HUB's LIST of medical fun facts

“Why do animals have sex, anyway?”
-Elizabeth Kolbert, The New Yorker, June 29, 2026

Procreation is necessary for evolution, but there are numerous ways for animals to procreate without having sex; fission, fusion (conjugation), budding, parthenogenesis, hermaphoriditism, even gender changes.  Darwin himself noted: “Why bother with sex at all when it is evidently possible to procreate without it.” (Ed.Note: Darwin married his cousin and had ten children)

“Sex must confer some important advantage. But what is it?”, writes Kolbert. Several theories have tried to answer that question: Muller’s ratchet (corrects the buildup of damaging mutations), Rec Queen hypothesis (protects against parasites), Vicar of Bray hypothesis (faster adaptation to environment changes), and most recently Size Matters (at least, gamete size and number; for instance, millions of swimming sperm looking for access to one big egg in humans) (1). Kolbert notes that “each of these theories have some appeal, yet all have been found wanting”.

Many species have more than two “mating types”. Single cell protozoa have seven distinct partners engaging in conjugation procreation. The common white-button mushroom has 18 mating types. The “winner” is the split-gill mushroom with the “astonishing 23,328 mating types”. Conjugation procreation (called isogamy) exchanges symmetrical, identical genetic material from each parent and off-spring are identical to both parents. Sex, as we know it, exchanges one-half of each parent’s genetic material to the off-spring, hence off-spring are either male or female. The off spring receive a different combination of genetic material than the parents. If the combination, or a mutation, provides a survival or procreation advantage to the offspring, evolution progresses. If the mutation or combination is harmful, to the offspring, it is not carried forward. So, sex is the engine of evolution in mammals.

Complicating this evolutionary binary appearance of male and female are species like Jesus birds where male birds sit on the eggs and the females go hunting, Brazilian lice where the females pierce the male’s abdomen with a penis-like probe to suck up its sperm, or the Clown fish where a male fish will transition into a female if the sole female in the group is lost. The Bluegill sunfish has one female and three male genders and females seem to prefer having their eggs fertilized by two bonded-males who then guard the nest. “There is no generally accepted definition of gender in the biological sciences.The  general trend in animal behavior is that every species that we observe thoroughly is way more complicated then it appears at first blush.” (2)

At least fifteen hundred species are known to engage in same-sex behavior (S.S.B.), including 80% of the twenty-two mammalian species studied.(3) Theorists speculate that S.S.B. emerged at the same time as sperm and eggs emerged, and that the behavior “proved adaptive enough to persist through the ages.”

Lents “finishes his book with a long section of the mating habits of humans” describing erotic liaisons for adolescent daughters to practice before they choose a husband in Cambodia; in New Guinea boys are taken from their mothers at age 9 years to live with men only and are expected to perform fellatio repeatedly; in the Himalayas woman marry the oldest son in a family and as the younger sons gain age they join the marriage, so that “who is the biological father is not really know, and nobody cares.”

Are there ANY benefits, other than procreation, for humans having sex? Studies have documented physiological benefits of the elderly (over 65 y0.) having sex,  including: improved immunity, lower systolic blood pressure, lower risks of heart attack, less pains, burns calories, improved sleep, eased stress, improved bladder control in women, lower prostate cancer risk, and improved cognition.  If this list looks familiar it is probably because of similar benefits are often listed for aerobic exercise. What about sudden death during sex? “Only 0.6% of sudden deaths recorded are associated with sexual activity, or one cardiac death (mostly men) per 10,000 person years” (JAMA 2011)

The moral of these studies of sex in Nature  is: “For every charming story of sex and gender fluidity, there are a number of stories involving rape, harassment, incest, traumatic sex, and infanticide. The natural world may be mesmerizing, but it doesn’t offer a reliable guide to what is—or should be—socially acceptable.”  Kolbert closes her essay with, “While copulating, female praying mantises often kill their mates and eat them. I understand the impulse, but I try to stop myself.”

References:
1. “On the Origin of Sex: The Weird and Wonderful Science of Reproduction”,  Lixing Sun, biologist, Central Washington University.

2. “The Sexual Evolution: How 500 Million Years of Sex, Gender, and Mating Shape Modern Relationships”, Nathan H. Lents, biologist, John Jay
College.

3. “Poking the Squid: What We Can Learn From Animal Sex,” Perrin Roosevelt Ireland,