THE LIVING WORLD
Unit Eight. The Living Environment
Different strategies have evolved among prey to deter predation. Some species use physical or chemical defenses. Organisms that contain toxins may advertise this fact with warning (aposematic) coloration. Interestingly, during the course of their evolution, many nonpoisonous animals have come to resemble distasteful or dangerous ones that exhibit aposematic coloration. Also protected species can mimic each other.
Batesian mimicry is named for Henry Bates, the nineteenth- century British naturalist who first brought this type of mimicry to general attention in 1857. In his journeys to the Amazon region of South America, Bates discovered many instances of palatable insects that resembled brightly colored, distasteful species. He reasoned that the mimics are avoided by predators, who are fooled by the disguise into thinking the mimic actually is the distasteful model.
Many of the best-known examples of Batesian mimicry occur among butterflies and moths. Obviously, predators in systems of this kind must use visual cues to hunt for their prey; otherwise, similar color patterns would not matter to potential predators. There is also increasing evidence indicating that Batesian mimicry can also involve nonvisual cues, such as olfaction, although such examples are less obvious to humans.
The kinds of butterflies that provide the models in Bates- ian mimicry are members of groups whose caterpillars feed on one or a few closely related plant families that are strongly protected by toxic chemicals. The model butterflies incorporate the poisonous molecules from these plants into their bodies. The mimic butterflies, in contrast, belong to groups in which the feeding habits of the caterpillars are not so restricted. As caterpillars, these butterflies feed on a number of different plant families unprotected by toxic chemicals.
One often-studied mimic among North American butterflies is the viceroy, Limenitis archippus (figure 35.25b). This butterfly, which resembles the poisonous monarch (in figure 35.25a), ranges from central Canada through much of the United States and into Mexico. The caterpillars feed on willows and cottonwoods, and neither caterpillars nor adults were thought to be distasteful to birds, although recent findings may dispute this. Interestingly, the Batesian mimicry seen in the adult viceroy butterfly does not extend to the caterpillars: Viceroy caterpillars are camouflaged on leaves, resembling bird droppings, whereas the monarch’s distasteful caterpillars are very conspicuous.
Batesian mimicry also occurs in vertebrates. Probably the most famous case is the scarlet king snake, whose red, black, and yellow bands mimic those of the venomous coral snake.
Figure 35.25. A Batesian mimic.
(a) The model. Monarch butterflies (Danaus plexippus) are protected from birds and other predators by the cardiac glycosides they incorporate from the milkweeds and dogbanes they feed on as larvae. Adult monarch butterflies advertise their poisonous nature with warning coloration. (b) The mimic. Viceroy butterflies, Limenitis archippus, are Batesian mimics of the poisonous monarch. Although the viceroy is not related to the monarch, it looks a lot like it, so predators that have learned not to eat distasteful monarchs avoid viceroys, too.
Another kind of mimicry, Mullerian mimicry, was named for German biologist Fritz Muller, who first described it in p 1878. In Mullerian mimicry, several unrelated but protected animal species come to resemble one another. Thus, different kinds of stinging wasps have yellow-and-black-striped abdomens, but they may not all be descended from a common yellow-and-black-striped ancestor. In general, yellow-and- black and bright red tend to be common color patterns that warn predators relying on vision. If animals that are all poisonous or dangerous resemble one another, they gain an advantage because a predator learns more quickly to avoid them.
Key Learning Outcome 35.12. In Batesian mimicry, unprotected species resemble others that are distasteful. Both species exhibit aposematic coloration. In Mullerian mimicry, two or more unrelated but protected species resemble one another, thus achieving a kind of group defense.
Invasion of the Killer Bees
One of the harshest lessons of environmental biology is that the unexpected does happen. Precisely because science operates at the edge of what we know, scientists sometimes stumble over the unexpected. This lesson has been brought clearly to mind in recent years, with reports of killer bees being discovered east of the Mississippi River.
In the continental United States, we are used to the mild-mannered European honeybee, a subspecies called Apis mellifera mellifera. The African variety, subspecies A. m. scutelar, looks very much like them, but they are hardly mild mannered. They are in fact very aggressive critters with a chip on their shoulder, and when swarming, they do not have to be provoked to start trouble.
A few individuals may see you at a distance, "lose it,” and lead thousands of bees in a concerted attempt to do you in. The only thing you can do is run—fast. They will keep after you for up to a mile. It doesn't do any good to duck under water, as they just wait for you.
They are nicknamed "killer” bees not because any one sting is worse than the European kind, but rather because so many of the bees try to sting you. An average human can survive no more than 300 bee stings. A horrible total of more than 8,000 bee stings is not unusual for a killer bee attack. An American graduate student attacked and killed in a Costa Rican jungle had 10,000 stings.
Killer bees remind us of the "law of unintended consequences” because their invasion of this continent is the direct result of scientists stumbling over the unexpected.
Killer bees were brought from Africa to Brazil 47 years ago by a prominent Brazilian scientist, Warwick Estevan Kerr. A famous geneticist, Kerr is the only Brazilian to be a member of the U.S. National Academy of Sciences. What he was doing in 1956, at the request of the Brazilian government, was attempting to establish tropical bees in Brazil to expand the commercial bee industry (bees pollinate crops and make commercial honey in the process). African bees seemed ideal candidates, better adapted to the tropics than European bees and more prolific honey producers.
Kerr established a quarantined colony of African bees at a remote field station outside the city of Rio Claro, several hundred miles from his university at Sao Paulo. Although he had brought back many queens from South Africa, the colony came to be dominated by the offspring of a single very productive queen from Tanzania. Kerr noted at the time that she appeared unusually aggressive.
In the fall of 1957, the field station was visited by a beekeeper. As no one else was around that day, the visitor performed the routine courtesy of tending the hives. A hive with a queen in it has a set of bars across the door so the queen can't get out. Called a "queen excluder,” the bars are far enough apart that the smaller worker bees can squeeze through. Once a queen starts to lay eggs, she never leaves the hive, so there is little point in slowing down entry of workers to the hive, and the queen excluder is routinely removed. On that day, the visitor saw the Tanzanian queen laying eggs in the African colony hive, and so removed the queen excluder.
One and a half days later, when a staff member inspected the African colony, the Tanzanian queen and 26 of her daughter queens had decamped. Out into the neighboring forest they went. And that's what was unexpected. European queen bees never leave the hive after they have started to lay eggs. No one could have guessed that the Tanzanian queen would behave differently. But she did.
By 1970 the superaggressive African bees had blanketed Brazil, totally replacing local colonies. They reached Central America by 1980, Mexico by 1986, and Texas by 1990, having conquered 5 million square miles in 33 years.
In the process they killed an estimated 1,000 people and over 100,000 cows. The first American to be killed, a rancher named Lino Lopez, died of multiple stings in Texas in 1993.
All during the following decades, the bees have continued their invasion of the United States. All of Arizona and much of Texas has been occupied. A few years ago, Los Angeles county was officially declared colonized, and it looks like African bees will eventually move at least halfway up the state of California.
Soon, however, the invasion is predicted to cease, on a line roughly from San Francisco, California, to Richmond, Virginia. Winter cold is expected to limit any further northward advance of the invading hoards.
For the states below this line, sure as the sun rises, the bees are coming, not caring one bit that they were unanticipated. The deep lesson—that unexpected things do happen—is being driven home by millions of tiny, aggressive teachers.