Male pheasant (photo: G. Göransson)
På
svenska, tack.
Mats
Grahn, Görgen Göransson, Torbjörn von Schantz and Håkan
Wittzell.
Molecular
Population Biology
Departments of Animal and Theoretical Ecology
Department
of Ecology
Lund University
Pheasants are polygynous and cocks defend a harem with several hens. There is a marked difference in plumage between the sexes and only the hen cares for the chicks. We studied pheasants in the Revinge area in SW Skåne in southernmost Sweden. No hunting or release of birds reared in captivity took place in the study area during the last 40 years. All adult birds had radio transmitters (approx. 20 g.) mounted as backpacks. By radio tracking we could follow the birds and collect data on behaviour and find the nests. From the size and mass of the eggs we could calculate the day of hatching. Pheasant chicks only stay a day in the nest so it is important to be there on the right day. The newly hatched chicks were marked with a small metal tag in the wing and we took a blood sample for paternity analysis by DNA-fingerprinting. The DNA testing showed that hens fairly often were untrue to the male that they spent most of the time with during the mating season. In 14 of 33 broods there were two or more fathers.
Radio-tracking data showed that the hens preferred males with long spurs. In addition to spur length we had also collected data on male tarsus, tail and wing length and body mass, social dominance, territory quality and age but none of these factors had any notable impact on male attraction on females (Göransson et al. 1990). 30% of the males had two or more females, 32% had one and 38% were without any females during a typical mating season.
Cocks with fake spurs
In order to test the relationship between spur length and mate attraction
we did an experiment where we changed the spur length on randomly selected
males. We had three groups; shortened,
elongated and a control group. Spurs in the control group were left
intact but the birds were otherwise handled the same way. From radio-tracking
the birds we found that males with artificially elongated spurs got more
females and hence, that the previous result could be confirmed. (Figure
1, von
Schantz et al. 1989).
But what about territory quality?
In another experiment we allowed one group of males to take up territories
three weeks prior to a second group of males. The result was that both
groups of males got nearly equal number of hens and we concluded that territory
quality had at the most only a minor impact on mate attraction. A not fully
unexpected result since females are free to place their nests disregarding
of male territories (Grahn
et al. 1993).
Spurs show male condition
Spurs are one of the most variable ornaments of male pheasants and
we found that males with longer spurs survived better than short spurred
ones and this was not a by-product of male-male competition (Grahn
1993). This observation indicates that spur length reflects male quality
and that females can use this trait to choose among potential fathers and
pick up only those with good genetic traits to mate with in order to increase
the quality of their young. According to this idea we found that offspring
from long spurred males seemed to have an increased survival (von
Schantz et al. 1994).
Hunting for the "good genes"
The advantageous genetic traits that females look for may thus be indicated
by the length of the spurs. Long spurred males carry genes that make them
more fit to current environmental conditions than males with shorter spurs
and females that do the right choice will therefore on average get better
surviving offspring. This agrees well with the theory by Hamilton
and Zuk (1982) on the importance of heritable resistance to parasites
and diseases to the evolution of male ornaments (exaggerated sexual traits
as the peacock's train or the stag's antlers) and female mate choice.
The eternal race
The idea is that if females choose males on their ornaments they will
transfer (by the genes) these males disease resistance to their young.
This will increase the proportion of resistant individuals in the population,
but only until the pathogen (e.g. a virus) has adapted to the new situation.
Hence, in a couple of generations usually good resistance traits will be
less effective and previously inferior resistance genes will give better
protection.
Ornaments reflect the immune systems
ability
Since females can not see male genotypes directly they rely on ornamental
traits that reflect male vigour. This gives that in generation after generation
different combinations of immune system genes are selected for or against,
depending on their effect on male condition, only because females prefer
to mate with the flashiest males. The Hamilton and Zuk theory argues that
most of what we find beautiful, or even bizarre, in nature is the product
of this evolutionary arms race between pathogens and their hosts.
We are currently looking at some of the genes that may affect disease resistance in the pheasant. These so called MHC genes regulates the immune systems ability to recognize foreign pathogens and are common to all vertebrate animals (Wittzell et al. 1994). You can find examples on pheasant MHC gene DNA sequence here.
In a sample of 110 pheasant males from our study area we found a significant difference in spur length between different MHC genotypes. In this sample we also found fewer than expected MHC homozygous individuals and different survival of different MHC genotypes (von Schantz et al. 1996). These are the first data that directly support the "good genes" hypothesis by Hamilton and Zuk (1982) predicting that females discriminate among males on the basis of secondary sexual characters in order to pass on genes for disease resistance that improve fitness in their offspring.
References
Created 1995-06-12 Updated 1997-09-11
Mats Grahn
Ecology Building
S-223 62 LUND
Sweden
