The reproductive success of drone honey bees seems to vary among drones depending on their body size. Honey bee colonies furnish their brood nests with two types of comb distinguished by cell size; large cells for rearing males (drone comb, 6.2-6.4 mm diameter, about four cells per linear inch) and smaller cells for rearing workers (worker comb, 5.2-5.8 mm diameter, about five cells per linear inch). Most drones are reared in the larger drone-size cells. Small drones are usually produced when the queen runs out of sperm and lays unfertilized eggs in worker cells or when laying workers in a queenless colony lay unfertilized eggs. The comb cell size determines the body size and weight of emerging drones. Drones emerging from worker cells are smaller in weight and size than those emerging from drone cells (Berg 1991, Berg et al. 1997, Schlüns et al. 2003). Because honey bees invest energy into building special comb with larger cells in which to rear drones and they preferentially rear and foster large drones, large drones must have a reproductive advantage.
Drones emerging from worker cells weighed significantly less than those emerging from drone-size cells, 151.8 mg and 260.8 mg, respectively (Berg 1991). The length of their wings was also significantly different 11.21 mm vs. 12.09 mm. Schlüns et al. (2003) found a mean wing length (distance between the two branching points in the wing venation pattern which reflects about 50 percent of the wing length) for large drones to be 6.03 mm and 5.27 mm for small drones. The wings of small drones that emerged from worker cells were about 13 percent smaller compared to the wings of large drones which emerged from drone cells. This is substantially more than observed by Berg et al. (1997) who reported a seven percent difference for wing length of small and large drones.
Drones reared in queenright (a colony having a healthy, egg-laying queen; opposite of queenless) colonies in drone combs and in laying worker colonies both in worker combs and in drone combs were compared with respect to their live weights at different ages, reproductive capacities, and differences in various body structures (Gencer and Firatli 2005). Depending on the comb and colony type, the weights of newly emerged drones varied. At emergence, the drones from drone comb in queenright colonies were 17 percent heavier than drones from drone comb in laying worker colonies. Drones from worker comb in laying worker colonies were 36.6 and 23.7 percent lighter than drones from drone comb in queenright and laying worker colonies, respectively. The differences between the two groups of drones produced in the colonies with laying workers may result from dietary differences. Drones sampled from drone comb in queenright colonies, drone comb in laying worker colonies, and worker comb in laying worker colonies lost 18.8, 16.1 and 13.3 percent of their initial weights during maturation from emergence to 18 days of age, respectively. Body weights (reflecting investment of workers into the male) of small drones reared in worker cells are 41.9 to 52.3 percent lower compared to the weights of large drones reared in drone cells.
Comparison of different sized drones indicated that large drones have larger mucous glands and seminal vesicles and produce more spermatozoa than small drones (Gencer and Firatli 2005). The average number of spermatozoa in drones from drone comb in queenright colonies (12.01 x 106) was significantly greater than that of drones from drone comb in laying worker colonies (10.17 x 106) and from worker comb in laying worker colonies (8.62 x 106). The other reproductive and body structures, such as weight of mucus glands and seminal vesicles and extent of hamuli (wing hooks), lengths of hind leg parts, total length of hind leg and head width, all had significantly higher values in drones from drone comb in queenright colonies, and the lowest values were from drones produced in worker comb in laying worker colonies.
Africanized honey bees and their comb cell sizes are normally slightly smaller in size than the European honey bee. Rinderer et al. (1985) reported that Africanized drones weighed significantly less (194.6 mg vs. 220.2 mg) and had significantly fewer spermatozoa than European drones (4.6 million vs. 5.7 million/seminal vesicle), although their seminal vesicles and mucous glands were not significantly different in weight. The reduced weight of Africanized drones possibly results from the smaller cell size and less feeding of drone larvae by Africanized nurse bees in comparison to European nurse bees.
The effect of drone honey bee’s body size on semen production was further evaluated (Schlüns et al. 2003). In the same colonies, drones were either reared in drone cells (large drones) or in worker cells (small drones). Wing lengths (size indicator) and sperm numbers of small and large drones were compared. Small drones (~13 percent reduced wing size) produce significantly fewer spermatozoa (7.5 ± 0.5 million) than normally sized drones (11.9 ± 1.0 million spermatozoa). The sperm numbers ranged from 1.09 x 106 to 30.31 x 106 spermatozoa and the overall mean was 9.19 x 106 spermatozoa per drone. There was a significant positive correlation between sperm number and wing size within the small drones and in both groups combined. In the large group alone no correlation was found. Small drones produce 20 percent more spermatozoa in relation to their body weight. The rearing investment per spermatozoan is lower for small than for normally sized drones because small drones produce more spermatozoa in relation to their body weight. Since colonies usually produce large drones, the enhanced investment must be outweighed by a mating advantage of large drones. Traits other than sperm numbers have to out weigh the costly investment in large drones. The lower flight performance of small drones in drone congregation areas or a potential difference in semen quality could contribute to the preference to rear large rather than small drones.
Sampling of drones in a drone congregation area in Germany with a drone trap and excluder grids to discriminate between large and small drones, over a three-week period, found an average of 9.14 percent (min = 7.2 percent, max = 13.2 percent) of the captured drones had developed in worker-size cells. Thus small drones appear frequently enough under natural conditions to play a substantial role in sexual competition among small and large drones (Berg 1991).
Berg et al. (1997) trapped drones within drone congregation areas (DCA) and were able to show that small drones originating from worker cells in comparison to large drones reared in drone cells had a reproductive disadvantage in drone congregation areas. Trapped drones from the drone congregation area were compared to drones emerging in an incubator. There was no significant difference in wing length between small drones from the DCA and small drones emerged from worker cells and large drones from the DCA and those emerged from drone cells. The difference in wing length between small and large drones was highly significant.
Several fitness components of regular and small-sized drones were studied (Jarolimek and Otis 2001). Flight characteristics and longevity of large and small drones were quantified. None of the attributes of drone flights: average flight duration, total flight duration, number of flights per drone per day, and maximum duration were affected by drone size on two days with good flight conditions (e.g., sunny, > 20°C). On a cooler day (~ 18° C), a higher proportion of large drones (61 percent) than small drones (7 percent) took mating flights, but because few if any queens take mating flights at these temperatures, this cannot explain the higher reproductive success of large drones. In one colony the large drones had higher longevity; in the other colony there was no difference, so those results were ambiguous. There was a highly significant positive correlation between number of sperm and drone size.
While beekeepers are unable to choose the actual drones that a queen mates with while she is on her mating flight(s), they can select the drones that will be used during instrumental insemination. The size of several body structures have been shown to have positive correlations with the drone’s body size and weight and indirectly number of spermatozoa. These characteristics e.g. wing length, could be used in selecting which drones would be used to supply semen for instrumental insemination.
It appears that body size and weight does not fully explain why large drones have a reproductive advantage over smaller drones during mating competition. Differential sperm numbers may primarily account for the greater reproductive success of large drones. Flight ability (Moritz 1981) has also been shown to vary between drones. Both traits clearly would affect individual reproductive success. Furthermore, honey bee drones may indirectly compete post copulation (semen quantity and quality) since honey bee queens mate with numerous drones. After mating, the semen of many drones is mixed and stored in the queen’s spermatheca, with a large proportion of the semen being expelled from her body.
When rearing queens and saturating a mating area with desirable drones, it would be advantageous to have big populations of large-sized, sexually mature drones available. In order to achieve this, drones should be reared in colonies with large quantities of drone comb, large nurse bee populations, and abundant supplies of pollen and nectar.
A similar approach in areas that have Africanized honey bees, saturating a mating area with slightly larger European drones, might reduce the probability of queens mating with Africanized drones and aid in slowing the Africanization process.
Berg, S. 1991. Investigation on the rates of large and small drones at a drone congregation area. Apidologie 22: 437-438.
Berg, S., N. Koeniger, G. Koeniger, and S. Fuchs 1997. Body size and reproductive success of drones (Apis mellifera L.) Apidologie 28: 449-460.
Gencer, H. and C. Firatli 2005. Reproductive and morphological comparisons of drones reared in queenright and laying worker colonies. J. Apic. Res. 44: 163-167.
Jarolimek, J.P. and G.W. Otis 2001. A comparison of fitness components in large and small honey bee drones. Am. Bee J. 141(12): 891-892.
Moritz, R.F.A. 1981. Der Einfluss der Inzucht auf die Fitness der Drohnen von Apis mellifera carnica. Apidologie 12: 41-55.
Rinderer, T.E., A.M. Collins and D. Pesante 1985. A comparison of Africanized and European drones: weights, mucus gland and seminal vesicle weights, and counts of spermatozoa. Apidologie 16: 407-412.
Schlüns, H., E.A. Schlüns, J. Van Praagh and R.F.A. Moritz 2003. Sperm numbers in drone honeybees (Apis mellifera) depend on body size. Apidologie 34: 577-584.