HOMOSEXUAL MATING DISPLAYS IN PENGUINS

1. HOMOSEXUAL MATING
DISPLAYS IN PENGUINS

2. Abstract
More than 50 yr ago, field studies recorded the same-sex
pairs (and trios) of penguins displaying to each other
during the mating season, using behavior patterns typical
of heterosexual mating displays. Such observations led to
a hypothesis that due to a lack of sex recognition pairing
occurs at random with respect to sex, an idea countered by
the argument that sex recognition is highly accurate. No
quantification of same-sex mating displays has tested the
frequency of such displays in penguins or tested the
hypothesis of random display partners with respect to sex.

3. During their mating season, the researchers studied
displaying and paired king penguins, Apenodytes
patagonicus, at Kerguelen Island and sexed them using a
DNA marker, to quantify any occurrence of this behavior.
Indeed, same-sex courtship displays were common
(28.3% of 53 displaying pairs), the great majority of which
were between males. Some homosexually displaying
males eventually paired with females, but such males
were significantly slower in heterosexual pairing than
males that did not display homosexually. In two
extraordinary cases, same-sex pairs learned each other’s
calls, an essential step in the pairing process.

4. The frequency of such pairs was much lower than
among displaying couples, significantly so for
males. Finally, the frequency of homosexually
displaying pairs was significantly lower than
expected from random assortment of displaying
birds, for both males and females. They examined
possible explanations for same-sex display and its
biological significance. A population sex-ratio bias in
favor of males and high concentration of male sex
hormones may help to explain non-reproductive
homosexually displaying pairs.

5. Methods
The researchers studied king penguins at Cape Ratmanoff
on the Courbet Peninsula of Kerguelen Island
(4914¢S,7034¢E) in a colony that contained more than
100,000 breeding pairs. In Dec. 2006, we captured pairs of
displaying but un-bonded individuals on courtship grounds
along the edges of the nesting colony, after observing these
individuals perform courtship calls for several minutes. In
Nov. and Dec. 2008, they sampled the pairs that showed
evidence of bonding. Pairing is a process that involves
ritualized interactions with several potential partners and
ends with a bonded pair. The sampled couples were chosen
haphazardly during searches for displaying birds.

6. The Researchers used the most commonly reported tri-
stimulus color variables for the study of bird coloration
(hue,chroma, and brightness). They calculated UV-hue,
UVchroma, and UV-brightness (from 320 to 450 nm) for
the beak, which has previously been shown to reflect UV
color (Jouventin et al. 2005, 2008; Dobson et al. 2008;
Nolan et al. 2010). Hue is a measure of color appearance
that gives the wavelength corresponding to what is in
everyday speech called, for example, ‘red’ or ‘blue.’ Hue is
the wavelength corresponding to the mid-point between
the lowest and highest reflectance for yellow-red colors
(Pryke et al. 2001).

7. Non-UV-hue and UV-hue were respectively estimated as
the values of the wavelength at the inflection point of the
spectral curve between 450 and 700 nm, and at the maxi
UV range of wavelengths (320–450 nm), given the range of
spectral sensitivity in birds (320–700 nm). Saturation, or
chroma, is a measure of color purity, calculated as the
difference between maximum and minimum reflectance,
divided by the average reflectance across the spectrum
([Rmax)Rmin] ⁄Raverage 320–700). Color brightness is a
measure of the total amount of light coming from a unit
area of a surface and was estimated by calculating the sum
of reflectance, expressed over all visible wavelengths
(Endler 1990; Hill & McGraw 2006a,b).

8. They used the frequency of males and females among displaying
birds to estimate the expected frequencies of types of displaying
couples (male–male, female–female, and male–female). Pair
expectations were estimated as in the toss of an unfair coin, where
p was the frequency of displaying males among un-bonded pairs
and q = (1)p) was the frequency of females. From this, they
predicted the frequency of pairs expected under random
assortment: male–male (p2), male–female (2pq), and female–
female (q2). Binomial tests were conducted separately comparing
the actual frequency of male–male or female–female pairs to the
expected frequency under random assortment. The frequency of
displaying male–male pairs in 2006 was compared to the
frequency of male–male pairs that had learned the partners call
from the sample taken in 2008 using Fisher’s exact test and the
actual number of pairs sampled in each year.

9. They examined latency to pair for males that were
observed displaying to other males and males observed
displaying to females with a non-parametric Kaplan–Meier
survival test (Klein & Moeschberger 1997). Unpaired
birds were treated in an analogous manner to ‘survivors’
and pairing as analogous to ‘death.’ The analysis
examines the rate at which the probability of becoming
paired changes over time for the two groups of displaying
males. A few males were ‘censured’ in this analysis,
because they were not paired by the end of the
observation period (censured males remain in the
analysis). Characteristics of color ornaments were
compared using non-parametric Mann–Whitney U-tests.

10. Results
Of 53 un-bonded couples for which we had DNA sexing
of both individuals in 2006, 26.4% were male–male
couples and 1.9% were female–female couples, leaving
71.7% heterosexually displaying couples (Fig. 1). The
sample of displaying birds had a sex ratio of 62.3%
males. This frequency was used to predict the
proportion of display pairings with males or females if
each individual displayed with other displayers at
random (38.8% male–male, 14.2% female–female, and
47.0% heterosexually displaying couples).

11. The frequencies of unisexual couples were significantly
less than expected at random for both males and
females (binomial tests, p = 0.041 for males, p = 0.003
for females). Of 75 bonded pairs in 2008, there was one
male couple (1.3%) and 1 female couple (1.3%), a
significant decline in the frequency of same-sex
couples among males from the un-bonded couples
(Fisher exact test, p < 0.0001). All four birds in these
two pairs were subsequently found paired to a new
partner and attending an egg in the colony during the
same mating Season.

14. THANK YOU AND GOD BLESS!