Imagine a social order where males possess elaborate weapons, do battle amongst themselves, and invade the dwellings of others. A world where victorious, dominant males gather their spoils and live in posh quarters. Where those with the most spacious digs frolic in the company of the finest females and mate with multiple partners each night. While the desolate losers retreat, perhaps to scheme alternate strategies, the victors reign supreme over their surroundings and their harems until a more powerful adversary commandeers their resources.
Fascinated? Welcome to the world of the Wellington weta, one of the most intriguing insect species found on earth. These primeval New Zealand giants have large heads and massive jaws with blunt teeth that can tear off and chew up insects and can inflict a painful bite in self-defence. They can grow as long as eight centimetres, with a three-centimetre-long head and jaw structure.
Like grasshoppers, locusts, katydids and crickets, weta are orthopterans. Orthoptera are widely used as subject animals by biologists studying Darwinian sexual selection. Clint Kelly, a PhD student at U of T, spent the better part of two summers living among these bush demons on Maud Island off New Zealand’s West Coast.
Kelly’s findings have provided valuable insight into the mating habits, social structure, and ecology of weta. Their mating system is referred to as resource defence polygyny. This means that males control access to more than one female by monopolizing the resources necessary for breeding. Such mating systems allow scientists to research relationships between operational sex ratio (the ratio of sexually active females to potential male suitors), resource defence strategies and the direction, and intensity of sexual selection.
When it comes to mating, females are usually the choosy sex because the potential rate of reproduction generally differs between the sexes. Males are capable of producing vast quantities of sperm while females expend time and energy securing costly resources for egg production. Since females tend to invest more time in individual zygotes, the ladies become a scarce prize for males. In many animal species, males compete for access to females.
Males generally take the initiative to court mates, and females closely inspect the courtship behaviour of their suitors to pick those that offer quality genes or other contributions. Other contributions by males may include postnatal offspring care, courtship gifts and the quality of resources they have to offer. One such resource is the males’ possession of a quality breeding site, nest, or territory.
The weta is a good example of theory in action. Male weta live in breeding sites called galleries in trees and dead wood. These galleries not only serve to attract females for mating but also act as refuge for the resident male and his harem. According to Kelly, breeding sites are somewhat difficult to come by in the weta world. Even though there may be an even ratio of sexually active males to females, the scarcity of breeding galleries skews the operational sex ratio towards males, since they hold dominion over the galleries. Single male weta fight other males to defend the galleries and the treasures they house. Males possessing large galleries hold court because the larger the gallery, the larger the harem size and the more opportunities there are for mating.
In order to study the weta’s mating system, Kelly conducted numerous field experiments on Maud Island, New Zealand. He discovered that larger males—especially ones equipped with large head structures—are better equipped to hold onto larger galleries and subsequently have the opportunity to mate with more females.
To defend their galleries, male weta use their vice-like jaws to fight with other males; the jaws of male weta are classified as weapons in Darwinian sexual selection theory. They are similar in function to the antlers on male deer. Like weta, deer use their weapons in violent combat when competing for females.
In weta, sexual competition occurs when a male has to ward off an intruder from his precious gallery of females. If the intruding male is victorious, he takes over the gallery and copulates with each female in the loser’s harem. Kelly’s data show males with larger heads tend to hold onto the galleries, but how exactly is yet to be determined.
Eventually, the small males will grow older, bigger, develop big, sexy heads and revel in sexual shenanigans.
“Well it’s not quite as simple as all that,” explained Kelly. In weta, as in most orthopterans, males go through several molts before achieving sexual maturity. Once they develop into sexually mature adults, they cannot grow any bigger. Hence, a small male—once past puberty—remains puny regardless of his environment.
So how do these small males compensate for their low resource-holding potential (dinky galleries) and low reproductive rate (smaller harems)? Well, this is where it gets interesting. Kelly is in the process of assessing several models of sperm competition—a term that refers to antagonism between the sperm of two or more males in pursuit of fertilizing a single female’s eggs. Kelly explained that while large-headed male weta reach sexual maturity after ten molts, smaller males may realize their impending size disadvantage on the mating scene, and rather than invest energy in going through extra molts, may somehow become sexually mature after only seven molts. This would give smaller males a head start in the mating game by giving them a longer sexual lifespan.
Another alternative mating strategy employed by small males may be forced copulation with females. Kelly has observed roving males pull females from galleries of other males and forcibly copulate with them.
Finally, Kelly is in the process of analyzing dried ejaculates from large and small males. He believes smaller males may compensate for their disadvantaged role through strategic ejaculation. This charming procedure works by the little guy inseminating more sperm per copulation than larger males, and by mating every chance they get. This would mean smaller males would have extra-large testes. This theory will only bear out once enough gonads are measured.
By studying weta behaviour and spending countless hours in the field, Kelly has not only begun to unravel the complex social structure of these insects but hopes to shed light on alternative mating strategies and on how the environment and genetics shape adult development and appearance. But most importantly: “I want to see if the big males do in fact get the job done!” said Kelly.
On that note, Kelly prepares to head off to New Zealand for another field season to check weta traps during the day and to prowl the New Zealand undergrowth with a headlamp in search of entomological antiquities at night.
