The next morning, she was not in sight. She had left her web and was on a ceiling beam toward the inside of the cabin, at almost the same spot where she had overwintered before. To get there from her lair by the window she had to cross three ceiling beams. She later moved to an even darker spot and there spun some silk surrounding herself, so she meant to stay. I was afraid she might dry up there because it was near the stove, which I would use all winter. So I installed her in a veggie crisper with some bark to hang on to and put her on a shelf.
When I checked on her three months later, her abdomen was discolored, flaccid, and shrunk. If she died because she had reached the end of natural life, she didn’t leave the expected egg case. Maybe I had chosen the wrong overwintering spot for her. With no possibility for intake of food or water, the overwintering home must be cool and moist enough (in the case of hibernators) to conserve energy and water. Survival then may depend on stomach contents. There are secrets here, but only other spiders like Charlotte will tell, if you ask her just right.
All of the spider lore that I read about, and heard from three spider experts, and as per E. B. White’s famous story of Charlotte, is that orb web spiders lay eggs in late summer that they ensconce in a fluffy silk cocoon, then die in “late autumn.” The eggs overwinter and hatch in the spring, and the almost microscopic spiderlings then “balloon” far and wide in the wind, being carried by a strand of silk they extrude. But, although it is well known that adult orb web spiders die in the fall after reproducing, I found no information about anyone who tracked individuals in a temperate seasonal environment from the time that they hatched until they reproduced, to find out if they accomplished that feat in one year, or two, or ten, or thirty.
My Charlotte left me with mysteries, and one of the main ones was how she could produce offspring, since she never gave signs of wanting to leave home. How could she have found a mate? Being saddled with a web might make it difficult to leave home, because what would she eat? Why didn’t she ever try to relocate to a new home, one where she might have found a mate? Males supposedly make no webs, but if a male is found in a web it is presumably because he had entered it to find a female. Because of this information I had been convinced that Charlotte had been a female, even though she had not left an egg case. More or less by chance, answers emerged the next summer from a new spider.
I had grown fond of Charlotte in her own special spot over two summers, and I missed her the next year, 2013, while wrapping up the writing of this book. I was, however, by then keeping a vigilant eye out for orb web spiders, especially at places that were cavelike, where those like Charlotte might make their homes. I ended up keeping many, of various sizes, at my cabin window and in cages. Most were various-size replicas of Charlotte in every way. But one was different.
In early May, when the first leaves started to appear on the trees and the first insects started to fly after a long cool rainy period, I found spider webs under a rock overhang at a road cut that I pass regularly on a daily run from my cabin. And, as hoped, another large spider that looked like Charlotte was perched above the web, clinging to the underside of the rock.
I checked on this spider several more times, and finally on June 9, thinking about some of the mysteries that Charlotte left me with, and also finding a discarded cardboard coffee cup there by the roadside, I stopped to catch the spider in the coffee cup, to bring it back to the cabin. It was time, once again, to have a spider housemate (and later several).
This spider’s pair of palps—the organs at the front end of a spider that are analogous to antennae in insects—were enlarged and clublike at the ends, indicating it was a male. But the day after I released him in front of a screened window in my cabin, he had built there a beautiful typical orb web—of about forty centimeters’ width. In every way, the web looked like Charlotte’s and the other females’. I captured insects, tossed them into his web, and he acted in every way like Charlotte had: capturing, rolling up, and pulling up prey to his lair on the ceiling.
Although I decided this spider was probably a male, I was not certain, because he was huge, and I had read that spider males are small relative to the females, and everyone I talked with informed me that male spiders do not build webs.
The new spider’s patterns of gray and black lines and white markings indicated it was an Araneus cavaticus, like Charlotte. However, aside from being more leggy than Charlotte, he had in comparison to her a tiny abdomen. So I kept throwing insects into his web. The last one I threw in, on June 18, right after he had made another huge new web, was a drone honeybee, whose body was about the same size as his. He silk-wrapped the drone, sucked it dry, and dropped the silk-wrapped corpse from the lair above his web onto my desk.
After the bee-drone meal, the spider stayed put in his lair, and he ignored all potential prey I threw into his gradually more tattered web. I thought he was sick, but a week later, on June 25, he molted and his already seemingly shrunken abdomen was now smaller still. On the other hand, his legs were a third longer than before the molt. His first leg, the longest, was 4.3 centimeters, giving him a maximum stretch of 9.2 centimeters. The dimensions of the cephalothorax were equal between male and female. He had, it seemed, metamorphosed into the body shape of a runner. He built no new web, and then he disappeared.
This could have been the end of his story, except that the fireworks on the Fourth of July kept me awake, and in my half-sleep I had thoughts that I wanted to write down. I got up, put on a headlamp, and, holding a pencil and notepad, sat in darkness on the couch downstairs and tried to scribble. There, within a minute or two, I felt a light, brushlike sensation on my bare skin, as though touched with a downy feather. I looked down at my thigh just in time to see a huge spider leave and, without a pause, run along the couch. I recognized the small abdomen and the oversize legs.
And this spider could run! In two seconds he had made it to the windowsill, and as soon as he sped along it I started counting: “thirty-one, thirty-two, thirty-three”—in three seconds he had traversed the length of the one-meter windowsill. At that speed he could do a thirty-minute mile. I looked at my watch—12:05 a.m. The spider, on reaching the corner, next raced up the wall and in seconds reached the ceiling. No female had ever acted remotely like this. Apparently male spiders of this species, after they mature, run around at night, and that is why the females don’t have to. Instead, they can stay put to continue catching prey, conserving and gathering resources to make their masses of eggs as their abdomens continue to expand. Males can then devote the rest of their lives to the mate chase. But if so, they can mate only once or perhaps twice. (Their pair of palps are not tactile organs like the female’s—they are instead sperm transfer organs. During mating one or both palps break off inside the female.)
Three days later I found him again, this time running around upstairs. Still, no web had been made, and I wondered if he would feed at all. I captured him and put him in a thirteen-cubic-centimeter cage to test if he would feed on the flies, bees, beetles, and grasshoppers that I offered live—he ignoring all (a female A. cavaticus I kept later in the same cage routinely captured prey). And, after a month without taking food, he was finally on his last legs and held still enough for me to sketch him.
The evolutionary “strategy” of this spider—where the female is sedentary and feeds and the male is highly mobile and does not feed when adult—is not an altogether unfamiliar one: in some species of moths (such as the aforementioned bagworm moths, the locally common tussock moth, Orgyia sp., and some winter-flying Geometridae) females lack wings and have only rudimentary legs, while the males have large wings but lack a digestive tract. They do not move beyond the cocoon out of which they emerge, deriving all of their food from the actively feeding larval stage. In ants and termites, there is (in addition to different body forms related to caste and sex) a serial change of body form and associated behavior instead; both sexes are highly mobile before reproducing but shed their wings as soon as they settle for the rest of t
heir lives into a home to reproduce.
Note: Having learned where spiders like Charlotte live and build their webs, I found twenty-three of them at a deserted camp complex in the woods in mid- to late August. At first I saw only huge ones, like Charlotte and the male had been, but the longer I searched, the smaller the size of these orb web weavers seemed to become. The tiniest individuals that I eventually found were mere specks. They appeared to hang in space, could be seen only if they happened to be in a web, and against a uniform dark background, and at just the right angle in the right light. With a hand lens, I saw that the tiniest of these spiderlings had created almost perfect replicas of adult webs, but these webs were woven of such a fine silk that they were, for all practical purposes, invisible.
Of the twenty-three individuals, nine had an abdominal diameter (as measured by a caliper) of 12 to 13 millimeters, two of 7 to 8 millimeters, three of 4 millimeters, two of 2 millimeters, seven of 1 millimeter or less. In terms of approximate volume, the spiders’ nearly round abdomens calculate to about 0.52 cubic milliliters for those of 1-millimeter diameter, to 905 cubic milliliters’ volume for the largest, those having 12-millimeter abdominal diameter. These size differences occurred near the end of August, and after the one month more that they had left before going into hibernation in that year, they had grown only slightly or not at all. All the largest ones laid a round packet of hundreds of yellow eggs, to which they clung until they dropped off dead in late October.
Spiders are the most common arthropods I encounter in hibernation in the winter woods, and I believe by a conservative estimate that Charlotte was at least five years old when I got her, and more likely very much older than that. The duration of the orb web spider life cycle has been hugely underestimated. I think that if Charlotte had matured from a hatchling to egg-laying in one Maine summer, she would have been an “extraordinary spider” indeed, one measuring up to E. B. White’s original.
The Communal Home
We become human only in the company of other human beings.
—Paul Rogat Loeb
THE NESTS OF MOST SONGBIRDS ARE SMALL AND HIDDEN, made by one bird or a pair, and function only to hold, hide, and protect one clutch of eggs and young. Those of the sociable weavers, Philetairus socius, of the Namib and Kalahari deserts of southern Africa, do much more. They are the world’s largest and most populated tree houses, or nests, which may weigh several tons and range up to six meters wide and three meters tall. Over a hundred nesting chambers or apartments contained in one of these communal homes are refurbished and reused and new ones added over successive generations, often for over a century; one generation inherits, builds on, and profits from an environment created by another.
In the sociable weaver’s native habitat of the Kalahari Desert, temperatures range from minus ten degrees Celsius to over forty-five degrees Celsius in the summer. Like a human apartment building (to which it is impossible not to draw comparisons), this weaverbird’s home is a year-round community’s living quarters that shelters all from direct sunshine and also protects them from rain, drought, and cold. Scorching summer temperatures demand water for evaporative cooling in many animals, and most birds regularly need water to get rid of excess body heat by gular fluttering, a process similar to panting where increased air movement in the moist throat area accelerates evaporative water loss. But by retreating into the communal nest at high temperatures in the summer, the sociable weavers save water otherwise needed for heat dissipation, enough so they can get by without drinking.
Not needing to be near water hugely affects the possible range where the weavers can live. Conversely, at low air temperatures in winter, when the birds spend nights in the nest, they save the energy otherwise needed for shivering to keep warm, thereby saving food. Some apartments used by pairs for rearing their young in winter become dormitories for up to five adults, and when there is frost outside and they huddle together, the inside temperature stays fifteen degrees Celsius above that outside. Thus the sociable weavers’ communal nest is a valuable resource other than just for breeding; it is also a year-round dormitory and a refuge. Perhaps understandably, the young of these already sociable birds are reluctant to leave their home after they fledge, and they stay, “earning their keep” by helping their parents feed subsequent broods.
Unlike most weaverbirds, the sociable weaverbirds don’t “weave.” Their nests look like huts complete with a thatched sloping roof made from grass stems that sheds rain. The nest grows through the years as the birds insert dry grasses into the bottoms and sides, as new apartments are added. Each of the separate compartments of over a hundred breeding pairs is lined with soft downy plant material and has a separate entrance about twenty-five centimeters long and three centimeters wide, built of downward-pointing spiky straws that help as snake excluders. As a result of the close proximity of nest entrances, the underside of the communal nest has a honeycomb appearance.
Communal nest of the sociable weaver. Entrances to individual compartments are on the nest underside (right).
Perhaps not surprisingly, what works well for the home-makers also works for some home crashers. The weavers’ communal home is also the almost exclusive nesting place of the pygmy falcon, Polihierax semitorquatus, and a favorite nesting place for red-headed finches, Amadina erythrocephela, and a parakeet, the rosy-faced lovebirds, Agapornis roseicollis.
The weavers’ nest-building drive, and their attachment to the communal home, is so strong that they not only live in and around their nest all year but also continue their home improvements throughout the year. At the San Diego Zoo, home of the only colony of these birds in the United States, the busy birds are daily provided with dry grasses for their year-round home-making.
When confronting phenomena as highly derived as a weaverbird’s communal nest, the honeybee’s language and social organization, or a skyscraper, we wonder how they evolved. These things may seem incomprehensible when the intermediate steps from “there” to “here” are obliterated, but possibilities can be visualized by comparisons with the well-known that exists. Weaverbirds’ nests vary widely, and an examination of their different kinds provides examples for a progression of possible stages from solitary to social behavior sometimes associated with social nesting. The progression from the original to the highly derived may not be entirely correct, but it provides scenarios that can be tested.
To begin with, weavers in general tend to be sociable. Many nest in colonies, and their nests are often in close proximity to one another. Nests of the white-billed buffalo weaver, Bubalornis albirostris, of Africa south of the Sahara, often touch each other. One bulky stick nest can become a convenient location to which the next one is attached, until several effectively become a communal nest with separate entrances. This arrangement is also found in another African weaver, the chestnut weaver, Ploceus rubiginosus, which nests in enormous colonies, and in other arid scrubland weavers of Africa, such as Plocepasser mahali and Pseudonigrita arnaudi, both of which tend to form clusters of about a dozen nests and are more socially tolerant than most other weavers.
Parrots provide a comparative example, converging on the same pattern as weavers but from a very different direction. Unlike most weaverbirds, which create their own cavities by weaving bag nests, parrots nest in the already-made cavities of tree holes. But there is an exception: the monk or Quaker parakeet, Myiopsitta monachus. This species is a true exception because it is the only one of the 350 parrot species not limited to nesting in cavities. Instead, it builds stick nests in trees, and not just little ones made by mated pairs but giant car-size communal homes.
Since all other parrots nest in cavities, we can infer that the monk parakeets’ communal tree nesting is a derived condition that has evolved from ancestors that were cavity nesters. The example then begs the questions of how their tree nest building evolved and why it has not evolved in any other parrot for which choosing cavities to nest in was highly advantageous and so became deeply ingrained in the birds’ ba
g of survival tricks. A radical change to a totally new mode, which would be akin to trying to rebuild a propeller plane into a jet, is unlikely. So how was the conversion from cavity nesting to building a stick nest made, and especially why was it again a communal nest, for specifically this species?
The monk parakeet’s ancestral nesting places were likely similar to those of the closely related cliff parakeet, M. luchsi, which nests in rock crevices rather than in tree holes. Cliff sites are partially enclosed by walls, and initially cliff nesters may have filled in some of the open space with perhaps a twig or two, and then maybe more. A cliff crack would have provided space for an adjacent nest, or two, or more. “Extra” space would have reduced competition at a nest site. Instead, a nearby nest might have helped to make a wide or long crack more like a cavity, so that a potential competitor was instead a “helper,” thus selecting for increased social tolerance. Increasing social tolerance in turn would permit the use of ever-wider crevices that could be substituted for cavities. Nesting next to or in between several other nests then became the monk parakeet’s substitute for nesting in partially enclosed spaces such as those on cliffs.
Nesting next to each other was a win-win condition for all, and at this (current) point in the parakeet’s evolution, hundreds of nests may be crowded next to each other to create a very large communal stick nest where each pair of birds has its separate “apartment” with a separate entrance. Like the homes of humans and those of the sociable weaver, the parakeets’ nests are used as a site for other birds to build their homes in. They include the spot-winged falconet, Spiziapteryx circumcincta, and two species of ducks, the Brazilian teal, Amazonetta brasiliensis, and the speckled teal, Anas flavirostris.
The Homing Instinct Page 19