We all know porpoises reproduce, but did you ever stop to think how they go about it?
They live in a 3D world where everything, including love making, is much more difficult!
Recently, the open-access book "Sex in cetaceans" was published and includes an entire chapter on porpoises. Let's go through the key bits together!
Although males are smaller than females, there are no clear characteristics that would allow us to distinguish the sexes at first sight (think, on the other hand, about the tall and erect dorsal fin of adult male orcas). If we catch a glimpse of the underside, though, it is possible to clearly see the mammary glands in females, who also have just one slit (genital/anal) while males have two distinct slits. This is the same for all cetaceans.
So, what about anatomical characteristics that play a role in the sexual behaviour of harbour porpoises? Some animals have small, raised epidermal tubercles on the back or dorsal fin. They seem to rub each other on these.
In 2018, William Keener and colleagues published a first study on the mating behaviour of any porpoise in the wild. The Golden Gate bridge in San Francisco (US) is a unique platform for observation of harbour porpoises without disturbing their natural behaviour. The authors described a "lateralised" behaviour: males ALWAYS approach the females from the left side. Soon other researchers elsewhere noticed the same!
Harbour porpoise behaviour in nature. (a) High leap, English Channel, UK. (b) Male left-sided mating approach with penis on the female’s right side, Prince Rupert, Canada. (c) Energetic male left-side mating approach, penis erect, female fluke lift and partial body roll, San Francisco Bay, California, USA. (d) Male with erect penis immediately after a left-sided mating approach, San Francisco Bay, California, USA. (e) Male left-sided approach perpendicular to the female, Salish Sea, Washington, USA. (f) Typical male mating approach to left side of the female, Black Sea, Romania. Photos: (a) Rebecca Knee/Marine Discovery Penzance; (b) Caitlin Birdsall/Ocean Wise Research; (c) Joey Meuleman/The Marine Mammal Center; (d) Marc A. Webber/The Marine Mammal Center; (e) Cindy R. Elliser/Pacific Mammal Research; (f) Romulus-Marian Paiu/Mare Nostrum NGO
What's that about?
Well, that's because the spiral shape of the vagina! the male would have no chance at mating if attempting to mate from the right side of the female.
Similarities in extensive vaginal folding of sexually mature harbour porpoises from the North Pacific (Alaska) on the left and the North Atlantic (Germany) on the right. Vaginal structures show asymmetry as a gauntlet for penis or sperm before reaching the cervix (truncated at top of images). Animals in dorsal recumbency with incision along ventral midline. Photos: Dara N. Orbach/Texas A&M University-Corpus Christi (left); Alexandra Rieger/University of Veterinary Medicine Hannover (right)
Penises of post-mortem sexually mature vaquita (lower left) and harbor porpoise (upper right), artificially inflated to simulate erection. Ruler = 15 cm. Photo: Dara N. Orbach/Texas A&M University-Corpus Christi
What's probably even more curious about this behaviour, is the seemingly explosive nature of the event. It all happens really fast - 2 or 3 seconds and it is al done! In most cases it is the male approaching the female, but not always. When the approach occurs, females do not always welcome the males, usually responding by rolling their bodies or lifting their flukes, although some animals also change directions or dive.
Interestingly, the sexual behaviour is not restricted between male and female, as there is at least one observation of male-male sexual behaviour. Annnnd, of a male calf displaying sexual behaviour with his mother!
This may be a form of learning/practice to know how to behave sexually when reaching maturity. That, though, is still to be discovered!
In Fjord&Bælt, in Denmark, there are currently 3 porpoises: 2 females (Freja and Saga) and 1 male (Eskild). He is a few years old, but began showing sexual behaviour towards the females before reaching sexual maturity, especially towards Saga, the younger of the two. As in the wild, Eskild approaches Saga and Freja from the left side, every time.
Harbour porpoise mating behaviour observed at Fjord&Bælt in Kerteminde, Denmark. Females were stationed at the surface near the edge of the pool enclosure. (a) Subadult male “Eskild” copulates with subadult female “Saga.” (b) Subadult male rubs penis against left flank of adult female “Freja.” Photos: Freja Jakobsen/University of Southern Denmark
These amazing discoveries were possible to a group of highly dedicated researchers (see full list below!) as well as with the use of emerging technology, especially drones and underwater cameras. Given that mating behaviour does not always involve activity at the surface, there is still much to about it - including the acoustic behaviour of the animals involved. What are they saying to each other?
Book:
Sex in Cetaceans. Morphology, Behavior, and the Evolution of Sexual Strategies.
Editors: Bernd Würsig and Dara N Orbach. 2023 - Springer.
Chapter 18:
Sexual Behavior and Anatomy in Porpoises
Authors: Marc A. Webber, William Keener, Magnus Wahlberg, Cindy R. Elliser, Katrina MacIver, Sara Torres Ortiz, Freja Jakobsen, Héloïse Hamel, Alexandra Rieger, Ursula Siebert, Holly Dunn, David Anderson, Anna M. Hall, Caitlin Birdsall, Kate Pielmeier, Romulus-Marian Paiu, Deborah D. Boege Tobin, and Dara N. Orbach.