Bigger isn’t always better in the wild
Some animals use their weaponry to kill. Think of wolves ripping their fangs through rabbit flesh. Animals also use their weapons for defence, as porcupines use their quills to fend off attacks.
Deer, elk and other ungulates use their antlers in epic feats of strength to claim harems.
Mammals didn’t evolve on Zealandia but there are native species with cool weaponry and distinct behaviours.
Invertebrates such as male sheet-web spiders grapple rivals with huge jaws. Male cavedwelling we¯ ta face away from each other and bash their rivals with enormously long legs.
Sheet-web spiders can barely see and engage in ‘‘blind bar fights’’ for the chance to mate, said Leilani Walker, a doctoral student in the School of Biological Sciences at the University of Auckland.
Females of the North Island species produce webs up to a square metre in size that drape horizontally above the forest floor, her paper explains. Wandering male spiders find a web, move in and often mate with the female. If another male arrives, he may fight the resident male to claim the female.
Walker staged contests between captured males in her lab. Males typically signalled their presence by shaking the web. If one didn’t retreat, the spiders approached and sparred by rearing up on their hind legs and pushing at each other with their forelegs.
If one didn’t then retreat, they would ‘‘grapple’’ – locking their oversize jaws together and pushing and thrashing. On two occasions out of 137 staged contests, a male spider stabbed the other in the head and thorax with a long fang and killed it.
The cave-dwelling male we¯ ta studied by Murray Fea, also of Auckland’s School of Biological Sciences, have body lengths of 30-35 millimetres but legs and antennae up to 350mm long. Both sexes feed outside at night and retreat in great numbers into North Island caves during the day. They pair up and mate ‘‘many times’’, until another male disrupts and fights develop.
Fea went into these caves to record the combat. The two males first intensely ‘‘antennated’’ (touched antennae). If one didn’t retreat, the aggressor lunged forward and struck the other. Then he faced away and lashed out with his enormously long hind legs. If there was still no retreat, both males faced away and raked and pushed the other with their hind legs. If that didn’t produce retreat, both males locked their legs together and pressed their bodies into each other as feats of strength. Eventually one retreated. None died.
In theory, the male with the biggest weapon should win fights most often. This leads to more mating and the large weapon trait should pass to offspring. Over generations, the weapon should become ever more exaggerated. At some point, however, an ever larger weapon becomes a liability that evolution no longer selects.
The young scientists got contrasting results on weapon size. Fea and colleagues found we¯ ta ‘‘males with longer hind legs were more likely to win contests, while body size did not influence contest outcome’’.
Walker and colleagues found that spider males with large jaws ‘‘were more likely to win contests, body condition and body size were better predictors of contest outcome’’.
This doesn’t suggest sheet-web spiders have evolved further than we¯ ta but that evolution is complex.