The Soldiers in Societies: Defense, Regulation, and Evolution

Abstract

The presence of reproductively altruistic castes is one of the primary traits of the eusocial societies. Adaptation and regulation of the sterile caste, to a certain extent, drives the evolution of eusociality. Depending on adaptive functions of the first evolved sterile caste, eusocial societies can be categorized into the worker-first and soldier-first lineages, respectively. The former is marked by a worker caste as the first evolved altruistic caste, whose primary function is housekeeping, and the latter is highlighted by a sterile soldier caste as the first evolved altruistic caste, whose task is predominantly colony defense. The apparent functional differences between these two fundamentally important castes suggest worker-first and soldier-first eusociality are potentially driven by a suite of distinctively different factors. Current studies of eusocial evolution have been focused largely on the worker-first Hymenoptera, whereas understanding of soldier-first lineages including termites, eusocial aphids, gall-dwelling thrips, and snapping shrimp, is greatly lacking. In this review, we summarize the current state of knowledge on biology, morphology, adaptive functions, and caste regulation of the soldier caste. In addition, we discuss the biological, ecological and genetic factors that might contribute to the evolution of distinct caste systems within eusocial lineages.

Workers, soldiers, and gynes – morphometric characterization and description of the female castes of Camponotus singularis (Smith, 1858) (Hymenoptera, Formicidae)

“…The high nutritional investment necessary for producing a distinct soldier subcaste suggests that these animals must serve a function greatly beneficial to colony survival and fitness (Tian and Zhou 2014). In many of the previously studied ant species the tasks of major workers or specialized soldiers do not only include nest defence, but also storage of food and liquid within their bodies: This was found to be true for the phragmotic major workers of Colobopsis nipponica (Wheeler, 1928) (Hasegawa 1993), large workers of the polymorphic Camponotus foreli Emery, 1881 (Espadaler et al 1990) and the highly specialized soldiers of Cataglyphis bombycina (Molet et al 2014).…”

An apparent mutualism between Afrotropical ant species sharing the same nest

“…The large P. conradti workers might be more suited to repel large arthropods or vertebrates in parallel with the defense specialization of large workers in polymorphic ant colonies against large enemies (Lamon and Topoff 1981;Hölldobler and Wilson 1990;Batchelor et al 2012). There is a vast amount of literature that stresses the specialization of morphologically distinct worker castes in nest defense (Jandt et al 2013;Tian and Zhou 2014), but here, we argue that morphologically distinct ant species can be analogously specialized in different tasks. These distinct morphs in the compound nest could be an alternative strategy for worker polymorphism in a single colony of an ant species.…”

“…Finally, we studied the potential benefits for both partners of engaging in the association. In species with size-polymorphic workers, it is well documented that workers of particular size cohorts specialize on carrying out specific tasks inside the colony, such as nest defense (Hölldobler and Wilson 1990;Tian and Zhou 2014;Parmentier et al 2015). Analogously, we hypothesized that a similar specialization in nest defense between the two ant partners that vary greatly in size and morphology could bring distinct benefits to the association.…”

Social buffering in a eusocial invertebrate: termite soldiers reduce the lethal impact of competitor cues on workers

“…While many solitary animals exhibit a negative correlation between morphological defense and the magnitude of their behavioral responses to risk, the presence of specialized castes may allow this tradeoff to occur at the colony level in eusocial species (Tian and Zhou 2014). While many solitary animals exhibit a negative correlation between morphological defense and the magnitude of their behavioral responses to risk, the presence of specialized castes may allow this tradeoff to occur at the colony level in eusocial species (Tian and Zhou 2014).…”

“…The two dominant termite castes are workers, the primary foragers and nest caretakers who are often injured or killed during interspecific interactions (Shelton and Grace 1996), and soldiers, defensive specialists who provide little foraging benefit (Tian and Zhou 2014). These colonially living insects communicate via chemical and vibrational cues, and their almost exclusive reliance on cellulose for nutrition prevents them from using other termite species as a food source.…”