Gentoo penguins belong to the pygoscelid or brush tail penguins that also include Chinstrap and Adélie penguins. Males and females look very much alike but females tend to be slightly smaller, particularly with regard to the beak depth and length.
Gentoo penguins stand about 60 cm tall with both feet on the ground and their heads pulled in. The colouration of the sexes is identical; head, throat, back and flippers are dark bluish-black while the chest, belly and underside of the flippers are white. The black and white body parts are clearly separated. Above the eyes are two white patches that often join across the crown. A dusting of white feathers is sprinkled around their head, nape and upper back.
The top of the beaks and their tips are black but the sides are orange to red. The feet are pinkish-orange to red and the irises are brown.
Juveniles can be distinguished from adults only in their first year when the white patches on their heads are discontinuous and the rings around the eyes are still white; some Gentoo penguins appear to retain the white eye rings into adulthood. Juveniles are also often smaller than fully grown adults
As in all penguins, the body mass of Gentoo penguins is highly variable throughout the year. At the beginning of their breeding cycle, these penguins weigh usually 4.8 – 5.7 kg. Post-breeding and prior to the moult they can reach a body mass of more than 8 kg.
Like Chinstrap and Adelie penguins, their tail feathers are much longer compared to other penguin species.
Gentoo penguins have a circumpolar distribution and their colonies are found at the Antarctic Peninsula and many sub-Antarctic islands. Where they go in the non-breeding period is poorly understood but some adults appear to remain in the vicinity of their colonies all year round. Vagrants were found as far north as 43°S along the Argentine coast and at the coasts of New Zealand and Australia. The largest breeding populations are found at the Falkland Islands and South Georgia.
The at-sea distribution varies throughout the year but Gentoo penguins rarely travel long distances away from their colonies. Their foraging trips usually last only hours rather than days but their duration increases as the chicks grow older and demand more food.
During the breeding season, Gentoo penguins tend to forage inshore within about 30 km of their colonies. But during winter they may go much farther afield; one penguin from the Falkland Islands, for example, travelled to 276 km from the coast. Generally though, they do not move far from their breeding grounds even outside the breeding season.
Gentoo penguins usually forage during the day. Although they can reach maximal depths of about 212 m (recorded in Marion Island in 1981), most of the time they forage at 40-80 m. Dives last on average 1-2 minutes but the longest time spent submerged is an astonishing 677 s recorded in South Georgia in 1989.
The onset of breeding among Gentoo penguins appears to be dependent upon the latitudes of the colonies. Populations south of 50°S start their breeding cycle in spring to early summer (Oct-Nov) while those breeding north of 50°S breed during the winter (Jun-Aug). Gentoo penguins do not necessarily breed every year. Every now and then an individual skips a season, especially when it was unsuccessful in the previous season or when environmental conditions are poor, e.g. a lot of sea ice is present. However, when they engage in reproductive activities the partners share the incubation duty and change over frequently, usually every 2-3 days. Breeding success is highly variable among colonies and between years but often only one chick is raised successfully. Nest failure is due to nest desertion, mismatched nest relief, infertility of eggs or predators.
The level of fidelity to a previous mate or nest site varies among years. In years when the return rate to the colony is low, few if any birds retain their previous partners. However, in years when many penguins attempt to breed mate fidelity can be as high as about 90%.
The main predators are skuas, giant petrels, Kelp gulls (Larus dominicanus), fur seals and, on some islands, feral cats. Southern elephant seals are occasionally observed to cause havoc among colonies at Macquarie Island.
Gentoo penguins breed in the ice-free areas of sub-Antarctic islands and the Antarctic Peninsula. Most colonies are in near the shores but at South Georgia Island some colonies are located some 2 km inland and about 200 m above sea level. Nests are either built from small stones or are prepared on the cushion plant Azorella or tussock (Poa spp.). In the sub-Antarctic, Gentoo penguins prefer vegetation as nesting material but in their southern colonies all nests are made of pebbles.
The diet of Gentoo penguins varies with location, as well as with season. Around the Antarctic Peninsula and South Georgia they mainly consume crustaceans, such as amphipods and Antarctic krill Euphausia superba while those at Macquarie Island prefer lantern fish (myctophids) and notothenid fish. At the Kerguelen Islands in the Indian Ocean, Gentoo penguins foraged mainly on fish in winter but on the krill Euphausia vallentini in summer.
Gentoo penguins lay clutches of two eggs which weigh about 130 g. The duration of the laying period also varies with location; at Crozet Island, laying can last up to 154 days compared to only 41 days at the South Orkney Islands. The two eggs are similar in size and shape although minor difference can exist. Eggs that are lost are usually not replaced. Eggs are laid 3 days apart and are incubated for 32-42 days.
The eggs hatch within one or two days of each other and the chicks are brooded for up to 10 days. Twin chicks appear to be brooded for longer and join the crèches at an older age than single chicks. Chicks are about 25-29 days old when they join a crèches which are often small comprising no more than 10 chicks. Depending on the colony chicks commence their moult at 39 to 85 days of age. Chicks usually receive less than 2 feeds per day and feeding chases are common once the chicks are old enough to join crèches. Overall it takes about 80-100 days to rear chicks from hatching to fledging.