Cannibalism

…all in the family?

Cannibals eat members of the same species as themselves. In many cases, the eaten individuals are already dead, from a variety of causes; eating them is more like scavenging. Another form of cannibalism might be called predatory cannibalism: the eaten individuals are killed (or badly damaged) just before being eaten. The animal kingdom includes many species that regularlyengage in predatory cannibalism, commonly among family members, and that is the subject of this essay. Here are a few examples; perceptive readers will notice that every example leads to more questions (stated or not):

–eating siblings: Occasional sib-eating is known to occur in owls and eagles and other species, but that is different from regular, customary sib-eating, which has evolved independently in many different kinds of animals, including spiders, sea stars, flour beetles, earwigs, ladybugs, and many more. A certain marine slipper snail, for instance, engages in multiple matings, after which the female broods her embryos in a capsule. Those embryos have multiple fathers, and the more fathers they have, the higher the intensity of sibling cannibalism. Sibling cannibalism usually results in the cannibalistic siblings growing faster, getting bigger, and surviving better than if they had not eaten their sibs. 

Sand tiger shark sibs eat each other before they are born. Females of this species have two uteri, each one holding multiple eggs. Females mate with several males, so the embryos have different fathers. Embryos from the first eggs to hatch in each uterus grow faster than the others and soon gobble them up (and then eat any unfertilized eggs too). This begs the question of what determines which embryos (fathered by which males) win out and thus have the advantage of becoming extra-large, thanks to their special diet.

Spadefoot toads breed in ephemeral pools; the tadpoles commonly eat detritus. Some tadpoles develop bigger and better jaws and teeth and start to eat fairy shrimp; then they eat their sibs. As a result, they reach metamorphosis more quickly and are more likely to become toads with the adult form before the pond dries up. The cannibals can discriminate their own kin from the tadpoles of other mothers, and they prey mostly on non-kin (at least when well-fed).

Larval tiger salamanders sometimes also become cannibals, with bigger heads and teeth than the non-cannibals. They too can discriminate between kin and non-kin. Larvae of different maternal lineages differ in discriminatory ability; lineages with greater tendencies to become cannibals are better at discriminating kin than lineages with a lower tendency to become cannibals. However, in over-crowded conditions, even cannibals are less discriminating and may eat anyone.

–eating parents: Even in the few species in which males take over parental care, I have not (so far) found any examples of young ones killing and eating their attentive fathers. However, the young of some insects, nematodes, pseudo-scorpions, and spiders commonly eat their mothers. Females of a desert spider normally regurgitate food to their young. Digestive enzymes of mated females increase and eventually digest the guts themselves; all that material is stored in the females’ abdomens. When the young are born, they puncture their mothers and suck them dry in just a few hours, leaving just a husk. The black lace spider female signals to her young to approach her; then they jump on her repeatedly and eventually she lets them suck out her insides; they finish her off by poisoning her before sucking all the juices out. Very young spiders of this species will eat any female, but after they are about four days old, they only eat their real mother (!).

–eating mates: Female spiders and mantises get public attention for their mate-eating behavior, but it turns out that this behavior is not as common as the myths would have it. Depending on the species, a lot depends on male behavior, on inherent aggressiveness of females, on fighting ability of the male, on availability of other prey and hunger levels of females, on the probability of finding another mate, and other factors. Males of some species can sometimes avoid being eaten by fighting back vigorously (springbok mantis), by wrapping her up in a silken shroud (a North American nursery-web spider), or just by being very cautious when approaching a female. And then there’s the little biting midge that lets a male begin copulating, but then jabs his head, digests his insides, and sucks him dry. His body husk drops off, except for his genitalia, which remain in place for a while, preventing access by other males.

In most cases, females that are cannibalistic gain reproductive advantages by laying larger, bigger eggs that survive better than those of non-cannibalistic females. Therefore their deceased mates also gain reproductive advantages. In a species of praying mantis, amino acids from the male’s body have been detected both in the female and in her eggs, so he is more than just a meal. Those indirect advantages to males have to be weighed against the probability of finding a second mate: for some species, males have such rare opportunities to mate that investing (their lives) in the young of one mating is the better reproductive strategy (leading to more offspring). 

–eating your own offspring: This happens sporadically in a variety of species, including spiders, rodents, snakes, fish, birds, and mammals: a parent eats one or more of the offspring in a brood, usually because it is not developing properly or adequately. It’s a way of regaining some of the resources initially spent on offspring production. 

However, regular consumption of one’s own offspring is obviously counterproductive. Evolutionary success is generally measured by the number of surviving offspring (and grandchildren, etc.) produced, and any family lineage that customarily ate its own children would not exist for very long. Nevertheless, under certain conditions, eating your own babies may be advantageous. In some cases, parental male fish may eat the young ones they are tending, especially if there has been lots of cuckoldry by other males and many of the young might not be their own; then they can start over with a new brood and hope for increased paternity.

Eating offspring is also a source of energy in times of food scarcity. A freshwater amphipod becomes more cannibalistic when it is parasitized and thus has increased nutritional needs. Juveniles are preferred prey, presumably including offspring of the cannibalistic adults. Can they distinguish their own young from others and prey selectively on the others?

Polar bears are currently losing access to their usual marine mammal prey, because the sea ice is breaking up. Researchers have observed that, as a result, the hungry bears have turned increasingly to predatory cannibalism. Male bears have been known to kill cubs occasionally (which brings the mothers back into heat and the males then may sire another cub), but now polar bear males are attacking females with cubs and eating the cubs significantly more often. In some cases, they may be eating their own offspring.

Some butterflies have caterpillars that feed on milkweed plants, from which they take up some toxic chemicals that help protect them from would-be predators. However, researchers have recently noted that males of certain of those species spend time and effort scratching the skins of caterpillars, which could be their own offspring. The skin oozes those toxic chemicals, which are sucked up by the scratching males, perhaps to be used as courtship attractants or part of the nuptial gift transferred to females along with the sperm. The caterpillars are seriously wounded and sometimes die. I wonder, then, if the intensity of scratching caterpillars increases when mating competition is fiercest.

Courtship and mating gifts

bonding techniques for enamored animals

Animal courtship often involves more than fancy displays and songs. Some male animals also give courtship ‘gifts’ to females, to help begin a mating relationship and help it on its way. But the gifts may also contribute to the well-being of the ensuing offspring and the mother. There may be both direct and indirect benefits to both giver and receiver.

Courtship gifts are often a food item; many birds, from songbirds to seabirds, engage in courtship feeding. Once a pair-bond has begun, the gift-giving often continues through the egg-laying period, when the female is fertile. Gifts of food come in a variety of forms. For instance, male kingfishers offer small fish to their females, and male shrikes commonly offer insects. The European great gray shrike with large gifts is reported to obtain better chances of copulating with his mate than one bringing small gifts—and he also gets more mating opportunities with other females when he goes gallivanting!

Females receiving gifts generally are in better condition, may lay more eggs, and are more likely to incubate successfully, so her reproductive success is enhanced. The direct advantage to the gift-giving male includes not only more copulations (and so, more eggs fertilized), but he also gains less direct benefits from the improved success of the female incubating his eggs. His gifts might also help keep her attention from wandering males looking for a chance to mate.

Male dance flies of some species capture another insect and present it to a female, copulating with her while she eats. If the prey is small, she may eat and run, before copulation is finished. Other species of dance fly (genus Hilaria) wrap the prey in a balloon of silk, which the female must unwrap in order to eat the prey. If he finishes copulating before she is finished eating, he sometimes takes back the gift and rewraps it for another female. In a related genus (Empis), some species just present a balloon of saliva bubbles that’s empty—and the females apparently accept that.

Male nursery web spiders in Europe give prey (usually silk-wrapped) to a female, who unwraps and bites it (instead of him). The male then copulates, spider style, by transferring sperm from his genitalia to his pedipalps (modified legs) and inserting the laden pedipalps in the female’s reproductive tract. He may keep one leg on his gift, lest she try to escape with it. However, sometimes the gifts are deceitful, just an inedible item wrapped in silk—that seems to work too, but perhaps not as well. 

Some male insects transfer nutrients to a female along with their sperm, in a package called a spermatophore. Depending on the species, the female may eat the part with nutrients or simply take it up into her reproductive tract. Experiments with fireflies and bush crickets have shown that spermatophore nutrients are indeed taken up by females and soon metabolized. In other species, the spermatophore contains protective substances. For instance, female fire-colored beetles take up spermatophores containing cantharadin (a poisonous and blistering compound); the females cover their eggs with it to protect them from predators.

In some cases, although there may be immediate reproductive advantages to both gift-giver and recipient, there may be other effects as well. For example, in the fruit fly (Drosophila melanogaster), the male’s gift may increase the female’s current reproductive success but decrease her long-term reproductive fitness.

There are still other ways of transferring things from one mating partner to another, such as the transdermal injections used by some invertebrates. For example, earthworms (which are hermaphroditic, the two partners exchanging sperm when they mate) have stiff hairs on the clitellum (the expanded segments near the front end of the body). Those hairs puncture the partner’s skin and may somehow manipulate the partner’s reproductive physiology. The famous ‘love darts’ of terrestrial,hermaphroditic snails and slugs are fired by both partners when they mate, just before sperm is exchanged. Apparently, mucus on the dart helps open the canal to the organ where incoming sperm is stored and improves sperm viability. Most snails produce one dart at a time, but some can produce many of them(they are made of calcium carbonate, chitin, or cartilage). Now the question arises: are these really gifts with the typical reciprocal benefits or does only the donor benefit?

Fall Colors

delights of a fading season

Photo by Mary Willson

Here in a rainforest, we don’t get the flamboyant displays of golden-leaved aspens or the flame-colored maples, although there may be some isolated cases of such brilliance dotted about our city streets. Our alpine zones are sometimes full of glorious color, but not all of us can get there. But we do love color. However, think not that we are deprived of these season visions; we have plenty of fall colors. They’re usually somewhat more subtle and on a smaller scale, but quite wonderful in their own way, when we bother to look. Attentiveness, as Robin Wall Kimmerer noted, is the key to seeing.

Here are a few examples of enjoyable displays of fall colors we’ve seen recently:

–on a hummock in a muskeg, a mat of sphagnum moss had turned partly red, still spangled with spots of gold and green. The mat was decorated with crimson leaves of cloudberry, a cluster of scarlet bunchberries, and bunchberry leaves in scarlet and green.

–the last stands of fireweed can be pink or red, or sometimes both of those colors grading into oranges and yellows and remnants of green.

–subalpine slopes are clothed in deer cabbage, offering a mosaic of yellow, golds, russets, and rich browns.

–high-bush cranberry shrubs often sport many colors—the whole bush may bear gorgeous red leaves. On others, each leaf can display every shade of red, orange, and yellow. Sometimes the whole show is high-lighted by those lovely red berries.

–the understory of the dark conifer forest is brightened by the broad, yellow leaves of devils club, even as they become dilapidated.

–cottonwood leaves often turn bronze or gold and flutter nicely in a breeze. How sad that the whole row of shapely young cottonwoods along Vanderbilt Hill Road has been destroyed.

–along one trail, I found a single salmonberry cane with every leaf a color-treat. An occasional leafy stem of goats-beard may be very red in the midst of others that are still green.

–in the forest edges, the heart-shaped leaves of mayflower turn yellow—or sometimes an unusual pattern of white with black lines.

–sometimes a single leaf displays a variety of color 

–and have you ever noticed that the upper sides of silverweed leaves can grade nicely from orange to yellows to tawny browns?

Summer leaves are green, because the cells contain lots of green chlorophyll that does the work of photosynthesis (making sugars). As days shorten and nights grow longer and cooler, chlorophyll gradually breaks down, exposing the yellow carotenoids that have been there all summer (absorbing light energy and transferring it to chlorophyll), concealed by the green. During those shortening days, some photosynthesis continues, but each leaf is gradually disconnected from the rest of the plant, so sugars are poorly transported to the rest of the plant and build up in the leaves. In bright light, they are built into colorful red-to-purple anthocyanins. A single leaf may sometimes be red on one half and yellow on the other, if one half was exposed to sunlight and the other was shaded.

That simplified explanation leaves many questions. Why build anthocyanins in a dying leaf?  Why do some species often produce lots of red leaves in fall, while other usually bear yellow leaves? Why do some of the typically yellow-leaved plants occasionally make red or orange leaves? Why do alder leaves just turn brown, with no bright colors? Readers can probably think of still more questions!

In any case, there is lots of color to enjoy, even in the rain.