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Fortress defense is one of the two factors that influences organisms to help their relatives rather than reproduce on their own. Fortress defenders nest and feed in protected enclosures that can accommodate many individuals, especially a class specialized as soldiers for territorial defense. The main advantage of grouping for these social insects is to defend the valuable resource of the nest since it provides protection and a source of food for the juveniles—the idea that two or more snaps, a defense mechanism of S. regalis , is better than one against intruders. Since food is already present in the nests, the focus of grouping in this case is not for foraging, but rather for protection against predators. Examples of other fortress defenders include mole rats (Sherman et al. 1991; Jarvis et al. 1998), social shrimp (Duffy 1996a), thrips (Chapman et al. 2000), aphids (Benton et al. 1992), beetles (Schuster et al. 1985), and termites (Thorne 1997; Bartz 1979).
The second factor is life insurance which arises in social insects that forage for food outside of the nest, which exposes them to predators ( [link] ). These insects must obtain nourishment outside of nest, unlike fortress defenders, since the young can not feed themselves and require food for development which the nest does not provide (Strassmann et al. 2007). The parent must undertake dangerous foraging for young in order for them to reach adulthood, but if the parent dies all of the offspring also die due to starvation—wasting the investment the parent had placed in the brood. However, an adult daughter can prevent this from occurring by staying in her natal nest to help protect and feed the dependent young. Therefore, if the parent dies while foraging the adult daughter will be able to take her place and raise the dependent brood, taking the role of reproduction as well. The different characteristics of Fortress Defenders and Life Insurers are portrayed in [link] (Queller and Strassmann 1998).
In the black dwarf honey bee, Apis andreniformis , the daughters of the queen care for the larvae, maintain and defend the hive, and forage for food outside of the nest (Picture 3). This species is a eusocial species of life insurers since they forage for food outside of the nest in order to feed the dependent juveniles (Arias et al. 2005; Keller et al. 1994). The queen bee smells each egg ensuring that all of the eggs are produced by the queen; if an egg smells foreign then it will be immediately removed from the nest by the queen (Pirk et al. 2004; Visscher et al. 1995). There can only be one queen and the chance of individual survival is very low making direct fitness unlikely for a solitary individual. The worker bee has a better chance of increasing its fitness through indirect fitness, rather than direct fitness, by helping the queen mother rear offspring.
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| A Queen Bee | A Drone Bee | A Worker Bee |
| Characteristics | Fortress Defense | Life Insurers |
| Taxa | Thrips, aphids, beetle, termites | Ants, bees, wasps |
| Main advantage of grouping | Valuable, defensible resource | Overlap of adult gen |
| Food | Inside nest or protected site | Outside nest |
| Juveniles | Active, feed selves and may work | Helpless; need to feed |
| Nonsocial ancestors | Not necessarily parental | Highly parental |
| First specialized caste to evolve | Soldiers | Foragers |
| Colony Size | Usually small | Often large |
| Ecological Success | Usually limited | extensive |
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