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Does structure of eyes help in finding a mate? Study on carpenter bees says so

Poto: Vidisha Kulkarni/Research Matters

“Beauty lies in the eye of the beholder”, goes a saying. A new research by scientists at Indian Institute of Science Education and Research, Thiruvananthapuram shows that this is literally true in the case of carpenter bees.

Dr. Hema Somanathan and her colleagues studied three species of carpenter bees (Xylocopa tenuiscapa, Xylocopa leucothorax and Xylocopa tranquebarica) in the Bhimashankar Wildlife Sanctuary, Maharashtra. Carpenter bees, unlike their better known relatives - the honeybees, do not build hives. Most of them are solitary and drill holes into wood using their mandibles to make nests, thus acquiring their name. Each nest may have a single female or small numbers of related females or both males and females. They are capable of long distance flight between patches of flowers, making them important pollinators of both crop and wild plants.

The researchers focused on how mate selection behaviour in these bees has an impact on the visual abilities and adaptations in males, an example of sexual dimorphism. Sexual dimorphism refers to the differences in size or appearance between males and females excluding their sexual organs. “Lots of insects actually have sexual dimorphism between males and females. The list would include bees, beetles, butterflies, moths, flies etc.”, says Dr. Somanathan. The current study investigates sexual dimorphism in eye size, a functional characteristic that is often overlooked, compared to generic features like size, colour, ornamentation and scent glands.

In this paper the authors show that bees that are active at different times – either day or night also differ in the manner in which they search and locate mates. The males of species X. leucothorax, usually active during the day, had similar-sized eyes as the females and patrolled in small areas within the canopy of trees to detect potential mates. The males of the nocturnal species X. tranquebarica has smaller eyes than females and also patrolled around flower bushes that females visited shortly after sunset, to pursue them for a chance to mate.

On the other hand, males of X. tenuiscapa, which are also active during the day, had bigger eyes than females of the same species and displayed a different behaviour in finding mates. They perched on vantage points like branches of shrubs and trees with extended wings and upright antennae to detect and pursue females in flight. They followed moving objects in the sky, but came back to their perch if the object was not a female.  “Perching is an interesting way of finding mates - probably the best strategy given that these bees nest and forage in open areas. Perching in open areas along the usual flight routes of females and chasing objects, females or otherwise, that appear against the horizon is probably a behaviour that could maximise the chance of encountering females”, explains Dr. Somanathan.

The scientists also conducted experiments to determine the ability of males to detect females from a distance. Bee-sized stones were thrown in an arc close to perching males to simulate a passing female. In 48% of trials, males alighted from their perch to pursue the stone. In one particular case, a perching male was observed to pursue a female from a distance of 20m!

But how could they see that far? The scientists found that eyes of males of X. tenuiscapa havehigh spatial resolution and contrast sensitivity, along with larger facet sizes, which allow them to detect females at a considerable distance. “Perching is likely an effective strategy in this species because the chances of encountering females are likely to be maximal as opposed to flying around searching for mates. For perching to succeed as a strategy, larger eyes are helpful since the visual field being monitored is bigger”, remarks Dr. Somanathan, explaining how larger eyes are linked to the more social and gregarious behaviour of this species. 

Nature is mysterious in many ways and this study uncovers just one of those hidden strategies employed by carpenter bees to find their mates. Scientists believe they may still be underestimating the bees’ ability to detect its mate and perhaps we may never uncover all their strategies. Maybe that's what makes it all the more interesting - that we will always have something new to learn.