Fróðskaparrit - 01.01.1992, Blaðsíða 18

22
THEINTERNATIONAL RESEARCH ...
evidence of infertile ovulations, since at
least 10% of mature females exhibited one
or several corpora lutea though no pregnan-
cy was detectable. The existence of a post-
reproductive phase is controversial. The
age-related decrease in ovarian activity and
in the proportion of pregnant females sug-
gested the existence of a post reproductive
phase while other analyses support the
hypothesis that active reproduction can
potentially continue throughout life. In any
case, the pattem of apparition of a post
reproductive phase would be different from
the pattem described for short-finned pilot
whales (Kasuya and Marsh, 1984; Marsh
and Kasuya, 1984; 1986; 1991). Long-
finned pilot whale females may become
post-reproductive much earlier, but in the
same time some females were still repro-
ductively active at an older age, with the
two oldest pregnant females of the Faroese
sample being 41yrs and 55yrs old respec-
tively.
There is uncertainty on the length of ges-
tation, though newly developed statistical
techniques resulted in a much shorter ges-
tation period than the 15.5 months usually
accepted for pilot whales (Sergeant,
1962a). The gestation seemed more likely
to last between 11 and 12 months.
The mean duration of lactation was
assumed to be 3.4 years, but the duration of
lactation increased dramatically with moth-
er age and may last more than 9 years in
female older than 40 years. Other indepen-
dent parameters indicated also that the lac-
tation period may be extended. Based on
the comparison between the number of lac-
tating females and the age composition of
juveniles in a school, the age at which
weaning was completed varied between 3
and 6 yrs according to schools and assump-
tions made. On the basis of stomach and
parasite examination, the mean age at
which pilot whales begin to catch prey was
calculated to be five months for both sexes,
with important individual variations. There
was, however, evidence for a protracted
period of partial suckling with more than
25% of calves in the age-classes 3 and 4,
and the observation of milk in the stomach
of a 7 year-old male and a 12 year-old
female.
The proportion of mature females giving
birth each year was assumed to range
between 0.21 and 0.17. The gross birth rate,
or annual reproductive output, was estimat-
ed to be 7.5% and the mean inter-birth
interval 5.3yrs.
Adult males showed a clear seasonal cycle
in testicular activity, with testis weights and
testosterone levels being in average 1.5 and
2.5 times higher respectively in the period
March-September than in the period
October-February. The annual profile of
testosterone concentration is, however,
bimodal with a significant decrease in July,
clearly suggesting two peaks of sexual
activity. The frequency distribution of con-
ception dates is also clearly seasonal. More
than 50% occurred in the period May-July
with a peak in June (a second one in
October in a bimodal pattern of breeding)
and few conceptions occurring in January
and February. However, since conceptions
were spread through nearly the entire year