Mutually Inclusive

We’re at the Point Reyes Dance Palace at the end of another Christmas bird count, the 32nd for this territory. Rich Stallcup has read off the litany of species that are usually seen, and now the individual parties are reporting the rarities. One party leader has also observed some unusual behavior: a western scrub-jay “hopping around on the back of a tule elk.”
David DeSante, who runs the Institute for Bird Populations in Point Reyes Station, California, offers an explanation. A few years back, he says, the Point Reyes Bird Observatory (PRBO) documented scrub-jays picking ticks from the hides of mule deer. The deer would come to the jays’ cleaning station to be de-ticked and seemed to really get into the process. Now it looks like the birds have discovered the reintroduced elk as a source of tasty ectoparasites.
I was intrigued enough by that story to track down DeSante’s article, which appeared in The Condor in 1985. DeSante and co-author Frank Isenhart described a series of interactions at PRBO’s Palomarin Field Station, in tick-ridden coastal scrub. The jays would land on the head, neck or back of a waiting deer and work their way all over its body, bucks’ antlers included. The deer stood stock-still during these ministrations, sometimes extending their ears so the jays could get at a hard-to-reach area. DeSante and Isenhart also noted that the deer would wait in line for the jays’ services. The behavior occurred only from late winter through spring, when ticks are most abundant.
If you’ve seen enough nature documentaries, this kind of thing will seem familiar enough. Two African species of oxpeckers, relatives of the starlings and mynahs, specialize in removing ticks from the bodies of antelope and other large ungulates. One of the celebrated Darwin’s finches of the Galapagos Islands does the same for marine iguanas.
Fish have evolved similar behavior. Small coral reef fish called cleaner wrasses maintain cleaning stations which larger fish like groupers and moray eels visit to have their external parasites removed. In Kenya’s Mzima Springs, a whole community of fish functions as hippopotamus cleaners. They’ve divided up the mobile territory: one species scrapes the hippo’s hide, another works on the soles of its feet, still others cover the bristles of its tail.
Skin-cleaning and tick-picking is just one example of what biologists call mutualism — a relationship between two species from which both benefit. And there’s a lot of it out there, running along a continuum from the intimate symbiotic partnership between alga and fungus that makes a lichen, to the loose association of songbirds in a multispecies feeding flock. Mutualist relationships cross the boundaries of phylum and kingdom, linking plants with fungi that nourish their roots and animals that pollinate their flowers and disperse their seeds. They’ve been a crucial evolutionary force: Distinguished biologist Lynn Margulis sees the very origin of complex life as the result of symbiosis between single-celled organisms (endosymbiosis).
The prevalence of mutualism seems at odds with the notion, going back to Darwin’s Origin of Species, of nature as an arena of fierce competition. But it’s a matter of perspective. To Darwin’s contemporary Pyotr Kropotkin, the Russian prince turned anarchist, cooperation was more central. His classic Mutual Aid catalogues many examples of cooperative behavior. Such behaviors within a species — the wolfpack or the termite colony — now have a convincing genetic explanation: an organism gains “inclusive fitness” by aiding the survival of close relatives whose genes it shares. When it comes to relationships between species, though, there has to be a direct quid pro quo, a tangible benefit for each partner.
The scrub-jay story reminded me of another mutualist relationship much closer to home than the coral reef or the African savanna. The small butterflies called blues, including local species like the endangered mission blue, have evolved a symbiotic association with ants, technically known as trophobiosis (“nourishing life”). Bert Holldobler and Edward O. Wilson give a fascinating account of this in their Journey to the Ants. The ants tend the blues in their caterpillar stage, sometimes lugging them around from food plant to food plant. When they’re not feeding, mission blue larvae rest in ant-excavated holes beneath their host lupines. Ants also offer protection against predators and parasitic wasps. Blue larvae experimentally deprived of their bodyguards had a much lower survival rate.
The payoff for the ants is a concentrated food source. On the seventh abdominal segment of the caterpillar is an organ called the Newcomer’s gland, which secretes honeydew, which ants find tasty. I haven’t seen a chemical analysis of the fluid produced by North American blues, but honeydew from a European blue species contains fructose, sucrose, glucose, protein and an amino acid, while an Australian blue yields 14 amino acids and a blend of sugars. A German entomologist calculated that a typical population of blue larvae could meet the entire nutritional needs of an ant colony. Other glands produce a pheromone that stimulates the ants to touch the caterpillar.
A good deal all around, but there’s a problem. Native ants in California (and elsewhere — notably in South Africa) are being displaced by the invasive Argentine ant, the kind you probably have in your kitchen. Argentines and native blues have no common evolutionary history. Although the invaders have been observed with the larvae of the rare Palos Verdes blue in Southern California, we don’t know if they can be as effective protectors as indigenous ants.
Inevitably, some species push the envelope of mutualism a bit. The larvae of a few Old World blues con the ants into taking them into their nests, where they feed on the ant larvae. And third parties sometimes exploit the relationship. The sabertooth blenny, a mimic of the cleaner wrasse, hangs around coral-reef cleaning stations and darts in to take a bite out of the unsuspecting cleanee.
But the interactions of the jays and the deer, the mission blues and the ants, are more typical. Looking at the natural world as a web of symbiotic and mutualist exchanges is a useful corrective to the “nature red in tooth and claw” business. There are a whole lot of organisms out there getting by with a little help from their friends.

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