Strange love

Insects go wild over romance too. From the fascinating to the feisty, The Straits Times looks at species with unique amorous techniques, in the spirit of Valentine's Day today.

It is common to see several snails copulating at the same time, or for snails to be attracted to a copulating pair even just as bystanders, likely due to the emission of a chemical known as pheromone.

Mr Tan Siong Kiat, a curator of molluscs, worms and other invertebrate collections at the Lee Kong Chian Natural History Museum in the National University of Singapore (NUS), said snails generally rely on chemical cues in slime trails or in the air to locate potential mates. Courtship behaviour may start only upon contact with a potentially receptive snail.

The giant African land snail is a hermaphrodite, meaning that it has both male and female reproductive organs, though not all snail species are hermaphrodites, he noted.

Some snails are seen with their white penial lobe (on the neck behind their right eyestalk) extended, which usually occurs before they initiate courtship behaviour. This means that the snail might have picked up the scent of a potential partner and is on its trail, he added.

Once the snail meets its love interest, the initiator will usually approach it from the rear and mount its shell, if the other snail responds favourably.

"This involves the lower snail arching its head back and waving its head and shell, while the snail on top will nibble the back of its neck, followed by both snails rubbing their penises together before simultaneously inserting it into each other," said Mr Tan.

By then, the snails will usually fall on their sides and stay in that position until the end of their sexual intercourse some six hours later.

Dung beetles, known as nature's soil engineers, are important to the natural ecosystem as they help to recycle nutrients into the soil when they bury animal faeces. They aid in dispersing undigested seeds that can be found in the dung and by removing the dung, they reduce the occurrence of parasitic flies in the area.

The Onthophagus dung beetles constitute about 30 per cent of global dung beetle species and are known to burrow underground, creating complex networks of tunnels that end in chambers that house balls of dung.

Singapore has at least 25 native dung beetle species, of which at least 23 are tunnellers that belong to this Onthophagus genus.

According to Assistant Professor Nalini Puniamoorthy from the department of biological sciences at NUS, male dung beetles hang out at the entrances of these underground tunnels, which are usually dug by females.

Larger males would fight one another for a chance to mate with the female, often using their large horns as a weapon to defend their territory. The winner gets to mate with the female, which lays her eggs in brood balls, which are developmental chambers that she builds out of dung. The victor continues to defend the female and her brood balls by fighting off other males at the tunnel entrance.

"This is driven by an evolutionary mechanism called sexual selection, where certain traits that increase an individual's reproductive fitness are selected. In this case, horn length and body size are important traits that increase mating success," said Prof Nalini.

That being said, many dung beetle species exhibit an interesting alternative mating strategy: the sneaky male tactic.

Small males (with no horns or small horns) can mimic females and sneak past the large males guarding at the entrance to mate with the female beetles in the tunnel. These male species usually "compensate" by having large testes, or more sperm, which in this case would be a key reproductive trait that influences fertilisation success.

"Different species of dung beetles display vastly different mating behaviours. In some rare instances, the tunnelling can be done by the male beetle as a nuptial gift, almost to say, 'I built a house for you, now you can come and have your offspring'," Prof Nalini added.

Known for diverse courtship rituals, the male black scavenger fly, also known as the Sepsid fly, begins his courtship by jumping and latching onto the female.

Prof Nalini, who leads the NUS' reproductive evolution lab, said most Sepsid males have elaborately modified forelimbs with spines that help them hook onto the base of the wings of the female. "This enables the male to hold on to the female during copulation, while they use their other limbs to actively court the female," she said.

Her research team has found that across several Sepsid species, male flies are able to use their limbs and other body parts to display more than 20 behavioural characteristics, which are used to win the female's interest. For instance, some males have a gland on their hind limbs which they use to rub the female to either "perfume" her or mark her to prevent other males from intercepting.

"Copulatory courtship is important in Sepsids because the female flies have different sperm storage organs, and they can mate multiple times with different male flies.

"Therefore, if the female does not enjoy the copulation process, she can decide not to use the sperm from a particular male and bias the paternity of her offspring. So in order to convince them, the males invest a lot in continuing to impress these female flies throughout the copulation process," she said.