Mating System
PARTNERS "FOR LIFE"
Adelie
penguins are monogamous, but pairs do not seem to spend the winter together, and
rather, only meet up at the rookery or colony where they last bred. For
Adelies, this is usually their natal breeding ground. It is unknown how
penguins are so well adept at finding their way back to these breeding grounds,
but it has been suggested that they are able to orient by the direction of the
sun. The males are the first to arrive and immediately begin building their
nest by smoothing out a crater in the ground and filling it with rocks,
feathers, and bones (Terraquest, 1995). Stone, or pebble, stealing is a
favorite pastime of Adelies and they will often take stones from their
neighbors' nests when they are not looking (Muller-Schwarze, 1984). After
males have established the nesting territory, they wait and try to locate their
mate and are usually able to recognize each other by their voices or cries
(Simpson, 1976). Pairs prefer to return to their old nesting site and each
other. Females may join a new male if they cannot find their mate at first, but
then leave for their original mate if and when he does show up at the colony.
If a female found her mate with another female, however, upon late arrival, she
would attack and drive the intruder away. (Penney, 1968)
For first time breeders, courtship lasts for about seven to twelve days and ends
with the female laying her eggs, usually sometime in November. When performing
courtship or mating rituals, male penguins will pump their chests, position
their head and flippers in an upright, erect posture, and produce a harsh, loud
braying sound that signals to other males to begin calling as well. Muller-Schwarze
(1984) noted that courtship behavior also includes the “ecstatic,” the “oblique
stare bow,” the “bill to axilla,” the “loud mutual” and the “quiet mutual”
displays. The male performs the “ecstatic” display, the female responds with
the “oblique stare bow” and the male typically then replies with the “bill to
axilla” display. The “loud” and “quiet” mutual displays are used when two birds
recognize each other from the previous year. Pair bonding can either be on a
trial basis, in which no offspring are produced from the copulation, or true
pairing, in which the two penguins actually raise offspring together (Muller-Schwarze,
1984). If birds come back together for repeated breeding seasons, they
are
considered reunited. If not, they are disunited or split, which happens when
one or both of the members of the pair dies. Reunited pairs, however, tend to
have higher breeding success as they are able to find each other relatively fast
and start breeding. If one becomes disunited or split, they may not be able to
find a new mate in time. (Penney, 1968)
Ainley (1978) studied a nesting population of Adelie penguins in Cape Crozier, Ross Island, Antarctica of non-breeding and breeding individuals to determine why a penguin may or may not breed. Typically, individuals at this site visited the breeding rookery at age two, but did not breed until three years old for the female, and four years old for the male. Some did not actually breed until they were five to seven years old. Some non-breeding males arrived just as early as the breeding males and should have been able to secure central territories within the colony, while all of the non-breeding females were tardy to arrive at the colony. As it turned out, it seemed as though the breeding males would become more active and social by displaying and defending their territory, as the number of days before pairing decreased, and thus increased their likelihood to pair. The males who would eventually breed maintained much higher quality nests, possibly revealing the amount of time they put into said nests. On the other hand, the eventual male non-breeders had significantly lower quality nests. It was assumed that more active males build better nests and thus are more able to attract females. In this particular study site, males greatly outnumbered females, meaning that there was a good deal of competition amongst males for mates. Thus, the only reason females probably did not mate was if they were too late in arriving at the colony. Non-breeding males, on the other hand, were considered to have just been inadequate competitors.
TRULY MONOGAMOUS?
Extra pair copulations, or EPC's, occur when a "monogamous" male or female believes they will enhance their reproductive success by engaging in them, without risking the loss of their other mate or offspring. In almost all seabird species, EPC’s are considered to be costly to the female because both parents are typically needed to successfully rear the young. Penguins particularly exemplify the need for biparental care as the female and male must share all parental duties, including incubation, so the other can forage at sea. By engaging in EPC’s, the female risks retaliation from her paired male if he suspects that he has been cuckolded by another male. However, it has been found that nearly 10% of female Adelie penguins engage in EPC’s. Even though EPC’s may occur, a female tends to copulate most frequently with her paired male, who will eventually help raise the chicks. In doing this, the male may be securing his paternity by being the last male to mate with her, and thus, EPC’s do not matter. (Pilastro et al., 2001)
Typical clutch size for pairs of Adelie penguins are two, but eggs suffer from
high predation rates. In a study by Pilastro et al. (2001), it was found that
10% of the Adelie offspring were not genetically related to the paired male who
provided parental care for them. It was also concluded that these were not
chicks that had wandered into the wrong nest, as they shared genetic markers
with the mother, just not the father. As it turned out,
about
21 – 32% of all females mated with more than one male within a single breeding
season, either through EPC’s or mate-switching. However, it appeared that
females traded copulations via EPC’s for extra stones for their nests. Thus,
females may simply be engaging in EPC’s to help gather nesting material and have
a bigger, better nest. For the males, it is difficult for them to prevent EPC’s
because they cannot leave the nest to defend her during the pre-laying stage as
the territories are saturated, and he would risk losing his spot (Pilastro et
al., 2001). Moreover, males may also enjoy the benefits of extra nesting
material acquired through EPC’s, as it improves the overall quality of the nest
and perhaps contributes to better fledglings. After the eggs have been laid,
the male also cannot leave to guard his female because he is responsible for
incubating the eggs, and predators would easily take advantage of an unguarded
nest. If one or both of those eggs should be lost, the female can lay more
eggs, so it is still worrisome to the paired male if his female engages in EPC's
(Simpson, 1976). Despite this, the male is more or less bound to the nest, as
ensuring the defense and raising of the eggs in the nest is his best chance of
passing on his genes.
COSTS OF BREEDING
Adelie penguins have many constraints on their reproductive abilities, including foraging endeavors, mode of transport, and extreme localities, so they must rely on stored fat reserves during the early parts of the breeding season for energy. Consequently, it is no surprise that an individual’s reproductive success is linked to how much body fat stores they have when they arrive at the colony for breeding season. All of these conditions have led to a highly synchronized breeding schedule that is relatively predictable. (Vleck and Vleck, 2002)
A study by Vleck
and Vleck (2002) found that from the time when males arrive until their first
attendance relief, they are usually forced to fast for an average of 37 days on
Torgersen Island, one location infamous for its breeding grounds. However, the
male has been known to last for up to 50 days of fasting before finally
abandoning the nest to go forage. Thus, it has been speculated that the
condition of the body may affect whether or not penguins choose to breed during
any given year.
In
part of the study, the body masses of the females that actually laid eggs were
considerably higher than those of the females who had not laid eggs and only
courted. It is suggested that females with lower body mass may not have
sufficient nutrients to lay eggs. Moreover, if an adult penguin is in poor
bodily condition, it will affect their ability to feed their chicks if they
choose to have any. Decreased chick feeding rates have been linked to poor body
condition of parents. Non-breeding penguins were also found to have had
significantly less body fat than those that did end up breeding.
Survival for Adelie penguins, which can live to be twenty years old, tends to decrease with each breeding incident, as breeding incurs great energy costs (Vleck and Vleck, 2002). Thus, it would seem to be a good strategy for an Adelie penguin to only breed when it is in optimal shape with good body fat stores. If they breed in sub-optimal body conditions, they may risk losing that year’s offspring, and may compromise future reproductive success.
WHY MONOGAMY?
Adelie penguins remain monogamous because both parents are needed to raise the offspring. In this way, Adelies are representative of the male assistance hypothesis of monogamy. Several of the above studies showed that one parent is not capable of providing enough food resources to the chicks and being able to sustain themselves as well. Since the pair returns to each other year after year, it is important that both remain healthy throughout the current breeding season, so they can meet back up and begin breeding with ease the next year after. The chicks require a great deal of care both in the egg stage and in the first few weeks of their lives. The parents must also forage during this time, so the switching on and off of parental duty is key to breeding success. Moreover, if one parent is forced to leave the nesting area, skuas will swoop down and eat the offspring.
