docs: def calc_seal_volume(mass_kg, dens_kgm3, length=None, girth=None):
      '''Calculate an animal's volume from mass and density or length and girth
  
      Args
      ----
      mass_kg: float or ndarray
          Mass of animal (kg)
      dens_kgm3: float or ndarray
          Density of animal (kg/m^3)
      length: float or None
          Length of animal. Default `None` (m)
      girth: float or None
          Girth of animal. Default `None` (m)
  
      Returns
      -------
      vol_kgm3: float or ndarray
          Volume of animal (m^3)
      '''
      if (length is not None) and (girth is not None):
          _, seal_vol = surf_vol(length, girth)
      else:
          seal_vol = mass_kg / dens_kgm3
  
      return seal_vol