Brief note: On the mass of the large fossil river dolphin, Pebanista yacuruna.

One of my ongoing side projects is developing a comprehensive database of weight data for small odontocetes ranging from belugas, dolphins, pygmy & dwarf sperm whales, etc. It is naturally very challenging as such data is more widely collected as opposed to that of large species. Every now and again, I may be encouraged to dedicate a post to a certain portion of this dataset. Now appears to be a good time with the recent discovery and description of a prehistoric species of river dolphin (Benites-Palomino et al., 2024).

Introduction

‘River dolphin’ is a term used to describe several species of small odontocetes that have adapted to freshwater and estuarine environments, and are found in various parts of Asia and South America. Despite their name, most of these species exist outside of the families of oceanic dolphins and porpoises. Four these genera were once united under one family, Platanistidae, however they were eventually placed in their own families (Zhou, 1982).

Modern phylogenetic trees suggest that the La Plata dolphin (Pontoporia blainvillei), boto (Inia spp.), and the Chinese baiji (Lipotes vexifiller) form their own clade, while the South Asian species (Platanista spp.) evolved from a very basal lineage of toothed whales (McGowen et al., 2020). The Tucuxi (Sotalia spp.) actually are ‘’true dolphins’’ of the Delphinidae family.

Below I provide a summary of the average size of physical maturity (L_∞) and largest reliable sizes for some of the major species of each genus (L_max).

Table 1: Size parameters for different species of River dolphins

Species	L∞ (cm) M/F	Lmax (cm) M/F	Source
Inia geoffrensisa	231.5 / 199.8	255 / 225	(Best & da Silva, 1984; Martin & Da Silva, 2006)
Platanista gangetica	205b / 250b	212 / 267	(Anderson, 1878; Braulik et al., 2021)
Pontoporia blainvilleic	130 / 155	152 / 177	(Barreto & Rosas, 2006; Beneditto & Ramos, 2001; Botta et al., 2010; Kasuya & Brownell, 1979)
Sotalia guianensis	185d	222 / 208	(De O. Santos et al., 2003; Meirelles et al., 2010; Ramos et al., 2010; Ramos & Lima, 2000)
Sotalia fluviatilis	—b / —b	149 / 152	(Silva, 1994)
Lipotes vexiffiler	—b / —b	229 / 253	(Brownell & Herald, 1972; Chen & Chen, 1975)

a. Corresponds specifically to those sampled from the Amazon River basin.
b. Data deficient.
c. Best reflects populations along coasts of southern Brazil, Uruguay, and Argentina. Populations from Rio de Janeiro, São Paulo, and Espírito Santo are quite smaller (Barreto & Rosas, 2006; Ramos et al., 2002)
d. There’s generally no consistent hints of sexual dimorphism as noted for other species (Lima et al., 2016; Rosas et al., 2003).

Caution should be placed in extending the values I report here to congeneric species or different morphs, as adult size and sexual dimorphism can vary considerably (Braulik et al., 2021; Silva, 1994; Silva et al., 2023). For Inia geoffrensis, older literature acknowledged a 274 cm male and 228 cm female respectively recorded in Peru (Layne, 1958) and the Orinoco river basin (Trebbau, 1975). However, it is suspected that these were not standardized measurements (Silva, 1994). So, for the time being, I will stick firmly with the largest individuals documented in more modern research.

Despite having been discovered in South America, the newly described fossil species Pebanista yacuruna is a close relative of the extant Ganges (P. gangetica) and Indus River (P. minor) dolphins (Benites-Palomino et al., 2024). On top of that very interesting detail, Pebanista is also quite large compared to any of the modern species, at an estimated range of 2.8-3.5 m based on the bizygomatic width (BZW) of these two specimens (Benites-Palomino et al., 2024). However, the authors consider these estimates to be conservative, as the BZW extrapolation is prone to underestimating the true size in similar taxa.

Mass data

I’ve currently assembled 414 individual weights for the above-mentioned genera that I’ve perused from existing literature and the USNM records. As can be seen in Figure 1, there’s a lot of variation at equal length. AIC model selection suggests that there’s a significant difference in the weight relationships between these species (ΔAICc= 72.09). This basically means that including the species as an additional variable justifies the cost of making the model more complicated, which is usually the case when the difference between two model’s AIC scores (ΔAICc) exceeds 2.

Figure 1: Mass data for River dolphin species.

As of now, I believe my dataset is comprehensive of the existing data for South Asian River dolphins, though it’s still a work in progress for other genera. I still lack a rather large sample (n=378) collected for the boto between 1994-2004 (Martin & Da Silva, 2006). I’m also aware of some larger datasets for Lipotes and Pontoporia, which I provide the existing regressions for in Table 2 (Brownell, 1984).

Update: 4:40 PM EST March 24th, 2024- I’ve since updated my dataset , which I can now proudly say is comprehensive for the existing weight data for the baiji (L. vexillifer) . I will leave Brownell’s regression, but will update the parameters for my baiji weight formula and the pooled sample. The corresponding mass estimates for Pebanista will also be updated.

Table 2: Mass-length formulae parameters for river dolphins

Genus	a*	b	N	r2	Source
Pooled sample	13.30	2.672	430	0.90	Current post
Inia	15.37	2.553	129	0.84	Current post
Platanista	12.45	2.519	141	0.90	Current post
Pontoporia	14.16	2.231	89	0.88	Current post
Pontporia	15.43 (M) 13.42 (F)	1.518 (M) 2.512 (F)	75 (M) 56 (F)	0.78 (M) 0.95 (F)	(Brownell, 1984)
Sotalia	15.47	2.491	42	0.90	Current post
Lipotes	20.23	2.370	28	0.76	Current post
Lipotes	17.47 (M) 3.739 (F)	2.445 (M) 4.218 (F)	12 (M) 8 (F)	0.91 (M) 0.90 (F)	(Brownell, 1984)

*All constants of proportionality converted to units of meters and kilograms.

The maximum weight recorded from a non-pregnant river dolphin was 207 kg for a male boto from the Amazon (Martin & Da Silva, 2006). The corresponding length was not cited, so it was not included in the regression. Two baiji weighing 224 kg and 237 kg appear to be the heaviest pregnant individuals recorded (Chen & Chen, 1975).

Table 3: Mass estimates for Pebanista

Formula type	Holotype (280 cm) Mean mass (kg) (95% Prediction interval)	MUSM 3953 (347 cm) Mean mass (kg) (95% Prediction interval)
Pooled sample	208.9 (173.8-251.1)	371.2 (308.1-447.2)
Inia	212.9 (172.0-263.6)	368.2 (295.2-459.3)
Platanista	166.7 (132.5-209.6)	286.1 (226.0-362.2)
Pontoporia	140.9 (114.4-173.5)	227.4 (182.1-284.0)
Sotalia	201.1 (158.8-254.6)	343.1 (265.9-442.7)
Lipotes	202.7 (162.9-252.0)	337.0 (266.0-426.9)

Table 3 clearly shows that the expected mass for Pebanista varies greatly depending on the sample. In my opinion, the mass of Pebanista is probably best represented by either its closest relative, Platanista, or its modern ecogeographical counterpart, Inia. This would likely mean the typical adult of this species ranged from 150-300 kg.

Concluding thoughts

Aside from its large size and unusual geographic location, Pebanista is a very foundational discovery for our knowledge on the independent marine-freshwater transitions within Cetacea. This was an interesting departure from my typical focus on larger species. I got this post out late due to me trying to compare all the growth studies for the La Plata and Guiana dolphins. I’m looking forward to future opportunities to share progress on what will probably remain an ongoing effort until I’m dead.

References

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