In many animals, there are often pronounced differences between the males and the females... In birds, the males often have large, exaggerated feathers; in many deer it is only the males that sport huge antlers; and female lions do not wear the magnificent manes boasted by the males. This is known as "sexual dimorphism" and it does not happen by accident. So are sharks sexually dimorphic? Why are they different? And how can you tell male and female sharks apart?
Boy Meets Girl
Sexual dimorphisms do not happen by accident, but arise as a result of millions of years of evolution. In many animals "sexual selection" causes the ornaments to become pronounced because these features win mates. For example, male peacock's tails have become so exaggerated not because they are especially useful in their lives (in fact, they are a hindrance, as they can slow them down when evading predators), but because a snazzy tail attracted mates, so the genes for a dazzling, elongated tail propagated throughout the population. In other species, like deer and some beetles, the ornaments are actually weapons used to fight for a mate.
Dimorphism =
Greek "di" : 'two' & "morph" : 'forms'
Sharks are incredibly diverse. In line with all the other things that can vary enormously, whether they are sexually dimorphic and how the differences arose, depends on the species. In some sharks, the contrast between males and females might be noticeable, in other species the differences are very subtle, and in some the dimorphisms are simply impossible for a casual observer to be able see (Compagno, 1984; Gayford & Sternes, 2024.
Size Matters
The most common dimorphism in sharks is a noticeable difference in size between the sexes, but what may surprise you, is that unlike many mammals and birds, in sharks it is the females that are often bigger than the males. This is known as "female-dominant sexual dimorphism". Whilst it's not seen in every single species, this is pretty common amongst sharks (Gayford & Sternes, 2024).
The size differences between the sexes are most pronounced in species of sharks that give birth to live young, as the females need more space in their bodies to be able to accommodate their pups as they grow during pregnancy. For example, female sandbar sharks (Carcharhinus plumbeus) reach sexual maturity at 13 years of age and can get as big as 2.5 metres in length, where the males mature younger (about 12 years old) and top out at only around 1.8 metres long (Compagno, 1984; Diatta et al, 2008; Gayford & Sternes, 2024).
For some species, the males and females also have differently shaped bodies. Female sandtiger sharks (Carcharias taurus), for instance, are much more butch than the males; becoming increasingly broad and developing an impressive hunch on their back as they mature (Compagno, 1984; Gayford & Sternes, 2024).
It's All In Your Head!
The bonnethead shark (Sphyrna tiburo) has an especially unique sexual dimorphism; the males and females have differently shaped "cephalofoils". Males have a prominent bulge on front of their iconic hammer head, which you do not find on females (Kariura et al, 2005).
Whilst bigger heads or bizarre ornaments are pretty common in male fishes, it's actually quite rare in sharks (Kariura et al, 2005).
Scientists think that the same chemical mechanism which is responsible for the elongation and hardening of the male's sexual organs at the onset of maturity also causes the bulging of the male bonnethead's cephalofoil cartilage on his head. Therefore, rather than having any particular function, this dimorphism is just a byproduct of the effects of his sex hormones (Kariura et al, 2005).
Bite Me!
In some species of sharks, their sexual dimorphism can be seen in their mouths; with males and females having different teeth. This is known as "gynandric heterodonty". For instance, male greater spotted catsharks (Scyliorhinus stellaris) teeth are a completely different shape to the females' (Berio et al, 2020).
There is no evidence that this is because the sexes have different diets. Instead, scientists suspect that the teeth have evolved this way to aid in mating! It is very common across many different species of sharks, for the male to bite down on the female's fins or flanks as they mate. Scientists suspect that in the great spotted catshark, the males' teeth have evolved their specialised shape to help him to successfully grip onto his mate (Berio et al, 2020).
Skin Deep
On the other side of the coin, a lot of female sharks have slightly different skin to the males. In many species, the blue shark (Prionace glauca) for example, the female's skin is much thicker than the male's, especially around her gills, fins and flanks - specifically to protect her from injuries that might be inflicted by frisky males during mating (Crooks et al, 2013).
In lesser spotted catsharks (Scyliorhinus canicula), the female's skin is not only thickened, but the morphology of the skin's structure is different to that of the males. Sharks' skin is made up of millions of hardened interlocking plates known as "dermal denticles" (to learn more about these, check out Taking the Rough with the Smooth). In the lesser spotted catsharks, the females' denticles are larger, wider and more dense around the fins and pelvic girdle. Scientists suspect this may offer additional protection for the female, but they also wonder if it might provide friction, assisting the pair to remain locked together as they mate (Crooks et al, 2013).
Glow Up
Some species of sharks are also sexually dimorphic in the patterns on their skin. In swell sharks (Cephaloscyllium ventriosum) the network of black patterns is different in males and females. This means that the corresponding "biofluorescence" (ability to glow in the dark) is also sexually dimorphic in this species (Gruber et al., 2016).
Scientists think their biofluorescence allows these sharks to identify members of their own species at great depth, where there is very little light. The differences in the light patterns between males and females also allows them to identify the sex of any sharks they come across, which is especially handy when they are on the look out for a potential mate. Therefore, in this case, this sexual dimorphism has evolved as a type of "visual communication" (Gruber et al., 2016). To learn more, you can head over to Never Break the Chain and Twinkle, Twinkle, Little Shark.
I'll Show You Mine...
Whilst each species has their own morphological variations, what is the same for all species of sharks, is the overarching differences between male and female reproductive anatomy. This means it is actually very easy to tell the sexes apart if you know what to look for...
Both sexes have a "cloaca" - an opening on their belly for defecation (peeing and pooing). The cloaca is also connected to the internal sexual organs and so it is used for mating in females. However, male sharks have very distinctive paired sexual organs called "claspers" (also known as "myxopterygia") on their bellies. These two organs are used for inserting sperm into the female during mating. To learn more check out Sharks Have Two Penises! If you are able to spot these distinctive organs, and know that you are definitely looking at a male, you might then be able to notice the more subtle differences between him and the females.
References
Berio F, Evin A, Goudemand N, & Debiais‐Thibaud M (2020). The intraspecific diversity of tooth morphology in the large‐spotted catshark Scyliorhinus stellaris: insights into the ontogenetic cues driving sexual dimorphism. Journal of Anatomy, 237:5. Access online.
Compagno LJV (1984). Sharks of the world. An annotated and illustrated catalogue of shark species known to date. FAO species catalogue. Access online.
Crooks N, Babey L, Haddon WJ, Love AC & Waring CP (2013). Sexual Dimorphisms in the Dermal Denticles of the Lesser-Spotted Catshark, Scyliorhinus canicula (Linnaeus, 1758). PLoS ONE 8:10, e76887. Access online.
Crooks N & Waring CP (2013). A study into the sexual dimorphisms of the Ampullae of Lorenzini in the lesser-spotted catshark, Scyliorhinus canicula (Linnaeus, 1758). Environmental Biology of Fishes, 96. Access online.
Diatta Y, Seck AA, Reynaud C, Guelorget O & Capape C (2008). New biological observations on the sandbar shark Carcharhinus plumbeus (Chondrichthyes: Carcharhinidae) from the coast of Senegal (Eastern Tropical Atlantic). Cahiers de Biologie Marine, 49:2, 103. Access online.
Gayford JH & Sternes PC (2024). The origins and drivers of sexual size dimorphism in sharks. Ecology and Evolution, 14:3, e11163. Access online.
Gruber DF, Loew ER, Deheyn DD, Akkaynak D, Gaffney JP, Smith WL, Davis MP, Stern JH, Pieribone VA & Sparks JS (2016). Biofluorescence in catsharks (Scyliorhinidae): Fundamental description and relevance for elasmobranch visual ecology. Scientific Reports, 6:24751. Access online.
Kariura SM, Tyminski JP, Forni JB & Summers AP (2005). The sexually dimorphic cephalofoil of bonnethead sharks, Sphyrna tiburo. Biological Bulletin, 209. Access online.
Whitney NM, Pratt HL & Carrier JC (2004). Group courtship, mating behaviour and siphon sac function in the whitetip reef shark, Triaenodon obesus. Animal Behaviour, 68, 1435–1442. Access online.
By Sophie A. Maycock for SharkSpeak
