Timing of Mineral Sequestration in Leg Bones of White-tailed Ptarmigan

Abstract

Birds are unique among vertebrates in that they protect their eggs with rigid, calcium-rich shells. Thus, for a short period of time during the annual reproductive cycle, birds experience extraordinarily high demands for calcium. Two strategies appear to exist for meeting those temporally high demands. Some birds apparently seek out calcium-rich foods immediately prior to and during egg laying whereas others may store calcium in their skeletons over a much longer period of time, mobilizing those reserves only when they are needed for production of eggshells. In this study, we used dual energy, X-ray absorptiometry and inductively coupled plasma mass spectroscopy to monitor annual shifts in bone mineral content in the legs of White-tailed Ptarmigans (Lagopus leucurus). The study organisms were known to live on calcium-poor soils. Despite an apparent shortage of calcium in their diets, the test subjects stored substantial amounts of calcium in their leg bones in months prior to reproduction. Those stores were subsequently depleted during the egg-laying period. We suggest ability to store calcium in the skeleton may afford this species more flexibility in selecting suitable breeding habitats than would be possible otherwise.

Mineral Retention, Medullary Bone Formation, and Reproduction in the White-Tailed Ptarmigan (Lagopus leucurus): A Critique of Larison et al. (2001)

“…Erroneous conclusions of this study resulted from imprecise "prelaying" classifi cation arising from the aforementioned methodological failings. I question whether Larison et al (2001) report long-term storage of calcium in this species; instead, I suggest that their fi ndings were consistent with the formation (and subsequent destruction) of medullary bone, a specialized type of bone that forms in the medullary cavities of long bones (e.g. femur, tibiotarsus).…”

“…Recently Larison et al (2001) described a study of White-tailed Ptarmigans (Lagopus leucurus) breeding in Colorado where dietary calcium availability was low. (Note that although the authors use "mineral" in the title of their paper, calcium appears to be the focus of their study and the main thrust of their discussions.)…”

“…During egg-laying, birds increase consumption of calcareous grit to obtain suffi cient calcium for eggshell formation (Sadler 1961). Although Larison et al (2001) provided no details of calcium availability within their study area for comparison with other studies of birds breeding in calcium-poor areas (e.g. Graveland 1995), their choice of study species and study area were appropriate to test predictions of longterm calcium storage for birds that "live where calcium is hard to fi nd…."…”

“…Of course, for females collected during egg-laying and postlaying periods, the authors could have quantifi ed the reproductive investment of birds from the number of eggs already laid or from the size of completed clutches. However, subsequently plotting bone mineral density against Julian date (see fi g. 1 in Larison et al 2001) is not as informative as plotting a temporal axis showing days since clutch completion or since laying onset for each collected bird. Placing the majority of collected birds in a time frame relative to egg-laying would have been possible with examination of reproductive tissues (see above).…”

“…The authors discussed bone mineral densities of females "before, during, and after egg-lay-ing…." However, from consulting fi gure 1 in Larison et al (2001), only one female was collected during egg-laying. To show that mineral sequestration in long bones of females met the "extraordinarily high demands for calcium" during reproduction, the authors needed to demonstrate that such accumulation of calcium occurred in birds that were laying at the time of collection.…”