Research Article ISSN 2336-9744 (online) | ISSN 2337-0173 (print) The journal is available on line at www.biotaxa.org/em

Native food spectrum, size-matching and foraging efficiency of the Mediterranean harvester wasmanni (: Formicidae)

TANJA TRAXLER

Department of Botany and , University of Vienna, Rennweg 14, 1030 Vienna, Austria e-mail: [email protected]

Received 20 September 2016 │ Accepted 11 November 2016 │ Published online 14 November 2016.

Abstract This study examined the food spectrum, size-matching and harvesting efficiency of the Mediterranean harvester ant Messor wasmanni Krausse, 1910, a major seed predator on annual grasslands, on the Croatian island of Cres. The ant workers collect available food items not only from the soil surface (post-dispersal seed predation), but also directly from the mother plant (pre-dispersal seed predation). The distributional pattern of the main food resources therefore changed constantly from May to October 2009. This investigation shows that food items were collected either from several distinct small regions, or from widely distributed zones of the foraging ground. Accordingly, the foraging strategies are a mixture of individual foraging and column foraging, adjusted to the ’ needs. Moreover, under natural conditions, M. wasmanni workers do not show size-matching at foraging trails. Little, if any, size variation of harvested seeds and fruits can be attributed to the foragers’ body size. Small workers did not appear to be constrained by load size, which indicates that size-matching may not be an adequate measure of colony foraging success for M. wasmanni. I further observed that the harvesting efficiency of media- and major-sized workers decreased considerably from May to October 2009, whereas the harvesting efficiency of minor-sized workers constantly increased over the year. This finding suggests that the worker size and the resulting difference in individual metabolism play an important role for the harvesting efficiency of M. wasmanni.

Key words: harvester ants, size matching, food spectrum, harvesting efficiency, granivory.

Introduction

The Mediterranean harvester ant Messor wasmanni Krausse, 1910 is a common seed predator on Mediterranean grasslands across southern Europe. Several studies revealed that granivory is a feature common to the whole Messor Forel, 1890 (Hahn & Maschwitz 1985, Hobbs 1985, Reyes-López & Fernándes-Haeger 2002). Typically, M. wasmanni workers collect food items, such as fruits and seeds, and store them in specific chambers within their nest. Within these chambers, metapleural glandular secretion prevents seed germination and decomposition by mushrooms and bacteria (Schildknecht & Koob 1971, Celli & Maccagnani 1994). This strategy helps the ants to preserve food items and to overcome periods of inactivity outside the nest and low seed production (Schildknecht & Koob 1971, Celli & Maccagnani 1994). Within a M. wasmanni colony, the sterile worker force shows allometric growth (Solida et al. 2007). This is defined as the occurrence of more than one form or type of individuals among members of a single , e.g., numerous ant species exhibit growth over a sufficient range to produce individuals of different sizes, body proportions, or both (Wilson 1953). Accordingly, the ant workers’ size

Ecol. Mont., 7, 2016, 451-463

FOOD SPECTRUM, SIZE-MATCHING AND HARVESTING EFFICIENCY OF MESSOR WASMANNI represents a limit for the quantity and quality of tasks an individual worker can perform in connection with colony life. Polymorphism also encompasses cooperative behavior among colony members (Bri