Mate Guarding in the Bearded Weevil

Photographed in situ [1] at Selva Verde, Costa Rica

A female bearded weevil (Rhinostomus barbirostris) digs fiercely into a fallen palm stem in preparation for oviposition while a male stands guard during the process.

Bearded weevils are a sentimental species for me because they are one of my earliest and most vivid beetle memories back when I first travelled to the New World tropics. Weevils are generally small at around 0.5–2 cm long, so just the size alone of this beetle struck me. In fact, they are one of the largest weevils in the world (ranked no.3 in one old paper…) reaching over 5.5 centimeters long including the beak. When I picked my first one up in Tarapoto, Peru, it grasped my fingers so tightly and rendered itself catatonic. Its grip strength was much more than what I’ve received from similarly-sized scarab beetles (and that’s saying a lot!), and the tibial claws fastened and secured its position on my hand. I ended up carrying the beetle around for ~10 minutes because there was no way I could possibly remove the bear-hugging insect without ripping its arms off. The first night the beetle kept me company I awoke to hear it flying around the room. I put the beetle back in the cage, patched up the spot I thought it had slipped through, and went back to sleep. Later in the night I heard a faint shearing sound and flipped on the light to find the beetle, in all its madness, slicing open the net cage with its scythe-like forearms. Clearly he wasn’t going to be the best companion to keep contained.

06/2015: My first bearded weevil, photographed after capture [5] in Tarapoto, Peru

Bearded weevils have a very distinct morphology. The rostrum is slender and dentate, and males have orange fuzzy hairs on the underside of the beak and thorax, giving them a “bearded” appearance. Before copulation the male will use his combed snout to gently stroke the dorsal surface of a female. This wiping behavior can have a duration of up to several minutes, immediately followed by tapping on the end of the female’s abdomen and a copulatory attempt. If the female moves away, the male’s behavioral sequence resets and wiping behavior is reinitiated. Many of the behaviors I discuss here are elaborately documented by Eberhard (1983) — see below for accompanying life history images. Once mating has occurred the male will usually stay by the female’s side, overseeing the oviposition process. Even afterwards, males will frequently guard the female during subsequent drilling and egg-laying without remating, suggesting that females can retain a single male’s sperm for multiple oviposition events.

Figures and description of adult beetle behavior from Eberhard (1983). Behavior of adult bottle brush weevils (Rhinostomus barbirostris) (Coleoptera: Curculionidae). Figure 4 nearly identical to the behavior documented in this blog post.

During these periods males will regularly be approached by contestants who seek to mate with the female before eggs are deposited. They will use their long scythed arms to grapple the substrate and tuck their beaks down low, aiming to knock competitors away by flicking upwards rapidly. These fights may last for long periods of time, with beetles crawling and flying back to their arena many times. Males will exchange blows repeatedly until one gives up, and usually the victor is the larger of the two males with the longer beak. In some cases the loser male may approach the defending male aggressively though recoil repetitively from direct contact. Similar male-male agonistic behaviors have been documented in horned scarab beetles, where sexual selection has also driven the evolution of male armaments.

Photographed in situ [1]

Male bearded weevils show substantial variation in size, length of the beak and forearms. In dyadic contests featuring a noticeable size difference, the smaller male sometimes performs what is called a clinging behavior. Although powerless to usurp the larger male, the smaller male will cling firmly to the larger one’s body. The grip strength and hooked distal tips of the tibiae make dislodging the beetle almost impossible, and this often results in both toppling over to the forest floor. This action may seem to hinder copulation success from both beetles because the female is left temporarily unguarded. However, if the smaller male has already mated with the female, it may allow for successful oviposition prior to any disturbance. While learning about this behavior I remembered back to my clinging story in Peru — if effective on a human surely it would be insurmountable for a fellow beetle. “Sneaker” (also referred to as dwarf) males also exist in this system. These tiny males pose little threat for direct competition and will attempt to wander underneath the female unnoticed, slipping in during fights that involve the defending male. After the larger male mates with the female, the sneaker will stealthily initiate wiping behavior from the underside and proceed with mating just in time for oviposition. All of these behaviors support a large role of sperm competition in this species, in which recently deposited sperm may provide an advantage in fertilizing offspring.

White weevil egg covered with a dark fibrous secretion; photographed in situ [1]

Finally, back to the female weevils… They are attracted to recently fallen palm logs (commonly coconut palms) to lay eggs. She will sway her head back and forth while antennating the surface repeatedly to scan for a suitable spot where perhaps the material is softer for facility of drilling or to provide moist plant tissue for developing larvae. Prior to drilling the female will hook onto the substrate firmly using her tibial claws and carve out a narrow deep incision into the palm stem with her beak. Periodically she will clear out bits of bark by flicking her beak, and eventually turn around to lay an egg. Afterwards she will coat the hole by a secretion that quickly hardens and protects the incision.

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Figure from Vaurie (1970): Weevils of the Tribe Sipalini (Coleoptera, Curculionidae, Rhynchophorinae) Part 1. The Genera Rhinostomus and Yuccaborus

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