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Mating for Life Part 4: Why are Animals Monogamous?

on July 29, 2014
  • Why are animals monogamous?

As we’ve seen in previous posts (about birds, mammals and insects), monogamy is common in some parts of the animal kingdom and rare in others, but the way in which it is practised varies a great deal. But what are the advantages of being faithful to a mating partner? Generally speaking, it makes sense for animals to be promiscuous; mating with a lot of partners increases the chance of producing a large number of offspring as well as increasing the chance of having the opportunity to mate with the most desirable partner. However, monogamy does have some advantages, as demonstrated by many of the species we have already discussed.

Warthog Family

Warthogs (Phacochoerus africanus) are promiscuous animals; the males wander around large areas, mating with any females they come across. They sacrifice selectivity for increased chances of producing young every breeding season. Photo Credits.

Monogamy does have its advantages: Many animals, such as Azara’s night monkey males (Aotus azarae) invest a huge amount of time and energy into raising their young and so they are motivated to minimise the risk of accidentally raising another male’s offspring; they do this by having a system of mating for life and staying faithful to their partners, ensuring that their chosen female will birth only his offspring. Some animals practice ‘mate guarding‘ in order to secure reproductive success; once a male has mated with a female, he stands guard over her to ensure that no other males will also mate with her. This may not entirely qualify as monogamy, however, it may explain why partners stay together for at least as long the gestation period (until the offspring is born).

Many animals spend a lot of time and energy acquiring mates; performing elaborate displays and courtship rituals. This behaviour ensures that animals select the best mates possible (strongest, fastest, most agile etc.), it makes sense for animals to stick with their partner once they have been selected so that the process does not have to be repeated; they have a reliable partner who can provide offspring without having to waste time and energy each mating season. This is best exemplified by albatrosses (family: Diomedeidae) who invest the first 10 – 15 years of their life practicing and then attempting to secure mates; once they get one, they stick with them for life.

Black Browed Albatross (Thalassarche melanophrys)

Albatrosses, such as these black-browed albatross (Thalassarche melanophrys), mate for life, eliminating the need to perform mating rituals every breeding season. Photo Credits.

One significant advantage of mating for life, or at least being monogamous for a significant period of time is that young animals can have both parents around in their early life to help raise them. This system of rearing young is efficient as parents can take turns finding food while the other stays behind to protect the infant(s) and is best demonstrated by the emperor penguin (Aptenodytes forsteri) where parents take turns doing a 200km round trip to get food from the sea and return to the inland colony in Antarctica.

Emperor Penguin Family

Emperor penguins (Aptenodytes forsteri) are serially monogamous meaning they form very close social bonds with their partners, but pick new ones each breeding season. Photo Credits.

There have been many studies that have attempted to demonstrate and explain the origins of monogamy in animals. It has been suggested that infanticide (the killing of youngsters) caused the development of monogamy. In some species, such as lions (Panthera leo), when a male takes control over a group of females from another male, he will kill all of his predecessor’s cubs so that the females will become sexually receptive once again. Monogamy counteracts this by evolving a system where males will stay with their females long-term in order to ensure that their offspring grow up to independence successfully.

African Lions (Panthera leo)

Male lions (Panthera leo) practice infanticide; when taking over a new pride (group) they will kill the offspring of their predecessor to make the females receptive to mating again. Photo Credits.

There is also evidence that hormones can contribute to monogamy. A chemical called oxytocin is found in mammals (including humans (Homo sapiens)) and seems to be released into the body during moments such as child birth, copulation and suckling. Oxytocin might be released in animals’ bodies to promote proximity and bonding between individuals. In humans, it’s the chemical that makes you feel good when thinking about people you love.

As always, the animal kingdom is full of complexity and fascinating behaviour. Although most animals do not show faithful monogamy to a partner, the ones that do have some very good reasons to do so. I hope you enjoyed this series and learned something along the way!

Much love,

-Nick

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One response to “Mating for Life Part 4: Why are Animals Monogamous?

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