Methods
Here we use post-reproductive representation (PrR) as a measure for
post-reproductive lifespan, which is measured as the proportion of adult
female years being lived by post-reproductive
females43. While a decline in fertility with age is a
general trait among animals44, the long
post-reproductive periods, often spanning more than a decade, observed
in humans and some toothed whales is a rare trait. Reports of
post-reproductive lifespan in other species often reflect individual
variation in senescence, rather than a general trait at the population
level or are calculated for populations living under artificial
conditions often with reduced mortality risk3,45–47.
A significant advantage of the PrR measure is that it is a
population-level measure that is directly comparable between species or
populations with different lengths of lifespan as it is the number of
female years lived post-reproductively out of all years lived by females
in a given population, while PrR also allows for a test of whether the
post reproductive lifespan is significantly larger than what is expected
by chance43.
Data type and collection
Long-term photo-identification data have been collected on three killer
whale populations with overlapping geographical ranges in the waters off
Washington State, the state of Alaska, USA and British Columbia, Canada:
Northern resident, Southern resident and Bigg’s killer
whales41 (Table 2). Photo-identification studies began
in 1972 for Bigg’s, 1973 for Northern residents and 1976 for Southern
residents. Data collection were boat-based and during each encounter
identification photos of dorsal fins and saddle were obtained from the
left side of each whale38,48. Sex was determined based
on the pigmentation of the ventral side of genital or mammary slits, the
presence of neonates or the size and shape of the dorsal fin of
adults41.