It's impossible to be really good at everything; specialization has its costs and they're often paid for in the currency of tradeoffs. This is well understood in the world of biomechanics where bones built to be tough cannot also be stiff, muscles good at contracting at high frequencies cannot generate high forces and fast animals can't often run long distances. Although such biomechanical tradeoffs necessarily lead to certain limitations on organismal performance they need not always be thought of as constraints. This point is beautifully illustrated by recent work on Darwin's finches by Anthony Herrel of Harvard University and his colleagues Jeff Podos, Bieke Vanhooydonck and Andrew Hendry. They show that tradeoffs between the force and speed of jaw movements in these birds might actually facilitate, not constrain, species diversification.
The beaks of Darwin's finches have received their fair share of attention and are known to be essential for at least two tasks with intimate ties to fitness: feeding and song production. However, big, strong beaks that can be used to generate large forces – good for feeding on hard seeds –are likely to be cumbersome and might not be nimble enough for seriously seductive singing. Herrel and his co-workers set out to quantify this potential tradeoff in nine species of finches and then address its possible evolutionary implications.
Working with ∼1000 banded birds of known size and beak morphology the research team encouraged every animal to bite a home-made force transducer multiple times, extracting the maximal bite force exerted by each bird. They also used high-speed video recordings of another 72 animals singing in the field to measure jaw movements during song production and relate them to beak size.
As predicted, large-beaked species move their jaws at slower speeds than species with small beaks: average maximal jaw closing velocities ranged between 0.11 m s–1 in the largest-beaked ground finch and∼0.29 m s–1 in the smallest-beaked warbler finch. In addition, birds with small, fast-moving beaks produced more complicated songs,but also significantly weaker bite forces, indicating a mechanical tradeoff in the jaws of these finches. At this point the authors aren't sure what accounts for this tradeoff but suggest that differences in the orientation of fibers relative to the axis of force production (i.e. pennation) in the jaw musculature likely plays a role. Highly pennate muscles are great at generating large forces but are limited in the distances (and hence velocities) they can contract over. Current studies quantifying jaw muscle architecture in these species are underway.
So, to summarize briefly, in Darwin's finches song quality hinges on beak velocity and gape, which depend on jaw muscles and beak size, which impact on bite force, which shapes diet. More simply, there appears to be a link between singing and eating; birds suited to cracking hard seeds will produce different types of song from less forceful feeders. Because mate selection is heavily influenced by singing, it is likely that specialization for distinct food types can severely limit the number of females attracted to a finch's song and lead to mating isolation, a major factor in speciation. The authors conclude that a biomechanical tradeoff between force and velocity may have played an important role in the diversification of Darwin's finches and could be a contributor to other songbird radiations as well.
