Beaver Ponds Research Articles

Evaluating the geomorphic channel response to beaver dam analog installation using unoccupied aerial vehicles

AbstractBeaver dam analogs (BDAs) are a stream restoration technique that is rapidly gaining popularity in the western United States. These low‐cost, stream‐spanning structures, designed after natural beaver dams, are being installed to confer the ecologic, hydrologic, and geomorphic benefits of beaver dams in streams that are often too degraded to provide suitable beaver habitat. BDAs are intended to slow streamflow, reduce the erosive power of the stream, and promote aggradation, making them attractive restoration tools in incised channels. Despite increasing adoption of BDAs, few studies to date have monitored the impacts of BDAs on channel form. Here, we examine the geomorphic changes that occurred within the first year of restoration efforts in Wyoming using high‐resolution visible light orthomosaics and elevation data collected with unoccupied aerial vehicles (UAVs). By leveraging the advantages of rapidly acquired images from UAV surveys with recent advancements in structure‐from‐motion photogrammetry, we constructed centimeter‐scale digital elevation models (DEMs) of the restoration reach and an upstream control reach. Through DEM differencing, we identified areas of enhanced erosion and deposition near the BDAs, suggesting BDA installation initiated a unique geomorphic response in the channel. Both reaches were characterized by net erosion during the first year of restoration efforts. While erosion around the BDAs may seem counter to the long‐term goal of BDA‐induced aggradation, short‐term net erosion is consistent with high precipitation during the study and with theoretical channel evolution models of beaver‐related stream restoration that predict initial channel widening and erosion before net deposition. To better understand the impacts of BDAs on channel morphology and restoration efforts in the western United States, it is imperative that we consistently assess the effects of beaver‐inspired restoration projects across a range of hydrologic and geomorphic settings and that we continue this monitoring in the future.

Emergence of Diptera (Brachycera) from a beaver pond in the upper reaches of the Khoper river in Penzenskaya Oblast of Russia

Emergence of Diptera (Brachycera) from a beaver pond in the upper reaches of the Khoper river in Penzenskaya Oblast of Russia

Impact of beaver dam analogues on hydrology in a semi‐arid floodplain

AbstractNatural beaver ponds help connect the stream to the floodplain, maintain late summer low flows and reduce peak flow during high flow events by offering temporary surface water (SW) storage. When beavers are extirpated from the landscape, stream degradation often ensues. This study assesses the impact of beaver dam analogues (BDA) as a stream restoration technique to help maintain low flow water levels and enhance stream‐floodplain interactions on a seasonal basis in Red Canyon Creek, Lander, WY. BDAs increased SW and groundwater (GW) levels, favoured the occurrence of flow reversals (i.e., stream‐to‐floodplain GW flow) during high flow events associated with mid‐winter and early‐spring thaw events, and reduced the groundwater‐to‐stream hydraulic gradient on an annual basis. Although GW temperatures varied seasonally, relatively cooler GW temperatures were observed in the BDA impacted reach compared to the control reach away from BDA influence. BDAs however did not significantly impact stream temperatures. Overall, results suggest that when installed in sequence, BDA complexes can successfully reconnect the stream to its floodplain, and ultimately increase SW‐GW exchange at the floodplain scale by allowing flow reversals to occur and by reducing the GW to stream hydraulic gradient. Although BDAs built with fence posts, willow branches, sediments and small boulders are naturally porous and require regular maintenance, this study also highlights the viability of small BDAs as a restoration practice to enhance landscape resilience to drought and high flow events in deeply incised channels where beavers would not come back naturally.

Technology as a socio-historical phenomenon

The article reveals the authors' vision of the essence of the technology as a sociohistorical phenomenon. It is based on the idea that technology is not only a set of technical devices but a segment of the general system – a society – located between a social medium and its natural surroundings in the form of a peculiar social technosphere, which simultaneously separates and connects them. The main objective purpose of the technosphere is to promote the effective rendering of society-generated entropy outwards; it defines the features of the technosphere as a sociohistorical phenomenon. The analogues of such material formations take place also in wildlife (from the spider-web to the beaver dam) but are very few and arise from the implementation of instinctive programs of the species. In a person's consciousness, such programmes are not given by “nature”, they are formed on the basis of “desobjectivation” of technical objects available in society. In the process of “desobjectivation” the essence, the “logic of the subject” becomes the achievement of a person and due to his abilities is filled with new meaning. As a result, the technology is a materially ideal phenomenon: on the one hand, it is a set of technical objects and on the other hand – technical thinking of a person, the highest manifestation of which is technical sciences nowadays. Properly technical objects are created by society to meet the individual and social needs of a person. These are primarily consumption items; due to their manmade nature, the question of production means development arises, which over time becomes increasingly important, especially by virtue of their significant impact on social relations (which in time also require certain technical devices for their implementation). The complex of these devices forms the techno sphere of society as a compound integrity. Not only groups of different in application technical objects become the constituent parts of the technosphere, but also their conglomerates designed to perform certain functions, which, similar to the biological branch, were called techno enosis; in the latter at the account of a peculiar “competition”, the development of these components in particular and the technosphere integrally takes place. However, despite consistency, the technosphere is a subsystem of a society, therefore, there is no perspective of creating certain laws of its development and an appropriate coherent periodization. For this reason, the scientific periodization of the development of technology as such is connected with the purpose of the given research and is defined by it.

Beaver creates early successional hotspots for water beetles

Beavers (Castor spp.) are ecosystem engineers that induce local disturbance and ecological succession, which turns terrestrial into aquatic ecosystems and creates habitat heterogeneity in a landscape. Beavers have been proposed as a tool for biodiversity conservation and ecosystem restoration. So far, most research has compared biodiversity in beaver wetlands and non-beaver wetlands, but few studies have explored how beaver-created succession affects specific taxa. In this study, we investigated how water beetles responded to different successional stages of wetlands in a beaver-disturbed landscape at Evo in southern Finland. We sampled water beetles with 1-L activity traps in 20 ponds, including: 5 new beaver ponds, 5 old beaver ponds, 5 former beaver ponds, and 5 never engineered ponds. We found that beaver wetlands had higher species richness and abundance than non-beaver wetlands, and that new beaver wetlands could support higher species richness (321%) and abundance (671%) of water beetles compared to old beaver wetlands. We think that higher water beetle diversity in new beaver ponds has resulted from habitat amelioration (available lentic water, shallow shores, aquatic vegetation, and low fish abundance) and food source enhancement (an increase of both dead and live prey) created by beaver dams and floods. We conclude that using beavers as a tool, or imitating their way of flooding, can be beneficial in wetland restoration if beaver population densities are monitored to ensure the availability of newly colonizable sites.

Short‐term impact of beaver dam analogues on streambank erosion and deposition in Semi‐Arid landscapes of the Western USA

AbstractRiver systems in the mountain western USA have been shaped by the presence of beavers for millennia. However, beavers have been extirpated from the landscape in many places, leading to excessive stream incision and streambank erosion. One common strategy to mitigate this issue is to deploy beaver dam analogue (BDAs) as a stream restoration technique. Although BDAs are intended to reduce erosion and stream incision, few studies directly document the impact of BDAs on stream channel geomorphology. This study, therefore, assesses how a complex of five BDAs along a 150 m long stream reach in Red Canyon Ranch near Lander, WY impacts stream bank erosion and deposition and channel evolution over a 1‐year period post‐installation. Relative to control locations not impacted by BDAs, the BDA reach showed greater spatial heterogeneity in erosion and deposition patterns than control locations, as well as less overall erosion. However, each BDA had unique effects on channel morphology. Large amounts of deposition were found at the most upstream BDA remaining after the first year at both inner and outer meander locations. High flow events created breaches that likely produced significant stream bank erosion observed immediately downstream of the BDA complex. From a design standpoint, the only BDAs that remained after a year were those built around fence posts inserted into the stream bed with a percussion fence post driver. Overall, our short‐term data indicate that BDAs can be successfully used as a stream restoration practice to reduce stream bank erosion and increase channel geomorphological heterogeneity.

Informed breeding dispersal following stochastic changes to patch quality in a pond-breeding amphibian.

The unidirectional movement of animals between breeding patches (i.e. breeding dispersal) has profound implications for the ecological and evolutionary dynamics of spatially structured populations. In spatiotemporally variable environments, individuals are expected to adjust their dispersal decisions according to information gathered on the environmental and/or social cues that reflect the fitness prospects in a given breeding patch (i.e. informed dispersal). A paucity of empirical work limited our understanding of the ability of animals to depart from low-quality breeding patches and settle in high-quality breeding patches. We examined the capacity of individuals to respond to stochastic changes in habitat quality via informed breeding dispersal in a pond-breeding amphibian. We conducted a 5-year (2015-2019) capture-recapture study of boreal toads Anaxyrus boreas boreas (n=1,100) that breed in beaver ponds in western Wyoming, USA. During early spring of 2017, an extreme flooding event destroyed several beaver dams and resulted in the loss of breeding habitat. We used multi-state models to investigate how temporal changes in pond characteristics influenced breeding dispersal, and determine whether movement decisions were in accordance with prospects for reproductive fitness. Boreal toads more often departed from low-quality breeding ponds (without successful metamorphosis) and settled in high-quality breeding ponds (with successful metamorphosis). Movement decisions were context-dependent and associated with pond characteristics altered by beaver dam destruction. Individuals were more likely to depart from shallow ponds with high vegetation cover and settle in deep ponds with low vegetation cover. The probability of metamorphosis was related to the same environmental cues, suggesting that boreal toads assess the fitness prospects of a breeding patch and adjust movement decisions accordingly (i.e. informed breeding dispersal). We demonstrated that stochastic variability in environmental conditions and habitat quality can underpin dispersal behaviour in amphibians. Our study highlighted the mechanistic linkages between habitat change, movement behaviour and prospects for reproductive performance, which is critical for understanding how wild animals respond to rapid environmental change.

A systematic approach and software for the analysis of point patterns on river networks

AbstractMany geomorphic phenomena such as bank failures, landslide dams, riffle‐pool sequences and knickpoints can be modelled as spatial point processes. However, as the locations of these phenomena are constrained to lie on or alongside rivers, their analysis must account for the geometry and topology of river networks. Here, we introduce a new numeric class in TopoToolbox called Point Pattern on Stream networks (PPS), which supports exploratory analysis, statistical modelling, simulation and visualization of point processes. We present three case studies that aim at inferring processes and factors that control the spatial density of geomorphic phenomena along river networks: analysis of a synthetic dataset of points on a stream network, the analysis of knickpoints in river profiles, and modelling spatial locations of beaver dams based on topographic metrics. The case studies rely on exploratory analysis and statistical inference using inhomogeneous Poisson point processes. Thereby, statistical and probabilistic procedures implemented in PPS provide a systematic approach for treating and quantifying uncertainties. PPS offers a consistent numeric framework for modelling point processes on river networks with a wide range of applications in fluvial geomorphology, but also other disciplines such as ecology.

Beaver dams: How structure, flow state, and landscape setting regulate water storage and release

Beaver (Castor canadensis and Castor fiber) are regarded widely as ecosystem engineers and the dams they create are well-known for their ability to drastically alter the hydrology of rivers. As a result, beaver are increasingly being included in green infrastructure practices to combat the effects of climate change and enhance ecosystem resilience. Both drought and flood mitigation capabilities have been observed in watersheds with beaver dam structures; however, how dams possess contrasting mitigation abilities is not fully understood since most studies neglect to acknowledge variation in beaver dam structures. In this study, an extensive cross-site survey of the physical and hydrologic properties of beaver dams was conducted in the Canadian Rocky Mountains in Alberta. This research aimed to improve the understanding of the hydrology of beaver dams by categorizing dams using their intrinsic properties and landscape settings to identify fundamental patterns that may be applicable across landscape types. The dam flow type classification from Woo and Waddington (1990) was evaluated in this new context and adapted to include two new flow types. The survey of intrinsic beaver dam properties revealed significant differences in dam structure across different sites. Physical differences in dam structure altered the dynamics and variance of pond storage and certain dam attributes related to the landscape setting. For instance, dam material influenced dam height and water source influenced dam length. However, a closer analysis of large rain events showed that the physical structure of dams alters seasonal dynamics of pond storage but not the response to rain events. Overall, this research shows that beaver dams can be both structurally and hydrologically very different from each other. Establishing broadly applicable classifications is vital to understanding the ecosystem resilience and mitigation services beaver dams provide.

Phosphorus Transport in a Lowland Stream Derived from a Tracer Test with 32P

Small streams in urbanized rural areas receive loads of P from various, often episodic, sources. This paper addresses, through a tracer test with 32P, retention and transport of a pulse input of phosphorus in a 2.6 km long stretch of a channelized second-order lowland stream. Tritiated water was introduced alongside the 32P-labelled ortophosphate in order to isolate the influence of the hydrodynamic factors on P behavior. Tracer concentrations in unfiltered water samples were measured by liquid scintillation counting. Four in-stream and five hyporheic breakthrough curves were collected at four points along the stream, two of which encompass a beaver dam impoundment. The overall retention efficiency of 32P along the studied reach was 46%. The transient storage transport model OTIS-P provided reasonable fits for in-stream breakthrough curves (BTCs) but failed at reproducing the hyporheic BTCs. The overall small effect of transient storage on solute transport was higher in the stretch with a more pronounced surface storage. Transient storage and phosphorus retention were not enhanced in the beaver dam impoundment.

Mammalian activity in the vicinities of beaver dams in the fall-winter season

European beavers, being so called a keystone species, transform their habitat according to their own needs while also creating favorable living conditions for other animals. The aim of the research was to find out how different species of mammals use beaver dams and the area directly adjacent to the watercourse, in the place where the dam was built, in the autumn and winter period. In the investigated areas the activity of 13 different mammal species were recorded, most often using the beaver dams as river crossing, either on or close to the dam itself. Between September-February the activity of mammals was evenly distributed over the period of each month with a slight increase in November. The study confirmed a great role of the european beaver in increasing the diversity of the local mammal species.

Thermal Characteristics of a Beaver Dam Analogues Equipped Spring-Fed Creek in the Canadian Rockies

Beaver dam analogues (BDAs) are becoming an increasingly popular stream restoration technique. One ecological function BDAs might help restore is suitable habitat conditions for fish in streams where loss of beaver dams and channel incision has led to their decline. A critical physical characteristic for fish is stream temperature. We examined the thermal regime of a spring-fed Canadian Rocky Mountain stream in relation to different numbers of BDAs installed in series over three study periods (April–October; 2017–2019). While all BDA configurations significantly influenced stream and pond temperatures, single- and double-configuration BDAs incrementally increased stream temperatures. Single and double configuration BDAs warmed the downstream waters of mean maxima of 9.9, 9.3 °C by respective mean maxima of 0.9 and 1.0 °C. Higher pond and stream temperatures occurred when ponding and discharge decreased, and vice versa. In 2019, variation in stream temperature below double-configuration BDAs was lower than the single-configuration BDA. The triple-configuration BDA, in contrast, cooled the stream, although the mean maximum stream temperature was the highest below these structures. Ponding upstream of BDAs increased discharge and resulted in cooling of the stream. Rainfall events sharply and transiently reduced stream temperatures, leading to a three-way interaction between BDA configuration, rainfall and stream discharge as factors co-influencing the stream temperature regime. Our results have implications for optimal growth of regionally important and threatened bull and cutthroat trout fish species.

Medieval Relict Beaver Ponds in the Polish Plain: Studies from the Tuchola Forest

Beavers have lived in the territory of Poland since the beginning of the Holocene, as testified by bone remains found in archaeological sites of different ages. The area inhabited by these animals has experienced continuing transformations of terrain relief, geological structure, hydrology and plant cover. In Poland, beavers are partially protected and their population has spread in virtually every part of the country (except in the highest mountain ranges). The authors of this paper wish to present the results of field works carried out since 2006 in the Tuchola Forest (Polish Plain). This paper aims to identify the potential sediments of relict beaver ponds and their sedimentological features. The studies are also backed up with a description of radiocarbon dating of samples. The results indicate that beavers used to live in the Tuchola Forest in the Middle Ages, as shown by the radiocarbon dates and sequences of mineral–organic deposits found in exposures and geological boreholes. The spatial distribution of organic and mineral deposits in wider sections of river valleys can be explained by the avulsion of the riverbed downstream of the pond and by the distribution of ponds in the Gołyjonka valley. The discovery of relict beaver pond sediments suggests that the activity of these mammals in the Middle Ages played a major part in shaping the landscape of the valley. The results of studies clearly indicate that analyses of the valley sediment facies of small watercourses should take into account the role beavers played in the past in shaping the landscape of the analysed valley. This highlights the insufficiency of studies concerning the activity of beavers in river valleys.

Source or sink? Quantifying beaver pond influence on non-point source pollutant transport in the Intermountain West

Non-point source (NPS) pollution remains high in many watersheds despite strategies aimed at reducing such pollution. Beaver (Castor canadensis) activity converts lotic systems to semi-lentic by impounding stream flow and trapping sediments, which have a high affinity for NPS pollutants such as nitrogen (N), phosphorus (P), and heavy metals. This study aimed to identify environmental conditions under which beaver ponds influence the fate and cycling of NPS pollutants. Dissolved and particulate nutrients were sampled upstream and downstream of three headwater beaver ponds differing in age and character through the summer season. Sedimentation rates and sediment concentrations of nutrients and metals were also determined. Results from this study suggest that beaver ponds can attenuate heavy metals at a rate 2 to 4 times greater than a riffle reach (p < 0.05). Metal sequestration scaled with pond age and sediment organic matter content. The oldest and youngest ponds had no significant effect on dissolved nutrients (NO3−, TDN and SRP) or total P (TP). The middle age pond was a significant TN sink in summer (0.6–0.8 g N m−2 d−1 [p = 0.03]) and influenced dissolved nutrient concentrations differently in spring (21% NO3− sink [p = 0.03], 61% SRP source [p = 0.05]) compared to summer (34% NO3− source, 7% SRP sink). This pond had little apparent effect on TP loads during the study period but accumulated a total of 146 g m−2 of phosphorus in the sediments suggesting that beaver ponds may reach their phosphorus sequestration potential within the first few years of pond development and then subsequently act as a weak SRP source. We use a theoretical relationship describing sediment-water interactions to show that biogeochemical processing in a beaver pond is optimized at intermediate levels of pond nutrient supply and residence time. If beaver ponds are to be considered as an option for landscape scale restoration, this theoretical relationship may be useful for predicting the effects of beaver ponds on water chemistry, and aid in the interpretation of variable water quality results from inherently heterogeneous environments.

Legacy effects of loss of beavers in the continental United States

Through their modifications of channels and floodplains, beavers are a premier example of ecosystem engineers. Historical and stratigraphic records suggest that hundreds of millions of beavers once modified small to medium rivers throughout the northern hemisphere. Where beavers actively modify the channel and floodplain with dams, ponds, and canals, their activities increase habitat abundance and diversity, biodiversity, nutrient uptake, attenuation of downstream fluxes of water and sediment, and resilience of the river corridor to disturbances. Loss of beavers through commercial trapping and habitat modification occurred simultaneously with other human modifications of uplands and river corridors. The cumulative effects of these human modifications have been to greatly reduce the ecosystem services provided by rivers. Contemporary efforts to re-introduce beavers in North America and Eurasia and to mimic the effects of beaver engineering with beaver dam analogues and Stage 0 restoration represent a good start, but fundamental questions remain about the extent of such restoration efforts needed to create and maintain significant increases in riverine functions.

Effects of land use change, wetland fragmentation, and best management practices on total suspended solids concentrations in an urbanizing Oregon watershed, USA

While many different watershed management strategies have been implemented to improve water quality, relatively few studies empirically tested the combined effects of different strategies on water quality in relation to land cover changes using long-term empirical data at the sub-basin scale. Using 10 years of total suspended solids (TSS) data, we examined how the conversion of wetland, wetland fragmentation, beaver dams, and Best Management Practices (BMPs) affect wet season TSS concentrations for the 25 monitoring stations in the Tualatin River basin, USA. Geographic information systems, FRAGSTATS, and correlation analysis were used to identify the direction of land cover change, degree of wetland fragmentation, and the strength of the relationship between TSS change and explanatory variables. Improvement in TSS concentrations was tightly coupled with the aggregation of wetlands, presence of beaver dams, particularly during the mid-wet season when flows were highest. Other BMPs effectively reduced TSS concentrations for the early and late-wet seasons when flows were not as high as in the middle wet-season. Aggregated wetlands were more effective for improving water quality than smaller disaggregated wetlands of similar total area when combined with the presence of beaver dams and BMPs. These findings offer important scientific and practical implications for management of urbanizing watersheds that seek to achieve the dual goals of improving environmental quality and land development.

Odonata species diversity, distributions, and status in a rare sand prairie-savanna wetscape

Inland sand areas scattered across the North American eastern deciduous forest and western tallgrass prairie ecotone are known for supporting pyrogenic early-successional vegetation and specially adapted terrestrial faunas. Many of these globally and regionally rare systems contain functionally connected wetland networks (“wetscapes”) potentially important for aquatic insects. Sampling adults, nymphs, and exuviae in a remnant sand prairie-savanna wetscape in Illinois, USA, I assessed odonate species diversity (alpha, gamma, beta), distributions (spatial, temporal, abundance), and rarity status. In one field season (12 sites, 12 visits) I found more than a third of Illinois odonate species and close to half of the state’s lentic breeding odonates, including a new state record (Erythemis vesiculosa). Richness averaged 25.8 species per site, reducing to 12.4 species with removal of nonbreeding occurrences. Three sites including a shrub swamp, beaver pond, and forested vernal depressions complex made significant contributions to beta diversity, dependent on general versus breeding occurrences. Majorities of Anisoptera species (70%) and Zygoptera species (53%) bred at three or fewer sites. Eight species flew during all or most of the study period (late May to early October) whereas 14 species were detected on a single survey. Status classification derived from the observed spatial, temporal, and abundance distributions resulted in 24 common or very common species, 20 uncommon or rare species, and 10 vagrants across the wetscape. These context-specific classifications may be combined with diversity and breeding patterns and other information in wetscape prioritization schemes.

Waterfowl use of mine tailing ponds in comparison with beaver ponds in boreal eastern Canada

Desjardins, E., L. Imbeau, M. Darveau, M. J. Mazerolle, and N. J. Fenton. 2021. Waterfowl use of mine tailing ponds in comparison with beaver ponds in boreal eastern Canada. Avian Conservation and Ecology 16(2):24. https://doi.org/10.5751/ACE-02003-160224

Beaver dams attenuate flow: A multi-site study.

Beavers can profoundly alter riparian environments, most conspicuously by creating dams and wetlands. Eurasian beaver (Castor fiber) populations are increasing and it has been suggested they could play a role in the provision of multiple ecosystem services, including natural flood management. Research at different scales, in contrasting ecosystems is required to establish to what extent beavers can impact on flood regimes. Therefore, this study determines whether flow regimes and flow responses to storm events were altered following the building of beaver dams and whether a flow attenuation effect could be significantly attributed to beaver activity. Four sites were monitored where beavers have been reintroduced in England. Continuous monitoring of hydrology, before and after beaver impacts, was undertaken on streams where beavers built sequences of dams. Stream orders ranged from 2nd to 4th, in both agricultural and forest‐dominated catchments. Analysis of >1000 storm events, across four sites showed an overall trend of reduced total stormflow, increased peak rainfall to peak flow lag times and reduced peak flows, all suggesting flow attenuation, following beaver impacts. Additionally, reduced high flow to low flow ratios indicated that flow regimes were overall becoming less “flashy” following beaver reintroduction. Statistical analysis, showed the effect of beaver to be statistically significant in reducing peak flows with estimated overall reductions in peak flows from −0.359 to −0.065 m3 s−1 across sites. Analysis showed spatial and temporal variability in the hydrological response to beaver between sites, depending on the level of impact and seasonality. Critically, the effect of beavers in reducing peak flows persists for the largest storms monitored, showing that even in wet conditions, beaver dams can attenuate average flood flows by up to ca. 60%. This research indicates that beavers could play a role in delivering natural flood management.

The Politics of Chastity

The Politics of Chastity Edward Feser Reinhard Hütter has authored an excellent and much-needed essay on the virtue of chastity, with a special focus on the unprecedented threat to this virtue posed by contemporary online pornography.1 His essay addresses the moral and spiritual aspects of the issue, as illuminated by both natural law and divine revelation. But there is also a crucial political dimension that the essay does not address, though it too is illuminated by Hütter’s insights. I propose in this essay to supplement Hütter’s account with some remarks on this dimension. Sex and Human Nature The political dimension I want to address is neither peripheral to chastity nor related to it only contingently. For, together with the moral and spiritual aspects of chastity, it follows directly and necessarily from our nature as rational social animals. This is evident from the traditional Thomistic natural law account of the foundations of sexual morality, so I will begin with an exposition of that. The fundamental way in which we are social animals is by being familial animals. And sex—both in the sense of there being two sexes, and in the sense of the sexual act—exists for the purpose of creating new families. In particular, the distinctive physiology and psychology of male human beings exists for the sake of making them fathers, and the distinctive physiology [End Page 1257] and psychology of female human beings exists for the sake of making them mothers. Of course, not all men and women actually become fathers and mothers, but the point is that that is what their being either men or women in the first place is for. If we did not reproduce in a way that required fathers and mothers, there would be no males and no females. Hence there would be no sex organs, no sexual arousal, and no sexual act. Now, the most obvious respect in which sex has this teleology is that male sexual physiology and arousal have the biological function of getting semen into the vagina, whereas female sexual physiology and arousal have the biological function of facilitating reception of the semen, so as to get the sperm it contains into proximity with an ovum, so that pregnancy will result. It is often assumed that getting this plumbing right is the main concern of the natural law theorist. Nothing could be further from the truth. To be sure, the natural law theorist does insist on getting the plumbing right, but that is because the plumbing ultimately exists for the sake of a larger and more important end—just like a beaver’s teeth ultimately exist for the sake of building shelters for beaver families, their function of gnawing trees so as to provide materials for beaver dams (which in turn provide the setting for the shelters) being merely an essential means to that end.2 The locus classicus for Aquinas’s treatment of these matters is the discussion in Summa contra gentiles III, ch. 2, nos. 122–26. There is a little bit there about emissions of semen and the like, but there is much, much more about what children and mothers need in order for family life to be possible, and how fathers have to provide it. That is to say, Aquinas’s treatment of what it is to be a man or a woman goes well beyond having sex organs of a certain kind and using them in a certain way, and that is exactly what we should expect given that we are social animals, and rational social animals. Sex is for making you a father or a mother, with all that that entails given our social and rational nature, and any deliberate use of sex that positively frustrates that end (with all that it entails given our rational and social nature) is as contrary to what is good for us as breaking off teeth or gnawing only rocks rather than trees is contrary to what is good for beavers. Now, one way this might happen is when a man sleeps with a woman to whom he has not committed himself in the way definitive of marriage. For...