Monogamy is easy

In the latest of our shortlisted entries to the 2011 Wellcome Trust Science Writing Prize, Fiona Lethbridge explains the reproductive strategies of the males of different insect species.

It’s hard enough having to spread yourself thinly during your normal daily activities – work, sustenance, childcare, rest, the list goes on. Luckily for us monogamous types, our efforts in the bedroom are most often directed towards one individual. Imagine, though, the dilemma of having to divide your reproductive resources between many partners. If you were a male seed beetle (Callosobruchus maculatus), you might face this very problem. You would have a limited supply of ejaculate, numerous females of differing ages and reproductive states, lots of rival males, and about a week to live. To fulfil your evolutionary potential and achieve reproductive success you need to prioritise your sexual encounters – do you allocate a little of your seed to several different females, which may offer fairly decent returns, or do you use up all your sperm on one ripe, virgin female in the hope of fertilising each one of her hundreds of eggs?

Sperm is not a limitless resource. Males often have to use it economically to maximise their lifetime reproductive success. In many insects the situation is complex because females store sperm internally from several different mates, much of which is surplus to requirement, so not all males that achieve copulation can be guaranteed paternity. However, males can sometimes bolster their chances, by adopting certain strategies to overcome this sperm competition.

As a promiscuous insect it is essential to assess your surroundings. For example, if you were a male cricket (Gryllus veletis) you might want to allocate lots of sperm when copulating if there is another male waiting his turn with the female, in attempt to father a greater share of the resultant clutch than he does. If there are ten rival males around, you’d probably be better holding onto your ejaculate for now and saving your sperm for other, less competitive situations.

Now imagine you’re a bush cricket (Kawanaphila nartee). That large female you can see might look appealing and you might think she has a lot to offer in terms of egg number and offspring quality. However, all the males think that. If you all mate with her many of you will lose out because she can’t use all the sperm. It might be wiser to reduce your sperm allocation and instead offer more of it to a smaller, less desirable female. With her, your sperm will be unlikely to face competition and you’ll probably father all her offspring.

Age also matters when it comes to females. If you were a meal moth (Plodia interpunctella), you’d assess female age upon mating and allocate your resources accordingly. Give more sperm to young females – they have more eggs in storage and more time to lay them. Your resources might be wasted on old females who could die before getting the chance to use your sperm on their few remaining eggs.

You also need to think about the non-sperm constituents of your ejaculate – water, nutrients and other chemicals. As a locust (Locusta migratoria), you could flush out sperm inseminated by males that have gone before you by allocating a large volume of water to your semen. It you were a swallowtail butterfly (Papilio machaon), you could delay a female copulating subsequently with a rival by inseminating a large ejaculate – she’ll be too full to accept another mating for a while.

Don’t forget about seasonality – the reproductive worth of females can change with the weather. As a small white butterfly (Pieris rapae), you can judge how many sperm and what quantity of nutrients to invest in a female depending on the period of the mating season. If you’re quick off the mark, the females you encounter are likely to be virgins so you can inseminate just enough sperm to fertilise all their eggs but include lots of nutrients to provide nourishment for your resultant offspring. Conversely, later in the season when females will have already mated with rivals, you should allocate more sperm but fewer nutritional resources – greater numbers of sperm will out-compete those of your rivals but there’s no point spending nutrients on offspring that might not be yours.

With all these things to consider you might be glad not to be an insect. If you are indeed a monogamous type things might appear more black and white. You might feel more empathetic towards the faithful Adélie penguin (Pygoscelis adeliae). Little does your partner know, however, that while allocating small numbers of sperm to her you are saving most of them for sneaky matings with others, in attempt to spread your genes far and wide.

As a male Homo sapiens you might think you have no control over the attributes of your ejaculate. However, with some suggestion that sperm numbers are increased when men return to a female partner having been away for a lengthy period, you might have more in common with an insect than you thought.

Fiona Lethbridge

This is an edited version of Fiona’s original essay. Views expressed are the author’s own.

Find out more about the Wellcome Trust Science Writing Prize in association with the Guardian and the Observer and read our ‘How I write about science‘ series of tips for aspiring science writers.

Over the coming months, we’re publishing the shortlisted essays in this year’s inaugural competition.

Image Credit: Richardus_H