Sexual dimorphism in organisms is nothing new; it has long been known that in certain species one sex is often larger, flashier, or somehow markedly different than the other. In some species like the Indian Peafowl (Pavo cristatus), the mail carries a brightly colored train that is used to advertise to prospective female mates, while in the deep-sea anglerfish Suborder Ceratioidea the males are absolutely minuscule when compared to the females, fusing to the bodies of the larger sex and ultimately becoming little more than a sperm supply. Such differences are contrasted with the almost non-existent dimorphism between male and female White-Handed Gibbons (Hylobates lar) where the only major difference between the sexes (outside of primary sexual characteristics) is a small tuft of hair on the male’s posterior, making it sometimes difficult to tell the sexes apart on sight. What, then, causes these differences?
Much of sexual dimorphism is tied to social and reproductive strategies of different species of animals, certain levels of dimorphism appearing to be associated with how intra- and intersexual contact takes place. In the almost sexually monomorphic White-Handed Gibbon, for example, adult males and females form socially monogamous bonds (although it should be remembered that this does not mean they mate for life or that they are entirely “faithful” to their mate), male/female pairs holding down an established territory in the canopy, males and females driving off intruding members of their own sex (indeed, female gibbons have large canines like males, a sexually dimorphic feature in other primates as we will soon see). Compare this system to that of the Olive Baboon (Papio anubis), an Old World Monkey where males that try to form sexual consortships with females in heat (as evidenced by the peak swelling of the perineum) but are often harassed by other males during this time. In this type of social system where there are multiple males and multiple females where females gain their place in the dominance hierarchy through heredity but males must compete for their place at the top. Such a system has likely caused the sexual dimorphism we now see in these primates, males being twice as large as females and bearing absolutely huge canine teeth, although coalitions and other social tools often allow males to shift the balance of power. The choice of mates and social “frienships” isn’t all up to the males, however. In Hamdryas Baboons (Papio hamadryas) males are even more dimorphic, sporting grey tufts of hair and controlling small groups of females. Females don’t always acquiesce to the coercion of the large males, though, and in one well-known case a particular female wanted to groom a male that was (obviously) not a member of her small group. The male sat behind a rock and over the course of about a half hour the female made her way over to the male, the female grooming the other male while still remaining partly visible to the male of her own group (dominant males get nervous when their females go out of sight) . While this event wasn’t about mating, it does show that these animals are capable of a startling level of deception, females not being entirely bound to the dominant males of their group.
Bovids, like baboons, show strong sexually dimorphic characters, most easily recognized in their ornamentation (i.e. horns). Males that attempt to hold down a territory through which females pass, for instance, often compete with other males for that territory, males with the largest horns often ending up as the possessors of the territories. Likewise, males that live with females often have to compete with each other for dominance, males with the most impressive set of horns usually being dominant (although there are ways to tips the odds in your favor if you’re a lower ranking male, like hitting another male when he’s not looking or mating with the females while the other males are competing like Bighorn Sheep [Ovis canadensis] do). Such systems have often led to the runaway selection for ever-larger horns in animals like the Greater Kudu (Tragelaphus strepsiceros), males with large horns having more regular mating success but also experiencing costs like becoming entangled in the brush as they try to escape a predator, getting their horns stuck with other males should a fight occur (leading to death if they cannot unlock themselves), and a heavy set of horns being a disadvantage when you’re being pursued by a predator, hunters of the !Kung San people often targeting males to hunt down as they will tire more easily for having to carry such large ornaments. Such considerations raise an important caveat to such forms of sexual selection; the ornaments or traits will only enlarge so long as they are not overly deleterious or make the organism overly vulnerable, traits like huge horns making the robust males more vulnerable in some ways. Indeed, this sort of directional sexual selection only works when horns are not used to fend off predators and are only used in intrasexual competition for mates, and in species where horns are used for predators, they do not grow to the same extremes as they must remain functional, females often bearing horns as well, i.e. the Sable Antelope (Hippotragus niger). It is also important to note that while males often have ornate armaments that help to establish hierarchies and dominance, this does not mean that only the dominant male gets to mate. Subordinate males can be tricky and overall have a good amount of mating success, passing along their genes just as the more robust males do, increasing the variation within the population of animals.
Clearly the selection pressures on males of some organisms are different than those encountered by females; a population may exist within the same ecological setting but social and reproductive systems within that setting have shaped traits in different ways. Part of these differences is allowed by maternal and paternal investment in offspring; females often invest an extremely high amount of energy into producing offspring (a limited number of eggs, gestation time, and postnatal care in some animals with the further energy strain of nursing in mammals), males instead often “investing” in larger size and more impressive ornamentation (although, like I noted, this various with social system and functional constraints). It is no surprise that males can produce a much larger amount of offspring than females can during a lifetime, males of many species contributing absolutely nothing outside of their genetic material to the next generation (indeed, even if females provide no postnatal care, gestation is still a very expensive process). What is interesting, then, is the example of Red Deer (Cervus elaphus); dominant, robust males often leave sons that have greater reproductive success while their daughters are not as successful as other females. Likewise, daughters of females that have a high amount of reproductive success tend to have more offspring, males of these mothers not doing as well when compared to other males. This suggests a divergence in life histories and how favorable traits are transmitted through a population, the divergence being kept in check by the fact that the disadvantaged individuals (i.e. a female sired by a robust male) do reproduce even if it is not with as much success, a population that produced only males or only females quickly leading to disaster.
The discussion so far has focused on adult animals and how they reproduce given social and environmental constraints, but what about differing pressures on infants and juveniles? Again, males and females of the White Rhinoceros (Ceratotherium simum) seem to be affected by differing circumstances, only this time it has to do with nursing. While the sexual dimorphism in White Rhinoceros is not as marked as in other species, the main difference being one of social structure (one dominant male holding down a territory). While there are various factors that lead to the large body size of these animals, male White Rhinoceros infants are given an extra boost by their mothers in getting more milk over a longer period of time, especially when nursing is critical to early growth. Female White Rhinoceros infants, by contrast, do not solicit suckling as much and stop suckling earlier than males, so the trend appears to be the result of the infant males being allowed to suckle more than females that aren’t as solicitous. While it might be intuitive to posit that the greater maternal investment in males (the longer suckling time resulting in less offspring for the adult females as nursing inhibits reproduction) leads to bigger, more robust males later in life, the difference seems to be made much earlier. Juvenile males fight and play, those that attained a great investment of milk from their mother often being the winners and eventually becoming dominant, those that did not receive as much investment often being injured or unable to establish themselves (thus being “weeded out”).
Young organisms have more things to worry about than who’s going to be an alpha when they grow older; infanticide is a problem faced by many organisms. This area of behavior has been especially contentious in primates, worries about morality (either being consciously invoked and subconsciously in place) making some researchers claim that when males kill infants it’s for the “good of the species” when resources run thin. Such an idea, however, has no actual support and studies of primates like langurs (Old World Monkeys belonging to the Subfamily Colobinae) have shown that there is a functional reason why males kill nursing infants but play with slightly older, weaned juveniles. As I noted earlier, nursing offspring represents a reproductive cost to the mother as a nursing infant prevents the mother from cycling, and the longer the nursing goes on, the longer she’ll go without cycling again. In the case of langurs, we’re often dealing with a one male, multi-female society, but males can be displaced by bachelor males (which sometimes work in groups to oust an established male, although why they should work together when only one is established is still a mystery). Females with infants often fight to make sure the established male stays in, but they are not always successful in preventing the coup. Such defense is the result of the fact that when the new male enters the group he will kill all the nursing infants that he did not sire, thus bringing the mothers back into their cycle (and they breed with the new male despite showing stress over the infant that was killed, females without infants usually having no problem with a new male and offering little to no defense). Defense of nursing mothers isn’t always enough, however, and the infants of some species have been adapted to have pelage patterns that differ from those of adults. This may help to confuse paternity, perhaps resulting in some infants being spared as they are so confusing (and it has been established that primates can detect relatedness and hierarchies).
Just as infants may not always have a form or pattern like that of adults, the differing sexes can be adapted to be confuse others and allow for an alternate route to reproductive success. One such example is the Bluegill Sunfish (Lepomis macrochirus), where males have different physical appearances and behavioral practices in mating. Indeed, it appears that are three different kinds of Bluegill males: large territorial males that make up about 15% of the population, “sneakers” that are a little smaller and lack orange coloration, and “satellites” which are essentially female mimics. Territorial males mate with females in the way of the traditional narrative, the territorial male (supposedly) driving other males away and mating with the females that enter his territory. Satellites, by contrast, have been adapted to get around the territoriality of the large males by looking like females, satellites drifting down between a male and female when a female enters the nest (the territorial male being fooled since the satellite doesn’t look like a male) , releasing its own sperm when the male and female release their gametes into the water thus leaving the territorial male to “foot the bill” of parental care for the developing eggs. Sneakers, in contrast to both these strategies, ambush mating pairs with a cloud of sperm timed at the right moment, departing just as quickly as they appeared. In both cases the resident male is not so easily fooled and attacks the interlopers, but the alternate ways of mating are successful enough that they are well established within these populations, sneakers and satellites becoming sexually mature earlier than males who must spend more time growing to a larger size in order to obtain a territory. While the sneakers and satellites are clearly taking advantage of the territorial males, they are really reliant on him to care for their offspring as without a territorial male protecting the offspring of the cuckolds, many of the eggs would probably be eaten or otherwise never make it to hatching. Still, these observations fly in the face of the traditional ideas and narratives that there is one alpha male who mates with females to the exclusion of all other males, there being no “cheating” as territorial males won’t allow it and females wouldn’t stray from their paragons of perfection.
This essay has only served as a brief sketch of a few mating strategies (mostly mammalian), but the point of it is that natural selection and sexual selection work on males and females in different ways, even causing individuals during a certain stage of life to undergo changes caused by natural selection. The costs of ornamentation, advertisement, and large size are many, and while an individual is an evolutionary dead-end if it does not reproduce sexual selection is not always about being the biggest or most ornate. Indeed, the majority of males who may not be able to become dominant or hold a territory will try to find ways to secretly rendezvous with females away from the angry eyes of the dominant males (or even, as we saw with the Bluegill, right under their noses), secretive tactics having enough success that even though subordinate males may not have as much mating success as dominant males over the course of their lifetimes, they definitely make a contribution to the next generation. It should really be no surprise that sex is such a complex and variable thing, evolution creating as many different strategies for mating as lifeforms that feel the urge to do so.