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.