Article

Evolutionary allometry of the canid baculum (Carnivora: Mammalia)

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Abstract

While the mammalian baculum shows enormous morphological variability, the baculum of canids is highly conserved, with most variation restricted to size. Here, we explore the allometric relationship between baculum length and body size in extant and extinct canids. Examination of 26 species in the extant subfamily Caninae using standard linear regression revealed isometry. Phylogenetic regression also revealed an allometric slope indistinguishable from isometry. This pattern differs from the substantially negative slopes seen in other mammalian clades. The strength of the canid allometric relationship (r2) is also greater than in other clades, suggesting functional constraints on their baculum size. The constraints may be related to the copulatory tie that is characteristic of canids, and/or their monogamous mating system. Complete bacula are known from just four extinct species. The two complete bacula from the extinct subfamily Borophaginae (Aelurodon ferox and Aelurodon stirtoni) fall on the same allometric relationship as the living canids. However, the baculum of the extinct dire wolf (Aenocyon dirus, from the extant subfamily Caninae) and from the extinct subfamily Herperocyoninae, Hesperocyon gregarius, are significantly longer than expected based on their body sizes, suggesting that they may have had a different reproductive biology from that of extant canines.

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Probable functions of the baculum are reviewed. Consideration of vectors of resistance of the vaginal orifice and of forces of the baculum reveals possible reasons for the specialization of the bacular tip in some carnivores and rodents. Coefficients of variation of length and some other measurements of some bacula of adult mammals are compared, and measures for Procyon and Rattus are correlated. Variations of length of baculum and width of tip are often exceeded by other bacular variations. The length is usually less variable than its components; this pattern of variation can be explained in part by (1) lack of correlation of the subsidiary measures, and (2) algebraic considerations. The ossicles of complex bacula show an increase in variation distally. The influence of sex on the baculum, and the function, form, and variation of the os clitoridis in several kinds of mammals are discussed.
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Male genitalia exhibit a taxonomically widespread pattern of rapid and divergent evolution. Sexual selection is generally believed to be responsible for these patterns of evolutionary divergence, although empirical support for the sexual selection hypothesis comes mainly from studies of insects. Here we show that sexual selection is responsible for an evolutionary divergence in baculum morphology among populations of house mice Mus domesticus. We sourced mice from three isolated populations known to be subject to differing strengths of postcopulatory sexual selection and bred them under common-garden conditions. Mice from populations with strong postcopulatory sexual selection had bacula that were relatively thicker compared with mice from populations with weak selection. We used experimental evolution to determine whether these patterns of divergence could be ascribed to postcopulatory sexual selection. After 27 generations of experimental evolution, populations of mice subjected to postcopulatory sexual selection evolved bacula that were relatively thicker than populations subjected to enforced monogamy. Our data thereby provide evidence that postcopulatory sexual selection underlies an evolutionary divergence in the mammalian baculum and supports the hypothesis that sexual selection plays a general role in the evolution of male genital morphology across evolutionary diverse taxonomic groups.
Article
The distribution, physical development, pelage, dentition, feeding behavior, reproduction, causes of mortality, and population structure of the Pacific walrus (Odobenus rosmarus divergens) were studied intermittently from 1952 to 1979. In winter, these animals tend to concentrate in north-central and southeastern Bering Sea, where sea ice conditions are most favorable for them. In summer, they concentrate mainly in northwestern and northeastern Chukchi Sea, along the edge of the ice. Most of the northward migrants are females and young; a large proportion of the adult males remains in the Bering Sea throughout the summer. Pacific walruses show strong sexual dimorphism; adult males are about 18% longer and 45% heavier and tend to have larger, more divergent tusks, as well as thicker, lighter-colored, and less hairy skin than adult females. As in other sexually dimorphic otarioid pinnipeds, males undergo secondary growth, beginning about the time of puberty and ending in full physical maturity about 15 years of age. The first pelage is a fine, white lanugo, which develops and is shed in utero. The second (natal) pelage, which resembles that of the adult, is shed and replaced 2 to 3 months after birth, in synchrony with the molt of older animals. The full primary and secondary dentitions include 38 and 30 teeth, respectively; 7 pairs of primary and 6 pairs of secondary teeth occur in less than 50 % of the animals. The permanent first upper premolar is a secondary tooth, preceded by an uncalcified primary tooth. The lower premolars appear to be homologues of P2-3-4, rather than P1-2-3. The cheek teeth grow in length very slowly, and the pattern of decrement of their crowns indicates no contact with molluscan shells except in the incisive area at the front of the mouth. The abrasion of the tusks indicates that they are dragged through the bottom sediments, rather than used for digging or raking. Food of the Pacific walrus consists of more than 60 genera of marine organisms, most of which are situated on or just beneath the surface of the sediments. The walrus apparently locates these tactually with its sensitive mystacial vibrissae and by “rooting” with its snout. Soft-bodied organisms are ingested directly, without mastication; the soft parts (siphon, foot) of mollusks probably are separated from the shells by suction. The intake of food is at least 5 to 7% of the total body weight per day. Most females ovulate for the first time at 5 or 6 years; males become fertile at 8 to 10 years but probably do not participate in mating until fully mature at 15 years. Walruses are polygynous; mating takes place mainly in mid-winter. Implantation of the blastocyst takes place about 5 months later, and the calf is born in the following spring, after a pregnancy lasting at least 15 months. Females tend to breed at 2-year intervals or less often and are most productive between the ages of 8 and 15 years. The principal causes of mortality appear to be predation, intraspecific trauma, and microbiological infections. The Pacific walrus population was severely depleted in the late 19th century and again in the mid-20th century by overharvests for commercial purposes. Over the past 20 years, it apparently has recovered rapidly, in response to reduced harvest. The sex ratio of breeding adults appears to be about 1 male:3 or 4 females. The crude birth rate in recent years is estimated at 17 ± 2%, and the survival of young to puberty appears to be very high.
Article
Crab‐eating zorros, Cerdocyon thous , in Amazonian Brazil weighed 5.2 kg (S.D. 0.6, n = 19), exhibited no sexual dimorphism, lived in social units of 2‐5 adults of > 1 year old, and occupied territories (restricted polygons) of 532 ha (range 48‐1042 ha, n = 21). The zorros were omnivorous, fruit being the most frequent food (occurring in 57% of 72 faeces in the dry season) comprising 26.4% by volume of undigested faecal remains. Insects were frequently eaten (86%), and vertebrates rarely (15.2%). Zorros were territorial. The ranges of neighbours overlapped minimally and were very stable in configuration, both from month to month and between dry and wet seasons. Members of each social unit shared on average 88.3% of 200 × 200m grid cells within their territory. Members of male‐female breeding pairs were within 100 m of each other for 63% of the activity period, their proximity being greatest during the mating season (July‐August). Parents travelled in close company with their adult‐sized offspring, and stayed within 100 m for up to 93.3% of the activity period. Territories differed in habitat composition. Overall, zorros spent most time in wooded savanna (33.9%) and scrub (30.5%). They did not utilize habitats in direct proportion to their availability, using some more (e.g. scrub) and some less (e.g. open savanna) than expected. Habitat preferences differed between wet and dry seasons, with elevated habitats being favoured during the wet season. Individuals differed in their habitat utilization, and parents had different habitat usage to their yearling offspring; this difference was exaggerated in the dry season when the yearlings used lowlying habitats apparently disfavoured by the parents. Differences in habitat utilization between group members were least in the wet season, when widespread flooding forced the shared use of higher ground. Each of two social units larger than two individuals comprised a pair and their three adult‐sized offspring (totalling five males and one female). In three cases, a yearling daughter was present in the breeding season, but none of these bred. Non‐breeding offspring were seen frequently in the whelping area and in close company with the breeding pair's cubs. This is the first proof of group‐living in South American zorros. Five dispersal events were monitored, and revealed a ‘good neighbour’strategy in which newly formed pairs set up their territories adjoining their natal territories. In each case individuals returned intermittently to their original territory where they were in close and amicable company with their parents. One male, having dispersed, returned home frequently without his mate to tend the next generation of his parents’cubs. Of four dispersing males, two subsequently returned to their natal group following the deaths of their mates at least 3‐13 months after their initial dispersal, in one case after breeding elsewhere.
Article
Despite the diversity of morphology and ecology in members of the Canidae, social behavior remains similar throughout the Family. Some specializations have occurred in groupliving species, serving to maintain group cohesion and to reduce intraspecific aggression, and these changes in behaviors and postures have been ones of degree rather than kind. The type of specialization differs in each species and is probably related to its ecology. Thus, the bateared fox has developed contact behaviors such as social grooming while the wolf has evolved more specialized agonistic postures that are used in the maintenance of a social hierarchy.
Article
The length of the baculum (os penis) was measured in 74 adult males representing 46 primate species. These data, and a review of previously published measurements, indicate that variation in baculum length among primates is related to taxonomic and behavioral differences. Thus, many New World monkeys have shorter bacula, relative to body weight, than Old World monkeys. The baculum is shorter in colobine monkeys than in cercopithecines. Among the great apes, reduction of the baculum is more pronounced in Pan and Gorilla than in Pongo. Very long bacula are found in some nocturnal prosimians (eg, Lorisidae) and also in Macaca arctoides. A comparison of baculum length relative to body weight was made in 34 species for which detailed information on copulatory behavi or was available. The presence of an elongated baculum was shown to correlate with copulatory patterns involving prolonged intromission and/or the maintenance of intromission during the postejaculatory interval (eg, Galago crassicaudatus, Loris tardigradus, M, arctoides). The evolutionary significance of these observations is discussed and it is suggeted that similar copulatory patterns may occur in species with elongated bacula (eg, Daubentonia, Perodicticus) for which behavioral data are lacking at present. The same hypothesis also applies to an extinct adapid primate which possessed a very large baculum.
Article
Comparative biologists often attempt to draw inferences about tempo and mode in evolution by comparing the fit of evolutionary models to phylogenetic comparative data consisting of a molecular phylogeny with branch lengths and trait measurements from extant taxa. These kinds of approaches ignore historical evidence for evolutionary pattern and process contained in the fossil record. In this article, we show through simulation that incorporation of fossil information dramatically improves our ability to distinguish among models of quantitative trait evolution using comparative data. We further suggest a novel Bayesian approach that allows fossil information to be integrated even when explicit phylogenetic hypotheses are lacking for extinct representatives of extant clades. By applying this approach to a comparative dataset comprising body sizes for caniform carnivorans, we show that incorporation of fossil information not only improves ancestral state estimates relative to those derived from extant taxa alone, but also results in preference of a model of evolution with trend toward large body size over alternative models such as Brownian motion or Ornstein-Uhlenbeck processes. Our approach highlights the importance of considering fossil information when making macroevolutionary inference, and provides a way to integrate the kind of sparse fossil information that is available to most evolutionary biologists.