After mating, the rodents spend as much time maintaining their relationships as they do their dams and lodges. The males and females co-parent their young and stay together until one partner dies. Attached beavers occasionally philander, but it's not enough to break up the family.
Unlike most reptiles, the shingleback skink of Australia only has eyes for one mate. Males make a series of moves—including caressing and licking females—before copulating. Courtship takes months, but partnered bliss can last over 20 years.
Some 90 percent of birds are socially monogamous, but that doesn't mean they're completely faithful to one mate. Barn owls, however, put all their eggs in one basket. Males woo females with screeches and gifts of dead mice. If the female responds with croaking sounds, she's basically saying, "I do. Long-distance relationships aren't easy, but bald eagles thrive in them. The birds fly solo during winter and migration, reconnecting with their mates each breeding season. We included them in our data set, bringing the total number of socially monogamous species to So, for each species we performed 2 or 4 different database searches.
We only included data from wild populations. For each species with genetic parentage data, we calculated up to three different indices of genetic monogamy because genetic parentage data is not always presented in the same way in the literature.
These three indices were: the proportion of a female's young sired by her social partner referred to as intra-pair young, hereafter IPY ; the proportion of all litters that were sired exclusively by a social pair referred to as intra-pair litters, hereafter IPL , i.
Although we used three different indices, it is important to note that these are not three different outcomes but rather different attempts to quantify genetic monogamy. Furthermore, although many papers also included information which allowed us to determine which males were paired with which females, this was not always the case e. For papers without this information, we calculated lower-end and upper-end estimates of our three indices of genetic monogamy based on the assumption that at least one of the sires of a litter was the male social partner when multiple-paternity was detected although we understand it is possible that none of the sires were the male social partner.
We then calculated one mean for each species from these lower and upper-end estimates and used these means in our models. For our list of socially monogamous mammalian species with genetic parentage data, we performed another set of literature searches to find information on variables that would allow us to examine functional hypotheses for genetic monogamy. For each variable of interest, we first searched the paper in which we had found the genetic parentage data for that species and the Mammalian Species account for that species when available.
If we could not locate the information in these sources, we searched the ISI Web of Science database following procedures similar to those used in our initial literature search. We found enough data to test 7 hypotheses: 4 life history hypotheses, 2 demographic hypotheses, and 1 environmental hypothesis Table 1 ; Supplementary Data Sheet 1. In our analyses, we controlled for the phylogenetic relatedness across all of our species to account for the lack of independence among closely-related species Garland et al.
This type of phylogenetic regression is similar to analyses in previous comparative studies on genetic monogamy Isvaran and Clutton-Brock, ; Huck et al. To accomplish this, we used a subset of a Mammalian supertree downloaded from TimeTree on Dec 14, , a publicly available phylogenetic tree synthesized from published phylogenies Hedges et al.
We analyzed our dataset using AICc model selection of phylogenetic least squares regression models that included different combinations of our predictor variables Martins and Hansen, ; Freckleton et al. We created 12 a priori models that consisted of different combinations of the life history, demographic, and environmental hypotheses for which we had data from a sufficient number of species Table 1 , including models based on the results of previous comparative studies on mammalian extra-group paternity see Supplementary Table 1 ; Clutton-Brock and Isvaran, ; Isvaran and Clutton-Brock, ; Cohas and Allaine, ; Lukas and Clutton-Brock, All of these variables were proportional data, therefore they were arcsine square-root transformed for the analyses.
All presented coefficients are back-transformed. These models included a phylogenetic correction following the Brownian model of character evolution Garland et al. The weights are the probability that the model is the best model, given the data and other models in the candidate set Wagenmakers and Farrell, Four other orders were represented by 1—3 species. Figure 1. Flow chart for the literature search for studies of socially monogamous mammals with genetic parentage data.
Sample sizes presented are the numbers of studies found. The variable we refer to as social structure of the species was present in 4 of the 6 Species that are group-living had lower levels of IPY, IPL, and GM pairs compared to species in which individuals lived in pairs or displayed intermediate social structure Figure 2 , Table 3. The level of male-female association appeared in 1 of the top models for GM pairs; species where pairs were more closely associated had lower proportions of IPY than species with intermittent levels of association between the male and female.
However, this outcome is likely to have been caused by multi-collinearity between pair association and social structure in the model, since this was the only model where pair association had this effect. Sex ratio appeared in two of the top models for GM pairs and population density appeared in one of them.
Each of these variables had a negative effect on the measure of genetic monogamy. None of the other variables or combinations of variables appeared in a top model.
Table 2. Figure 2. The proportion of intra-pair young IPY produced by species of socially monogamous mammals that live primarily in pairs, groups, or that are intermediate between these two types of social structure. The proportions presented are based on the coefficients from the top model for IPY Tables 2 , 3 and control for phylogenetic relatedness, differences in the level of paternal care, and are weighted by sample size.
Table 3. Model coefficients and standard errors from the top phylogenetic regression models explaining three indices of genetic monogamy GM. Figure 3. The proportion of intra-pair young IPY produced by species of socially monogamous mammals with differing levels of paternal care, ranked from species that provide no paternal care to those that provide intensive paternal care.
The regression is based on the coefficients from the top model for IPY Tables 2 , 3 and controls for phylogenetic relatedness, differences in social structure, and is weighted by sample size. Our results show that no one model was consistently the best for all our indices of genetic monogamy. Life history variables, specifically social structure and paternal care, were the only type of predictor variables for IPY and IPL and the life history variable referred to as social structure was in two of the top models for GM pairs.
Demographic variables, specifically sex ratio and population density, only appeared in two of the top models for GM pairs. The only predictive variable found for all three indices of genetic monogamy was social structure Table 2 , specifically living in a socially monogamous pair or intermediately i.
Individuals that lived as a member of a socially monogamous pair showed higher levels of genetic monogamy compared to individuals found primarily in a group with other sexually mature conspecifics. Living as a member of a pair was an important predictor of genetic monogamy either by itself or in combination with other life history variables such as paternal care, the amount of male-female association or sex ratio.
Sex ratio and population density were the only demographic variables in any top model. They were found in the top models for GM pairs but not in models for any other index of genetic monogamy.
No other demographic or environmental variables were in any of our top models. These results suggest that the levels of genetic monogamy across socially monogamous mammals are likely not driven by merely one variable but a combination of variables, and different variables may be more important for different species see also Klug, Our study, including twice as many species as prior studies, is consistent with results from the largest previous comparative study of socially monogamous mammals Cohas and Allaine, that showed that group-living species had higher levels of EPY than pair-living species.
Although there are numerous possible benefits to group-living, including increased group vigilance resulting in increased protection from predation, foraging benefits, increased production, or survival of offspring due to helping by other group members Krause and Ruxton, ; Ward and Webster, , it appears that a major cost to group living is being unable to prevent one's mate from engaging in EPCs Suter et al.
A high level of paternal care was also associated with increased levels of genetic monogamy for all models in which it was present Table 3. The positive relationship between levels of paternal care and IPY is consistent with the results of Huck et al.
Previous studies show that paternal care likely evolved after transitions to social monogamy Komers and Brotherton, ; Lukas and Clutton-Brock, For this reason, we suggest that genetic monogamy may have co-evolved with paternal behavior when selection initially favored males that were more affiliative toward females and offspring and, subsequently these affiliative behaviors became modified into paternal care Komers and Brotherton, ; Dillard and Westneat, Selection might then have favored males that provided parental care only when paternity was certain to allow males to limit investing in offspring sired by other males.
If females engaged in EPCs and males were able to detect this, then these females could face high costs from losing male investment and may then have decreased or refrained from engaging in extra-pair mating to ensure male care. Over evolutionary time, females might have produced offspring that were increasingly reliant on male care or male care might have resulted in greater fitness benefits through the quality or number of offspring.
Either of these scenarios could result in stabilizing selection for male care and genetic monogamy Smith, ; Dunbar, ; Wade and Shuster, ; Stockley and Hobson, This may be the explanation for the high level of genetic monogamy in the California mouse Peromyscus californicus , for example, where providing paternal care and remaining genetically monogamous results in greater reproductive success for a male than the alternative Gubernick and Teferi, ; Ribble, Demographic variables were not good predictors of IPY or IPL but the combination of sex ratio and either social structure or population density were two of the top models for GM pairs, with increases in population density or the proportion of adult males in the population having a negative effect on GM pairs.
This was consistent with our predictions and also has been found in previous avian studies Westneat and Sherman, Higher population densities may provide greater opportunities for EPCs since there would be a greater likelihood of encountering opposite-sex conspecifics, thus lower costs of pursuing EPCs while resulting in increased difficulty in guarding ones' own mate to prevent EPCs.
A male-biased sex ratio likely has similar effects because unpaired or subdominant males may pursue matings with paired females. There are a number of possible reasons that we found different top models depending on the index of genetic monogamy that we examined.
Variation in the number of offspring produced per reproductive bout could influence the outcome of model selection. Additionally, the length of the studies and of the pair-bonds differ greatly among species in the data set.
Although some studies provided data from multiple breeding seasons or years e. Finally, although our three indices of genetic monogamy are each different ways to quantify genetic monogamy, they are not necessarily of equal biological importance. Previous studies have focused on EPY or IPY for our study , and this may be the best index from an evolutionary perspective because it is the closest to reproductive success, which is often used to index fitness Gimenez and Gaillard, We had to exclude several potentially biologically important variables from our analyses due to the lack of field data from a sufficient number of species.
These variables include the role of female spacing, sexually transmitted disease, relatedness between members of a pair, potential genetic benefits of EPC, and other environmental variables that may influence the interactions between unpaired individuals e. Female spacing has been proposed to be a very important driver of social monogamy Komers and Brotherton, ; but see Dobson et al. Furthermore, under certain circumstances the presence of sexually transmitted diseases may selectively favor genetic monogamy because mating with one only one opposite-sex conspecific allows individuals to decrease the probability of being infected Loehle, ; Thrall et al.
Additionally, relatedness between members of a pair and the genetic benefits of EPY can influence the levels of EPCs in a variety of socially monogamous species Blomqvist et al. If we were able to include additional variables in our analysis, we might have found different variables in our top models. Future studies focusing on these potentially important variables may increase our understanding of mammalian genetic monogamy or increase our confidence that the variables in our top models are the most important drivers of genetic monogamy.
Furthermore, some of these variables might interact, be highly related to each other, or be bidirectional, making it challenging to unravel the most important predictors of genetic monogamy. For example, estrous synchrony and spatial distribution of females could interact to affect the probability of males obtaining EPCs. If all females are fertile or in estrus at the same time but are dispersed as opposed to being clustered, the prospects for a male seeking EPCs would differ.
Additionally, variables such as population density and seasonality may be highly related, as seen in vole populations where population density typically is lower in winter and spring than in the fall Getz et al. Finally, some variables may be bidirectional i. For example, if females engage in EPCs we would predict the level of paternal care to decrease. Conversely, the level of paternal care could also influence the likelihood of females seeking EPCs Westneat et al.
These examples highlight some of the challenges in trying to encapsulate relevant factors into a model that predicts genetic monogamy across numerous species.
Although a comparative approach can allow us to determine predictors of genetic monogamy across mammals, we realize that all mammalian species do not fit the patterns found. Tight synchronous breeding Platner, ; Hilgartner et al.
Although comparative studies may detect overall evolutionary or environmental trends, the results are not expected to adequately explain the complex processes that result in genetic monogamy in every species. Another important consideration is how genetic monogamy might differentially benefit conspecifics within or between populations. Differential benefits among individuals may complicate the interpretation of data analyzed at the species-level. Some hypotheses proposed to explain genetic monogamy e.
Search ». Turtle doves and swans Although commonly held up as symbols of love and fidelity, turtle doves and swans are only socially— not sexually —monogamous. Snowy owls Pairs of snowy owls typically mate for life. About Cell Mentor. Subscribe to Cell Mentor.
Browse Cell Mentor by Topic and Type. Blog posts Videos How-tos Handbooks. Contact Cell Mentor. Stay connected:. Research Journals. Trends Reviews Journals. Here are seven animals that are in it for the long haul. In a grey wolf pack, it is usually only the male and female alpha who are allowed to breed, The pair mate for life as a way to cement their position as pack leaders, producing a new litter each year and ensuring the stability of the rest of the pack.
As explained by David W. Macdonald et al in Monogamy: Cause, Consequence, or Corollary of Success in Wild Canids , monogamy makes it easier for alphas to display their strength and superiority over the other male members of the pack. Defending one mate against advances is much easier than trying to protect numerous partners, assuring their place as top dog.
For Eurasian beavers, monogamy is more about practicality than romance. Unlike their North American counterparts who have been known to mate outside their bonded partners, Eurasian beavers team up for life as a way to increase their chances of survival.
Although one of their primary sources of food is tree bark, its lack of nutritional value means that beavers need to eat a lot of bark to keep themselves going. So, by pairing up, couples can split their workload and ensure everything gets done. In a study by Sam M J G Steyaert, Andreas Zedrosser and Frank Rossel for Oecologia , research found that tasks divided between pairs included maintaining their dams and lodges, guarding their territory and sharing the parenting duties.
Paired gibbons perform 'duets', a series of vocal cries that combines their individual mating calls into a single song that becomes intrinsic to their relationship.
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