Evolutionary biologists often use phenotypic differences between species and between individuals to gain an understanding of organismal design. The focus of much recent attention has been on developmental plasticity - the environmentally induced variability during development within a single genotype. The phenotypic variation expressed by single reproductively mature organisms throughout their life, traditionally the subject of many physiological studies, has remained under-exploited in evolutionary biology. Phenotypic flexibility, the reversible within-individual variation, is a function of environmental conditions varying predictably (e.g. with season), or of more stochastic fluctuations in the environment. Here, we provide a common framework to bring the different categories of phenotypic plasticity together, and emphasize perspectives on adaptation that reversible types of plasticity might provide. We argue that better recognition and use of the various levels of phenotypic variation will increase the scope for phenotypic experimentation, comparison and integration.

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Phenotypic flexibility and the evolution of organismal design

Spring temperatures have increased over the past 25 years, to which a wide variety of organisms have responded. The outstanding question is whether these responses match the temperature–induced shift of the selection pressures acting on these organisms. Organisms have evolved response mechanisms that are only adaptive given the existing relationship between the cues organisms use and the selection pressures acting on them. Global warming may disrupt ecosystem interactions because it alters these relationships and micro–evolution may be slow in tracking these changes. In particular, such shifts have serious consequences for ecosystem functioning for the tight multitrophic interactions involved in the timing of reproduction and growth. We determined the response of winter moth ( Operophtera brumata ) egg hatching and oak ( Quercus robur ) bud burst to temperature, a system with strong selection on synchronization. We show that there has been poor synchrony in recent warm springs, which is due to an increase in spring temperatures without a decrease in the incidence of freezing spells in winter. This is a clear warning that such changes in temperature patterns may affect ecosystem interactions more strongly than changes in mean temperature.

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Warmer springs disrupt the synchrony of oak and winter moth phenology

Organ structures and correlated metabolic features (e.g. metabolic rate) have often taken as fixed attributes of fully grown individual vertebrates. When measurements of these attributes became available they were often used as representative values for the species, disregarding the specific conditions during which the mesurement were made. Evidence is accumulating that the functional size of organs and aspects of the metabolic physiology of an individual may show great flexibility over timescales of weeks and even days depending on physiological status, environmental conditions and behavioural goals. This flexibility is a way for animals to cope successfully with a much wider range of conditions occurring during various life-cycle events than fixed metabolic machinery would allow. Such phenotypic flexibility is likely to be a common adaptive syndrome, typical of vertebrates living in variable environments.

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Rapid reversible changes in organ size as a component of adaptive behaviour

The time in spring when a male kestrel rapidly increases his daily hunting time and his hunting yield, and thereby the amount of food delivered to the female, determines the date when she lays the first egg. Food experiments in free-living and captive kestrels gave a significant advance in laying date. Clutch size, which decreases with progressive laying date, did not change independent of date in response to food manipulation. These effects are in agreement with most other feeding experiments. Photoperiod experiments in kestrels advanced the reproductive cycle in constant long days, and a similar seasonal decline in clutch size was found. It seems that there is an internally preprogrammed decrease in clutch size within an annual "reproductive window". A proximate control model for the seasonal decline of clutch size is proposed, modified from an earlier model by HAFTORN (1985). This incorporates an increasing tendency to incubate the first eggs with progression of the season, an egg contact-incubation positive feedback loop, and the resorption of further follicles in the ovary when the laying female incubates 50% of the time. This follicle resorption fixes the clutch size ca. four days before the last egg is laid. the 50% incubation level is reached earlier in late females and consequently resorption starts earlier and the resulting clutch is smaller than in early females. Experiments in kestrels with removal and addition of eggs, in combination with measurements of incubation behaviour are discussed in relation to the model. Plasma prolactin data of female kestrels show that this hormone is a serious candidate for a physiological component relaying time of year in our model for clutch size regulation.

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Family planning in the kestrel (Falco tinnunculus): the ultimate control of covariation of laying date and clutch size

This paper reviews information from ecological and physiological studies to assess how extrinsic factors can modulate intrinsic physiological processes. The annual cycle of birds is made up of a sequence of life-history stages: breeding, moult and migration. Each stage has evolved to occur at the optimum time and to last for the whole duration of time available. Some species have predictable breeding seasons, others are more flexible and some breed opportunistically in response to unpredictable food availability. Photoperiod is the principal environmental cue used to time each stage, allowing birds to adapt their physiology in advance of predictable environmental changes. Physiological (neuroendocrine and endocrine) plasticity allows non-photoperiodic cues to modulate timing to enable individuals to cope with, and benefit from, short-term environmental variability. Although the timing and duration of the period of full gonadal maturation is principally controlled by photoperiod, non-photoperiodic cues, such as temperature, rainfall or food availability, could potentially modulate the exact time of breeding either by fine-tuning the time of egg-laying within the period of full gonadal maturity or, more fundamentally, by modulating gonadal maturation and/or regression. The timing of gonadal regression affects the time of the start of moult, which in turn may affect the duration of the moult. There are many areas of uncertainty. Future integrated studies are required to assess the scope for flexibility in life-history strategies as this will have a critical bearing on whether birds can adapt sufficiently rapidly to anthropogenic environmental changes, in particular climate change.

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Control of the annual cycle in birds: endocrine constraints and plasticity in response to ecological variability.

This book provides detailed coverage of the many mechanisms birds have evolved to cope with seasonal variations in the availability of resources. Readers will find discussion of the hypophysial and other endocrine systems, the role of hormones in reproductive behavior, energy budgets in relation to reproduction, egg and clutch size, breeding seasons, circadian and ultradian rhythms and their control, sexual selection, and the pair bond. Factors which influence avian survival rates are distinguished from the processes of population dynamics, and some constraints on evolutionary change are considered.

R. K. Murton

Papers

Avian breeding cycles