Introduction
Across the natural world, it is commonly observed that as individuals get older, they are more likely to die and invest less in their offspring (Nussey et al. 2013; Hoekstra et al. 2019; Zajitschek et al. 2019). Broad patterns of senescence have been well described for a wide range of taxa (Nussey et al. 2013; Hoekstra et al. 2019; Zajitschek et al. 2019), and extensive variation in the onset and rate of ageing, both within and across populations, has also been observed (Holand et al. 2016; Rodríguez-Muñoz et al. 2019; Cayuela et al. 2020). The drivers of senescence and the causes of variation in the timing and intensity of the ageing process are only just beginning to be determined (Gaillard & Lemaître 2019).
Life-history theory predicts that senescence is driven by trade-offs between reproduction and other physiological processes (Kirkwood 1977; Boggs 2009; Baudisch & Vaupel 2012; Davison et al. 2014). A central argument to this theory is that resources are limited and must be allocated either to reproduction or somatic maintenance (Partridge 1987; Boggs 2009). Reproductive senescence could also, however, occur through physiological damage incurred directly from reproduction, or be adaptive to prolong survival (McNamara et al. 2009).
Controlled experiments are useful for quantifying patterns of senescence and identifying causal factors. Experiments where the age at which females are mated and their access to resources can be varied contribute to tests of whether reproduction causes senescence, either directly or by resource allocation trade-offs. In testing various senescence hypotheses, insects are useful organisms as they have relatively short generation times, can be reared in large numbers, and access to mates and resources can be easily manipulated.
By varying the age at which females were mated, experiments with Lepidoptera have shown that delayed mating reduces fecundity but extends longevity (Unnithan & Paye 1991; Jiménez-Pérez & Wang 2009). However, virgin females still produce eggs, and in these studies, females of the same age but mated at different times were not compared. The effects of reproduction on reproductive senescence cannot therefore be fully evaluated using these data.
By manipulating nutrition, researchers have shown that, generally, females with access to fewer resources have lower overall reproductive output but longer lifespan (De Sousza Santos & Begon 1987; Ernsting & Isaaks 1991; Kaitala 1991; Chippindale et al. 1993; Tatar & Carey 1995; Curtis Creighton et al. 2009). These studies support the theory that senescence is caused, at least in part, by either direct or indirect costs of reproduction. To our knowledge, no studies have compared reproductive output of females of the same age, but mated at different ages, in the same experiment as females under nutritional stress, to compare directly the contributions of reproductive history and resource availability on reproductive senescence.
Here, we present a study on senescence in tsetse flies (Glossinaspecies), where we quantify the effects of maternal age on offspring quality, under nutritional stress, delayed mating and control (standard insectary) conditions. Tsetse are vectors of human and animal trypanosomiasis in Africa. They have an unusual reproductive ecology, giving birth to a single live larva weighing the same as the mother (Hargrove & Muzari 2015), approximately every nine days and living for up to 200 days (Hargrove 2004). With immature stages that receive only energy and nutrients from the mother and a relatively long adult lifespan for their small size, tsetse present an alternative model system to study reproductive and survival senescence.
Evidence for age-related changes in maternal investment in field and laboratory tsetse is, to date, mixed (Jordan et al. 1969; Langley & Clutton-Brock 1998; McIntyre & Gooding 1998). Key limitations to existing studies are that flies were kept only under optimal laboratory conditions, not tracked individually and frequently grouped across ages.
In this study, we used a novel method of housing tsetse females to track, for individual mothers, how senescence patterns vary if we alleviate the costs of reproduction or impose nutritional stress. We manipulated nutritional stress by feeding adult females on highvs low quality diets and changed reproduction stress by delaying the age at which females were mated. In each treatment we measured maternal mortality, reproductive output and offspring survival. We hypothesised that offspring from older mothers would be of lower quality and would have lower starvation tolerance. If reproduction contributes to senescence, either directly by physiological damage or by resource allocation trade-offs, we hypothesised that: i) mothers mated later would experience senescence later, and potentially slower senescence; and ii) nutritionally stressed mothers would have an earlier, and potentially steeper decline in senescence. Alternatively, if senescence occurs through physiological damage during reproduction, nutritionally stressed mothers may senescence more slowly, due to an overall lower reproductive output.