Egg-size Dimorphism Research Articles

The breeding biology of erect-crested penguins, Eudyptes sclateri: Hormones, behavior, obligate brood reduction and conservation.

Erect-crested penguins are the least studied of all penguins. They breed on two isolated subantarctic island groups, the Antipodes and Bounty Islands. Sporadic nest counts indicate a dramatic decline in numbers of erect-crested penguins over the last 50 years. Here we present data from a study undertaken in 1998 on the breeding biology, behavior and hormones of erect-crested penguins. It represents, even today, by far the most detailed data available on this species. The penguins exhibited extreme reversed egg-size dimorphism, whereby the first-laid A-egg was much smaller than the second-laid B-egg. A-eggs were lost before (42.3%) or on (37.8%) the day the B-egg was laid, and none survived more than 7 days after that. The penguins were in a low state of reproductive readiness, as evidenced by low levels of copulation, fighting, and testosterone in males during the courtship/laying period when, curiously, plasma levels of testosterone were at least as high in females. The laying interval (5.4 days) is the longest recorded for any penguin species, and incubation was highly variable until clutch completion. Most nests (91.2%) contained no nesting material and eggs were laid directly onto the ground. A-eggs were lost mainly by rolling out of the nest. However, even when prevented from doing so by an experimental manipulation, A-eggs survived no longer than those in control nests. Testosterone levels in males increased after clutch completion, when they remained in attendance at the nest for up to 13 days, despite females assuming most of the incubation duties. The bills of males were significantly larger than those of females and probably help with guarding the nest. We discuss explanations for obligate brood reduction in crested penguins and the options for conservation in light of our census results, which indicate that this enigmatic penguin species could be in trouble.

Compensatory effect of egg size dimorphism on hatching asynchrony in Magellanic penguin

Egg laying is one of the most important phases in a female bird's breeding cycle. Its cost is high because eggs contain all the resources needed for the development of an embryo. Variation in size and quality of eggs can have important long‐term consequences for offspring survival. Hatching asynchrony is known to influence sibling competition in many bird species. Last‐hatched chicks will have a competitive disadvantage throughout the pre‐fledgling period because they are smaller. The aim of this study was to analyze the effect of hatching asynchrony and egg size variation on the growth and fledging success of Magellanic penguin Spheniscus magellanicus chicks after disentangling the effects of parental condition. We simultaneously manipulated egg size dimorphism, hatching asynchrony and parental condition by performing a cross‐fostering experiment, creating broods with controlled egg size dimorphism and hatching asynchrony in a colony of Magellanic penguins located in Isla Quiroga, Santa Cruz, Argentina. We found that hatching asynchrony had a negative effect on last‐hatched chicks, but this disadvantage was mitigated by egg size dimorphism in their favor. Moreover, females in good condition invest more in second than in first chicks, which, added to a greater investment by foster fathers, leading to offspring fledging in good condition. On the contrary, for the first‐hatched chicks, we found that body condition of the biological father was an important factor for their growth. We conclude that raising more than one chick seems to be a decision based on parental condition throughout the breeding season.

Extreme intra-clutch egg size dimorphism is not coupled with corresponding differences in antioxidant capacity and stable isotopes between eggs

Oviparous females need to allocate resources optimally to their eggs in order to maximize their fitness. Among these resources, dietary antioxidants, acquired by females and transferred to the eggs during egg formation, can greatly affect the development and survival of the embryo and chick. In crested penguins, incubation starts after the second and last egg is laid and, as opposed to many other bird species, this egg hatches first, thereby enhancing the survival of the chick. Here, we assessed whether antioxidant and isotopic composition could underlie these differences between eggs within clutches of southern rockhopper penguins (Eudyptes chrysocome chrysocome). The second-laid egg had higher total antioxidant capacity than the first-laid egg, although this was not due to higher antioxidant concentration but to its higher mass. This suggests that resources are allocated by females at a constant rate in both eggs within clutches. Accordingly, we found a strong correlation for isotopic compositions between eggs suggesting that resources were allocated similarly to each egg within the clutch. Overall, we found little evidence for a significant role of antioxidant and isotopic compositions to explain differences in terms of embryo/chick development between eggs in crested penguins. However, since our results suggest a constant rate of antioxidant transfer from females to eggs, limiting the mass of the first-laid egg might represent a strategy for females to spare antioxidant defences and preserve self-maintenance.

Migratory life histories explain the extreme egg-size dimorphism of Eudyptes penguins

When successive stages in the life history of an animal directly overlap, physiological conflicts can arise resulting in carryover effects from one stage to another. The extreme egg-size dimorphism (ESD) of Eudyptes penguins, where the first-laid A-egg is approximately 18-57% smaller than the second-laid B-egg, has interested researchers for decades. Recent studies have linked variation in this trait to a carryover effect of migration that limits the physiology of yolk production and egg sizes. We assembled data on ESD and estimates of migration-reproduction overlap in penguin species and use phylogenetic methods to test the idea that migration-reproduction overlap explains variation in ESD. We show that migration overlap is generally restricted to Eudyptes relative to non-Eudyptes penguins, and that this overlap (defined as the amount of time that egg production occurs on land versus at sea during homeward migration) is significantly and positively correlated with the degree of ESD in Eudyptes In the non-Eudyptes species, however, ESD was unrelated to migration overlap as these species mostly produce their clutches on land. Our results support the recent hypothesis that extreme ESD of Eudyptes penguins evolved, in part, as a response to selection for a pelagic overwinter migration behaviour. This resulted in a temporal overlap with, and thus a constraint on, the physiology of follicle development, leading to smaller A-egg size and greater ESD.

Two eggs, two different constraints: a potential explanation for the puzzling intraclutch egg size dimorphism in Eudyptes penguins.

Phenotypic plasticity and phenotypic stability are major components of the adaptive evolution of organisms to environmental variation. The invariant two-egg clutch size of Eudyptes penguins has recently been proposed to be a unique example of a maladaptive phenotypic stability, while their egg mass is a plastic trait. We tested whether this phenotypic plasticity during reproduction might result from constraints imposed by migration (migratory carry-over effect) and breeding (due to the depletion of female body reserves). For the first time, we examined whether these constraints differ between eggs within clutches and between egg components (yolk and albumen). The interval between colony return and clutch initiation positively influenced the yolk mass, the albumen mass, and the subsequent total egg mass of first-laid eggs. This time interval had only a slight negative influence on the yolk mass of second-laid eggs and no influence on their albumen and subsequent total masses. For both eggs, female body mass at laying positively influenced albumen and total egg masses. Female investment into the entire clutch was not related to the time in the colony before laying but increased with female body mass. These novel results suggest that the unique intraclutch egg size dimorphism exhibited in Eudyptes penguins, with first-laid eggs being consistently smaller than second-laid eggs, might be due to a combination of constraints: a migratory carry-over effect on the first-laid egg and a body reserve depletion effect on the second-laid egg. Both these constraints might explain why the timing of reproduction, especially egg formation, is narrow in migratory capital breeders.

Parental investment in eggs and its effect on nestling growth and survival in Magellanic Penguins

Life-history variables evolve in response to cost-benefit trade-offs. For birds, larger eggs are thought to be beneficial for development of offspring but are energetically costly to produce. Further, egg-size dimorphism within or between clutches can vary with proximate and ultimate causes. We undertook a correlational study to evaluate parental investment in eggs by Magellanic Penguins (Spheniscus magellanicus) and how it affects the growth and survival of nestlings in Puerto Deseado, Argentina, over 3 years. We evaluated the variables that affected egg-volume and yolk-area (using a nondestructive technique), and determined the effects of egg-volume and yolk-area on growth and survival of young. Females in good body-condition laid larger second eggs and, in good years (i.e. years of high reproductive success in the colonies of the study area), yolk-area of second eggs was larger than that of first eggs. We found a positive association between egg-volume and nestling body-size and yolk-area was positively related to nestling survival. Our results suggest that the size of eggs within clutches varied with year and female body-condition. Moreover we demonstrate for the first time that yolk-area is a strong predictor of nestling survival in Magellanic Penguins.

Extreme Intraclutch Egg-Size Dimorphism inEudyptesPenguins, an Evolutionary Response to Clutch-Size Maladaptation

Eudyptes penguins (six species) are uniquely characterized by a two-egg clutch with extreme intraclutch egg-size dimorphism (ESD): the first-laid A-egg is 17.5%-56.9% smaller than the B-egg. Although A-eggs are viable, they almost never produce fledged chicks (genus average <1%). Using classical life-history theory and phylogenetic comparative methods, we demonstrate a marked slowdown in the life history of Eudyptes: age of first reproduction is 52% later and annual fecundity 48% lower compared with other two-egg clutch penguin species. All six Eudyptes species have retained a two-egg clutch, despite this pronounced life-history slowdown; this suggests evolutionary mismatch between clutch size and chicks fledged per clutch. Consistent with this, we show that Eudyptes fledge 43% fewer chicks per clutch than other two-egg clutch penguin species. Extreme intraclutch ESD in Eudyptes is associated primarily with a uniform (5%) increase in relative B-egg size, and B-egg size has evolved in accord with life history. We further show that intraclutch ESD is positively correlated with age of first reproduction in Eudyptes but not in other two-egg clutch penguin species. We argue that Eudyptes' persistent failure to evolve a one-egg clutch constitutes a unique genus-wide evolutionary maladaptation and that extreme intraclutch ESD evolved as a correlated response to selection favoring a slower life history imposed by their extreme pelagic overwintering and migration ecology.

Migratory constraints on yolk precursors limit yolk androgen deposition and underlie a brood reduction strategy in rockhopper penguins

Hormonally mediated maternal effects link maternal phenotype and environmental conditions to offspring phenotype. The production of lipid-rich maternal yolk precursors may provide a mechanism by which lipophilic steroid hormones can be transported to developing yolks, thus predicting a positive correlation between yolk precursors in mothers and androgen levels in eggs. Using rockhopper penguins (Eudyptes chrysocome), which produce a two-egg clutch characterized by extreme egg-size dimorphism, reversed hatching asynchrony and brood-reduction, we examined correlations between circulating concentrations of the primary yolk-precursor vitellogenin (VTG) and levels of yolk androgens. Previous work in Eudyptes penguins has shown that egg-size dimorphism is the product of migratory constraints on yolk precursor production. We predicted that if yolk precursors are constrained, androgen transport to developing yolks would be similarly constrained. We reveal positive linear relationships between maternal VTG and androgens in small A-eggs but not larger B-eggs, which is consistent with a migratory constraint operating on the A-egg. Results suggest that intra-clutch variation in total yolk androgen levels depends on the production and uptake of yolk precursors. The brood reduction strategy common to Eudyptes might thus be best described as the result of a migratory constraint.

Poecilogony and Population Genetic Structure in Elysia pusilla (Heterobranchia: Sacoglossa), and Reproductive Data for Five Sacoglossans that Express Dimorphisms in Larval Development

Credible cases of poecilogony, the production of two distinct larval morphs within a species, are extremely rare in marine invertebrates, yet peculiarly common in a clade of herbivorous sea slugs, the Sacoglossa. Only five animal species have been reported to express dimorphic egg sizes that result in planktotrophic and lecithotrophic larvae: the spionid polychaete Streblospio benedicti and four sacoglossans distributed in temperate estuaries or the Caribbean. Here, we present developmental and genetic evidence for a fifth case of poecilogony via egg-size dimorphism in the Sacoglossa and the first example from the tropical Indo-Pacific. The sea slug Elysia pusilla produced both planktotrophic and lecithotrophic larvae in Guam and Japan. Levels of genetic divergence within populations were markedly low and rule out cryptic species. However, divergence among populations was exceptionally high (10-12% at the mitochondrial cytochrome c oxidase I locus), illustrating that extensive phylogeographic structure can persist in spite of the dispersal potential of planktotrophic larvae. We review reproductive, developmental, and ecological data for the five known cases of poecilogony in the Sacoglossa, including new data for Costasiella ocellifera from the Caribbean. We hypothesize that sacoglossans achieve lecithotrophy at smaller egg sizes than do related clades of marine heterobranchs, which may facilitate developmental plasticity that is otherwise vanishingly rare among animals. Insight into the environmental drivers and evolutionary results of shifts in larval type will continue to be gleaned from population-level studies of poecilogonous taxa like E. pusilla, and should inform life-history theory about the causes and consequences of alternative development modes in marine animals.

Intra-clutch ratio of yolk progesterone level changes with laying date in rockhopper penguins: a strategy to influence brood reduction?

Hatching asynchrony in avian species generally leads to a size hierarchy among siblings, favouring the first-hatched chicks. Maternally deposited hormones affect the embryo and chick's physiology and behaviour. It has been observed that progesterone, a hormone present at higher levels than other steroid hormones in egg yolks, is negatively related to body mass in embryos, chicks and adults. A differential within-clutch progesterone deposition could therefore be linked to the size hierarchy between siblings and to the resulting brood reduction. We tested whether yolk progesterone levels differed between eggs according to future parental ability to feed the entire clutch in wild rockhopper penguins Eudyptes chrysocome. This species presents a unique reversed egg-size dimorphism and hatching asynchrony, with the larger second-laid egg (B-egg) hatching before the smaller first-laid egg (A-egg). Yolk progesterone levels increased only slightly with female body mass at laying. However, intra-clutch ratios were not related to female body mass. On the other hand, yolk progesterone levels increased significantly with the date of laying onset for A-eggs while they decreased for B-eggs. Early clutches therefore had proportionally more progesterone in the B-egg compared to the A-egg while late clutches had proportionally less progesterone in the B-egg. We propose that females could strategically regulate yolk progesterone deposition within clutches according to the expected food availability during chick growth, an adaptive strategy to adjust brood reduction to conditions. We also discuss these results, relating to yolk progesterone, in the broader context of other yolk steroids.

Egg components vary independently of each other in the facultative siblicidal Black-legged Kittiwake Rissa tridactyla

Egg composition varies both within and between clutches, and mothers are expected to alter their deposition of resources to the egg depending on environmental conditions and breeding strategies. Within-clutch variation in egg composition has been proposed to reflect an adaptive maternal strategy influencing sibling competition. In species with brood reduction, mothers should reinforce brood hierarchies due to hatching asynchrony and favour senior chicks by making first-laid eggs larger, richer in nutrients, with higher testosterone and carotenoid levels and lower corticosterone concentrations than last-laid eggs [parental favouritism hypothesis (PFH)]. Moreover, mothers that are of better quality and/or experience better feeding conditions during laying are expected to increase their deposition of resources to the egg, resulting in differences between clutches [investment hypothesis (IH)]. Several components may act together to provide an optimal reproductive strategy, but studies of variation in different egg components in the same egg are relatively rare. We analysed egg size, testosterone and corticosterone concentrations and carotenoids measured as yolk colour between and within clutches for the facultative siblicidal Black-legged Kittiwake Rissa tridactyla. First-laid eggs were larger, contained lower testosterone, higher yolk colour score, and similar corticosterone levels than last-laid eggs. Thus, only differences in egg size and yolk colour supported the PFH. We used within-clutch egg size dimorphism as an indicator of the quality of the mother or the feeding conditions during laying. In support of the IH, we found that mothers of better quality or that experienced better feeding conditions deposited more corticosterone into their eggs. High corticosterone levels may benefit nestlings when there is no brood reduction but high sibling competition is present. We found no support for the hypothesis that egg components are mutually adjusted to each other and we discuss the possible reasons for this.

A Carryover Effect of Migration Underlies Individual Variation in Reproductive Readiness and Extreme Egg Size Dimorphism in Macaroni Penguins

Where life-history stages overlap, there is the potential for physiological conflicts that might be important in mediating carryover effects. However, our knowledge of the specific physiological mechanisms underlying carryover effects remains rudimentary, and specific examples remain rare. Here we show that female macaroni penguins (Eudyptes chrysolophus) initiate vitellogenesis and yolk formation while at sea during return migrations to breeding colonies; yolk formation takes approximately 16 days, but females lay only 7-14 days after their return. Once on land, Eudyptes penguins show a unique reproductive pattern of extreme egg size dimorphism in which the smaller, first-laid A-egg is 55%-75% of the size of the larger B-egg. We show that the degree of egg size dimorphism is inversely correlated with time between arrival and laying; that is, females that begin reproductive development well in advance of their return produce more dimorphic eggs. Furthermore, late-arriving females that produce the most dimorphic eggs have lower plasma levels of the yolk precursor vitellogenin on arrival; that is, they show lower reproductive "readiness." These data support the hypothesis that extreme egg size dimorphism in Eudyptes penguins is due to a physiological constraint imposed by a migratory carryover effect and argue against small A-eggs having a specific, adaptive function.

Is the reduced incubation time for B-eggs in Rockhopper Penguins Eudyptes chrysocome linked to egg density variation?

Crested penguins (genus Eudyptes) present a unique reversed egg-size dimorphism and hatching asynchrony, with the larger second-laid egg (B-egg) hatching before the smaller first-laid egg (A-egg). Both a higher water vapour conductance and parental favouritism during incubation could explain the shorter incubation period for the B-egg than for the A-egg. Because the incubation period is increased by the presence of a sibling for A-eggs, but not for B-eggs, and because both egg categories have the same incubation period when they are incubated alone, it has been suggested that the difference in incubation period was largely driven by the parental favouritism for B-eggs during incubation. We tested whether A- and B-eggs show a difference in laying density, density at the beginning of incubation and in density decrease during incubation according to the presence of a sibling. Although density at the start of incubation was significantly higher for B-eggs than for A-eggs, the decrease in density during incubation had the same slope for both egg categories. Moreover, the presence of a sibling did not influence densities. We additionally provide two equations that allow the back-dating of laying dates for a clutch of SouthernRockhopper Penguin (Eudyptes c. chrysocome) and we discuss the precision of the method (3.04 ± 2.29 days for A-eggs and 2.73 ± 2.10 days for B-eggs) for penguins which are increasingly being used as marine environmental sentinels.

Geographical variation in egg size dimorphism in rockhopper penguins

All crested penguins present a unique reversed hatching asynchrony: the larger second-laid egg (B-egg) hatches before the smaller first-laid egg (A-egg). Although both eggs often hatch, the A-chick generally dies of starvation within days after hatching. However, within rockhopper penguins, the population at the Falkland Islands is unique in that some birds manage to raise both chicks. Although it has been suggested that the egg size dimorphism between A- and B-eggs may explain how long both eggs and chicks survive, this hypothesis has never been explicitly tested. We expect that both eggs are retained longer in the less dimorphic clutches than in the more dimorphic ones. In this paper, we have compiled egg measurements for three rockhopper penguin species (Eudyptes chrysocome, E. filholi and E. moseleyi) in order to compare the intra-clutch egg size dimorphism among these species. Furthermore, we have collected new data to compare egg size dimorphism between two populations of E. chrysocome (Falkland Islands versus Staten Island). A-egg volumes are more variable between species and populations than B-egg volumes. E. chrysocome and especially the population from the Falkland Islands produces the largest A-eggs and the least dimorphic eggs. Nevertheless, as differences in A-egg volumes between species and between the populations of Falkland Islands and Staten Island are stronger and more significant than differences in egg dimorphism, we suggest that A-egg volume, more than egg dimorphism, could be one of the factors influencing the prevalence of twins. A large A-egg and/or reduced egg dimorphism is probably necessary to enable rockhopper penguins to raise two chicks, but other reasons may also be involved which enable them to keep both eggs and chicks.

Egg size and laying order in relation to offspring sex in the extreme sexually size dimorphic brown songlark, Cinclorhamphus cruralis

In some bird species, mothers can advantage the offspring of one sex either by elevating them in the laying order to promote earlier hatching or by allocating greater resources to eggs of the preferred sex. In size dimorphic species, the predictions as to which sex should benefit most from such pre-laying adjustments are ambiguous. The smaller sex would benefit from an initial size advantage to help compensate for the faster growth rate of the larger sex. However, an early advantage to offspring of the larger sex might have a greater effect on their lifetime reproductive success than an equivalent advantage to offspring of the smaller sex. We investigated these hypotheses in the polygynous brown songlark, Cinclorhamphus cruralis, which is one of the most sexually size dimorphic birds known. We conducted within-clutch comparisons and found that females hatched from larger eggs and were initially heavier (but not structurally larger) than their brothers. This may afford females an early competitive advantage, as egg volume remained correlated with chick mass until at least 5 days of age. Similarly, we found that hatch order was still positively associated with nestling mass and size when the brood was 10 days of age, but there was no clear relationship between offspring sex and hatching order. During this study, food was plentiful and there were few obvious cases of nestling starvation. When food is limited, we suggest that the greater nutrient reserves of female hatchlings could not only help compensate for their slower growth, but could also give them a survival advantage over their brothers early in the nestling period. Consequently, egg size dimorphism may be an adaptation that facilitates an early shift in brood sex-ratio towards cheaper daughters in conditions of low food availability.

Multiple Mechanisms of Reversed Hatching Asynchrony in Rockhopper Penguins

Brood reduction in birds is frequently associated with hatching asynchrony, wherein incubation commences before clutch completion, causing last-laid eggs to hatch after earlier eggs. In crested penguins (Eudyptes spp.), second-laid eggs typically hatch before first-laid eggs, but the mechanisms behind this reversal are unknown. Through a multifactor field experiment with rockhopper penguins (Eudyptes chrysocome Forster 1781), I examined several factors that might influence incubation length : laying order (smaller first-laid eggs vs. larger second-laid eggs) ; incubation onset (exposure to immediate vs. delayed incubation) ; intraclutch dimorphism (incubated with an equal-sized vs. larger nest mate) ; laying date ; egg mass ; and egg composition (yolk and albumen content). Three of these factors - laying order, intraclutch dimorphism and laying date-contributed significantly to variation in incubation length ; first-laid eggs produced early in the season and incubated with a larger nest mate took longest to hatch. Differences in incubation onset did not contribute to incubation length when tested independently. The albumen content of both first and second eggs increased in proportion to fresh egg mass, but egg mass had no effect on incubation length. Although Eudyptes parents can potentially adjust the degree of hatching asynchrony through variation in egg position, the effects of laying date and laying order are probably beyond facultative control. Together, these effects may contribute to the unique reversal in both hatching asynchrony and egg-size dimorphism characteristic of crested penguins.

Feeding Behavior, Size Asymmetry, and Food Distribution in Magellanic Penguin (Spheniscus magellanicus) Chicks

How food is distributed among siblings has long been of theoretical importance because the interests of siblings differ (Trivers 1974). The manner in which food is apportioned among siblings appears to be important in the determination of chick mortality patterns. One of the factors that could be affecting this food distribution is the size relationship between siblings. Penguins vary in their degree of hatching asynchrony, egg-size dimorphism, and fledging success (Lamey 1990). In Spheniscus penguins, hatching asynchrony creates size asymmetries that can lead to brood reduction (Lamey 1990, Boersma 1991, Boersma and Stokes 1995). Magellanic Penguins (Spheniscus magellanicus) at Punta Tombo, Argentina, show high variability in breeding success among years, mainly as a result of changes in food availability (Boersma et al. 1990). During years with low food availability, most pairs raise no offspring or only one as a result of the starvation of one or both chicks (Boersma and Stokes 1995). When one sibling dies, the surviving chick receives all the food brought to the nest by the parents, resulting in an increased growth rate (Boersma 1991). We describe the feeding behavior of Magellanic Penguin chicks, quantify the way food brought to the nest is apportioned among siblings, and analyze how this distribution relates to chick size asymmetries. Study area and methods.-The study was conducted in the Punta Tombo Provincial Reserve, Chubut, Argentina (44002S, 65?11'W), during January 1990 and 1991. Punta Tombo is the site of the largest continental colony of Magellanic Penguins (Boswall and McIver 1975), with 225,000 breeding pairs (Boersma et al. 1990). Magellanic Penguins have a seasonal breeding schedule in which they begin returning to the colony in late August and early September, and lay two eggs in October that are laid three to four days apart (Boersma et al. 1990). Eggs are always nearly equal in size with an average difference of 5 g between the larger and smaller eggs (Boersma and Stokes 1995). Chicks hatch in November approximately two days apart, and fledge in late January and February. Both adults defend the nest site, incubate

Crested Penguin Egg Dimorphism: Reply to Williams

We welcome T. D. Williams' data bearing on the interesting question of egg size dimorphism in crested penguins (Eudyptes spp.). As pointed out in Johnson et al. (1987), there are two questions that should be considered: 1) the existence of an extra egg/chick that is never reared and 2) the unusual, small size of the first egg. Warham (1975) suggested answers to both questions. He speculated that the extra egg functions as insurance against egg loss and that the size discrepancy facilitates brood reduction.

Aggression, Incubation Behaviour and Egg-Loss in Macaroni Penguins, Eudyptes Chrysolophus, at South Georgia

In crested penguins Eudyptes spp., the first-laid (A) egg is very much smaller than the second (B) egg and, although viable, rarely survives to hatching. I tested the recent hypothesis that this is an adaptation to cope with the high degree of egg loss resulting from selection for male aggression. In 60 nests of macaroni penguins 60% of A-eggs were lost on the same day and 23% on the day before the B-egg was laid. During the laying interval adults spent increasing time incubating prone (i.e. providing maximum protection for the A-egg). Aggression rates peaked before A-eggs were laid. Both results are contrary to the suggestion that intra-specific aggression and poor egg protection cause high A-egg loss and are the main factors favouring egg-size dimorphism. It is hypothesised that successful laying of the B-egg may have a causal effect on A-egg loss and that the A-egg may function as insurance against unsuccessful laying (immediate loss) of the B-egg.