Optimal moult strategies in migratory birds

Avian migration, which involves billions of birds flying vast distances, is known to influence all aspects of avian life. Here we investigate how birds fit moult into an annual cycle determined by the need to migrate. Large variation exists in moulting patterns in relation to migration: for instance, moult can occur after breeding in the summer or after arrival in the wintering quarters. Here we use an optimal annual routine model to investigate why this variation exists. The modelled bird's decisions depend on the time of year, its energy reserves, breeding status, experience, flight feather quality and location. Our results suggest that the temporal and spatial variations in food are an important influence on a migratory bird's annual cycle. Summer moult occurs when food has a high peak on the breeding site in the summer, but it is less seasonal elsewhere. Winter moult occurs if there is a short period of high food availability in summer and a strong winter peak at different locations (i.e. the food is very seasonal but in opposite phase on these areas). This finding might explain why only long-distance migrants have a winter moult.     

Modelling and Analysis of Planar Cell Polarity

Planar cell polarity (PCP) occurs in the epithelia of many animals and can lead to the alignment of hairs, bristles, and feathers. Here, we present two approaches to modelling this phenomenon. The aim is to discover the basic mechanisms that drive PCP, while keeping the models mathematically tractable. We present a feedback and diffusion model, in which adjacent cell sides of neighbouring cells are coupled by a negative feedback loop and diffusion acts within the cell. This approach can give rise to polarity, but also to period two patterns. Polarisation arises via an instability provided a sufficiently strong feedback and sufficiently weak diffusion. Moreover, we discuss a conservative model in which proteins within a cell are redistributed depending on the amount of proteins in the neighbouring cells, coupled with intracellular diffusion. In this case, polarity can arise from weakly polarised initial conditions or via a wave provided the diffusion is weak enough. Both models can overcome small anomalies in the initial conditions. Furthermore, the range of the effects of groups of cells with different properties than the surrounding cells depends on the strength of the initial global cue and the intracellular diffusion.     

Improved excitation light rejection enhances small-animal fluorescent optical imaging

Small-animal fluorescence-enhanced imaging involves the detection of weak fluorescent signals emanating from nanomolar to picomolar concentrations of exogenous or endogenously produced fluorophore concurrent with the rejection of an overwhelmingly large component of backscattered excitation light. The elimination of the back-reflected excitation light of the collected signal remains a major and often unrecognized challenge for further reducing the noise floor and increasing sensitivity of small-animal fluorescence imaging. Herein, we show that the combination of three-cavity interference and holographic super notch filters with appropriate imaging lenses to collimate light improves rejection of excitation light, enabling more accurate imaging. To assess excitation leakage, the "out-of-band (S(lambda x))" to "in-band (S(lambda m) - S(lambda x))" signal ratio from phantom studies and the target-to-background ratio (TBR) from in vivo animal imaging was acquired with and without collimating optics. The addition of collimating optics resulted in a 51% to 75% reduction in the ratio of (S(lambda x))/(S(lambda m) - S(lambda x)) for the phantom studies and an improvement of TBR from 11% to 31% and of signal-to-noise ratio from 11% to 142% for an integrin-targeting conjugate in human glioma xenografts.     

The Value of a hoard: not just energy

We present a stochastic dynamic programming model of the contributionof stored food to winter survival. Using the acorn woodpecker(Melanerpes formtcivorus) as a model organism, we demonstratethat a hoard of small energetic value can impart considerableimprovement in the probability of surviving the winter and soimprove fitness. With this model we hope to resolve Koenig andMumme's paradox, that acorn woodpeckers expend much time andeffort to create and maintain hoards of acorns which provideno more that 16% of their energetic needs over the period inwhich they are used. We further demonstrate that the contributionof hoarded acorns to survival depends on the variability inforaging outcome, independently of the energetic value of thehoard in absolute terms. We point out that the results applyin principle to all hoarding animals and suggest a number offurther elaborations of the model.     

A theoretical investigation into the direct and indirect effects of state on the risk of predation

As well as there being a direct physical effect of the state (for example fat reserves, or size) of an animal on the risk of being caught by a predator, state also has an effect on predation risk indirectly through changes in behaviour. We present a mathematical model which looks at these two components of the effect of state on predation risk. We focus on two different models, (i) where the animal must achieve a fixed state and its fitness depends on the time at which this state is reached and (ii) where the animal must survive until a fixed time and its fitness depends on its final state. We investigate conditions under which the indirect effect of increased state is to increase or decrease predation risk, and give some numerical illustrations. Under certain conditions in the fixed-state model, the indirect effect of state is to increase predation risk, whereas under certain conditions in the fixed-time model the indirect effect of state is to decrease predation risk. We discuss the implications of our results for empirical investigations into the effect of state on predation risk.     

Alternative roles for putative ice-binding residues in type I antifreeze protein

Two sets of variants of type I antifreeze protein have been synthesized to investigate the role of Leu and Asn in the activity of this 37-residue alpha-helix. Leu and Asn flank the central two of four regularly spaced ice-binding Thr in the i-1 and i + 3 positions, respectively. All three residues project from the same side of the helix to form the protein's putative ice-adsorption site and are considered in some models to act together as an "ice-binding motif". Replacement of Asn by residues with shorter side chains resulted in either a small loss (Ala) or gain (Thr) of antifreeze activity. However, substitution of Asn by its slightly larger homologue (Gln) abolished thermal hysteresis activity. The Gln-containing peptide was very soluble, largely monomeric, and fully helical. Of the three variants in which Leu was replaced by Ala, two of the three were more active than their Leu-containing counterparts, but all three variants began to precipitate as the peptide concentration increased. None of the seven variants tested showed dramatic differences in ice crystal morphology from that established by the wild type. These results are consistent with a primary role for Leu in preventing peptide aggregation at the antifreeze protein concentrations (10 mg/mL) normally present in fish serum. Similarly the role for Asn may have more to do with enhancing the solubility of these rather hydrophobic peptides than of making a stereospecific hydrogen-bonding match to the ice lattice as traditionally thought. Nevertheless, the dramatic loss of activity in the Asn-to-Gln replacement demonstrates the steric restriction on residues in or near the ice-binding site of the peptide.     

Credible threats and promises

We consider various implications of information about the other player in two-player evolutionary games. A simple model of desertion shows that information about the partner's behaviour can be disadvantageous, and highlights the idea of credible threats. We then discuss the general issue of whether the partner can convince the focal player that it will behave in a specific way, i.e. whether the focal player can make credible threats or promises. We show that when desertion decisions depend on reserves, a player can manipulate its reserves so as to create a credible threat of desertion. We then extend previous work on the evolution of trust and commitment, discussing conditions under which it is advantageous to assume that a partner will behave in a certain way even though it is not in its best interest.