Growth Patterns and Developmental Strategy in the Black-Bellied SandgrousePterocles orientalis

Abstract

As a result of convergent evolution, sandgrouse share similarities with some other precocial landbirds, namely Galliformes and precocial waders (Charadriiformes). They are birds of arid lands and hence their chicks face problems of predation risk and shortage of food and water, exacerbated by the patchy and unpredictable distribution of these resources. We therefore hypothesised that sandgrouse would be more precocious at hatching than precocial galliformes and charadriiformes living in more favourable habitats. Patterns of growth for body mass, wing, leg and sternum were described for the black-bellied sandgrouse Pterocles orientalis by a logistic model. Post-hatching development was characterised by the ratio of exponential growth rate (EGR) to relative size (Ln(W/A)) (W: Weight and A: Asymptotic size) at a given age. Developmental trajectories, indicating the degree of the development of precocity, were derived from these relationships as the slopes of least square regression of EGR on relative size. We also characterised the post-hatching development of body size in terms of maturation time. Growth parameters were compared to those of the Galliformes and precocial Charadriiformes using interspecific allometric relationships. Black-bellied sandgrouse hatched with relatively well developed body mass and legs, which accords with their faster post-hatching maturation. The development of body mass and wing reached their asymptotic sizes on steeper developmental trajectories than did the leg. The mean growth rate constant for the wing was the highest compared to those of the sternum and leg which were similar. In contrast to the leg, wing and sternum showed biphasic growth patterns. The black-bellied sandgrouse shows growth rates and a maturation degree at hatching that are intermediate between their convergent Galliformes and precocial Charadriiformes. It exhibits a developmental strategy in which both precocial legs and wings allow early development of either walking or flying. This constitutes an adaptive strategy to arid environments with low and unpredictable resource availability and high predation risk.