“Birds do it, bees do it, even educated fleas do it…” [Cole Porter]
Paper hearts, sentimental rhymes, romantic meals, and images of “lovebirds” — it must be Valentine’s Day!
For many of us, Valentine’s Day involves romance or light-hearted celebrations of pair bonding: the selection of a mate and the long- or short-duration connection with that mate. Humans spend a great deal of energy in setting up and maintaining such connections; many other species do, too.
Why are there males and females in the first place? The biological advantage of sex is to mix and recombine the population’s genetic material. Such mixing diversifies the population over the generations and makes it more adaptable to change. That genetic variety is key to the long-term success of the species and is the prime mover of evolution. That mixing is so valuable that even bacteria exchange genetic material.
If you tallied all animal species, from worms to whales, you’d find that most indulge in rather rudimentary or short-term sexual relationships. Most marine invertebrates, such as seastars and clams, for example, simply cast their eggs and sperm into the water to mix and combine at random. The biggest challenge in such cases is to get the cues or timing right.
Internal fertilization sets the stage for more elaborate sexual behavior, likely because of the higher biological investment by the parents. And when offspring require significant care, pair bonding becomes more important because it better guarantees both parents will be around to help raise the young.
Male and female animals that bond in reproductive pairs use a variety of strategies to do so. While each species has its own approach, there are a handful of key patterns.
The norm for invertebrates, and for most fish, amphibians, and reptiles, is promiscuity. Promiscuity is also common among mammal species (perhaps because the female mammal’s production of milk means she carries the primary burden of infant care). In promiscuous species, each adult of both sexes has many mates and each pair’s relationship is limited to the actual mating.
Promiscuity is much less common in birds, perhaps because male and female parent birds can share the care of offspring. In mammals, rodents, shrews, and bats tend to be highly promiscuous. Males and females in those groups are generally the same size, and the males have little to do with the offspring they sire.
Another tactic is for individual adults of one sex mating with several partners, called “polygamy.” In the most common style of polygamy, “polygyny,” one male mates with several or many females; in some polygynous species, like elephant seals and elk, males collect a harem of females. In highly polygamous species, males tend to be larger than females.
The reverse of polygyny is “polyandry,” where females have several mates, but males have only one. Species that practice this rare tactic include the Western snowy plover and phalaropes.
Relatively rare in the animal kingdom, “monogamy” (one male mating with one female each reproductive season) is most common in groups of animals that have high-maintenance offspring. Monogamy involves various lengths of pair commitment: about 90 percent of bird species are monogamous for at least one season and a few species, such as certain geese, mate for life — a better record than many humans.
Pair bonding, especially for long-term monogamous species, often involves rituals that establish or confirm the bond. Some birds are noted for their elaborate greeting rituals, rituals that may include mirrored postures, feeding or grooming a mate (“billing”), or sharing songs (“cooing”).
Such human-appearing pair-bonding rituals — and birds’ affinity for monogamy — almost certainly play a role in our selecting birds to symbolize romantic love.
The hearts are another story.