The Breeding Biology Of Eudyptes Penguins With Particular Reference To Egg Size Dimorphism

ABSTRACT

The breeding biology of Macaroni Penguins  and of Rockhopper Penguins was

studied at Marion Island (46°54'8, 37°45'E) in the austral

summers of 1974-75 and 1976-77.

The clutch comprises two eggs, the first-laid (A-) egg

being markedly smaller than the second-laid (B-) egg.

Components of B-eggs are heavier than the same components

of A-eggs. Incubation begins after the B-egg is laid.

Incubation of B-eggs lasts 34 and 36 days in Rockhopper and

Macaroni Penguins respectively. Parents treat the larger

egg preferentially. In Macaroni Penguins, less than 1% of

all A-eggs hatch and 54% of all A-eggs are lost before the

B-egg in the same clutch is laid. In Rockhopper Penguins,

both eggs are retained until one hatches at 30% of all nests,

but both eggs hatch at only 12% of all nests. The B-egg

hatches before the A-egg.

Hatchling weight and the growth of chicks for the first

35 days after hatching are correlated with egg weight. B-egg

chicks grow significantly faster than A-egg chicks and are

heavier within 24 h of .''fledging". Chicks of both species

are fed at intervals of 36-92 hours and receive about 30 meals

during the 70-day chick-rearing period. Except in exceptional

circumstances, Macaroni Penguins rear no A-egg chicks. When

two Rockhopper Penguin chicks hatch from dimorphic eggs, one

dies of starvation within 12 days but in experiments chicks

hatched from two eggs of similar size co-existed for up to

57 days. Chicks are raised successfully from 3% of all

A-eggs laid by Rockhopper Penguins, and from 34% and 43%

of all B-eggs laid by Rockhopper and Macaroni Penguins

respectively. Only one chick can be reared from each clutch.

Egg-size dimorphism predetermines the offspring most likely

to survive.

Eudypte~ penguins may forage 100 km from the colony when

feeding chicks. However, the ancestral Eudypte~ was probably

an inshore-feeder laying two eggs of similar size and capable

of raising two chicks. Egg-size dimorphism has probably

developed through major enlargement of the B-egg, and possibly

arose in the course of the transition from inshore to offshore

foraging.