The Evolution of Love

Post provided by Chloe Robinson

The sending of letters under the pen name ‘St. Valentine’ began back in the middle ages as a way of communicating affection during the practice of courting. Fast forward to 2020 and Valentine’s Day is a day for celebrating romance, but now it typically features the exchange of gifts and cards between lovers.

Credit: Pixabay

As humans, we have a grasp on the concept of love. While this can be represented in different forms (i.e. love for a pet, a family member, a partner), we’re confident of love when we see or experience it. Love is deep-rooted, in the sense it causes changes to our very biology. But finding the basis of love has stumped scientists for years. Previously, the basis of love was thought to have originated in genetics and/or psychology, but new research points towards love having evolved for reproductive success (as measured in number of children produced).

In the animal kingdom, ‘love’ (in the traditional sense) is impossible to confirm. We see animals courting, competing for the opposite sex, forming bonds (sometimes lifelong) and producing offspring, but can we call this behaviour romantic love? There’s evidence that a range of animal species are capable of experiencing a similar array of emotions to us, but we can never be sure that this is romantic love. In addition to love, parental care and investment looks very different in the animal kingdom. In this blog post, we’re going to explore some of the methods for investigating animal mating and parental care systems through the lens of human emotions.

Costs of Mating: Female Choice

In animal populations, mate choice is a key element which forms the basis of many mating systems. Females are often much pickier when choosing their mate compared to males, due to the higher energy investment females put into sexual reproduction. As well as the physical energy costs of producing offspring, mating can be a costly event to females in terms of risk of aggression or disease transmission, which can overall cause harm to the individual and the likelihood of reproductive success.

Female mallard ducks often experience aggression from male mates during the mating season. Credit: Richard Bartz.

When humans choose a partner, often factors such as wealth, age, proximity and, of course, physical appearance strongly influence the likelihood of a relationship developing initially. Despite being a trivial factor for some, the science of physical attraction is actually highly complex and deep-rooted in our genetics. The term assortative mating (as previously described in ‘A statistical methodology for estimating assortative mating for phenotypic traits that are labile or measured with error‘) refers to a form of sexual selection where humans with similar physical characteristics have a higher likelihood of forming a relationship and mating than is expected by chance. This is not unique to humans. Individuals of brightly coloured bird species pair with other brightly coloured birds, and duller coloured birds of the same species are seen to pair up.

Mating Systems in Animal and Human Populations

Often, when we think of human relationships, we’re referring to social monogamy, which is the exclusive pairing of a single individual with another individual. But in animal populations, we can typically see many different forms of mating system.

In addition to monogamy, there are polygyny, polyandry, polygynandry and promiscuity mating systems. Polygyny typically refers to one male with multiple females, which is commonly seen in mammals as a way to maximise male reproductive success. Polyandry is a group with one female and many males, which helps to ensure female reproductive success. Thirdly, polygynandry is a mating system with very loose male-female bonds, where multiple males and females mate with each other and share the care of broods. This is commonly seen in chimpanzees.

Snowshoe hare. Credit: NPS.

Promiscuity is a system which, as humans, often has a different meaning. In the biological sense, a promiscuous mating system is one which has no pair bonds and mating occurs almost randomly. This system is often seen in species where environmental conditions are unpredictable, such as the snowshoe hare, which undergo dramatic population cycles. In humans, promiscuity is considered more in a social context, referring to the behaviour exhibited without the intent for sexual reproduction.

Compared to other vertebrate species, which often only have one mating system, humans are much more flexible. Existing mating systems in humans have been devised to describe systems of marriage, and despite the most represented system in human culture being monogamy, one estimate states that 83% of human societies are polygynous, 0.05% are polyandrous, and the rest are monogamous.

Parental Investment and Care

In many species, females are more likely to be the sex that provides the main parental investment and care to offspring. This investment can range from the internal biological energy invested to produce a gamete to the act of physically guarding territories and directly caring for offspring. Depending on the species and mating system at play, biparental (e.g. burying beetle) or paternal care (e.g. Californian mouse) is also observed.

Burying beetles exhibit biparental care. Credit: Syuan-Jyun Sun.

In many species, the parents influence their offspring both genetically (through the passing on of genes) and non-genetically (through offspring care). This combination of genetic and non-genetic influence can determine the fitness and survival of offspring. In ‘Measuring selection when parents and offspring interact‘, Thomson and Hadfield looked at evaluating a mixed fitness measure. It was comprised of parental fecundity (i.e. number of offspring produced) and offspring survival as a method for estimating phenotypic selection (i.e. when individuals with certain characteristics produce more surviving offspring than individuals with other characteristics). They determined that mixed fitness measures can give misleading results on phenotypic selection and looking at number of offspring produced is a more accurate measure.

Population Genetics and Estimating Abundance

Brook trout outside the Fluvarium in Nagle’s Hill Brook. Credit: JaySo83.

As highlighted in Thomson and Hadfield’s contribution to the How to Measure Natural Selection Special Feature, it can be difficult to know the number of wild offspring which have survived and successfully made it to adulthood. Determining population abundance, particularly in terms of population growth, has always been difficult in ecology and conservation, particularly for aquatic species.

But, the genetic transfer from parents to offspring can enable us to ‘recapture’ the genetic material of the parents when sampling offspring, to generate a better picture of population size. A Methods in Ecology and Evolution study investigated how to use close‐kin mark-recapture (CKMR) to identify the number of parent-offspring pairs (POPs) in seven brook trout Salvelinus fontinalis populations. They found the CKMR method to be an appropriate method for estimating population size for populations that have a thorough mixing of individuals (i.e. for fish).

Brown bear cub. Credit: Pixabay.

Understanding how genetic information is shared between parents and offspring across a wide spatial scale is important when referring to management and conservation of species. In ‘Multi‐level patterns in population genetics‘ Schregel et al. developed a statistical model to describe the spatial genetic patterns of Norwegian and Swedish brown bear populations. In this study, they identified barriers and corridors for genetic exchange and determined that bear home range is likely responsible for the existing genetic patterns of brown bears.

Summary: Love is All Around

As you can see, our idea of ‘love’ in the animal kingdom is a complex and costly process. The traditional sense of ‘love’ can take on many forms, in terms of mating systems, parental care and the influence of parents on offspring survival. To better understand these dynamic and variable systems, the continuing development of methods to measure processes such as parental investment and genetic exchange is much needed.

To read more about the statistical method developed for estimating assortative mating, visit the Methods in Ecology and Evolution article ‘A statistical methodology for estimating assortative mating for phenotypic traits that are labile or measured with error’

To learn more about selection when parents and offspring interact, visit the Methods in Ecology and Evolution article ‘Measuring selection when parents and offspring interact’

To find out more about parental condition-transfer, visit the Methods in Ecology and Evolution article ‘What are parental condition‐transfer effects and how can they be detected?’

For more on close-kin mark recapture methods, visit the Methods in Ecology and Evolution article ‘Validation of close‐kin mark–recapture (CKMR) methods for estimating population abundance’

To read more about the use of variograms for detecting population genetic structure of brown bears, visit the Methods in Ecology and Evolution article ‘Multi‐level patterns in population genetics: Variogram series detects a hidden isolation‐by‐distance‐dominated structure of Scandinavian brown bears Ursus arctos.

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