Of kings and queens
Sexual dimorphism in monarch butterflies
I initially started writing a follow up about monarch migration in regards to strategies for fall, but got slightly derailed. After taking photos of several monarchs last week, I came back home to edit them and noticed that some monarchs had a pair of black dots on their wings while others didn’t. Which got me thinking: there was a clear morphological difference, and differences like this that occur within a species are either due to age, sex, or, in rare cases, diet.
Monarch demographics
It turns out that it’s not an uncommon pattern for male butterflies in some species (e.g. clouded sulphurs) to have a pair of scent glands on their wings that attract females. As with the clouded sulphurs, female monarchs use olfactory cues from the males to select among the stock of potential mates (source). The scent glands are small black scales that are raised up on the top of the wing. Despite the clear difference, I found my initially ID of a female was often wrong. As I started getting better at telling the difference in the field I noticed that the scent glands are faintly visible on the underside of the wing when the wings are folded up, and occasionally the body of the butterfly will obscure the glands so waiting for the butterfly to shift positions was helpful. Check out the photos below to get a better sense of the difference.
Sex Ratios in Monarchs
Part of the delay in sending this out was that I wanted to head back out to get good photos of both the males and females. I headed out to a big meadow along the Winooski River where there’s goldenrod and joe-pye weed aplenty. It was a somewhat informal survey, but of the 32 monarchs that I spotted, 21 were male and 11 female. It’s probably too small of a sample size to really draw any meaningful conclusions, but was enough of a skew that I wondered whether the populations might be male-biased. Turns out I’m not the only one. A 2009 study found that, oddly, female populations of monarchs were dwindling faster than males. Between sampling from 1976-1985 and 2009, the population had shifted from 53% female to 43% female. The researchers didn’t draw any definitive conclusions, but suggest that a parasitic protozoan, which preferentially attacks females, may be the culprit.
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