Mineral retention, medullary bone formation, and reproduction in the white-tailed ptarmigan (Lagopus Leucurus): A critique of Larison et al. (2001)

The Auk 120(1):224-228, 2003

FOR MANY YEARS, calcium has received little attention in studies of the nutritional requirements for successful reproduction in free-living birds even though it constitutes 98% of the dry mass of the avian eggshell (Romanoff and Romanoff 1949). Despite the importance of calcium in avian reproduction (see Reynolds and Perrins 2003), ornithologists have been more concerned with macronutrient requirements (i.e. fat, protein, carbohydrate) for successful reproduction in birds (see review in Carey 1996); only in the past few decades has there been interest in how birds meet the high demands for calcium during reproduction (e.g. Simkiss 1967, Hurwitz 1978, Ankney and Scott 1980). Furthermore, only in the last two decades has the focus shifted to investigation of micro- (e.g. essential amino acids, vitamins, and minerals) as well as macronutrient limitations on breeding performance of birds (for accounts and tests of calcium-limited reproduction, see Drent and Woldendorp 1989, Graveland et al. 1994).

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.) By radiotracking birds, they were able to define the laying periods for the 1998 and 1999 breeding seasons. They collected males and females during prelaying, laying, and postlaying and dissected femora and tibiotarsi from each bird. Using X-ray absorptiometry (DXA) to examine bone-mineral density of right bones and inductively coupled plasma mass spectrometry (ICP-MS) to analyze chemical composition of left bones, they suggested that females (but not males) stored calcium prior to the onset of laying. Furthermore, those stores were depleted during egg formation. The authors concluded that long-term calcium storage may afford White-tailed Ptarmigans the opportunity to select breeding habitats where dietary calcium availability is low. The authors also inferred that by storing calcium months in advance of reproduction, this species has reduced its reliance on exogenous calcium during egg-laying, thereby gaining access to a wider selection of suitable breeding habitat.

At first glance, these findings are exciting because gallinaceous birds in particular are sensitive to dietary calcium availability, and their distributions may reflect availability of sufficient exogenous calcium for their reproductive requirements (Dale 1955, Wilson 1959). During egg-laying, birds increase consumption of calcareous grit to obtain sufficient 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 find...." Furthermore, they employed rigorous and sensitive laboratory methods. The study's main shortcomings lie in methodological flaws that make their conclusions questionable.

Central to their study was the classification of collected females as "prelaying". The authors monitored timing of breeding for birds carrying radio transmitters during the two years of study. Presumably, birds were then categorized as prelaying or laying according to when they were sacrificed in relation to the laying period of the radiotagged birds the authors observed. Because a destructive technique was used to examine the skeletal structure of birds, breeding status could have been determined more accurately from examination of the carcasses. At the simplest level, birds could have been categorized as reproductive or nonreproductive on the basis of the extent of recrudescence of reproductive tissues (e.g. Krementz and Ankney 1995). Furthermore, for birds collected a few days before the first egg was laid and after the clutch was completed, unequivocal determination of laying date would have been possible through measurement of the size of pre- and postovulatory follicles. Sizes of pre- and postovulatory follicles of the study species can be measured at various times before and after egg-laying onset, respectively. Subsequently, the size of the largest yolk follicle and the largest postovulatory (regressing) follicle would have allowed prediction of clutch initiation dates for hens sacrificed during prelaying, laying, and postlaying (for details see Jones and Ward 1976).

No additional information about prelaying females was provided. For example, information about the ages of prelaying birds and, for nonyearlings, details of clutch sizes in previous breeding seasons would have allowed the reader to hazard an educated guess about the likelihood of birds breeding and their clutch sizes. The main drawback with sampling a population in that manner is that there is no guarantee that birds would have bred had they not been collected. Of course, for females collected during egg-laying and postlaying periods, the authors could have quantified 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 fig. 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). To reach the conclusion that calcium storage occurred "months prior to reproduction," the authors were faced with an impossible task of showing that females would have bred months after they were collected. Such long-term calcium storage in the skeleton could only be proven through repeated sampling of birds across pre- and postlaying periods with a nondestructive technique capable of fine-scale resolution of skeletal structure. To my knowledge, such an analytical tool is not currently available.