That leftmost of the five graphs, the one that shows in its shaded area the proportion of the hydra sample that still lives, confuses me. What could possibly cause a linear decrease in population? A constant probability of death given previous survival (podgps) would produce a decreasing exponential curve, but this requires a podgps for each individual that is exactly hyperbolically growing over time, for example through an internal candleclock that is at birth set to a uniformly distributed time between 0 and 1400 years.

(Yes, hyperbolically. As the time interval lengths during which we record whether the hydra died go to 0, the ratio of podgps between the last time segment and the, say, 3rd goes to infinity. The crux of the matter lies in "given previous survival" and if we leave that part out, the result would indeed be a constant line.)

Maybe the researchers found by experiment or hearsay that the hydra has a constant podgps, the graphmakers failed to record that last bit and simply took the negative integral of that absolute probability of death to generate that ridiculous population curve and then the authors found that that integral curve hits 0 at 1400 years and postulated that as their livespan.

(By the way, it's also troubling that they also left out the bit of the curve that would have indicated infant mortality.)