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Spatial dynamics and the evolution of social monogamy in mammals


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Social monogamy is an uncommon mating system among mammalian species, and several hypotheses have been suggested to explain its evolution. It is generally thought that low local population densities and widely spaced female home ranges, particularly small home ranges, may facilitate social monogamy. We tested these expectations with a complete data set on local density, home range area, and body mass for 64 mammalian species, 22 of which were described as socially monogamous and 42 as not socially monogamous (socially polygynous or polygynandrous). Larger samples were examined separately for local density (84 species) and home range size (129 species). We found that with or without statistical adjustments for body size and phylogeny, socially monogamous and nonmonogamous species appeared similar in local density and home range area. Thus, we found no support for the idea that low population densities and wide dispersion of small home ranges have favored the evolution of social monogamy. Given support for different hypotheses in studies of different species, we suggest multiple causes of social monogamy among mammalian species.
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F. Stephen Dobson and Claude Baudoin
Department of Biological Sciences, Auburn University, 336 Funchess Hall,
Auburn, AL 36849, USA
Laboratoire d'Ethologie Expérimentale et Comparée, Université Paris-Nord,
99 avenue Jean-Baptiste Clément, 93430 Villetaneuse, France
Abstract: Social monogamy is an uncommon mating system among
mammalian species, and several hypotheses have been suggested to explain its
evolution. It is generally thought that low local population densities and wide
dispersion of female home ranges may facilitate social monogamy. We tested
this expectation with data on local density, home range area, and body mass for
65 mammalian species, 24 of which were described as socially monogamous and
41 as polygynous or promiscuous. We found that monogamous species
generally had lower local densities and smaller home ranges, though these
differences were not statistically significant and species broadly overlapped.
With statistical adjustments for body size and phylogeny, the species appeared
even more similar in local density and range area. Local density and home range
area showed a negative association, but this was mainly due to influences of
body mass and phylogeny. Thus, we found no support for the idea that low
population densities and wide dispersion of home ranges have favored the
evolution of monogamy. Given support for different hypotheses in individual
studies of different species, we suggest multiple causes of social monogamy in
Key Words: body mass, female home range, local population density,
monogamy, mammals, phylogeny, polygyny, promiscuity
Social monogamy has come to refer to the tendency of some species to
exhibit heterosexual pairs living in social cooperation, often with some sort of
pair bond (e.g., Mock and Fujioka 1990; van Schaik and Kappeler 2003).
Estimates of the proportion of mammals that exhibit social monogamy are about
5-15% of studied species (Kleiman 1977; Dobson 1982). Genetic monogamy, in
which a male and female have an exclusive mating relationship, appears much
rarer, given the mounting evidence of rates of multiple paternity and cuckoldry in
socially monogamous species (e.g., Keane et al. 1994; Sillero-Zubiri et al. 1996;
Girman et al. 1997; Goossens et al. 1998; Fietz et al. 2000). Yet, even without
the strictures of consistent mating and paternity patterns among couples,
conditions under which social monogamy evolves are poorly understood,
especially for mammalian species (Reichard and Boesch 2003). In part because
it is an unusual social system for mammals, social monogamy has attracted
considerable research attention (e.g., Whittenberger and Tilson 1980; van Schaik
and Dunbar 1990; Mock and Fujioka 1990; Komers and Brotherton 1997; Allainé
2000; Brotherton and Komers 2003; van Schaik and Kappeler 2003).
There are several hypotheses concerning the evolution of social
monogamy in mammals, and these are ably summarized by Brotherton and
Komers (2003). An early and perhaps simplest hypothesis was suggested in a
seminal review by Emlen and Oring (1977) that neither sex has the opportunity
or ability to monopolize multiple members of the opposite sex for mating, either
directly or through resource defense. In monogamous species, the potential for
polygyny should be limited because individuals are widely spaced over a
relatively uniform environment. This hypothesis suggests, then, that local
population density should be lower in socially monogamous species than in
polgynous or promiscuous species. This idea has not been directly tested with
information concerning mating system and population levels.
Other ideas, however, have been tested, and these hypotheses in general
have little support. The idea that biparental care is required for successful
reproduction has received much attention for explaining monogamy in some
species (e.g., Clutton-Brock 1989; Ribble 2003), but this idea was rejected as a
generalization in a phylogenetic analysis (Komers and Brotherton 1997). Two
other appropriate hypotheses involve the way that space is used: that males
maximize reproductive success by guarding a single female (which also might
suggest low local population density), or that joint defense of territory is
essential for successful reproduction. A final idea is that kin selection lowers
competition among offspring that are full siblings, and that this benefits mothers.
Both of the last two ideas have been rejected as impractical explanations for
social monogamy in mammals (Komers and Brotherton 2003). Thus, we are left
with two hypotheses that produce an expectation of lower local population
density in socially monogamous than polygnyous or promiscuous species.
Komers and Brotherton (1997) tested the idea that monogamous species
should be broadly dispersed by examining home range size in monogamous and
polygynous mammalian species, where females were dispersed into non-
overlapping ranges (viz., those that are not group-living). They found that
socially monogamous species had significantly smaller home ranges than socially
polygynous and promiscuous species. This information was difficult to interpret.
It was argued that small ranges of socially monogamous females should be more
clumped than in the polygynous species, thus facilitating association of a male
with more than one female. But the dispersion of the smaller ranges was not
tested, nor was local density examined. Lower population density should favor
monogamy, if it leads to less opportunity to monopolize multiple mates (Emlen
and Oring 1977). On the other hand, according to the logic of Komers and
Brotherton (1997) higher local density could produce greater dispersion of small
female home ranges, thus promoting social monogamy. Nonetheless, Brotherton
and Komers (2003) rejected space use as an explanation of social monogamy,
preferring a hypothesis based on mate guarding by males. Lower local
population densities should, other things being equal, facilitate mate guarding.
The purpose of our study was to examine both local population density
and female home range size in mammalian species. By re-examining home
range size and adding information about local density, we tested the conclusion
that female space use is the best predictor of social monogamy (Komers and
Brotherton 1997). To test this idea, we gathered information about local density
and augmented the data of Komers and Brotherton (1997). Two factors might
bias comparisons of home range size and local density in socially monogamous
versus polygynous or promiscuous species. The first is body size, which varies
widely among mammals (Silva and Downing 1995a), and the second is
phylogeny (Dobson 1985; Felsenstein 1985; reviewed by Garland et al. 2005).
We used regression techniques to examine and statistically remove the
influences of both of these potential biases, thus producing an examination of
the way that home range size and local density change with alternative mating
systems in mammals.
We began with the data set of 184 species that Komers and Brotherton
(1997) examined, and augmented it with additional information from Mammalian
Species Accounts (American Society of Mammalogists). We also added in
information from Silva and Downing (1995b) on local density of mammalian
species. We augmented these data with limited information from the primary
literature. These sources produced 65 mammalian species with complete data
on body mass, local population density, and female home range area. These
species spanned the orders Artiodactyla, Carnivora, Lagomorpha, Primates,
Rodentia, and Soricomorpha.
Body mass, local population density and home range area were tested for
normality using the Shapiro-Wilk statistic. Body mass was taken into account via
individual ANCOVAs of local population density and home range area, with
monogamous and polygynous species the grouping variable and body mass
entered as the covariate. The same comparisons were made using the software
PDAP (Garland et al. 1992, 1993, 1999, 2005), with phylogeny taken into
account statistically via PDTREE and ANCOVA comparisons made with PDSINGLE.
Statistical distributions of expected F statistics, given the phylogeny used, were
estimated via 1000 simulations of random data using PDSIMUL and PDANOVA. F
statistics from ANCOVA were compared to those from the simulations to
determine statistical significance.
The phylogeny of mammalian orders was taken from Reyes et al. (2004)
and Springer et al. (2004). The Artiodactyla tree was taken from Matthee and
Robinson 1999, Matthee and Davis (2001), and Price et al. (2005); Carnivora
from Bininda-Emonds (1999); Lagomorpha from Stoner et al. (2003); Primates
from Purvis (1995) and Masters et al. (2006); Rodentia from Debry (2003),
Jaarola et al. (2004), Jansa and Weksler (2004), and Steppan et al. (2004); and
Soricomorpha from Symonds (2005).
For the 65 species with complete data for all three quantitative variables,
monogamous species (n = 24) did not differ significantly from polygynous
species (n = 41) in body mass, population density, or range area (Table 1). The
rank Wilcoxon test yields the same test statistics for the raw and log-transformed
data. Non-parametric tests were used for these comparisons, because none of
three variables were normally distributed: body mass (Shapiro-Wilk test, W =
0.44, p < 0.0001), local population density (W = 0.29, P < 0.0001), and home
range area (W = 0.36, P < 0.0001). With log-transformation, however, the data
conformed much more closely to a normal distribution: body mass (Shapiro-Wilk
test, W = 0.97, p = 0.21), local population density (W = 0.98, P = 0.35), and
home range area (W = 0.97, P = 0.19). In addition, variances were much closer
to equality (log-mass, F23,40 = 1.47, P = 0.33; log-density, F23,40 = 2.67, P =
0.01; log-range, F23,40 = 1.58, P = 0.25). Since log-mass and log density had
higher means and variances for polygynous species, the differences in variance
for log-density between monogamous and polygynous species were not likely to
confound further analyses. Nonetheless, we conservatively tested under the
assumption of unequal variances. We used log-transformed data in all further
We examined whether body mass might be biasing the above
comparisons with ANCOVAs in which each of the two remaining variables (local
density and range area) were regressed against body mass, with social mating
system as the grouping variable (Figure 1). First, we found significant influences
of body mass on both local population density and home range area (local
density, R2 = 0.476, F1,63 = 57.22, P < 0.0001; range area, R2 = 0.705, F1,63 =
150.32, P < 0.0001). Second, for both local density and range area, slopes were
not significantly different for socially monogamous and polygynous or
promiscuous species (interaction of mating system and body mass: local
density, F1,61 = 2.83, P = 0.10; range area, F1,61 = 0.74, P = 0.39). Adjusted
means of monogamous and polygynous species were also not significantly
different (main effects of mating system: local density, adjusted means = 1.19
and 1.18, F1,62 = 0.00, P = 0.99; range area, adjusted means = 0.50 and 0.79,
respectively, F1,62 = 1.84, P = 0.18).
The above analyses could be biased by the structure of the historical
relationships between the species. Without adjustment for phylogeny, local
density and body mass were significantly associated (r = -0.690, n = 65, P <
0.0001), and range area and body mass were significantly associated (Pearson’s
r = 0.835, n = 65, P < 0.0001). We used phylogenetically independent contrasts
to examine the relationship between body mass and local density, and between
body mass and range area. Even when phylogeny was taken into account via
phylogenetic regression, local density and body mass were still significantly
associated (r = -0.345, n = 64, P < 0.01), as were range area and body mass (r
= 0.598, n = 64, P < 0.0001).
We took body size into account in a phylogenetically adjusted ANCOVA of
the monogamous and polygynous species. Socially monogamous species were
not significantly different in local population density from polygynous species
(compare F1,62 = 0.00 to F1,62 = 1.14 for random data, P = 1.00), nor were they
significantly different in home range size (compare F1,62 = 2.16 to F1,62 = 1.32 for
random data, P = 0.39). In both cases, slopes of local density or range area
regressed on body mass of socially monogamous and polygynous species were
not significantly different (local density, compare F1,61 = 0.59 to F1,61 = 1.18 for
random data, P = 1.00; range area, compare F1,61 = 2.82 to F1,61 = 1.25 for
random data, P = 0.21).
Because Komers and Brotherton (1997) had found that socially
monogamous species had significantly smaller ranges than polygynous or
promiscuous species, we also examined the complete data set for this difference.
In unadjusted data, body mass and range area were not significantly different
for socially monogamous and polygynous species (mass, means = 5.9 and 16.7
kg, n= 50 and 79, respectively, Wilcoxon statistic 3481.5, P = 0.12; range area,
means 1037.4 and 1116.0 ha, n = 50 and 79, respectively, Wilcoxon statistic
3272.5, P = 0.92). After log-transformation and taking body mass into account,
the overall regression was still highly significant (R2 = 0.623, F2,123 = 101.80, P <
0.0001), but there was no significant difference between the adjusted means for
monogamous and polygynous species (adjusted means = 0.90 and 1.10,
respectively, F1,123 = 1.38, P = 0.24). Slopes of socially monogamous and
polygynous species were not significantly different (F1,122 = 0.34, P = 0.56).
Finally, we examined the relationship between local population density
and home range area in the mammalian species for which we had complete
data. These variables were strongly and negatively correlated in unadjusted but
log-transformed data (r = -0.719, n = 65, P < 0.0001), and for socially
monogamous (r = -0.648, n = 24, P < 0.001) and polygynous or promiscuous
species (r = -0.745, n = 41, P < 0.0001). We regressed local density and range
area separately on body mass, and used the regression residuals from each
analysis as a “body mass adjusted” index of local density and range area. These
indices were also significantly correlated, but to a lower degree (r = -0.356, n =
65, P = 0.004). We examined these indices using phylogenetic independent
contrasts, and the correlation dropped even further and became non-significant
(r = -0.213, n = 64, P = 0.09).
There appeared to be substantial differences between local population
density and home range area of socially monogamous and polygynous
mammalian species, but these were not significant (Table 1). Both variables
were non-normally distributed and substantially influenced by body mass.
Accounting for both of these problems did not change the result of little
difference in range area and local density for monogamous and polygynous
species, but rather made it clear how little difference there really was (Figure 1).
If anything, accounting for phylogeny brought socially monogamous and
polygynous species even closer together in their range area and local density.
Under no conditions were socially monogamous and polygynous species
significantly different in local density or range area. We conclude from these
results that there are probably not great differences in use of space by socially
monogamous and polygynous species of mammals, at least given present data.
Rejection of the idea that monogamous species occur sparsely on the
ground casts doubt on the idea of Emlen and Oring (1977) that environmental
potential for polygyny is lower in monogamous species. In fact, several socially
monogamous species are genetically promiscuous (e.g., review by Allainé 2000).
Thus, there is little evidence from either local population density or home range
size that males of socially monogamous species cannot contact more than one
female during the breeding season. Other explanations must thus be sought to
explain social monogamy, at least for some species. A key might come from the
fact that although such species initially appeared to be genetically monogamous
from behavioral observations outside of the day of estrus of females, various
levels of promiscuous genetic contribution occur. Thus, mating opportunities
must be more abundant than the apparent social monogamy implies.
Remember that for the above comparisons, only species where females
are dispersed in home range (i.e., largely non-overlapping females) were
considered. There are a few species, however, in which females overlap spatially
and live under social monogamy: e.g., Alpine marmots (
Marmota marmota
Allainé 2000), acuchis (
Myoprocta exilis
, Dubost 1988), dwarf mongooses
Helogale parvula
, Creel and Waser 1991), wolves (
Canis lupus
, Sillero-Zubiri et
al. 1996; Jedrzejewski et al. 2007), and African wild dogs (
Lycaon pictus
, Frame
et al. 1979). In these species, it appears that there is reproductive suppression
if females are forced to produce young together, something that may also occur
in polygynous species, though perhaps to a lesser degree (e.g., Armitage 1998).
Nonetheless, when females commonly live in overlap, the common mating
system is polygyny or promiscuity (Komers and Brotherton 1997). Reproductive
suppression may reflect competition among females that must occur for group-
living species to exhibit social monogamy, otherwise they would be polygynous.
Our results supported Brotherton and Komers (2003) rejection of spatial
dispersion as an explanation of monogamy. Contrary to the suggestion of
Komers and Brotherton (1997), there was little evidence to suggest that female
space use is the best predictor of monogamy. If lower home range size of
socially monogamous species were associated with greater population density,
then greater dispersion of females might occur, thus supporting the spatial
dispersion prediction. While there was generally a tradeoff of home range size
and local population density among both monogamous and polygynous species,
this turned out to be due primarily to differences in body size among the species
and to phylogeny.
The explanation that Brotherton and Komers (2003) preferred was that of
mate guarding as the primary factor leading to mammalian monogamy, and our
finding that spatial dynamics exhibit great variation but no consistent pattern
with respect to local density and range area do not support their hypothesis.
Unfortunately, mate guarding occurs in polygynous species as well as
monogamous species (e.g., Kummer 1968). One might think that species with
mate guarding that occur at lower densities might by default end up as socially
monogamous. Though this might occur in some cases, there is no general
evidence for the requirement of relatively lower density or greater dispersion of
home range that would facilitate mate guarding of single females in those
species that are socially monogamous.
Mating system concepts have been fitted into an old concept of
monogamy and polygyny (e.g., Emlen and Oring 1977; Wittenberger and Tilson
1980). In some ways this is unfortunate, because it may bias our view of the
essential elements of mating interactions, and the behaviors and environments
that surround them. It might be more accurate and informative to consider the
roles of the sexes separately. Males may be examined along a scale of pair
bonding, from strongly bonded males to un-bonded males. Strongly bonded
males might have single mates or multiple mates, as occur in different human
cultures. Un-bonded males are those without attachments outside of mating
interactions with females, in species such as tree squirrels or North American
pikas (e.g., Koprowski 1988; Smith and Ivins 1986). Intermediate species would
have shorter and weaker bonds, such as males that attend females just before
their estrus periods, but not at other times (e.g.,
Spermophilus columbianus
Manno et al. 2007; F.S. Dobson and A. Nesterova, unpublished observations).
Species along this axis might be constrained by their genetic and neurobiological
inheritance, or might show phenotypic flexibility according to environmental
Females could also be placed along a scale, from those that usually mate
singly to multiple mating females. Females that mate singly could be associated
with paternal investment in young or with male defense (e.g., Richardson 1987;
Brotherton and Komers 2003; Ribble 2003). Such females produce either
monogamy, depending on whether males are mating singly or multiply as well,
or polygyny. Females that mate multiply might be attracting benefits such as
paternal care from more than one male, or they could be improving the genetic
composition of their cohort of young, depending on the number of young that
the species usually produces. Naturally, the mating patterns of both sexes would
be constrained by the social or ecological environment, such as whether males
use resources to attract females. Thus, our suggestions are not in conflict with
ideas about mate or resource defense. Rather, the division of the sexes allows a
context for understanding influences on mating systems that produce an
incredible diversity of mating and fertilization patterns, and the patterns of
parental care that attend them.
In conclusion, none of the currently described hypotheses presents a
satisfying general explanation of social monogamy in mammalian species. This
may indicate that a new approach is needed. Although five reasonable
hypotheses have been suggested, additional hypotheses may be discovered or
imagined. Alternatively, social monogamy may represent somewhat different
phenomena which have evolved independently in different groups of mammals,
and for which a unique or nearly unique set of circumstances occurs. In
addition, multiple causes may occur, and this hypothesis has been supported for
other patterns of behavior such as dispersal from the natal area (e.g., Dobson
and Jones 1985). In either event, generality is not to be expected, and one or
more of the existing hypotheses may be supported for specific cases. For
example, paternal care in the nest appears to be fairly important to monogamy
of California mice (Ribble 2003, 2007). However, in elephant shrews and Kirk’s
dik-dik, where paternal care is absent, mate guarding may prove to be the
strongest factor favoring monogamy (Brotherton and Komers 2003; Rathbun and
Rathbun 2006). Furthermore, male defense against infanticide could lead to
social monogamy in some primates (van Schaik and Dunbar 1990). The best
generality may be that many factors are involved in the evolution of social
monogamy in mammals.
We thank N. Rajamani and H. Trevino for assistance with searching the
literature and with data entry. This study was funded by invited professorships
from Université Paris-Nord in 2004 and 2007.
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Table 1. Body mass, home range area, and local population density for
mammalian species that are socially monogamous or are polygynous or
promiscuous. Sample sizes are 24 socially monogamous and 41 polygynous or
promiscuous species.
± 1SD
± 1SD
Wilcoxon test
Figure 1. a) Local population density regressed against body mass. b) Home
ranges area regressed against body mass. Circles are socially monogamous
species and diamonds are polygynous or promiscuous species. Linear regression
lines are fit: the shorter lines are the monogamous species. All data were log-
transformed before analyses. Statistical comparisons and adjusted means
appear in the text. For both comparisons, 24 socially monogamous and 41
polygynous or promiscuous species of mammals are represented.
-3 -2 -1 0 1 2 3
Log Mass
Log Density
-3 -2 -1 0 1 2 3
Log Mass
Log Density
-3 -2 -1 0 1 2 3
Log Mass
Log Range
-3 -2 -1 0 1 2 3
Log Mass
Log Range
... The resulting size of female home ranges then, in turn, precludes males from monopolizing access to more than one female (Brotherton & Komers, 2003;Dobson et al., 2010). This hypothesis is often considered in combination with the "male mate-guarding" hypothesis, which postulates that a male's best reproductive strategy is then to associate with only one female, defend her, and ensure he sires all of her offspring (Brotherton & Komers, 2003;Komers & Brotherton, 1997;Lukas & Clutton-Brock, 2013). ...
... Key to evaluating whether and how some of these drivers contribute to the expression of pair-living and sexual monogamy in a taxon is characterizing that taxon's ranging behavior and space use (Dobson et al., 2010;Fernandez-Duque, 2016;Munshi-South et al., 2007;Sabol et al., 2018;Schubert et al., 2009;Tecot et al., 2016). ...
... These three hypotheses all predict territoriality, with low overlap among the territories of neighbors (Brotherton & Komers, 2003;Dobson et al., 2010). Territoriality-defined as the consistent defense of an entire home range, or a large portion of it, against the intrusion of conspecifics-implies not only spatial exclusivity, but also the expression of defensive and/or monitoring behavior that may deter incursions by opponents (Burt, 1943;Hinsch & Komdeur, 2017;Maher & Lott, 1995). ...
Patterns of ranging behavior and space use are key for evaluating current ideas about the evolution and maintenance of pair-living and sexual monogamy as they provide insights into the dispersion of females, the potential for territoriality, and whether males are limited to defending an area that can support only one female and her offspring. We examined ranging behavior and space use to evaluate the potential for territoriality in five groups of red titi monkeys (Plecturocebus discolor) during a 10-year study in Ecuadorian Amazonia. Mean home range size, calculated using a time-sensitive local convex hull estimation procedure, was 4.0 ± 1.4 ha. Annual home ranges of neighboring groups overlapped, on average, 0%-7%. Mean daily path length was 670 ± 194 m, resulting in defendability indices of 2.2-3.6 across groups. Groups visited, on average, 4 of 12 sections of their home range border area per day, but that was not more often than would be expected by chance, and intergroup encounters were infrequent. We did not find evidence of active monitoring for intruders in border areas, in that groups did not travel either faster or slower when at the border than when in central areas of their range. The absence of overt monitoring might be compensated for by engaging in loud calls, which the study groups did throughout their home ranges; these calls may serve as an advertisement of occupancy and a deterrent to intruding conspecifics. Our finding that red titis have a high potential for territoriality is consistent with several of the main hypotheses proposed to explain pair-living in mammals.
... However, genetic fingerprinting revealed that extra-pair paternity is common in many pair-living species (Cohas & Allainé, 2009), leading to the realization that seemingly monogamous relationships do not necessary predict genetic outcomes (i.e., genetic monogamy). As a result, some researchers introduced the term "social monogamy" (Dobson et al., 2010;Gowaty & Buschhaust, 1998) to distinguish social behavior within pairs from genetic monogamy. Recent reviews from multiple research groups advocate abandoning the term "social monogamy" and using the term "monogamy" only in the context of mating systems Garber et al., 2016;Huck et al., 2020;Kappeler & Pozzi, 2019;Kvarnemo, 2018;Tecot et al., 2016). ...
... We therefore expected that variability in social organization will increase with variability in habitats . Population density is the factor most emphasized to have influenced the evolution of pairliving in elephant-shrews (Rathbun & Rathbun, 2006) and mammals more broadly (Lukas & Clutton-Brock, 2013 but see Dobson et al., 2010), such that we predicted pair-living to be associated with low population density, making it difficult for a male to associate with more than one female. ...
Elephant‐shrews (Macroscelidea) have long been considered the only mammalian order to be completely monogamous, based on observations of their pair‐living social organization. We reviewed primary studies on the four components of social systems (social organization, mating system, social structure, and care system) in elephant‐shrews to evaluate whether they truly are monogamous. To identify gaps in our knowledge of their social system, we reviewed evidence for a pair‐living social organization, mate fidelity (mating system), pair bonds (social structure), and biparental care (care system). Field data were available for eight species and seven were often pair‐living. However, these seven species exhibited intra‐specific variation in social organization; two of these species were also solitary living, two species were also group‐living, and the remaining three species were both solitary and group‐living. The eighth species was exclusively solitary. We reconstructed the ancestral social organization of Macroscelidea using Bayesian phylogenetic mixed‐effects models and found that variable social organization, rather than exclusive pair‐living, was the most likely ancestral state, though there was high uncertainty. No socio‐ecological factors (body size, population density, and habitat) predicted a specific social organization. Observations of mating have been rare, such that no firm statements can be made. However, one unpublished study indicated high levels of extra‐pair paternity. Regarding social structure, there was no evidence of pair‐bonding, but there was evidence of mate guarding. Only maternal care has been observed, with females having very short nursing bouts. Evidence suggests that despite having often a pair‐living form of social organization, Macroscelidea should not be described as a monogamous order, as little or no evidence supports that designation, nor are they exclusively pair‐living (social organization) and we urge further field studies on Macroscelidea social systems. Elephant shrews have often been claimed to be the only monogamous mammalian order. Here we show that this statement is wrong. Our literature review shows that no data are available to determine their mating system and that their social organization is variable, often including pair‐living.
... Social monogamy is an uncommon mating system in mammals (about 9% of species; Lukas & Clutton-Brock, 2013), yet a high proportion of plateau pika families were socially monogamous (26% in 2 years; see also Table 5). Multiple hypotheses have been proposed to explain the evolution of social monogamy (Clutton-Brock, 2016;Dobson et al., 2010;Lukas & Clutton-Brock, 2013, 2020. Lukas and Clutton-Brock (2013) proposed that social monogamy should be associated with low female density and occur in situations where breeding females are intolerant of each other. ...
... Additionally, the proximity of adult females in adjoining families and the high rates of affiliative behaviours among females run counter to the idea that females should be intolerant of each other. As suggested by Dobson et al. (2010), multiple factors may lead to the expression of social monogamy in mammals. ...
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We investigated factors leading to variation in social complexity or ‘social systems’ among plateau pika family groups within a contiguous local population across 2 years. Plateau pikas are small, diurnal, nonhibernating, sexually monomorphic lagomorphs that occupy family home ranges on open alpine meadows on the Qinghai-Tibetan Plateau. Expression of the social organization, social structure, mating system and parental care system in plateau pikas did not follow expectations from traditional ecological or evolutionary explanations. Variability in plateau pika family group size and the transitions of group size between years allowed us to investigate potential advantages and disadvantages of group living. Evidence that group living served to protect pikas against predation was weak. Although social huddling could have minimized thermoregulatory costs during the extremely cold Tibetan winters, there was no correlation of overwinter survivorship among pika families of different sizes. There was no apparent group-living benefit with regard to foraging, and the occurrence of cohesive social families on the flat, continuous meadow contradicts the hypothesis that sociality is related to patchiness of critical resources. Cost of maintaining burrows appeared unrelated to group size. Most interactions between pikas occurred within family groups and were affiliative (99% of adult interactions; 97% of adult–juvenile interactions), and most interactions between adult males of different family groups were aggressive (96% of interactions). Matings were primarily within families (88% of copulations). Pikas also possess a complex vocal repertoire that enhanced interactions within social families. Demographic constraints associated with variable overwinter survivorship appeared to be the dominant precondition that produced a given family size and mating system type, coupled with selective dispersal by some pikas before the start of the breeding season. Paternal care enhanced juvenile survival, and thus led to an equalization of reproductive success among adults in families with different mating combinations.
... Bércouni jsou obecně považováni za monogamní, což je u savců poměrně vzácný jev (Dobson et al. 2010). Trvalé soužití samce a samice se však u bércounů projevuje především využíváním společného teritoria a samec se samicí příliš mnoho společného času netráví. ...
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Bércouni (řád Macroscelidea) představují nepočetný řád savců, který vyniká mnoha jedinečnými vlastnostmi. Jejich název poukazuje na prodloužené zadní končetiny, které jim při běhu umožňují dosáhnout skutečně nevídané rychlosti. Celá řada zajímavostí se pak pojí srozmnožováním a vývojem mláďat, která se rodí vysoce prekociální. O pokročilé vyvinutosti novorozených bércounů jsme se nedávno přesvědčili při prvním odchovu bércouna afrického (Macroscelides proboscideus) v Zoo Praha. Cílem tohoto příspěvku je podělit se o zkušenosti získané tímto odchovem a zasadit je do širšího kontextu dosavadních poznatků o rozmnožování a postnatálním vývoji bércounů.
... The critical contrast lies in whether females live socially in groups or forage alone (with or without dependent young) (Emlen & Oring 1977; see also Rutberg 1983) and whether, when females forage alone, males are forced to choose between attaching themselves to one female in a monogamous relationship or opting for a more promiscuous roving-male strategy Historically, three principal hypotheses have been proposed as explanations for monogamy in mammals, namely (1) biparental care, (2) female overdispersion and (3) protection against external threats (such as predation or infanticide). This issue continues to attract debate with several recent analyses (Dobson et al. 2010;Shultz et al. 2011;Opie et al. 2013;Lukas & Clutton-Brock 2013;Kappeler & Pozzi 2018) disagreeing with each other. ...
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Explanations for the evolution of monogamy in mammals typically emphasise one of two possibilities: monogamy evolves when females are overdispersed (such that males cannot defend more than one female at a time) or when males provide a service to the female. However, the first claim has never been directly tested. I test it directly at three levels using data from primates and ungulates. First, I show that the females of monogamous genera do not have territories that are significantly larger, either absolutely or relatively, than those of polygamous genera. Second, using both the Mitani-Rodman and Lowen-Dunbar inequalities, I show that, given their typical day journey lengths, males of most monogamous species could easily defend an area large enough to allow them to monopolise as many as 5-10 females if these ranged solitarily. Finally, I use a model of male mate searching strategies to show that, unlike the males of socially-living polygamous species, the opportunity cost that monogamous males incur is typically more than five times the reproductive success they have by being obligately monogamous. This suggests that the selection pressure dissuading them from pursuing a roving male strategy must be very considerable.
... In the 1980-2000s, the costs and benefits of monogamy to males and females received a lot of attention (Wittenberger and Tilson, 1980;van Schaik and Dunbar, 1990;Hames, 1996;Reavis and Barlow, 1998), and numerous hypotheses were proposed and tested in different species. Since then, great advances in phylogenetic reconstruction have paved the way for a better understanding of the evolution of social monogamy, including the causal factors that led to social monogamy and the factors that were consequences of this social/mating system (Dobson et al., 2010;Lukas and Clutton-Brock, 2013). Previous phylogenetic studies on factors influencing genetic monogamy have not reached consistent conclusions for a number of reasons, including differences and limitations in methodology and the species included in analyses (Clutton-Brock and Isvaran, 2006;Cohas and Allaine, 2009;Huck et al., 2014;Dobson et al., 2018). ...
... We only recorded the association between one measure of sociality and mating and reproductive success in one year and the effects of the number of social connections on fitness could be altered when environmental conditions change. For example, the female dispersion hypothesis would predict that if our measure of sociality reflects the socially monogamous behaviour of male prairie voles, it should be positively correlated with male mating success when females are spatially clumped as males that have more social connections with clumped females should have higher mating success (Shuster & Wade, 2003;Dobson, Way, & Baudoin, 2010;Lukas & Clutton-Brock, 2013). In natural populations of prairie voles, density varies between years (Getz et al., , 2001 and some previous observational studies of prairie voles in field settings suggested that socially monogamous behaviour is more common at low densities (McGuire, Pizzuto, & Getz, 1990;Solomon et al., 2009; but see . ...
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Comparative studies aid in our understanding of specific conditions favouring the initial evolution of different types of social behaviours, yet there is much unexplained intraspecific variation in the expression of social behaviour that comparative studies have not yet addressed. The proximate causes of this individual variation in social behaviour within a species have been examined in some species but its fitness consequences have been less frequently investigated. In this study, we quantified the fitness consequences of variation in the sociality of prairie voles, Microtus ochrogaster. We characterized sociality of voles in seminatural enclosures using an automated behavioural tracking system paired with social network analyses to quantify the degree of spatial and temporal co-occurrence of different voles. We then assessed the relationship between sociality and both mating success (number of different conspecifics with which an individual produced offspring) and reproductive success (total number of offspring surviving to first capture). We measured the number of social connections each individual had with all voles and with only opposite-sex voles (unweighted degree) through social network analyses. Both female and male voles varied in the number of social connections they had with all conspecifics and with opposite-sex conspecifics. In both analyses, females and males with an intermediate number of social connections had higher mating success overall and, for the analysis with all connections, produced more offspring. Males with many or few social connections also had the lowest average body mass. Overall, our results suggest some limit on the fitness benefits of sociality. Although there was substantial individual variation in our measure of vole social behaviour, intermediate levels of social connections may be most favourable.
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As an essential biomedical model organism, house mice have been studied intensely under laboratory conditions, yet they evolved to survive and reproduce in complex and dynamic environments. There has been recent interest in the study of ‘rewilded’ mice reared in complex outdoor environments, particularly for understanding the brain and behavior. Yet little work has examined lab mouse behavior under free-living conditions. Here, we characterize the emergent spatial and social structure of replicated populations of C57BL/6J (C57) mice over 10 days in large outdoor field enclosures and compare them to populations of recently wild-derived outbred house mice under the same conditions. We observed shared aspects of space use and social structure across all trials but found that C57 societies differed from those emerging from outbred mice across multiple dimensions. Males of both genotypes rapidly established and then defended territories. Female C57 mice spent more time with other individuals and explored more space relative to all other groups. These behavioral differences resulted in C57 mice rapidly forming less stable, but more densely connected, social networks than outbred wild-derived mice. These data suggest that laboratory domestication has had larger effects on female mouse social organization than their male counterparts. Importantly, this work demonstrates that C57 mice recapitulate many, but not all, aspects of social structures generated by wild mice in outdoor conditions. Rewilding allows for tractable, replicable, and ecologically realistic approaches to studying mouse behavior and can facilitate the study of the biological basis of higher order social organization. HIGHLIGHTS We describe emergent spatial and social structures of rewilded C57BL/6J (C57) lab mice across replicated trials in outdoor field enclosures and compare them to wild-derived outbred mice Both C57 and outbred males rapidly establish and maintain territories C57 females explore the field enclosures substantially more than any other group With the exception of C57 females, most mice spent the majority of their recorded time alone The resulting societies formed by C57 mice are less modular, more densely connected, and less stable than those formed by wild-derived outbred mice
Mammal societies Although a few types of structures prevail, social systems among mammals are relatively varied. New techniques, from monitoring to genetics, have allowed for a deeper understanding of this variation, how it is related to the environment, and how it has evolved. Clutton-Brock reviews the forms of and drivers of the different types of breeding systems and how they have been shaped by ecology and history. The author discusses how mammalian social interactions may be affected by human activities that are driving environmental change. —SNV
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Parental allocation of resources into male or female offspring and differences in the balance of offspring sexes in natural populations are central research topics in evolutionary ecology. Fisher (1930) identified frequency‐dependent selection as the mechanism responsible for an equal investment in the sexes of offspring at the end of parental care. Three main theories are proposed for explaining departures from Fisherian sex ratios in light of variation in environmental (social) and individual (maternal condition) characteristics. The Trivers‐Willard (1973) model of male‐biased sex allocation by mothers in the best body condition is based on the competitive ability of male offspring for future access to mates and thus superior reproduction. The local resource competition model is based on competitive interactions in matrilines, as occur in many mammal species, where producing sons reduces future intrasexual competition with daughters. A final model invokes advantages of maintaining matrilines for philopatric females, despite any increased competition among females. We used 29 years of pedigree and demographic data to evaluate these hypotheses in the Colombian ground squirrel (Urocitellus columbianus), a semi‐social species characterized by strong female philopatry. Overall, male offspring were heavier than female offspring at birth and at weaning, suggesting a higher production cost. With more local kin present, mothers in the best condition biased their offspring sex ratio in favor of males, and mothers in poor condition biased offspring sex ratio in favor of females. Without co‐breeding close kin, the pattern was reversed, with mothers in the best condition producing more daughters, and mothers in poor condition producing more sons. Our results do not provide strong support for any of the single‐factor models of allocation to the sexes of offspring, but rather suggest combined influences of relative maternal condition and matriline dominance on offspring sex ratio.
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The aim of this review is to consider the potential benefits that females may gain from mating more than once in a single reproductive cycle. The relationship between non-genetic and genetic benefits is briefly explored. We suggest that multiple mating for purely non-genetic benefits is unlikely as it invariably leads to the possibility of genetic benefits as well. We begin by briefly reviewing the main models for genetic benefits to mate choice, and the supporting evidence that choice can increase offspring performance and the sexual attractiveness of sons. We then explain how multiple matin!: can elevate offspring fitness by increasing the number of potential sires that compete, when this occurs in conjunction with mechanisms of paternity biasing that function in copula or post-copulation. We begin by identifying cases where females use precopulatory cues to identify mates prior to remating. In the simplest case, females remate because they identify a superior mate and 'trade up' genetically. The main evidence for this process comes from extra-pair copulation in birds. Second, we note other cases where pre-copulatory cues may be less reliable and females mate with several males to promote post-copulatory mechanisms that bias paternity. Although a distinction is drawn between sperm competition and cryptic female choice, we point out that the genetic benefits to polyandry in terms of producing more viable or sexually attractive offspring do not depend on the exact mechanism that leads to biased paternity. Post-copulatory mechanisms of paternity biasing may: (1) reduce genetic incompatibility between male and female genetic contributions to offspring; (2) increase offspring viability if there is a positive correlation between traits favoured post-copulation and those that improve performance under natural selection; (3) increase the ability of sons to gain paternity when they mate with polyandrous females. A third possibility is that genetic diversity among offspring is directly favoured. This can be due to bet-hedging (due to mate assessment errors or temporal fluctuations in the environment), beneficial interactions between less related siblings of the opportunity to preferentially fertilise eggs with sperm of a specific genotype drawn from a range of stored sperm depending on prevailing environmental conditions. We use case studies from the social insects to provide some concrete examples of the role of genetic diversity among progeny in elevating fitness. We conclude that post-copulatory mechanisms provide a more reliable way of selecting a genetically compatible mate than pre-copulatory mate choice. Some of the best evidence for cryptic female choice by sperm selection is due to selection of more compatible sperm. Two future areas of research seem likely to be profitable. First, more experimental evidence is needed demonstrating that multiple mating increases offspring fitness via genetic gains. Second, the role of multiple mating in promoting assortative fertilization and increasing reproductive isolation between populations may help us to understand sympatric speciation.
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In many communally breeding species, only the dominant female normally breeds, while subordinates tolerate reproductive suppression (these are “despotic” species, in the terminology of Brown, 1987; Macdonald and Moehlman, 1983; Vehrencamp, 1983). Yet in many species for which reproductive suppression is the norm (across a wide variety of taxa), subordinates do occasionally breed. Because reproduction by subordinates is atypical for these species, it is often regarded as simple failure of the normal mechanisms of suppression. An alternate hypothesis is that subordinate pregnancies represent an evolutionary compromise between dominant and subordinate, in which dominants concede their monopoly on reproduction in order to retain helpers. We use data from a long-term study of dwarf mongooses (Helogale parvula) in Serengeti National Park, Tanzania, to test this hypothesis, using an inclusive fitness model adapted from one by Vehrencamp (1983). We find that the incidence of subordinate pregnancy closely matches that predicted by the model, suggesting that the mechanisms that underlie reproductive suppression in dwarf mongooses are finely adjusted to the social and demographic environment. [Behav Ecol 1991; 2: 7–15]
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Predictions made by previous allometric analyses of the relationship between population density and body mass were tested using data on ecological density of 987 terrestrial mammal populations. The relationship is not log-log linear as previously postulated. Only populations of mammals with body mass between 0.1 and 100 kg had allometric exponents approaching the value of -0.75 proposed by previous studies. Different trophic groups showed divergent relationships between density and body mass. Previous global analyses have disagreed with relationships between density and body mass in individual communities partly because of this nonlinearity. Analyses of 45 mammalian communities show positive, negative, or even no relationship between density and body mass, depending on the trophic groups and body sizes of community members and the range of sizes. Population energy use is inequitably partitioned among populations, with populations of large mammals using more than 100 times more energy than the smallest mammals. Herbivorous mammals can use 25 times the energy used by carnivores, and populations of small insectivores use only 10% of the energy used by other carnivores of equivalent body mass.
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The behavioral tactics and mating behavior of male fox squirrels (Sciurus niger) were studied in a small population of uniquely marked, free-ranging individuals during the winter breeding season in 1986 to 1990. Twelve mating bouts were observed with an average of 5.83 males. Two alternative male reproductive tactics were active pursuit and satellite. Active-pursuit males were the most dominant squirrels that fought for and defended proximity to the estrous female. Satellite males were subordinates that remained dispersed in the estrous female's home range but avoided interaction with active-pursuit males. Active pursuit accounted for more copulations than the satellite tactic (0.83 vs 0.23 copulations/male/bout) with the copulations distributed more evenly among active-pursuit males (CV = 133.7) than among satellite males (CV = 191.3). Satellite males copulated with a female after she avoided the contest competition among active-pursuit males. Although the tactics were dominance-based, dominance rank was not directly correlated with mating success. However, highranking, dominant males gained access to and mounted more females than lower ranking males. The alternative mating tactics of male fox squirrels may be important in mediating intermale mating success.
The genus Peromyscus (deer mice) is an attractive group in which to study the evolution of social and mating behaviours. This genus includes over 50 species (Carleton, 1989) that are widely distributed across North and Central America from coast to coast and from the northern subarctic to Panama (Kirkland&Layne, 1989). The diversity in body sizes among Peromyscus ranges from 13 to 77 g (Millar, 1989) and exceeds that of most other genera. Phylogenetic relationships among species of Peromyscus are relatively well understood (Stangl&Baker, 1984), although the systematics of Peromyscus is an active area of study (e.g., Rogers&Engstrom, 1992; Bradley et al., 2000). Most relevant to this chapter, populations and species of Peromyscus exhibit a variety of social behaviours and mating systems (Wolff, 1989), with social monogamy, and particularly reproductive monogamy, being relatively rare. Since monogamy is rare among Peromyscus, those Peromyscus species that exhibit monogamous behaviours may reveal important factors in the evolution of the genus. One of the best studied monogamous species within the genus is P. californicus (California mouse). Association patterns, biparental care, and mating exclusivity indicate that this species is socially and reproductively monogamous, and I will begin by reviewing these elements. Furthermore, recent field experiments demonstrate that male care is critical for offspring survival and is the salient feature of monogamy in this species. I will then review the ecology of female and male home range use and spatial organization and paternal care in other Peromyscus species.
This review considers the behavioral, ecological, and reproductive characteristics of mammals exhibiting monogamy, i.e., mating exclusivity. From a discussion of the life histories of selected species of monogamous primates, carnivores, rodents and ungulates, several trends emerge. Two forms of monogamy occur, Type I, facultative, and Type II, obligate. The selective pressures leading to these two forms of monogamy may have been different. Facultative monogamy may result when a species exists at very low densities, with males and females being so spaced that only a single member of the opposite sex is available for mating. Obligate monogamy appears to occur when a solitary female cannot rear a litter without aid from conspecifics, but the carrying capacity of the habitat is insufficient to allow more than one female to breed simultaneously within the same home range. Within both types of monogamy, the following traits are typically seen: (1) adults show little sexual dimorphism either physically or behaviorally: (2) the adult male and female exhibit infrequent socio-sexual interactions except during the early stages of pair bond formation. Additional trends specific to mammals exhibiting obligate monogamy are: (1) the young exhibit delayed sexual maturation in the presence of the parents, and thus only the adult pair breeds; (2) the older juveniles aid in rearing young siblings; and (3) the adult male (father) aids in the rearing of young by any or all of the following: carrying, feeding, defending, and socializing offspring.
Dwarf mongooses, Helogale parvula, are group-living carnivores in which the oldest male and female dominate reproduction, while subordinates tolerate reproductive suppression and provide care for the offspring of the oldest pair. Subordinates of both sexes, however, mate and subordinate females occasionally become pregnant. In addition, a model of 'power-sharing' in social groups (Vehrencamp 1983, Anim. Behav., 31, 667-682) predicts that dominant individuals should cede some fitness to subordinates, particularly older, higher-ranking individuals not closely related to the dominants, in order to retain them within the group. Human multilocus probes (Jeffreys' 33.15 and 33.6) were used to 'fingerprint' dwarf mongooses from nine different packs in the Serengeti National Park, Tanzania. The results demonstrate that subordinates of both sexes obtain direct fitness: 24% of young had subordinate fathers, while 15% had subordinate mothers. Multiple paternity can occur within a single female's litter. Those subordinates that reproduced were of high social rank and tended to be distantly related to the same-sex dominant, as predicted by Vehrencamp's model. The number of yearlings produced by subordinate females was similar to that predicted by the model. Subordinate males father a relatively high proportion of offspring given their frequency of mating, while subordinate females mate relatively more often than they produce offspring, consistent with previously inferred differences between the mechanisms of male and female reproductive suppression (Creel et al. 1992, Anim. Behav., 43, 231-245). Even though DNA analyses indicate that subordinates produced a substantial proportion of young born in this population, the magnitude of direct fitness obtained by subordinates was still low compared with the magnitude of indirect fitness obtained by helping.