Desire under the figs: an Amazonian bird's unusual diet results in an even more unusual mating system

Natural History, May, 2002 by Peter T. Sherman

Thump! It sounded as though something the size of a baseball had hit the ground about thirty feet away from me. I scanned the layer of dead leaves covering the rainforest floor but saw nothing. The group of white-winged trumpeters that I was observing reacted immediately to the sound, running over to the point of impact while giving the loud, squawking alarm call these birds generally reserve for ground-dwelling predators.

Thump! Something else hit the ground close to where the first object had fallen. The seven members of the trumpeter group were working themselves into a frenzy, giving repeated alarms and fluffing out their feathers so that they would appear larger and more threatening than their normal chicken-sized selves. Unable to see what was going on, I was feeling frustrated and a little bit on edge. I knew from twelve months of following trumpeters through the rainforests of Manu National Park, in southeastern Peru, that their usual response to danger (which generally comes in the form of an ocelot or a jaguar) is to fly up into the safety of the trees. These birds were behaving very strangely.

Thump! This time my eyes went straight to the spot where something had just hit the ground. Nothing. I walked closer, and as I did so, the trumpeters faced off against me, fanning their wings over their backs, raising their necks with heads cocked for attack, and giving a hair-raising call that was a cross between a growl and a scream and that meant "stand back." It dawned on me that the trumpeters--normally indifferent to my presence--had been directing their alarm calls at me, and I finally saw why: sitting on the ground behind the protective guard of adults sat a stunned brown ball of well-camouflaged fluff. After about thirty seconds of blinking and periodic head-shaking, the trumpeter chick stood up and shakily stumbled off, adults packed closely around it. This chick and its two siblings had stepped boldly forth from the protective confines of a nesting cavity in a tree--where they had hatched the previous day--and plummeted forty feet to the ground, entering the outside world with a bang.

These free-falling chicks were the products of an extremely rare mating system known as cooperative polyandry, which involves several males (in the case of the trumpeters, usually three--one dominant individual and two unrelated subordinate males) that all mate with the group's single breeding female. The males then stick around to help out with incubation and, once the chicks leave the nest, to provide them with food for several months and protect them from predators such as boa constrictors and ocelots.

A typical trumpeter group also contains several other members: the group's sexually immature offspring, which help raise their new brothers and sisters, as well as an unrelated adult female, which doesn't. Monogamy is the rule for the majority of bird species (though members of a pair often "cheat" on each other), so what led to the evolution of cooperative polyandry in white-winged trumpeters? Looking for the answer to that question involved consideration of a number of factors in the lives of these birds, including the unusual ecological role they play in the forest, the need for young trumpeters to leave home when they reach adulthood, and the advantages of having comrades to protect the home turf from invaders.

If you spent all day beneath a fig or other fruiting tree in the Amazonian rainforest, craning your neck to look up into the canopy, you would likely see a round-the-clock procession of toucans, guans, capuchin monkeys, squirrel monkeys, and many other mammals and birds moving through the branches fifty to a hundred feet above the ground and feasting on soft, fleshy tropical fruit. Once they have eaten their fill in one tree, these animals move on to others, eventually either regurgitating the seeds or releasing them undigested in their feces (which then may serve as fertilizer for the germinating seeds).

The tree benefits only if its seeds are thus dispersed. But many fruits end up on the forest floor uneaten. Monkeys, the kings of sloppy eating, often grab up handfuls of fruit, sample a few, and throw the rest to the ground. What happens to the seeds of these fruits? Few species of birds and mammals include fallen fruit in their diet, and almost all that do, grind it up and digest the seeds right along with the pulp. Any uneaten fruits will have been bruised by the fall and quickly rendered inedible by bacteria and fungi. The few seeds that survive these animal and microbial assaults may germinate where they land but will be quite literally overshadowed by the parent tree, reducing the odds of their survival.

This is where the trumpeters come in. Alone among ground-dwelling creatures attracted to fruiting trees, trumpeters disperse--rather than consume and destroy--the seeds in the fruits they eat. As the fruit passes through their digestive tract, the muscular gizzard massages it until the pulp separates from the seeds, leaving them naked, intact, and--once they have been excreted--ready for germination.