All organisms experience mechanical forces which shape their body size and morphology. However, mechanical forces vary between species within a given lineage, between populations of a given species, between different sexes, and even within an individual organism over time. Here, I explore how variable mechanical forces influence bird morphology across these different scales. First, I explore how the presence or absence of hovering behavior alters bone morphology across a lineage of birds. Second, I look at how remaining sedentary or migrating influences morphology within a single species. Third, I study how egg-laying behavior alters female bone morphology over time. By studying how mechanical forces influence morphology we can gain an understanding of the mechanisms that control organism shape.
Chapter 1: Morphological Adaptations to Hovering in a Remarkable Radiation of Old World Nectar-Eating Birds: the Sunbirds (Nectariniidae)
Hovering is a unique form of locomotion that allows an animal to remain stationary in the air. While hummingbirds hover almost exclusively to obtain nectar from flowers, other nectar-eating birds vary in whether and how often they hover. This tendency may be constrained by morphology. Hummingbirds have several morphological adaptations to hovering, including long wings, short tarsi, and shortened proximal wing bones.
I hypothesized that the morphology of hovering birds converges on hummingbird morphology. Specifically, I predicted that hovering birds would be lower in mass and have longer wings, reduced tarsi, and longer tails. I also predicted that hovering birds would not vary in mass across elevations, but that higher elevation species would have relatively long wings. To test these predictions, I measured mass, elevation, wing length, tail length, and tarsus length in a group of birds that includes species that hover and species that do not hover: the sunbirds (Nectariniidae).
In contrast to my predictions, hovering sunbird species were heavier than those that did not hover. Female hovering sunbirds did have relatively long wings, but males did not. Hovering sunbirds did not have relatively short tarsi or long tails. However, male sunbirds in general did have relatively short tarsi and long tails. Hovering species did not vary in mass across elevation, Females but not males had longer wings with increasing elevation.
These results suggest that nectar-eating behavior, not hovering behavior, may select for hummingbird-like morphologies such as long wings and short tarsi. Additionally, hovering behavior seems to apply weaker selective forces on the morphology of most birds than it does in hummingbirds. A deeper understanding of the morphological requirements for hovering will aid in our understanding of the evolution of nectar-eating and its association with hovering behavior.
Chapter 2: Influence of Migratory Behavior on Bone Morphology in the Dark-Eyed Junco (Junco hyemalis)
Migratory behavior requires birds to expend increased energy as they spend a greater proportion of the day flying. To prepare, birds increase body mass by 20% or more, increase the masses of muscles associated with flight, and shrink organs that are not used during migration such as the stomach. This simultaneous increase in body mass, muscle mass, and the number of loads applied to the body each day has been associated with increased microcrack formation and risk of fatigue fracture in humans. Is migratory behavior in birds associated with any adaptations in bone structure?
To answer this question, I compared bone morphology of resident (J. h. carolinensis, J. h. pontilis) and migrant (J. h. hyemalis, J. h. montanus, J. h. aikeni) subspecies of the Dark-Eyed Junco (Junco hyemalis). Specifically, I looked at trabecular and cortical bone morphology in the humerus and femur using micro-computed tomography and linear mixed effects models.
I found that migratory birds had humeri that were thinner and wider, but these changes were not associated with a difference in geometric stiffness. In contrast, migratory femora were thinner, resulting in reduced geometric resistance to bending. Therefore, migrant femurs are less stiff under loading, but migrant and resident humeri have similar whole bone stiffness properties.
Taken together, these results suggest that residents and migrants have similar demands on the humerus, but that migrants have reduced demands in the femur. This may be due to resorption of muscle mass during migration, relatively increased evolutionary pressures to reduce body mass in migrants, or other differences in selection between residents and migrants. Further research should be performed to explore what mechanisms drive differences between resident and migrant birds.
Chapter 3: Microstructure and Mechanical Properties of Bird Bone During Egg-Laying
In the week prior to laying an egg, a female bird creates a unique calcified tissue inside her long bones: medullary bone. Medullary bone is primarily thought to function in calcium storage, as females draw heavily from it when producing an eggshell. However, it also increases overall bone mass and alters whole bone mechanical properties, and thus may influence avian energetics and behavior. What is the structural contribution of medullary bone to resisting forces during bending, and how might it influence behavior?
To answer this question, I gave male zebra finches (Taeniopygia guttata) an estrogen-eluting implant in order to generate a predictable model of medullary bone. Using micro-computed tomography scans of the humerus and femur, I created models with and without medullary bone, and used finite element analyses to apply bending forces (resulting in 1% axial displacement) and measure the load held in each bone.
I found that the addition of medullary bone resulted in a 36 – 41% increase in bone mass but an increase in whole bone stiffness of only 24 – 30%. It also had minimal influences on the load held in the cortex. I confirmed these results in similar models of female birds during egg-laying.
My results align with those of previous studies, which showed that medullary bone increases whole bone strength, but that increases are not concomitant with its increase in volume. Medullary bone therefore represents an ideal compromise between the need to store calcium for use during egg-laying while maintaining bone loading and bone mechanical integrity.
Conclusion
In summary, variations in mechanical forces influence morphology across varying scales. Specifically, the high forces experienced during hovering may select for longer wings, while the large energy expenditures during migration may select for reduced femur mass. In addition, these studies demonstrate that birds can be a useful system in which to understand how mechanical forces influence morphology. Future work should explore the nuances and potential mechanisms by which mechanical forces shape morphology.
