Males are bigger than females, right? Generally, this is true of humans - imagine the extremes of Dwayne "The Rock" Johnson and singer Kylie Minogue. It is also true of other familiar mammals including pets, such as cats and dogs, and livestock such as sheep and cows.
But a new study by US scientist Kaia Tombak and colleagues found that, in many mammal species, males are not larger than females. In fact, in a comparison of 429 species in the wild, 50% of species including rodents and some bats - which make up a large proportion of all mammal species - showed no difference in body size between the sexes. Male-biased size dimorphism (where males are larger than females) was found in only 28% of mammal species.
So, why do a lot of people have a misconception that males are normally larger than females?
Anisogamy is the term used to describe the difference in sex cells - small, numerous, sperm, compared to relatively large eggs. Males can produce sperm throughout most of their lifespan, whereas females are born with a finite number of eggs. Therefore, females (or rather, their eggs), are a scare resource for which males compete for access. Generally, in species where females are a limited resource that males need to fight over, males are larger than females.
In terms of evolution, most males have been shaped to be larger, bolder, heavier, more adorned and have more weaponry than females. This is due to males fighting to acquire females - a larger stag with bigger antlers would do much better in a fight, known as a rut, than a small stag with tiny antlers. So, bigger usually wins.
This includes species such as lions and baboons, where size is an advantage when competing physically for mates. Male northern elephant seals, who fight for access to harems of females, show the largest male-biased size dimorphism, being over 3.2 times heavier than females. These are the animals that tend to attract research
The strange world of fish
But, what happens in species where males don't fight for access to females? Generally, females are larger than males. This is because larger females usually produce more offspring. Indeed, Tombak's study noted that larger female rabbits usually have multiple litters each mating season. Being a larger female is much more advantageous in terms of reproductive success. But more so when offspring do not need extended parental care and when gestation periods are short.
The most extreme sexual size dimorphism is found outside of mammals. Cichlid fish (Lamprologus callipterus) males are up to 60 times larger than females. The males protect empty snail shells for the females to breed in. Larger females can produce more offspring but they need larger shells and therefore a larger male to defend those shells.
In mammals, the largest female-biased size dimorphism is found in peninsular tube-nosed bats, where females are 1.4 times the size of males. However, more dimorphism in body size is seen in fish, reptiles and insects. For example, the female orb-weaving spider (Nephila plumipes) has a much larger body size than the male, reaching up to ten times his size. Size dimorphism also shows a correlation with cannibalism, where larger females are more likely to eat their male partner.
Anglerfish that typically live at the bottom of oceans, are an example of extreme sexual dimorphism in body size. While the females look like typical fish, the males are tiny, basic organisms. In order to survive, the male needs to fuse with a female, tapping into her nutrients to produce enough sperm to fertilise her. Female deep-sea anglerfish (Ceratias holboelli) are 60 times longer and half a million times heavier than males.
But, the most extreme sexual size dimorphism is found in rhizocephala, types of barnacle where the male looks like a larvae. Once a male finds a mate, he inserts himself into the females, transforming into nothing more than a mass of cells.
What about mammals?
So, why isn't sexual size dimorphism seen in more mammals? Mammals tend to have fewer offspring than other species such as fish or spiders. They only have a few offspring at a time, and often have long gestation periods or extended periods of parental care. In addition, the majority of mammals are monogamous, so there is less need for males to fight over females. That's why species such as lemurs, golden moles, horses, zebra and tenrecs, usually have similar sized males and females.
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It is thought that biases in the scientific literature may have led to the misconception that males are normally bigger as research historically focused on species considered "charismatic", such as primates and carnivores, that attract funding. These are some of the few mammalian species where males compete for mates, and so gain an evolutionary advantage if they are larger.
There was also a bias of male scientists conducting research. And, although a study in 1977 by a female scientist found that species with little sexual size dimorphism were frequent in mammals, the research was drowned out by studies on charismatic species with a bias towards large males. Perhaps if there had been more female scientists at the time, we might have had a different preconception about body size in the animal kingdom.
Louise Gentle, Principal Lecturer in Wildlife Conservation, Nottingham Trent University
