Limb posture dominates femoral deformation
The femora of both the two birds showed dorso-ventral bending under radial loading and medio-lateral bending under axial loading; the bending orientation of the femora explicitly corresponded with the loading conditions. During terrestrial locomotion, the angle at which the femur bears external force is related to the posture of the hind limb. A crouched posture of the hind limb tends to cause radial loading on the femur, while an erect posture tends to cause axial loading on the femur (Biewener 1989, 2005). Our results showed that femoral stress, strain, and SED due to radial loading was significantly larger than that of axial loading in all of the individuals, suggesting that the femur bears a high risk failure under radial loading. Furthermore, in the femora of both pigeons and Cabot’s Tragopans, the compressive strain was larger than tensile strain under axial loading, whereas the two strain types were similar to each other under radial loading. As the acting orientation of axial loading parallels to the longitudinal axis of femora, the main proportion of the total load due to axial loading is born in compression rather than tension. Therefore, an erect posture of limbs will lead to a net decrease in the tensile deformation of femora, which makes good use of the outstanding compressive resistance of bones, and therefore achieves a high locomotor safety factor of femora (Currey 2002). Corresponding kinematic studies found that the hip joint movement of birds increases significantly with increasing speed during terrestrial locomotion (Cracraft 1971; Gatesy 1999; Abourachid et al. 2011; Stoessel and Fischer 2012). It makes the femora of birds more erect at the mid-stance of the stride. Because the mid-stance is generally the moment when the net ground reaction force reaches its maximum, the erect posture of the femur will enhance its resistance to loads during strenuous locomotion. Additionally, Biewener (2005) argued that the crouched posture will cause a reduction in the muscle’s moment arm relative to the moment arm of the ground’s reaction to the force of the limbs. This decreases the effective mechanical advantage of limbs. During locomotion, larger muscle force is needed in limbs with low effective mechanical advantage than that in limbs with high effective mechanical advantage.
Moreover, although the crouched posture of limbs leads to a low safety factor in bones, it enhances the maneuverability and accelerative capability of animals (Biewener 1989; Alexander 2003). Several studies suggested that terrestrial vertebrates with large body weight prefer an erect limb posture, while small animals prefer a crouched posture (Alexander et al. 1979a; Biewener 1982, 1989, 2005). The elastic modulus of long bones of vertebrates falls within a relatively narrow range (Erickson et al. 2002), and the breaking strengths of long bones are quite similar among mammals and birds across a huge range of body sizes (Biewener 1982). By adopting more erect postures, large animals decrease bone deformation and enhance the effective mechanical advantage of their limbs to overcome the locomotor limit of their large body weight. In contrast, small animals have a much smaller body mass relative to their bone strength than large animals (Biewener 1982). As such, the safety factor of bones may not be the primary consideration for them. However, small size also presents disadvantages for small animals. For example, small animals are more vulnerable to predators and experience a more diverse terrain (Hedenström and Rosén 2001; Herrel and Gibb 2006). Nevertheless, small animals have a high power-to-mass ratio, which makes them more maneuverable than large animals (Dial et al. 2008). Meanwhile, the crouched limb posture not only enhances their maneuverability but also their accelerative capability. These enhancements allow them to contend with large animals and escape from predators efficiently (Alexander 2003; Dial et al. 2008). This is probably the predominant reason that small animals prefer crouched postures.
Species-specific mechanical performance
The femoral strain and SED of young Cabot’s Tragopans are lower than those of young pigeons, which suggest a higher femoral mechanical performance of young Cabot’s Tragopans. Furthermore, the strain and SED of young and adult Cabot’s Tragopans are similar, while these parameters sharply decline during growth in pigeons. This suggests that the femoral precocity of young Cabot’s Tragopans is higher than that of young pigeons. The different femoral mechanical performance between the two birds corresponds well to the fact that precocial birds have higher locomotor demands than altricial birds. Furthermore, the femoral midshafts of 4- and 7-day-old pigeons have a relatively higher strain than that of older pigeons. The narrow midshaft in 4- and 7-day-old individuals may be responsible for their high femoral stress and strain. The narrow midshaft is very deformable and is, thus, not suitable for weight-bearing. However, pigeons cannot stand until they are 7 days of age, and, thus, the easily deformable shape is prevented from damaging the locomotion of young pigeons. Of note, the weak locomotor ability of young pigeons is also reflected in the structural characteristics of their femora.
This study has revealed that femoral stress increases with growth in both pigeons and Cabot’s Tragopans. Stress is often adopted by studies to evaluate bone strength. As bones will fail at a certain stress level, the ratio of failure stress to load-induced stress is used as the safety factor of bones (Biewener 1982). However, the skeletal material property develops with animal growth and thereby failure stress increases as well (Currey 2002). We cannot clarify the ontogenetic change of the femoral safety factor based on the stress value alone. For the same reason, the finding that the femoral stress of Cabot’s Tragopans is higher than that of pigeons does not imply that the femoral safety factor of pigeons is higher than that of Cabot’s Tragopans. Stress is equal to the product of strain and elastic modulus. During the growth of the two birds, the femoral strain decreased while the stress increased. The femoral strain of 4-day-old pigeons under axial loading was ~ tenfold that of adults, but the stress increased by ~ 350% during growth. Therefore, the increase in stress with growth may be caused by the enhancement of the elastic modulus.
The femora of precocial hatchlings are built to ensure locomotion ability
Wei and Zhang (2019) observed the negative allometry of the breaking load of quail femora; the loading resistance of young quails was higher than that of adults. Similarly, the results of the present study showed that the femoral SED of young Cabot’s Tragopans was lower than that of adults. The robust femora of young individuals are thought to be an adaptation to the intense locomotion required during their early growth stages (Wei and Zhang 2019). Due to weak parental care, precocial birds must forage and avoid predators independently or partially independently. Because of their small body size and immature sensory-motor system, young animals suffer the highest mortality rate and therefore the strongest selection pressure (Carrier 1996; Herrel and Gibb 2006). The locomotor ability of young animals is enhanced to cope with this strong selection pressure, which increases their survival rate. Therefore, the high mechanical performance of the femora of quails and Cabot’s Tragopans is an adaptation to the high functional requirements of their bones that are needed to enhance the locomotion of young animals.
The femoral strain of young Cabot’s Tragopans was lower than that of pigeons, which suggests that precocial birds have a high resistance to skeletal deformation. However, the strain of young Cabot’s Tragopans was not lower than that of adults, unlike SED. This implies that young Cabot’s Tragopans may have different preferences between deformation resistance and energy efficiency. The development of an organism is an energy-consuming event, and the available energy for young animals is limited (Ricklefs 1979a). Compared to altricial species, precocial species not only obtain little energy from parental birds but also consume more energy with their activity (Ricklefs 1979a). The rational distribution of energy is therefore critical for precocial animals. For this reason, the skeletal mechanical performance should not be enhanced without limit, as with this, energy may be wasted on unnecessary functions. Bones modulate their modeling and remodeling processes based on specific mechanical stimulus to optimize their mechanical performance (Carter 1984). Carter et al. (1987) argued that the apparent bone density is positively correlated to the apparent SED. Combining this conclusion with our results, we hypothesize that SED is the bone modeling/remodeling stimulus. SED is affected by both stress and strain. A high SED value implies high stress and/or strain, which harms the performance of bones. Nevertheless, an extremely low SED value may reflect excessive bone volume, and therefore more weight and consequentially higher energy consumption. Skeletal SED may be maintained within a reasonable range to meet the mechanical performance requirements of strength, rigidity, and lightweight together with limited energy sources.
The femora of Cabot’s Tragopans showed decreased strain with growth. In contrast, two studies employing in vivo strain tests have observed another ontogenetic trajectories in the bone strain of birds. The strain of the tibiotarsus remained constant during chick growth (Biewener et al. 1986), while the strain of the femur and tibiotarsus increased with emu growth (Main and Biewener 2007). Although the ground-dwelling birds were used as the research objects in these two studies as well, they adopted the in vivo strain test which is different from our study. The results of in vivo strain tests reflect the effects of locomotion on bones, while the results of the present study reflect the mechanical performance of bones. Therefore, the different strain results between the present study and the two abovementioned in vivo studies are not contradictory. Furthermore, if the in vivo femoral strain of Cabot’s Tragopans maintains or decreases with growth, which is just like that of the chick and emu assessed in Biewener et al. (1986) and Main and Biewener (2007), respectively, young Cabot’s Tragopans must have some mechanisms that decrease bone strain, because our results showed that the femoral strain decreased with growth under body weight loading alone. Biewener and Taylor (1986) suggested that an animal’s gait and speed during locomotion is limited by the magnitude of bone strain. Therefore, if young Cabot’s Tragopans indeed have some strain-reducing mechanisms, they can achieve a relatively higher speed than adults under the same conditions of bone mechanical performance. The strain-reducing mechanisms decrease the functional demands on the mechanical performance of bones, which improves the locomotor ability of young Cabot’s Tragopans.
The robust femoral structure of altricial hatchlings adapts to rapid growth
Pigeons hatch with weak femora, which corresponds to their low demands for locomotion. However, the femoral structure of hatchling pigeons is relatively more robust than that of hatchling Cabot’s Tragopans. This implies that the weak mechanical performance of femora is not simply the result of a correlation between low performance and low functional demands. Carrier and Leon (1990) found that in California Gulls, the growth of wing bone length begins immediately after hatching, whereas other wing tissues do not develop until shortly before fledging. Carrier and Leon (1990) suggested that the linear growth of bones may require more time than other tissues, and that therefore the linear growth of long bones could be the rate-limiting factor in limb development. After that, Carrier and Auriemma (1992) found that the fledging period of birds has a strong positive correlation with relative bone length. This result supports the hypothesis of Carrier and Leon (1990). Cosman et al. (2019) then revealed the reason for this phenomenon, that is, rapid linear growth of bone leads to changes in the organic bony matrix composition of the bone, which makes the bone brittle and susceptive to fracture by even the smallest deformation. Thus, bone growth must be initiated earlier so as to prevent longer maturation durations. Therefore, the high structural precocity of pigeon femora may be an adaptive trait to rapid growth.
As altricial birds, pigeons exhibited rapid growth rates regarding femoral mechanical performance. Their femora were extremely deformable at hatching; however, the bone strain reached adult levels at 28 days after hatching, which exactly coincides with the fledging time. In contrast, the femoral strain of Cabot’s Tragopans reached adult levels at the sub-adult stage (individual SA1). According to the growth curve of Cabot’s Tragopans in Wen and Zheng (1998), it can be estimated that the age of the Cabot’s Tragopan SA1 was ~ 70–80 days. The long developmental period of Cabot’s Tragopans provides sufficient time for bones to grow slowly, while pigeons need robust bone structures at hatching that enable maturation in just 4 weeks.
California Gulls show constant bone growth already from hatching onwards (Carrier and Leon 1990), while pigeons not only exhibit constant bone growth but also a robust bone structure at hatching. Additionally, the development of the cross-sectional bone structure was delayed in California Gulls but not in pigeons. This difference may be caused by the different demands on the rapid growth between the two birds. Altricial pigeons have more rapid growth than semi-precocial California Gulls; pigeons may, thus, have better developed bone structures at the time of hatching to support their rapid growth in comparison to gulls. This may imply that more altricial birds exhibit more robust bone structures. If a negative relationship between the precocity of hatchling birds and the precocity of long bone structures is proven, the type of hatchling birds can be inferred based on the bone structure. These results have potential applications for studies regarding developmental patterns of fossil avian species.
The methods used in the present study have some limitations as in other previous similar works. First, the elastic modulus is calculated from the three-point bending test using equations from beam theory. Because the shape and geometry of the femur are complex and irregular, beam theory only provides rough estimation of material properties (Jepsen et al. 2015). Second, the mechanical test requires the sample to be machined to standard size and tested in a particular loading mode, but this standard protocol cannot be applied to birds’ femur because of the small size. Fortunately, it has been proved that the beam theory combined with the mechanical test has high precision in estimating elastic modulus of bones (Arias-Moreno et al. 2020), which enhances the credibility of the present research. In particular, these methods are more suitable for comparative and ontogenetic analysis on the biomechanical performance of animals’ limb bone.