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Reproductive timing shifted in German harbour seals over three decades

A harbour seal mother beside her two-day-old pup on a sandy beach
Research · Reproduction & development

A study of harbour seals (Phoca vitulina) collected along Germany’s North Sea and Baltic Sea coasts has found changes in the seasonal timing of ovarian activity over three decades. Harbour seals normally undergo embryonic diapause, a temporary pause after fertilisation before the embryo implants in the uterus, making the timing of ovarian activity an important part of their tightly seasonal reproductive cycle. The researchers also reported age-related patterns that differed from earlier estimates for the species.

The study covered 1,523 females that were found dead or mercy-killed between 1995 and 2024. Ovaries from 383 of them were examined: 80 adults, 77 juveniles and 226 neonates. The tissue was cut into two-millimetre slices and checked for tertiary follicles, corpora lutea, corpora albicantia and scars. A corpus luteum forms in the ovary after ovulation, while a corpus albicans is the scar-like remnant left as it regresses, so these structures provide clues to recent reproductive activity and history. Results were compared between 1995–2015 and 2016–2024.

Sexual maturity appeared later and reproductive decline earlier than previous estimates

The first evidence of sexual maturity occurred at 3.5 years of age, six months later than previously reported. Reproductive activity declined from the age of 13, which was earlier than previously assumed.

Peak ovarian activity shifted from March to December

Peak ovarian weight shifted from mid-March in the earlier period to late December after 2015, suggesting that reproductive activation began earlier. The period of lowest ovarian activity also occurred later, indicating a longer reproductive window. Variation in the presence of corpora lutea during the expected period of embryonic diapause provided another indication that reproductive timing had changed.

The carcass material also contained a consistently high proportion of young animals, most of which died before reaching reproductive age. The authors highlight this juvenile mortality as a potential bottleneck for population growth.

The authors say the timing shifts may be linked to factors such as prey availability or population structure, but the study did not isolate a single cause.

Sources and image credit

  1. Schmidt, B., Steingrefer, F., Kesselring, T., & Siebert, U. (2026). Changing reproductive dynamics in harbour seals of the North and Baltic Seas. Theriogenology Wild, 8, 100152. https://doi.org/10.1016/j.therwi.2026.100152Primary peer-reviewed research article; accepted 13 March 2026 and first available online 14 March 2026.

Image: Bob Tilden — wild harbour seal mother with a two-day-old pup in Monterey, California, photographed 5 April 2022; representative species image, not an animal from the German study. Source: Wikimedia Commons. · CC BY 2.0 · Source

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