New paper out in Ecology Letters!

My final PhD thesis chapter is now out in Ecology Letters: Linking life‐history theory and metabolic theory explains the offspring size‐temperature relationship. This work was in collaboration with my two PhD supervisors, Dustin Marshall and Craig White, along with Robert Bryson-Richardson at Monash University. We combined experimental work on a bryozoan (Bugula neritina) and zebra fish (Danio rerio) with two meta-analyses on over 70 ectotherm species to determine how the costs of development scale with temperature.

One common pattern in life-history theory is that among and within species, mothers in cold environments produce larger offspring. This pattern was first observed by Gunnar Thorson, a marine larval biologist in the 1950’s, and since then, tens of studies have reported that when mothers are reared under cooler temperatures, they produce larger offspring:

Relationship between the magnitude of change in offspring size with temperature (Hedges’ g) and change in experimental temperature. For the majority of 34 ectotherm species, an increase in maternal brooding temperature results in a decrease in offspring size.

One potentially general explanation for this pattern is how the costs of development scale with offspring size. If we assume that mother’s must provision their offspring with enough energy to sustain them throughout development (i.e fertilisation until onset of feeding), then offspring size must be proportional to the costs of development. Here, we measured the costs of development as development time x metabolic rate. We expect that as temperatures increase, development time decreases and metabolic rate increases:

A schematic showing how we expect development time (i.e in hours) and metabolic rate (i.e in mJ per hour) to change with increasing temperatures from T4 to T1

We reared individual larvae (Bugula neritina) and embryos (Danio rerio) across 4 temperatures and measured development time and metabolic rate. We found that while metabolic rate does increase with temperature, development time decreases at a much faster rate, such that overall, the costs of development decrease with temperature. We think that mother’s may be offsetting these costs under cooler environments by provisioning their offspring with more energy, thus making larger offspring under cooler temperatures.

To see whether this pattern holds more generally, we conducted a meta-analysis on over 80 species of ectotherms and found the same pattern for the vast majority of species:

Proportional change in the costs of development with a 10% increase in
temperature (relative to natural temperature range for each species), for 72 species. A less than proportional change in the costs of development (values <1) signifies a decrease in the costs of development with a 10% increase
in natural mean temperature

Interestingly, we also found that at temperature beyond those that we tested, the costs of development begin to increase. This is because the temperature sensitivity of metabolic rate becomes higher than that of development time. Thus, our results have implications for global changes in temperature – under extreme temperature increases, we expect developmental costs to increase, and mothers may need to make larger offspring. Such effects would reduce the productivity of ectotherms across a wide range of species. An important next step is to determine how the temperature sensitivity of development time and metabolic rates can evolve over short and long time scales.

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