- There are many different mating systems and social systems in the animal kingdom.
- Monogamy is when one male mates with one female in a breeding season.
- There are benefits and costs to a monogamous mating system.
- Monogamy exists in all vertebrate classes, but is rare in fish, reptiles, and amphibians.
In humans today, many cultures are monogamous, and many cultures are not. In ancient human societies and in prehistoric human species, mating with multiple partners was commonplace. For the animal kingdom, monogamy exists as well. In fact, it can be found in all five major vertebrate classes – mammals, fish, amphibians, reptiles, and birds. Mated pair-bonds may seem cute or endearing, but from a scientific perspective, they are not very straightforward. This article will investigate why monogamy may be helpful to a species and why it can also be a handicap. Then, we will explore five animals, one from each vertebrate class, that is monogamous.
Animal Mating Systems
In the animal kingdom, many species have different social organizations that are optimal for their ecosystems. These social organizations come with various costs and benefits related to important factors of survival. Social systems have consequences that affect feeding competition, mate availability, and vulnerability to predation. Therefore, depending on different contextual factors like resource density, predator population density, the duration of a species reproductive cycle and many more, different social and mating systems are more fitting for different species. Other than monogamy, the social systems common in the animal kingdom are polygyny, polyandry, polygynandry, and solitude.
Polygyny is a common mating system that many species employ. This is when males of one species mate with multiple females. An example of a polygynous species is the gorilla! One dominant male gorilla provides protection to a harem of females and in return, he has access to them as mates. Each female in the harem only mates with one male, but the male mates with multiple females.
Polyandry is the opposite of polygyny. In polyandrous species, one female mates with multiple males. An example of polyandry in the animal kingdom is the African jacana! The African jacana is a polyandrous bird species that lives throughout sub-Saharan Africa. The female mates with a male and lays an egg. Then, the male guards and incubates the egg while the female leaves to mate with another male.
Polygynandry is essentially a combination of polygyny and polygynandry. In a polygynandrous species, females mate with multiple males and males mate with multiple females. One polygynandrous species is the olive baboon. In this species, to maximize reproductive success females and males mate with multiple partners.
Solitary animals live most of their lives outside of breeding season alone. An example of a solitary species is the polar bear. Infants and sub-adults live together with their mother, but once they reach sexual maturity, they leave their families. Solitude is advantageous in this context because of resource scarcity in the Arctic.
What is Monogamy?
Monogamy describes a mating system where a single female mates with a single male. In these social units, both sexes usually invest in the offspring. Monogamous pair-bonds often live together and rear young together outside of breeding season when the female is not receptive. Social monogamy is most common in birds and exists in 90% of species. In mammals, however, it is less common and exists in only 3 to 5% of species. It is even less common in fish and is restricted mostly to the tropics and subtropics. Monogamy is incredibly rare in reptiles and amphibians.
What Are the Advantages and Disadvantages?
There are many contexts where monogamy can be beneficial to the survival of a species. If populations are small or are dispersed over a large area, it may be advantageous to find one mate and stick to them because finding multiple mates will be unlikely. Also, in species that produce vulnerable or highly dependent young, monogamy would allow for the consistent presence of two investing parents. In polygyny and polygynandry, females often carry the burden of rearing young. Monogamy is advantageous to species that require high investment in their offspring because two parents will be present. Two parents can more effectively provide protection, collect more food, maintain better nests, and raise more vulnerable young.
A notable disadvantage of monogamy is that it limits reproductive opportunities to the times when one individual is reproductively receptive. In chimpanzees, males optimize their reproductive success by mating with as many females as possible so when one female is pregnant, the male will mate with others who are not. For monogamous animals, there is no reproductive opportunity available during pregnancy. In many monogamous animals, however, there are occasional “extra-pair” interactions where a male will sneak a copulation with a different female outside of his mated pair.
There are many other costs and benefits of monogamy depending on an animal’s ecosystem and related biological factors. The following five animals, however, have put it into practice! Let’s introduce five animals, one from each vertebrate class, that are monogamous.
Pygmy Marmoset (Mammal)
An example of a mammal that is monogamous is the pygmy marmoset. Marmosets are New World monkeys native to South America. There are 22 species of marmosets that live in Bolivia, Brazil, Colombia, Ecuador, Paraguay, and Peru. Two species of pygmy marmosets live in the rainforests of the Amazon Basin, the western pygmy marmoset and the eastern pygmy marmoset. These little monkeys are typically 4.6-6 inches long, not including their tail, and weigh around 3.5 ounces. Social units consist of a mated-pair and their offspring, which are always born in sets of twins. The mated-pair stay together for life.
Throughout the animal kingdom, species that have high levels of reproductive competition are often sexually dimorphic. Sexual dimorphism is when males and females have distinctly different physical features, like a peacock’s tail or a male gorilla’s body size. Sexual dimorphism is very common in monkeys and apes but does not appear in marmosets. This is because monogamous species don’t face the pressures of sexual selection to the same extent that polygynous species with greater competition do. Males don’t have to prove to females that they are the strongest or most attractive suitor before every reproductive opportunity to win a mate, so they did not evolve energetically expensive, flashy traits.
The next monogamous animal we will discover is the penguin, however there are some exceptions. Many species of penguins are monogamous and mate for life, including the macaroni penguin. Some penguins, however, are “serially monogamous”, like the emperor penguin. The emperor penguin is loyal to one mate each year, but only 15% of those pairs stick together for multiple years. Similarly, the gentoo penguin is monogamous and has up to three breeding seasons, but after, they usually find a new mate. Also, in some penguin species males might remain faithful, while females might have one to three partners in a season.
For penguins, males and females both need to invest in their offspring to ensure their survival in harsh conditions. Male macaroni penguins look after their chicks after they hatch while the female hunts, for example. In emperor penguins, the female transfers her egg to the male after it has been laid and he must incubate it while she returns to the sea to feed. Another interesting behavior seen in penguins is the performance of “ecstatic displays”. These dances are done when a penguin sees their mate or when they are trying to attract a mate. The displays vary based on species, but they often include a loud call, extending the neck, and waving the head around.
Convict Cichlid (Fish)
Convict cichlids are a species of monogamous fish that earn their name from their black and white stripe pattern. Similar to emperor penguins, convict cichlids are serially monogamous. They most often stick with one mate each breeding season and then find a new mate for the next breeding season. Only rarely do they mate with the same partner for multiple seasons. These fish are native to Central America and live in freshwater lakes and streams. Feral populations also exist outside of their natural range in Australia, Japan, Mexico, Colombia, Taiwan, and the USA.
Monogamy is very rare in fish. Cichlids are unique because they have an unusually long period of parental care of their young and they are highly territorial. In territorial species, monogamous pairs can more effectively defend their territory from invaders. Monogamy is also helpful to a species that requires greater parental investment because then the work can be shared by two parents. In convict cichlids, each parent has specific tasks relating to parental care and they cooperate to complete them.
Blue-tongued Lizard (Reptile)
The next monogamous animal is the blue-tongued lizard, or blue-tongued skink. These lizards belong to the skink family, Scincidae, one of the most diverse families of lizards. Blue-tongued skinks are becoming increasingly popular as pets and have a reputation of being non-aggressive, easy to handle, and more docile than some of their skink relatives. They also are known to often form close bonds with their owners.
Blue-tongued skinks are primarily monogamous animals, which is incredibly rare in reptiles. Before mating, a pair will spend 6 to 8 weeks together. During this time, they foster a bond by travelling everywhere together and living together before reproducing. After mating, the pair separates, and gestation occurs over the following 5 months. The mother births and raises the offspring in her own territory, and then the mated pair will reunite the following year.
Mimic Poison Frog (Amphibian)
The final monogamous animal we will explore is the mimic poison frog. Monogamy is so rare in amphibians that the mimic poison frog is actually the very first amphibious species found to be monogamous! A paper in 2013 was published outlining how scientists in Peru confirmed the species to be the first monogamous amphibian. The researchers discovered that 11 of 12 pairs they monitored remained faithful to their partners and the primary factors influencing this novel behavior were the use of small breeding pools and intensive parental care including egg feeding.
Monogamy in this species is not well understood because of how scarce the behavior is throughout the amphibian class. For the closely related poison dart frog, after a female lays her eggs and a male fertilizes them, the female leaves and the tadpoles are left to fend for themselves. In mimic poison frogs, however, males guard the pools of tadpoles and females repeatedly return to deposit nutrient eggs for the tadpoles to eat. This is one of the only examples of biparental care in amphibians on record. The nutrient availability in the small pools and the highly dependent offspring are two important factors as to why this species is monogamous when scientists know of no other monogamous amphibians.
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- Wikipedia, Available here: https://en.wikipedia.org/wiki/Callitrichidae
- Wikipedia, Available here: https://en.wikipedia.org/wiki/Marmoset
- Wikipedia, Available here: https://en.wikipedia.org/wiki/Mimic_poison_frog
- Wikipedia, Available here: https://en.wikipedia.org/wiki/Macaroni_penguin
- Wikipedia, Available here: https://en.wikipedia.org/wiki/Emperor_penguin
- Wikipedia, Available here: https://en.wikipedia.org/wiki/Convict_cichlid
- Wikipedia, Available here: https://en.wikipedia.org/wiki/Monogamy_in_animals
- Wikipedia, Available here: https://en.wikipedia.org/wiki/Blue-tongued_skink
- Stephen M. Shuster; https://doi.org/10.1073/pnas.0901132106 (2009) Sexual Selection and Mating Systems
- Nancy Burley; https://doi.org/10.1086/283732 (1981) Mate Choice by Multiple Criteria in a Monogamous Species
- GA Schuiling; doi: 10.3109/01674820309042802 (2023) The benefit and the doubt: why monogamy?
- National Science Foundation, Available here: https://beta.nsf.gov/news/animal-attraction-many-forms-monogamy-animal