66 research outputs found
False holes as camouflage
Long noted by naturalists, leaf mimicry provides some of the most impressive examples of camouflage through masquerade. Many species of leaf-mimicking Lepidoptera also sport wing markings that closely resemble irregularly shaped holes caused by decay or insect damage. Despite proposals that such markings can either enhance resemblance to damaged leaves or act to disrupt surface appearance through false depth cues, to our knowledge, no attempt has been made to establish exactly how these markings function, or even whether they confer a survival benefit to prey. Here, in two field experiments using artificial butterfly-like targets, we show that false hole markings provide significant survival benefits against avian predation. Furthermore, in a computer-based visual search experiment, we demonstrate that detection of such targets by humans is impeded in a similar fashion. Equally contrasting light marks do not have the same effect; indeed, they lead to increased detection. We conclude that the mechanism is the disruption of the otherwise homogeneous wing surface (surface disruptive camouflage) and that, by resembling the holes sometimes found in real leaves, the disruptive benefits are not offset by conspicuousness costs.Funding provided by: Engineering and Physical Sciences Research CouncilCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000266Award Number: EP/M006905/
The jet and circumburst stellar wind of GRB 980519
We present extensive multi-colour (UBVR_CI_C) photometry of the optical
afterglow of GRB 980519. Upon discovery, 8.3 hours after the burst, the source
was decaying as a power law, (t-t_GRB)^alpha, with a rapid decay rate alpha_1 =
- 1.73+-0.04. About 13 hours after the burst a steepening of the light-curve to
alpha_2 = -2.22+-0.04 was observed. Within the framework of current afterglow
models, we argue that the rapid initial decline, the `break' in the light
curve, and the spectral properties of the afterglow are best interpreted as
being due to a collimated ultra-relativistic jet of fixed opening angle
expanding into an inhomogeneous medium. In this scenario, we find that the
circumburst medium has a density structure that goes as r^(-2.05+-0.22). This
is characteristic of a preexisting wind expelled from a massive star. A
possible physical scenario is that the progenitor star collapsed to form a
black hole (i.e., a `collapsar'), producing the observed burst and afterglow.
However, the supernova signature expected in the light curve in such a scenario
is not detected. This either implies that the redshift of GRB980519 is greater
than 1.5 or that supernovae accompanying GRBs are not standard candles.Comment: 11 pages, including 2 tables and 3 figures. Accepted for publication
in ApJ, part
'Round the Clock Observations of the Q0957+561 A,B Gravitationally Lensed Quasar
An observing campaign with 10 participating observatories has undertaken to
monitor the optical brightness of the Q0957 gravitationally lensed quasar for
10 consecutive nights in January 2000. The resulting A image brightness curve
has significant brightness fluctuations and makes a photometric prediction for
the B image light curve for a second campaign planned for 12-21 March 2001. The
ultimate purpose is to determine the gravitational lens time delay to a
fraction of an hour, and to seek evidence for rapid microlensing.Comment: 8 pages, AASTeX 4.0, accepted by the Astrophysical Journa
- …