Love in a cold climate

Natural History
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Do butterflies like the Mourning Cloak overwinter with fertilized eggs to get a quick start in spring?

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Hey @GilesR , as always, thanks a lot for your questions. They really ake me delve deeper into the sience and learn new things! :slight_smile:

So, what I found is that Mourning Cloak butterflies (Nymphalis antiopa) employ a strategic overwintering method to ensure a quick start in spring, but they do not overwinter with fertilized eggs. Here’s the breakdown of their life cycle and reproductive strategy:

  1. Overwintering Stage: Mourning Cloaks overwinter as adults, seeking sheltered locations (e.g., tree cavities, under bark) and entering dormancy. This allows them to emerge early in spring, often before other butterflies.
  2. Mating Timing: They typically mate in late summer or fall before hibernation. Females store sperm in a specialized organ called the spermatheca during winter.
  3. Egg-Laying in Spring: Upon emerging in early spring, females use the stored sperm to fertilize eggs just before laying them. This enables immediate oviposition on host plants (e.g., willow, elm), giving their larvae a head start on fresh foliage.
  4. Advantage: By bypassing the need to mate in spring, Mourning Cloaks expedite reproduction. Their larvae develop into adults by mid-summer, and this generation may either enter dormancy or produce another brood, depending on climate.

So, while Mourning Cloaks do not carry fertilized eggs through winter, they do carry sperm while hibernating allowing rapid egg-laying in spring, aligning with their ecological niche as early-season emergers.

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Yesterday (March 19) at a woodland in Toronto I observed two Mourning Cloaks chasing one another for several minutes. They then dove to the ground behind a broken-off tree limb. I manoeuvred in the woods to get a clear photo and first found them side-by-side. While I watched they joined together so it was a male and a female. The upper butterfly then opened its wings. I took photos, then waited a while and then left. I assumed this was normal behaviour in spring. It was only when I saw this discussion in the forum that I realized this must have been aberrant behaviour. Perhaps it was a female who had not mated in the fall or the male simply overpowered her. Posted this observation yesterday in eButterfly with photos.

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The female may also benefit in the Spring from additional nurients passed by the male during copulation. Maybe that is why she is receptive to his advances.