Should mothers provision their offspring equally? A manipulative field test

Authors: Hayley Cameron, Keyne Monro, and Dustin J Marshall

Published in: Ecology Letters, volume 20, issue 8 (August 2017)

Abstract

Within-brood variation in offspring size is universal, but its causes are unclear. Theoretical explanations for within-brood variation commonly invoke bet-hedging, although alternatives consider the role of sibling competition. Despite abundant theory, empirical manipulations of within-brood variation in offspring size are rare.

Using a field experiment, we investigate the consequences of unequal maternal provisioning for both maternal and offspring fitness in a marine invertebrate. We create experimental broods of siblings with identical mean, but different variance, in offspring size, and different sibling densities.

Overall, more-variable broods had higher mean performance than less-variable broods, suggesting benefits of unequal provisioning that arise independently of bet-hedging. Complementarity effects drove these benefits, apparently because offspring-size variation promotes resource partitioning.

We suggest that when siblings compete for the same resources, and offspring size affects niche usage, the production of more-variable broods can provide greater fitness returns given the same maternal investment; a process unanticipated by the current theory.

Cameron H, Monro K, Marshall DJ (2017) Should mothers provision their offspring equally? A manipulative field test. Ecology Letters, PDF DOI 

Boldness traits, not dominance, predict exploratory flight range and homing behaviour in homing pigeons

Authors: Steven J Portugal, Rhianna L Ricketts, Jackie Chappell, Craig R White, Emily L Shepard, and Dora Biro

Published in: Philosophical Transactions of the Royal Society B, volume 372, issue 1727 (August 2017)

Abstract

Group living has been proposed to yield benefits that enhance fitness above the level that would be achieved through living as solitary individuals. Dominance hierarchies occur commonly in these social assemblages, and result, by definition, in resources not being evenly distributed between group members. Determinants of rank within a dominance hierarchy can be associated with morphological characteristics, previous experience of the individual, or personality traits such as exploration tendencies.

The purpose of this study was to investigate whether greater exploration and positive responses to novel objects in homing pigeons (Columba livia) measured under laboratory conditions were associated with (i) greater initial exploration of the local area around the home loft during spontaneous exploration flights (SEF), (ii) faster and more efficient homing flights when released from further afield, and (iii) whether the traits of greater exploration and more positive responses to novel objects were more likely to be exhibited by the more dominant individuals within the group.

There was no relationship between laboratory-based novel object exploration and position within the dominance hierarchy. Pigeons that were neophobic under laboratory conditions did not explore the local area during SEF opportunities. When released from sites further from home, neophobic pigeons took longer routes to home compared to those birds that had not exhibited neophobic traits under laboratory conditions, and had spontaneously explored to a greater extent.

The lack of exploration in the neophobic birds is likely to have resulted in the increased costs of homing following release: unfamiliarity with the landscape likely led to the greater distances travelled and less efficient routes taken. Birds that demonstrated a lack of neophobia were not the dominant individuals inside the loft, and thus would have less access to resources such as food and potentially mates. However, a lack of neophobia makes the subordinate position possible, because subordinate birds that incur high travel costs would become calorie restricted and lose condition.

Our results address emerging questions linking individual variation in behaviour with energetics and fitness consequences.

Portugal SJ, Ricketts RL, Chappell J, White CR, Shepard EL, Biro D (2017) Boldness traits, not dominance, predict exploratory flight range and homing behaviour in homing pigeons. Philosophical Transactions of the Royal Society of London B, PDF DOI

Skin sloughing in susceptible and resistant amphibians regulates infection with a fungal pathogen

Authors: Michel E B Ohmer, Rebecca L Cramp, Catherine J M Russo, Craig R White, and Craig E Franklin

Published in: Scientific Reports

Abstract

The fungal pathogen Batrachochytrium dendrobatidis (Bd) has been implicated in amphibian population declines globally. Given that Bd infection is limited to the skin in post-metamorphic amphibians, routine skin sloughing may regulate infection. Skin sloughing has been shown to reduce the number of cultivatable microbes on amphibian skin, and Bd infection increases skin sloughing rates at high loads. However, it is unclear whether species specific differences in skin sloughing patterns could regulate Bd population growth on the skin, and influence subsequent infection dynamics.

We exposed five Australian frog species to Bd, and monitored sloughing rates and infection loads over time.

Sloughing reduced Bd load on the ventral skin surface, in all five species, despite wide variation in susceptibility to disease. In the least susceptible species, an increase in sloughing rate occurred at lower infection loads, and sloughing reduced Bd load up to 100%, leading to infection clearance. Conversely, the drop in Bd load with sloughing was only temporary in the more susceptible species.

These findings indicate that the ability of sloughing to act as an effective immune defence is species specific, and they have implications for understanding the pattern of Bd population growth on individual hosts, as well as population-level effects.

Ohmer MEB, Cramp RL, Russo CJM, White CR, Franklin CE (2017) Skin sloughing in susceptible and resistant amphibians regulates infection with a fungal pathogen, Scientific Reports, PDF DOI 

Invasive beetles travel further on longer legs

Individuals within a species can vary greatly in their ability to move around and disperse within their environment. This variation in movement ability can often be described by physiological, morphological and behavioural traits that are related to dispersal, but the associations among such traits are not always clear-cut or predictable. Previous studies, including work on the invasive cane toad in Australia, have identified that individuals that are larger and have longer legs relative to their body size are able to move farther and faster than smaller or proportionate individuals.

The Centre For Geometric Biology’s Craig White and colleagues Pieter Arnold and Phillip Cassey used an invasive insect species, the red flour beetle, as a model to investigate how movement characteristics related to morphological and physiological traits.

Individual beetles were run through a maze that simulated a complex environment to assess their movement. The researchers identified that movement ability could be described along an axis; individuals that scored positively on this axis moved at higher speed, travelled longer distances, moved continuously, and reached the edge of the maze quicker. Leg-length relative to body size was strongly correlated to movement ability. That is, beetles with relatively long legs had positive movement ability scores, presumably because longer legs allow for longer stride length and therefore a greater movement ability. Surprisingly, body size and metabolic rate (energy expenditure) were unrelated to movement ability. The results suggests that dispersal may be more strongly related to the muscles and structures that directly affect locomotion, rather than body size overall or energy-related traits.

It will be important for future studies to consider locomotor morphology as a foundation for studying variation in movement and dispersal, especially when investigating the ecology and evolution of traits in invasive or pest species.

This research was published in Functional Ecology.

Travel news

The Centre for Geometric Biology’s Dustin Marshall and Hayley Cameron are returning to Oxford this month to finalise a project they have been working on with Tim Coulson, from the Zoology Department at the University of Oxford, as well as re-visit some favourite haunts, including the famous Eagle and Child pub and University Parks.

Oxford’s iconic Eagle And Child is one of Dustin’s favourite places to visit.
Hayley prefers a stroll through the picturesque University Parks.

This study investigates how phenotypic variation within species can mediate competitive dynamics among species.  In particular, they are interested in how life history traits (such as fecundity, growth and survivorship) are effected by competitive interactions and influence population dynamics and selection on body size.

As with many other CGB research projects, Hayley and Dustin have used sessile marine invertebrates as a model system and draw on the expertise of Professor Coulson to implement structured population models to formally integrate empirical estimates of trait-specific competition with competition theory.

Dustin will also be visiting the Netherlands to further the research collaboration with Andre de Roos who visited the CGB late last year, as well as attending an editorial meeting for Oikos in Sweden.

Investigating movement in the laboratory: dispersal apparatus designs and the red flour beetle, Tribolium castaneum

Authors: Pieter A Arnold, Michelle A Rafter, Rokhsareh Malekpour, Phillip Cassey, Gimme H Walter, and Craig R White

Published in: Entomologia Experimentalis et Applicata, volume 163, issue 1 (April 2017)

Abstract

The natural dispersal of Tribolium castaneum Herbst (Coleoptera: Tenebrionidae) has been emulated in the laboratory for more than 50 years, using a simple dispersal apparatus.
This has typically comprised of a starting container (initial resource or patch) connected by tubing, which contains thread for the animals to climb into a tube and hence to an end container. That is, beetles move to a new viable resource or patch from an inter-patch zone or non-viable habitat.

We modified this basic apparatus design to test the effect of tubing length and tubing insertion angle on the dispersal rate and proportion of successful dispersers.

We expected that the proportion of successful dispersers would be repeatable within each apparatus design, and that increasing tubing length and steepness of the insertion angle would reduce dispersal rate and success across apparatus designs.

Dispersal increased linearly through time, similarly so for both males and females. The design with the most vertical tubing insertion angle had a lower proportion of successful dispersers. Tubing length also had a negative relationship with dispersal success (as judged by insects reaching the end container), but a significant reduction in dispersal success was only apparent between the shortest and longest tubing between containers.

We suggest that locating and climbing the vertical section of string before they can enter the tubing between containers restricts dispersal and that at higher densities, insects exhibit greater inclination to climb.

This type of apparatus has flexible design tolerances and further potential to study the dispersal of other small insect species that primarily use pedestrian locomotion.

Arnold PA, Rafter MA, Malekpour R, Cassey P, Walter GH, White CR (2017) Investigating movement in the laboratory: dispersal apparatus designs and the red flour beetle, Tribolium castaneum. Entomologia Experimentalis et Applicata PDF DOI 

Functional traits in red flour beetles: the dispersal phenotype is associated with leg length but not body size nor metabolic rate

Authors: Pieter A Arnold, Phillip Cassey, and Craig R White

Published in: Functional Ecology, volume 31, issue 3 (March 2017)

Summary

Individuals vary in their ability to disperse. Much of this variation can be described by covarying phenotypic traits that are related to dispersal (constituting the ‘dispersal phenotype’ or ‘dispersal syndrome’), but the nature of the associations among these traits is not well understood. Unravelling the associations among traits that potentially constitute the dispersal phenotype provides a foundation for understanding evolutionary trade-offs due to variation in dispersal.

Here, we tested five predictions pertaining to the relationships among physiological, morphological and movement traits that are associated with dispersal, using a species with a long history as a laboratory model for studying ecological phenomena, red flour beetles (Tribolium castaneum).

We identified a dominant axis of movement ability that describes variation in dispersal-related movement traits. Individuals that scored positively on this axis moved at higher speed, travelled longer distances, had lower movement intermittency and dispersed quicker to a specified area.

Relative leg length, but not body size nor routine metabolic rate related positively with movement ability, indicating a likely mechanistic relationship between increased stride length and movement ability.

Our data suggest that the dispersal phenotype may be more strongly linked to morphological traits than physiological ones. We demonstrate that associations among many functional traits do not necessarily conform to a priori expectations, and predict that the substantial intraspecific variation in trait values may be important for selection. Movement is a complex behavioural trait, but it has a mechanistic basis in locomotor morphology that warrants further exploration.

Arnold PA, Cassey P, White CR (2017) Functional traits in red flour beetles: the dispersal phenotype is associated with leg length but not body size nor metabolic rate. Functional Ecology, PDF DOI