Why Do Pheidole Oxyops (Forel, 1908) Ants Place Feathers Around Their Nests?
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Ecological Entomology (2019), DOI: 10.1111/een.12722 NATURAL HISTORY Why do Pheidole oxyops (Forel, 1908) ants place feathers around their nests? INÁCIO J. M. T. GOMES,1 DIOGO F. SANTIAGO,2 RICARDO I. CAMPOS1 and HERALDO L. VASCONCELOS3 1Programa de Pós-Graduação em Ecologia, Departamento de Biologia Geral, Universidade Federal de Viçosa, Viçosa, Brazil, 2Programa de Pós-Graduação em Ecologia e Recursos Naturais, Departamento de Ecologia e Biologia Evolutiva, Universidade Federal de São Carlos, São Carlos, Brazil and 3Instituto de Biologia, Universidade Federal de Uberlândia, Uberlândia, Brazil Abstract. 1. The ant species Pheidole oxyops (Hymenoptera: Formicidae) has the particular behaviour of placing feathers along the edge of entrances to its nests. Here two hypotheses concerning this behaviour were tested: (i) P. oxyops, due to its essentially carnivore diet and particular nest structure (which acts as pitfall traps), uses feathers to enhance the capture of arthropods; and (ii) P. oxyops uses feathers as a strategy for obtaining water, as feathers could act as retainers of night moisture. 2. To test the first hypothesis, two plots with 20 pitfall traps were established, and inside each plot feathers were placed around half of these traps. It was expected that pitfall traps with feathers would collect a larger number of individuals and species of arthropods. To test the second hypothesis, all feathers were removed from 28 nests and water was supplied (wet cotton balls) to half of these nests. It was expected that colonies with access to an artificial water source would collect fewer feathers than control colonies. 3. Only the first hypothesis was supported. Our results indicate that feathers have the potential to enhance prey capture by P. oxyops providing the colonies with a more diverse and abundant diet. The ants’ habit of placing feathers around the nest, which can be considered as a low-cost foraging strategy, could be particularly important during periods of low food resource availability. Key words. Dominant species, foraging behaviour, Formicidae, water stress. Introduction In spite of being conspicuous, one as yet un-investigated behavioural trait is found in the Neotropical ant species Pheidole The life histories of ants comprise an enormous range oxyops (Forel, 1908), which places bird feathers (e.g. from of behavioural attributes that allow them to exploit virtu- doves) around the entrances of its nests (Fig. 1a). Colonies of ally all terrestrial habitats (Hölldobler & Wilson, 1990; Kaspari, P. oxyops are frequently observed collecting feathers in the 2000). However, we know little about how those behavioural vicinity of their nests when these are available within their features influence local habitat adaptations and, consequently, foraging area (I.J.M.T. Gomes, unpublished). This behaviour is ant species distribution (Laskis & Tschinkel, 2009; Rosumek, possibly associated with the ant’s foraging strategy, but the exact 2017). Adaptation to a given habitat includes, among many other mechanisms that trigger it are still unknown. requirements, an efficient foraging strategy for gathering food Pheidole is by far the richest ant genus in the Neotropics, with and water (Ribeiro & Navas, 2008 and references therein; Lanan, 702 described species (AntWeb; http://www.antweb.org). In the 2014). Because of that, ants have developed a myriad of different Neotropical savannas, P. oxyops ants play a highly important foraging strategies that can vary according to species, resources ecological role, because they are essentially predators and are or environmental conditions (Traniello, 1989; Lanan, 2014). regarded as both numerically and behaviourally dominant (Cam- pos et al., 2011; Czaczkes et al., 2011). This predominantly predatory behaviour may be associated with the behaviour of Correspondence: Inácio Gomes, Prédio da Entomologia, sala 210, placing feathers around the nest. Previous studies have shown Campus Universitário, Universidade Federal de Viçosa, 36570-000, that nests of P. oxyops have a peculiar structure, with a wide, Viçosa-MG, Brazil. E-mail: [email protected] deep, and smooth entrance tunnel (Fig. 1b), which is thought to © 2019 The Royal Entomological Society 1 2 Inácio J. M. T. Gomes et al. areas. We identified P. oxyops using the taxonomic key provided by Wilson (2003), and voucher specimens are deposited at the Zoological Collection of the Federal University of Uberlândia, Uberlândia, Brazil. To verify the occurrence of feathers around the nests of P. oxyops and the possible effect of seasonality on this behaviour, we performed two exploratory samplings. First, in the campus of the Federal University of Uberlândia, we counted the number of feathers around 17 P. oxyops nests in the dry season (in 2010) and around another 19 nests in the wet season (in 2018). Second, Fig. 1. Nests of Pheidole oxyops with (a) or without (b) bird feathers to test if the number of feathers collected by the ants over 24 h around its entrance. The wide, deep and smooth entrance of the nest (b) was dependent on the number of initial feathers, for those same is thought to work as a pitfall trap. Photographs: Ricardo Solar. [Colour nests and also in both seasons (dry and wet), we offered 15 new figure can be viewed at wileyonlinelibrary.com]. feathers positioned 30 cm away from their entrances and counted the number of new feathers that had been collected after 24 h. work as a pitfall trap, enhancing the capture of arthropod prey (Fowler, 1987; Forti et al., 2007; Supporting information, File Hypothesis 1: Feathers increase prey capture S1). Feathers may thus further enhance prey capture, given that they may work as bait (due to their smell, colour or shape) to To test whether feathers increase the number of arthropods some arthropods that end up falling into the nest entrance tun- falling into the nests and thus potentially become prey to P. o x y - nel. In addition, it is possible that feathers directly benefit ants, ops, we established in February 2011 two sampling plots inside as they could obtain food from small residues of bird tissue or the savanna reserve described earlier. The plots were located from prey present in the feathers. 100 m away from each other and inside each plot we installed Pheidole oxyops is a species typical of Brazilian savannas 20 pitfall traps (diameter 5.2 cm, volume 70 ml) along four lines (Wilson, 2003). As such, it often faces water shortage during (5 m apart) with five traps each (5 m apart). The pitfall traps were the dry season (Alvares et al., 2013). The same restriction buried in the ground, so their openings were level with the soil is experienced by Diacamma rugosum (Le Guillou, 1842), a surface. To simulate a P. oxyops nest entrance, we used pitfall common and conspicuous ant species that lives in dry areas traps with approximately the same entrance size as mature nests in Asia. Interestingly, this species was also found to place (diameter 4.3 cm). The openings of the pitfall traps were closed feathers around its nest entrances, and previous studies have for 48 h to minimise the effects of soil disturbance in attracting shown that D. rugosum consumes the water that accumulates on soil arthropods into the traps (dig-in effect). After 48 h, the traps feathers overnight (Moffett, 1985). Therefore, it is reasonable to were opened and partially filled with water and a few drops suppose that, similar to D. rugosum, P. oxyops may use feathers of soap to break the water’s surface tension. Within each plot to obtain water. In addition, we have observed that ants are we randomly selected 10 pitfalls where we placed 15 feathers recruited quickly and in large numbers to moistened cotton balls around them (‘feathers-addition group’). The remaining 10 traps (I.J.M.T. Gomes, unpublished), which indicates that P. oxyops served as controls (‘no-feathers group’). These experimental experiences water shortage during the dry season. If that is the feathers were obtained from our study site and, before placing case, one would expect that feather retrieval rates would increase them around the pitfalls, we carefully checked all feathers for during periods of severe water shortage. the presence of small animals. After 48 h, we collected the pitfall Aiming to identify the factors that trigger this behaviour and traps and took them to the laboratory, where the arthropods were its possible importance in the foraging strategy of P. oxyops,we sorted into morphospecies. If feathers do enhance prey capture, attempted to answer the following question: why do P. oxyops we expected that traps with feathers would collect more arthro- ants place feathers around their nests? In order to answer this pod species and individuals than would traps with no feathers. question, we tested two non-excluding hypotheses: (i) feathers We performed a complementary sampling to compare the increase arthropod prey capture by P. oxyops colonies; and (ii) arthropod species composition and abundance between the feathers act as moisture retainers, and thus increase the supply of ‘pitfall trapping’ and ‘active foraging’ strategies performed by water to colonies, being collected more frequently when water P. oxyops. For this purpose, we selected five nests and, using a is in short supply. forceps, removed from the ants’ mandibles all the food items brought back the nest. This procedure was always done between 09.00 and 10.00 hours in five nests and over three consecutive Materials and methods days. Food items were preserved in 70% ethanol