Xavier University Investigates the Evolution of Certain Traits in Species Discussion

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An experimental test of the effect of male attractiveness
on maternal investment in Dark-eyed Juncos
Author: Ferree, Elise D.
Source: The Auk, 136(1) : 1-12
Published By: American Ornithological Society
URL: https://doi.org/10.1093/auk/uky009
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AmericanOrnithology.org
Volume 136, 2019, pp. 1–12
DOI: 10.1093/auk/uky009
RESEARCH ARTICLE
An experimental test of the effect of male attractiveness on maternal
investment in Dark-eyed Juncos
Elise D. Ferree*
Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, California, USA
Current address: Keck Science Department, Claremont Colleges, Claremont, California, USA
* Corresponding author: eferree@kecksci.claremont.edu
Submitted May 25, 2018; Accepted November 20, 2018; Published February 5, 2019
ABSTRACT
While social mate choice is an obvious point where sexual selection acts on males, cryptic selection can occur based on
how much females invest in a male’s offspring. I tested 4 hypotheses about the relationship between female investment
and social mate attractiveness by manipulating a sexually selected trait, white tail feathers (“tail white”), in male Dark-eyed
Juncos (Junco hyemalis thurberi): (1) the differential allocation hypothesis predicts that females optimize lifetime reproductive
success by maximizing parental investment in the offspring of ornamented mates and minimizing investment when mated
to less attractive males; (2) instead of positive differential allocation, females could show negative differential allocation by
investing more in less attractive males to compensate for their lower quality; (3) female care could also be influenced directly
by paternal investment, in particular what appears to be positive differential allocation could be compensation for reduced
care by attractive males; or (4) females could reduce care when paired with socially attractive males that are “good parents”.
To evaluate these hypotheses, I examined the impact of tail white manipulation on clutch size, female incubation and
brooding, and female and male provisioning. Females did not differentially allocate in relation to social mate attractiveness,
nor were females responsive to patterns of male investment, even although artificially attractive males provisioned offspring
significantly more than did controls. Naturally bright males, who were also the largest males, did not provision at higher
rates, highlighting the need to experimentally separate the role of specific traits from other correlated factors. Tail whitemanipulated males were involved in rare cases of polygyny and at times were actively guarded by their mates. Together,
these results suggest that tail white influences reproductive behaviors in this junco population, but because of the potential
for extrapair paternity, additional data are needed to accurately investigate the trait’s role in female investment.
Keywords: behavior, evolution of reproductive effort, mate choice, parental investment theory
Evaluación experimental del efecto del atractivo del macho en la inversión maternal de Junco hyemalis
thurberi
RESUMEN
Mientras que la elección de la pareja social es un aspecto obvio donde la selección sexual actúa sobre los machos, la
selección críptica puede ocurrir vinculada a cuánto invierten las hembras en la descendencia de un macho. Evalué cuatro
hipótesis sobre la relación entre la inversión de la hembra y el atractivo de la pareja social mediante la manipulación de
un rasgo seleccionado sexualmente, las plumas blancas de la cola (“cola blanca”), en machos de Junco hyemalis thurberi.
(1) La hipótesis de asignación diferencial predice que las hembras optimizan el éxito reproductivo de toda la vida
maximizando la inversión parental en la descendencia de machos ornamentados y minimizando la inversión cuando
se aparean con machos menos atractivos. En lugar de asignación diferencial positiva, (2) las hembras podrían mostrar
asignación diferencial negativa, invirtiendo más en machos menos atractivos para compensar por su menor calidad.
El cuidado de la hembra también podría estar influenciado directamente por la inversión paternal. En particular, (3) lo
que parece ser asignación diferencial positiva podría ser compensación por un menor cuidado por parte de machos
atractivos, o (4) las hembras podrían reducir el cuidado cuando se juntan con machos socialmente atractivos que son
“buenos padres.” Para evaluar estas hipótesis, examiné el impacto de la manipulación de la cola blanca en el tamaño
de la nidada, en la incubación y cría de la hembra, y en el aprovisionamiento de la hembra y el macho. Las hembras no
asignaron diferencialmente en relación con el atractivo del macho social, ni respondieron a los patrones de inversión
del macho, aunque los machos artificialmente atractivos proveyeron a la descendencia significativamente más que los
controles. Los machos naturalmente brillantes, que fueron también los más grandes, no aprovisionaron a tasas más altas,
subrayando la necesidad de separar experimentalmente el rol de los rasgos específicos de otros factores correlacionados.
Los machos con colas blancas manipuladas estuvieron involucrados en casos raros de poliginia y a veces fueron vigilados
activamente por sus parejas. En conjunto, estos resultados sugieren que la cola blanca influencia los comportamientos
reproductivos en esta población de J. h. thurberi, pero debido al potencial de paternidad extra pareja, se necesitan datos
adicionales para investigar con precisión el rol de este rasgo en la inversión de la hembra.
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2
Maternal investment and mate attractiveness
E. D. Ferree
Palabras clave: comportamiento, elección de pareja, evolución del esfuerzo reproductivo, teoría de inversión
parental
INTRODUCTION
Sexually selected traits can evolve through the effects of
intra-sexual competition for mates, or when females exhibit
preferences for certain males. While direct selection of one
male over another is the most obvious way female choice
acts on male traits, more subtle means, termed cryptic choice, could be equally influential (Eberhard 1996).
Cryptic choice occurs after social mate choice and can
come about via sperm sorting, extrapair mating and biased
maternal investment in breeding attempts (Eberhard 1996,
Sheldon 2000). This last phenomenon has been the focus
of what is traditionally called the differential allocation
hypothesis (DAH; Burley 1988). In more recent use, the
DAH proposes that because individuals experience a tradeoff between investing in current vs. future reproduction,
the attractiveness or quality of one parent could shape the
optimal tradeoff and therefore investment of the other parent (Sheldon 2000, Ratikainen and Kokko 2010, Haaland
et al. 2017). The focus of the DAH is normally on female
investment in relation to her mate’s attractiveness, but the
hypothesis could apply to either parent.
As an adaptive female behavior, differential allocation
of reproductive investment is expected, per its definition,
only when 2 main assumptions are met. First, the DAH
assumes that producing and caring for offspring represent major costs of reproduction that affect an individual’s lifetime reproductive success (Williams 1966, Trivers
1972). The tradeoff between investing in parental care vs.
self-maintenance within a given breeding attempt can ultimately affect the total number of offspring produced over
a lifetime. The second assumption of the DAH is that some
aspect of male quality indicates either a genetic or direct
benefit to the young. For example, males could signal their
attractiveness, which will be passed to the offspring, or
the quality of their territory, which would directly influence offspring fitness (Ratikainen and Kokko 2010). To
maximize long-term reproductive output females should,
therefore, adjust their investment based on what their mate
has to offer, which is the basis for the differential allocation hypothesis (Sheldon 2000). For example, female Barn
Swallows (Hirunda rustica) responded to manipulation
of their mate’s tail length, which is under sexual selection,
by provisioning their offspring more and having a greater
number of broods per season when mated to males with
elongated tails compared with females mated to controls
and those with shortened tails (de Lope and Møller 1993).
Findings like these illustrate that cryptic female choice
does occur in some organisms, but there are also many
instances where the DAH is not supported (Sanz 2001,
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Mazuc et al. 2003). More studies are needed that simultaneously test the predictions of additional hypotheses
related to mate attractiveness and parental investment, particularly studies with an experimental approach (Maguire
and Safran 2010). Here, I manipulated male attractiveness in a common songbird to test the DAH and 3 other
hypotheses for how male attractiveness influences female
reproductive investment. Two of the 4 hypotheses make
predictions about how maternal care relates to male attractiveness when paternal investment is constant. The other 2
hypotheses explicitly address the fact that male attractiveness might itself correlate with male investment, and hence
make predictions about how females should respond.
As described above, the first hypothesis, the DAH, has
traditionally been interpreted as positing that females
should increase investment with attractive males to maximize the benefits they receive from breeding with them,
so called positive differential allocation (Sheldon 2000,
Ratikainen and Kokko 2010, Haaland et al. 2017; Table
1). Differential allocation, however, could occur either in
favor of attractive males or non-attractive males (Gowaty
2008, Harris and Uller 2009, Ratikainen and Kokko 2010).
The second hypothesis, sometimes framed in terms of
compensation but what I will call the negative differential
allocation hypothesis, predicts that in certain situations
females could benefit from maximizing investment with
unattractive mates as compensation for their lower genetic
quality (Gowaty 2008, Harris and Uller 2009, Ratikainen
and Kokko 2010, Haaland et al 2017; Table 1). For example, female Barn Swallows (H. rustica) paired with tailshortened males supplied their eggs with greater amounts
of immunity-boosting carotenoids than did females with
control and tail-elongated mates, possibly to help the
developing young cope with the greater likelihood of parasitism that offspring of tail-shortened males experience
(Saino et al. 2002). Since the DAH was formally introduced by Burley (1986), evidence has grown for increased
but also decreased investment by females with attractive
males. Only recently has it been explicitly recognized that
although they make opposite predictions, the 2 hypotheses
both represent differential allocation based on mate quality (Ratikainen and Kokko 2010, Haaland et al. 2017).
The other 2 hypotheses account for variation in male care,
since maternal care could be influenced by male quality in a
direct way if attractiveness relates to a male’s own parental
investment. For example, if high-quality males withhold care
in favor of seeking extrapair copulations (Sanz 2001), females
would need to increase their own investment to maintain
constant levels of care for their offspring. This response
would be consistent with positive differential allocation,
2019 American Ornithological Society
E. D. Ferree
Maternal investment and mate attractiveness
3
TABLE 1. Hypotheses and predictions evaluated in this study examining patterns of maternal investment in relation to whether
females were mated to control or enhanced males.
Hypothesis
Predicted pattern of female care: when mated to
attractive males relative to
controls
Accompanying
change in male
care: attractive
males relative to
controls
Positive differen- More care when mated to
tial allocation
attractive males
None
Negative differential allocation
Less care when mated to
attractive males
None
Compensation
More care when mated to
attractive males
Less care when mated to
attractive males
Reduced
Good parent
Increased
Reasoning for female pattern
Sources
Females maximize high perceived offspring
quality when mated to attractive males relative to
females mated to controls.
Females mated to control males try to compensate for lower perceived offspring quality
by investing more relative to females mated to
attractive males.
Attractive males themselves provide less care than
control males, for which females compensate.
Attractive males themselves provide more care
than control males, which relieves females of
needing to invest as much.
1, 2, 3
3, 4, 5
3
6
1. Burley (1988); 2. Sheldon (2000); 3. Ratikainen and Kokko (2010); 4. Gowaty (2008); 5. Harris and Uller (2009); 6. Hoelzer (1989).
as females allocate more energy when mated to attractive
males, but actually be a form of compensation (Ratikainen
and Kokko 2010). The third hypothesis, the compensation
hypothesis, therefore predicts that females should compensate with increased investment if attractive males themselves
provide less care (Ratikainen and Kokko 2010; Table 1). In
Common Yellowthroats (Geothlypis trichas), for example,
male ornament size is negatively correlated with paternal
care: males with bigger bibs or masks invest less in offspring
than males with smaller ornaments (Mitchell et al. 2007). In
one population, female yellowthroats compensated by providing more care as their mates provided less (Mitchell et al.
2007). Again, while consistent with positive differential allocation, the fact that male care is in some cases correlated with
attractiveness makes it difficult to say whether females are
favoring attractive males or simply compensating to maintain a minimum level of care for their offspring.
Lastly, the good parent hypothesis accounts for instances
where male quality correlates positively with parental abilities (Table 1). Under this hypothesis, females could subsequently reduce their share of the total parental investment
when mated to high-quality, attractive males (Hoelzer
1989). Orange bill color in blackbirds (Turdus merula) is a
sexually selected trait, and in one study males with brighter
bills provisioned nestlings at higher rates than males with
duller bills (Préault et al. 2005). The link between male bill
color and care is consistent with the good parent hypothesis, however females did not reduce their investment
in response to increased male care (Préault et al. 2005).
Parental investment is similarly correlated with measures
of attractiveness in other, but not all, studies in which it
has been examined, and a subsequent effect on female
care is even rarer (Smiseth et al. 2001, Siefferman and Hill
2003). Nevertheless, as with the compensation hypothesis,
without considering male investment, accurate interpretation of female investment is not possible.
I manipulated a plumage signal, the degree of white on
the outer tail feathers (hereafter “tail white”) in Dark-eyed
Oregon Juncos (Junco hyemalis thurberi) to test the above
hypotheses for how female parental investment relates
to male tail white. I created a subset of experimentally
enhanced males with tail white at the maximum of the natural range and compared investment by their mates to investment by females paired to control males. I also accounted
for potential differences in paternal care between the treatment groups when examining female care.
Dark-eyed Juncos are ideal for this type of experiment
partly because tail white is easy to manipulate (Holberton
et al. 1989, Hill et al. 1999, Ferree 2007). More importantly, previous work on juncos highlights the need for a
more thorough understanding of whether female preference plays a role in the evolution or maintenance of tail
white. For example, in one study of the Carolina junco subspecies, females in an aviary study preferred males with
experimentally enhanced tail white over control males, but
subsequent studies in the field found that natural tail white
did not predict male reproductive success (J.h. carolinensis, Hill et al. 1999, McGlothlin et al. 2005). Experimental
studies that address the role of tail white in the wild are
lacking. Finding, for example, that females in the population studied here differentially allocate based on tail white
would provide evidence for inter-sexual selection favoring
males with whiter tails. Also needed is further comparative
work that will shed light on the role of tail white in the several Dark-eyed Junco subspecies, which all have white tail
feathers but in varying amounts (Ferree 2013).
Tail white could also relate to the behavior of the males
themselves, particularly being impacted by male-male
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E. D. Ferree
competition since tail white is known to relate to male
dominance in some populations (J. hyemalis, Balph et al.
1979, Holberton et al. 1989). Dominance status related to
tail white could then have a positive or negative impact on
a male’s investment in parental care and seeking extrapair
copulations. In terms of the developmental basis for tail
white, in some populations tail white shows enough heritable variation so that maternal differential allocation could
enhance offspring fitness when genetic benefits are at stake
(J.h. carolinensis, McGlothlin et al. 2005; J.h. thurberi, Yeh
2004). At the same time, the amount of white on the tail
feathers is partly dependent on diet, which means that tail
white could reflect the ability of parents to gather food for
their offspring (J.h. carolinensis, McGlothlin et al. 2007a).
In turn, the amount of food provided to the young by both
the female and male could have a direct impact on the offspring’s subsequent tail white expression and hence fitness.
In summary, I considered 4 hypotheses for how maternal investment relates to ornamentation in male Dark-eyed
Juncos to gain insight into the mechanisms by which female
choice can act, and therefore also to better understand the
function of white tail plumage in juncos (Table 1). In this
study, the first 2 hypotheses apply when male investment
can be ruled out as directly affecting female investment,
i.e. to stages of female reproductive investment in which
males are not directly involved (clutch size, incubation and
brooding) or when there is no difference in male provisioning between the treatment groups: (1) positive differential
allocation: females mated to tail-white enhanced males
should increase the number of eggs laid and the amount of
care provided to young compared with females with control
mates; (2) negative differential allocation: females mated
to controls should invest more relative to females mated
to attractive males because they are compensating for the
lower quality of their mates. The remaining 2 hypotheses
take into account how differences in male provisioning in
relation to attractiveness could directly influence female
investment: (3) compensation: females should increase
provisioning effort when paired to tail-white enhanced
males if attractive males provision less compared with
control males; (4) good parent hypothesis: females should
reduce investment when mated to tail-white enhanced
males if attractive males are better parents that provision
more than control males. By evaluating these 4 hypotheses
with behavioral data from both females and males, I can
determine whether females cryptically favor bright males
through variation in parental investment.
1,944 m in elevation) in the 2003, 2004 and 2005 breeding
seasons. A detailed description of the study system can be
found elsewhere (Ferree 2007). In brief, I caught males and
females as they began pairing in early to late May, banded
them and measured flattened wing chord to the nearest
1 mm and mass to the nearest 0.5 g. I scored tail white
by visually estimating to the nearest 0.1 the proportion
of white on each tail feather (Hill et al.1999, Ferree 2007,
Ferree 2013; Figure 1A). Juncos in this subspecies usually
have white on the 3 outer tail feathers and show the most
METHODS
FIGURE 1. (A) Image of junco tail feathers (rectrices 4–6) ranging
from 1.0 to 0.1 proportion that is white. (B) A male junco with
2.1 white feathers on each side of the tail, for a total of 4.2 white
feathers and (C) an experimentally enhanced male with 3.0 white
tail feathers on each side of the tail. The third feather in (rectrix
4) was replaced with a fully white tail feather using a cut-andpaste technique (see Methods).
Study System and General Methods
I studied Oregon Juncos (J.h. thurberi) in the Sierra Nevada
at Sagehen Creek Field Station, University of California
Natural Reserve, California, USA (39°25’N, 120°14’W,
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E. D. Ferree
Maternal investment and mate attractiveness
variation on the third feather from the outside (rectrix 4).
I estimated tail white on both right and left sides of the tail
and summed these estimates for a measure of the total number of white feathers on the tail, the number being out of
the 12 total tail feathers (Figure 1B). Estimates of tail white
were significantly repeatable when I recorded tail white for
30 museum specimens 3 times each without awareness of
my previous estimate(s) (repeatability = 95.8%, P < 0.001, Lessells and Boag 1987). Tail white scores based on visual estimation are also highly correlated with those estimated using digital photography technology (r > 0.90, J. W. Atwell
personal communication, Yeh 2004).
I located and monitored each pair’s nest throughout the
season to monitor reproductive success and conducted
nest observations (as described below) to verify the identity of the social parents and quantify parental investment. The first egg date was calculated as the day a female
laid her first egg in a given breeding attempt, counted in
days since the first egg date of the first nest of that year.
Pairs laid replacement eggs after predation events that
occurred early enough in the season, but otherwise were
single-brooded. The population consisted of ~40 pairs per
year, and each male and female was included only once in
analyses.
Tail White Manipulation
I carried out the tail white manipulation after males had
paired but before nest building, with pairing status determined through daily nest searching and observations
of each male’s territory. Paired males were regularly and
repeatedly seen for extended periods with a female and
were accompanied by a female when responding to conspecific song during capture. A few males were treated
within 1 or 2 days after a failed early-season breeding
attempt, after which they re-nested with the same female.
Tail white can be easily manipulated via a cut-and-paste
technique (Holberton et al. 1989, Hill et al. 1999, Ferree
2007). To carry out the white addition, I first obtained
white tail feathers from living male juncos in a distant
Oregon junco population (Santa Cruz County, California,
USA), cut these feathers 1 cm from their base, and hollowed out the feather shaft with a needle. On the bird to
which the feather would be attached, I cut its fourth rectrix at an angle 1.5–2 cm from the base, applied a drop of
superglue (Instant Krazy Glue, High Point, North Carolina,
USA) to the extra feather, slid this feather over the experimental bird’s feather stub, and lastly spread a thin layer of
glue around the attachment site. I waited several min for
the glue to dry, and if necessary I trimmed newly attached
white feathers to the length of original tail feathers, then
released the bird (Figure 1C). Finally, during the study
I periodically captured a subset of males in the experimental and sham groups, and found that the manipulated
feathers were still in place (n = 5), which was also evident
when using binoculars to observe males spreading their
tail feathers in social displays (Nolan et al. 2002).
I assigned junco males to increased tail white, control
or sham treatments in random order as they were captured. Males in the study population had a mean (± SE) of
4.75 ± 0.06 total white tail feathers with only 2 individuals
having ≥ 6.0 white feathers (n = 79, Ferree 2007). I gave 30
males (n = 15 in both 2004 and 2005) 3 white feathers on
each side of the tail (total = 6 white feathers), the maximum of the natural range. I treated sham males in a similar
manner but cut and reattached their own fourth rectrices
(n = 17 in 2004, n = 6 in 2005), while control males were
un-manipulated (n = 20 in 2003, n = 9 in 2004, n = 12 in
2005). Due to the random assignment of tail white treatments, males of different treatments were distributed
equally throughout the study site and each nest bordered
several territories.
Quantifying parental investment. I checked nests every
other day to determine the first egg date, clutch size, number of hatchlings and number of fledglings for each breeding attempt. To quantify parental care I conducted 2 2-hr
nest observations per nest: 2–3 days after incubation began
and when nestlings were 2–3 days old. I also observed
6–7 day-old nestlings, but sample size at this stage was low
due to nest predation. The results for older nestlings were
consistent with those shown but not significant, and provisioning rates at 2–3 days old correlated roughly with those
at 6–7 days old (number of feeding trips per nestling per hr,
females: Pearson r = 0.37, n = 25, P = 0.055; males: Pearson
r = 0.36, n = 25, P = 0.064). I observed nests between 1400
and 1800 hours and never in inclement weather. Incubation
and brooding rates were calculated as the proportion per
hr that females spent incubating or brooding, respectively,
and then arc-sin transformed. I calculated provisioning
rates as the number of feeding trips per nestling per hr
made by the male and female and also calculated the total
feedings per nestling per hr for each brood as a measure of
overall effort (male feeds plus female feeds divided by the
number of nestlings per hr).
Statistics
I first analyzed clutch size, percentage of eggs hatching, fledgling number and measures of female and male
care for normality and homogeneity of variance/covariance. I evaluated normality with the Shapiro-Wilks test.
To test the assumption of homogeneity of variance I used
Levene’s test for equality of variances for univariate tests
and Box’s test of equality of covariance matrices for multivariate tests. Based on these assessments, I used MannWhitney U-tests to compare clutch size, hatching rate and
fledgling success between treatment groups and used general linear mixed models (GLMMs) to test for significant
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E. D. Ferree
significant multivariate effects. Treatment was retained in
all final models. Finally, I tested for a relationship between
tail white and wing length using all males in the population,
including the non-experimental year (2003), using parametric correlation. Analyses were conducted in SPSS 17.0
(SPSS Inc, Chicago, Illinois, USA), all tests were 2-tailed,
and means are presented with standard errors.
RESULTS
FIGURE 2. Relationship between pre-manipulated male tail
white and wing length among all males captured during this
study (n = 77, 2003–2005). Overlapping data points are slightly
staggered.
maternal and paternal investment differences, in terms
of incubation, brooding and provisioning, between treatment groups and among years. In GLMMs, treatment was
a fixed factor and year was included as a random factor.
Pre-manipulated male tail white and wing length, female
tail white and wing length, and date and time of day for
nest observations were included as covariates. I considered
a male’s natural tail white to be an indicator of his underlying quality and wing length as a measure of body size independent of the tail white manipulation (McGlothlin et al.
2005, 2007a). Another reason I included pre-manipulated
male tail white in the models was to account for the possibility that females would not respond to the manipulation of tail white that occurs after the pair bond is formed.
I included female tail white and body size because females
with more white or longer wings may be in better condition and invest more heavily in their broods than smaller
females with less white (Ferree 2007).
When testing for an effect of the tail white treatment,
I included only data collected in the years of the manipulation, 2004 and 2005. Starting with maximum models containing all predictor variables and interactions, I removed
interactions and then factors in a backwards stepwise fashion and used appropriate follow-up analyses to examine
I combined sham and control males in analyses after verifying that they did not differ significantly in body size, tail
white, clutch size, hatching rate, number of fledglings,
incubation rate, brooding rate, or male and female provisioning rate, and collectively called them controls (independent t-test, all P > 0.38). Experimental and control
males and their mates did not differ initially in tail white
or wing length (Ferree 2007). In the population as a whole,
male wing length and tail white were positively correlated
(Pearson’s r = 0.26, n = 77, P = 0.02, 2003–2005, Figure
2). Nests in the 2 treatments did not differ in mean first
egg date (controls: 15.18 ± 1.86 days; enhanced tail white:
16.19 ± 2.39 days, t = 0.34, df = 73, P = 0.74). Likewise,
clutch size, the number of eggs hatching and the number
of fledglings from all attempts and from only successful
attempts did not differ between experimental and control
broods (Table 2).
I analyzed incubation rates independently of behaviors
observed during the nestling stage because of unequal
sample sizes. When controlling for pre-manipulated
male tail white in a univariate GLMM, females mated to
experimentally enhanced males incubated 6% less than
females mated to control males, a non-significant difference (F = 2.80, df = 1 and 40, P = 0.10, 2004–2005, Figure
3A). The negative relationship between female incubation
and her mate’s un-manipulated tail white was, however,
significant: female incubation decreased as her mate’s premanipulated, natural tail white increased (F = 10.33, df = 1
and 40, P = 0.003, 2004–2005, Figure 3B). This correlation
was significant within each treatment group (enhanced
males: Pearson r = −0.43, n = 20, P = 0.05; control males:
Pearson r = −0.48, n = 23, P = 0.02, 2004–2005, Figure 3B).
Year, time of day, date, male wing length, and female tail
TABLE 2. Median, minimum and maximum clutch size, percentage of eggs hatching and number of young fledging from broods of
experimental and control males, and the results of Mann-Whitney U-tests comparing treatment groups.
Clutch size
Percentage of eggs hatched
Number of fledglings
All broods
Successful broods
Experimental broods
Control
broods
Mann-Whitney U (total n)
P
4; 3–5
100%: 67–100%
4; 3–5
100%; 75–100%
315.0 (48)
206.0 (38)
0.39
0.32
0; 0–4
4; 1–4
2.5; 0–4
4; 1–4
168.5 (34)
38.5 (19)
0.36
0.61
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E. D. Ferree
Maternal investment and mate attractiveness
7
TABLE 3. Post-hoc analyses from a multivariate general linear
mixed model examining the effect of male tail-white treatment
and covariates a on female brooding and feeding, male feeding,
and total feeding rate.
Factor
Dependent variable
Tail white treatment Female brooding
Female feeding
Male feeding
Total feeding
Male wing length
Female brooding
Female feeding
Male feeding
Total feeding
Error
df
F
P
1 0.15
0.70
1 0.18
0.67
1 10.93 < 0.001 1 3.53 0.07 1 2.05 0.16 1 0.01 0.90 1 11.14 < 0.001 1 5.50 0.03 30 Year, time of day, date, male tail-white, and female tail-white and wing length did not relate significantly to any measures of nestling care (in multivariate analysis, all P > 0.20).
a
FIGURE 3. (A) Effect of tail white manipulation on mean (+ SE)
proportion of an hour that females spent incubating when
mated to control or tail-white enhanced males (control, n = 23;
enhanced, n = 20) and (B) relationship between the proportion
of an hour that females spent incubating and their mate’s premanipulated tail white in 2004 and 2005 (n = 51).
white and wing length were not significantly related to
female incubation rate (all P > 0.24).
I tested for a treatment effect on parental care during
the nestling stage in a multivariate GLMM that included
female brooding and provisioning rate, male provisioning rate and total provisioning rate as the dependent variables. Both tail white treatment and the covariate male
wing length had significant overall effects (tail white
treatment: Pillai’s trace multivariate F = 4.23, df = 3 and
28, P = 0.01, male wing length: F = 3.60, df = 3 and 28,
P = 0.03). In relation to the tail white treatment, enhanced
males provisioned nestlings at significantly higher rates
than control males, providing an extra third of a feeding
per nestling per hr (Table 3, Figure 4A). While male provisioning increased, female provisioning declined slightly,
leading to a non-significant increase in total provisioning to nestlings in experimental broods compared with
control broods (Table 3, Figure 4A). The male tail white
treatment did not affect female brooding rate (female
brooding: enhanced white = 35.0 ± 0.05% per hr, n = 18;
control = 34.0 ± 0.04% per hr, n = 19; Table 3).
As a covariate, natural male tail white was not related significantly to either male or female provisioning, although
it tended to be negatively correlated with male provisioning (Pearson r = −0.23, n = 43, P = 0.13, Table 3, Figure
4B), but male wing length was. As wing length increased,
male provisioning by both experimental and control males
declined significantly (enhanced males: Pearson r = −0.53,
n = 18, P = 0.02; control males: Pearson r = −0.47, n = 19,
P = 0.04, 2004 and 2005; Figure 5). Total provisioning rate
also showed a significant negative relationship with wing
length (Table 3).
DISCUSSION
The results of this study did not clearly support any of the
4 hypotheses tested about how female investment could
relate to male attractiveness or care patterns. First, females
in the enhanced group laid the same number of eggs as
those mated to control males. Once incubation began,
females socially mated to naturally bright males incubated
less than females with naturally dull mates, but incubation rates in relation to the tail white manipulation did not
differ. Subsequent hatching success was equal between
groups. During the nestling stage, neither female brooding nor provisioning rates differed between the treatment
groups, but the provisioning rate of enhanced males was
significantly higher than that of control males. This outcome was unexpected because males with longer wings,
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E. D. Ferree
FIGURE 5. Relationship between male provisioning and wing
length in 2004 and 2005 for control and enhanced males (control,
n = 19; enhanced, n = 18).
FIGURE 4. (A) Effect of tail white manipulation on mean (± SE)
feeding trips per nestling per hr provided by females mated to
control and tail-white enhanced males, by the males themselves,
and by females and males combined, and (B) relationship
between male provisioning and a male’s pre-manipulated tail
white in 2004 and 2005 for control and enhanced males (control,
n = 19; enhanced, n = 18).
who also had brighter tails, provisioned less than smaller,
duller males. Finally, although the offspring of enhanced
males tended to receive more food than those of control
males, fledging success did not differ between groups.
Male Provisioning
The most striking outcome of the tail white manipulation, summarized in the preceding paragraph, was that
enhanced males fed almost 30% more than did control
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males. On the surface, this result is consistent with the good
parent hypothesis, which states that certain traits indicate
a male’s ability to provide parental care (Hoelzer 1989). In
juncos, tail white could be an honest indicator of foraging
efficiency, especially considering that its development is
partly dependent on a high-protein diet (McGlothlin et al.
2007a). Apparent support for the good parent hypothesis
is weakened, however, by the fact that natural tail white did
not relate to male provisioning in the same way that the
tail-white manipulation did (Figure 4A, 4B).
A discrepancy in the relationship between provisioning
and natural tail white on the one hand, and provisioning
and experimental tail white on the other, could be explained
by the fact that tail white and wing length are positively
correlated (Figure 2). Wing length was negatively related to
male provisioning (Figure 5), and so by association, I found
a weak negative relationship between non-manipulated tail
white and male provisioning. In other junco populations,
high testosterone levels also characterize males with bright
tails, and those males are more aggressive and provide less
parental care compared with duller males (J.h. carolinensis, McGlothlin et al. 2007b). In this study the experimental addition of white tail feathers, however, uncoupled the
relationships between tail white and wing length, and tail
white and testosterone. This uncoupling thereby relaxed
the potentially constraining influences of body size and
testosterone on parental care decisions in relation to tail
white. This reasoning is also consistent with the finding
that the tail-white treatment effect was only significant
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Maternal investment and mate attractiveness
when controlling for wing length, and confirms the need
for experimental studies to uncouple traits that might covary naturally among individuals.
Why, however, would experimental males provision at
higher rates compared with control males? This result was
likely not an artifact of the experiment itself. If the manipulation impaired tail function or interfered with mobility,
manipulated males should provide less, not more parental care. Furthermore, a direct effect of the manipulation
would reduce the magnitude of the behavioral difference
between the groups, since both sham males (included in the
control category) and experimental males underwent the
feather manipulation. Increased provisioning by enhanced
males may relate to certainty of paternity, with theory predicting that a male should only invest in young that he has
sired (Whittingham et al. 1992). Unfortunately, I was not
able to examine paternity, but natural tail white was not
related to within-pair paternity in another population of
the same subspecies (Price et al. 2008). Experimentally
enhanced male juncos in this study had 3 white feathers on
each side of the tail, which was within, but on the extreme
end of the natural range of variation (Ferree 2007). Perhaps
having such an abundance of super-attractive males in the
population each breeding season had unique effects on
male and female interactions, paternity, and hence paternal behavior.
Another potential explanation for enhanced provisioning rates of attractive males is that these ornamented
males themselves acted to maximize their offspring’s success. For example, because tail white has a relatively large
heritable component, males could benefit from producing high-quality attractive sons (McGlothlin et al. 2005).
Further, because the development of white tail feathers is
also dependent on dietary protein, males could optimize
the amount of tail white their offspring develop by providing them with more food (J.h. carolinensis, McGlothlin
et al. 2007a). This explanation would require that males
had information about and differentially allocated based
on their own attractiveness. Males could use the behavior
of conspecific males or females to assess their own attractiveness or status and modulate their behavior accordingly
(Pied Flycatchers [Ficedula hypoleuca] Sanz 2001; Great
Tits [Parus major] Galván and Sanz 2009). For example,
male Pied Flycatchers with reduced white forehead patches
apparently compensated for their own lessened attractiveness by providing more care than controls or enhanced
males (Sanz 2001). In this junco population, males able
to assess their tail white could then maximize their offspring’s fitness with increased nourishment (McGlothlin
et al. 2005, 2007a). Finally, because of the positive association between tail white and dominance (Balph et al.
1979, Holberton et al. 1989), males with added tail white
might spend less time in territorial defense compared with
control males and, therefore, could invest relatively more
energy into their offspring.
Female Care
The investment patterns of females suggest a potentially
complex response to their mate’s tail white. In relation to
the first hypothesis, while the data were not consistent with
positive differential allocation, females could have differentially invested in ways I did not measure, such as via variation in egg size or contents (Gil et al. 1999, Rustein et al.
2005, Uller et al. 2005). Females might also cryptically favor
certain males, including their own social mate, by either
participating in or refusing extrapair copulations (Rustein
et al. 2005). Forty-four percent of broods contained extrapair young, and 28% of young were sired by extrapair males
in a one-year study of J. thurberi (Price et al. 2008), while
in a 4-year study of J. carolinensis, 31–74% of broods had
extrapair young and 21–56% of young were identified as
extrapair (Ketterson et al. 1998). The fact that extrapair
paternity is relatively common in Dark-eyed Juncos suggests that some of the variation in maternal investment in
this study could be explained by the quality of a female’s
extrapair mate rather than her social mate. Finally, producing male-biased broods when mated to attractive males
would maximize the fitness benefits of producing attractive sons (Fisher 1930). Partially consistent with this prediction, in this study population females with naturally
high amounts of tail white themselves had male-biased sex
ratios, while their mate’s tail white (pre- and post-manipulated) had a slight but non-significant positive effect
(Ferree 2007, Price et al. 2008). So while female juncos did
not show evidence of cryptic selection in favor of high tailwhite males through the behaviors measured here, a more
thorough examination of female investment is needed to
exclude the possibility.
Recent theoretical work has outlined the scenarios
under which we would expect females to differentially allocate based on male quality (Haaland et al. 2017). Females
would not benefit from positive differential allocation if
male quality has an additive effect on offspring fitness: the
offspring of high-quality males have higher fitness than
those of low-quality males, but increases in female investment influence offspring fitness at the same rate. If male
quality, on the other hand, influences offspring fitness by
increasing the marginal returns of maternal investment, a
multiplicative fitness effect, females should show positive
differential allocation (Haaland et al. 2017). The results of
this study could be better understood if data were available to fully assess whether female juncos would benefit
via offspring fitness from positive differential allocation in
relation to their mate’s tail white. Because white on the tail
feathers is partly dependent on diet in another junco subspecies, I anticipated that female investment could be an
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Maternal investment and mate attractiveness
important determinant of offspring fitness via its influence
on offspring tail white (J.h. carolinensis, McGlothlin et al.
2007a). The fact that I found no evidence for positive differential allocation could indicate that male quality (here
expressed as tail white) has a relatively strong additive
effect on offspring fitness via its genetic influence (J.h. carolinensis, McGlothlin et al. 2005; J.h. thurberi, Yeh 2004).
These predictions could be tested with data on offspring
tail white, survival and reproductive success in relation to
their father’s tail white and mother’s parental investment.
The negative differential allocation hypothesis states that
females overcome fitness costs that stem from pairing with
unattractive males by investing heavily in their offspring
(Gowaty 2008, Ratikainen and Kokko 2010). The most obvious way that females could partly overcome the disadvantage
of being mated to a dull male would be by providing more
food to their offspring, since high-protein diets enhance
development of white on the tail feathers (McGlothlin
et al. 2007a). Instead of increasing provisioning rate, however, females with duller mates were more attentive during
the incubation stage compared with females with brighter
mates. The eggs of control males were incubated 6% more
than eggs of enhanced males, or ~4 additional min per hr,
a non-significant increase (Figure 3a). Female incubation
rates varied significantly, however, in relation to her mate’s
pre-manipulated tail white, being 90% per hr for females
paired with the least white males and as low as 36% per hr
with the brightest mates (Figure 3b). While incubation effort
could in theory impact nestling development and hatching
rate, I did not find a hatching advantage in the clutches of
control males (Nilsson et al. 2008, Table 3). An alternative
hypothesis is that females reduced the time spent incubating
when mated to relatively attractive males in order to mateguard, something discussed in more detail later.
Within a compensation framework, females mated to
enhanced males would be predicted to reduce their own
provisioning rates, which they did not. Females with naturally attractive males reduced attentiveness during the
incubation stage, but because junco males rarely feed their
mates during incubation, females are likely not compensating for a lack of male assistance at this time (Nolan et al.
2002, E. Ferree personal observation). Lastly, the good
parent hypothesis could be relevant if male nest defense
relates to female incubation. Males with more tail white
are normally larger and more dominant than duller males
(Balph et al. 1979, Holberton et al. 1989, McGlothlin et al.
2005), but these and other attributes of aggression in male
juncos do not necessarily relate positively to attentiveness
at the nest (Cawthorn et al. 1998, McGlothlin et al. 2007b).
Females could reduce the time spent incubating if highly
ornamented males provide better defense compared with
control males, but data on male nest defense in this population would be needed to test that possibility.
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E. D. Ferree
Together these data do not support the idea that female
juncos use parental care behaviors to cryptically favor
males with brighter tails. At the same time, anecdotal
evidence suggests that females and males noticed and
responded to the tail-white manipulation in occasionally surprising ways: I observed 2 instances of polygyny in
an otherwise socially monogamous species, both involving a tail-white enhanced male, as well as female guarding. While theory traditionally focuses on males guarding
females to prevent paternity loss, in some species there
is evidence that females guard their mates as a means of
maintaining monogamy (Great Tit [P. major], Slagsvold
1993; European Starling [Sturnus vulgaris], Sandell 1998).
A re-occurring scenario of guarding in this study population involved 3 color-banded individuals, 2 females and 1
tail-white enhanced male. Over several days an unpaired
female (with no mate or active nest that I could find)
approached the male whose mate already had a full clutch
of eggs. The mated female chased away the approaching
female each time. Similar female mate defense behaviors
were also documented in an aviary study in another junco
subspecies as well as in another wild population of the
subspecies studied here (J. h. carolinesis, Jawor et al. 2006;
J. h. thurberi, Reichard et al. 2018). In the captive population, females housed with their mate aggressively defended
him from an introduced intruder female (Jawor et al. 2006).
In a wild population of J. h. thurberi in San Diego, southern
California, females also approached caged female intruders, diving and lunging at them and chasing their own mate
away from the vicinity (Reichard et al. 2018). Given that
the instances of polygyny and mate-guarding involved tailwhite enhanced males, these observations suggest that the
tail white had some effect on the interactions and behaviors of males and females. The fact that at least one female
participated in mate-guarding during the incubation stage
also provides a potential explanation for why female incubation effort was negatively related to male attractiveness
as measured by the tail whiteness.
Conclusions
In summary, while the results of this study did not conclusively support any of the 4 hypotheses considered, they do
exclude the possibility that female juncos in this population use the measured behaviors to differentially allocate,
and hence cryptically select for male tail white. At the same
time, several emerging patterns indicate that tail white may
still play a role in determining male-female interactions in
the reproductive cycle. In addition to the observance of
polygyny and mate-guarding involving tail-white manipulated males, the most unexpected result was that males
with enhanced tail white provided more care than control
males. Further, the difference in the tail-white manipulation’s effect on male provisioning and the relationship
2019 American Ornithological Society
E. D. Ferree
Maternal investment and mate attractiveness
between pre-manipulated tail white and male provisioning
confirms the need for experimental studies to separate out
potentially confounding variables, like body size, which is
associated with the most ornamented males. Future investigations into the role of mate quality and investment in
parental investment should continue to test competing
hypotheses that consider the traits and investment of both
sexes. Finally, in relation to the Dark-eyed Junco species
complex, further experimental work will be needed to
assess the function of tail white, a shared but highly variable trait among the junco subspecies.
ACKNOWLEDGMENTS
I wish to thank L. Chiavola-Larson, T. Gonzalez, A. Johnson,
J. Mehlhaff and E. Sievers for assistance in the field, B. Lyon
for advice throughout the study and J. Dickinson and
G. Pogson for helpful comments. This work was performed at
the University of California Natural Reserve System (UCNRS)
Sagehen Creek Field Station, California, USA.
Funding statement: This work was supported by a Mildred
E. Mathias UCNRS Graduate Student Research Grant, The
American Museum of Natural History Chapman Memorial
Fund, Sigma Xi and the Achievement Reward for Collegiate
Scientists Northern California chapter.
Ethics statement: This research was conducted with approval
from the University of California Chancellor’s Animal
Research Committee and the California Department of Fish
and Game.
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