Why Do Men Have Two Testes Instead of One Large One?

Why Do Men Have Two Testes Instead of One Large One?


“Why do men have two testes instead of one large one?” This is a question that perhaps most will not have given any thought. I wouldn’t be surprised if many people live their full lives without ever even bringing this question to mind. However, it is a question which once asked, refuses to be forgotten until it is answered. Well, for me anyways. But let’s not beat around the metaphorical bush and get straight into the heart of the matter. The solution to this testicular query can be split into three main reasons, as I will describe below:

Bilateral symmetry

By far the most convincing argument for the existence of paired testes lies within the innate symmetry of our entire body: this characteristic is something many of us take for granted, and with good reason. Greater than 95% of all animals are symmetrical, with the exception of species such as the crossbill bird, American lobster, and the flatfish. Even so, these species are still largely symmetrical with only minor deviations from the general trend, implying that symmetry is an important, naturally selected characteristic with great evolutionary success. But what exactly is bilateral symmetry and why is it naturally selected?

An image of the flattering flatfish with its asymmetrical features

We as humans belong to the large family of bilaterians, any species of organism containing only one plane of symmetry, which divides it into 2 rough mirror images of each other. All bilaterians share the same rough ‘body plan’ which consists of: a) a continuous digestive tract which begins with a mouth and ends in an anus; and b) possessing clear ‘front’ and ‘back’ sides, the ‘front’ side being where the sensory organs such as eyes, nose and ears are grouped.

Associated with bilateral symmetry are many advantages: The most notable being the ease of effective, directional and purposeful movement. Movement is a fundamental feature of all living organisms – even a Fourth could tell you that from the acronym MRS H GREN. However, what is perhaps understated by most people is the importance of movement for land animals specifically. (sea animals are not as bound by gravity so movement is more forgiving). In order for land animals to survive, they must be able to catch prey and/or evade predators. This is where bilateral symmetry comes in: because most animals have evolved to be symmetrical, their weight is evenly distributed along their entire body. This allows for the most efficient muscle activation (muscles are also evenly distributed along the body) in order for the organism to move smoothly and with the least energy. Furthermore, the symmetry also allows for streamlining of the body, which is particularly important for airborne animals such as birds and sea-bound animals such as fish. All of these factors allow for effective movement of the symmetrical organism.

I mentioned earlier that all bilaterians have a clear ‘front’ and ‘back’ end. This by itself already defines a sense of ‘direction’ for movement. As the sensory organs are grouped facing ‘forwards’ on the head, this allows the organism to focus on points of interest, which in turn allows for purposeful movement, for example moving towards a source of water which has was detected by the eyes, or away from a chasing predator. An interesting theory suggests that because we had evolved for sensory organs to be grouped facing forwards on our heads, the high density of sensory nerves concentrated in our small heads led to the development of a brain (which is just a group of nerves) and eventually a central nervous system.

Some other notable advantages from bilateral symmetry include more efficient embryonic formation, and mirror image learning – the theory that our brain is adapted to ‘translate’ experiences that happened to one half to another – an example would be being able to snap your fingers on your left hand after learning how to snap your fingers on your right hand. However, it is clear that we bilaterians have evolved this characteristic for the purpose of effective movement primarily, and as a consequence, it seems that the testes have followed this trend as well. After all, I am sure that many of my male readers would be able to understand the discomfort of having to walk with lopsided testicles.

Natural redundancy

Another argument that I would like to bring forward to the table is the idea of natural redundancy – in other words, having spare parts. It may come as no surprise to you that we are able to survive without some of our organs. This is again another evolutionary trait which has been positively selected for, as in the wild skirmishes between prey and are commonplace, and it is quite easy to sustain damage to vital organs. However, many organs have been evolved to be redundant – you may have heard that we can function without a kidney, most of our liver and even without a whole lung. This allows for a greater chance of survival for primitive species, such that they can reproduce for as long as possible.

A diagram showing how the liver can regenerate a large majority of itself

But how does this apply to our topic at hand? The most observant of our readers may have noticed that the testes are very much symmetrical and identical. This is because they both have the same function within the male body: the production of testosterone and sperm. Testosterone is important because it regulates sex drive, muscle mass, bone density and sperm production. Sperm production in particular is important because the goal of all primitive life is to reproduce. It makes sense then, that there are two of these sites of sperm and testosterone production, so that in the (God forbid) event that one of them are damaged or rendered unusable, there is still a backup to allow the organism to continue reproducing. But why then do we not have more testes in order to have more backups and produce more sperm? This transitions smoothly onto my third and final point.

Optimisation of resources

Given now that the testes are a fundamentally important organ for the survival of a species, the question now has changed to why we don’t have more of them. – and the answer is simple. This is because evolution has shown that the optimal number of testes that any male can have is two. Each testicle requires energy and resources to maintain and grow, and it is simply not worth it to evolve any more than two. Although it is true that an organism would be more likely to succeed in reproduction, there would be less resources allocated to other vital organs, leading to a decrease in survivability. There is no point in investing in greater reproductive power if you wouldn’t be alive to use it, so it only makes sense that most organisms prioritise survival over reproductive ability.



And there we have it. Another great mystery of life solved. Another case closed. I must admit that when I first started writing this article (as a joke) I was not expecting to get this far. However, what I have learnt is that as a whole is a greatly underappreciated area of research. Since it is a key trait that most organisms share, it holds the potential to be used to trace for LUCA – the last universal common ancestor from which all life on earth came from, which is one of the (actual) unsolved mysteries of biology. Perhaps that was the true lesson all along – sometimes deep revelations can come from the most unexpected of places.