Measures of offspring quality
In addition to spontaneous
abortions, we recorded the wet weight of viable pupae and, for a subset,
we measured either fat, or whether they emerged and their starvation
tolerance upon emergence (Fig. 1). Wet weight of pupae correlates with
pupal volume in field flies and does not require destructive sampling
(Hargrove 1999). We recorded emergence as we hypothesised that offspring
with more reserves would be more likely to emerge. We measured
starvation tolerance because, in the wild, newly emerged tsetse have a
high probability of mortality (Hargrove et al. 2011), likely due
to failure to find a blood meal before running out of energy reserves.
From preliminary data on the tsetse colony at LSTM (S1 File), we
hypothesised that there would be an increase in the probability of
abortion with age. If resource-allocation trade-offs exist, we would
also expect that the increase would be steeper for the nutritionally
stressed treatment and potentially shallower for the mating delay
treatment, relative to the control. However, for offspring wet weight
and starvation tolerance we predicted that there would be an initial
increase, reflecting the requirement for newly emerged flies to also
divert energy and nutrients to the development of thoracic muscles
(Anderson & Finlayson 1973), followed by a decline due to senescence.
Lastly, we hypothesised that the effect of maternal age on starvation
tolerance would be determined largely by energy reserves, as indirectly
measured by offspring wet weight. Heavier offspring would have more
reserves and therefore would be more likely to emerge and have higher
starvation tolerance. Maternal age could also influence offspring
starvation tolerance, independent of effects on wet weight, through the
quality of the resources provided.