Observations of Weddell seal ( Leptonychotes weddellii ) supernumerary nipples

A wide variety of nipple locations and configurations exist among pinnipeds. Like all marine mammals, pinnipeds can have supernumerary nipples that form in utero. Supernumerary nipples have been documented in several species of pinnipeds, the only taxonomic group of marine mammals with variation in nipple number. We document multiple observations ( n = 4) of female Weddell seals ( Leptonychotes weddellii ) with supernumerary nipples in Erebus Bay, Antarctica, including a female Weddell seal with four nipples that was observed nursing two pups. Intraspecific variation in the number of supernumerary nipples observed included both one and two supernumerary nipples. The majority of the observed supernumerary nipples were nursed on by pups, but lactation was unable to be confirmed. These are the first documented observations of supernumerary nipples in Weddell seals.


Introduction
Marine mammals exhibit significant variation in nipple location and configuration (Stewart and Stewart 2002).The only taxonomic group of marine mammals with variation in nipple number is Pinnipedia, which includes seals, sea lions, fur seals, and walruses.Otariids (i.e., sea lions and fur seals) and odobenids (i.e., walruses) have four nipples on the abdomen: one pair caudal to and the other pair rostral to the umbilicus (King 1983).Most phocids (i.e., true seals such as harbor seals and Weddell seals) have two nipples on their abdomen caudal to the umbilicus (King 1983;Oftedal et al. 1987).The exceptions within the Phocidae family are the bearded seal (Erignathus barbatus) and the three species of monk seal (genera Neomonachus and Monachus), each of which have four nipples in the same configuration as that found in Otariidae and Odobenidae (King 1983;Riedman 1990).The most ancestral phocids are thought to be relatives of the bearded seal, suggesting that four nipples are an ancestral trait derived from a shared ancestor (Schulz and Bowen 2005;Paterson et al. 2020).
For all mammals, the number of teats is correlated with litter size, with average litter size equal to half the number of teats and maximum liter size equal to the total number of teats present (Koyama et al. 2013;Rawal 2019).This holds true in pinnipeds who generally give birth to only one pup and have two nipples (one pair; Boness and Bowen 1996;Nowak 2003).However, as noted above, there are exceptions such as the Hawaiian monk seal (Neomonachus schauinslandi) that has four functional teats but typically only gives birth to one pup (Kenyon and Rice 1959).The maximum litter size for pinnipeds is two, though twinning is infrequent (King 1983).In Weddell seals (Leptonychotes weddellii), twinning occurs in about 0.1% of births (Gelatt et al. 2001).Pinnipeds have also been found to foster and nurse abandoned pups, but again, this happens at a low rate (Fogden 1971;Stirling 1975;Riedman and Le Boeuf 1982).Pinnipeds follow the mammalian pattern for teat number, and it seems that additional nipples in pinniped species are a result of an ancestral trait as opposed to an increased litter size.
Supernumerary nipples are the result of accessory mammary tissue (Cedenilla et al. 2022).They may be nonfunctional (polythelia) or contain mammary glands with the potential to produce milk (polymastia; Galli-Tsinopoulou et al. 2001).Accessory nipples form during embryogenesis, arising from the pair of mammary ridges that extend from the mid-axilla (i.e., upper chest) to inguinal (i.e., groin) area along the ventral body wall (McLaughlin et al. 2008).In humans, polythelia is more common than polymastia and may be present in all stages of life, while polymastia may only be evident during or after pregnancy or lactation (Galli-Tsinopoulou et al. 2001).Polythelia and polymastia have not been extensively studied in other mammals, so the frequency at which each occurs is unknown.
The literature is sparse with observations of supernumerary nipples in pinnipeds.Laws (1956) observed a southern elephant seal (Mirounga leonina) with two supernumerary nipples.Two Mediterranean monk seals (Monachus monachus) were observed, each with one supernumerary nipple (Cedenilla et al. 2022).Supernumerary nipples were found in three northern fur seal (Callorhinus ursinus) hides examined, two with one extra nipple and one with two extra nipples (Scheffer 1962).In the same study, there was a field observation of a female with one supernumerary nipple that appeared to be in use, but it is unclear if the nipple was, in fact, suckled on or produced milk (Scheffer 1962).The limited literature may be a result of the infrequency of supernumerary nipples or simply a lack of published observations.The objective of this paper is to document the occurrence of supernumerary nipples in Weddell seals and to add to the limited literature on this phenomenon.

Methods
The largest population of Weddell seals is found in Erebus Bay, Antarctica (− 77°62′ to − 77°87′S, 166°3′ to 167°0′°E) and has been intensively studied in a capture-mark-recapture project for over 50 years (Stirling 1969;Siniff et al. 1977;Cameron and Siniff 2004;Hadley et al. 2006;Paterson et al. 2018;LaRue et al. 2021;Rotella 2022).During each field season from October to December, hundreds of female adults and pups are observed daily as part of the tagging work associated with the long-term project, but their morphology is not analyzed frequently as it is not the main research focus.Opportunistic field observations of supernumerary nipples were made during three field seasons (2017,2018,2022) while researchers were completing capture-mark-recapture work.This brief note aims to provide descriptions on four incidental observations of supernumerary nipples of Weddell seals in Erebus Bay, Antarctica; however, the supernumerary nipples of the individual seals were not monitored annually nor were their physiology studied.
Like most phocids, Weddell seals tend to give birth to just one pup, although twinning and adoption occur rarely (Stirling 1975;Gelatt et al. 2001).Weddell seals usually have two nipples.On a full-grown adult, each nipple is about 15 cm caudal to the umbilicus and six cm from the ventral midline (Tedman and Bryden 1981).Additional nipples are considered supernumerary.

Results
The first observation of supernumerary nipples occurred in 2017 when two pups were observed nursing on a female Weddell seal (unique identifying number: 17725) with four nipples (Fig. 1).Two of 17,725's nipples were in standard body locations and appeared to be of typical formation for Weddell seal females (Tedman and Bryden 1981).The two supernumerary nipples were located caudal to the normal nipples, one on each side of the midline.The extra nipple on her right side was approximately six cm from the midline, which is the same distance as a normal nipple from the midline (Tedman and Bryden 1981).The extra nipple on her left side was located a little further caudally and laterally from the umbilicus.We did not make any measurements in the field but used ImageJ software to make measurements on photos taken in the field (Schneider et al. 2012).The observed female with four nipples was 11 years old when observers first noted her supernumerary nipples.She was recorded three times in 2017, always with two pups.It is unknown if she gave birth to both pups or if she adopted a second pup shortly after parturition.All four nipples were observed to have been suckled on by the pups as evidenced by the wet patch of fur surrounding the nipples and the engorged nipples, though we did not physically test if all four were lactating.She had previously given birth to one pup in the study area in 2014, but researchers did not document the presence of supernumerary nipples at that time.
During the field season of 2018, we observed two different female Weddell seals that each had three nipples.The first (unique identifying number: 25065) was of unknown age and was observed with a pup in 2018.She had been observed with one pup in a previous year, but the presence of supernumerary nipple was not noted at this previous observation.The second (unique identifying number: 14591) was 18 years old at the time she was noted with supernumerary nipples and observed with a single pup.Despite having had 11 previous pups across 11 different years, the presence of a supernumerary nipple had not been documented in previous years.The supernumerary nipples on both females were in very similar locations: on the left side of the midline, almost directly rostral to the normal nipple, and just over halfway towards the umbilicus.When first observed, the supernumerary nipples did not appear to be suckled on because there was no visible wet fur around the nipples and the nipples were flush to the body instead of engorged and extruded as occurs when suckling happens (Tedman and Bryden 1979).However, on November 28, 2018, we observed that the supernumerary nipple on seal 14,591 had been suckled on because it became engorged and had wet fur surrounding it, but we do not know if lactation occurred because no milk was observed passing through the nipple.
During the field season of 2022, we observed one 9-yearold female Weddell seal (unique identifying number: 22187) with three nipples that gave birth to her first pup.This female's supernumerary nipples were in a similar location to those on females 25,065 and 14,591.Again, the supernumerary nipple was on the left side of the midline, rostral to the umbilicus, and appeared to have been suckled on as determined by a wet patch of fur surrounding the nipple and a slightly engorged nipple, but lactation was not confirmed (Fig. 2).
In total, four different Weddell seals were observed with one or two supernumerary nipples and a total of three to four nipples instead of the typical two (Table 1).Each of the female seals was observed with at least one pup, and most of the supernumerary nipples were suckled on.We were unable to confirm lactation for the four different observations of supernumerary nipples in Weddell seals, and without further physiological studies, it is impossible to determine if the supernumerary nipples were functional (polymastia) or nonfunctional (polythelia).

Discussion
Despite the fact that the Erebus Bay Weddell seal population is the most heavily studied population of Weddell seals in the world (Rotella 2022), these confirmed sightings provide the first known record of Weddell seals with supernumerary nipples.Approximately 600-1500 female Weddell seals are observed each year in Erebus Bay, and, given that only a few females have been observed with supernumerary nipples, the frequency of supernumerary nipples appears to be quite low, substantially less than 1% of the population.Since these observations were incidental, it is probable that there are more females with supernumerary nipples in the population than are noted here.Further work is required to accurately estimate the frequency of supernumerary nipples in the population.
It is improbable that supernumerary nipples hold any fitness benefit for the female seal or evolutionary strategy for raising more than one pup.Previous work in humans has shown an association with supernumerary nipples and malformations in the urinary tract as well as cardiovascular issues (Pellegrini and Wagner 1983;Brown and Schwartz 2003).If this is the case for Weddell seals and other pinniped species, mothers with supernumerary nipples may have decreased fitness.Additionally, in humans and primates, there is an association with supernumerary nipples and a higher frequency of twinning, although this has not  looked at in pinnipeds (Speert 1942;Hsu et al. 2000).Because of the high maternal investment in Weddell seals, it is difficult for a mother to successfully raise two pups (Macdonald et al. 2020).As such, supernumerary nipples may decrease an individual's fitness if the extra nipples result in a higher likelihood of twinning.More work is needed to determine if supernumerary nipples incur a fitness cost in Weddell seals.

Fig. 1
Fig. 1 Seal 17,725, an 11-year-old female, observed in 2017 with two supernumerary nipples.In Image A, one pup nurses on a supernumerary nipple circled in black with the normal nipple located rostrally in a blue triangle.In Image B, the two supernumerary nipples are circled in black with the two normal nipples in blue triangles.Photo credit: Alissa K. Anderson, taken under NMFS permit No. 21158

Fig. 2
Fig. 2 The supernumerary nipple on 22,187, a 9-year-old Weddell seal, in 2022.The third nipple, circled in black, is located roughly halfway between the normal nipple (blue triangle) and the umbilicus on the left side of midline.The head of the seal is to the left and the tail is to the right.Photo Credit: Parker M. Levinson, taken under NMFS permit No. 26375