clutton-Brock 1982). We therefore examined whether females engaged in antler combats against males to obtain or defend food, and if so, how successful they were. We studied a small population of woodland caribou for two winters. The caribou were the descendents of animals re-introduced to an area from which caribou disappeared around 1925 (Vandal 1983). The study site (47o30' N, 70o50' W) was a lichen woodland habitat (Rowe 1984) with an area of 310 km 2. We made 1959 10-min focal observations (Altmann 1974) on individual caribou that were in groups of 6-59. With the help of ear tags, neck collars, antler shape or pelage features, we could record the identity of all the animals observed. In all of the sparring matches we could clearly distinguish the initiator from the target. Snow craters, dug by the caribou, expose the ground lichens (e.g. Cladina spp.) that constitute by far the main food of caribou in our population during the winter (Vandal & Barrette 1985). Immediately before a sparring match, the target either was, or was not, in possession of a crater and we distinguished between resource and no-resource sparring matches on this basis. During the rut, in October-November, females did not initiate sparring (232 matches observed) and were the targets of sparring only six times. None of the sparring matches during the rut was a contest over resources because there was less than 20 cm of snow on the ground, making competition for ground lichens unnecessary (Vandal & Barrette 1985). We recorded 1232 sparring matches in the