Why do so many species have two sexes

Well, it’s partly because having separate male and female body parts, each dedicated to making either sperm or eggs, helps prevent inbreeding. Those two reproductive cells or ‘gametes’ each contribute half the genetic material (DNA) needed to build a new organism, and they share similar features due to common functions in an embryo: eggs nourish, sperm fertilise.

What is an egg?

The female gamete is an unfertilised embryo, generally a giant cell that either contains or is surrounded by a stockpile of materials for growing that embryo.

An egg’s size is influenced by where an embryo will develop. In true mammals, a mother supplies her baby with nutrients during an internal pregnancy before she gives birth to live young (viviparity), whereas reptile and bird eggs are bigger as they’re incubated externally after being laid (oviparity) and enclosed within a shell, nourished by proteins and fat in yolk. A human egg is about 0.1mm wide – 10 times larger than an animal body cell; ostrich eggs can be more than 15cm tall.

How is reproduction similar in animals and plants?

The life-cycle of land plants is more complex than that of animals: generations alternate between reproducing asexually via spores (clones) and making male and female gametes. And yet in both groups, sexual reproduction occurs when a sperm fuses with an egg cell to make an embryo or ‘zygote’ with a paired set of genes.

Plants that produce seeds also have a structure called the ovule in which the fertilised egg forms a seed containing the embryo – along with a food supply that’s functionally equivalent to the nutritious yolk in oviparous animals.

Do eggs really need sperm?

Not always! Environmental factors such as temperature can prompt an embryo to develop without fertilisation, via asexual parthenogenesis or ‘virgin birth’.

Sperm are not only used to trigger embryonic development, either: in flowering plants, sperm ‘fertilise’ cells of the ovary so they form a fruit. In principle, eggs could simply duplicate their DNA, but the majority are activated by sperm.

What are sperm?

The male gamete is typically a tiny cell that, like a space rocket, is stripped of all but essential components, improving the chances of delivering its DNA cargo.

In some living things, sperm are non-motile: they have no propulsion system and rely on the environment to carry them to an egg. The most obvious example is the sperm in pollen grains, which are transported by the wind or insect pollinators, but algae and immobile animals such as coral are also at Mother Nature’s mercy.

The sperm of many organisms are motile and propel themselves through an aquatic medium by beating a tail, the flagellum. While the sperm of ancient plants such as ferns and gingko use multiple flagella for swimming, the majority of animal sperm have a single tail – usually connected to a head (containing DNA) via a ‘midpiece’ filled with energy-generating mitochondria to power its movement.

Why and how do sperm compete with one another?

It’s driven by natural selection – survival of the fittest sperm. Ejaculation evokes imagery of a race between tadpole-like cells, of the strongest swimmers with winning genes. But while competition can weed-out the weakest sperm in an individual’s ejaculate, it also occurs between gametes from different males (a female who mates with more than one partner can store their sperm for later).

Males have evolved strategies to try and outcompete rivals, such as delivering a greater number of sperm (per ejaculate or by frequent mating) or making various forms, including larger cells. The seminal fluid of mammals can contain millions or even billions of sperm, for instance, whereas the fruit fly Drosophila bifurca makes only a few male gametes nearly 6cm long (incredibly, over 20 times the length of the fly itself!) that coil-up and block other sperm from entering a female’s reproductive tract.

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Discover more about the fascinating subject of reproduction, from what is dimorphism, a stingray’s virgin birth and ageing, immortality and lifespan in plants and animals explained.