Lasioglossum hemichalceum

Taxonomy and phylogeny

The Lasioglossum hemichalceum is part of the genus Lasioglossum. The two species that are most closely related to L. hemichalceum are the Lasioglossum florale and Lasioglossum lanarium. Other species within this genus include Lasioglossum malachrum, Lasioglossum zephyrum and Lasioglossum clarum. The next most closely related species is the Lasioglossum zonulum. Unlike some species within the Halictidae family, this particular genus is characterized by very fragile wings, which can sometimes inhibit the L. hemichalceum from flying, and have small veins near the outer portions of the wings. Lasioglossum species which occupy parts of Africa and central Europe demonstrate varying types of social behavior—ranging from solitary to communal and even semi-social. Variability in sociality is correlated with both the altitude and latitude that the bees occupy. As altitude and latitude increase, the amount of solitary behavior also increases. The L. hemichalceum is in the company of nearly 350 other species that fall within the genus of Lasioglossum and are also native to Australia. However, the greatest amount of variability among bees within this subfamily of halictinae is seen in Africa and Madagascar.

Morphology

Lasioglossum hemichalceum can be identified by its medium long antennas, often off-white in color, and triangular head. While many are dull black in color, there are also members that can be various shades of purple, green, or copper. Males typically have thinner bodies than females and also have less hair on their hind legs for carrying pollen. Females usually range from 4.7–5 mm in length, and have wider heads than males (being about 1.5-1.6 mm in width). Male members have slightly shorter body lengths, and heads that are more narrow in length (approximately 3.8-4.7 mm in length, and 1.3-1.5 mm in width). In addition, the legs of the L. hemichalceum tend to be dark brown or black in color, although there have been instances of red-brown pigmentation. In terms of coloring, there are not significant differences between female and male members. Furthermore, because the L. hemichalceum is a communal species, there are not queens who are distinguishable from workers—almost all members appear the same in appearance.

Male dimorphism

There are two possible morphs that male Lasioglossum hemichalceum can take on: either that of a male bee with typical proportions and small in size, or the other which is referred to as macrocephalic. Macrocephalic morphology is characterized by a disproportionately sized head and mandibles. In some species of bees there is variation between different morphologies, however, in the species of L. hemichalceum, bees are predominantly dimorphic. Macrocephalic males have wider heads than both females and small males and also have a heavier weight when they are pupae than most female pupae. Small males typically spend more of their time away from the nest gathering pollen than macrocephalic males as well.

Nest

Female Lasioglossum hemichalceum are known to build their nests underground. They will construct burrows and carry the dirt that was removed to the surface. At the beginning of the colony initiation, the females will construct a large number of tunnels; however, as females get ready to overwinter, they will participate less and less in nest construction. They also do not coordinate with one another when constructing the tunnels, but do so independently of one another.

Distribution and habitat

While members of the genus Lasioglossum can be found in nearly every part of the world and are one of the most common bees in North America, the L. hemichalceum is specifically endemic to specific regions of Australia. Examples of cities in which the L. hemichalceum can be found include New South Wales, Victoria, and Queensland. Their environment in southern Australia is one that has wet winters that are fairly mild and summers that are warm and dry. Nests are not frequently built above ground and are typically constructed by separate groups of egg-laying females.

Colony initiation

Colony initiation starts late in the month of November and typically ends in the early spring, around February or April, after females have overwintered. The individual members of the group have two options in terms of where they would like to initiate a new colony. Their first option is to find depressions in soft ground that are appropriate for building a nest and build a new nest in accordance with the description above. However, they may also choose to re-use the nest from the year before, which happens about 40% of the time. In certain cases, members may even choose to add on to old nests so that a mixture of both old and new nests are created for the colony. New nests will be created regardless of whether or not there are other colonies who are occupying the same nest. This process of initiation is conducted sequentially, in which one female will start digging and other females will join her (this number is variable). The number of total bees that inhabit the nest will typically reach a couple hundred in size, with a larger percentage of females than males. However, it is important to note that these females are also unrelated, which is different from other communal and eusocial species.

Colony growth and decline

During this time, the reproductive females will raise two separate generations of broods, one that is reared by the mother, and the second which is reared by both surviving mothers and their daughters (which originated from the first brood). However, the number of broods that are initiated depends on climate conditions. For example, if it is rainy rather than dry, only one brood will be raised rather than two (which is more typical). Furthermore, if new nests are created, it typically happens between the first and second brood initiations. In this way, the colony initiation cycle and nesting cycle are in rotation with one another. Once the eggs have been laid, the egg-laying females will construct individual brood cells, each with a certain amount of nectar and pollen. An interesting fact is that this pollen is rolled into a small ball, and the egg is laid on top of the ball before the cell is sealed. Differences in the size of these pollen balls (which are the only food that is provided to the growing larvae) is thought to contribute to differences in weight for adult members. The pupal stage lasts 18 days alone, however, the L. hemichalceum's complete journey to adulthood will span approximately 6 weeks. Colony decline happens during the summer months between the end of egg production and overwintering. However, because the females of L. hemichalceum migrate between different nests, more than one nest exists at a time.

Communal behavior

The Lasioglossum hemichalceum demonstrates behavior that aligns with that of a communal species. This means that members will occupy the same nest and share the duties of caring for a brood regardless of whether or not it is their own, and they all have the capacity to reproduce. They often have colonies that are small in size, and relatively well concealed. Furthermore, unlike any other known species of bee, members of the L. hemichalceum will share nectar with one another via trophallaxis regardless of whether they are nestmates or not. They also demonstrate remarkably cooperative behavior towards each other even though the majority of the population members are not related. This has demonstrated that interactions between group members are not restricted to kin, and that L. hemichalceum tend to differ from solitary, eusocial species. Also unlike most communal species of bees, members of the L. hemichalceum do not occupy nests over the course of multiple generations, however they may re-use a nest at some point in their life cycle.

Dominance hierarchy

Because L. hemichalceum is a communal species, it displays egalitarian behavior. This means that members do not display a distinct division of labor or hierarchy. All individual females are also able to reproduce, which is probably related to the highly cooperative behavior and egalitarian behaviors that these bees display. In accordance with their lack of hierarchy, members rarely display threatening behaviors and do not discriminate in terms of genetic relatedness for these threatening behaviors. Thus, agonistic displays are not directed towards individuals based on their familiarity as nestmates or familial members. This egalitarian caste system stands in stark contrast to that of the L. zephyrum which is both eusocial and has a division of labor based on reproductive capacity.

Mating behavior

L. hemichalceum displays different mating behaviors for different morphologies. Males who are born with the typical small body morphology tend to display mating behaviors away from the nest. This contributes to outbreeding and makes the genetic pool much more diverse. However, males that display the microcephalic body morphology will remain in the nest and mate with female colony members there. In order to win the right to mate with resident females, microcephalic males will fight each other until they are severely injured, and even kill each other. Because microcephalic males are able to mate with several females, their offspring will be genetically related, thus, the females migrate away from their natal nests in order to reduce intra-colony relatedness. Females will typically overwinter and will not start reproducing until the months of the spring season. Some seasons are worse than others in terms of food availability and predation, so some females will decide to stay and mate in the colony rather than leave, although this is less typical behavior for adult females. If a female needs to exit the nest to obtain nectar or pollen, it is sometimes better to take advantage of this opportunity and mate outside the colony.

Genetic relatedness

Genetic relatedness among colonies of L. hemichalceum differs depending on the type of relationship. Because the L. hemichalceum is a communal species, there are not queen-worker dynamics that can be identified in terms of genetic relatedness. Genetic relatedness is determined by using mean relatedness values (r) among various sizes of colonies (most samples had approximately 200 member colonies). For mother-daughter relationships, genetic relatedness is low in comparison to other species of bee (r=.75). Although mothers and daughters share the minimum of one allele, only 11% of daughters who were immature shared the colony with their mother. This indicates that most young females do not have their mother present. However, this statistic was even lower for mother-son relationships (r=.125). Indeed, only about 20% of all juvenile members in the colony had any type of closely related adult present. These statistics seem to reveal that parental care is highly generalized and occurs in the absence of kin selection and is probably related to the fact that members act indiscriminately towards each other. Nest switching and migratory behaviors also result in the decreased genetic relatedness among adult members and their respective kins.

In terms of genetic relatedness among sister-sister relationships and sister-brother relationships, however, there is a substantial difference. L. hemichalceum tends to avoid inbreeding whenever it is possible because of their sex determination system, which uses a single locus mechanism. Should members of the same family breed with one another, there is the possibility that the male offspring will either be haplodiploid (normal phenotype) or diploid (sterile phenotype). Thus interbreeding is a highly avoided behavior by many of the female members. This lack of interbreeding impacts the genetic relatedness of siblings. For sister-sister relatedness there tends to be a significant number of full sister relationships within the colony among juvenile members. This may be attributed to the fact that juvenile members do not need to avoid their female siblings in the same way that they do male siblings in order to avoid inbreeding. Along the same lines, sister-brother relatedness is low within the same nest because the females tend to leave if there are male relatives nearby.

Costs and benefits to sociality

There are many advantages and disadvantages for the L. hemichalceum in terms of living communally in a large colony. While many members may be living with unrelated individuals (about 50% of adult females do not have daughters or sisters of any type nearby in the colony), they gain many advantages. One of the primary benefits is constant care for larvae and offspring. It takes approximately six weeks for a larvae to reach adulthood, and by having other adults nearby that are able to look after the brood, the risk of predation from ants is significantly reduced. Another benefit is life insurance. Foraging is highly risky activity, and should a mother be killed while foraging, her offspring will be cared for by other members of the colony. Thus, forming a colony that is larger in size (the typical colony is about 200 members), is advantageous in terms of brood rearing and protection. However, there are disadvantages as well. While the presence of adult L. hemichalceum is effective at warding off unwanted predators, such as ants, there is no active defense behaviors. This makes it less advantageous to be in a large colony in moments when active defense is needed. Furthermore, another disadvantage are cheaters who exploit the cooperative protection that the colony offers. It is thought that communal protection of unknown broods prevents exploitation, however this theory is still being explored.

Kin recognition

While it is thought that L. hemichalceum displays kin recognition, it is yet to be confirmed. Originally the cooperative behavior and act of trophallaxis that is displayed towards both genetically related kin and non-related kin was attributed to lack of kin recognition. However, it is now thought that members simply do not show special treatment towards their own kin. This is exemplified most saliently by the L. hemichcalceum's lack of guarding against unknown nestmates, and the migration of female members between different nests. Thus, this attitude of universal acceptance has led scientists to conclude that there is either the absence of recognition cues, or lack of responsive behavior to those cues. Furthermore, while there are no differences in behavior pattern that can be attributed to relatedness, there are differences associated with the grouping of several nests, suggesting that there may be environmental cues for behavior.

Agonistic behavior

Unlike other species of Lasiolgossum, members of the species L. hemichalceum rarely display agonistic behaviors towards another. Agonistic behaviors typically include patterns such as lunging or moving their bodies in the shape of a C while pointing the stinger and mandibles towards the offender. Typically the C-shaped body (also known as C-posture) behavioral pattern was observed in about 19% of conflicts, and even fewer in terms of the lunging pattern. This demonstrates that although members are capable of displaying agonistic behaviors, they choose not to. Lack of agonistic behavioral patterns is a characteristic of the L. hemichalceum that is very different from L. hemichalceum's close relative, L. zephyrum.

Variable pupae nutrition

For L. hemichalceum larval nutrition was one of the largest environmental factors in determining male morphology. When the female members prepare to lay eggs, they first construct individual cells and place small pollen balls (that are a mixture of nectar and pollen) into the cells. However, the variation in both the size of the cell and the pollen ball seems to impact whether or not male morphology is macrocephalic or that of a normal small body. The factors impact one another; for example, the dimensions and volume of the cell dictate the amount of nectar and pollen that females gathered, and the subsequent size of the pollen balls for the cells. However, the size of the pollen balls also affect the pathway for development. Typically females will place fertilized female eggs on pollen balls that are proportionately bigger than the ones given to males. However, it is hypothesized that macrocephalic males may be the result of females unintentional placement of unfertilized eggs (male eggs) onto pollen balls that are intended for female eggs. By providing males with additional nutrition, they are able to attain heavier weights as pupae, and develop into larger, bulkier adult males. Thus, dimorphic male development in L. hemichalceum is thought to be impacted by cell size and pollen ball size, rather than just genetic differences between males.