The Wiring of Desire

From the September issue of Seed:

I met a leopard gecko the other day. He sat on my hand, scarcely moving, for a full 15 minutes. So we took a good look at each other—I gazed into his still, dark eyes, and he seemed to gaze at mine.

He was unlike any gecko I had seen before. He had smooth, pale skin spattered with dark spots, a large wedge-shaped head and a tail of pronounced fatness. (He cannot, however, walk on ceilings—unusually for a gecko, he doesn’t have adhesive feet.)

But the fatness of his tail and the non-stickiness of his feet were not the reasons I was interested to meet him. My interest stems from the fact that leopard geckos—more formally known as Eublepharis macularius—provide the clearest example of how the early environment can sculpt the brain and affect sexual desire.

Leopard geckos breed easily in captivity. Which is lucky; to encounter one in the wild, you’d have to head to the rocky deserts and arid badlands of Iraq, Iran, Pakistan and Afghanistan. There, they snooze in burrows during the infernal heat of the day, and venture forth at night to dine on insects, spiders and scorpions.

As is often the case for lizards, leopard geckos have their sex determined by the temperature at which the eggs develop. The particular temperatures vary from one species to another. In leopard geckos, eggs incubated at 26° C (78.8° F) all hatch female. At 30° C (86° F), 30% of the eggs will hatch male and 70% female. Raise the temperature another 2.5° C, and that sex ratio flips—a third of the geckolings will be female and two-thirds, male. But go up another 1.5° C and 95% of hatchlings will be female. In other words, you find cool chicks and hot chicks, but tepid fellows. (And yes, if it gets unseasonably cold, the population will be all female.)
As it happens, incubation temperature has profound effects, not simply on the gonads a gecko carries, but on the structure of its brain—and thus, on myriad aspects of its behavior. Females that hatch at intermediate temperatures—the temperatures that predominantly produce males—tend to be big and highly aggressive, far more so than females from “female” temperatures. Males from cool temperatures are less aggressive than usual—but are much more sexually rambunctious. They also have more processing power in those parts of the brain that are important for wooing.
Interestingly, egg incubation temperature also accounts for differences in male perceptions of which females are attractive. In short, it affects the wiring of sexual desire. Given the possibility of consorting with a hot female or a cool female, males from cool temperatures prefer hot females. Tepid males, in contrast, like their mates to be cool.  A fellow’s lust, therefore, is affected not only by his own early environment, but by the early environment of his mate.

How can they tell who’s who? It’s not clear, but there are several possible cues. Egg temperature affects looks: Cooler temperatures produce individuals with darker spots. And it probably affects scent. Females that come from different temperatures have different hormone profiles—so they probably smell different. When a male flicks his tongue through the air, his attrac­tion to one female over another may, quite literally, be a matter of taste.
It turns out that an embryo’s environment has similarly powerful effects in other species. Mice, for example, give birth to more than one pup at once—and the behavior of adult mice is affected by whom they were next to in the womb. A female who was between two sisters is more docile as an adult, and males tend to find her more attractive than other females. She is also more likely to be attacked if she rejects a male’s advances. A female who was sandwiched between two brothers will be more aggressive—and less attractive to males. Among gerbils, females lust after males who were between two brothers in the womb; yet such males are much less likely to engage in childcare than males who were between their sisters.

All of which is a salutary reminder that early environment can have effects that are just as profound as any gene; long before an animal first opens its eyes, events wire its brain and have subtle but indelible effects on the attitudes and tastes of the adult.

Originally published October 9, 2006


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