Why Penguin Eggshells Are So Thick
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 1984-2001 only 2.6% of 10,023 eggs in our study areas were broken or cracked. Most eggs were broken in unusual or catastrophic events, mainly penguin fights and rainstorms. Low breakage rates appear to be due 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 these 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 the stomach contents of birds during the egg period (settlement, egg laying and early incubation) and the postegg 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, while the proportions of males and females with mollusk shells did not differ in the post-egg period. The thick eggshells of Magellanic Penguins, along with selective ingestion of mollusk shells, appears to be an adaptive response that reduces egg breakage.
Birds’ eggshells are mainly composed of calcium carbonate (Romanoff and Romanoff 1949). Consequently dietary calcium is important to female birds at the time of egg production (see Simkiss 1961). Female birds 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 the eggs of most other birds, the eggshells of some seabird species constitute a larger proportion of total egg mass. (Schönwetter 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.
The eggs of penguins (Spheniscidae), cormorants (Phalacrocoracidae) and some alcids (Alcidae) such as murres (Uria) and guillemots (Cepphus) have the highest proportions of shell mass among the seabirds (Schönwetter 1960; Williams et al. 1982). A thick eggshell was at one time thought to correlate positively with the developmental level (precocity) of the chick, but Ar and Yom-Tov (1978) found no correlation when they controlled for egg size. Williams et al. (1982) likewise found no correlation across seabird species between eggshell mass and chick development at hatching, nor is total egg mass correlated with the proportion of total mass formed by the eggshell. High eggshell mass may be an adaptation to hard nesting substrates. In seabirds with a calcium-rich diet, thick eggshells may be an inexpensive way to reduce egg breakage. Penguins are large birds that nest on hard substrates, often use little or no soft nesting material, and incubate for a prolonged period (30-64 days; Williams 1995). Although female penguins often fast for several days to weeks before egg laying, they may be able to obtain calcium through ingestion of mollusk shells prior to fasting. These shells digest slowly and may release calcium that can be used for eggshell formation. Females may also ingest shells shortly after egg laying to replenish depleted calcium levels.
To test the hypothesis that eggshell thickness is an adaptive feature of penguin nesting ecology, we investigated eggshell mass, rates of egg breakage, and ingestion of mollusk shells in Magellanic Penguins (Spheniscus magellanicus) nesting at the largest breeding colony of this species. Magellanic Penguins typically lay a clutch of two eggs on hard surfaces with little or no nesting material and incubate the eggs for approximately 42 days (Boersma et al. 1990). In this region, their diet is dominated by schooling fish, primarily anchovy Engraulis anchoita (Scolaro et al. 1999; Boersma pers. obs.).
We hypothesized that: 1) eggshells of this species should be thick relative to those of other types of birds, 2) egg breakage rates should be low and comparable to those of birds that do not nest on hard surfaces, 3) female penguins should ingest more mollusk shells than do males around the time of egg laying, 4) after egg laying, males and females should ingest similar, small amounts of shell, 5) females should ingest more shells during the egg-laying period than later in the breeding season, and 6) ingestion of shells by males should be independent of stage of breeding season.
