Two puzzles: Hurricanes and menopause

 

Last Monday, the Department was very excited to welcome Prof Shripad Tuljapurkar for the weekly departmental lecture. This time the lecture was special: Prof Tuljapurkar’s impressive body of work using mathematical models in biology was recognised with by the annual Weldon Memorial Prize. In his award lecture, Prof Tuljapurkar discussed two unsolved puzzles in ecology and evolution as examples of the large repertoire of questions that he has tackled in his career: the role of hurricanes in plant population dynamics and the evolution of senescence.

First, Prof Tuljapurkar addressed how to optimally examine the effects of extreme events, like hurricanes, on the response of different species. With the changing climate, the occurrence of high scale extreme events is likely to increase. The effect of this can be modelled by comparing a stochastic model based on the current generalised extreme value distribution value distribution (GEV) with a stochastic model with an adjusted GEV. With this approach, Prof Tuljapurkar and his collaborators were able to show that a change in extreme events is not always gloom and doom for all species.

In the second part of his talk, Prof Tuljapurkar addressed the mysteries around the existence of menopause, whereby there is decreased fertility with continued survival. The current fitness of an individual depends on the age specific fertility rate and its survival probability. After menopause, the age specific fertility rate goes to zero, and so individuals should run into a wall of death, whereby their contributions to the population make them irrelevant.  So, how is it possible that some species life long after their age at menopause?

The clue seems to be that older individuals may be able to “borrow” fitness from the young. This phenomenon occurs mostly among males. Males with an age older than the age of female menopause reproduce with younger females which results in a fitness value for post-menopause aged males. Which results in selection against age-dependent mortality due to mutations, which is also beneficial for females. Another possible explanation that specifically targets female menopause is the grandmother hypothesis; according to this hypothesis, older females still have a fitness after they stopped reproducing, by helping younger females to raise their offspring.

Before we knew it, time was up. Overall, Prof Tuljapurkar give us a glimpse of his outstanding work and it was a great honour to have him over. Also, the questions afterwards showed us that there are a lot more puzzles in ecology and evolution that we can continue to puzzle over.

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