Senescence, or the physical decline in physiological functioning with age, superficially appears non- adaptive and so presents a puzzle to evolutionary biologists (Abrams and Ludwig 1995, Medawar 1952, Williams 1957). It poses a problem for theories of evolution and natural selection. How can a process which is reducing fitness be selected for? Senescence in multicellular organisms is paradoxical, given that natural selection supposedly causes the evolution of increased fitness, and so many biologists have taken the view that senescence reflects an inevitable process of damage accumulation (Charlesworth 2000). One of the key predictions of the adaptive explanation of senescence is that it should be universally present in organisms with delayed reproduction (Bennett and Owens 2002, Williams 1957, Hamilton 1966). Senescence suggests that with an increasing age there is an increasing probability of death, and senescence differs in different organisms. The process of senescence is complex and many theories and mechanisms have been put forward to explain it. This essay is going to review the patterns of senescence in birds.
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Birds (class aves) are a diverse taxonomic group (9000 species) and are generally known for long life- spans and slow aging rates relative to mammals; many live up to 3 times longer than mammals of equivalent body mass (Holmes et al 2001). Finch (1990) organized aging patterns into 2 general models ; rapid senescence with sudden death, characteristic of a semelparous animal and, secondly, gradual senescence with a finite life span which is characteristic of birds, which he feels show negligible senescence. Smaller animals have shorter life spans and so it would be expected smaller birds face more rapid senescence (Speakman 2005).
There is controversy surrounding the senescence of birds: a large number of authors have suggested senescence is either very rare or entirely absent in birds, although mammals do senesce (Botkin and miller 1974; Partrige 1989; Williams 1992p.151). Holmes and Austad (1995) suggest that senescence is negligible as they state most birds continue to reproduce until they die, have no menopause and there is little evidence of declining condition in old individuals of many species. Ricklefs (2000) also suggests that birds show negligible senescence; they retain a high level of physical fitness to old age and eventually succumb to intrinsic disease processes that kill rapidly. He suggests extrinsic mortality is more constant throughout the life cycle of birds because any reduction in physiological function is balanced by increasing experience and acquired immunity. Extrinsic factors such as starvation are independent of age and cause the same amount of death between young and old. Intrinsic factors such as cancer may increase with age.
Theories of Senescence
Historically senescence was just put down to wear and tear: when organisms get older they become weaker (Williams 1957). However, this would not explain the differing lifespans within biological groups such as mammals (Bennett and Owens 2002). Williams (1957) states the wear and tear hypothesis lacks factual support and poses the dilemma why would an organism not just maintain itself , the morphogenetic process of creating a new organism is a lot more complex than just maintaining what is already formed. There are parts of an organism that literally wear out such as teeth, but this is not senescence but the loss without replacement. Comfort (1979) suggests that many biologists have taken the view that senescence reflects an inevitable process of damage accumulation with age. Due to the complexities regarding why senescence occurs, there have been many theories put forward to explain it, I will introduce the most popular ones.
Weismann was one of the first in 1891 to advocate a theory of senescence. He believed that natural selection had caused a mechanism which eliminated the old worn-out members of the population, and that organisms must show a decline analogous to that of mechanical devices, although he did not give a clear explanation of how this was occurring (Williams 1957).
Medawar's mutation accumulation in 1952, was probably the first model of senescence that provided a mechanism of how senescence could work. Medawar stated that reproductive values decline over much of adult life, and so selection will be more effective in improving performance earlier rather than later in life (Charlesworth 2000). This means that alleles with deleterious effects restricted to later stages of life will equilibrate at higher frequencies at mutation-selection balance than alleles that act earlier (Charlesworth 2000). This suggests that most unfavourable mutations only show their effect later on and there will be increased accumulation of these mutations which causesaging.
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The competing hypothesis to explain senescence is William's theory of antagonistic pleiotropy put forward in 1957 (Williams 1957).Pleiotropy means one gene controls more than one phenotypic trait and antagonistic pleiotropy means the gene has both a beneficial and detrimental effect on an organism. This suggests genes that have a beneficial effect on fitness early on in the life cycle will have pleiotropic deleterious effects in later life, but are still favoured by natural selection and that there is a decreasing probability of reproduction with increasing adult age (Williams 1957, Hamilton 1966). As increased performance in life early on will lead to earlier senescence and, therefore, lower fitness such as reproduction later, fitness is high when Fisher's reproductive value is high. Pleiotropic effects lead to both phenotypic and genetic correlations between life-traits; this differs from the mutation accumulation theory which relies on uncorrelated deleterious mutations (Charmantier et al 2006). The adaptive theory as proposed by Charlesworth (1994) is based on pleiotropy, it states senescence occurs because of selection for traits with positive effect that have pleiotropic negative effects later (Abrams and Ludwig 1995).
Another well-known theory is the Disposable soma theory which was suggested in 1977 by Thomas Kirkwood. This suggests senescence arises from an optimal balancing of resources between reproduction and somatic repair (Abrams and Ludwig 1995). Energy is diverted from repair to reproduction resulting in accumulation of damage throughout much of the lifespan (Abrams and Ludwig 1995).
Are the patterns seen linked to these theories?
Bennett and Owens (2002) reviewed the pattern of reproductive and mortality senescence in wild populations of birds using a comparative analysis of age-specific data, to test two predictions of the adaptive theory. Firstly that senescence is present in organisms with juvenile and adult forms and secondly the rate of senescence is positively related to the instantaneous rate of mortality. They found evidence of reproductive senescence for 13 out of the 16 species tested, and the ages of onset for reproductive senescence ranged from 4- 21 years. Evidence of mortality-related senescence was found in 13 out of the 17 species for which they had data; these results support the two theoretical predictions and show that senescence does occur (Bennett and Owens 2002). Their comparative study showed that in longer-lived species, with low instantaneous mortality rates reproductive senescence is delayed and that senescence related to mortality should begin immediately after the onset of breeding. This seems to be different to other mammals where senescent increases do not begin until well after the age of first breeding, further evidence is needed (Bennett and Owens 2002, Promislow 1991). Bennett and Owens strongly support the pattern that senescent decline is caused by natural selection.
Charmantier et al (2006) monitored the relationship between first reproduction and late reproduction in the free-ranging mute swan (Cygnus olor) over 36 years. They show that both traits are strongly selected in opposite directions through the multivariate analysis, and genetic variance that is associated with an early start to reproductive life also causes an early end to reproduction. This provides support for the antagonistic pleiotropy model as it is shows a heritable trade off: increased performance early on is coupled with faster senescence (Charmantier et al 2006). Gustafsson and Part (1990) provided empirical support for the theory of antagonistic pleiotropy with an experiment on the flycatcher (Ficedula albicollis) which showed costly reproduction accelerates senescence for fertility.
Exceptional longevity of birds as a group suggests they have evolved special mechanisms to protect against more rapid aging; the comparative evidence is consistent that longevity is associated with the evolution of flight and ability to escape predators (Holmes & Austad 1995). One of the critical tests of senescence theory is the comparison of flying birds' vs mammals. Maximum recorded longevities of wild birds are on average 1.7 times greater than those of captive mammals, and captive birds on average outlive captive mammals by a factor of three ( Lindstedt and Calder 1976, Austad and Fischer 1991, Holmes and Austad 1995). This suggests that flightless birds have high rates of extrinsic mortality, and the ability to fly is an evolutionary mechanism to increase senescence. However, intrinsic mortality does also play a part in avian aging, and avian diseases are similar to those seen in mammals, including cancers (Holmes et Austrad 1995).Maximum recorded longevities in birds provide many examples between delayed reproduction and longevity as predicted by senescence theory (Holmes and Austad 1995).
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To conclude senescence patterns do not seem to be as well documented in birds as in other mammals; and some authors have even suggested senescence is not present. Comparative studies by Bennett and Owens (2002) have clearly shown that senescence does occur in this group. The work done has not conclusively linked senescence of birds with a particular theory but Bennett and Owens (2002), Charmantier et al (2006) and Gustafsson and Part (1990) provide some support for the theory of antagonistic pleiotropy and the fact natural selection can work to favour traits that are deleterious to the individual. Comparative studies of flying bird's vs. mammals supported the fact senescence occurs but did not link it to a senescence theory. The physiological adaptation of birds to fly and reduce early predation, explains why their small body size does not mean rapid senesce like in other mammals. In my opinion more research and data needs to be collected on wild birds to conclusively see what theory of senescence birds are following.