Social Monogamy in Geese
Social monogamy -- when two animals form a lifelong pair and rear offspring together but might not be sexually exclusive -- seems to offer geese the best of both worlds [source: Bryner]. Males spread their sperm by engaging in extra-pair copulations with the occasional female; females enhance the fitness and health of their brood by getting a sampling of sperm from a collection of males; and both sexes benefit from an indispensable partner that will help raise their young.
The monogamy practiced by geese helps to ensure their young will survive to adulthood. Young geese are highly susceptible to harm, as are most baby birds, and adults must spend weeks or months caring for their well-being in order to make sure they survive past infancy. Without the help of a mate, for instance, females wouldn't be able to replenish nutrient reserves to sustain their new brood.
Having two parents split domestic duties also significantly reduces the strain of protecting eggs and caring for hatchlings. Adult males staunchly defend the pair's nesting territory so the females can focus on finding food and incubating their eggs. In addition, pairs of geese have higher social rankings than those of unmated geese, so they gain access to better foraging opportunities and better nesting sites. As you've probably witnessed when feeding geese at a pond, these birds are highly competitive and it can be difficult for a single bird to acquire adequate resources on its own.
Entire families are regarded even higher on the social totem pole, so perhaps it's not surprising that the young of paired geese are also more successful than the offspring of a one-night stand. They're more likely to grow up to breed successfully, continuing their parents' lineage [source: Raveling]. Perhaps you could compare it to being the offspring of a well-paid movie star or head of state -- you're not guaranteed success, but you definitely get a head start on the competition.
Along with the above benefits, studies show that geese with strong familial bonds produce more offspring than those without such bonds. Due to the "mate familiarity effect," paired geese see their reproductive success increase for the first six to 11 years of the partnership [source: Black]. Biologists think this occurs because they're able to fine-tune their behaviors and coordinate their efforts to acquire optimal resources. Established pairings also share a familiarity with frequently visited sites, so they can draw on each other's expertise of the sites' associated predators, available food and nesting areas.
Further cementing the value of social monogamy in geese is the fact that their reproductive cycles occur in such a short time span because of the limited duration of available resources. Mating begins almost immediately upon finding good nesting sites, and the growth of the goslings is timed to occur at the same time that their food -- sedges and grasses -- grows. Having to waste time looking for a mate each season would cut into the small window of time that geese have to breed. The fact that most geese have established pair bonds allows them to begin nesting without delay.
For better, for worse; for richer, for poorer -- in a world where it's all about spreading one's seed as prolifically as possible into the next generation, these birds do whatever it takes to make sure their DNA lives on.
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