Fortress defense

The benefits to being altruistic come in two ecological modes: “life insurers” and “fortress defenders”.^[2] Most Hymenoptera, the large majority of social insects, are life insurers, where eusociality is adapted as a safeguard from decreased life expectancy of offspring.^[3] Termites, as fortress defenders, benefit from working together to best exploit a valuable ecological resource, in the case of Nasutitermes corniger a vast wood gallery. Fortress defense is sufficient to evolve eusociality when three criteria are met: food coinciding with shelter, selection for defense against intruders and predators, and the ability to defend such a habitat.^[4] Termite colonies are generally large enclosed nests or mounds that house large supplies of wood for the termites to exploit, fulfilling the first criteria for fortress defense. In N. corniger, the soldier caste has had their heads modified to spew a noxious, sticky liquid when under attack from Tamandua anteaters.^[5] The secretion contains pinene, limonene and other high molecular weight compounds that deter the anteater from returning. The termites then remain on guard near the breach for several minutes. This adaptive morphology and defense of the habitat are sufficient for satisfying the second two criteria for fortress defense. The fortress defense strategy necessitated the evolution of soldiers first, which has resulted in the unique specialization of the nasute termites.

Associations

Numerous species of ants cohabit the nests of N. corniger or colonise them once the termites have abandoned them. Some species prey on the termites but others do not. Studies with radioactive tracers have shown that when cohabiting nutrients flow both ways between the ants and the termites. Monacis bispinosa, also known as Dolichoderus bispinosus is one of the most common ant species to cohabit with the termites but is susceptible to their chemical defences and cannot prey on live termites. Camponotus abdominalis associates with termites less often but is an aggressive predator of the termites. Camponotus species and Dolichoderus diversus have been found to inhabit N. corniger nests that have been abandoned.^[10]

Crematogaster brevispinosa rochai is one subspecies of ant whose interaction with N. corniger has been studied. C. b. rochai lives in areas of caatinga in Brazil. No queens of the ant have been found in the nests but their larvae of all castes and sexes have been. Nests that contain C. b. rochai do not have a termite queen in either. It can therefore be concluded that both the ants and termites are members of polydomous colonies that each have numerous nest sites. The ants and termites are segregated within the nest and do not normally come into contact with each other. C. b. rochai plug channels at the boundary of the areas they occupy to cause this segregation. On occasions when the ants and termites do come into contact with each other (e.g. if the nest is broken into) they are rarely aggressive and tend to avoid each other instead. It has been hypothesised that the hydrocarbon content of their cuticles may have changed to allow them to live together relatively peacefully.^[10]

The termites are thought to benefit from the association as the ants leave debris in the nest containing nitrogen and that this increases the availability of this important nutrient in an environment where it is scarce. They may also benefit from the ants protecting the nest from predators. The ants benefit as the termite nests provide an ideal location to raise broods, particularly of reproductive castes. The climate is suitable for this and the nests are easily defended against predators. Other Nasutitermes species have been found to produce anti-fungal compounds and these would also be beneficial to the ants although it is not known if N. corniger do produce such compounds.^[10]

The White-throated Round-eared Bat, Lophostoma silvicolum, roosts inside the nests of N.corniger. Males excavate the roost themselves, expending considerable energy whilst doing so. They consequently gain reproductive success as a harem of females will join them in the roost. The termite nest is an ideal temperature for raising young and provides protection from predators of the bats. N. corniger repair the damage made to the nest by the bat meaning that the males have to constantly maintain the roost. Once the bats leave the cavity is filled by the termites within a few weeks.^[11] Scientists are currently investigating how the bats are able to create the roosts without being attacked by the termites.^[12]

Several bird species including trogons, puffbirds and parakeets also form nests in termite nests.^[13] These can be distinguished from those made by bats as they have a horizontal entrance whereas those made by bats have a vertical entrance at the base of the nest.^[14]

Symbioses

The entire gut flora of a termite very closely related to N. corniger has been analysed using metagenomics to determine the function of different microbes in their gut.^[15] Typical to all wood-feeding higher termites, bacterial gut microbiota in the guts of N. corniger are dominated by insect-specific members of TG3 (candidate_phylum), Fibrobacterota, and Spirochaetota.^[16] It has also been shown that the same bacterial lineages are preferentially enriched in the cellulolytic bacterial community that is associated with wood particles in the gut.^[17] In addition to a role in fiber digestion, Symbiotic bacteria in N. corniger have also been shown to fix nitrogen at a rate of 0.25-1.0 mg N per colony per hour. This suggests a nitrogen doubling time of 200–500 days making it possible for the whole population of the colony to be replaced once or twice each year.^[18]