Sexual dimorphism

• 1. Peacocks and Peahens

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  Sexual dimorphism is the condition in which males and females of one species differ in their bodies, colors, markings or even behavior, well beyond the organs of reproduction. Consider the peafowl. At first sight, a peacock and a peahen could be mistaken for two entirely different species. The male carries vibrant colors and an enormous spread of plumage, while the peahen is a subdued brown.

  That plumage is a hindrance in flight and makes the male conspicuous to predators. Still, it is the very thing that draws the peahen in. Birds of paradise and argus pheasants play out the same puzzle.

  The pattern runs through most animals and some plants, wherever the sexes are separate individuals. Why would nature reward a feature that endangers its owner? Why are females sometimes the larger or showier sex? What becomes of a male who shrinks to almost nothing? And what can fossils say about dinosaurs that left only bones? The opposite condition has a name too, monomorphism, where the two sexes cannot be told apart by their bodies.

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• 2. Color and Display

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  Nestling blue tits carry a signal that humans cannot see. Males are more yellow than females, a difference thought to come from eating green moth and butterfly larvae rich in the carotenoids lutein and zeaxanthin. The same diet shapes colors in the ultraviolet range, so a male that looks yellow to us actually wears violet-tinted plumage in the eyes of females. That plumage may advertise parenting ability, because it shows skill at finding food. Tail and breast feather color tracks body condition, and carotenoids support the immune system, so the colors may also advertise health.

  The frog Hyperolius ocellatus shows how sharp the split can be. At sexual maturity, males turn bright green with white lines along their sides. Females are rusty red to silver with small spots. The male's brightness attracts females and also warns predators. Frog color differences come in two types. Ontogenetic species, the more common kind, change permanently, while dynamic species such as Ranoidea lesueuri change color only in the breeding season.

  Male guppies have colorful spots and ornaments, while females are generally grey, and females prefer the brighter males. In the two-spotted goby, Gobiusculus flavescens, the roles flip. Females develop colorful orange bellies in the breeding season, and males favor them. Males of this species invest heavily in the young during incubation, and the orange carotenoid color hints at egg quality.

  Male painted dragon lizards, Ctenophorus pictus, are brightly conspicuous when breeding, but their color fades with age. It appears to reflect an innate capacity to resist oxidative DNA damage, a major part of aging. A female reading that color may be reading the health of a mate's DNA.

  Some male specialties are invisible. In redlip blennies, only the male grows an organ near the anal-urogenital region that produces antimicrobial substances. He rubs it over the inner surfaces of the nest to shield the eggs from infection, one of the most common causes of death in young fish.

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• 3. Weapons and Contests

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  Beetle horns show what rivalry can build. In Onthophagus taurus, horns are outgrowths of the head or thorax that only males develop. Copris ochus and the Japanese rhinoceros beetle, Allomyrina dichotoma, carry equally striking versions. Longer horns track larger bodies, and larger males gain better access to mates. In some other beetle species, both sexes wear horns.

  Teeth built for fighting and heads hardened into battering rams are further weapons that males have evolved against rivals. The cichlid fish Lamprologus callipterus pushes this to an extreme, with males up to 60 times larger than females. Males collect and defend empty snail shells, and each shell becomes a nursery for one female. Bigger males can haul in bigger shells.

  The female must stay small enough to squeeze inside a shell to lay her eggs. She may even adjust her growth rate to the shells available. Large males also win fights over territory and steal shells from smaller rivals.

  Anolis sagrei lizards illustrate a smaller gap. The average male measured 53.4 mm, against 40 mm for females. In the bee Anthidium manicatum, larger males may have been favored because of their aggressive territorial behavior and the mating success it brings.

  Ray-finned fish show the widest range of dimorphism of any animal class. Sizes run from dwarf males to males more than 12 times heavier than females, according to Fairbairn. Some fish even change sex. Where one male dominates many females, a female often becomes male when no dominant male remains. The ones who switch are often those who gained an early size advantage.

  Sockeye salmon split their differences between outside and inside. Males deepen their bodies, grow a hump and lengthen the snout. Females change little on the outside, but their gonads swell to about 25% of body mass.

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• 4. When Females Are Larger

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  Leopard seals, raptors, spiders and certain waders share a reversal. In each, the female is larger or more ornamented than the male. Among waders, this often travels with reversed color differences and swapped sex roles. Owls, the spotted hyena and the blue whale also have the larger sex on the female side.

  Theory points to the cost of reproduction. A bigger female can make more offspring and care for them longer. The black myotis bat, Myotis nigricans, fits the idea. Females are substantially larger in weight, skull and forearm length. They nurse their young until the pups are nearly adult size, and they must still fly and catch prey while carrying that extra mass. Smaller males may be more agile.

  Another route to large females is sedentary living. Fritz Vollrath and Geoff Parker argue that when females stay put, males must search for them, and the search favors smaller males. In the red mason bee, Osmia rufa, males measure 8-10 mm and females 10-12 mm. Females eat more pollen, which explains the difference in size.

  Spiders add a darker twist. In the wolf spider Tigrosa helluo, hungry females cannibalize more often. Males therefore choose larger females, which are both more fertile and less likely to eat them. Males of some spiders answer with tricks. They bind the female in silk, grow proportionally longer legs, alter her web, mate while she feeds, or bring a nuptial gift. In peacock spiders such as Maratus volans, males wave a colorful fan.

  The most extreme cases erase the male. In some anglerfish, the male is a tiny creature with a stunted digestive system. He fuses to a female and lives parasitically as little more than a sperm-producing body. The Zeus water bug, Phoreticovelia disparata, has a female with a glandular patch on her back that can feed a clinging male. In Sacculina barnacles, the male injects himself into the female and becomes only sperm-producing cells. The whole group, Rhizocephala, was once mistaken for hermaphroditic.

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• 5. Plants and Pollinators

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  About 6% of flowering plant species have separate male and female individuals, a condition called dioecy. Most flowering plants are hermaphroditic. Insect-pollinated species usually keep the sexes looking alike, because the nectar reward teaches pollinators to move on to a similar flower.

  Catasetum orchids break that rule. Males violently attach pollinia to euglossine bees. The bees then avoid other male flowers but may visit the female, which looks different. Loxostylis alata also has visibly different sexes, and the contrast lets pollinators use the most efficient strategy on each flower type.

  Some Geranium species show dimorphism over time rather than between plants. A flower opens with its anthers presented, sheds them after a day or two, and may change color as the pistil matures. Some then change appearance once fertilized. That spares the developing fruit and saves the pollinator from unrewarding visits.

  Vallisneria americana is an aquatic plant. Its females float on long stalks and are fertilized when they touch one of the thousands of free-floating flowers released by a male. Wind-pollinated plants such as Leucadendron rubrum tend to be dimorphic because males are selected to scatter pollen and females to catch it. In hemp, Cannabis sativa, males photosynthesize faster while growing, but females overtake them once the plants mature sexually.

  The deepest dimorphism sits inside every seed plant's life cycle. Each pollen grain can be seen as a male plant in its own right. It produces a sperm cell and looks dramatically unlike the female plant, the embryo sac that makes the female gamete.

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• 6. Dinosaur Evidence

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  Finding sex differences in dinosaurs requires complete, articulated skeletons, and fossilization rarely delivers them. Decay and geography both erode what survives. Some paleontologists turn to statistics and to comparisons with related living animals.

  The long-necked sauropods offer one clue. Female Apatosaurus and Diplodocus had interconnected tail vertebrae that may have let them keep their tails raised during copulation. The fusion appeared in only 50% of Apatosaurus and Diplodocus skeletons and 25% of Camarasaurus skeletons, which suggested a sexually dimorphic trait.

  Theropods raise a more contested question. Some researchers hypothesized that males had a retractable penis, as modern crocodilians do. Male crocodiles have significantly larger tail chevrons, which anchor the penis muscles. Critics have challenged the findings, and the matter remains debated.

  Hadrosaurs invite study through their cranial crests, which probably served in display. A biometric study of 36 skulls found dimorphism in the crests of 3 hadrosaurid species. Crests could be sorted into full, taken as male, or narrow, taken as female.

  Ceratopsids lack convincing evidence of dimorphism in size or mating signals. According to Scott D. Sampson, any dimorphism would likely show in horns and frills. The earlier ceratopsian Protoceratops andrewsi had frill and nasal differences, but other researchers favor individual and growth variation as the explanation. Soft features such as color or dewlaps would rarely be preserved.

  A 2015 study of Hesperosaurus mjosi described two shapes of dermal plate, one short, wide and oval, the other taller and narrower.

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• 7. Mammals and Humans

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  Among mammal species, males are larger in 45%, the sexes are the same size in 39%, and females are larger in 16%. Marine mammals show some of the biggest gaps. Male sea lion pups are already about 10% heavier and 2% longer at birth. The southern elephant seal, Mirounga leonina, is among the most dimorphic mammals. Most anthropoid primates differ by sex in body size, canine teeth, skull shape and voice, while strepsirrhines and tarsiers are mostly monomorphic.

  Clark Spencer Larsen reports that average body mass in modern humans differs by roughly 15% between the sexes. Daly and Wilson wrote that the sexes differ more in humans than in monogamous mammals, but much less than in extremely polygamous ones. In the United States, adult men are on average 9% taller and 16.5% heavier than adult women.

  Testosterone drives much of the divide. Adult men have 10-20 times more of it than women, at 300-1,000 ng/dL against 15-70 ng/dL. Even at ages 5-10, boys are about 17% stronger in the upper body and 8% in the lower body. Among 14-17 year olds, the gap grows to 50% and 30%. Bone mineral density averages about 3.88 g/cm² in men and 2.90 g/cm² in women.

  Trained athletes narrow the strength gap. In Olympic weightlifting, the absolute difference in records is about 30%, 492 kg against 348 kg for unlimited classes. A study of world rankings from 1980 to 1996 found men ran about 10% faster. Wider hips in women create a larger Q-angle, which affects force transmission. Women, however, resist fatigue better at submaximal effort.

  Immune function leans female across many animals, most strongly in insects. Human females have more white blood cells and make antibodies faster. Testosterone acts as an immunosuppressive agent, and mammal males suffer more frequent and severe infections. Women show higher rates of autoimmune disorders.

  The brain is the most contested territory. Haier and colleagues reported in 2004 that men and women achieve similar IQ results with different brain regions. A graph analysis of 36 women and 36 men found women's structural connectome better connected, even though every man's brain was smaller than every woman's. At the gene level, about 6,500 genes show sex-differential expression in at least one tissue.

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• 8. Why Costly Traits Survive

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  In 1871, Charles Darwin advanced the theory of sexual selection and tied it to sexual dimorphism. The first step was far older than any horn or tail. Eggs and sperm differed in size, a split called anisogamy. Small sperm can be made in huge numbers, which set off fierce sperm competition. Volvocine algae help researchers study these origins, and the cowpea seed beetle Callosobruchus maculatus, whose females are larger, is used to probe the genetics.

  Where males gain much from many matings and females gain little, selection favors any trait that wins a male more matings. Weapons and size solve the fighting problem simply. Mate choice is harder to explain. Females may choose males who look strong and healthy and so carry good alleles. Sometimes they choose traits that do not help offspring survive, and may even harm it.

  Two hypotheses cover that puzzle. Under the sexy son hypothesis, a preference that began by helping survival becomes self-sustaining. A female who ignores it will have sons that most females find unattractive. Under the handicap principle, a male who survives despite a burden proves his other genes are good, so the burden is a signal that is hard to fake.

  The costs are real. In Denmark, dimorphic coloration increased the vulnerability of bird species to European sparrowhawks. Showier ornaments may also suppress immune function. A trait still spreads if the offspring it earns outweigh the survival it costs, even when males die earlier. Some birds drop the display outside the breeding season, molting into duller plumage to favor survival.

  The red-backed fairywren shows the environment at work. Healthy males produce more androgens and become black breeders. Less healthy males become brown auxiliaries, so each year's conditions help set reproductive success.

  Differences between the sexes can also expose one sex to harm. In Switzerland, female whinchats breed in intensely managed grasslands, and earlier harvests of the grass during the breeding season killed more of them.

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