Discussion
While there is substantial evidence of actuarial senescence in insects
and some evidence for reproductive senescence (Nussey et al.2013; Zajitschek et al. 2019), the drivers of variation in
senescence patterns are not fully understood. Here we manipulated both
age at mating and nutrition to quantify reproductive senescence in
tsetse, a viviparous fly with high maternal allocation and iteroparous
reproduction. Both offspring weight and starvation tolerance declined
with maternal age, after a peak, yet these patterns of senescence were
similar across treatments. Contrary to predictions from life history
theory, therefore, neither changes in maternal allocation nor resources
affected the timing and rate of reproductive senescence, in terms of
offspring quality.
We did observe a steeper increase in the hazard of mortality with age
for nutritionally stressed mothers (S5 File). This suggests that females
do not have a survival benefit from reduced reproductive effort, as
nutritionally stressed mothers had higher probability of abortion and
produced smaller offspring at any age compared with mothers in the
control and mating delay groups. This contrasts with findings from
studies of other insects where nutritional stress both reduced
reproductive output and either maintained or even extended lifespan
relative to a control group (De
Sousza Santos & Begon 1987; Ernsting & Isaaks 1991; Kaitala 1991).
Taken together, it may be that
factors other than the direct costs of reproduction through
physiological damage, or indirectly through resource allocation
trade-offs, impose higher mortality rates. Alternatively it may be that,
given the extreme maternal allocation in tsetse, even though females
produce relatively smaller offspring they still pay a high cost of
reproduction in terms of physiological damage; and females on a poor
quality diet experience this cost to a greater extent in terms of impact
on mortality. This is supported by data from field-caught tsetse, where
smaller females invest relatively more of their fat in their offspring,
even though their offspring were smaller (Hargrove et al. 2018).
We find that females experiencing nutritional stress have a relatively
higher rate of abortion. Hargrove and Muzari (Hargrove & Muzari 2015),
using field collected G. pallidipes , showed that transfer of the
majority of fat to the larva occurs only after c. 80% pregnancy has
been completed. Therefore, a female could potentially abort a larva if
there are not enough fat reserves for a full-term pregnancy.
Evolutionary models tailored to tsetse life-history, with high
investment in single offspring across multiple reproductive bouts, could
yield insights into whether such spontaneous abortion is an adaptive
strategy to retain reserves for future reproduction, or a result of
physiological constraints that limit the reserves available (McNamaraet al. 2009).
Our study highlights the benefits gained from individual-level data to
understand senescence. The bell-shaped relationship of offspring quality
with age may have contributed to the relatively small effects of age
evident in previous studies where grouped ages and mean values were
used, rather than tracking reproductive output from individual females
(Langley & Clutton-Brock 1998; McIntyre & Gooding 1998). The
bell-shaped pattern observed here is strikingly similar in shape across
treatments and reflects the general bell-shaped pattern of reproductive
senescence observed across diverse taxonomic groups e.g. (Velando et al.
2006; Sharp & Clutton-Brock 2010).
As summarised by (Monaghanet al. 2020), a bell-shaped relationship between maternal age and
reproductive output, or offspring quality, can arise from
population-level effects, due to selective disappearance, but also from
individual-level effects. By tracking individual mothers, here we
provide evidence that, for tsetse, the bell-shaped relationship between
offspring wet weight and maternal age is a consequence of individual
effects. The initial increase in offspring wet weight could be a
consequence of mothers accruing reserves each time she takes a
bloodmeal, so that at each sequential reproductive event she has more
energy reserves that can be provided to the offspring. The subsequent
decline could then be explained by senescence.
Tracking
individual mothers provided insights into individual variation in
maternal allocation. There was marked variation in offspring wet weight
among mothers, particularly for the mating delay treatment and variation
in senescence patterns for wet weight, particularly for nutritionally
stressed mothers. Some females in
the mating delay treatment consistently produced smaller than average
offspring, across all ages, and these females contributed more to the
individual heterogeneity than those producing consistently heavier
offspring. For nutritionally
stressed mothers, variation between individuals in offspring wet weight
increased as mothers aged. These observations suggest that variation in
offspring quality is affected not only by mother size but also
unmeasured aspects of her condition. The large amount of variation
between individuals, in offspring weight and changes in the extent of
variation with maternal age in this study, was unexpected, suggesting
that future studies quantifying the relative roles of mother size and
physiological condition on offspring size across different ages would be
valuable.
Offspring from young mothers that were nutritionally stressed had the
lowest starvation tolerance. We also found that maternal age affected
offspring starvation tolerance independently of age-effects mediated
through wet weight, suggesting that there may be other factors
associated with maternal age that influence the quality of offspring.
More subtle effects of maternal
age on the quality of resources transferred to offspring warrant further
investigation in tsetse and other species. For tsetse, during late
stages of pregnancy females not only transfer fat but also amino acids.
It may be that young nutritionally-stressed females are limited in these
amino acids. High amounts of tyrosine in the gut contents of third
instar larvae, which is involved in tanning of larval and adult cuticles
have been observed (Cmelik et al. 1969). The authors reasoned
that the tyrosine and phenylalanine obtained from a single bloodmeal is
unlikely to be sufficient to meet the amount required by offspring and
that a surplus stored from previous bloodmeals may be required.
This suggests that size may be
limited when nutrients are limited, irrespective of the amount of energy
available. It also demonstrates that resource allocation processes are
likely more complex, and more nuanced studies on the effects of the
quality of resources as well as quantity may be required to understand
the ageing process.
We focused on reproductive senescence in this study. One limitation is
that we did not continue the experiment beyond 100 days to quantify more
fully mother survival for all three treatments. Our evidence of
actuarial senescence in the nutritional stress treatment supports
analysis of mark-recapture studies of G. m. morsitans in the
field (Hargrove et al. 2011) which also showed an increase in
mortality as a function of age. Considering this, an additional
experiment to test whether females mated later experience a delayed
onset in actuarial senescence would be informative, given the similar
rates of reproductive senescence observed across treatments.
We also note that our conclusions
with respect to reproductive senescence are constrained to this 100-day
period. While, in principle, it would be optimal to run the experiment
until all females had died, the >1/4 of offspring being
aborted and fewer females surviving would result in relatively small
sample sizes beyond this time point. However, we are confident that our
study timeframe captures an ecologically relevant period to study
age-dependent allocation in tsetse, given that field studies show that
90% of females have died before 100 days (Hargrove et al. 2011).
Lastly, we acknowledge that our
conclusions may not necessarily extend to wild flies, and further study
of tsetse in the field would need to be carried out to confirm this.
To conclude, our results provide evidence of a bell-shaped relationship
between maternal age and offspring quality at the individual-level for
the iteroparous tsetse fly. This complements the existing body of work
from other species, that has predominantly shown a similar curve for the
number of offspring produced as mothers age (Monaghan et al.2020). Contrary to predictions from life history theory, however, our
study did not find evidence of a decline in offspring quality as a
function of direct costs of reproduction or resource allocation
trade-offs.