Alice Cassidy

Hamilton and Zuk meet heterozygosity? Song repertoire size indicates inbreeding and immunity in song sparrows (Melospiza melodia)

Hamilton and Zuks influential hypothesis of parasite-mediated sexual selection proposes that exaggerated secondary sexual ornaments indicate a males additive genetic immunity to parasites. However, genetic correlates of ornamentation and immunity have rarely been explicitly identified. Evidence supporting Hamilton and Zuks hypothesis has instead been gathered by looking for positive phenotypic correlations between ornamentation and immunity; such correlations are assumed to reflect causal, additive relationships between these traits. We show that in song sparrows, Melospiza melodia, a males song repertoire size, a secondary sexual trait, increased with his cell-mediated immune response (CMI) to an experimental challenge. However, this phenotypic correlation could be explained because both repertoire size and CMI declined with a males inbreeding level. Repertoire size therefore primarily indicated a males relative heterozygosity, a non-additive genetic predictor of immunity. Caution may therefore be required when interpreting phenotypic correlations as support for Hamilton and Zuks additive model of sexual selection. However, our results suggest that female song sparrows choosing males with large repertoires would on average acquire more outbred and therefore more heterozygous mates. Such genetic dominance effects on ornamentation are likely to influence evolutionary trajectories of female choice, and should be explicitly incorporated into genetic models of sexual selection.

Song repertoire size predicts initial mating success in male song sparrows, Melospiza melodia

Male song sparrows sing repertoires of 4–13 distinct song types and have proved a valuable model for testing hypotheses concerning the function and evolution of song complexity. Captive female song sparrows solicit more copulations in response to playback of larger repertoires, yet it remains unclear whether male repertoire size influences female mate choice in natural situations. We used long-term data from free-living song sparrows inhabiting Mandarte Island, British Columbia, Canada, to investigate whether male song repertoire size predicted three components of reproductive performance during the first year: territory acquisition, mating success and laying date. Across males whose song was recorded, males with larger repertoires were not more likely to acquire a territory, to acquire a larger territory or to settle sooner. However, after we controlled for territory size and between-year variation in the population sex ratio, first-year males with larger repertoires were more likely to mate. This was because they were more likely to pair with newly settled females, not because they were more likely to acquire territories where older females were already resident. After we controlled for territory size and between-year variation in breeding date, newly settled females laid earlier when mated with males with larger repertoires. Together with the results of previous mate choice experiments, these patterns are consistent with the hypothesis that male song repertoire size is a sexually selected trait that influences female mate choice in song sparrows.

Fitness Correlates Of Song Repertoire Size In Free-Living Song Sparrows (Melospiza Melodia)

Models of sexual selection propose that exaggerated secondary sexual ornaments indicate a male’s own fitness and the fitness of his offspring. These hypotheses have rarely been thoroughly tested in free‐living individuals because overall fitness, as opposed to fitness components, is difficult to measure. We used 20 years of data from song sparrows (Melospiza melodia) inhabiting Mandarte Island, British Columbia, Canada, to test whether a male’s song repertoire size, a secondary sexual trait, predicted overall measures of male or offspring fitness. Males with larger song repertoires contributed more independent and recruited offspring, and independent and recruited grandoffspring, to Mandarte’s population. This was because these males lived longer and reared a greater proportion of hatched chicks to independence from parental care, not because females mated to males with larger repertoires laid or hatched more eggs. Furthermore, independent offspring of males with larger repertoires were more likely to recruit and then to leave more grandoffspring than were offspring of males with small repertoires. Although we cannot distinguish whether observed fitness variation reflected genetic or environmental effects on males or their offspring, these data suggest that female song sparrows would gain immediate and intergenerational fitness benefits by pairing with males with large song repertoires.

Spatial variation in winter survival of Song Sparrows Melospiza melodia

The resident population of Song Sparrows breeding on Mandarte Island, British Columbia fluctuated markedly between 1975 and 1989. A major population crash occurred during a week of unusually severe winter weather in early February 1989. Over 90% of Song Sparrows alive on the island in mid-January 1989 disappeared by 9-12 February 1989. The high mortality was accompanied by directional natural selection favoring female birds with longer wings. No selection was detected among males. We addressed the alternative hypothesis that Song Sparrows died of disease rather than poor weather. The population of Fox Sparrows Passerella iliaca wintering on Mandarte also declined sharply during the same period as Song Sparrows. The contemporaneous decline in the population size of these two species supports the conclusion that severe weather caused the population crashes. However, Fox Sparrows survived the poor weather better than Song Sparrows, possibly because of greater fat reserves in the former species. The effect of cold weather on population size of Song Sparrows was local. Sexspecific adult mortality was highest on Mandarte, lowest at a nearby mainland site provided with supplemental winter food, and intermediate on similar islands near Mandarte.

Using social network analysis to measure the impact and value of work that takes us beyond institutional boundaries

Abstract In educational development, accountability is paramount, in particular for activities that take educational developers away from their institutional boundaries to local, national, and international venues. Educational developers must demonstrate the benefits of such work to the home institution and its constituents. We asked educational developers from around the world to comment on the nature and value of their external work. Their responses made clear that such work expands valuable social networks and, concomitantly, builds social capital. Using these observations, we explain how social network analysis can be used to assess and demonstrate the impact of external work.