the tubing and the other end is then sealed. One advantage of this technique is
that dose rate remains constant and is directly proportional to the length of
tubing used. For example, doubling the dose rate can be achieved by doubling
the length of the implant or by using two implants of the same length.
Miniosmotic pumps. ALZET®osmotic pumps (Alza corporation, Palo Alto,
California) continuously deliver test substances at controlled rates. They are
available in three sizes with 100 l, 200 l, and 2 ml reservoirs and operate for
various periods, from 1 day to 4 weeks. Delivery rate, in terms of volume, is
determined by the model. The required dose rate can be achieved by calculating
the appropriate concentration of substance in the reservoir. These pumps are best
suited to substances that can be delivered as aqueous solutions.
Both types of implant should be positioned subcutaneously—intraperitoneal
implants often become encapsulated in connective tissue and they may cause
hemorrhage. Subcutaneous implants should not be placed on the back because
they can rupture the skin. They can be implanted on the flank or side of the
thorax. Gaunt and Oring (1999) recommend that the back of the neck should
not be used because the implants can penetrate the thoracic cavity. Personally,
I have not found this to be a problem. Subcutaneous implants in birds are rela-
tively easy because the skin is only loosely attached to the underlying tissue.
A small incision (slightly longer than the diameter of the implant) is made in the
skin, the implant is passed through and the incision is sutured. In free-living
birds, care should be taken that long-term treatment does not interfere with vital
functions. For example, short-term use of testosterone implants can be useful to
examine the behavioral effects of testosterone, but long-term use may prevent
molt or migration. Ideally, the implants should be removed at the end of the
experiment. If this is not possible, the implant should be designed so that all of
the test substance has diffused before it can have negative effects.
9.2.9Laparotomy
Laparotomy is a major surgical procedure and should be carried out under general
anesthesia. In the United States laparotomy of free-living birds is sometimes done
without anesthesia, or with local anesthesia. The justification is that, for birds that
are to be released quickly back to the wild, general anesthesia causes more trauma
than the surgery. In the United Kingdom, such a procedure would not normally
be permitted. Laparotomy was frequently used to determine sex in monomorphic
species, but this has largely been superseded by the use of molecular markers in
blood or other tissue samples. The procedure is still used to assess sexual maturity
by measuring testicular size or the size of ovarian follicles. The procedure has been
220 |Techniques in physiology and genetics