Waste poses a significant threat to the health of social groups due to the potential transmission of diseases. In response to this risk, various animals, including those in eusocial societies with large colonies, have evolved behavioural strategies to manage waste. Eusocial insects, such as ants and stingless bees, generate substantial waste, including excreta, food remnants, cocoon casings, and deceased members (ants: (Hölldobler and Wilson 1977; Hölldobler and Wilson, 1990; Möglich and Hölldobler 1975; Rettenmeyer 1963; Weiss 2006); stingless bees: (Kolmes and Sommeijer, 1992); other subsocial insects: (Tallamy and Wood 1986)). The accumulation of such waste in proximity to colony members increases the likelihood of infections and the spread of diseases caused by fungi, bacteria, or parasites (Cremer 2019). Recognizing the importance of waste management in preventing disease, this study explores the behaviour of Central Australian red honey-pot ants, Melophorus bagoti, in their disposal of waste, especially comparing the performance of experienced and naive ants in waste dumping.
Before foraging, inexperienced naive red honey ants walk around the nest to acquire visual information about the surrounding terrestrial panorama through a series of learning walks (Deeti and Cheng 2021a). These ants take 3–7 such walks, increasing the area and distance covered on each successive walk, before setting off to forage. The knowledge gained from learning walks is then employed for navigation when the ant commences foraging (Cataglyphis: Fleischmann et al. 2018; Melophorus: Wystrach et al. 2012; Deeti et al. 2020). Foraging ants also use another well-studied navigational strategy of path integration (PI), through which they continuously keep track of a vector pointing to their starting point (Wehner and Srinivasan 2003). When travelling, the animals combine their compass information with estimates of speed or distance constantly to update a home vector, which they can use at any time to guide them directly back to their starting point of journey, typically the nest (Collett et al. 1998; Haferlach et al. 2007; Webb 2019). When ants are uncertain about the nest position, or in the absence of PI and learned visual cues, foragers employ systematic search as a back-up mechanism, in which they perform looping search walks until they find information, whether local, panoramic, or celestial, which will aid navigation (Schultheiss and Cheng 2011; Schultheiss et al. 2013; Schultheiss et al. 2015; Wehner and Srinivasan 1981). While we know much about the naive learning walks of foragers, we do not know how naive dumpers overcome the navigational challenges of learning the nest surroundings.
In learning walks, desert ants frequently engage in body rotations with saccades to fixate in different directions, enabling them to learn and remember the nest panorama. Previous studies on Cataglyphis noda, Cataglyphis aenescens, and Melophorus bagoti have identified two types of body rotations: pirouettes and voltes (Deeti and Cheng 2021a; Fleischmann et al. 2017). During pirouettes, ants turn their bodies completely (± 360 deg) or partially (± 180 deg) to face different directions, intermittently stopping and fixating their gaze for at least 100 ms. This behaviour likely facilitates the learning of views of the distant panorama around the nest entrance. Voltes involve ants walking a small circle in a rotational movement without stopping, potentially allowing the calibration of celestial cues for navigational purposes. However, the precise role of these turns in navigation remains unclear.
In navigation, ants combine various sources of navigational information flexibly. In their initial inbound journey, ants primarily rely on path integration (PI)-based guidance and goal-directed image-matching information acquired near the nest (Freas and Cheng 2018). The resulting route memories enable ants to navigate precisely even in the absence of a celestial compass (Collett 2012; Collett et al. 2003). Experiments introducing conflicting information from two sources demonstrate that ants generally integrate both guidance systems to steer their path (Collett 2012; Bregy et al. 2008). In certain situations, foragers seem to strategically weigh different information sources following an optimal Bayesian weighting, where the weighting assigned to a source depends on its reliability (Legge et al. 2014; Hoinville and Wehner 2018; Narendra 2007; Wehner et al. 2016; Wystrach et al. 2015). Thus, all systems operate simultaneously, and their outputs are integrated, depending on the certainty and reliability of sources and on the motivational state of the animal.
To our knowledge, all the studies on navigation and learning walks in desert ants have been conducted foragers. But ants also perform another duty outdoors that requires significant travel, that of dumping waste. An earlier study on the dumping behaviour of red honey ants measured the distance at which they disposed of experimentally provided materials (Deeti et al. 2023a). Except for ant larva shells, the items of animal origin were dumped at average distances exceeding 5 m. Conversely, items with little or no animal content were dropped at average distances less than 1.2 m (Deeti et al. 2023a). However, how naive ants, unfamiliar with their surroundings, learn enough navigational information to suceessfully carry out dumping is unknown. For the distances of travel in dumping, it seemed to us that dumper red honey ants would need to execute learning walks before starting off on any dumping trip.
The red honey-pot ant Melophorus bagoti is a most thermophilic ant on the Australian continent (Christian and Morton 1992). The ants perform outdoor activities during the daytime on hot summer days, the activity ceasing during the twilight evening, entire night and early morning. Each evening a single worker closes the nest entrance from the outside and remains stationed nearby until the next day when the ants inside open the entrance (personal observations). Foragers primarily scavenge deceased arthropods but also collect sugary plant exudates and seeds (Muser et al. 2005; Schultheiss and Nooten 2013). Foraging ants work individually for short durations, covering distances up to 50 m from the nest, relying on path integration and terrestrial visual landmarks without utilizing any chemical trails (Cheng et al. 2009). Besides foraging, some workers dedicate themselves to excavating the nest throughout the day. Nest excavators remove sand from the nest, depositing it within a 10 cm radius outside (Deeti et al. 2023a, Deeti et al. 2024 in prep). A separate study examined the behaviour of naive excavators, while this study focuses on waste dumpers, carrying out the third kind of outdoor task.
In waste management, these red honey ants employ two distinct strategies: transporting waste materials away from the nest during the active above-ground summer months and depositing waste in subterranean latrine channels/tunnels, approximately 10 to 12 cm below ground level (personal observations), a behaviour likely persistent throughout the year (Deeti et al. 2023a). This study characterizes M. bagoti workers’ paths in their dumping behaviour, for the first time examining the role of experience in waste disposal. We investigated the paths taken by both naive (inexperienced) and experienced ants when they engaged in removing waste from inside the nest and depositing them away from the nest. Considering the distances at which materials are dumped, our initial hypothesis posited that naive dumpers would likely cover shorter distances compared to their experienced counterparts in the cluttered terrain. We also thought that naive ants would conduct at least one learning walk before dumping, as foragers conduct multiple learning walks before foraging (Deeti and Cheng, 2023a). Another question is: do these dumpers prefer to dump materials in the same sector of directions from the nest or in scattered, random directions across individuals? To understand all these issues, we documented the characteristics of dumpers’ walks, including their structure, duration, and the distance traversed from the nest site, then compared the paths of naive and experienced dumpers.