Selection and reproduction

by Giacomo Acerbi

Small portions of the sky…
le Drone congregation area – Dca

Near Near – When I will be – Near Near – And I will enter – Nei
Your borders – When I will be – Near near

Amedeo Minghi

We continue our “buzzing journey” in the evolution of knowledge on some important behavioral characteristics of the drone, on its preparatory and preparatory activities for mating, on how and in what way the gathering areas manifest themselves and finally on their role for the diffusion and specialization of bees and other hymenoptera on earth.

Is it a pile-up? No!

The drone congregation area (DCA) is anything but a banal “gathering of males” is, in fact, the specific way in which thousands of drones and virgin queens meet and where fertilization takes place. It is therefore and mainly a social behavior, a typical and almost exclusive dynamic of the Apis mellifera and of some species of polyandri meliponins: Scaptotrigona (Paxton 2000), Trigona Collina (Cameron 2004)…; this is achieved under certain environmental conditions: in a space from 30 to 200 meters in diameter (Ruttner 1965), usually from 15 to 40 meters
of height
from the ground (Ruttner 1966).

The DCA takes place in very precise air boundaries: the drones totally ignore the pheromone of the queens a few meters outside this circumscribed area (Ruttner and Ruttner 1965). In addition , the colonies of an area remain faithful to the same DCA: different generations of drones of the same hive attend the same DCA every season (Laidlaw and Page 1984). At the same time the males of a colony can be distributed in different DCAs surrounding their nest: their simultaneous presence has been recorded in 10 distinct gathering areas (Ruttner 1975). Each hive tries to participate in the genetic future of different bee populations, according to a precise survival strategy: avoid endogamy (1) and maximize genetic variability. The presence of a DCA in the same place has also been recorded for more than 50 years (Jean Prost 1957).

Brawl in the gallery by Umberto Boccioni

Brawl in the gallery by Umberto Boccioni, 1910

Several generations of hive drones attend the same DCAs each season. But the males of a colony may prefer different areas of gathering, near the hive. The same DCA can be visited by drones of different subspecies (Ruttner 1972) but at different times (Benstead 2009)

A gathering can be considered a DCA if attended by at least a thousand males and if there is the formation of drone comets; and finally its presence was recorded for at least two non-consecutive days, separated by at least two weeks (Loper 1992).
The number of males in a DCA can vary, depending on climatic conditions (temperature and wind) and the density of colonies in a territory of about 5 km radius (Ruttner 1976). The amount of drones of a DCA, in areas with a high density of colonies, is (simultaneous presence calculated by hourly average), from 2,145 to 11,750 drones up to a recorded maximum of 15,290 (Koeniger 2005).

The manifestation or not of a gathering area during the breeding season can be conditioned by a multitude of factors: territorial conformation, shelter from the winds, distance from the hives of origin of the males, solar irradiation and, not least, by the specific behavioral and ethological characteristics peculiar to the drones that intervene in it: by their ability to orient, flight, communication… In addition to the attitudinal aspects that may affect the election or not of a specific area to DCA, there are also discriminatory elements of a purely geographical and morphological nature of a territory linked to its conformation, the presence or absence of watercourses, forests, urbanized areas, fields.

Research and field experience have shown that the DCAs are located near open ground surrounded by trees or from a very high vegetation and that the presence of waterways, intersections of streets, the presence of great “obstacles” (for example: a centuries-old tree…), the alternation of areas without vegetation… probably they are among the factors that greatly facilitate the orientation of males and virgins and therefore contribute to the existence of DCAs (Loper 1992). Surprisingly, in the results of the research conducted by Galindo-Cardona, in 71% of the DCAs there was a portion of the territory with urban coverage: this may suggest that virgin drones and queens use houses, palaces, roads … as reference points, for orientation.

In addition, the majority of the areas examined in which a DCA was detected have a shelter from the winds blowing from the North and are characterized by a free horizon line, open, with few reference points (10%) and normally with a flat underlying terrain, with a maximum slope of 19%.

Mare ballerina by Gino Severini

Mare = ballerina by Gino Severini, 1914

The orientation for the identification of DCAs is connected not only to the recognition of landscape and morphological characteristics of the territory, but to a specific sensitivity of bees to terrestrial geomagnetism

Puerto Rico Apiaries

Figure A shows the map of Puerto Rico with the apiaries on the island (black triangles) and our apiary (star) identified. Figure B shows where there are DCAs (No. 8 and identified with a black pin) and where they are absent (blue squares) – from the Galindo-Cardona 2012 study

“Welcome to the South!”

The important and fascinating sense of orientation of bees is consequent not only to the recognition of landscape and morphological characteristics of the territory, as well as to the distribution of light and wind direction (Hempel 2009), but to a specific sensitivity to terrestrial geomagnetism (Hsu 2007). It is the fascinating phenomenon of magnetoreception (Yoshi 2009)(2), thanks to which many living species are placed in space, orient themselves, “trace routes” to travel, return, mate, reproduce, provide for offspring etc … Many studies have confirmed that bees, as well as bumblebees, have an internal compass to identify and trace routes in the right direction (Merlin 2011), also based on the distribution of light and wind direction (Hempel 2009). The attitude of the drones, based on the circadian rhythm (3), expresses and includes numerous “navigation” skills.

Dca lease diagram

The diagram of a topographic section representing the slope and conformation facing south of a Dca. The star indicates the exact location of the Dca. N and S represent the North and the South – from the study of Galindo-Cardona 2012

A study (Galindo-Cardona 2012), which investigates the landscape characteristics of the DCA, highlights numerous aspects on the territorial conformation of a gathering area, but above all reveals that, in the choice of the geographical place of a DCA, the ethology of the drone plays a crucial role: its ability to fly, to orient itself… and his predilection for going “south”! The localities in which the gathering areas were detected have, in fact, a territorial conformation with orientation towards the South. Drones use the sun as a compass (von Frisch 1967) and therefore fly oriented to the South/South-East as a function of solar radiation (McCune and Keon 2002).

Who comes first? The virgin or the drones?

The drones gather and constitute DCAs regardless of the presence or absence of virgin queens (Ruttner 1966, Jean Prost 1957), the female presence is not crucial for their formation, even in this case … the fair sex becomes… wait (Koeniger 2004).

They are important reference points for the gathering of drones (Ruttner 1985), the horizon line (mountains, treetops …), anomalies in the Earth’s magnetic field etc … but, fundamental, it is the production of pheromones by drones to attract others. Recent studies have ascertained that a bouquet of volatile substances, probably emitted by the labial glands or by those located in the antennae, constitute an “odorous” signal, perhaps fundamental for the constitution of the gathering areas (Bastin 2017). The emission of a series of olfactory stimuli of the drones could explain both the very limited geographical boundaries of the DCAs, and also how virgin queens recognize and are attracted to areas crowded by drones (Bastin 2017).

Golconde by Renè Magritte

René Magritte, Golconde, 1953

The drone able to express greater fitness and physical vigor reproduces thanks to the air competition that combines strength, strategy, speed and readiness

Competition. Ready, go… road!

The drones create flight formations in the DCA, with a shape similar to comets, therefore called “drone comets” or “fertilization comets” (Koeniger 2005).

They can manifest and melt in a few seconds (Gary 1962) and determine the most important breeding behavior of the bees: here the “pole position” is decided and, as in a car race, fundamental results: acceleration, speed, ability to maintain a dominant position compared to the others … The race, the competition for mating does not allow margins of error, here it is determined and discriminated who will pass their genes to the offspring or not. The total number of drones “competing” within a comet ranges from 20 to 41 (Koeniger 2005), with a unit weight (of each individual specimen) ranging from 79 mg to 223 mg, with average flight speed between 2.6 meters / second and 4.6 meters / second, and an acceleration capacity of 10 meters / second (Koeniger 2005).

The average length of stay of a drone in the comet varies between 0.7 and 1.7 seconds: they enter and exit continuously from the flight formation, it is, in fact, very limited the time space to reach an “interesting” position for the coupling and the continuous replacement in the comet can perhaps be interpreted as the momentary loss of hope of placing in a winning position and the attempt to regain it. This has important implications of a genetic-hereditary nature: the subject able to express greater fitness and physical vigor reproduces and in the drones this is determined by an air competition that combines strength, strategy, speed and readiness. Confirming that the flight capacity of drones and all related characteristics are one of the key elements that determine the inheritance in the offspring and the reproductive capacity of the bees. The distance of the virgin queen from the drones in the comet, varies from 4 to 15 cm, but only those who manage to approach less than 10 cm, flying at its height and remaining in an orbit of 2,000 cubic centimeters, will, perhaps, be able to grasp it (Koeniger 2005).

Marilyn Monroe sculpted hives

The return to the ancient traditions of Baroque folk art in Germany and Poland. Carved hives: with the life force of bee fertility as the main theme of figurative representation. Happy Birthday Marilyn Monroe

No physical contacts have been observed between the drones but rather adjust their position according to that of the neighbor while defending it and in 73% of cases they begin to chase the queen in groups: fighting and bumping into each other in flight, in fact, could result in the loss of the queen’s trajectory (Koeniger 2005). However, the behavior of drones in comets is also determined by the different stages of mating.

The courtship sees the queen fly quickly and reach a certain height from the ground (Koeniger 1989): from here the “chase” comes to life and begins, culminating with one of the “well placed” drones that manages to grab the virgin and mate; during this phase there is a considerable reduction in speed that allows the other drones to try to get a better position in the comet.

Fertilization takes place in the air at a height of 15 to 60 meters (Loper 1992), at a speed that can reach 12 km / h (Oertel 1956).

The duration of copulation is less than two seconds and, probably not surprisingly, this time interval corresponds with the average frequency of stay of a drone inside a comet (0.7 -1.7 seconds).

In the frenzy mistakes can happen, and it is a frequent occurrence the chase in the pre-copulation position and the attempt to grab between the drones themselves (Gary 1963). This can be determined by a distorted visual perception (Gary 1963) due, for example, to the distance from the virgin queen in the comet.

Circa, 1851, Carl Vogt, in Studies of Animals States

“The kingdom of bees” His Majesty decided to serenely multiply (Translated from Old German)

About Carl Vogt

Without dcas? What would become of the bees?

These small “portions of the sky” have guaranteed for millions of years reproduction and survival of bees, thanks to their precious role of development and maintenance of the genetic variability of which the DCAs are the cradle and in which the drones play a crucial role. A queen can be fertilized from 6 to 28 males with a statistical average of about 15 (Holm 2010); simple and at the same time excellent leverage to counterbalance parthenogenesis (4) and ensure a high heterogeneity to the offspring. Complexity, considerable energy consumption, investment of specific and peculiar biological functions, ability to comply with environmental trends… they constitute an articulated “dynamic” and “phenomenon” that is the basis of the adaptive success of Apis mellifera, of its extraordinary longevity and widespread presence at different latitudes on this planet.

Identifying, knowing and conserving the areas of gathering can be very useful for: studying the different populations of bees within a territory (Loper 1992), allowing their characterization according to genetic diversity (Collet 2009), estimating their genetic structure (5) (Collet 2009), identify the presence in the colonies of a territory of diseases or possible adaptive immunities (6) thanks to the study of drones as samples of the genetic variability of a given area (Evans 2006, Robinson 2008). Knowing the aspects of the reproductive behavior of bees, identifying how and where it occurs, is essential to, in the first place, protect these precious areas from anthropogenic pressure (agrochemicals, loss of biodiversity, etc …).

Finally, consideration and awareness of the ethology of drones can be strategic and functional to try to implement more effective, punctual and functional beekeeping practices for conservation, genetic improvement and genealogical selection (Ruttner 1976): activities for which the genetic and qualitative role of the paternal component is the basis for obtaining and consolidating satisfactory results. On the other hand, as we have had the opportunity to verify, it is precisely the peculiar behavior of the drones that expresses the set of inclusive or discriminatory mechanisms that allow … or not to a hive to pass on its genes to future generations. It is in the ability to fly, orientation, speed, etc …, in the physical and reproductive qualities of the “sperm with wings” that the composition, imprint and genetic impact of a population of bees is determined. So who are the drones? They are the queen, “copy apparatuses”, but each one different from the other, like the spermatozoa in other living beings …

LEGEND

(1) Endogamy: sexual reproduction between individuals of the same parental nucleus, therefore consanguineous.

(2) Magnetoreception: a kind of biological compass included in some living species: many birds, lobsters, whales, dolphins, sharks, manta rays, bees, as well as microorganisms and some plants. It allows these life forms to detect the Earth’s magnetic field, create territorial maps, with relative altitude, position, direction and then orient themselves.

(3) Circadian rhythm: the complex biological clock that determines, in animals and humans, a series of innate synchronizing stimuli (sleep-wakefulness / hormone production …) in the 24 hours of alternation of day and night.

(4) Parthenogenesis: form of sexual reproduction of plants or animals that does not require fertilization, amphigonic; it does not require the union of male and female gametes.

(5) Genetic structure: the genetic constitution of a population in quantitative terms (allelic and phenotypic frequencies) and qualitative (allelic variants in a population).

(6) Adaptive immunity: acquired ability of the immune system to respond based on the formation in the body, of memory cells for a rapid response to an antigen.

VOCABULARY

Drone

A word in vogue today, but it is only since 1946 that, in English, the word ‘drone’ means, also, unmanned radio-controlled aircraft. Until that moment it had the simple meaning of ‘drone’, that is, of male bee, and at most, since the drones do not make honey, it came to mean ‘lazy, tired’. However, it is not known why the name of the drone was chosen; probably for the misunderstanding that the drone would be the component of the colony more expendable than the female one. An unmanned flying object on board, with the possibility of carrying cargo, was operated for the first time in the war in 1849, to bomb Venice, one of the officers of the Austrian general Von Radetzky, had the idea of launching an attack with balloons launched from a ship to anchor, unmanned and with about fifteen kilograms of explosives. A crude charcoal timing device and cotton trigger thread would have released the bombs over Venice, but… unfavorable weather conditions and irregular winds caused most of the balloons to return to the Austrian lines.

In musical jargon drone instead indicates a note or a continuous chord of accompaniment, which arouses different effects. Famous are those of Beethoven’s Scherzo della Pastorale and Haydn’s Finale of Symphony 104. In continental Europe this type of sound is known as ‘bordone’: an imitative origin, which recalls a low and continuous sound.

Radetzky drones

The first “drones”, used in 1849 by Radetzky’s army to try to bomb Venice