Why do geese mate for life?


Love at first sight. See more pictures of birds.
Digital Zoo/Digital Vision/Getty Images

For every 100 marriages in Iceland, there are 31 divorces; in Belgium, the divorce rate is almost 60 percent; and for every two marriages in the United States, another one falls apart [source: CDC, NationMaster.com]. Perhaps humans could learn a thing or two from the birds on this one: More than 90 percent of all birds are monogamous, meaning they maintain an essentially exclusive relationship, or pair bond, with just one member of the opposite sex [source: Gill].

Geese are especially fastidious when it comes to their loyalty. They're well known for the long-term pair bonds they form. So what's their secret? What makes these water birds so successful in their relationships, and why would geese mate for life?

Advertisement

As nice as it would be to attribute the birds' success to their undying love for one another, it's probably just pure and simple selfishness. In his Second Edition of Ornithology, Frank Gill concludes that monogamous relationships may simply be "a temporary truce between selfish, competitive individuals" rather than a "spirited, cooperative partnership."

The reason for this is relatively straightforward -- mating systems are generally determined by whichever arrangement maximizes the participants' breeding opportunities. In the case of monogamy, a relatively exclusive relationship appears to be the optimal arrangement for both sexes.

Just like human relationships, avian pairings involve a series of sacrifices and tradeoffs. Although the gander may be tempted to stray and arrange a few trysts on the side, doing so would leave his own partner vulnerable to other males. Such adulterous behavior could also incite his partner to either desert him or avenge herself by having a fling of her own -- thus diluting his paternal contribution to the upcoming brood [source: Gill].

In addition, time spent flirting with other geese takes away from the important task of helping to care for the young. Similarly, females tempted to stray risk alienating their mates and being forced to raise the brood alone. Unlike other animals such as reptiles, adult birds don't just take off once the eggs are laid.

This isn't to say, however, that geese -- or birds, for that matter -- are as virtuous as they may seem. DNA testing has shown that in some cases, male birds unknowingly raise young that aren't even related to them, up to 40 percent of the chicks [source: Holladay]. These studies have shown that a large percentage of "monogamous" birds occasionally engage in extra-pair copulations. Essentially, socially monogamous animals create strong domestic partnerships with one mate but aren't always faithful.

Social Monogamy in Geese

One big happy family
One big happy family
Duncan Usher/Foto Natura/Minden Pictures/Getty Images

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.

Advertisement

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.

Related HowStuffWorks Articles

 

More Great Links

 

Sources

  • Akesson, Thomas R and Raveling, Dennis G. "Behaviors Associated with Seasonal Reproduction and Long-Term Monogamy in Canada Geese." The Condor. Vol. 84, no. 2. 1982. (Aug. 22, 2008)http://elibrary.unm.edu/sora/Condor/files/issues/v084n02/p0188-p0196.pdf
  • Barash, David P. "Deflating the Myth of Monogamy." Trinity University. (Aug. 25, 2008)http://www.trinity.edu/rnadeau/FYS/Barash%20on%20monogamy.htm
  • Black, Jeffrey M. "Fitness consequences of long-term pair bonds in barnacle geese: monogamy in the extreme." Behavioral Ecology. Vol. 12, no. 5. 2001.
  • Bryner, Jeanna. "Are Humans Meant to be Monogamous?" Life's Little Mysteries. LifeScience. (Sept. 10, 2008)http://www.livescience.com/mysteries/080319-llm-monogamy.html
  • Gill, Frank B. "Ornithology." New York: W.H. Freeman and Company. 1990.
  • Holladay, April. "Philandering animals, fast spins, faster circumnavigations." USA Today. Feb. 25, 2005. (Aug. 28, 2008)http://www.usatoday.com/tech/columnist/aprilholladay/2005-02-25- wonderquest_x.htm
  • "National Vital Statistics Report." CDC. Vol. 54, no. 20. July 21, 2006. (Aug. 28, 2008)http://www.cdc.gov/nchs/data/nvsr/nvsr54/nvsr54_20.pdf
  • "People Statistics: Divorces per 100 marriages (most recent) by country." NationMaster.com. 2000. (Aug. 28, 2008)http://www.nationmaster.com/graph/peo_div_per_100_mar-people-divorces-per- 100-marriages
  • Raveling, Dennis G., et al. "Reproductive success and survival in relation to experience during the first two years in Canada Geese." The Condor. Vol. 102, no. 4. Nov. 2000.
  • Sinervo, Barry. "Mating Systems and Parental Care." UCSC Department of Ecology and Evolutionary Biology. 1997. (Aug. 22, 2008)http://bio.research.ucsc.edu/~barrylab/classes/animal_behavior/MATESYS.HTM