Wild Things
There are many cases of mutualism in nature. Surprisingly, as with humans, connections are driven as much by personality as biology
There are many cases of mutualism in nature. Surprisingly, as with humans, connections are driven as much by personality as biology
MUTUALISM IS A WONDERFUL THING: two species engaging in specific behaviours that create benefit for each. Fish like the bluestreak cleaner wrasse are a perfect example. Small groups of them wait at one of their so-called cleaning stations on the reef for a client fish to arrive. When the client is in view, the wrasses greet it with a signature move of their rear ends, and the client fish responds by positioning itself so that the cleaners can pick off parasites clinging to the client fish’s skin. The cleaners gain some nourishment, and the client swims away with a much-reduced parasite load.
Cows and their gut bacteria and ants protecting aphids in return for the aphid secretion called honeydew are other welldocumented examples. Now biologist Rob Found of the University of Alberta has uncovered an example where the personalities of the two species govern the mutual relationship.
The species are magpies and elk; their mutualism is based on the birds landing on the elks’ backs and picking off parasites, playing the role of cleaner fish of the air. Elk can be plagued by winter ticks, and the magpies will eat them, but Found discovered it’s not every magpie nor every elk that participates. He was able to show that in every flock of magpies or herd of elk, there’s a range from shy to bold. Bolder animals (or birds) would allow Found to approach closer and/or would flee less far. Bolder elk were socially dominant and less fearful of novel situations. Found evaluated magpies’ comfort with novelty by evaluating behaviours like the time it took them to perch on a bicycle decorated with tape and ornaments.
These “personality” factors determine which bird lands on which elk. Bold magpies are much more likely to land on an elk; shy elk are much more likely to accept them.
But this seems counterintuitive: if picking off ticks is beneficial to the elk, why wouldn’t all of them, even the bold ones, accept magpie visits? Why wouldn’t shy magpies be able to access all the animals if the benefits flow in both directions?
This disparity in behaviour between the two species raises a variety of issues: perhaps winter ticks are not the life-threatening problem for elk that they are for moose, and so an irritated elk that drives away a magpie might not be losing very much. Maybe shy elk are accepting because by doing so they can marginally increase their health by having their ticks removed.
Or perhaps this charming little scene of two species helping each other isn’t quite what it appears. The case of the red-billed oxpecker in Africa suggests there might be more to this than meets the eye, or especially the casual observer’s assumptions. The oxpecker, as the name suggests, feeds heavily on African cattle, consuming ticks, mucus, dead skin and, according to one scientific paper, “blood, sweat and tears.”
Nearly 20 years ago, Paul Weeks of Cambridge University conducted an experiment in Zimbabwe with oxpeckers and their targets, in this case a herd of 22 oxen that were hosts to five different kinds of ticks.
The results were startling: Weeks found that whether the birds were allowed access to the oxen or not made no difference to the animals’ tick load, and he estimated the oxpeckers spent only 5 per cent of their time removing ticks. However, they did spend quite a lot of time taking blood meals at skin wounds. They busied themselves enlarging wounds caused by ticks or scratches by wire fences — day after day. Weeks suggested that oxpeckers might more accurately be called parasites.
I reference oxpeckers (as did Rob Found) because the usual assumption about their relationship to the oxen wasn’t as straightforward as it seemed, in that they gained much more than they provided. There’s no doubt trickery exists: on the reef, some cleaner fish clients offer themselves to phony cleaners, who rip little pieces of flesh from them. Could something similar be true of magpies and elk?
Magpies can be villainous. Found has encountered elk calves, still alive, with their eyes pecked out by them. But he suspects that parasitism is just one part of this complex relationship. Shy elk don’t risk too much by letting a magpie land on their back but do benefit from the removal of ticks; bold elk really just don’t like anyone in their space, human or bird, and don’t lose much by excluding magpies because their tick load doesn’t really compromise their health. Shy magpies might lose out on a potential food source, but if the elk aren’t carrying that many ticks anyway, it might not matter. We’re left with one certainty: there are subtleties that have yet to be teased out.a
“PERSONALITY” FACTORS DETERMINE WHICH BIRD LANDS ON WHICH ELK. BOLD MAGPIES ARE MUCH MORE LIKELY TO LAND ON AN ELK; SHY ELK ARE MUCH MORE LIKELY TO ACCEPT THEM