To distinguish males from females by appearance, there are two clues...
- Males (upper in photo) have an enlargement of the vein on each rear wing. It looks something like a spot. It's actually a scent gland that helps the female identify a male by his odor.
- Females tend to have more intense orange coloration, while the black veins on the wing tend to be wider. So, they appear more vibrant.
The vibrant coloration of the female isn't a reliable indicator, because a newly eclosed male may appear more colorful than an old female, which has lost many of her colorful scales.
There are reliable details on the abdomen but observing them requires having the monarch in hand.
- The male has appendages for mating towards the end of his abdomen called "claspers."
- The female has a slit on the under surface of her abdomen near the posterior tip.
However, you can make a pretty good guess about a monarch's sex based on location and behavior.
On display grounds like the Odana Golf Prairie, about 90% of the monarchs there are males. You can tell it's a display ground because you see many monarchs chasing one another. Or you can make a fairly reliable guess about sex based on behaviors seen in your garden.
Behavioral differences
I've been watching a lot of monarchs this summer. I watched a male orbit around two yards for 1.25 hours. I followed a female down a rural road as she found milkweeds along the shoulder.
Males are looking for females at a display ground, or at smaller patches of milkweed or nectar flowers, such as a small garden.
Males tend to fly faster, higher, and more aggressively than females. I call this "patrolling." They have a "circuit" that tends to repeat itself roughly, but not exactly. If they see another male, they will give chase or even attack. Sometimes they don't make an overt attack but just swoop by the other.
One male attacks another that was nectaring on the flowers.->
Females are more laid back. Their behavior may look somewhat like patrolling, but they are looking for and evaluating milkweed--so they are flying a little more slowly and lower. They may fly past or circle around milkweed plants, without stopping. A fairly reliable sign it's a female is if she dips down into the foliage, especially foliage of milkweeds.
If the butterfly stays around your garden for several hours (sometimes a day or two), it's probably a male. You can distinguish individual butterflies by photographing them and looking for individualistic damage to their wings.
In contrast, females show up, make a few passes around your garden, maybe lay and egg or two, and are gone--even if you have lots of milkweed. This is because the female's strategy is to spread 700 or so eggs widely across the landscape, making them harder for predators to find. This "vanishing act" is one reason why females are so hard to catch for breeding.
Both males and females alternate the above behaviors with nectaring, resting, or basking. The amount of nectaring depends on temperature, with more nectaring (probably) on very hot days. Resting on a shrub or tree becomes more frequent towards the end of the day as they slowly wind down. Basking--spreading the wings in the sun to warm up--is a way of managing their temperature (thermoregulation), especially when it's cool.
Chasing by males (edited from Copilot AI)
Males patrol milkweed areas, which are prime spots for encountering females.
They often chase away other males and may even attempt to mate with them by mistake.
These aerial “twirling chases” can look aggressive, but they’re more about mate access than true territorial defense.
I've observed that some males are more aggressive than others. One old male with tattered wings was extremely feisty, even though he was smaller than most. Yes, insects have personalities.
So while it’s not territoriality in the classic sense—no fixed boundaries or long-term defense—it’s definitely a form of competitive behavior aimed at maximizing reproductive success.
Female Mate Choice in Monarchs
Males chasing females can force them to the ground or even grab them in the air. At first it seems like the males are forcing themselves on females--that the females have little choice. But there may be more going on...
Monarch females are surprisingly selective when it comes to mating, even though males often initiate the process with aerial chases and pheromone signals.
- Visual cues: Females may prefer males with brighter coloration and more vigorous flight—signs of good health and genetic fitness.
- Pheromones: Males release chemical signals to attract females, but females don’t always respond. They can reject advances by flying away or resisting mating attempts.
- Spermatophore quality: After mating, females store the male’s spermatophore (a packet containing sperm and nutrients). The size and quality of this packet can influence her future reproductive success.
- No cryptic female choice after mating: A study found that females don’t seem to favor sperm from unrelated males over brothers after mating, suggesting limited postcopulatory selection.