L8 - Developmental trade offs

L8 - Developmental trade offs

1. A shortage of nutrients during early development can have extraordinary long-term consequences in addition to the more immediate and obvious effects on early growth rates. Compensation itself can carry diverse and counter-intuitive costs paid over a variety of time scales

   • During development, there can be a hierarchy of tissue preservation, with certain key, irreplaceable cells having priority, thereby preserving neuronal tissue if pregnant rats are supplied with a diet that is deficient in protein, the foetus will maintain same level of investment in brain by diverting investment away from the heart - can be up to 1/3 of its normal size in mammals whilst brain is normal size- pecking order of the organs.
     • It has been suggested that responses such as this are an adaptive strategy: if the pregnant female experiences a low protein environment, the offspring develop so as to be adapted for a nutritionally poor environment (the ‘thrifty phenotype’ hypothesis) However, this phenotype does less well if conditions subsequently improve, because it then suffers from a range of metabolic disorders in adult life and has a reduced lifespan a and b. Given that environmental conditions for rodents are rarely stable, it seems unlikely that this metabolic imprinting at such an early stage would be advantageous. Rather than an adaptive strategy, it might simply be a physiological constraint, because, in most mammalian body organs, cell number and distribution become fixed at a very early stage c and are influenced by nutritional conditions in pregnancy.
       • It might initially seem paradoxical that, in mammals, food restriction early in life seems to shorten lifespan, whereas a similar restriction later in life increases it 27. However, it is possible that the first effect is a result of the tradeoff between short-term benefits and long-term physiological costs of growth compensation, whereas the second effect might simply be a consequence of the tradeoff between adult survival and reproductive investment (because reproduction often ceases in low ration adults)
     • What determines which strategies are pitted against on each other
       • Proximate
         • Proximity: The location of structures and how close they are to other structures can influence which organs vs which, e.g. in different species of dung beetle the location of the horns varies, when horns are based on the head they are more likely to be in a trade off against eye size.
         • Hormonal basis: endocrine system may influence which are pitted against one another as the same hormone can regulate different aspects of physiology; hormones commonly have antagonistic effect on the two traits in competition, promoting resource allocation to one and restricting to the other; levels of juvenile hormone influence trade offs between flight muscles and ovaries in firebugs
           • Hormones provide a link between an animals environment and physiology and can act as developmental switches, controlled/fine tuned by the eenvironment
       • Ultimate
         • strategic developmental trade offs: 2 morphs representing two different strategies e.g. in male Onthophagus beetles trade off between testes size and horn size, larger horns means more success gaining mates but the accompanying smaller testes = less success in fertilisation, alternative strategy = small horns so have to sneak matings but lager testes means need fewer attampts
           • What is invested in in the trade off reflects what is optimal for a particular life history trait of that organism e.g. in longer lived species of caddis flies where flight is more important, investment in the thorax is prioritised rather than the abdomen as in shorter lived species. It is adaptive due to selection
             • ecology and selection determine which way the trade off will go
               • Strategic trade offs can exist between sexes: in excess of resources male crickets invest super-optimally in sexual traits but die sooner than males on a lower quality diet whilst females invest in longevity over fecundicity
                 • Reflects limitations of sex; females limited by egg production (need time to make eggs), males limited by number of partners
   • Animals that have experienced a period of retarded growth to then enter a phase of growth acceleration when conditions improve (Fig. I). This compensatory phase is usually achieved through hyperphagia
     • As experimental work on compensatory growth demonstrates that animals are capable of growing faster than they generally do, it seems likely that compensatory responses themselves carry significant costs. Whereas costs that act at the physiological/cellular level are usually paid in the longer term the ecological costs of compensatory responses tend to be more immediate. This is highlighted in recent experimental work on the rate of early development in butterflies. Caterpillars hatching at a time that appears (through photoperiod manipulation) to be late in the season exhibit accelerated growth rates, and so are still able to reach a size whereby they can pupate before winter1. However, this faster growth rate increases their risk of being predated, possibly because it necessitates more foraging activity and thereby increased exposure. As fast growth in caterpillars has been found to be associated with low resistance to starvation during periods of food shortage19, such acceleration
       • For compensation to occur at all, the benefits must outweigh the possible short- and long-term costs. Although compensation might shorten life, this can still be adaptive if it increases overall reproductive success
         • Costs of compensation might in many cases not be paid until very late in life. Thus rats born to a mother given a reduced protein diet during pregnancy actually have a better glucose tolerance than do controls when reaching adulthood (at 12 weeks of age), but much poorer tolerance in old age (15 months) Similarly, although people that recover in childhood from a low birth weight have an increased risk of coronary heart disease compared to those that remain relatively small (Box 2), this cost is usually paid after reproduction has ceased and so might have little effect on darwinian fitness
           • The probable fitness benefits of a given compensatory response might differ between individuals. For example, observed sex differences in compensatory growth might arise from differences between the sexes in investment priorities or compensatory costs; similar differences might arise between animals of differing social status
           • However, designing appropriate experiments thatpin down costs to compensation itself is difficult, because there might be individual differences in the strength of the compensatory response that might relate to the magnitude of the deficit experienced. It is then unclear whether any subsequent costs are a result of the deficit or to the compensatory response itself. Therefore, experiments must manipulate the opportunity to compensate as well as the extent of the original deficit.