Abstract 110 – Humming of a captive polar bear cub in the maternity den

Homare Yamamoto, Yusuke Sano, Kenji Aburaya, Sakura Ito, Aoi Maruyama, Maki
Yamazaki, Noriko Katsu, Kazunori Yamada

Animals rear their young in a variety of places, some of which are challenging for researchers to
observe. Wild and captive polar bears (Ursus maritimus) rear their cubs in dens for about four
months after their birth. In zoos, keepers often set up cameras in the maternity den during this
period. However, the cubs are rarely seen on camera because the mother’s body hides them. It
is known that cubs produce humming when they are suckling on their mother’s nipples. We
examined developmental changes in humming to comprehend the growth of cubs in dens,
which were difficult to observe directly. The subjects were a polar bear mother and her female
cub at Tennoji Zoo, Japan. A camera was set up on the ceiling of the maternity den, and video
was continuously recorded for four months after birth. The total observation time was 720 h.
To investigate the acoustic structure, we analyzed the spectrogram. A pulse (< 0.1 s) was
produced in rapid succession, and pulses composed a pulse train (range: 1.0–8.0 s). A series
consisted of two or more pulse trains, with a short inhalation after each pulse train. We
compared the frequency, duration, and interval time of series among each cub's age in months.
The proportion of series (< 10 s) was significantly lower at ages 2 and 3 months than at ages 0
and 1 months (p < 0.01). The frequency of series per hour decreased significantly with age of
the month (p < 0.01). The inter-series time increased significantly with age of the month (p <
0.01). Polar bear’s humming changed with its development. In 2023, three polar bears were
born in Japan, but all of them died. We compare the humming of surviving and deceased cubs
and examine whether humming can be used as an indicator of the physical condition of the
cubs.