Cannibalism: A Perfectly Natural History(15)
As it relates to cannibalism, the most common example of sexual dimorphism is body size, and among the invertebrates, females are often substantially larger than males. But why is this so?
The benefits appear related to the ability of larger mothers to better carry, protect, and provide for their young. Relatively small body size can also provide males with a gravity-related biomechanical advantage. Since gravity is less of a constraint on lightweight bodies than it is on heavier ones, this is especially evident in males that must climb in order to reach females. Likewise, in other species, tiny adult males are able to “balloon” like spiderlings, using the wind as an energy-efficient means of travel. Presumably this increases their chances of finding a potential mate.
On the other side of the web, but still supporting the small-get-cannibalized rule, are the rare instances in which male spiders are larger than females. This spider role reversal occurs in two species that exhibit some very unspiderlike behavior. In the sand dwelling wolf spiders (Allocosa brasiliensis), females undertake risky visits to burrows built by the larger males. Because these structures represent a high reproductive investment, male wolf spiders become extremely picky when females show up and initiate courtship—which they do by alternately waving their forelegs around in the universal signal for “Pick me! Pick me!” In many cases, though, researchers noted that instead of mating, the females were often attacked and cannibalized.
To determine why, arachnologist Anita Aisenberg and her colleagues performed experiments in which twenty male spiders were consecutively exposed to one virgin and one previously mated female (in alternating order). Findings revealed that only 10 percent of the virgin females were cannibalized while 25 percent of the mated females were eaten, especially those exhibiting lower body condition indices. In other words, male wolf spiders chose their mates based on looks and sexual history. The researchers concluded that by selecting younger, fitter females, male spiders maximized the likelihood that their mate would survive to produce a successful batch of spiderlings. Older, less fit females, also served a purpose—as food.
Cannibalism by males also occurs in the aptly named water spiders (Argyroneta aquatica), the only living arachnids that exist completely underwater. In this species, females spend most of their lives inside web-shrouded air bells, where their smaller bodies require less oxygen than their male counterparts. Natural selection may favor larger body size in male water spiders by providing them with enhanced swimming and diving abilities. While females are ambush predators, males are active hunters, and although their diets consist primarily of insect larvae, they will kill and cannibalize smaller males during intense competition for females. The female water spiders’ preference for larger males can also turn deadly in a hurry, specifically during failed mating encounters in which females are chased down and consumed.
These two examples illustrate that when cannibalism occurs, it’s size, rather than sex, that is the key determinant, with the smallest individuals (usually the males) ending up on the menu. This generalization also extends from invertebrates like spiders to some of the most familiar and beloved creatures on the planet. But before you come away thinking that praying mantises and spiders have maintained their stranglehold on cannibalistic copulatory behavior, we need to drop in on yet another member of the animal kingdom.
When terrestrial snails cross paths (or more accurately, slime trails), the potential for bizarre sexual encounters can rival a bachelor party in a Hangover film. For the snails, the high hook-up ratio stems from the fact that most of the participants are simultaneous hermaphrodites, enabling them to exchange sperm while at the same time having their own eggs fertilized. And while this particular sexual orientation increases the likelihood that any two individuals that meet will be able to mate, things can go downhill quickly once the lovers begin biting chunks out of each other.
Snails and slugs, their shell-challenged relatives, are mollusks, a biologically diverse invertebrate group that also contains the bivalves (clams, oysters, and their shelly relatives) and cephalopods (squid, octopuses, and cuttlefish). Known collectively as gastropods, the approximately 85,000 species of snails and slugs have a worldwide distribution, inhabiting a variety of marine, freshwater, and terrestrial environments. To put this into perspective, there are approximately 17 times as many gastropod species on the planet as there are mammals.
Along with their popularity as escargots or scungilli, gastropods are renowned for their slow-footed locomotion—a point celebrated annually by pub-going “researchers” in the UK. At the World Snail Racing Championship in 1995, a garden snail named Archie rocketed across a 13-inch course at an average speed of 0.0053 miles per hour, the fastest ever recorded for a snail. Scientists believe that snail speed (or lack thereof) is actually an adaptation related to energy efficiency. In other words, by devoting less energy to locomotion, gastropods can spend more of it involved in alternative behavior—like mating.
Although snail sex can last for up to six hours in some herbivorous species, this is definitely not the case in certain carnivorous gastropods, where foreplay can turn into cannibalism in the blink of a turreted eye. In these species, since even copulating individuals will bite their mates, each potential sexual partner is also a potential predator. As a result, they often employ the wham-bam-scram approach during sexual encounters, which can sometimes linger on for as long as six seconds.
An even more cringeworthy behavior is exhibited by banana slugs (genus Ariolimax), which become so entwined during sex that they sometimes chew off their partner’s corkscrew-shaped penis in an effort to disengage. During this process, which is known as apophallation, penises are slurped down spaghetti-style, occasionally by their owners. Although this usually puts an end to the festivities, the fact that the penises do not grow back presents fewer problems than one would expect. The hermaphroditic slugs simply carry on the remainder of their lives as females.