by Joseph DeSisto
In one of many vain attempts to show me that birds and mammals are far superior to invertebrates, a good friend once reminded me that swans only love once, in reference to some pop song or another. I reminded her that many spiders, too, only love once — for very different reasons.
Being a spider with parental care sounds like a good deal. Mother spiders of the Mediterranean lined spider (Stegodyphus lineatus) raise their young in silken nests and guard them as they grow, feeding them and reducing the likelihood that they will be found by predators. This is all well and good, but beneath it is a story, one of murder and suicide, sex and deceit, loyalty and sacrifice.
By the time a female lined spider has reached adulthood, her web is well-established (Maklakov et al. 2005). Rather than build a single, simple web like many other spiders do, lined spiders builds a shelter out of silk on a bush and then constructs many tangled webs, all radiating from the shelter. This way, she can capture prey from multiple locations, and when her young emerge, they have plenty of webs from which to harvest prey.
Males, on the other hand, spend most of their time wandering around looking for females. When a male finds one, he approaches with caution. She is much bigger than him, and may attack him if she is not in the mood for company. If, for example, she already has a clutch of eggs, or if she is not old enough to mate, or if she is just feeling hungry, he might be out of luck.
If, however, she is receptive, the male will mate with her and then stay for a few days, to prevent any other males from approaching. When he (or she) decides it is time to go, he rushes out to avoid being attacked, and then continues wandering about, in case he should find another female. This hardly ever happens. Almost invariably, male spiders only love once.
When she lays her eggs, the female spider wraps them in a silken ball called an “egg sac.” This she guards for up to two weeks until the young hatch. Although they are guarded by a ferocious mother spider, the eggs are very vulnerable. Ants are common predators and, in their armies of hundreds, can easily overwhelm a single spider.
Perhaps the greatest threat, though, is more familiar: male spiders. Just as male lions and polar bears kill the young of rivals, male lined spiders frequently steal and destroy the egg sacs of females (Schneider and Lubin 1997). Why do they do this? First, stealing a female’s egg sac removes competition — his offspring will not have to compete with the product of another male’s successful mating. Second, once a female loses her eggs she becomes able to mate again, allowing the infanticidal male to replace her former partner as the father of her progeny.
This seems cruel, but male spiders are dealt a hard lot in life. While a female may encounter many males in her lifetime, and indeed will mate with many of them (Maklakov et al. 2005). Her ability to do so, however, is because of her lifestyle as a sedentary, shelter-dwelling female. Males, on the other hand, suffer a high mortality due to their wandering habits, and many males will only encounter a single female in a lifetime. So, when he encounters her, if she already has an egg sac, he will do whatever it takes to make her receptive again, even if it means killing the young she already has.
The female has ways of preventing this from happening. She can lay her eggs later in the season, when all the males have either mated or died (Schneider 1999). Or, she can simply chase away/beat up/kill any male that tries to bother her after she lays eggs.
Either way, if her eggs are fortunate enough to survive, she can look forward to caring for the young for a few weeks, catching food for them from the web and mashing it up so their adorable little mouthparts don’t get damaged. During this time, the young are completely dependent on their mother, but as they get older, they venture out onto the web themselves to harvest their own prey.
Meanwhile, the mother undergoes major changes. Her digestive organs begin to degrade, and ultimately disintegrate as she stops feeding herself (Salomon et al. 2015). By the time her young are ready to leave the nest, her internal organs have begun to liquefy. It is time for one last gift to her offspring: her body. The spiderlings, which she protected from infanticidal males, carefully incubated as eggs, and offered her own food as tiny spiders, now eat their mother as their last meal before leaving the web and heading out into the world (Salomon et al. 2005).
Why does she do this? Believe it or not, it is to her genetic advantage to give her life to ensure her progeny’s survival. The world is a harsh place for spiders, and not only are her chances of successfully raising a second clutch next to nothing, but each of her spiderlings has at best a meager chance of survival. Offering her own body gives them the best possible head start in life, and ensures that at least a few of them will be able to grow, mate, and raise young themselves.
Maklakov, A.A., T. Bilde, and Y. Lubin. 2005. Sexual conflict in the wild: Elevated mating rate reduces female lifetime reproductive success. The American Naturalist 165(S5): S38-S45.
Salomon, M., E.D. Aflalo, M. Coll, and Y. Lubin. 2015. Dramatic histological changes preceding suicidal maternal care in the subsocial spider Stegodyphus lineatus (Araneae: Eresidae). Journal of Arachnology 43(1): 77-85.
Salomon, M., J. Schneider, and Y. Lubin. 2005. Maternal investment in a spider with suicidal maternal care, Stegodyphus lineatus (Araneae, Eresidae). Oikos 109(3): 614-622.
Schneider, J.M. 1999. Delayed oviposition: A female strategy to counter infanticide by males? Behavioral Ecology 10(5): 567-571.
Schneider, J.M. and Y. Lubin. 1997. Infanticide by males in a spider with suicidal maternal care, Stegodyphus lineatus (Eresidae). Animal Behavior 54: 305-312.