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Bonter D.N.,Cornell University | Bonter D.N.,Shoals Marine Laboratory | Moglia M.C.,Cornell Univ | Defisher L.E.,Cornell Univ
Journal of Avian Biology | Year: 2016

Skewed adult sex ratios sometimes occur in populations of free-living animals yet the proximate mechanisms, timing of sex-biases, and the selective agents contributing to skew remain a source of debate with contradictory evidence from different systems. We investigated potential mechanisms contributing to sex biases in a population of herring gulls with an apparent female skew in the adult population. Theory predicts that skewed adult sex ratios will adaptively lead to skewed offspring sex ratios to restore balance in the effective breeding population. Parents may also adaptively bias offspring sex ratios to increase their own fitness in response to environmental factors. Therefore, we expected to detect skewed sex ratios either at hatching or at fledging as parents invest differentially in offspring of different sexes. We sampled complete clutches (n = 336 chicks) at hatching to quantify potential skews in sex ratios by position in the hatch order, time of season, year, and nesting context (nest density), finding no departure from equal sex ratios at hatching related to any of these factors. Further, we sampled 258 chicks at near-fledging to investigate potential sex biases in survival at the chick stage. Again, no biases in sex ratios were recorded. Male offspring were favored in this population via greater maternal investment in eggs carrying male embryos and greater parental provisioning of male offspring which reached greater sizes by fledging. Despite the advantages realized by male offspring, females were equally as likely to fledge as males. Thus, biased adult sex ratios apparently arise in the post-fledging and pre-recruitment stage in our population. © 2016 Nordic Society Oikos.

Covino K.M.,University of Southern Mississippi | Covino K.M.,Shoals Marine Laboratory
Wilson Journal of Ornithology | Year: 2015

Many species of passerines are sexually monochromatic and thus sex cannot be determined based on plumage characteristics. Northern Waterthrushes (Parkesia noveboracensis) and Swainson's Thrushes (Catharus ustulatus) are two such species. The objective of this study is to examine morphological differences between males and females of both species and determine whether this information may be used to determine sex. With genetically sexed birds, I indicate wing chord values can be used to determine sex with 95% confidence. This information would allow field researchers to determine sex of 58% of Northern Waterthrushes and 33% of Swainson's Thrushes with 95% confidence of correct assignment. If age is taken into account, the proportion of individuals for which sex can be determined increases to 62% and 38%, respectively. This information may be used by avian ecologists in future studies of behavioral ecology, conservation biology, or evolutionary biology these species. © 2015 The Wilson Ornithological Society.

The ocean predators use their buzz saw mouths to efficiently dismantle prey, ranging from marine mammals and sea turtles to seabirds and—as Hollywood likes to remind us—an occasional human. There are more than 400 species of sharks in the world and each has a unique tooth shape. Some are simple triangles, while others are deeply notched or spear-shaped. But despite this variety, scientists haven't detected a difference in how different shark teeth cut and poke tissue. A recent University of Washington study sought to understand why shark teeth are shaped differently and what biological advantages various shapes have by testing their performance under realistic conditions. The results appeared in August in the journal Royal Society Open Science. "When you have all these different tooth shapes, there should be some functional reason. That issue was fundamentally troubling to me," said senior author Adam Summers, a UW professor of biology and of aquatic and fishery sciences. "It seemed likely what we were missing is that sharks move when they eat." Sharks shake their heads rapidly when they bite their prey, so evaluating how teeth perform while in a side-to-side motion was critical to the study tests, which took place during a summer marine biology course at the UW's Friday Harbor Laboratories on San Juan Island. Summers and his collaborators affixed three different types of shark teeth to the blade of a reciprocating power saw, then cut through thick slices of Alaska chum salmon at a speed that mimicked the velocity of head-shaking as a shark devours its prey. "Sure enough, when we cut through salmon, different teeth cut differently," Summers said. "We found a way to distinguish between this huge morphological difference we see among shark teeth in nature." The researchers also noticed that some species' teeth dulled more quickly than others. Two kinds of teeth, belonging to tiger and silky sharks, dulled after only several passes of the saw blade over tissue, meaning that it's possible these sharks in the wild must replace their teeth every time they kill prey. Teeth from the bluntnose sixgill shark didn't cut as well, but they also didn't dull as quickly as the other teeth. "There's this tradeoff between sharpness and longevity of the tooth edge," Summers explained. "It looks like some sharks must replace their teeth more often, giving them a consistently sharp tool." This might shed light on the feeding patterns of different sharks, the authors explain. For example, bluntnose sixgill sharks with duller, longer-lasting teeth might be swallowing their prey whole. Tiger sharks that eat a larger range of prey such as sea turtles, dugongs and seabirds usually bite their prey to pieces before eating it and would need sharper teeth to puncture a sea turtle's rigid shell, for example. When tissue is punctured and twisted side-to-side as prey is during a shark attack, the prey's tissue doesn't always behave the same way. This is not unlike a child's Silly Putty that will stretch into a long, stringy piece when slowly pulled apart, but break in two when yanked at a much faster speed. Biological tissues behave in the same unpredictable way when pulled, prodded or strained. It was this nuance that the research team tried to capture using experiments that involved movement. They believe it's the first study of its kind for mimicking how sharks hunt and kill. "It is really important to test biological materials at strain rates that are high enough to mimic how the predator and prey tissues would actually behave in real life," said co-author Stacy Farina, a postdoctoral researcher at Harvard University and an adjunct lecturer at Shoals Marine Laboratory. Farina was a teaching fellow at Friday Harbor Labs when the research was conducted. The experiments for this study were designed and carried out during an intensive five-week course at Friday Harbor Labs in summer 2014. Katherine Corn, now at the University of California, Davis, used epoxy from a local hardware store to glue shark teeth to the reciprocating saw blades. The materials worked surprisingly well. "We asked ourselves, how do we safely and effectively move these teeth back and forth quickly? The quick and dirty way was, glue them onto a power saw," Farina said. "It was a simple solution to a complicated problem." A video of the cutting apparatus. A blade with teeth from the bluntnose sixgill shark (Hexanchus griseus) on its fifteenth use. Explore further: Taking the bite out of shark DNA More information: Katherine A. Corn et al, Modelling tooth–prey interactions in sharks: the importance of dynamic testing, Royal Society Open Science (2016). DOI: 10.1098/rsos.160141

Keogh C.L.,University of Georgia | Keogh C.L.,Shoals Marine Laboratory | Sanderson M.E.,Shoals Marine Laboratory | Sanderson M.E.,Long Island University | Byers J.E.,University of Georgia
Oecologia | Year: 2016

Local adaptation may optimize an organism’s investment in defenses in response to the risk of infection by spatially heterogeneous parasites and other natural enemies. However, local adaptation may be constrained if recruitment is decoupled from selective pressure experienced by the parent generation. We predicted that the ability of three intertidal littorinid snail species to defend against trematode parasites would depend on prior levels of population exposure to parasites and on larval dispersal mode, a proxy for population openness. In a common garden experiment, for two snail species with direct development and localized recruitment (Littorina obtusata and Littorina saxatilis), hosts from sites with high trematode infection risk were less susceptible to infection than hosts from low-risk sites. However, this relationship was not apparent for a third host species with broadcast larvae (Littorina littorea), suggesting that broad larval dispersal can impede local adaptation; alternatively, the lack of response in this species could owe to other factors that limited experimental infection in this host. Our findings support that locally recruiting hosts can adapt their defenses to scale with localized infection risk. © 2015, Springer-Verlag Berlin Heidelberg.

Covino K.M.,University of Southern Mississippi | Covino K.M.,Shoals Marine Laboratory | Morris S.R.,Shoals Marine Laboratory | Morris S.R.,Canisius College | Moore F.R.,University of Southern Mississippi
General and Comparative Endocrinology | Year: 2015

Preparation for breeding may overlap extensively with vernal migration in long-distance migratory songbirds. Testosterone plays a central role in mediating this transition into breeding condition by facilitating changes to physiology and behavior. While changes in testosterone levels are well studied in captive migrants, these changes are less well known in free-living birds. We examined testosterone levels in free-living Nearctic-Neotropical migrants of three species during their vernal migration. Testosterone levels increased during the migratory period in males of all three species but significantly so in only two. Testosterone levels in females remained the same throughout their migration. Our results support the extensive overlap between vernal migration and breeding preparation in male songbirds. The pattern of testosterone changes during vernal migration is far from clear in females. © 2015 Elsevier Inc.

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