Discussion
There is substantial evidence of survival senescence in insects and some
evidence for reproductive senescence. Here we manipulated both age at
mating and nutrition to quantify senescence patterns in tsetse flies.
The decline in offspring weight and starvation tolerance with mother
age, after the peak at c. 60 days, was similar across treatments.
Contrary to predictions from life history theory, therefore, neither
changes in reproductive investment 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. This is despite nutritionally
stressed mothers having higher probability of abortion and producing
smaller offspring at any age compared with mothers in the control and
mating delay groups. The increase in the hazard of death with age for
nutritionally stressed females contrasts with findings from studies of
other insects where nutritional stress reduced reproductive output but
either maintained or 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 reproduction cause
reproductive ageing. Alternatively it may be that, given the extreme
maternal investment 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. In an analysis
of tsetse caught in the field, smaller females were shown to 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 spontaneous abortion, particularly at later ages.
Hargrove and Muzari (Hargrove & Muzari 2015), using field collectedG. 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 concave pattern observed here is strikingly
similar in shape across treatments, reaching a peak at c. 60 days and
declining thereafter, and reflects the general concave pattern of
reproductive senescence observed across diverse taxonomic groups e.g.
(Velando et al. 2006; Sharp & Clutton-Brock 2010). The concave
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). In addition, tracking individual
mothers provided insights into individual variation in reproductive
investment. There was marked variation in offspring weight between
mothers and variation in senescence patterns for this trait,
particularly for nutritionally stressed mothers. There was more
variation in offspring weight between individual mothers for the mating
delay group relative to the control, predominantly contributed from
mothers producing smaller than average offspring. There was less
variation in offspring weight for young nutritionally stressed mothers,
but this increased as they aged. These observations suggest that
variation in offspring quality cannot be explained by variation in
mother size alone. The large amount of variation in offspring size in
this study was unexpected, suggesting that future studies quantifying
the relative roles of mother size and condition on offspring size would
be valuable.
Offspring from young mothers that were nutritionally stressed had the
lowest survival under starvation. That our analyses showed an effect of
mother age on offspring survival separate from its effect via energy
reserves, as measured by wet weight, indicates that there may be other
factors associated with mother age that influence the quality of
offspring. While the resource-allocation framework of senescence focuses
on absolute resources, there may be more subtle effects of mother age on
the quality of those resources transferred to offspring that 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. Cmelik et al. (Cmelik et al. 1969) observed
high amounts of tyrosine in the gut contents of third instar larvae,
which is involved in tanning of larval and adult cuticles. They 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 reserves of essential nutrients could also regulate the
size of the offspring – if there are only reserves sufficient for
offspring of a given size. It also demonstrates that the resource
allocation theory is 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.
Evidence that smaller pupae do not survive starvation for as long as
larger pupae is in contrast to studies showing that smaller sized
insects are potentially more competitive, or survive longer under times
of stress because their overall energy requirements are lower than
larger offspring (De Sousza Santos & Begon 1987).
We focused on reproductive senescence in this study. One limitation is,
therefore, that we did not continue the experiment beyond 100 days to
quantify more fully mother survival for all three treatments. Our
evidence of survival senescence in the nutritionally stressed group
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 survival senescence would be informative, given the similar
rates of reproductive senescence observed across treatments.
With respect to tsetse as vectors of trypanosomes, an increase in the
hazard of death of an insect vector, with age, affects pathogen
transmission rates and may therefore need to be accounted for in
quantitative assessments of control strategies (Bellan 2010; Ryanet al. 2015). Moreover, older mothers often produce compromised
offspring, in terms of their own lifespan (Lansing 1947) or immunity
(Clark et al. 2017). For tsetse, it has been shown that
nutritionally stressed individuals can be more susceptible to
trypanosome infection (Kubi et al. 2006). Maternal age effects on
offspring nutritional status therefore has consequences for modelling
the dynamics of the spread of tsetse populations and the pathogens they
transmit (English et al. 2020). At present, we still understand
very little about the extent of senescence in insect vector populations
and the underlying drivers of variation in these life history patterns.