Mute swans are one of the most recognizable waterfowl species due to their large size, all white plumage, and orange bills. They are native to Eurasia but have also been introduced in North America and elsewhere. Mute swans form lifelong pair bonds and exhibit behaviors associated with monogamy such as mutual preening and defending their mate and territory. However, there is some debate over whether mute swans are truly monogamous or just mostly monogamous. In this article, we will explore the evidence for and against monogamy in mute swans.
What is monogamy?
Monogamy refers to a mating system where individuals form exclusive pair bonds and mate only with their partner. True monogamy is rare in the animal kingdom. Most monogamous species still exhibit some level of infidelity such as extra-pair copulations. Monogamy in birds typically serves to improve reproductive success through increased parental care and nest defense. Since both parents invest in raising offspring, monogamy helps ensure the male is caring for his own genetic offspring.
Pair bonding behaviors
Mute swans form strong pair bonds that can last for many years. Though some pairs may stay together for life, most bonds last around 5 years on average. Swan pairs engage in mutual behaviors that help maintain their bond such as synchronous head bobbing, mutual preening of head and neck feathers, and calling when separated. They also synchronize their movements when feeding, resting, and swimming together. These behaviors likely help reinforce the pair bond.
Territory defense
Mute swan pairs will aggressively defend their nesting and feeding territories against intruders. One member of the pair will threaten or attack while the mate provides backup support. Their territory ranges from 2-12 acres in size depending on food availability. Mute swans will threaten any animal entering their territory including other swans, geese, ducks, and humans. This mutual territory defense helps protect resources and mates.
Nest building and egg incubation
Mute swans build large mounds of aquatic vegetation in shallow water for their nests. The male and female contribute equally to nest building. Once complete, the female lays 4-8 creamy white eggs and both parents take turns incubating them for about 36 days until they hatch. The male closely guards the nest while the female incubates and takes over incubating to allow her to feed. This shared nest care provides further evidence of the strong pair bond.
Parental care
Mute swan pairs work together to raise their cygnets (babies). The female broods and feeds the cygnets for the first few weeks after hatching. Both parents then escort the cygnets on the water and share food with them by allowing the cygnets to reach into their throat pouch and take food from them. The cygnets remain with their parents for nearly a full year until the next breeding season. This extended parental care of offspring requires commitment from both parents.
Evidence against strict monogamy
While mute swans exhibit very strong pair bonding and parental behaviors, there is some infidelity that may contradict strict monogamy.
Divorce and re-pairing
Though many swan pairs remain bonded for years or life, about 25% get “divorced” and find new mates. Usually the male will leave the female and offspring and may even attack them to drive them away before finding a new mate. Females may also abandon males and cygnets. This demonstrates their bonds can change over time.
Temporary infidelity
In some cases, a mute swan may temporarily mate with another swan other than its partner for one breeding season. They may reunite with their original mate the next year. DNA analyses have shown cygnets do not always match their social father, evidencing some infidelity occurs.
Rape
Extra-pair mating is sometimes forced. Male mute swans have been observed sexually pursuing and coercing females other than their mate to copulate. Even paired females near their nests may be harassed by persistent unmated males. These rapes contradict monogamous behaviors.
Polygyny
Rarely, some male mute swans may have multiple simultaneous mates. They may drive off an original mate and cygnets and then pair with new females to potentially have more offspring. This polygyny again defies monogamy.
How common is infidelity?
Despite the rare behaviors challenging strict monogamy, most evidence suggests the majority of mute swan pairs remain faithfully bonded each season.
One study in Sweden found only 16% of cygnets did not match their social father using DNA fingerprinting, suggesting 84% did result from monogamous mating.
Other research in England showed 7% of mute swan broods had extra-pair cygnets.
So while infidelity does occur at low rates, most mute swan pairs do mate monogamously based on available paternity data.
Conclusion
The preponderance of evidence suggests mute swans predominantly display monogamous behaviors and breeding patterns. Their strong pair bonding, synchronized behaviors, joint territory defense, shared nesting duties, and biparental care of cygnets demonstrate a high level of commitment between mates. These monogamous behaviors likely evolved to improve reproductive success.
However, mute swans are not 100% sexually exclusive based on documented instances of divorce, temporary infidelity, rape behaviors, and rare polygyny. These rare extramarital copulations indicate they may not meet the strictest definition of true monogamy in which pairs mate exclusively.
Thus, the most accurate way to characterize mute swan mating patterns is mostly monogamous or predominantly monogamous rather than purely monogamous. Their social and sexual behaviors strongly favor monogamy with only infrequent exceptions. Overall the mute swan’s high degree of monogamy is quite unique among birds and waterfowl. Their lifelong bonds and synchronous behaviors provide a classic example of monogamous pairing in the animal kingdom.
Behavior | Evidence for Monogamy | Evidence against Strict Monogamy |
---|---|---|
Pair bonding | – Long-term bonds lasting years or life | – About 25% of pairs “divorce” |
Synchronized behaviors | – Synchronous head bobbing, swimming, feeding | – None known |
Territory defense | – Joint defense of feeding and nesting territories | – None known |
Nest building | – Shared contribution of materials | – None known |
Egg incubation | – Both parents incubate eggs | – None known |
Parental care | – Both parents feed and escort cygnets | – One parent may abandon offspring |
Infidelity | – Unknown | – 16% of cygnets unmatched to social father in one study |
Polygyny | – Unknown | – Rare instances of males with multiple mates documented |
In summary, the preponderance of evidence supports predominantly monogamous pairing in mute swans, with occasional exceptions. Their strong bonds and synchronized behaviors provide a classic example of avian monogamy. Though they fail tests of 100% sexual exclusivity, genetic analyses confirm the large majority of mute swan pairs remain faithful each breeding season. Mute swans are rightfully considered one of the most monogamous waterfowl species, albeit with rare infidelity in some pairs.