Exoneura robusta

Exoneura robusta is a species of the primitively eusocial allodapine bee, belonging to the genus commonly referred to as 'reed bees'. Their common name derives from their use of the soft pith of dead fern fronds as a nesting material. They are native to southeastern Australia, living in both montane and heathland habitats. E. robusta do not have a fixed pattern of sociality, but rather they are capable of adapting their social strategy to different environments. While typically univoltine (producing one brood per season), populations living in warmer habitats (such as those in higher latitude heathlands) are capable of producing two broods per season. This leads to the incidence of sibling rearing and eusocial behavior. E. robusta lack strict morphological castes, thus allowing for their plastic social behavior and dominance hierarchies. Exoneura robusta is a species of Australian allodapine bee. American zoologist Theodore Dru Alison Cockerell first described E. robusta in 1922. It belongs in Apidae family within the order Hymenoptera, which consists of ants, wasps, sawflies, and bees. The species was initially classified as the montane population of Exoneura bicolor, but was more recently reclassified as E. robusta. Bees in the genus Exoneura are commonly referred to as “reed bees,” due to the substrate in which they build their nests. E. robusta have a black head and thorax, and an orange-brown abdomen. Their legs are yellow-orange, and they have hind wings. They are capable of delivering a small sting. They can be identified by their nest site, as they almost exclusively build their nests in the dead fronds of the tree fern Cyathea australis. Newly founded nests can be identified from a reused nest due to their clean appearance that shows only traces of pollen. In contrast, reused nests have a dark coloration caused by the aging of pollen and nectar from the previous year's brood-rearing activities. E. robusta are native to southeastern Australia. They can be found in both montane and heathland habitats. In montane habitats, they build their nests in the dead fronds of C. australis, while in heathland habitat they tend to nest in eucalyptus branches. Their nests are constructed in the pithy center of frond stems, and consist of a single, undivided burrow. E. robusta is an important pollinator in the forests of southeastern Australia. As will be discussed in detail in a later section, the sociality of E. robusta is influenced by its habitat. New nests are founded only during a two-week period in early spring. These bees typically do not stray far from their birth nest during colonization, leading to a low dispersal rate. An unusual behavior for most bees, E. robusta colonies will be co-founded by two to eight females. These co-foundresses are typically related, but if no kin is available they will co-found a colony with unrelated females. Egg laying occurs during the end of winter and throughout the spring, and new adults emerge from their pupae (eclose) during the summer. Adults overwinter, and nests can be reused for six to ten years by the descendants of the founders. E. robusta is generally univoltine, meaning they produce only one brood per season. However, this does depend on habitat, which will be discussed in a later section. Colony sizes tend to be small, with an average nest containing 6.7 offspring. The most productive females produce an average of 4.7 offspring in a season. E. robusta have very adaptable (plastic) behavior compared to other bee species. They have no morphological castes and females have the option of nesting alone or in groups. However, in larger colonies they do show behavioral differentiation in which colony members will specialize as guards, nurses, or foragers. It is important to note that these differences are behavioral, not morphological, meaning that every colony member has the biological capacity to perform any role. The social structure of E. robusta is polyphenic, meaning different behavioral phenotypes can arise from the same genotype based upon environmental conditions. Depending on habitat, these bees can exhibit solitary, semisocial, quasisocial, or eusocial colony structure. The type of sociality found in this species of bee largely depends upon the number of broods produced per season. In most cases, only one brood is produced per season, leading to a quasisocial organization. However, some heathland populations have been shown to produce two broods per season, allowing for the incidence of sibling rearing and therefore the presence of eusociality. Essentially, social polyphenism allows E. robusta to respond to changing environments by keeping their behavior plastic. Reproductive hierarchies do not occur in newly founded nests, whereas a social dominance structure is developed in reused nests. In newly founded nests, a quasisocial organization is found in which all founding members are reproductive and a social hierarchy does not exist. Once a nest enters its second or later year of use, the first female to eclose takes on a reproductively dominant status resulting in a semisocial organization. This difference in eclosion rate can be as close as a few days; a female emerging from her pupa only a day before the other offspring in the nest is sufficient to establish the reproductive hierarchy. Females that eclose later tend to become foragers and will have higher mortality rates than reproductive females. First eclosed females take on a guarding role, which increases their risk of danger in from predators and competitors at the nest entrance. However, the increased risk of guarding pays the reproductively dominant females because it allows them to regulate the reproduction of their nestmates. Dominants are much less likely to allow a female to return into the nest after she has interacted with a foreign male. In contrast, eviction from the nest has not been observed in newly founded nests because the cofoundresses behave in an egalitarian, cooperative manner. It has been suggested that dominants control reproduction in nestmates through pheromonal signals that inhibit ovary development in the non-dominant females. Since reproductive status is not conferred through strict morphological differences, a dominant female is capable of passing on her dominant status to her daughters through her own behaviors. By preventing her nestmates from breeding through nest guarding (or evicting them if they do), she can ensure that her daughters will be the first eclosed and therefore will gain dominant status as she did. Secondary reproductives will either lay their inseminated eggs later, leading to their daughters also becoming secondary, or they will lay uninseminated eggs that will become males who do not compete for dominance. One reason why subordinates may acquiesce to this system is because group living is very important in E. robusta. Therefore, it may better pay a subordinate to refrain from mating, remain in the nest, and produce sons rather than trying to found a new nest solitarily. One question raised by the eviction of inseminated females is why male E. robusta would produce a scent marker at all. If this scent marker is what prevents subordinate females from re-entering the nest, it would hypothetically make sense for males to not produce a scent marker so the female could rear his offspring within the colony. The most likely explanation is that the scent is an unavoidable consequence of mating, a cue that cannot be avoided. Since females mate only once, it is not at all likely that the scent has anything to do with discouraging other males from mating with a given female. Another possibility is that the scent marker is what attracts a female to mate with a male in the first place, thus making it quite necessary. The presence of the scent could be adaptive or it could be a by product, not adaptive for the producing male.