Love is in the air! It’s that time of year… It’s February and as we humans are planning outings with our sweethearts the animal kingdom is starting to buzz as well. While squirrels and birds may not be heading out to the newest restaurant, they are engaging in unique courtship rituals. Although not exactly romantic, these mating systems and preferences can tell us a lot about a species.
Animal Love 101
First, readers must separate their ideas of human practices from what animals do. In many cases, they are not at all the same, and so people are often surprised at what goes on in the animal kingdom.
There are some basic rules and definitions that apply to animals and mating. The goal of most animals’ lives is to reproduce and for their offspring to successfully make it to adulthood so that they can reproduce. Biologists thus refer to an animal’s “fitness” as how many offspring they produce and how well-equipped their offspring are to in turn to produce their own. This is important to understand because often animals make mating choices in an attempt to maximize their own fitness.
A basic rule that applies broadly to animal mating is Bateman’s Principle. Bateman’s principle states that females are often the “choosier” sex – having their pick of available males – because females cannot produce as many young as males (who can easily impregnate multiple females). This choosiness on the part of the females means that male reproductive success can be quite variable – some males may be able to mate very frequently while some may have very few mating opportunities, or none at all. This leads to often high levels of male-male competition for female attention.
There are many exceptions to Bateman’s Principle, but it is very broadly applicable, and so for the rest of the article I will discuss most mating systems in the context of choosy females and competitive males, unless otherwise specified.
How to Impress the Ladies
There are many models of mate choice, or explanations of specific ways that animals choose mates. One is referred to as the Direct Benefits model. In this case, females choose males based on what they can provide for her – usually something like how much food they can bring or how much protection they can offer. An example of this that occurs in many insect species is called “nuptial gifts,” in which males bring pieces of food to females and mate with them while the females consume the food.
Another model of mate choice is the Good Genes model, in which females choose males with the genes best suited to their particular environment, in hopes that their offspring will inherit these traits. There are a number of ways that females can determine male genetic components: smells, colorations, etc. Barn owls have been thought to do this – biologists suspect that males are attracted to females with more spots, because higher numbers of spots indicates immunity to a number of diseases.
A third mate choice model is Runaway Sexual Selection. This is an interesting occurrence in which males possess a certain physical characteristic that females prefer (tail length, colors, etc.). Females prefer the males with the most extreme of this characteristic (the longest tail, the brightest blue, etc.) The trait itself gets passed down from father to son, and the preference for the trait gets passed down from mother to daughter, until the trait slowly becomes so extreme that it hinders a male’s physical fitness (tails that inhibit efficient flight, etc.).
A final mate choice model is called Sensory Bias. This occurs when females have a pre-existing aptitude to notice a certain trait, and so male characteristics evolve to exploit this. For example, certain guppy species evolved to be very good at spotting the color orange, because their only food sources were orange in color. As this occurred, males also evolved to have orange spots, because they knew that females could better see them with orange on their bodies.
So You’ve Found the One for You
So there are clearly many ways that animals choose mates, but the way that they interact with mates once they have been chosen is a whole other story. There are four main animal mating systems: monogamy, polygyny, polyandry, and promiscuity. The first, monogamy, is fairly self-explanatory; one male mates with one female. There are two types: serial monogamy, in which individuals stay with one mate for the duration of a breeding season but find a new mate at the beginning of the next breeding season, and lifetime monogamy, in which individuals stay with one mate for their entire lives. Usually in monogamous pairings, partners form “pair bonds,” which involves a lot of complex brain chemistry compelling the members of the pair bond to fiercely defend their partner. In cases of serial monogamy, the pair bond is temporary and fades at the end of the season.
Many monogamous pairs are not exactly “faithful.” While a pair of animals may be socially monogamous – they are pair-bonded with each other and may or may not raise young together – frequently animals will partake in extra-pair copulations, or EPCs. This is a fancy phrase for an individual who mates with another individual outside of their pair bond. This increases the fitness for both individuals and thus benefits them, but not all monogamous pairs partake in EPC. Researchers are not sure why some male animals would choose to be mated for their entire lives when they could instead increase their own fitness by inseminating many females. Some biologists have theorized that males want to have a part in raising the young, to ensure that they will survive to pass on their genes.
The next two systems, polygyny and polyandry, involve one individual mating with multiple members of the opposite sex. Polygyny occurs when one male mates with multiple females – a fairly common occurrence. There are many theories as to how this evolved, but most studies point towards resource availability. By “resources,” biologists are referring to things like food and habitats – things that animals need to survive. Because females are often choosier and males often compete for females, male fitness depends on the number of available females, while female fitness depends on the resources she is able to procure. So in an area where resources are spread out evenly, females are also spread out evenly, and monogamy tends to occur because males can only easily defend one female at a time. But in a location where resources are clumped in one area with gaps in between, females are all grouped at these clumps close together and it is easier for males to defend multiple females at once.
Polyandry occurs when one female mates with multiple males at a time. It has been documented in many birds as well as in relatives of the seahorse. It is not as common as polygyny because it rather works against Bateman’s Principle, but when it occurs there are often some very interesting sex-role reversals that occur, such as females competing for male attention, and females developing the flashy secondary sexual characteristics that are so often seen in males of other species (think bright colors).
The final mating system is a “promiscuous” mating system, which is when polyandry and polygyny occur in the same population at the same time. The name is not a value judgment, it’s just a way to describe it. There are two forms: in one, both males and females mate with many partners and no pair bonds are formed. In the other type (called polygynandry), several males and several females form pair bonds simultaneously.
So while animals may not be the most romantic types, their mating systems have allowed them to survive for hundreds of years. These unique sets of behaviors are something we as humans can appreciate as being very different from our own, and yet still miraculous in their efficiency. –Samantha West
Samantha West is a senior Zoology major at Ohio Wesleyan University. She has loved animals and wildlife since childhood, and hopes to work in species conservation upon graduation.