Sexual Dimorphism in Titi Monkeys’ Digit (2D:4D) Ratio is Associated with Maternal Urinary Sex Hormones During Pregnancy

The second‐to‐fourth digit (2D:4D) ratio is a sexually‐dimorphic biomarker for prenatal sex hormone exposure. We investigated whether titi monkeys (Plecturocebus cupreus) exhibit sexually‐dimorphic 2D:4D ratio, and whether variation in 2D:4D ratio correlates with maternal testosterone and estrogen levels during early pregnancy. Subjects were 61 adult titi monkeys (32 males, 29 females). For 26 subjects, maternal urine samples were collected approximately 15–20 weeks before birth and assayed for testosterone and estrone conjugate (E1C). Titi monkeys exhibited a human‐like pattern of sexual dimorphism in right‐hand 2D:4D ratio, with females exhibiting higher 2D:4D ratio than males (β = −0.29, p = 0.023). For left‐hand 2D:4D ratio, high levels of maternal E1C predicted low offspring 2D:4D ratio (β = −0.48, p = 0.009). For right‐hand 2D:4D ratio, high levels of testosterone (β = −0.53, p = 0.005) and testosterone‐to‐E1C ratio (β = −0.41, p = 0.028) predicted low offspring 2D:4D ratio. For 2D:4D ratio asymmetry (right‐hand – left‐hand), high levels of testosterone (β = −0.43, p = 0.03) and testosterone‐to‐E1C ratio (β = −0.53, p = 0.003) predicted low (right‐biased) asymmetry. This is the first report of sexually‐dimorphic 2D:4D ratio in New World monkeys, and the results support a growing literature suggesting prenatal sex hormones may modulate offspring 2D:4D ratio.