Sex goes back about 2 billion years, and is generally agreed to be good.
When monogamy as opposed to indiscriminate relations began, we do not know, but for a beautiful example of lifelong devotion you need look no further than the Israeli wood louse.
The desert isopod of Israel is an arthropod with a segmented body, as arthropods have, and seven sets of legs. This denizen of the desert mates for life, which lasts in its case for about a year.
The couple may have up to 60 to 70 kids.
- Moreover, procreation isn’t done by laying the eggs in the sand and callously abandoning them. No! The lady’s eggs go into her marsupial-style “brood pouch,” where they hatch and where the infant isopods live until they are ready to launch into the burrow. There they dwell with their doting parents for a few months until ready to sally into the world.
‘Tis an idyllic scene in their burrows under the sands of the Negev, is it not?
Not if you’re the 90-milligram weakling louse getting sand kicked in your face by a bigger rival and left the inferior habitats where predators lurk.
The question addressed by Viraj Torsekar, Moshe Zaguri and Dror Hawlena of the Hebrew University of Jerusalem in their new paper in Ecology was mating patterns in woodlice, and chiefly – whether the proximity of a predator affects the choice, or mechanism, or pattern, call it what you will.
Specifically, the researchers were asking how predation risk affects size assortative mating in the Israeli wood louse, if it does.
Who eats the desert woodlice of Israel? The golden scorpions of Israel.
Size assortative mating is when the male and female mate with someone the same size as themselves more often than random choice would dictate.
The attraction in the biggest females is that it correlates with the number of young she can produce, by the way.
- The biggest female woodlice and biggest male woodlice end up together – which begs the first question, before getting to that golden enemy: who is choosing what in the woodlouse world?
It’s a lousy world
- This is what woodlice do. When a female is ready, she digs a burrow, but it isn’t exactly that she chooses her mate. Males realizing what is at stake come to fight, which then begs another question – what happens if a weedy little male somehow vanquishes the Goliath of Isopodia? Does the big lady accept him? Is she even involved in the mating choice process?
- Well, sometimes the smaller males win but about 75 percent or maybe 80 percent of the time, in other words, usually, it is the big male that thrashes the smaller one, Torsekar explains.
Interestingly, when the wee louse gets there first, he has a better chance of beating the bigger competition, he adds – the “prior resident effect” applies. So theoretically a plucky little one can beat a bigger rival but in practice, the larger the male, the better he can defend his new “territory” – the female and her burrow.
Whether the female has choice: “We don’t know. The literature suggests the female inside the burrow will not reject any males, but she may make the male wait before she lets him into her burrow,” Torsekar says. That could suggest she may be hopefully waiting for a top-quality, beefier male. But there is no actual evidence of that, he hastens to stress.
So maybe if a feisty titch shows up and flexes its 14 legs, she ignores him like in high school. Okay.
Now let us add the scorpion to the mix. To study how scorpion proximity affects the sex life of the wood louse, the researchers set up two experimental controlled habitats. Both had females of the same size, but males of different sizes – and in one habitat they added soil that stank of scorpion. In the other they put normal soil.
- Let us be clear that the golden scorpion of Israel also spends its life in a burrow and when it digs one, that tunnel smells of scorpion, Torsekar explains.
When an animal it could eat, such as a wood louse, walks on the roof of its burrow, like a trapdoor spider it leaps out and attacks. “Prior work in the lab shows isopods are well aware of scorpion presence from just the soil mound,” he says.
Apropos smell, some people keep woodlice as pets, feeding them any vegetable matter, and they don’t smell like roses. Note that as crustaceans they are not related to insects but are distant cousins of crabs, lobsters and aquatic isopods which can be pretty monstrous in size and since they stemmed from marine creatures, they still breathe through (adapted) gills.
Louse nous
The bottom line is that the big males clearly preferred the no-scorpion scenario and when the whiff of one lurked, he might well leave the high-risk habitat for the small male to take, the researchers found. No altruists in the house of the louse.
And come winter, the pair goes into the burrow and stays there until winter is over, when the new generation nurtured in the brood pouch comes out and the cycle begins anew.
It makes sense to avoid a habitat featuring a predator and bears adding that while tigers for instance move around to find prey, perhaps rendering themselves less of a bucket-list item to avoid when choosing a cave to live in – not so the golden scorpion.
- The latter is “anchored in space,” as Torsekar puts it – it stays in its lair. In this study the researchers have shown how individual choices can end up affecting the mating pattern of a whole population.
“It’s fascinating what the possibilities are just by a predator being close by,” he says.
In fact there’s another aspect in which the isopods of the Negev sound totally like thrill-seeking teens. “Sometimes these isopods actually climb the mound smelling of scorpion. Maybe they do it to gain more information about the presence of an animal inside,” Torseker says. “They seem to have a different response to a mound with no smell.”
Source: Ruth Schuster – TOI