Understanding insect life histories and senescence through a resource allocation lens

Abstract

Summary 1.Resource acquisition and allocation are the physiological mechanisms integrating foraging and life-history traits. An understanding of the patterns of acquisition and allocation in different environments and organisms is critical to a predictive theory of life history. 2. Here I develop an allocation framework, which provides a template for conceptualizing the interactions among resource acquisition, allocation and life-history traits. The framework describes the process through which food is taken in by an organism at specific life stages, then allocated to growth, survival (including maintenance, defence, dispersal, etc), reproduction and further foraging. 3. I use the allocation framework to examine allocation to life-history traits in insects under both benign and stressful environments. Stressful environments result from resource scarcity or harsh environmental conditions. I consider effects of consistent stress or variable stress across time. 4. Several broad generalizations emerge from empirical studies, viewed in the allocation framework. First, resource congruence, or the requirement for specific nutrient ratios in, for example, eggs, results in different limiting nutrients for each life-history trait. Second, the timing of resource acquisition affects both allocation patterns and the identity of limiting nutrients for a given life-history trait. Third, physiological trade-offs may occur across, not just within, life stages. Fourth, apparent trade-offs may be driven by differences among traits in resource congruence constraints and deleterious effects of excess nutrients on a particular trait. Fifth, allocation response to environmental stress shows age-specific and sex-specific patterns. Sixth, physiological trade-offs are often more pronounced under environmental stress. Finally, even within insects, there is considerable variability in allocation response to environmental stress. We do not yet have sufficiently diverse and thorough case studies to understand why this is so. Studies in the wild, or relating laboratory conditions to wild environments, are also needed. 5. Senescence can also be understood in an allocation framework. The present approach provides a necessary functional basis for understanding patterns of senescence in diverse organisms and environments. 6. The allocation framework fosters a mechanistic understanding of life-history patterns, and the beginning of an understanding of the processes underlying those patterns.

Male field cricket song reflects age, allowing females to prefer young males

“…Large males probably have a fitness advantage over small males. However, this is unexpected from a resource allocation perspective because large males should expend a greater proportion of their daily energy budget on metabolic maintenance owing to higher metabolic subsistence costs (Boggs 2009) and should therefore have less energy available for calling. Greater metabolic heat loss by smaller males (greater surface area:volume ratio) is unlikely to explain why large males can call more, as male G. bimaculatus do not elevate their thoracic temperature significantly above ambient (Toms et al 1993) as do some tettigoniids (Heath & Josephson 1970).…”

How to get the most bang for your buck: the evolution and physiology of nutrition-dependent resource allocation strategies

“…The Y model of resource allocation, as a framework to explain proximate mechanisms underlying life-history trade-offs [10,85], has in recent years been criticized by some as inadequate, leading some to seek revision of life-history theory (see exchanges in [86 -89]). The challenge to a resource-centered model is based on new empirical data showing that: (i) abrogation of reproduction does not always extend lifespan; (ii) some mutations that extend lifespan do not affect, or in fact, increase fecundity; and (iii) male and female organisms of several species respond differently to interventions that increase lifespan.…”

Quantitative genetic variation in the control of ovarian apoptosis under different environments

“…We expect animals to have evolved mechanisms by which energy saved by a reduction in reproduction can be used to increase survivorship (Bell and Bohm 1975). Oosorption, the ability to resorb eggs, is thought to be an adaptive mechanism by which nutrients can be reallocated into survival when opportunities for reproduction are limited (Bell and Bohm 1975;Papaj 2000;Boggs, 2009). For example, in butterflies a reduction in food leads to a reduction in fecundity, an increase in oocyte resorption, but no reduction in lifespan (Boggs and Ross 1993).…”

“…Another possibility is oosorption, the reallocation of reproductive resources to somatic maintenance under conditions of environmental stress. Studies addressing the evolution of loss of fertility have focused on the selective pressures that may be acting on the cessation of reproduction before senescence (Peccei, 2001;Reznick et al, 2006) or on oosorption (Boggs 2009). A complementary approach, which has been largely ignored so far, is developing an understanding of the nature of variation in the target of selection and how this may contribute to its evolution.…”

“…However, the underlying physiological and genetic control of this trade-off has not been explored. Insight into patterns of resource allocation in varied environments is essential to developing a predictive theory of life history (Boggs 2009), including the relationship between early reproductive history and later life fecundity.…”