Spartan defense in the Thermopylae pass: Strategic defense by aggregations of Pseudomyrmex spinicola (Hymenoptera, Formicidae) on the trunk of Acacia collinsii (Mimosaceae)

Abstract

Acacia ants (Pseudomyrmex spinicola) usually prune all of the vegetation growing in the vicinity of their host plant (Acacia collinsii). This lack of branches in contact with the host tree makes the main stem of the acacia the only point of access for ants invading the resident colony. The present study assessed whether the intrusion site affects the probability of the intruder being detected. It also investigated whether ants that remain inactive on the main trunk base (a) are involved in defensive duties and (b) are morphologically different from other colony members (e.g. foraging ants). In the field, intruder ants were placed on the trunk and leaves of acacias inhabited by P. spinicola; colony members on the trunk base and the leaves were marked and monitored; and head length, head width, and thorax length of ants that were either foraging on the leaves or immobile on the trunk were measured. P. spinicola ants found intruder ants entering their host acacia via the trunk more rapidly than those entering by means of the leaves. Ants on the base of the trunk do not exchange tasks with foragers: between 5 and 47 % of the 1063 ants originally marked on the trunk were re-observed there, whereas only two of the 804 ants marked on the leaves were seen on the trunk. However, ants on the trunk were similar to foragers in morphology, suggesting that defensive ants may be part of a behavioral caste.

Behavioral responses of acacia ants correlate with age and location on the host plant

“…The tasks regularly performed by leaf-marked ants could explain their bias toward carrying items. Ants on the trunk seem, in contrast, to be involved in defensive tasks (Amador-Vargas, 2008), and this may be associated with their more aggressive responses. Crematogaster nigriceps associated with Acacia trees in Africa also have a worker caste that readily leaves the swollen thorns to defend the tree and it is physically distinguishable from other workers that tend to remain inside the spines (Stapley, 1999).…”

“…The size of the circle represents the number of cases. The line shows the logit function from the logistic regression spinicola do not appear to have physical castes, and at least trunk-and leaf-marked ants are not different in thorax length or head size(Amador-Vargas, 2008).…”

Ecologia comportamental e de interações entre <i>Pseudomyrmex concolor</i> Smith (Hymenoptera, Formicidae, Pseudomyrmecinae) e sua planta hospedeira <i>Tachigali myrmecophila</i> Ducke (Fabaceae: Caesalpinioideae) na Amazônia amapaense

“…Em outros sistemas mutualistas formiga-planta (e.g. Acacia-Pseudomyrmex) operárias patrulhando as folhas de sua planta hospedeira também são menos ágeis em detectar intrusos, provavelmente porque as formigas dessa região estão associadas com a coleta de alimento disponibilizado pela planta (NEFs e corpúsculos proteicos) e não diretamente com a defesa como as formigas do caule (Janzen, 1966;Amador-Vargas, 2008;. Apesar de aparentemente T. myrmecophila não disponibilizar nenhum recurso alimentar direto à colônia de P. concolor, suas operárias estão frequentemente investigando as folhas da planta.…”

“…Porém, parte do material analisado nesses estudos foca em exemplares de museus que, obviamente, desconsideram variações etárias e de localização das operárias dentro na colônia. Apesar disso, ainda que escassos, alguns trabalhos têm demonstrado que formigas do gênero Pseudomyrmex em associações mutualistas com plantas mirmecófitas apresentam divisão de trabalho baseado no tamanho da colônia, idade e localização espacial das operárias em sua planta hospedeira (Amador-Vargas, 2008;2012a;Amador-Vargas et al, 2015). Em P. concolor, operárias presentes no caule ficam frequentemente paradas e com suas cabeças direcionadas para superfície do solo, aparentemente resguardando essa via de acesso em T. myrmecophila (observação pessoal).…”

“…de Acacia collinsii (Amador-Vargas, 2008). Nas folhas, as operárias de P. concolor permanecem paradas raspando a superfície do limbo foliar de sua planta hospedeira ; capítulo 1).…”