Julie Christie with newborn North American river otter pup - Tilly, the North American river otter at the Oregon Zoo, gave birth to her second pup at Oregon Zoo (Nov 8)
The new arrival weighed just shy of 5 ounces at birth and has nearly tripled that thanks to mom's naturally high-fat milk.
Julie Christie, the zoo's senior North America keeper said: "We're pretty sure this pup's a male. But Tilly is very protective, so we can't be positive until our vets conduct a more thorough exam."
Tilly and her pup are currently in a private maternity den, and it will be another month or two before visitors can see them in their Cascade Stream and Pond habitat. Young river otters usually open their eyes after three to six weeks, and begin walking at about five weeks.
Julie Christie says: "Young river otters are very dependent on their moms, and Tilly has been very nurturing.
"She did a great job with her first pup, Mo, earlier this year. She raised him up from this tiny, helpless creature into the sleek, agile, full-grown otter he is today. We're confident Tilly will be a great mom to her new pup as well."
Keepers have yet to decide on a name for the new pup, though it is likely he will be named after a local river or waterway. Older sibling Mo is short for Molalla, after the Molalla River, whilst mum Tilly is named after the Tillamook River - where she was found orphaned near Johnson Creek in 2009. at just four months old.
North American river otters typically give birth from late winter to spring, but Tilly seems to be on her own schedule, keepers say. The breeding season for river otters is December through April, and actual gestation only lasts a couple of months. Unlike their European cousins however, North American river otters usually delay implantation so that the time between conception and birth can stretch to as much as a year. That hasn't been the case with Tilly.
Christie said it is also unusual -- though not unheard of -- for an otter to give birth to a single pup, as Tilly has now done twice. Litters - Portland, Oregon, United States - Wednesday 27th November 2013 (1 Picture)