Odd Couples: Extraordinary Differences between the Sexes in the Animal Kingdom
“. . . a perfect gift for the budding zoologist in your family.”
Dr. Daphne Fairbairn began her career by studying deer mice, which led to an interest in evolution and how sexual differences can affect an animal’s body shape, size, life history, behavior, and ecology.
Odd Couples is a non-sexy book about sex that addresses extremes in species size dimorphism—species having large males and small females or small males and large females. In some species the size difference is so great that males and females were even not recognized as belonging to the same species until observed mating or emerging from the same batch of eggs.
The technical term for a species’ separation into male and female is called dioecy and the vast majority of animal species are dioecious. Hermaphroditism exists in less than 0.16% of living animal species. Dr. Fairbairn raises and answers questions such as: What is special about dioecy, and why does it work so well?
The mechanism of dioecy is by genetics, and gender chromosomes across different species can vary. For humans, this is typically (but not always) XX chromosomes for females and XY chromosomes for males. The differentiating Y chromosome is inherited from the father for humans but not for all animals. Some species lack a Y-chromosome altogether, and there is either an XX or a single X (called X0) chromosome instead. For these species the number of X chromosomes determines gender. This is true for birds, butterflies, snails, slugs, and some fish and reptiles.
Some animals lack sex chromosomes altogether; male or female is determined by genetic cues. This is true for ants, bees, wasps, and some beetles. For these species unfertilized eggs hatch males and fertilized eggs hatch females.
Where evolutionary factors come into play is that within a species the males and females can sexually mature at different ages, sizes, have different lifespans, and spend much of their time in different places. A key factor in dimorphism is in having different growth rates. In one species the male can be 13 times heavier than the female, in another the female 500,000 times heavier than the male.
Most extreme differences between males and females occur in species with no obvious genetic differences—the mechanisms by which these patterns are orchestrated are still not fully understood. It is believed that sex specific trajectories are triggered by environment or social cues at critical stages in development. One example occurs in the spoon worm where if a larva settles on an empty patch of ocean bottom, it becomes female but if it lands on a female, the larva becomes male.
Dioecy can also lead to common patterns of form and behavior. For example, where there are a plentiful number of males for females to choose from, we see competition among males leading to larger males, we also see displays for attention, colorful plumage, aggressive temperament, the offering of gifts of food, providing secure nesting sites, and the defending and caring for eggs or offspring.
The environment plays its part, too. In isolated areas such as the open ocean and the deep sea floor there is less competition because there is greater difficulty in simply finding a mate. Males tend to be smaller and specialize in mate-searching capabilities. Being smaller requires less energy, so males tend to be highly mobile and have exaggerated sensory organs.
Females use bioluminescence and produce well-provisioned eggs. The author notes the speculative possibility that deep-sea females might also disperse chemo-attractants such as pheromones.
Dr. Fairbairn first introduces species that exhibit dimorphism that the reader is familiar with: seals, birds, fish, spiders, and octopuses but then advances to the more exotic: giant seadevils, a newly discovered species of bone-eating marine tubeworm, and shell-burrowing barnacles.
Each chapter’s subtitle provides a clue as to the relationship between the sexes. Rather than address all, this review will pick and choose but provide detail for the last two because of their exceptional weirdness. The reader can guess the gender-specific behavior based on each chapter subtitle:
• Elephant Seals: Harems, Hierarchies, and Giant Males.
• Great Bustards: Gorgeous Males and Choosy Females.
• Shell-Carrying Cichlids: Protective Males and Furtive Females
• Yellow Garden Spiders: Sedentary Females and Roving Males
• Blanket Octopuses: Drifting Females and Dwarf Males
• Giant Seadevils: Fearsome Females and Parasitic Males
• Bone-Eating Worms: Female Tubeworms with Harems of Minuscule Males
• Shell-Burrowing Barnacles: Sac-Like Females with Harems of Phallic Males
Elephant seals were nearly hunted to extinction in the 1880s yet survived and now thrive in great numbers. Male seals weigh seven to eight times more than females. In elephant seal society males and females tend to feed in different parts of the ocean, going to land only to molt, mate, birth, and raise their young. Males and females lives overlap only in the breeding rookeries.
On land a dominant male will tend to a harem of 50 to 100 females, and within a harem females also form a dominance hierarchy; more dominant females in the center and less dominant females at the edges where they are more likely to be bothered by stray males. Females care for pups; males do not.
Male elephant seals mature later than females and often die before maturity. Most adult mortality occurs in the sea from sharks and killer whales. A male seal’s goal is to survive long enough to mate.
Male elephant seals display to show off their size and power and older males are more successful at mating because they are larger. Few males will mate even once in a lifetime. Coupling is aggressive and often brutal though seldom lethal (to the female).
Although Odd Couples is a serious book it is not without moments of humor. In describing the mating ritual of great bustard birds where groups of males put on elaborate displays for groups of females, Dr. Fairbairn likens the birds’ interactions to “speed dating” and congregating in close proximately allows females to “comparison shop.”
Dr. Fairbairn notes octopuses “can open jars, play with toys, even recognize their keepers. Who would have thought this of a mollusk? Clearly one should be careful not to judge an animal by the phylum it keeps.” (This reviewer imagines the author snort-laughing at her clever turn of phrase.)
Seadevils, bone-eating worms, and shell burrowing barnacles are simply bizarre creatures, and seemingly more out of a science fiction writer’s wildest imaginings than your typical run-of-the-mill zoo animal. These last three species have dwarf males, something that tends to occur where animal life is low density with respect to the environment. Note too, as the open ocean and ocean depths are less accessible to the scientist and as animal species become more difficult to study, the information available becomes less detailed and more speculative.
Bone-eating tubeworms deserve special attention in that they were not even known as a species until 2002 when they were discovered by a remotely operated submersible that was studying animals feasting on a deceased whale carcass.
The tubeworms are not just living on the bottom of the ocean but are anchored and immobile like plants, “. . . every population goes extinct as the carcass disappears. The worms literally eat themselves out of house and home.”
To further their study, a whale carcass was intentionally sunk in the shallows and recorded over time. You can find this recording on YouTube by using the keywords: “weird worms eat bones.”
Tubeworm larvae (before being anchored) are neither male nor female; gender is environmentally determined. Those that settle on bone become female. Those that settle on females become male. Dr. Fairbairn’s points out, “. . . some females have tens of hundreds of males in their tubes.” Just in case this last sentence wasn’t clear enough, because of their extreme size dimorphism hundreds of male tubeworms can live inside a single female tubeworm.
The last example of extreme dimorphism is the shell-burrowing barnacle. Barnacles are almost all sessile, sea-bottom dwelling animals that are anchored to one spot throughout their adult lives. Barnacles harvest drifting plankton using their legs, delivering food to their mouths, hidden inside their shells. As both males and females are cemented in place, mating should be considered difficult.
The distinguishing feature of male barnacles is that they have the largest penises relative to their body size for any animal, a length up to eight times the diameter of its body. And as the male is immobile, the penis searches to mate, and once mated the male sticks to the female for life.
Shell-burrowing barnacles are somewhat different from other barnacle species in that the distance between shells is so great that the dwarf male cohabits with the female inside the female’s shell-burrow, attached permanently to the female, outside her body.
Females are ten times the length and 500 times the weight of males. Their size dimorphism starts during the larval stage and continues to sexual maturity. By adulthood, both sexes share virtually no morphological features. If the males were not found living on the females and fertilizing them, they might have never be identified as the same species. The photo of the male shell-burrowing barnacle is not to be missed.
The next to last chapter, The Diversity of Sexual Differences is the nerdiest, containing tables and charts and scientific terminology that will most likely appeal only to students and professional biologists and zoologists.
In this chapter, Dr. Fairbairn unchains herself, putting her knowledge on full-afterburner. While this reviewer was suitably impressed he was also thankful that the previous chapters are what makes the book worthwhile, and so did not feel too badly in skimming it, whew! In the acknowledgements she lets the reader know that her editors have restrained her from turning this book into an encyclopedia. And on the basis of this chapter, this reviewer thanks those editors.
The last chapter of Odd Couples, Concluding Remarks is a satisfying summing up. Dr. Fairbairn tells us that she has detected no universal pattern of sexual differentiation among the species beyond the basic mechanics of producing sperm or eggs and “. . . we should be cautious about drawing generalities about sexual differences from the relatively few kinds of animals that we know well.”
Everything concerning sexual differences, she says is “up for grabs” and, “. . , there is clearly no one way of being a male or female animal.”
The one obvious question remaining is: Where do we as humans fit into the grand scale of dioecy and dimorphism? Dr. Fairbairn puts humans are low in the dimorphism scale: a low 3 on of a scale of 1 to 10, explaining that for humans, cultural dimorphism exceeds the biological. “We are simply not extraordinary” when compared to the 1.4 million species in the animal kingdom.
In the acknowledgments Dr. Fairbairn tells the reader that Odd Couples has been a labor of love, something that clearly shows—printed on quality paper stock with charts, tables, color photos, notes, appendices, references, and a glossary, Odd Couples was written for but does not talk down to the nonspecialist.
Odd Couples is a perfect gift for the budding zoologist in your family.