Birds have a body plan that clearly distinguishes them from the rest of the vertebrates. However, when we take a closer look, we can infer birds’ locomotor styles based on their body shape, limbs, and foot type. For aquatic birds, diving is a specialized skill that can be performed to acquire food or to escape from predators. Diving can be facilitated by either using the wings, as in penguins and alcids; or the legs, as in the cormorants, shags, darters, loons, grebes, and certain ducks. We can infer these species are divers since they have a distinctively narrow and streamlined body shape, a narrow sternum and pelvis, and legs located very posteriorly¹. Their femora are short and craniocaudally curved, and their toes are connected by webbing².

But, despite our attempts to categorize and generalize the relationship between form and function of all living things, there are always exceptions. This is the case of Coots (Fulica), birds with a worldwide distribution (except Antarctica) within the Rallidae family and related to aquatic environments³. Coots appear to be generalists in terms of their body shape, akin to wading or swimming birds since they have strong legs and large, lobed feet (where digits are separated from each other, but each digit has leaf-like membranes along the edges)³. However, Coots are capable of foot-propelled diving, and some of them base their feeding strategy on this type of locomotion. In particular, the Red-gartered Coot Fulica armillata is a frequent and skilled diver⁴.

The morphology of specialized divers with typical body shape has been extensively studied and is well known^1,2,5,6. However, knowledge of the morphology of more generalist birds that are not specialized but capable of diving, such as Coots, is scarce. Does the skeletal morphology of Coots resemble that of their close non-diving relatives (other rallids), or is it more similar to other unrelated foot-propelled diving birds?

In this paper, we´ve addressed these research questions by describing and comparing the postcranial osteology of Fulica with other (non-diving) rallids, considering osteological characters typical of foot-propelled divers, as well as the proportions of the hind limb bones and the femoral splay angle. Our results showed that Coots are significantly disparate from other rallids.

Many osteological traits considered adaptations to diving are absent in Coots, especially concerning the femur and tarsometatarsus bones. However, other traits related to diving are noticeable in Coots when compared with other rallids, especially those in the tibiotarsus and the pelvis. This could indicate a stabilizing selection of characteristics associated with a generalist morphology in Coots and reinforces the importance of studying early adaptation or adaptation without specialization in relation to their relatives that have other lifestyles.

One of the major challenges in evolutionary biology and paleontology is to generalize evolutionary patterns and processes from observations of osteological traits. A step forward in that direction is to study adaptations in generalist taxa, thereby broadening the understanding of ecomorphologically significant features. Coots were a well-suited group for exploring the origin of diving adaptations. Now… which bird taxa will be next?

References

1. Dabelow, A. Die Schwimmanpassung der Vögel. Ein Beitrag zur biologischen Anatomie der Fortbewegung. Jb. Morph. Mikroskop. Anat. 54, 288–321 (1925).
2. Raikow, R. J. Evolution of diving adaptations in the stifftail ducks. Univ. Calif. Publ. Zool. 94, 1–51 (1970).
3. Winkler, D. W., Billerman, S. M. & Lovette, I. J. Rails, Gallinules, and Coots (Rallidae), version 1.0. In Birds of the World (eds. Billerman, S. M. et al.) (Cornell Lab of Ornithology, 2020). https:// doi. org/ 10. 2173/ bow. ralli d1. 01.
4. Taylor, B. Red-gartered coot (Fulica armillata), version 1.0. In Birds of the World (eds del Hoyo, J. et al.) (Cornell Lab of Ornithology, 2020). https:// doi. org/ 10. 2173/ bow. regco o1. 01.
5. Johnsgard, P. A. Diving Birds of North America (University of Nebraska, 1987).
6. Lapansky, A. B. Aquatic Locomotion in Birds—Biomechanics, Morphometrics, and Evolution (ScholarWorks at University of Montana, 2021).