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Sexual Selection & Sex Differences in Lifespan

1:21:14 recording · AUTO · 2 speakers

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Brief overview

Zoo records across mammals and birds show no single rule for which sex lives longer.

  1. No universal rule on which sex lives longerEither sex can live longer depending on species; 11 species showed no difference and over 70% of canids in zoos break the expected female advantage.
  2. Zoos remove pressures but add their ownPredation, starvation and hunting are off the table, but contraceptives, generations of captivity and zoo-to-zoo variation confound the comparison.
  3. Observational data cannot settle the mechanismVincent stresses that confounding variables make conclusions impossible, and Nels wants chromosomal manipulation in model systems such as mice.
Executive Summary AI
  • Vincent brings Nels a paper titled "Sexual selection drives sex difference and adult life expectancy across mammals and birds", prompted by the roughly ten-year female advantage in his US life-expectancy teaching graph.10:26
  • Nels lays out the three leading explanations the authors weigh: the deleterious effects of the heterogametic sex chromosome (XY males in mammals, ZW females in birds), the intensity of sexual selection, and the cost of reproduction.17:07
  • The data come from the Species 360 Zoological Information System (ZIMS), lifting earlier samples of roughly a hundred mammals and a couple of hundred birds to almost 530 mammals and 650 bird species, covering 20 of 29 mammalian orders and 31 of 42 bird orders, plus 69 wild mammals and 41 wild birds for comparison.24:42
  • They score each species on a minus-one-to-one advantage metric where 0.5 means a 50% female advantage, and find mammal and bird distributions that barely overlap yet full of exceptions: carnivores show a male advantage in zoos, over 70% of canids break the expected female advantage, 11 species show no difference at all, and lemurs and large flightless ratites such as ostriches, tinamous and emus s32:23
  • The authors say their results only partially support the heterogametic sex hypothesis, point to pre-copulatory costs in non-monogamous mammals, and close on Swedish records where male and female life expectancy were similar in the 1700s and diverged mainly as modern medicine cut maternal mortality in childbirth.41:28
Key Quote
“Drive fast, take chances, encode a Y chromosome.”
— Nels21:06
Key Quote
“You can never, ever make conclusions from an observational study because you can't get rid of all the confounding variables.”
— Vincent28:18
Key Quote
“So you can't just make a generalized statement.”
— Vincent44:34