Organisms have very diverse
lifestyles. Even amongst mammals and birds there is enormous variation in size
and lifespan. In addition animals have different ways of dealing with the
environment. Some hibernate in winter, some migrate. True seals are
‘capital breeders’, accumulating large energy surpluses before
feeding pups rich milk for a short time. In contrast sea lions and fur seals
provision each pup for a long period of time from ‘income’ (i.e.
energy obtained from foraging). Some albatrosses breed every year, other
species every two years. Life history theory attempts to understand the
lifestyles of animals in evolutionary terms; that is
as a product of the action of natural selection, so that lifestyle is
approximately optimal given suitable constraints. I contend that previous work
in this area has paid insufficient attention to the links between physiology,
behaviour and the environment. These links are necessary for a full
understanding because physiology and behaviour co-evolve, with the selection
pressure on both depending on the environment.
Body design and the strategy of the
animal over its lifetime are both subject to selection. By body design I mean
features that are (relatively) fixed at maturity, such as whole body size,
brain size, and the type of skeletal muscle fibres. By the animal’s
‘strategy’ I mean the rule specifying how the decisions made over
its lifetime depend on circumstances. The strategy includes a specification of
when and where to forage and whether to migrate, but also more physiological
‘decisions’ such as how fast to grow and the scheduling of
reproductive effort over the lifetime. Since some aspects of physiology such as
digestive capacity are highly plastic in some species, the rule specifying a
plastic physiological response can be considered as part of an animal’s
strategy.
There are trade
offs in body design. For example, a large body may make the animal
better able to catch prey or avoid predators, but at the cost of a greater rate
of energy expenditure and a longer time to grow to maturity. Various trade offs also act within the
lifetime of an animal. High reproductive effort may increase current
reproductive success but decrease the probability of future survival. A
migratory bird that gains an advantage by reducing the size of its digestive
apparatus before a migratory flight has reduced flight costs but also reduced
capacity to process food once it arrives at its destination. For given body
design these within-lifetime trade offs determine the
strategy that will evolve.
However, design trade offs and
within-lifetime trade offs are related. For example,
consider the relationship between body size and the foraging strategy of an
animal. The foraging strategy is a contingent rule that specifies where and
when to forage depending on the animal’s energy reserves and other
aspects of its circumstances. For a given body size the best strategy will
depend on the food availability and predation risk in the environment. However,
body size is liable to affect food needs and predation risk. Thus for a given
environment the best strategy will depend on body size. Thus as body size
evolves so will the foraging strategy. The body size/ foraging strategy
combination that evolves will depend on the environment.
In general we expect that within a given environment
selection acts so as to produce a body design that is best given the strategy
and to produce a strategy that is best given the design, with different
environments selecting for different design/strategy pairs. Thus a system
biology approach in which there is an integration
across levels is required. We need to simultaneously consider behaviour,
physiological changes within the lifetime of an animal, body design and the
environment. Carrying out this integration is challenging, but I believe it is
the direction in which life history theory needs to move.
References
McNamara, J.M. and Houston, A.I. (1996). "State-dependent life histories". Nature, 380 pp215-221.
McNamara, J.M., Welham, R.K. and Houston, A.I. (1998) The timing of migration within the context of an annual cycle. J. Avian Biol., 29, 416-423.
McNamara, J.M., Welham R. K., Houston, A. I., Daan, S. & Tinbergen, J. M. (2004) The effects of background mortality on optimal reproduction in a seasonal environment. Theor. Popl. Biol 65, 361-372.
Bateson, P., Barker, D., Clutton-Brock, T., Deb, D., Dudine, B., Foley, R. A., Gluckman, P., Godfrey, K., Kirkwood, T., Lahr, M. M., McNamara, J., Metcalfe, N. B., Monaghan, P., Spencer, H. G. and Sultan, S. E. (2004) Developmental plasticity and human health. Nature 430, 419 – 421.
McNamara, J. M. and Buchanan, K. L. (2005) Stress, resource allocation and mortality. Behavioral Ecology 16, 1008-1017.
Barta Z., Houston, A. I., McNamara, J. M., Welham, R. K., Hedenström, A., Weber, T. P. & Feró, O. (2006) Annual routines of non-migratory birds: optimal moult strategies. Oikos 112, 580-593.
Houston, A. I., Stephens, P. A., Boyd, I. L., Harding, K. C. and McNamara, J. M. (2007) Capital or income breeding? A theoretical model of female reproductive strategies. Behavioral Ecology 18: 241-250.
Houston, A.I., McNamara, J.M., Barta, Z. and Klasing, K.C. (2007) The effect of energy reserves and food availability on optimal immune defence. Proc R Soc Lond B 274, 2835- 2842.
Hedenström, A., Barta, Z., Helm, B., Houston A.I., McNamara, J.M. and Jonzén N. (2007) Migration speed and scheduling of annual events by migrating birds in relation to climate change. Clim Res 35, 79-91.
McNamara, J.M.,and Houston A.I. (2008) Optimal annual routines: behaviour in the context of physiology and ecology. Phil. Trans. R Soc B 363, 301-319.
Barta, Z., McNamara, J.M., Houston, A.I., Weber, T.P., Hedenström, A. and Feró O. (2008) Optimal moult strategies in migratory birds. Phil. Trans. R Soc B 363, 211 229.
Fero O., Stephens P.A., Barta Z., McNamara J.M. & Houston A.I. (2008). Optimal annual routines: New tools for conservation biology? Ecol. Appl., 18, 1563-1577.
McNamara J.M., Barta Z., Wikelski M. & Houston A.I. (2008). A theoretical investigation of the effect of latitude on avian life histories. Am. Nat., 172, 331-345.
McNamara J.M. & Houston A.I. (2008). Introduction. Adaptation to the annual cycle. Philosophical Transactions of the Royal Society B-Biological Sciences, 363, 209-210.
Bauer S., Barta Z., Ens
B.J., Hays G.C., McNamara J.M. & Klaassen M.
(2009) Animal migration: linking models
and data beyond taxonomic limits. Biol. Lett. 5
433-435.
McNamara, J.M., Houston, A.I., Zoltan, B., Scheuerlein, A. & Fromhage, L. (in press) Deterioration, death and the evolution of reproductive restraint in late life. Proceedings of the Royal Society B-Biological Science.