Is There Any Association Between Digit Ratio And Natural Occurring Depression Macaca Fascicularis?

Background:The second-to-fourth (2D:4D) digit ratio has been associated with human behavioral dysfunction including autism and dyslexia. However, the relationship between the digit ratio and depression in primates remains unclear. This investigation aimed to provide evidence (if any) of a relationship between the digit ratio and depression status in the cynomolgus monkey (Macaca fascicularis).Method:Firstly, to select the typical depressed Macaca fascicularis as potential samples, and validated via Shiverly reported criteria via behavior observation from each free enclosures; to further confirm by compare the behavior differences from socialisolation induced depressed monkeys and controls. Secondly, Finger lengths from the right and left paws of both the forelimbs and hind limbs of 15 depressed monkeys and 16 randomly-selected control subjects were measured. Finger length and digit ratio comparisons between the two groups were conducted using Student's t-test.Results:On the forelimb, the ring and little finger were significantly longer in the left and right paws in the control group as compared to the depressed monkeys. In the hind limb, the middle, ring, and little fingers on the left paw were significantly longer in the control group relative to the depressed monkeys; as for the right paw, only the middle finger was significantly longer in the control group as compared to the depressed monkeys. The 2D:4D,2D:5D, and 3D:4D in the left paw in the forelimb, 3D:4D in the right paw in the forelimb,2D:4D in the left paw in the hind limb, and 2D:3D,2D:4D, and 2D:5D in the right paw in the hind limb were significantly lower in the control group as compared to the depressed monkeys.Conclusions:This is the first study to reveal significant differences in finger lengths and digit ratios between depressed monkeys and healthy controls.