By Carolyn Shea

What can you tell me about Mexican jumping beans? Are they extinct?

Esther Rice, Hemet, California

 

Jonathon Carlson

The Mexican jumping bean has a somewhat overblown reputation as a jittery legume with a penchant for wild ricocheting, a kind of vegetal Super Ball. In truth, it's not a bean at all but the seed casing from a subtropical spurge that's native to an expanse of desert around Alamos, Mexico, the jumping bean capital of the world. Neither has the jumping bean broken any vaulting records--its motion being more shake, rattle, and roll--although by one calculation, it bounces 500,000 times before its "battery" runs down.

What propels this little circus act of nature is the larva of the jumping bean moth, which exploits the pod as a pupating chamber. The history of these host-specific moths took an unfortunate turn when someone saw entertainment value in their pupal habits and began exporting them wholesale to novelty vendors around the globe where, alas, there are no jumping bean shrubs. The moths are not extinct, but this year's bug-powered crop was disappointing, reports Oz Svehaug, a marketer in Chula Vista, California. "They're dwindling," he says ruefully. "They used to be all over the place. Now a lot of Mexicans don't even know what they are."

When a species declines, there's talk of a reduced gene pool and inbreeding. Why isn't this a problem in small, isolated ponds?

James F. Schatz, Bruceton, Tennessee

According to the mendelian scheme of life, the greater the genetic diversity in a population, the greater the probability that different combinations of genes will be passed on to descendants, enabling at least some individuals to withstand environmental changes. Whenever a population is drastically reduced, it is squeezed through a genetic "bottleneck" and loses precious hereditary wealth.

The consequences are not always bad. Take the cheetah, a species that harbors little genetic variation but is superbly adapted to its niche. In this best-case scenario, unhealthy genes are slowly weeded out over time. On the other hand, a shallower gene pool can lead to a precarious condition known as "inbreeding depression" and the inheritance of genetic defects by offspring. This is what's happening to the population of endangered Florida panthers, which is now down to fewer than 50 animals and is suffering from a drop in fertility and the spread of cardiac abnormalities.

Groups that thrive despite having little genetic variability often inhabit stable environments. But one hurricane or new disease could wipe them out. The bottom line is that genetic uniformity amounts to living on a knife's edge.

Closely related pupfish populations that have persisted in secluded desert pools for millennia are in this position. Still, an isolated pond that appears small to us may contain a genetically rich school that is well equipped to weather what storms may come, provided the pond remains. Which brings up a crucial point, notes Matt Powell, a University of Idaho research biologist studying the genetics of threatened and endangered fish. "What we should be rightly trying to conserve is not the species itself," he says. "It is the species within its environment."

Do fish drink water?

Elise Marie, Marysville, Washington

Jonathon Carlson

Regardless of the expression "drink like a fish," many members of the piscine population do not actively ingest water, despite being engulfed by the stuff. Without going into excruciating details involving chemistry and cell biology, suffice it to say that bony fish are osmoregulators. In plain English this means that they regulate, through physiology and behavior, the essential balance of water and elements in their bodies, which are affected by osmosis. Since freshwater species are saltier than their surroundings, water is constantly being sucked into their bodies through their gills and skin. They don't need to drink any more. Instead, they must rid themselves of the excess or they risk swelling up from fluid retention. Their kidneys are designed to excrete copious amounts of extremely diluted urine. Marine fish, which are less salty than the seawater they inhabit, face the opposite problem. The salt in the ocean is constantly drawing out their bodily fluids, so they have to drink lots of water to replenish their internal supply. Otherwise they become dehydrated. Special cells in their gills pump out excess salt, and they eliminate small amounts of salt-laden urine. Fish that spend part of their lives in freshwater and part in the sea, such as salmon, can go both ways, thereby raising the mundane process of osmoregulation to an art form.