Sex-Based Differences in Bees

Male-Female Variations in Solitary & Social Taxa

Did you know that ~20% of males in the solitary bee genus Megachile (“leafcutter bees”) have modified forelegs that serve as pheromone-secreting glands for attracting a female, or that male longhorn bees have antennae that exceed their body lengths? Well, you do now. Honey bee biology is a mature field and well documented, it is the solitary bees that surprise you. In Apis mellifera, the visual differences in “sexual dimorphisms” means you can easily tell a male drone apart from a female worker (and those less experienced may confuse a drone for a queen).

What about solitary bees, can you tell which is male and female? “Yes, by looking at signs of hair on their abdomen/hind legs which means they can carry pollen”. Okay, good start. How about in cases where the bees are sparsely-covered or hairless, or if they transport pollen internally? What about in the case of mimics, whereby a bee looks like a wasp and vice versa? If your response is “these are niche cases with minimal real-world applicability”, you are correct and should stop reading.

This article explores several variations between the two sexes and describes their purposes, through the lens of sexual selection and/or natural selection. Several references are made to the social bees, as these are the most studied taxa.

Physical Dimorphisms: Larger Females, Brighter Males

Appearance is the first and most obvious difference between the sexes. Some males are larger than the females (e.g. honey bee drones), other species have brightly coloured males whilst their females are darker (see the carpenter bees; Xylocopa spp.). In the leafcutter bees (Megachile spp..), the males possess modified forelegs that release pheromones for female attraction, and a final example is antennae size: male longhorn bees (tribe Eucerini) have antennae that can be as long as the rest of their body, owing to successive sexual selection by the ladies (Streinzer et al., 2013).

Sexual selection is driven by females of the species, as they are the ones who must bear the cost of rearing offspring, so why should she go through the efforts of building a nest only to be inseminated by a (seemingly) low quality male? I say “seemingly” because it is the best advertisement that wins, not necessarily the best the product.

(An avian [and common] example are how the male peacock’s feathers and their sizeable plumage are a fitness risk, as that bird may not be as effective as avoiding predators compared to a bird with less encumberments – only the high quality males can afford to produce and maintain such traits, hence they are more likely to be selected by the females [on the other hand, surviving and showcase “such vast plumage” is a signal to females that you are still alive despite your burdens, which is a positive signal all up… tradeoffs!])

The downsides of long horns in the Eucerini bees could be that more energy is allocated to sensory organs in the antennae, making for weaker fliers relative to males of the same size from other tribes/families. Perhaps they could be a predatory disadvantage, as there is “more bee to see”, and thus become more likely to be picked off by a passing bird or amphibian.

For Males, the Physical Variations are Advantageous for Mating

Consider the functions of a male versus female bee: one gathers pollen, nectar, water, nesting material, lays eggs, build, defend nests, amongst (many) other functions, whilst the other waits to chase a foraging female for copulation. The female bee’s workload is more demanding than the male, and so their bodily structure reflect their Jack(Jill?)-of-all-trades lifestyle, and one could say they have been naturally selected to be “good” at everything, with variations in between species.

Males have the luxury of overspecialisation. Look at the drones in Apis spp.– their defining features are large compound eyes and a “thicker” body (A. mellifera). The larger eyes make for more precise flight and spotting of the virgin queen during the mating comet, and the larger abdomen houses the drone’s (relatively) sizeable genitalia. The drawback is that they are not much better at complex, non-reproductive tasks, like transferring their experience across domains¹. There’s also the well-known beekeeping facts of drones being unable to feed themselves, relying on workers for trophallaxis; the males have a probiscis, it’s just not long enough for them to forage and feed independently. (Solitary male bees do not have this problem and will feed themselves nectar).

Males and Females Will Not Look Identical When Form Follows Function

All other parameters aside, a bee’s physical form reflects its function², and a “unique” example are the Aztec cuckoo leafcutter bees (Coelioxys aztecus), whose abdomen ends with a sharp tip that they use to break their hosts’ brood cell walls and then lay their own eggs. Another cuckoo example is of the chequered cuckoo bee Thyreus caeruleopunctatus, whose blue-and-black spots mimic their target host, the blue banded bees (Amegilla spp.).

You’re right to think that “bees recognise one another via chemical cues” and this will be discussed later on. If (from afar) the chequered cuckoo bee looks and flies like another blue banded bee, then the host is less likely to engage in defensive posturing/attack the scouting T. caeruleopunctatus. What I would like to know is if the cuckoo was caught at the nest, would it be attacked by the mother bee, or would it attempt to get around through chemical means (odour mimickry/matching/foreign odour reduction)? Probably the latter, as this has been documents in other cuckoos like the mason wasps’ Chrysis mediata and Pseudospinolia neglecta (Wurdack et al., 2015).

While chemicals matter for the insect-level detection, colouration is camouflage for the field, and a dull-er looking bee will have a higher chance of survival relative to a brighter counterpart from predators or an aspiring taxidermist. Females invest more energy into growing and maintain hairs, as well as energy into powering enzymes that digest pollen (as in A. mellifera workers in preparation for making of royal jelly).

Physiological Dimorphisms: Body Mass & Flight, Olfaction, Stingers, Digestion

The larger the body, the more muscles, and the more muscles, the better the thermoregulations (bees “shiver” to keep warm). In females, a larger body mass probably leads to enhanced foraging capability, as the bee can warm up faster/increase her flying ability in less optimal conditions, carry larger forage loads and visit further foraging locations. A larger-sized male also matters for thermoregulations (some ladies like the cold), and more so for finding and mating with said females. With solitary bee species, the enhanced flight capabilities helps them find nectar to fuel their activities.

Olfactory detection in males centres around female sex pheromones and is extremely sensitive to a narrow range of compounds. For honey bee drones, they are attuned to the virgin queen’s pheromones (such as 9-ODA; Wanner et al., 2007), as well as the mandibular pheromones of other males (which drives the formation of drone congregation areas). For females, it is the floral odours, brood cues and nest scents that are prioritised, which results in very (very) complex learning networks. This complexity is greater in social bees, as workers are attuned to the scents of their sisters, the queen, the hive and all of the above under changing environmental conditions.

Are Male Bees Defenceless Because They Lack Stingers?

A very-short-and-not-quite-correct answer is “Yes”. It’s not-quite-correct as you have to define “defencelessness”. Defenceless against me? Yes. Defenceless against other males or an adventurous ant? Not quite. The absence of a stinger means a male bee cannot strike in defence of himself or his hive (by the way, bee species outside of the Apis genus can sting multiple times; only honey bees “die” after stinging), and so he would be helpless against my comfortably-gloved hand. Against other bees, or insects of a similar size the male bee can chase and bite in self- or territorial-defence, which is why the long answer is “no”, they are not defenceless despite lacking a stinger.

What About Immune Defensibility?

Studies on the variations between male and female immune systems are done in social bees because of sample size constraints (good luck catching a few hundred leafcutter bees…). Early work by Gätschenberger & colleagues found that drones can initiate an immune response like worker bees, with stronger responses in adult drones relative to their larval form (Gätschenberger et al., 2012). A drone’s activation of an immune response (nodulation, antimicrobial peptides) is independent of vitellogenin, unlike queens and workers who have high titres of this protein.

Both sexes had reduced immune function as they aged, however this depends on the type of “challenge” the drone faces – bacterial injection (e.g. lipopolysacchride) versus cuticle wounding (via needles or sham injections with insect Ringer solution – Laughton, Boots & Siva-Jothy, 2011). That said, drones are more sensitive to abiotic stressors than workers (cold exposure, pesticides), and their findings suggest that drones are “built different” when it comes to stress management (McAfee et al., 2022). This variation is proposed to be a part of the “haploid susceptibility hypothesis”³, as drones were more sensitive to imidacloprid (a neonicotinoid pesticide) than workers, despite having higher baseline levels stress-response proteins (i.e. they are already “ready” for life’s stressors). From the authors:

[W]e suggest that the drone’s stress response may be a result of sex-specific rewiring of the stress responses, such that most of their amino acid reserves are constantly mobilized to support high baseline levels of proteins that help mitigate damage due to short-term stress…. It is somewhat confusing for drones to so easily die in the face of challenging conditions if their stress response is already primed. One explanation is that, while they generally express high levels of stress response proteins, they have no further amino acid or energy reserves to amplify that response, should a severe intensity or duration of stress be encountered.” (McAfee et al., 2022).

If you’ve skimmed past the above and want a quick answer: “Yes, male (honey) bees have an overall weaker immune system than the females but this is very nuanced and contextual so you should skim texts less, and skim milk more”.

Habitual Dimorphisms: Aggregations & Roostings

In social bees, the males and females nest together in the hive, whether in cavities (e.g. A. mellifera), in exposed comb (A. dorsata, A. cerana) or underground (Bombus spp.). In the Apis species, drones are reared as part of the swarming reproduction process and are taken care of by their sister worker bees. Once the swarming season is over, they are (violently) evicted and the helpless male dies from exposure/starvation/predation (“bad luck”).

For the bumble bees, drones are reared when it’s time to reproduce the future queens (gynes) towards the end of the season (“autumn”) and eventually the whole colony ends after the now-mated gynes have departed to enter diapause (“hibernate”) underground in wait of the next spring.

In Solitary Bees, Females Build Nests, Males Sleep Outside Alone or in Aggregations

In solitary bees, females search for a suitable site, excavate/build said site and then stockpile food for the future brood. Males do not nest, instead flocking to temporary roosting sites (pun intended). This division is a psychological (physiological?) variation: the females’ nest reflects her reproductive role, whereas the males’ is for territorial maintenance and/or safety in numbers.

Depending on the bee species, females may nest in soil (e.g., Andrena spp., Halictus spp.), pre-existing cavities (e.g., Osmia spp.), plant (“pithy”) stems or wood (e.g., Megachile spp.).  The same cavity is modified to suit her needs, either with tunnels and branching brood chambers, or partitioned with materials like leave cuttings or resin. Once complete, she begins provisioning it with food (pollen/nectar) and then lays an egg. Throughout this process the female “sleeps” at the nest entrance, which protects her from the elements, and protects her nesting site/offspring from competitors or predators/parasites.

Variations arise as the degree of sociality advances⁴, multiple females may nest in the same locality (“gregarious nesting”) or share the same nest entrance (with multiple side tunnels/chambers belonging to a different mother). More advanced (but still “primitive”) sociality would see multiple females collectively cooperating to rear brood and guard nests, and some may have proto-division of labour, where a lesser (yet still reproductive) bee will act as a guard while the matriarch “forages”… the variations in solitary bees are many, and I will not touch on how some species have males that stay near their birth nest (“philopatry”), either for easy access to reproductive females reproduction or as nest “guards”.

The male nesting (“resting”) sites are in plant stems, branches, under leaves or inside flowers (inside a bloom may support one or a few bees; not many). Sleeping in flowers is self-explanatory and cozy. When males choose stems and branches, they grasp them with their mandibles and then proceed to rest (see image below). Males show preference for the same roosting site, and oftentimes if a branch holds bees one night, you’re very likely to see those sleeping males again the next night. Some species choose the same spot year on year, making for excellent comparative photos.

Variations in Body Chemistry: Cuticle Hydrocarbons, Glandular Secretions, Pheromones

If you are surprised to know that there are sex-based differences in chemistry, you have not been paying attention to introductory biology. A classic paper on the topic is by Howard & Blomquist, who explore the insect cuticle hydrocarbons as a whole and showed that in solitary and social insects, the cuticle hydrocarbons are relevant for distinguishing whether the individual is a male or female or whether they are of the same species (Howard & Blomquist, 2005).

In small social colonies, the cuticle also provides nestmate recognition. Once that number of nestmates grows, then other signals and/or cues are required (e.g. queen mandibular pheromones in honey bees, which I would argue is “carried” on the bee’s cuticle).

For female solitary bees (and virgin queens), they produce volatile sex pheromones that males track in flight. Some of those pheromones can be followed for long distance (e.g. the aforementioned 9-ODA of A. mellifera queens), or can be used for nearby signalling, as in tarsal/mandibular pheromones of solitary bees that denote “a receptive female was here”.  In females, it is the mandibular (& Nasonov glands in A. mellifera) that output the major secretions, and the Dufour gland is for “egg marking”, as in “this is a queen laid egg”. Further reading on pheromones in bees here.

In solitary bees these glands are present to varying degrees depending on the species, with (I presume) similar functions; perhaps an added step of “nest marking” that signals to other bees a spot is taken, and also to cuckoo/parasites of a potential host.

Since females are the pheromone producers, males adapted to be the pheromone detectors, and their antennae have evolved strong olfactory sensitivities to the Scent of a Woman⁵. Solitary males themselves can release contact pheromones during courtship to signal their presence to the female (“I’m still here”). The male secretions are (probably) mandibular-based and given the variations in bee taxa some may be tarsal, or from yet-undescribed specialised glands.

Hydrocarbon Profiles of Drones, Workers & Queens

This part is technical and may be skipped by readers with better things to do

In general, males and females broadly differ in chain length distribution, branching patterns, alkene positions & component abundance. Using the Asian honey bee A. cerana as an example (Rahman et al., 2016):

• In drones, 21–37 carbon chain length hydrocarbons comprising of a complex mixture of alkanes, alkenes, mono-, di-, tri-methyl alkanes and alkadienes were detected. Alkanes of drones resemble those found in workers except pentacosane and triacontane which was not detected in the latter.
• Workers showed pentacosane, heptacosane, 11-Methyl-heptacosane, 13-Methyl-heptacosane, nonacosane, nonacosene, 13-Methyl-nonacosane and 15-Methyl-nonacosane as major CHCs. Workers had an equal number of straight alkanes and alkenes but differing in their proportions and also containing fewer branched hydrocarbons with chain lengths ranging from 21–37 carbons.
  • Further, workers had significantly higher concentration of nonacosene, pentatriacontene, heptatriacontene as compared to drones. In queens much simpler and straight chain hydrocarbons with carbon chain ranging from 21–37 were detected. Several branched hydrocarbons as compared to drones and workers were absent.
• Queens had a higher concentration of alkanes than workers and drones.

The above is a direct extract from the authors’ abstract, as I was unable to access their unfortunately-niche paper using three different methodologies… yaaarrrr me hearties.

Variations in Honey Bee Anatomy: Workers, Drones and Queens

I conclude by restating that form follows function: form follows function (at least in honey bees). Broadly, a male bee’s primary purpose is to mate, and so their body has been selected (whether naturally or sexually) for maximum chances of reproduction. Solitary female bees carry the reproductive load, and so their skills are spread between foraging and guarding, as well as everything in between (navigation, weight bearing, pattern recognition, pollen digestion etc…). More-social castes that have a colonies headed by a queen sees her majesty being optimised for egg laying and sperm storage (I’m not getting into the specifics of Bombus spp.).

Apologies for the gap between articles, every year I seem to struggle to publish in March

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Footnotes

1. One example are Bombus terrestris drones, covered by Lichtenstein, Sommerlandt & Spaethe’s 2015 paper. They found that while drones can form memories as good as the workers, they failed to apply this “experience” onto other tasks, in this case discriminating colours in a Y-maze test following conditioning to said colours with sugary rewards. Many such cases in school students, who think that memorisation is the same as application. ↩︎
2. A disclaimer: This “form is function” statement breaks down around mimicry. There are more creatures that mimic bees than bees that mimic something more harmful, since our buzzers possess a stinger that will hurt/dissuade the average predator. Where there are bees that mimic wasps, you can see in the colouration – the European wool carder bees (Anthidium manicatum) have similar abdomens with spotting to that of social wasps in the Polistes species. The mimics are harmless, they just evolved to mirror someone harmful and improve their chances of survival. ↩︎
3. The haploid susceptibility hypothesis states that haploid individuals are more susceptible to parasites and pathogens because they lack the genetic diversity that comes from being diploid – when you are made up of genes from both your mother and father, you are more resilient to stressors (due to increased genetic variations) than someone whose genes are 100% from their mother, as in the case of male bees.
   An oversimplified example: if “short wings” was a recessive trait and you are a haploid individual, then you are doomed to have short wings. If you were diploid, there is a chance you end up with “normal wings”, if one of your parent would possess the allele for a normal wing, and passes this one down to you. ↩︎
4. I can get technical and reference the spectrum of sociality in the sweat bees (Halictidae families), who can range from solitary to gregarious to primitively social depending on their geography, however such a description deserves its own article so I will summarise it as “they are more social in warmer climates with longer daytime”; Michels et al., 2024. ↩︎
5. This sensitivity has been hijacked by plants, as in the famous case of the bee orchids who release female bee pheromones that trick male longhorn bees (Eucera longicornis) into attempting to mate with them, only to release they’re a dud and leave the flower covered in its pollen… and then get fooled by the next flower shortly afterwards, thus cross-pollinating the orchids, see Schiestl, 2005. ↩︎