The reproductive tract of harbor porpoises
Keywords: porpoise, harbor, odontocete, marineThe reproductive tract of a the odontocete (toothed) Harbor porpoise, Phocoena phocoena.
An immature specimen:
It was estimated that this porpoise was approximately 9 months old, being born in late May of 2013.
Image size: 1199 x 676px
Notable features of the tract included the following:
- Slit like vulva lips and a small but prominent clitoris (yellow arrow)
- A long and anteriorly pointed bladder (green arrow) with prominent round ligaments.
- Length and position of the vestibule, external urethral opening and total length of the vagina that were proportional to those of domestic ruminants and horses.
- A very prominent and broad cervix, very much wider in proportion to that of any domestic animal.
Image size: 1000 x 654px
In addition, in the immature specimen there was a...
- Transverse, interdigitating cervical rings, reminiscent of those in domestic ruminants and unlike that of a mare.
This is also shown in a cow (mature female porpoise) during advanced pregnancy:
Image size: 1000 x 750px
The immature specimen also showed...
- A "T" shaped uterus, more akin to the shape of an equine uterus that that of a ruminant uterus.
- An intercornual ligament, intermediate in proportion to that of a ruminant (obvious and with both dorsal and ventral components) and that of a mare (small and with only a single component).
- Lengths of the uterine horns that were proportionally similar to those of mares and ruminants and much shorter in proportion to those of a Pilot whale as seen in this LORI entry.
- Ovaries that were elongated and perhaps more similar to those of a cow than those of a mare. Interestingly, the ovarian bursae were proportionally huge in comparison to those of domestic farm animals, akin to those of the Pilot whale, another cetacean.
Notes: The Harbor porpoise is a small porpoise, being about 70 cm long at birth and 1.5 meters in length at maturity. They weigh about the same as a standard (70kg) human but females are generally larger than males. This species tends to be found in the Northerm latitudes and is well adapted to life in both salt and fresh water, sometimes being found in rivers, long distances from the sea.
Harbor porpoises reach puberty at about 60% of their mature body weight, at about 3 years of age. Their breeding season is usually strictly seasonal and on the east coast of North Amnerica, calves are born towards the end of May and breeding occurs in late June or July. In males, the exceptionally large testicles (about 5% of body weight during breeding) regress in size in late summer.
From data on unintentional catches, it was shown that large and apparently functional corpora lutea were present in the ovaries for about a month after breeding, yet embryos or fetuses were not seen during that time. Only in August were the smallest fetuses visible. The assumption therefore, was that that harbor porpoises experiences embryonal diapause of perhaps a month in duration. As mentioned, term fetuses are born in late May, suggesting that the active period of gestation is about 9.5 months with a total gestation length of about 10.5 months.
It should be noted however that the suggested presence of embryonal diapause was not consistent between the studies cited below. Porpoises in the Nordic and Canadian studies had slight genetic differences which could account for that difference.
The ability of most (but not all) harbor seals to calve every year and the simultaneous presence of lactation and large corpora lutea in the ovaries suggest that these animal are similar to equids in that they do not experience postpartum lactational anestrus. Interestingly, in only one of 122 females studies was there was evidence of bilateral ovulations; otherwise all corpora lutea and corpora albicanti were found in the left ovaries. Also, in the 13 pregnancies in those animals, a single fetus was only occurred in the left uterine horns. In agreement with those findings, there was a single corpus luteum in the left ovary, supporting a single pregnancy on the same side.
Image size: 1126 x 1264px
Although the occurrence of unilateral ovulations is surprising, the presence of single pregnancies in left uterine horn is the same as that in camelids.
This image also shows that the harbor porpoise has diffuse placentation, akin to that in mares.
References:
1.Read, A. J. 1990. Reproductive seasonality in harbour porpoises, Phocoena phocoena, from the Bay of Fundy. Can. J. Zool. 68: 284-288.
2.Read J. and Hohn A.A. 1995 Life in the fast lane:The life history of
harbor porpoises from the gulf of main. Marine Mammal Science. 11: 423–440.
3.Lockyer C. et al 2001 Age, length and reproductive parameters of harbour porpoises
Phocoena phocoena (L.) from West Greenland. ICES J. Marine Science. 58: 154-162.
