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Mating for Life Part 4: Why are Animals Monogamous?

  • 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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Mating for Life Part 3: Monogamy in Insects

We’ve seen in part 1 (birds) and part 2 (mammals) of this series that there is a lot of variation in monogamy in the animal kingdom. It is worth mentioning one more group of animals before we carry on.

  • Do insects mate for life?

In the invertebrate world, there is a countless variety of mating techniques, but even amidst this variety there are some examples of monogamy and mating for life. Termites (infraorder: Isoptera) are colonial animals where a single queen produces all the offspring that then grow up to service the collective. A termite queen constantly produces offspring, most of which become workers and soldiers that serve the colony, but each year, the queen will produce a generation of breeding individuals. These ‘breeders’, called alates, are males and females with wings that fly away from the home colony to found their own. Females breed and then head underground where they will stay for the rest of their lives producing young. In many species of termite, a queen will keep a single male at her side, a king, who will mate with her throughout her life and he will be her sole partner and father to the entire colony.

Termites surrounding a queen

A termite queen is a huge, egg producing machine (she is the blob in the middle of this picture); all termites in her colony are her offspring but sometimes she is accompanied by a king for her entire life. Photo Credits.

Many species of insect, such as mosquitoes (family: Culicidae) are monogamous because they do not live long enough to be anything else! They hatch from their eggs as larva and spend most of their lives in a larval form. Eventually, the larvae transform into their adult form and emerge, they will then seek to find a single mate with whom they will reproduce and then die. The adult stage of many insects’ lives are dedicated to the sole purpose of producing a single batch of eggs. You could argue whether this counts as monogamous or is just the force of circumstance.

Mosquito

Mosquitos are monogamous mostly because they die not long after they mate for the first time! Photo Credits.

The invertebrates make up the majority of animals on Earth and there is of course a lot of variation within their behaviours, however, monogamy and mating for life is very much uncommon.

In the next post, we will look more at the advantages of monogamy and faithfulness to a mating partner and some explanations as to why some animals have adopted this mating technique.

Much love,

-Nick

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Mating for Life Part 2: Monogamy in Mammals

In the previous post we saw how birds are, generally speaking, the most faithful animals to their mating partners, but which other animals are monogamous? Although loyalty to a partner is reasonably common (although varied) in the bird world, it is far less common elsewhere, for example, only 3% of mammal species show any sort of monogamy.

  • Which mammals mate for life?

One mammal species that was thought to have mated for life was the siamang (Symphalangus syndactylus), along with some other species of gibbon. Intensive research has shown that they actually practice something called ‘social monogamy‘ as opposed to sexual monogamy, in other words, they are swingers (and not just from tree to tree)! Siamangs pair off and form close social bonds with their partners, often spending their whole lives together and raising families together, however, they quite frequently will mate with other individuals they meet during their routine travels around their forest home, apparently with no consequence.

Siamangs in Duet

Siamangs (Symphalangus syndactylus) form lifelong partnerships which are reinforced by ‘singing’ together, sometimes accompanied by the rest of the family. Photo Credits.

The prairie vole (Microtus ochrogaster) is a small North American rodent that is commonly studied for its monogamous behaviour. These voles do indeed seem to mate for life, living together, sharing household duties and responsibility for the offspring, however, they too cannot seem to resist the temptation to mate with desirable individuals that seem to come their way.

Family of Prairie Voles

Prairie voles (Microtus ochrogaster) are monogamous and share parental responsibilities. Photo Credits.

The mammal that wins the title of ‘most faithful to their partner‘ is Azara’s night monkey (Aotus azarae); this small South American primate mates for life and appears to never stray. An 18 year long term genetic study of night monkey infants found that all the youngsters were the offspring of both parents, none of them were step-siblings meaning that the parents never mated (or at least conceived) with any other partners.

Azara's Night Monkey

Azara’s night monkey (Aotus azarae) are nocturnal and so hard to photograph well, but lengthy genetic studies on this species have show that they are perhaps the mammals which are most faithful to their partners. Photo Credits.

Many mammals have complex social systems and it can be hard to label or categorise a species’ mating technique. Male lions (Panthera leo), for example, will take control of a pride and then will be the sole mating partner for each of the females within that pride, however, he is not exclusive to any one of them; the male is not faithful to one partner whilst the female is but they are all socially bonded none the less. Other mammals have a form of social structure that is dependant on monogamy such as African wild dogs (Lycaon pictus) and meerkats (Suricata suricatta) where social order is kept by an alpha couple; a male and female who are partners for life and the only members of the group who are permitted to reproduce.

African Wild Dogs

Groups of African wild dogs (Lycaon pictus) have an alpha pair who are the only two permitted to breed, the rest of the group cooperatively raise the young. Photo Credits.

Although monogamy is rare in mammals, there are a few notable examples as we have seen. However, animal behaviour is rarely ‘black and white’ and though some animals stick with a partner, they are not always entirely faithful. In the next post, we will look at invertebrates which practice some form of monogamy.

Much love,

-Nick

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