WHY PENGUIN EGGSHELLS ARE THICK

Auk, The, Jan 2004 by Boersma, P Dee, Rebstock, Ginger A, Stokes, David L

ABSTRACT.-Like most other penguin species, Magellanic Penguins (Spheniscus magellanicus) are large-bodied birds that incubate their eggs for a prolonged period on hard substrates with little nesting material - all circumstances that could lead to high rates of egg breakage. However, Magellanic Penguin eggs at Punta Tombo, Argentina are seldom broken. From 1984 to 2001, only 2.6% of 10,023 eggs in our study areas broke or cracked. Most of those were broken in unusual or catastrophic events, mainly penguin fights and rainstorms. Low breakage rates appear to be attributable to thick eggshells. Shells of Magellanic Penguin eggs averaged 0.81 mm without the egg membranes - at least 56% thicker than expected for bird eggs of similar mass. The calcium required for those thick eggshells cannot be supplied by normal food intake because females lay eggs during a fasting period. It is also unlikely that sufficient skeletal calcium can be mobilized. An alternative potential calcium source is mollusk shells. To determine whether female penguins were selectively ingesting calcium to form thick eggshells, we examined stomach contents of birds during the egg period (settlement, egg laying, and early incubation) and the post-egg period (late incubation and chick rearing). Both females and males were more likely to have mollusk shells in their stomachs during the egg period than during the post-egg period. However, females were much more likely than males to have shells in their stomachs during the egg period, whereas the proportions of males and females with mollusk shells did not differ in the post-egg period. Selective ingestion of mollusk shells by Magellanic Penguins, resulting in thick eggshells, appears to be an adaptive response that reduces egg breakage. Received 16 January 2003, accepted 5 October 2003.

RESUMEN. - Como la mayoria de las demas especies de pinguinos, Spheniscus magellanicus es un ave de tamano corporal grande que incuba sus huevos por un periodo prolongado sobre sustratos duros y utiliza poco material de nido. Todas estas circunstancias podrian conducir a altas tasas de ruptura de hvievos. Sin embargo, es raro que los huevos de S. magellanicus en Punta Tombo (Argentina) se rompan: entre 1984 y 2001, solo el 2.6% de 10,023 huevos en nuestras areas de estudio se rompieron o se rajaron. La mayoria de los huevos se rompieron en eventos poco usuales o catastroficos, principalmente peleas entre los pinguinos y tormentas de lluvia. Las bajas tasas de ruptura parecen ser debidas a los cascarones gruesos de los huevos. El grosor promedio del cascaron en S. magellanicus, sin incluir las membranas del huevo, fue de 0.81 mm, lo que es al menos un 56% mas grueso de lo que se esperaria para huevos de aves de masa similar. El calcio necesario para formar estos cascarones gruesos no puede ser provisto por la tasa normal de ingestion de alimento porque las hembras ponen los huevos durante un periodo de ayuno. Tambien es poco probable que sea posible movilizar suficiente calcio esqueletico. Una fuente alternativa de calcio potencial son las conchas de moluscos. Para determiner si las hembras estaban ingiriendo calcio selectivamente para formar cascarones gruesos, examinamos los contenidos estomacales de aves durante el periodo de postura de huevos (establecimiento, postura e incubation temprana) y el periodo posterior a la postura (incubation tardia y crianza de los pichones). Fue mas probable que los machos y las hembras tuvieran conchas de moluscos en el estomago en el periodo de postura que en el periodo posterior. Sin embargo, la probabilidad de tener conchas en el estomago fue mucho mayor en las hembras que en los machos durante el periodo de postura, mientras que las proporciones de hembras y machos con conchas en el estomago fueron similares en el periodo posterior a la postura. El cascaron grueso de los huevos de S. magellanicus, junto con la ingestion selective de conchas de moluscos, parece ser una respuesta adaptativa que reduce la ruptura de huevos.

BIRDS' EGGSHELLS ARE composed mainly of calcium carbonate (Romanoff and Romanoff 1949). Consequently, dietary calcium is important to female birds at the time of egg production (see Simkiss 1961). Females of many species ingest calcium-rich items - such as mollusk shells, calcareous grit, calcareous ash, and bones - before egg laying (e.g. Johnson and Barclay 1996). Lack of calcium can reduce reproductive success in passerines, and birds breeding in areas with calcium-poor soils may not be able to obtain sufficient calcium for eggshells from their diet (Graveland and Drent 1997).

Compared to eggs of most other birds, the eggshells of some seabird species constitute a larger proportion of total egg mass. (Schonwetter 1960, Williams 1981, Williams et al. 1982). Despite the large mass of their eggshells, seabirds are nearly absent from the list of species known to selectively ingest calcium-rich items before egg laying (but see Brenninkmeijer et al. 1997), probably because their diet of fish and shellfish is high in calcium.