A key focus of ecologists is explaining the origin and maintenance of morphological diversity and its association with ecological success. We investigate potential benefits and costs of a common and varied morphological trait, cuticular spines, for foraging behavior, interspecific competition, and predator-prey interactions in naturally co-occurring spiny ants (Hymenoptera: Formicidae: Polyrhachis) in an experimental setting. We expect that a defensive trait like spines might be associated with more conspicuous foraging, a greater number of workers sent out to forage, and potentially increased competitive ability. Alternatively, consistent with the ecological trade-off hypothesis, we expect that investment in spines for anti-predator defense might be negatively correlated with these other ecological traits. We find little evidence for any costs to ecological traits, instead finding that species with longer spines either outperform or do not differ from species with shorter spines for all tested metrics, including resource discovery rate and foraging effort as well as competitive ability and anti-predator defense. Spines appear to confer broad anti-predator benefits and serve as a form of defense with undetectable costs to key ecological abilities like resource foraging and competitive ability, providing an explanation for both the ecological success of the study genus and the large number of evolutionary origins of this trait across all ants. This study also provides a rare quantitative empirical test of ecological effects related to a morphological trait in ants.

These data were collected at the Xishuangbanna Tropical Botanical Garden (XTBG) in southern Yunnan in southern China. During June and July of 2017 and 2018, we collected Polyrhachis colonies in and around XTBG. We kept the colonies in the field station laboratory in plastic containers (17 × 11.5 × 10 cm), and regularly provided each colony a standard liquid sugar diet (50% sugar/50% water solution) as a rough approximation of their natural sugary food sources. Species were identified in the laboratory using the primary literature and images from AntWeb.org (accessed June–July 2017). Species voucher specimens were deposited in the Field Museum of Natural History in Chicago, IL, USA, and the Southwest Forestry University Specimen Hall in Kunming, Yunnan, China.

We then conducted a number of resource discovery rate, foraging effort, and competitive ability trials, as well as anti-predator ability trials using the myrmecophagous spide Siler semiglaucus. See the associated manuscript for further details about the trials we conducted.

The analyses we conducted should run in a self-contained manner, as described in the R Script incldued as a file here.