Fur-Cleaning Beetle Jumps From One Exclusive Designation to Another
When mammal expert Robert Timm collected some unusual beetles buried deep in the neck fur of a Costa Rican cloud-forest mouse, he was very suspicious.
For more than a century, scientists had claimed that these wingless beetles, called amblyopinines, were the world’s only blood-sucking beetles--parasites feeding exclusively on the vital fluid of living animals. One naturalist had found mammal blood inside an amblyopinine. Others reported the rodents’ skin appeared irritated where the beetles lodged.
But Timm thought that some things about these suspected parasites were wrong.
‘Way Too Large’
“Probably what keyed us to the problem was that they were way too large,” said Timm, who is an assistant professor and curator of mammals at the Museum of Natural History at the University of Kansas in Lawrence.
The beetles were two-fifths of an inch long--huge for a parasite on the little rodents. If a human were infested with a parasite comparable in relative size, the pest would be almost a foot long, Timm said in a telephone interview.
But there were other disturbing things about this so-called parasite: “One of the most unusual things was that the host fully tolerated their presence. All animals--like our dogs and cats--scratch at fleas and ticks” and other parasites, Timm said.
But the Central American mice and rats infested with amblyopinines “would tolerate the beetles strolling right across their face and eyes,” Timm said. “They seemed to ignore it.”
Even heavy loads of up to 13 beetles did not seem to bother the rodents, which carried an average of four beetles each.
“The sight of 10 or more of these large beetles on the head of a small mouse is unnerving,” Timm wrote in a recent article in Natural History magazine.
During a stay in Costa Rica in 1983, Timm teamed with James Ashe, head of the insect division at the Field Museum of Natural History in Chicago, to take a close look at the two species of amblyopinines found in cloud forests, the damp, dwarf woods high in the mountains and perpetually shrouded in clouds.
The pairing of mammal and insect expertise proved essential to unraveling the mystery. Entomologists looking for amblyopinines before had considered them extremely rare, with most findings being made by mammalogists, who in turn knew too little about insects to understand the beetles’ peculiarities.
The pair set live traps overnight to collect the nocturnal cloud-forest deer mouse and Tome’s rice rat. When they picked up the traps at dawn, they found both types of rodent well-festooned with its own distinct species of beetle. Then they took the animals back to a lab to examine the mammals for damage from the beetles and to conduct behavioral observations of host and parasite, night and day.
This vigil proved to be the key. During the daytime, as the rodents slept, the researchers were amazed to see the beetles relax their tenacious grip on the rodents’ fur and come to life.
The rat’s beetle began roving around on its host, while the deer mouse’s beetle slipped off the animal and roamed around the mouse’s nest.
“The beetles left the mice and were active in the nest all day long. That’s their standard behavior pattern. They remain attached to the mouse all night and active in the nest all day,” Timm said.
Inspecting the rodents for damage, the pair found that “infested hosts were in excellent condition. Their fur was rich and luxuriant and their skin healthy.”
When they looked for puncture wounds from the supposed parasites, they found none and concluded that the injuries seen by earlier naturalists had been caused by the naturalists themselves when they tried to wrest the beetles from their hosts.
The pair found that the beetles bury themselves head down in the rodents’ fur, then use their vice-like mandibles “to grab the base of their hair very tightly. So they don’t puncture the skin at all.”
But when the researchers observed the beetles’ daytime activities, the biggest piece of the puzzle fell into place. “What is happening is, the beetles are not feeding on the mouse at all. They feed on fleas and lice in the mouse’s nest,” Timm said.
The blood the earlier researchers had found in a beetle had likely been the recent meal of a flea or tick the beetle had eaten.
The pair believe that the rat’s beetle spends the day gleaning and eating the real parasites from the mammal’s fur.
Although the beetles have lost their status as the world’s only blood-sucking beetles, they have gained an equally exclusive designation: the world’s only beneficial, fur-cleaning insects. They join species of cleaner shrimp and an African bird, the oxpecker, as Earth’s only animal nit-pickers. The shrimp clean parasites off reef fish, and the oxpeckers groom giraffes, elephants and other large game.
After the researchers realized that the beetles were helping, not harming the rodents, they began to see other curious aspects of the relationship as elegant adaptations of mutualistic species to one another’s behavior.
For example, the cloud-forest deer mouse maintains several nests. Exactly where it will end up after a night of activity is unpredictable, from the beetle’s point of view. To guarantee it will always be where the action is, the beetle must stick with its mouse all night.
Tome’s rice rat, on the other hand, does not maintain nests, so its beetle must stay on its host night and day.
Since reporting their findings in Natural History, the researchers have made another discovery, finding young amblyopinines for the first time. They found the larvae in the nest of a Chilean rodent known as the tropical tuco-tuco. The researchers are combing other rodent nests for larvae and wondering whether the beetle may time its reproduction to that of its provident rodent.
Examination of eight of the 38 other species of amblyopinine beetles has shown each lives on just one species of South American rodent. As the team explores the intricacies of these other relationships, Timm said, “Perhaps greater mysteries lie ahead.
“Preliminary findings suggest the biology of the other species will prove as interesting,” Timm said, declining to elaborate.
Timm said the beetles studied so far could not be raised for natural pest control on livestock and pets because the cool, moist conditions of the fragile Central American cloud forest could not be matched elsewhere.
Unfortunately, he said, “these cloud forests are rapidly disappearing,” with clearing of trees for grazing land.
“As soon as you cut down the forest for cattle, you’ve destroyed habitat for beetles,” he said, noting that the forest does not grow back easily or quickly.
“There could be some species of amblyopinines beetles that are already extinct that we never knew because this habitat is disappearing so quickly,” Timm said.