By Rex Graham
If a body of water splits a population of one species of mammal, bird or insect from another, one species can become two. The striking “island effect” of speciation helped Darwin devise his theory of evolutionary change.
A team of researchers from Arizona and Missouri discovered new evolutionary islands, of sorts. Actually, a bunch of islands.
The islands are not on a Google map. Each is the 5,000 or so feathers of a single Galápagos Hawk (Buteo galapagoensis). A bird louse (Degeeriella regalis) feeds on the keratin of the hawks’ feathers and dead skin. (The louse also infests Swainson’s Hawks.)
The louse doesn’t have wings of its own or means of transport to a new island of feathers.
All birds preen to delouse, thereby maintaining the insulating properties of feathers, and males have an added incentive – to enhance their sex appeal to females. However, no matter how much a Galápagos Hawk preens, plenty of lice are left to be passed from parent to chick.
Patricia Parker, a zoology professor and chair of the Department of Biology at the University of Missouri–St. Louis, and Noah Whiteman, assistant professor of biology at the University of Arizona, have been studying the lice and other parasites and pathogens of Galapagos birds for more than 15 years.
Parker and Whiteman wanted to know if each louse population genetically diverged on its own, or diverged in tandem with its hawk host. Jennifer Koop, a postdoctoral fellow in Whiteman’s lab, analyzed genetic data gathered by Parker and Whiteman from Galápagos Hawks and the D. regalis lice collected from them. Karen DeMatteo, adjunct research associate at the Saint Louis Zoo’s WildCare Institute, was part of the team that generated genetic fingerprints for individual bird and louse.
Passing down genes and lice
“To the lice, each bird is an island, and their populations are very different from bird to bird,” Whiteman said in a news release. “We found the lice are passed on from mother to babies during brooding, almost like genes.”
In a paper published August 6, 2014, in Biology Letters of the Royal Society, Koop, DeMatteo, Parker and Whiteman, reported that the lice living on any one bird and its offspring are more closely related than the lice living on a different bird. A population of hawks is an archipelago of islands to the lice, which have no way to island hop.
The birds of the Galapagos Islands are popular attraction for birding tours that cater to customers worldwide. Many of the birds there are not plentiful, but usually stable.
Galápagos Hawks, listed as a vulnerable species on the IUCN Red List, hunt large insects, centipedes, and small lizards, snakes and rodents. “They also eat birds, and pretty much anything they can get their talons on that isn’t a plant,” Parker said “That includes scavenging things like sea lion placentae and dead goats left by the goat eradication in the early-2000s.”
Fewer than 50 Galápagos Hawks live on the Galápagos island of Marchena about 1,000 km west of Ecuador. The hawks studied there have an unusual, polyandrous mating system composed of territorial family units of two to five males and one female. Territorial families claim the best habitats and defend them against non-territorial interlopers.
“The hawks are polyandrous on most, but not all islands,” Parker said. “The largest group sizes occur on Santiago, where each female resides in a stable group with as many as eight males. For reasons no one understands, hawks on Espanola are strictly monogamous. This correlates with the Espanola hawks also being by far the most genetically distinct and clearly isolated from the rest of the archipelago. It’s as if they took a different path.”
Parker and her co-investigators used genetic analysis to study polyandry in the hawks. For many bird species, the idea that females pair-bond with the same male for life has eroded as more genetic studies document multiple paternity of chicks in single nests.
Even Darwin noted exceptions to the simplistic notion of a group of males competing fiercely to win the sole right to mate with a coy female. One benefit of polyandry is increased heterozygosity, or gene diversity, in offspring. Greater disease resistance could be one of several benefits. Actually, male reproductive cooperation is one of the least studied areas of animal behavior. The Galápagos Hawk and Acorn Woodpeckers use cooperative polyandry in which males also cooperate in raising the young.
The Galápagos Hawk breeds on eight islands within the Galápagos National Park. Parker estimates a total population of about 500 birds. The researchers discovered that their parasites diversified genetically with their host families. The findings help explain the rapid rate of parasite evolution: about half of all known species are parasites.
1 louse bad, 2 worse
Actually, Parker and Whiteman discovered earlier that the Galápagos Hawk is infested with a second, more virulent and numerous louse (Colpocephalum turbinatum), which consumes blood and live tissue. They found that the blood-consuming louse, which is more easily spread by bird-to-bird contact, is more plentiful on Galápagos Hawks that haven’t established home territories, possibly because they spend more time looking for food and less time preening than hawks with territories. (The blood-consuming louse also infests 46 other species of raptors and domesticated pigeons.)
Virulent parasites and disease could, in theory, play a role in depleting the population of Galápagos Hawks. Of the 13 main islands of the Galápagos Islands, the hawk is no longer breeding on Daphne and four human-inhabited islands – Baltra, Floreana, San Cristóbal and Santa Cruz. Parker said the largest subpopulations are on the islands of Santiago and Isabela, with about 200 hawks on each. Marchena has about 50 hawks and the islands of Santa Fe and Pinzon have the smallest subpopulations of about 30 each.
Extinctions of birds endemic to islands accounts for about 90 percent of all bird extinctions. Because of the Galápagos Hawk’s small population and highly inbred genetics, Parker said the outlook for the species could hinge on its vulnerability to disturbance, disease or parasitism.