A morphologically specialized soldier caste improves colony defense in a neotropical eusocial bee

Abstract

Division of labor among workers is common in insect societies and is thought to be important in their ecological success. In most species, division of labor is based on age (temporal castes), but workers in some ants and termites show morphological specialization for particular tasks (physical castes). Large-headed soldier ants and termites are well-known examples of this specialization. However, until now there has been no equivalent example of physical worker subcastes in social bees or wasps. Here we provide evidence for a physical soldier subcaste in a bee. In the neotropical stingless bee Tetragonisca angustula, nest defense is performed by two groups of guards, one hovering near the nest entrance and the other standing on the wax entrance tube. We show that both types of guards are 30% heavier than foragers and of different shape; foragers have relatively larger heads, whereas guards have larger legs. Low variation within each subcaste results in negligible size overlap between guards and foragers, further indicating that they are distinct physical castes. In addition, workers that remove garbage from the nest are of intermediate size, suggesting that they might represent another unrecognized caste. Guards or soldiers are reared in low but sufficient numbers (1-2% of emerging workers), considering that <1% usually perform this task. When challenged by the obligate robber bee Lestrimelitta limao, an important natural enemy, larger workers were able to fight for longer before being defeated by the much larger robber. This discovery opens up opportunities for the comparative study of physical castes in social insects, including the question of why soldiers appear to be so much rarer in bees than in ants or termites.Meliponini | caste evolution D ivision of labor among workers is nearly universal in eusocial insects (1-5). In most species, this division is based on temporal subcastes, in which workers specialize in different tasks at different ages ("age polyethism") (2, 4, 5). Physical worker subcastes (i.e., groups of workers with morphological adaptations for particular tasks) are less common (4-6). Only ∼15% of all ant genera contain species with physical worker subcastes, and no species of bee or wasp is known to have a division of labor based on physical subcastes (4-6). Where physical worker subcastes occur, they appear to benefit the colony by increasing the efficiency of key tasks such as defense, but have the disadvantage of preventing a colony from rapidly adjusting caste ratios if necessary (4-6). In addition, extreme subcastes, such as large-bodied soldiers, are more costly to rear and have a limited task repertoire (4). It also has been argued that individual-level selection might prevent the evolution of worker polymorphism if workers of different subcastes have varying opportunities to reproduce (5, 7). Given the number of ant and termite species with physical worker subcastes, the lack of evidence for physical worker subcastes in bees and wasps is an intriguing and ongoing puzzle in the study ...

Context affects nestmate recognition errors in honey bees and stingless bees

“…Each colony had a wax entrance tube built by the bees. Guards stood around the tip of the entrance tube [see fig.1A in Grüter et al (Grüter et al, 2012)], where their behaviour and interactions with experimentally introduced conspecifics could be observed. We used six discriminator colonies (F-K) plus three additional colonies (L-N) as sources of non-nestmates.…”

Abelhas-sem-ferrão amazônicas defendem meliponários contra saques de outras abelhas

“…No caso de T. angustula, Grüter et al (2012) demonstraram a existência de diferenciação morfológica entre as operárias, sendo as abelhas maiores as responsáveis pela defesa da colônia. A resistência aos saques seria assim explicada pela existência, em espécies resistentes, de um grupo diferenciado de operárias responsáveis pela defesa da colônia.…”

Decreasing worker size diversity does not affect colony performance during laboratory challenges in the ant Temnothorax nylanderi

“…This has been shown in species where workers are highly variable morphologically or where a discrete soldier caste exists such as harvester ants (Wilson 1984;Arnan et al 2011), turtle ants (Powell 2009) and leaf-cutter ants (Evison et al 2008) among others (e.g. Porter and Tschinkel 1986;Foster 1990;Hasegawa 1993a;Hasegawa 1993b;Passera et al 1996;Harvey et al 2000;Perry et al 2004;Toth and Duffy 2008;Grüter et al 2012). However, Fjerdingstad and Crozier (2006) highlighted the fact that worker size diversity is limited in the majority of ant species, including some of the most successful species.…”