Parasites can also use hormones to signal them when it’s time to leave. The fleas on a female rabbit’s skin can detect hormones in the blood they drink from her. They can tell when she’s about to give birth, and they respond by rushing to the front of her face. Once she has delivered her babies and is nuzzling and licking them, the fleas leap onto the newborns. Baby rabbits can’t groom themselves yet, and their mothers clean them only when they visit their nest once a day to nurse. That makes the baby rabbits wonderfully tranquil homes for fleas. The fleas immediately start feeding on the babies, mating, and laying eggs. The new generation of fleas grows up on the babies, but when they sense that the mother is pregnant again, they hop back on her. There they wait to infect her next litter.
Getting to a new host can become a huge challenge when a parasite’s species of choice is a solitary creature. Dig a few feet down into the hard summer dirt of an Arizona desert, for example, and you may a find a toad. It is the spadefoot toad Scaphiopus couchi, and it is sleeping away the eleven-month drought that dominates every year. It sits underground, not eating, not drinking. Its heart barely beats, but its cells still have to purr metabolically along, and it stores its wastes in its liver and bladder. In July or August the first rains come, monsoons that roar down and break up the soil. On the first wet night the toads come alive and crawl out.
The toads gather in ponds, where the males outnumber the females ten to one. They attract the females by singing in floating choruses, croaking so passionately that their throats bleed. A female drifts among the males until she finds the voice she likes and nudges the male. He climbs on her and they lock together, the female letting slide a raft of eggs that the male fertilizes with his sperm. By four in the morning the courtship is over. Before the hot sun rises, the toads have crawled back down a few inches into the ground. Only when the sun sets again (and only if there’s enough water) will the toads return to the surface. When they aren’t mating, the toads are eating enough food to tide them over for the rest of the year. A toad can eat half its weight in termites in one night. Meanwhile, their offspring grow frantically from egg to toadlet in only ten days, since the rainy season is only a few weeks long. As the rains taper off the toads all disappear underground, having spent a few days out of the earth, and return to their life of sleep.
With so little opportunity to go from host to host, a spadefoot toad might seem a bad choice for a parasite. There are, in fact, hardly any parasites that have gotten a foothold inside the spadefoot, and most of them can only mount feeble infections. But one parasite positively revels in the spadefoot life, a worm named Pseudodiplorchis americanus. Pseudodiplorchis belongs to a group of parasites called monogeneans, delicate blobby worms that almost always live on the skin of fish and travel from host to host in the comfort of ever-present water. Yet, half of spadefoot toads carry the monogenean Pseudodiplorchis, and each toad carries an average of five.
Of all places, Pseudodiplorchis chooses the toad’s bladder to live during the long sleep. As the toad pumps more salts and other wastes into the bladder the parasite goes on with its life, sucking blood and mating. Within each female Pseudodiplorchis, hundreds of eggs mature into larvae. They sit inside her for months, waiting for the toad to rouse. The parasites will wait as long as the toad waits, even if the rains don’t come until the next year. When the rains do fall, the parasite is caught in a deluge of its own. After the toad has clawed its way to the ground, its skin soaks up water, which floods through its bloodstream, scouring out all the poisonous waste that has built up in its body over the year, through its kidneys and into its bladder. This torrent of urine suddenly turns the parasite’s habitat from a salty ocean to a freshwater pool. Pseudodiplorchis holds tight during the torrent and goes on waiting. It waits out the male choruses and the female inspections. Only when their toad host is sexually aroused as it tries to mate with another toad does a mother Pseudodiplorchis send her hundreds of young rushing out of the bladder and into the pond. When they reach the water, they rip out of their egg sacs and swim free.
Now, after their eleven-month wait, the parasites have to race. They have only a few hours to find another host in the mating pool before the toads crawl back underground and the sun rises and any stranded parasites fry. As they swim through the pond they have to be sure that they don’t crawl onto one of the other species of desert toads that crowd the water as well. Some kind of unique skin secretion from the spadefoot probably guides them to their host. Pseudodiplorchis has an awesome homing ability in its ponds. For many parasites, it’s not unusual for only a few out of thousands of larvae to find a host in which they can mature. Pseudodiplorchis has a success rate of 30 percent. As soon as it hits its host, a Pseudodiplorchis larva starts crawling up the toad’s side. It comes out of the water altogether, climbing as high as it can go. It ends up on the toad’s head, and once there, it can find the nostrils and slip inside.
The race goes on further: Pseudodiplorchis still has to get into the toad’s bladder before the rainy season ends. And within the toad, Pseudodiplorchis faces conditions just as murderous as the desert sun. It travels down the toad’s windpipe, drinking blood as it goes, until it gets to the lungs. There it lives for two weeks, fighting off the toad’s efforts to cough it up, maturing into a young adult about a tenth of an inch long. It leaves the lungs and crawls into the toad’s mouth, only to turn around and dive down its esophagus and into its gut.
The acids and enzymes the toad uses to digest its food should dissolve such a delicate parasite. If you pull a newly arrived Pseudodiplorchis out of a toad’s lung and stick it directly into its intestines, the parasite will die in minutes. But in its two weeks in the lungs, it can prepare itself for the trip by storing up a collection of liquid-filled bubbles in its skin. When it dives into the toad’s digestive tract, it lets the bubbles burst, spilling out chemicals that neutralize the compounds trying to digest it. Yet, even with this protection, Pseudodiplorchis doesn’t dawdle: it charges through the entire digestive tract of the toad in half an hour and makes its way into the bladder. The entire trip, from nose to lung to mouth to bladder, takes no more than three weeks, and by then the host toad has finished its annual mating and feasting and is back underground.
The spadefoot toad is one of the few hosts that leads a life as isolated as its parasites; together they spend a year in the ground waiting for the chance to see their kind again.
Parasites have colonized the most hostile habitats nature has to offer, evolving beautifully intricate adaptations in the process. In this respect, they’re no different from their free-living counterparts, much as that might horrify Lankester. And I haven’t even had room in this chapter to talk about the most remarkable adaptation that parasites have made: fighting off the attack of the immune system. That fight demands a chapter of its own.
3
The Thirty Years’ War