Evidence for a postreproductive phase in female false killer whales Pseudorca crassidens

TL;DR

This study provides morphological and statistical evidence that female false killer whales experience a significant postreproductive lifespan, making them the third non-human mammal with confirmed PRLS in wild populations.

Contribution

It demonstrates the presence of postreproductive lifespan in false killer whales using new morphological and quantitative measures, expanding understanding of mammalian aging.

Findings

Morphological evidence of ovarian activity cessation in older whales.

Calculated PrR indicates significant postreproductive lifespan.

False killer whales show PrR values comparable to humans and other mammals.

Abstract

A substantial period of life after reproduction ends, known as postreproductive lifespan (PRLS), is at odds with classical life history theory. Prolonged PRLS has been confirmed in only two non-human mammals, both odontocete cetaceans. We investigate the evidence for PRLS in a third species, the false killer whale, Pseudorca crassidens, using a quantitative measure of PRLS and morphological evidence from reproductive tissue. We examined specimens from false killer whales from combined strandings (South Africa, 1981) and harvest (Japan 1979-80) and found morphological evidence of changes in the activity of the ovaries in relation to age. Ovulation had ceased in 50% of whales over 45 years, and all whales over 55 years old had ovaries classified as postreproductive. We also calculated a measure of PRLS, known as postreproductive representation (PrR) as an indication of the effect of inter-population demographic variability. PrR for the combined sample was 0.14, whereas the mean of the simulated distribution for PrR under the null hypothesis of no PRLS was 0.02. The 99th percentile of the simulated distribution was 0.08 and no simulated value exceeded 0.13. These results suggest that PrR was convincingly different from the measures simulated under the null hypothesis. We found morphological and statistical evidence for PRLS in South African and Japanese pods of false killer whales, suggesting that this species is the third non-human mammal in which this phenomenon has been demonstrated in wild populations. Our estimates for PrR in false killer whales (0.12-0.37) spanned the single values available for the short-finned pilot whale (0.28) and the killer whale (0.22) and are comparable to estimates for historical or hunter-gather human populations (0.3-0.47).